Part 2_free And Forced Convection (1)

TD1005 Free and Forced Convection

User Guide

© TecQuipment Ltd 2015 Do not reproduce or transmit this document in any form or by any means, electronic or mechanical, including photocopy, recording or any information storage and retrieval system without the express permission of TecQuipment Limited. TecQuipment has taken care to make the contents of this manual accurate and up to date. However, there are any errors, please let us know so we can rectify the problem. TecQuipment supply a Packing Contents List (PCL) with the equipment. Carefully check the contents of the package(s) against the list. If any items are missing or damaged, contact TecQuipment or the local agent.

DB/ad/BW/0518

Symbols used in this manual

NOTE

CAUTION

WARNING

User Guide

Important information.

Failure to follow these instructions can damage the unit, other equipment, personal property or the environment.

Failure to follow this instruction may cause injury.

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TD1005 Free and Forced Convection

Contents Introduction Description

.................................................................. 1

................................................................... 3

The Main Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 The Heat Transfer Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Versatile Data Acquisition System (VDAS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Technical Details

............................................................ 9

Main Unit and Duct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Heat Transfer Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Noise Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Assembly and Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11

Location and Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrical Connection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Connection to VDAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11 15 16 16

Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heat Transfer by Conduction, Radiation and Convection . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Conductivity of Air (kair) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of Airflow and Pressure Drop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrical Power, Heat Quantity and Heat Transfer Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Lag and Gradient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Inertia (or Thermal Mass) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Horizontal and Vertical Finned Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Heat Transfer Coefficient (Convective) (hc) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nusselt Number (Nu). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applying Theory to the Duct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

17 18 19 20 21 21 22 23 23

Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

27

Safety. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Useful Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . To Fit a Heat Transfer Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . To Fit or Remove the Fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magnetic Cover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . To Use the Probe Traverse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Using the Surface Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experiment 1: Free Convection - Fixed Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

27 27 28 28 29 29 29 31

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Experiment 2: Free Convection - Quick Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experiment 3: Free Convection - Power and Temperature Relationship . . . . . . . . . . . . . . . Experiment 4: Forced Convection - Fixed Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experiment 5: Forced Convection - Flow Obstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experiment 6: Temperature Distribution Across the Extended Surfaces . . . . . . . . . . . . . . . Experiment 7: Forced Convection - Effect of Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experiment 8: Heat Transfer Coefficient and Nusselt Number . . . . . . . . . . . . . . . . . . . . . . . Experiment 9: Free Convection from Horizontal and Vertical Surfaces . . . . . . . . . . . . . . . .

33 37 39 41 43 47 49 53

Typical Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Experiment 1: Free Convection - Fixed Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experiment 2: Free Convection - Quick Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experiment 3: Free Convection - Power and Temperature Relationship . . . . . . . . . . . Experiment 4: Forced Convection - Fixed Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experiment 5: Forced Convection - Flow Obstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . Experiment 6: Temperature Distribution Across the Extended Surfaces . . . . . . . . . . Experiment 7: Forced Convection - Effect of Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . Experiment 8: Heat Transfer Coefficient and Nusselt Number . . . . . . . . . . . . . . . . . . . Experiment 9: Free Convection from Horizontal and Vertical Surfaces . . . . . . . . . . .

55 56 57 58 58 59 60 61 63

Useful Textbooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Maintenance, Spare Parts and Customer Care . . . . . . . . . . . . . . . . . . . . . . . 67 Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Spare Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 Customer Care . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

TD1005 Free and Forced Convection

User Guide Introduction

Figure 1 The Free and Forced Convection Apparatus (TD1005) This product works with VDAS®

Engineers learning about thermodynamics and heat transfer need to know how well different surfaces and shapes convect heat. They can use this information to predict how heat energy convects from the surfaces of their own designs and which surface works best for any given purpose. TecQuipment’s Free and Forced Convection Apparatus shows students how heat convects from a choice of three different surfaces, both under free (or ‘natural’) convection and under forced convection. To automatically record experiment results and save time, the apparatus works with TecQuipment’s Versatile Data Acquisition System (VDAS®). VDAS is a registered trademark of TecQuipment Ltd. TecQuipment Ltd

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TD1005 Free and Forced Convection

User Guide

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TD1005 Free and Forced Convection

Description

Figure 2 The Free and Forced Convection Apparatus (TD1005)

The Main Unit The Main Unit is a compact bench-mounting frame that connects to a suitable electrical supply. It has a vertical duct assembly and a main ‘Control Panel’ with electrical controls and displays. Each of the three heat transfer surfaces (supplied) fit into the back of the vertical duct, at just above halfway up the duct.

NOTE

Only test one heat transfer surface can be tested at a time.

The vertical duct allows air to pass over the heat transfer surface, both by free convection, or by forced convection using a removable variable speed electric fan at the top of the duct. A fixed thermocouple probe measures the inlet (ambient) air temperature in the duct. A movable thermocouple probe in a traversing mechanism allows measurement of the temperature distribution across the duct at the outlet. This allows students to find the bulk outlet temperature for the more advanced calculations. An anemometer measures air velocity in the duct. Each heat transfer surface includes a built-in thermocouple to measure its surface temperature. The equipment also includes a hand-held thermocouple probe for heat distribution measurement along the finned and pinned heat transfer surfaces. The user inserts the probe tip into a selection of six equallyspaced holes in the side of the duct (see Technical Details for distances). A magnetic cover allows the holes to be covered completely or so that only use one at a time can be used, reducing stray convection caused by the other holes.

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TD1005 Free and Forced Convection

Figure 3 The Magnetic Cover The Base Unit supplies safe, low-voltage electrical power to the heater (heat source) in each heat transfer surface and a variable supply for the fan at the top of the duct. The thermocouples in the duct and the thermocouple on the heat transfer surface connect to sockets on the front of the control panel. For heat distribution experiments with the hand-held probe, the user connects the probe to any unused thermocouple socket (not all are used for each experiment). A display on the control panel shows the electrical power supplied to the heater in the heat transfer surface, the air velocity in the duct and the temperature at each of the three thermocouples connected. A socket on the control panel allows connection to TecQuipment’s optional VDAS for data acquisition from this equipment, with the use of a suitable computer (not supplied).

User Guide

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TD1005 Free and Forced Convection

The Heat Transfer Surfaces TecQuipment include three different heat transfer surfaces to allow students to compare their performance. Each heat transfer surface fits quickly and easily into a square opening in the back side of the duct. A foam gasket around each surface helps to seal it from stray convection when fitted to the duct. Each heat transfer surface has two sides: An exposed front side. It has a light grey bare metal finish. This helps to ensure more accurate experiment results because most heat transfer is then by convection rather than radiation (a dark matt black surface increases heat loss by radiation). In actual applications, heat transfer surfaces are often coated matt black to maximise heat transfer by both convection and radiation. An enclosed back side. This has a surface-bonded electrical heater and a centrally-mounted thermocouple. The heater includes a thermal safety cut-out switch to help prevent overheating. Insulation surrounds the back side. This prevents unnecessary heat loss which would affect the experiment results.

Flat Plate

Figure 4 The Flat Plate This is simply a flat aluminium plate. This surface is unique from the other two, in that it fits completely flush with the inner wall of the duct and has no extra fins or pins that penetrate the duct airflow.

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TD1005 Free and Forced Convection

Finned Surface

Figure 5 The Finned Surface This is a popular surface design used in for ‘heat sinks’ to transfer heat away from components in electrical and electronic circuits. It is also used on air-cooled combustion engines or compressors to help transfer heat away from the cylinder walls. It effectively increases the available heat transfer surface area to help transfer more heat into the surrounding air (or capture heat from the surrounding air if used in reverse). This surface is useful for a demonstration of free convection both vertically (up from the fins) and horizontally (with fins horizontal). The holes in the side of the duct allow the user to probe the temperature along a fin, to see the change in temperature (heat distribution) along it.

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TD1005 Free and Forced Convection

Pinned Surface

Figure 6 The Pinned Surface This is a popular surface design used in ‘heat exchangers’, where one fluid flows along the pins (usually hollow tubes) at right angles to the flow of another fluid that passes around the pins. The heat energy in the hotter fluid passes through the surface of the pins or tubes into the colder fluid. Again, as with the finned surface, this surface effectively increases the available heat transfer surface area to help transfer more heat into the surrounding fluid (or air as used in the experiment). The holes in the side of the duct allow the user to probe the temperature along a pin, to see the change in temperature (heat distribution) along it.

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TD1005 Free and Forced Convection

Versatile Data Acquisition System (VDAS)

Figure 7 The VDAS Hardware and Software TecQuipment’s VDAS is an optional extra for the Free and Forced Convection equipment. It is a two-part product (Hardware and Software) that will: •

automatically log data from experiments

•

automatically calculate data

•

save time

•

reduce errors

•

create charts and tables of data

•

export data for processing in other software

NOTE

User Guide

A suitable computer (not supplied) is needed to use TecQuipment’s VDAS.

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TD1005 Free and Forced Convection

Technical Details Main Unit and Duct

Item

TecQuipment Ltd

Details

Operating Environment

Indoor (laboratory) Altitude up to 2000 m Temperature range 5°C to 40°C Maximum relative humidity 80% for temperatures up to 31°C, decreasing linearly to 50% relative humidity at 40°C Overvoltage category 2 (as specified in EN61010-1). Pollution degree 2 (as specified in EN61010-1).

Nett Dimensions (assembled)

550 mm front to back, 850 mm wide and 1200 mm high

Nett Weight (assembled)

Main unit: 26 kg (with no heat transfer surface fitted)

Electrical Supply

Single phase 50 Hz to 60 Hz 100 VAC to 120 VAC at 1.2 A or 220 VAC to 240 VAC at 0.6 A Specified on order

Fuse

20 mm 6.3 A Ceramic Type F

External connections

Heater, air velocity sensor, thermocouple and VDAS sockets  - Extra Low Voltage (<25 VDC) Fan socket (rear of equipment) - 0 VAC to mains supply voltage.

Thermocouple inputs

3 off type K Displayed resolution 0.1°C

Heater output and display

Maximum power approximately 100 W Displayed resolution 0.1 W

Duct

Nominal internal cross section: 128 mm x 75 mm = 0.0096 m2 Approximate length: 850 mm Nominal air velocity: Greater than 3.8 m.s-1 with flat plate. Normal experiment velocity: 3.5 m.s-1or less. Probe positions (away from heat transfer surface): 7.5 mm, 19.5 mm, 31.5 mm, 43.5 mm, 55.5 mm and 67.5 mm

Anemometer Range

0 to 3.8 m.s-1

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Heat Transfer Surfaces

Experiments Flat Plate

Details Nett Dimensions: 160 mm x 140 mm x 55 mm and 810 g Plate material: 3 mm thick Aluminium Total surface area: 106 mm x 106 mm = 0.0112 m2 K- type thermocouple on back side of plate surface.

Finned Surface

Nett Dimensions: 160 mm x 140 mm x 125 mm and 1227 g Flat plate with six fins at right angles to the plate. Plate material: 3 mm thick Aluminium 106 mm x 106 mm Fin material: Stainless Steel Fin dimensions: 90 mm x 73 mm x 1.5 mm thick. Total surface area: 0.092 m2 (including ends of fins) K- type thermocouple on back side of plate surface.

Pinned Surface

Nett Dimensions: 160 mm x 140 mm x 125 mm and 1836 g Flat plate with 18 pins at right angles to the plate. Plate material: 3 mm thick Aluminium 106 mm x 106 mm Pin material: Stainless Steel Pin Dimensions: 12 mm diameter x 73 mm Total surface area: 0.027 m2 (including ends of pins) K- type thermocouple on back side of plate surface.

Noise Levels The noise levels recorded at this apparatus are less than 70 dB (A).

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Assembly and Installation The terms left, right, front and rear of the apparatus refer to the operators’ position, facing the unit.

NOTE

A wax coating may have been applied to parts of this apparatus to prevent corrosion during transport. Remove the wax coating by using paraffin or white spirit, applied with either a soft brush or a cloth. Follow any regulations that affect the installation, operation and maintenance of this apparatus in the country where it is to be used.

Location and Assembly Use the Free and Forced Convection equipment in a clean, well-lit laboratory or classroom type area. Put the equipment on top of a solid, level workbench, away from doors, windows or ventilation machines.

NOTE

WARNING

For consistent and sensible results, always use the equipment in a room that has a stable temperature of around 20°C and lowest possible air movement.

Use assistance to assemble and move the equipment.

The Base Unit uses a bench area of 550 mm x 850 mm. If using the optional VDAS, allow room nearby for the bench-top unit (VDAS-B) and a computer. TecQuipment may have removed the legs from the Main Unit for Transport. Use the fixings supplied to fit them to the side of the Main Unit, then use the adjustable feet to level the Main Unit.

Figure 8 Fitting the Legs

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Duct Assembly and Duct Probes The Duct Assembly, Duct Traverse Probe and Fan may be removed from the Base Unit for transport. To fit them: 1. Put the Base Unit onto the desk or workbench. 2. Carefully hold the duct against the four fixing holes to the front right hand side of the Base Unit. Fit the four fibre washers between the duct fixings and the Base Unit (see Figure 9). 3. See Figure 10. Use the nuts supplied to fix the duct to the Base Unit, then connect the short earth cable from the duct to its stud on the Base Unit. 4. Push the anemometer probe into its socket in the duct so the hole in its tip is in the middle of the duct and rotate it so its hole aligns with the air flow (see Figure 11).

NOTE

The anemometer may have an arrow on its end indicating flow. Make sure it points in the correct direction (upwards).

5. If the fan is not already fitted, leave it resting on the workbench until needed in the experiments. The experiment section shows how to fit the fan. 6. Use a flat-blade screwdriver and the large screw (with a hole drilled through it) to fix the traverse probe to the duct (see Figure 12). 7. Carefully slide the probe into the traverse and fit the small thumbscrew (see Figure 13). 8. Insert the duct inlet probe (Figure 2) fully into the duct (so that the tip is approximately 20 - 25 mm from the duct wall). Ensure that the compression fitting is locked. 9. Plug the inlet duct cable into the socket labelled T1 on the front of the apparatus.

CAUTION

User Guide

Do not adjust the small grub screw that holds the nylon probe bush to the probe (see Figure 13). TecQuipment set this to calibrate the probe position correctly.

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Figure 9 Fit the Fibre Washers

Short earth cable.

Four fixings that hold the duct to the main unit

Figure 10 View from Behind Duct

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TD1005 Free and Forced Convection

Figure 11 Fit Anemometer

Figure 12 Use a Flat Blade Screwdriver to Fit the Duct Traverse Probe

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TD1005 Free and Forced Convection

Do not adjust this Figure 13 Insert the Probe and Fit the Small Thumbscrew

Installation 1. Use assistance to put the assembled Base Unit on the workbench.

WARNING

The assembled equipment is heavy (see Technical Details). Use assistance to move or carry it.

2. If using the optional VDAS, put the VDAS-B Hardware onto the workbench and connect as shown in ‘Connection to VDAS’. 3. Connect the electrical supply cable from the back of the Base Unit to the electrical supply (see Electrical Connection). 4. Fit the heat transfer surface as shown in the ‘Experiments’ section. 5. If necessary, fit the fan to the duct as shown in the ‘Experiments’ section. 6. Recheck the level of the equipment and make sure the duct is perfectly vertical. 7. The equipment is now ready for use.

NOTE

TecQuipment Ltd

The experiments show which thermocouples to fit into which sockets, so these may be left disconnected until needed.

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Electrical Connection Use the cable supplied to connect the equipment to the electrical supply. Connect the equipment to the electrical supply through a plug and socket. The apparatus must be connected to earth. WARNING

The mains inlet to the rear of the equipment and the plug and socket form the mains disconnect device. Make sure the user can easily reach them.

These are the colours of each individual conductor:

GREEN AND YELLOW:

EARTH E OR

BROWN:

LIVE, L1 or Hot 1

BLUE:

NEUTRAL

Connection to VDAS If using the optional VDAS, read the VDAS User Guide and connect the socket (marked ‘VDAS’) on the Control Panel to any of the six sockets (marked ‘Digital Inputs’) on the VDAS-B Interface.

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Theory This section only gives the basic information needed to do the experiments. For full theory, refer to the textbooks listed in Useful Textbooks on page 65.

Notation

Symbol

Units

Ad As

Cross-sectional area of the duct Cross-sectional area of the surface

m2

hc

Convective heat transfer coefficient

W/m2K

kair

Coefficient of thermal conduction of air or Thermal Conductivity

W/mK

L

Length or thickness

m

Mass and mass flow

kg and kg.s-1

c

Heat Capacity

J/K

Q

Heat Quantity

J

air

Air density

kg/m3

Cpa

Specific Heat Capacity of Air

kJ/kg K Nominally 1005 between 0 and 40°C

Qꞏ

Heat energy transferred for a unit time (heat transfer rate)

J.s-1 or W

W

Electrical energy

W

T

Temperatures: Difference in temperature Surface temperature Inlet (ambient) temperature Outlet downstream temperature Mean temperature (logarithmic)

K (or °C where shown)

Time

Seconds

m,

mꞏ

T TS Tin Tout Tm t

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Definition

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TD1005 Free and Forced Convection

Heat Transfer by Conduction, Radiation and Convection Heat or internal energy is the kinetic energy of the molecules in a substance (solid, liquid or gas) as they vibrate or move around. Kinetic activity increases with temperature (the molecules move faster). When a hot substance touches a colder substance, the kinetic activity passes on through the contact point. The molecules in the hotter substance pass their energy to the molecules in the colder substance. The hotter substance cools and the colder substance warms up. Heat transfers from the hotter substance (the heat source) to a colder substance (the heat sink), until the two materials reach equilibrium.

Figure 14 Heat Energy Transfer Between Hot and Cold Substances Heat transfers from one body to another by three methods, conduction, convection and radiation. Most real-world heat transfer uses elements of all three. •

In conduction, heat transfers from one molecule to the next.

•

In convection, heat transfers by a fluid flow (liquid, air or gas).

•

In radiation, heat transfers through electromagnetic radiation.

Figure 15 Conduction, Convection and Radiation When trying to find the heat conduction properties of a material, always try to minimize the energy lost from the material by radiation and convection, as the results will be wrong. Insulation helps to stop these losses.

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TD1005 Free and Forced Convection

Free Convection This is when the heat transfers from the object under the influence of fluid (air) density changes. The heat energy around the object causes the air density around the surface of the object to decrease. The reduced density air is more buoyant than the surrounding air and rises, transporting the heat energy away naturally. In normal conditions gravity is the main force affecting buoyancy and therefore convection. However, where the object forms part of a rotating machine, centrifugal force can be a driving force for convection.

Forced Convection This is when an external force moves air around or across the surface. The movement of air transports the heated air away from the object. The higher the air velocity, the faster it transports heat away from the object.

Thermal Conductivity of Air (kair) Some materials (including fluids) are better heat conductors than others; their chemical and atomic structure affects the rate of heat transfer. This effect is its thermal conductivity (k). It is a measure of how quickly heat energy travels along a unit length of material of a unit cross-sectional area. The thermal conductivity of air increases almost linearly with temperature over the range 0 to 100°C.

Thermal Conductivity of Air 0.032

Thermal Conductivity (W/mK)

0.031 0.03 0.029 0.028 0.027 0.026 0.025 0.024 0

20

40

60

80

100

Temperature (°C)

Figure 16 Thermal Conductivity of Air

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TD1005 Free and Forced Convection

Comparison of Airflow and Pressure Drop

Figure 17 Comparison of Airflow Through the Pinned and Finned Surfaces The Flat Surface has little or no effect on airflow and sits flush with the duct wall. The main body of the airflow does not pass the surface. The Pinned Surface sits directly in the main body of airflow. It causes turbulent airflow and varied velocities in and around its pins. This surface creates an air pressure difference (drop) between upstream and downstream airflows. The airflow at its inlet may be uniform, but the airflow immediately downstream is turbulent. It causes a greater obstruction to airflow and pressure drop than the flat or finned surfaces. The Finned Surface sits directly in the main body of airflow. It has a less pronounced effect on airflow and pressure drop than the Pinned surface, with a relatively uniform airflow into and out of the fins.

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Electrical Power, Heat Quantity and Heat Transfer Rate Assuming ideal conditions, the electrical power supplied to the heater on each experiment gives a direct and accurate value of the heat energy that it emits or conducts. The electrical power into the heater is the product of the voltage and current supplied to it, so: Electrical Power (W) = Voltage (V) across the heater x Current (I) passing through the heater. Or

W = VI Heat quantity or heat energy (Q) is an amount of energy, usually specified in Joules (J). A rate of heat energy transfer is a given amount heat energy transferred in a given time, or Q/t. To simplify the equations, put a mark above the symbol Q to represent this, so:

Q ---- = Qꞏ t The unit (Watt) of electrical power is also a measurement of rate of energy transfer (one Joule per second), so:

W = Qꞏ

(1)

This shows that the rate of electrical energy supplied to the heater is equal to the energy (heat) transfer rate from the heater.

Thermal Lag and Gradient

Figure 18 Thermal Lag - Assuming no losses Using Figure 18 for reference, as shown earlier in this theory, when heating one end of a conductor, the heat energy moves along the conductor by conduction. It takes time for the heat energy to move from one part of the conductor to another - determined by the thermal properties of the material and its dimensions (its thermal conductance or resistance). Assuming no losses, that heat energy spreads around the conductor until the whole conductor reaches equilibrium and all parts are at the same temperature. The time taken for the heat energy to move from T1 to T2 is the thermal lag.

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In real applications, losses to radiation and convection affect the amount of heat energy that reaches the furthest parts of the conductor and therefore their temperature, so the furthest points may never reach the same temperature as the point near the heat source. The conductor then has a thermal gradient along its length.

Figure 19 Thermal Gradient

Thermal Inertia (or Thermal Mass) Q = mcT

(2)

From the textbook Equation (2), it can be seen that the heat energy (Q) needed to raise the temperature (T) of an object is proportional to the mass (m) of the object (c remains fixed). From this it can be seen that when there are two objects of the same material, the one with the largest mass needs more heat energy to raise its temperature. Inversely, when two objects of the same material but of different mass have the same temperature, the object with the largest mass could contain or store the most heat energy. It has a larger thermal mass. In terms of heat flow therefore, a larger mass takes more time to reach a given temperature than a smaller mass when supplied at the same rate. Again, inversely, a larger mass takes more time to lose energy than a smaller mass when the loss is at the same rate. It has a larger thermal inertia. In mechanical engineering, a flywheel helps to store energy (mechanical inertia) and help damp out transient changes in demand. In electronic engineering, a capacitor helps to store charge and help damp out transient changes in current or voltage. In thermodynamic engineering, a large thermal mass helps to store heat energy to help damp out transient changes in temperatures or heat supply.

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Horizontal and Vertical Finned Surfaces

Figure 20 Vertical, Flat and Horizontal Finned Surface Under natural (free) convection, flat/vertical and vertical/vertical fins have similar performance and allow better heat transfer to the surrounding air than vertical/horizontal fins (this is not true for forced convection). When used as a heat sink for electrical applications, a vertical/horizontal finned surface has as much as 70% less efficiency than a flat/vertical or vertical/vertical surface in free convection. Note however, that even a vertical/horizontal finned surface transfers heat better than a simple flat surface, due to the extended surface area of its fins.

Heat Transfer Coefficient (Convective) (hc) Heat transfer coefficient is a material’s ability to conduct heat to another material. Convective heat transfer occurs between the surface of a material and a moving fluid. Typical values of heat transfer to air are: 5 to 25 W/m2K in free convection 10 to 200 W/m2K in forced convection (showing that heat transfers to air better in forced convection than it does in free convection).

Qꞏ ----------------hc = As  Tm

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Where Tm = logarithmic mean temperature and

Qꞏ is the heat flow from the surface to the air.

T OUT – T IN - (log is natural loge or ln) T m = ------------------------------T S – T IN log ----------------------T S – T OUT

(4)

Nusselt Number (Nu) A Nusselt number applies to heat transfer. It is a dimensionless value of the ratio of convective to conductive heat transfer across a boundary. It can also give an indication of convective flow - a low number (near to 1) shows that flow is laminar, while a high number (greater than 100) shows that flow is turbulent.

hc  L Nu = ------------k air

(5)

Where L is the length of the surface over which the air moves (for the flat plate, this is simply the length of the plate).

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Applying Theory to the Duct For Equations 3 to 5 to work properly it is necessary to first know the heat flow input at the boundary ꞏ between the surface and the air Q , and the temperature leaving the duct (Tout). When applied to a duct this gives two problems:

1. Measuring Heat Flow to the Air In a perfect theoretical application heat flow to the air could be found from:

Qꞏ = mꞏ  C pa  T But the air velocity in the duct may not be even across the duct, giving a velocity profile and a variation ꞏ (see Figure 21). in air mass flow m The air flow sensor measures air flow directly, but as Figure 21 shows, a single point measurement may not give accurate values of velocity (and therefore mass flow) in ducts, especially if the probe itself affects the geometry of the duct. Probe manufacturers usually give a correction factor to allow for certain duct geometry to account for this. In the TD1005, with a heat source towards one side, the problem is made worse when considering that different velocity conditions (for example from free and forced convection) change the velocity profile.

Figure 21 Velocity Profile TecQuipment make the equipment to have minimal heat loss - insulating materials minimise loss by conduction and the bare metal surface helps minimise loss by radiation. This means that losses to the local environment will be small and the electrical input to the surface can be considered almost equal to the heat applied to the air in the duct, so

W  Qꞏ However, the electrical input power flow will always be slightly more than the heat flow applied to the air due to some minor heat losses, this must be allowed for when analysing results. Using this method will give more sensible results than trying to find it from the mass flow rate.

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2. Measuring Bulk Air Temperature Downstream (Tout) Due to the heat source on one side, the air temperature in the duct downstream of the heated surface may not be even across the duct, giving a temperature profile across the duct and a variation in Tout (see Figure 22).

Figure 22 Temperature Profile As with the velocity probe, a single point measurement of downstream temperature may not give an accurate value for the bulk temperature of the air. The theory and experiments show that the extended surfaces give a thermal gradient along their length which will transfer to the air passing over them and therefore up the duct. The geometry, thermal conduction of the extended surfaces and the air velocity all affect the temperature profile. However, the upper probe allows a traverse across the duct so simple averaging or integration of the temperature values give sensible values of Tout. TecQuipment has used both methods, giving results within a few percent of each other.

Tout by Simple Averaging If traversing across the duct in equal and very small steps (1 mm), the temperature profile becomes very precise and can be simply averaged by the number of steps to find Tout. fewer steps gives a less accurate profile and less accurate results.

Tout = sum of all temperatures/steps However, this can take time.

Tout by Integration Time will be saved byf traversing across the duct in small steps at the back of the duct (where the profile changes rapidly) and large steps towards the front of the duct (where the profile changes less rapidly). However, the simple averaging method cannot be used due to the uneven steps. By using a spreadsheet software package the equation of the curve can be found. Integration of the curve over the limits of the duct width will give an accurate average to find Tout.

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Experiments Safety

If the equipment is not used as described in these instructions, its protective parts may not work correctly. WARNING

Never look directly into the fan when it turns. Disconnect the fan when it is not in use. Wait for heat transfer surfaces to cool down to ambient temperature before handling them. The Finned Heat Exchanger has sharp corners to maximise heat transfer area. Handle with care.

Useful Notes Local Temperatures and Air Flow Local ambient temperature, direct sunlight and air flows will affect the results. Make sure to do the experiments in a place that has a reasonably constant ambient temperature and no moving air. TecQuipment’s results are based on a standard room temperature of approximately 20°C and nonmoving air conditions.

NOTE

If using a laptop style computer, keep it away from the inlet of the duct. The computer’s cooling fan may blow warm air towards the duct and affect the results.

Thermal Equilibrium and Time for Accurate Results For most experiments, the results will be more accurate if time is taken for the temperatures to stabilize before taking readings. This is when the experiment reaches ‘thermal equilibrium’ and can take several minutes in forced convection tests or up to an hour for free convection tests.

NOTE

While waiting for equilibrium, check the inlet (ambient) temperature to ensure that any increasing or decreasing trends are not simply caused by changes in ambient conditions.

Thermal Switches During the tests, try to keep the surface temperatures of the surfaces below 95°C, or the thermal switches may operate, switching off the heater power temporarily and affecting the results.

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Constant Power As the heaters in the surfaces warm up, their electrical resistance changes. For this reason, to keep the power constant, it may be necessary to make one or two small adjustments to the power control during the experiment.

The Anemometer and Fan Speed Control This device is very sensitive and works using an element at the tip which heats up and cools down. Because this is a thermal process, the device reacts slowly to changes in velocity. Therefore, slow and careful adjustments of fan speed must be made to reach the air velocity needed.

CAUTION

Never insert anything into the hole in the anemometer probe tip. Make sure the anemometer locking screw never enters the hole in the tip.

To Fit a Heat Transfer Surface 1. Disconnect the Base Unit electrical supply and make sure the heater control is set to minimum and the heater switch is set to off. 2. Carefully insert the Heat Transfer Surface into the square hole in the duct and secure it with the four thumbscrews supplied.

NOTE

Tighten the thumbscrews to compress the gasket slightly so that the surface fits flush inside the duct. The ends of the finned and pinned surfaces will touch the clear window on the opposite side of the duct.

3. Connect the heater and thermocouple cables (supplied) between the heat transfer surface and the sockets on the control panel (refer to the experiments for the correct thermocouple connections). 4. Reverse this procedure to remove the Heat Transfer Surface.

To Fit or Remove the Fan To fit the fan: 1. Disconnect the Base Unit electrical supply and make sure the fan speed control is set to minimum (fully anticlockwise). 2. Carefully fit the fan to the top of the duct and secure it with the eight fixings supplied. 3. Connect the fan supply plug to its socket in the back of the Base Unit. To remove the fan: 1. Turn the fan speed to minimum. 2. Disconnect the Base Unit electrical supply and unplug the fan supply plug.

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3. Undo the fixings that hold the fan and carefully lower the fan to the desk or workbench.

Magnetic Cover Unless otherwise stated, make sure the magnetic cover blocks all six holes in the side of the duct while doing the tests. If holes are left open, a stray air path could be introduced that could affect the results.

To Use the Probe Traverse

Leading edge is datum (Shows 45 mm in this image) Figure 23 Using the Probe Traverse TecQuipment calibrate the nylon bush around the probe so that the leading edge of the nylon bush (the datum) is at the 0 (zero) mark when the probe tip touches the opposite wall of the duct. The image shows the probe at 45 mm from the opposite wall. To use the probe simply loosen the thumbscrew and slide the probe across the duct, using the leading edge of the nylon bush as the distance measurement from the opposite wall.

Using the Surface Probe The surface probe has a spring loaded mechanism to protect the thin probe tip when not being used. To use it: 1. Put a small amount of the thermal paste (supplied) onto a piece of non-absorbent material. 2. Push the probe tip out slightly. 3. Dip the end of the probe into the paste.

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4. Insert the probe into the hole in the side of the duct so that its tip gently touches the pin or fin.

CAUTION

Do not push too hard on the probe tip. It, or the fin that is pushed against it could break.

5. Dip the end of the probe into the paste before inserting it into each hole.

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Experiment 1: Free Convection - Fixed Power

NOTE

This experiment works for all three heat transfer surfaces.

Introduction In most applications a ‘heatsink’ cools a critical component such as an engine cylinder head or electronic component. Therefore, a suitable and simple comparison of the surfaces is to apply a fixed input power and air flow (natural), while measuring surface temperature. The surface that reaches the highest surface temperature will be the least effective at transferring heat to air. Therefore the surface that reaches the lowest temperature will be the most effective at transferring heat to air.

Aims To compare the maximum temperature each surface reaches for a given input power when in free convection.

Procedure 1. Remove the fan from the top of the duct as shown in To Fit or Remove the Fan on page 28. 2. Fit the chosen heat transfer surface as shown in To Fit a Heat Transfer Surface on page 28. 3. Create a blank results table, similar to Table 1. If using VDAS, the software will create a table automatically when readings start being taken. 4. Increase the power to 15 Watts. 5. Wait for the temperatures to stabilise while readjusting the power if necessary and record the maximum temperature each surface reaches.

NOTE

Under free convection, it may take up to 30 minutes for temperatures to stabilize. If after 30 minutes the temperatures do not seem to have stabilized, then record the values at this time.

6. Record the inlet (ambient temperature). 7. Switch off the heater and allow the surface to cool down to near ambient temperature. 8. Repeat the experiment for the other heat transfer surfaces.

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Power = 15 W

Heat Transfer Surface

T2

T1

Surface Ts (°C)

Duct Inlet (ambient) Tin (°C)

Difference TS - TIN (°C)

Flat Plate Pinned Finned

Table 1 Blank Results Table

Results Analysis For each set of results, subtract the inlet temperature from the heat transfer surface temperature to complete the results tables. The temperature difference gives a value with respect to ambient, helping to allow for changes in local conditions. Compare the results. Which surface created the highest temperature difference in free convection? What does this say about this surface?

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Experiment 2: Free Convection - Quick Comparison

NOTE

This experiment works for all three heat transfer surfaces.

Introduction The first experiment made a direct comparison of the heat transfer surfaces by allowing them to reach equilibrium. This can take some time. An alternative is to apply a set heat input and record the time taken to reach a given surface temperature. This transient test is much quicker because there is no waiting for equilibrium. However other factors affect the outcome of this experiment when compared to the earlier experiment.

Aims •

To compare the time taken for each surface to reach a given temperature for a fixed input power.

•

To understand the different thermal inertia characteristics of each heat transfer surface for free convection.

Procedure 1. Remove the fan from the top of the duct as shown in To Fit or Remove the Fan on page 28. 2. Fit the chosen heat transfer surface as shown in To Fit a Heat Transfer Surface on page 28. 3. Create a blank results table, similar to Table 2. If using VDAS, the software will create a table automatically when readings start being taken. 4. Take readings of the surface and inlet temperatures. 5. Find a stopwatch or other suitable timing instrument. Switch on the heater and quickly set it to 90 Watts power, then start the timer. If using VDAS, the timed data capture feature works well for this experiment (refer to the VDAS User Guide). 6. Record the surface and inlet temperatures at intervals of twenty seconds until the surface temperature reaches approximately 70°C. 7. Switch off the heater and allow the surface to cool down to near ambient temperature. 8. Repeat the experiment for the other heat transfer surfaces.

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Heat Transfer Surface: Power = 90 W

Time (seconds)

T2

T1

Surface TS (°C)

Duct Inlet (ambient) TIN (°C)

Difference TS - TIN (°C)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Table 2 Blank Results Table

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Results Analysis For each table of results, subtract the inlet temperature from the heat transfer surface temperature to complete the results tables. The temperature difference gives a value with respect to ambient, helping to allow for changes in local conditions. Create a chart of temperature difference (vertical axis) against time. Add to this chart the results from all three surfaces to see the relationship and compare results. Do not use the first line (zero seconds) results in the chart, as this is only for reference. Which surface took the most time to reach 70°C? Look at the Technical Details for the surfaces and compare the different masses. Each surface is made of the same material and the back side of each is of identical construction and mass, so do the results confirm the thermal inertia theory? Why can this test only be used as a general comparison? What does this test show about the importance of waiting for equilibrium (as in the earlier experiment)?

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Experiment 3: Free Convection - Power and Temperature Relationship

NOTE

This experiment only works suitably with the Pinned and Finned heat transfer surfaces. Under free convection, the Flat Surface becomes very hot even at low powers, giving only a few readings (not enough to create a sensible chart).

Introduction The earlier experiments compare the effectiveness of the heat transfer surfaces in free convection for a fixed power input. They show that the flat plate is not effective; reaching high temperatures even with low power input. Therefore, to compare the surfaces at a range of powers excludes the flat plate as a workable option. This experiment uses either the pinned and finned surfaces, varying their power input while recording the surface temperature to establish the relationship between the two.

NOTE

This experiment can take approximately 35 minutes to stabilize at each power setting, so there may only be time to test one of the two suggested surfaces. The conclusions are the same for both but the finned surface has the lowest thermal mass, reaching equilibrium quickest.

Aims To show the link between power and temperature on the pinned and finned heat transfer surfaces when in free convection.

Procedure 1. Remove the fan from the top of the duct as shown in To Fit or Remove the Fan on page 28. 2. Fit the Pinned or Finned heat transfer surface as shown in To Fit a Heat Transfer Surface on page 28. 3. Create a blank results table, similar to Table 3. If using VDAS, the software will create a table automatically when readings start being taken.

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Heat Transfer Surface: Pinned/Finned

Power (W)

T2

T1

Surface TS (°C)

Duct Inlet (ambient) TIN (°C)

Difference TS - TIN (°C)

0 5 10 15 20 25 30 35 40 45 50

Table 3 Blank Results Table 4. For reference only, take readings of the surface and inlet temperatures with no power applied. 5. Switch on the heater and set to 5 Watts power. 6. Wait for the temperatures to stabilise and then record surface and inlet temperatures. 7. Repeat for several more heater powers as shown in the results table, stopping before the surface reaches 95°C. 8. Switch off the heater and allow the surface to cool down to near ambient temperature. 9. If there is time, repeat the experiment for the other heat transfer surface (Pinned or Finned).

Results Analysis For each table of results, subtract the inlet temperature from the heat transfer surface temperature to complete the results tables. The temperature difference gives a value with respect to ambient, helping to allow for changes in local conditions. Create a chart of temperature difference against power (horizontal axis). Add to this chart the results from both surfaces to see the relationship and compare results. How would darkening the heat transfer surface affect the results (think of other modes of heat transfer)?

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Experiment 4: Forced Convection - Fixed Power

NOTE

This experiment works for all three heat transfer surfaces.

Introduction In free convection, naturally-created small convective currents limit the heat transfer rate. As input power increased, so did surface temperature and the convective currents. However, this restriction prevents dissipating more power with a lower surface temperature for any given surface. Therefore artificially increasing the airflow across the surface (using a fan) helps to increase the heat transfer rate, giving a reduced surface temperature for any given power input.

Aims •

To compare the surface temperatures of the heat transfer surfaces in forced convection for a fixed input power.

•

To find the surface temperature.

Procedure 1. Fit the fan to the top of the duct as shown in To Fit or Remove the Fan on page 28. 2. Fit the chosen heat transfer surface as shown in To Fit a Heat Transfer Surface on page 28. 3. Create a blank results table, similar to Table 4. If using VDAS, the software will create a table automatically when readings start being taken. 4. Make sure fan speed is at zero. 5. Switch on the heater and set it to 15 Watts power. 6. Wait for the temperatures to stabilise and then take readings of the surface and inlet temperatures. 7. Increase the fan speed to give an air velocity of approximately 2 m.s-1. 8. Wait for temperatures to stabilize and take readings of surface and inlet temperatures. 9. Switch off the heater and allow the surface to cool down to near ambient temperature (use the fan to help cool down the surface if necessary). 10. Repeat the experiment for the other heat transfer surfaces.

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Power = 15 W Air Velocity = 2 m.s-1

Heat Transfer Surface

T2

T1

Surface Ts (°C)

Duct Inlet (ambient) Tin (°C)

Difference TS - TIN (°C)

Flat Plate Pinned Finned

Table 4 Blank Results Table

Results Analysis For each table of results, subtract the inlet temperature from the heat transfer surface temperature to complete the results tables. The temperature difference gives a value with respect to ambient, helping to allow for changes in local conditions. Compare the results with those from free convection. Which surface improves the least with forced convection and which improves the most, explain this.

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Experiment 5: Forced Convection - Flow Obstruction NOTE

This experiment works for all three heat transfer surfaces.

Introduction The earlier experiments show the comparative heat dissipation efficiency of the different surfaces, and the improvement due to forced convection. However, they have not shown how each surface affects the air flow and therefore pressure drop along the duct. This simple test uses the flat plate as the datum and compares the relative drop in velocity caused by the other surfaces. A lower velocity indicates a higher pressure drop for a fixed fan setting.

Aim To compare the obstruction to flow caused by each heat transfer surface.

Method Heat Transfer Surface

Airflow (m.s-1)

Flat Finned Pinned

Table 5 Blank Results Table

Procedure 1. Fit the fan to the top of the duct as shown in To Fit or Remove the Fan on page 28. 2. Fit the flat heat transfer surface as shown in To Fit a Heat Transfer Surface on page 28. 3. Create a blank results table, similar to Table 5. 4. Set the airflow to approximately 3 m.s-1. 5. Do not adjust the fan speed control. 6. Change the flat surface for the finned surface and note the reduction in airflow. 7. Do not adjust the fan speed control. 8. Change the finned surface for the pinned surface and note the reduction in airflow.

Results Analysis Note the difference in airflow. How would this affect the choice of heat transfer surface for any application.

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Experiment 6: Temperature Distribution Across the Extended Surfaces

NOTE

This experiment only works for the pinned and finned heat transfer surfaces.

Introduction Previous experiments show that the surfaces with extended surfaces (pinned and finned) work better to transfer heat to air. For these surfaces to be 100% efficient, the extended surfaces should be at the same temperature as the base plate. As shown in the theory (see Thermal Lag and Gradient) this never happens and a thermal gradient occurs along the surface. A steeper gradient indicates a lower efficiency. This experiment also shows the link between heat transfer methods, as the thermal conduction of the extended surface material also affects the results.

Aim To show the temperature distribution (gradient) along a pinned or finned heat transfer surface in both free and forced convection.

Procedure 1. Remove the fan from the top of the duct as shown in To Fit or Remove the Fan on page 28. 2. Fit the Pinned Heat Transfer Surface as shown in To Fit a Heat Transfer Surface on page 28. 3. Create a blank results table, similar to Table 6. If using VDAS, the software will create a table automatically when readings start being taken. 4. Switch on the heater and set it to 30 Watts power. 5. Wait for the temperatures to stabilise - this may take 30 to 40 minutes. 6. Using the sliding magnetic cover, uncover just the first hole (furthest left and nearest the heat transfer surface).

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Heat Transfer Surface: Power = 30 W Air Velocity: Free or Forced Convection: Position 1 (7.5 mm)

Position 2 (19.5 mm)

Position 3 (31.5 mm)

Position 4 (43.5 mm)

Position 5 (55.5 mm)

Position 6 (67.5 mm)

Probe Temperature (Tp) T3 Surface Temperature (Ts) T2 Inlet Temperature (Tin) T1 Difference (Tp-Tin)

Table 6 Blank Results Table 7. Add a small amount of the thermal paste (supplied) to the probe tip. 8. Insert the probe into the first hole, so that it touches the pin.

CAUTION

Do not push too hard on the probe tip. It or the fin that is pushed against it may break.

9. Record the probe, surface and inlet temperatures. 10. Now move the probe into each of the other holes, waiting for temperatures to stabilize each time and recording the temperatures. Remember to slide the magnetic cover to block the other five holes. 11. Now fit the fan to the top of the duct as shown in To Fit or Remove the Fan on page 28. 12. Set the fan to 3 m.s-1 and repeat the experiment. 13. Switch off the heater and allow the surface to cool down to near ambient temperature (use the fan to help cool down the surface if necessary). 14. Repeat the experiment for the Finned Heat Transfer surface.

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Results Analysis Create a chart of temperature (vertical axis) against probe position along the pin or fin. The temperature must the difference between the probe and ambient (Tp-Tin) to allow for changes in ambient conditions. Add the free and forced convection results. Note the difference in gradient between free and forced convection for each surface. Do the results match the conclusions from earlier experiments? The extended surfaces are made of stainless steel, which is a relatively poor conductor. How would using aluminium or copper (better conductors) affect the temperature gradient and the overall efficiency?

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Experiment 7: Forced Convection - Effect of Velocity

NOTE

This experiment works for both the finned and pinned heat transfer surfaces.

Introduction Earlier experiments show that increased air flow improves effectiveness, particularly on the extended surfaces. This experiment helps to show this more clearly.

Aim To show how increased air flow improves the effectiveness of heat transfer.

Procedure 1. Fit the fan to the top of the duct as shown in To Fit or Remove the Fan on page 28. 2. Fit either the finned or pinned heat transfer surface as shown in To Fit a Heat Transfer Surface on page 28. 3. Create a blank results table, similar to Table 7. If using VDAS, the software will create a table automatically when readings start being taken.

Heat transfer Surface: Power:

Air velocity (m.s-1)

T2

T1

Surface Ts (°C)

Duct Inlet (ambient) Tin (°C)

Difference TS - TIN (°C)

Table 7 Blank Results Table 4. Set the fan to give an air velocity of 1 m.s-1. 5. Set the heater power to 50 W.

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6. Wait for the temperatures to stabilise. 7. Record the surface and inlet temperatures. 8. Repeat for increased air velocities of approximately 1.5, 2.0, 2.5 and 3.0 m.s-1. 9. Repeat for the other surface.

Results Analysis For each table of results, subtract the inlet temperature from the heat transfer surface temperature to complete the results tables. The temperature difference gives a value with respect to ambient, helping to allow for changes in local conditions. For each surface, create a chart of Ts - Tin (vertical axis) against velocity. What does the chart say about temperature and velocity? Which surface has the coolest temperature for any given air velocity?

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Experiment 8: Heat Transfer Coefficient and Nusselt Number

NOTE

This experiment works for all three heat transfer surfaces, but only shows the method for the flat surface.

Introduction Earlier experiments compare performance of the surfaces in free and forced convection by measuring surface temperature. However, a more scientific test needs calculations of coefficients that show the effectiveness of heat transfer. In this case the convective heat transfer coefficient and Nusselt number are important (see Theory section). The earlier experiments showed that forced convection gives better results than free convection. This experiment quantifies this more scientifically for the flat plate. It could be tried with the other surfaces but the results analysis is beyond the scope of this guide.

Aims To show how to find a value for heat coefficient and Nusselt number for a heat transfer surface in a duct for both free and forced convection.

Procedure 1. Make sure the duct is perfectly vertical, as this will affect the results. 2. Remove the fan from the top of the duct as shown in To Fit or Remove the Fan on page 28. 3. Fit the flat heat transfer surface as shown in To Fit a Heat Transfer Surface on page 28. 4. Create a blank results table, similar to Table 8. If using VDAS, the software will create a table automatically when readings start being taken. 5. Set the heater to 20 Watts. 6. Move the duct traverse probe so it reads 0 (zero) (see To Use the Probe Traverse). Check that its tip touches the opposite wall of the duct at this position. Now move it to the 1 mm position. 7. Wait for the temperatures to stabilise and then take readings of the surface, inlet and outlet (duct probe) temperatures. 8. Choosing to either move in equal steps (if time allows) or larger steps (see 2. Measuring Bulk Air Temperature Downstream (Tout)), take readings of the temperatures across the duct using the traverse. Stop when on reaching 74 mm (the tip is almost fully retracted into the near side wall of the duct at this point). Recheck the inlet and surface temperatures as this is done. 9. For forced convection, repeat the experiment with the fan fitted and airflow of 3 m.s-1.

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Duct Traverse Probe Position (mm)

User Guide

T1

T2

T3

Ambient Temperature (Probe) Tin (°C)

Heat Transfer Surface Temperature Ts (°C)

Duct Traverse Probe Tp (°C)

50

Ts-Tin (°C)

Tp-Tin (°C)

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Duct Traverse Probe Position (mm)

T1

T2

T3

Ambient Temperature (Probe) Tin (°C)

Heat Transfer Surface Temperature Ts (°C)

Duct Traverse Probe Tp (°C)

Ts-Tin (°C)

Tp-Tin (°C)

Table 8 Blank Results Table

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Results Analysis For each set of results (free and forced): 1. Produce a chart of Tp-Tin to see the outlet temperature profile with respect to inlet (this allows for changes in ambient temperature). 2. Find Tout using either of the methods shown in the theory section. 3. Find average values for the other temperature readings. 4. Use the Tout and the average readings to find the logarithmic mean temperature difference Tm using Equation 4. 5. Use this to find the heat transfer coefficient (hc) (assuming heat flow to the air is equal to the power applied). 6. Find the thermal conductivity (kair) for air at the average inlet temperature. 7. Use the values of hc and kair to find the Nusselt number. 8. Compare the values with those given in the theory.

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Experiment 9: Free Convection from Horizontal and Vertical Surfaces

NOTE

This experiment works best with the finned plate heat transfer surface.

Introduction As shown in the theory section, the relative orientation (horizontal, flat or vertical position) of some heat transfer surfaces affects their efficiency in free convection. This experiment uses the finned plate to show this effect.

Aim To compare the heat convection from a horizontal finned surface with that from a vertical finned surface.

Procedure

NOTE

Keep the air around the experiment as still as possible. Any air movements (even breathing) near the surface will affect the results.

1. Carefully rest the finned plate heat transfer surface face upwards (vertical) on the desk top, just in front of the main unit.

Vertical

Horizontal

Figure 24 Finned Plate Vertical and Horizontal 2. Create a blank results table, similar to Table 3. If using VDAS, the software will create a table automatically when readings start being taken.

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Finned Plate Power = 30 W

Heat Transfer Position

T1 or T3

T2

Ambient Temperature (Probe) Tin (°C)

Heat Transfer Surface Temperature Ts (°C)

Ts-Tin (°C)

Vertical (fins upwards) Horizontal (fins horizontal)

Table 9 Blank Results Table 3. Connect the heater cable and the surface temperature thermocouple. 4. For ambient temperature reference, connect the probe to T1 or T3 and rest it on the desk in front of the main unit. 5. Set the heater power to 30 W.

NOTE

Determined by the local conditions, the surface may start to exceed 95°C. If this happens, try a lower power. For best results keep the air still around the surface.

WARNING

Do not touch the heated surface.

6. Wait for the temperatures to stabilise and then take readings of the surface and ambient temperatures. 7. Switch off the heater and allow the surface to cool down to near ambient temperature. 8. Carefully rest the flat plate unit on its side, so that its fins are horizontal. 9. Repeat the experiment, keeping the power the same. 10. Switch off the heater and allow the surface to cool down to near ambient temperature.

Results Analysis For each table of results, subtract the ambient temperature from the heat transfer surface temperature to complete the results tables. The temperature difference gives a value with respect to ambient, helping to allow for changes in local conditions. Compare the temperature difference for vertical and horizontal convection of finned surfaces. Does it match the information shown in the theory section?

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Typical Results Note: These results are sample results only, actual results may be slightly different. All results are based on experiments in a room with stable conditions and at approximately 20°C.

Experiment 1: Free Convection - Fixed Power

Free Convection Maximum Temperature Difference at 15 W Input Power 60 50

Ts-Tin (°C)

40 30 20 10 0 Flat

Pinned

Finned

Surfaces

Figure 25 Typical Results The results show that the flat surface becomes much hotter than the other two surfaces in free convection for the same power input. The other two surfaces have a reasonably similar temperature. This shows that in free convection, the flat surface is least likely to transfer its heat energy to the surrounding air, compared to the two surfaces that have the extended surface area.

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Experiment 2: Free Convection - Quick Comparison

60 50

Ts-Tin (°C)

40 30 20

Pinned Surface Finned Surface

10

Flat Surface

0 0

50

100

150

200

Time (seconds)

Figure 26 Typical Results The results show that for a fixed input power, the Flat Surface reaches any given temperature more rapidly than the other two surfaces. The Pinned Surface takes longer to reach any given temperature. The given weights of the surfaces give an indication of the thermal mass of each surface, showing why they take different times to reach given temperatures. This test should only work as a guide, as other factors affect how quickly each surface heats up apart from thermal mass. These include its physical construction and how it allows air to pass around it for cooling in free convection. The other tests should help to teach more about this.

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Experiment 3: Free Convection - Power and Temperature Relationship

Pinned and Finned Surfaces Free Convection - Varied Power 80 70 Pinned Surface

60

Finned Surface

Ts - Tin (°C)

) 50 C °( n i 40 T s T 30 20 10 0 0

10

20

30

40

50

Power (W)

Figure 27 Typical Results The results should show that the surface becomes hotter as more power is applied. Also, as the surface gets hotter, it can dissipate more power in free convection due to the increased effectiveness of the convective air currents. Figure 27 also shows straight lines under each curve to show the non-linear trend. Darkening the surfaces would improve their ability to transfer heat energy, but more by radiation rather than convection.

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Experiment 4: Forced Convection - Fixed Power

-1

Forced Convection at 2 m.s Maximum Temperature Difference at 15 W Input Power 50 45 40

Ts-Tin (°C)

35 30 25 20 15 10 5 0 Flat

Finned

Pinned

Surfaces

Figure 28 Typical Results The results show again that the Flat Surface is least likely to transfer its heat energy to the air even in forced convection. All surface temperatures have reduced compared to free convection results, showing that forced convection helps to improve heat transfer. Note that the Pinned Surface has shown the most improvement in that it has the coolest surface when in forced convection.

Experiment 5: Forced Convection - Flow Obstruction

Heat Transfer Surface

Airflow (m.s-1)

Flat

3 (reference)

Finned

2.75

Pinned

2.4

Table 10 Typical Results The results should show that the Pinned Surface reduces the flow rate (causes the greatest obstruction to flow). In real applications, the obstruction is important, as it affects the size (and power) of fan needed to push air past the heat transfer surface in forced convection.

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Experiment 6: Temperature Distribution Across the Extended Surfaces Pinned and Finned Surface Probe Free Convection

70

Tprobe-Tin (°C)

60 Finned Surface

50

Pinned Surface

40 30 20 10 0 0

20

40

60

80

Distance Along Surface (mm)

Figure 29 Typical Results

Pinned and Finned Surface Probe Forced Convection 70

Tprobe-Tin (°C)

60 Finned Surface

50

Pinned Surface 40 30 20 10 0 0

20

40

60

80

Distance Along Surface (mm)

Figure 30 Typical Results The results should show a decreased gradient (and lower overall temperatures) for forced convection, showing that forced convection gives an even spread of temperature along the extended surfaces, making them work more efficiently. If the extended surfaces were made of a perfect conductor, the surface would be near perfectly efficient and the gradient would be near horizontal.

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Experiment 7: Forced Convection - Effect of Velocity

Pinned and Finned Surface at 50 W - Varied Air Velocity 80 70

Ts-Tin (°C)

60 50 40 30

Pinned Finned

20 10 0 0

0.5

1

1.5

2

2.5

3

3.5

-1

Velocity (m.s )

Figure 31 Typical Results The results should show that the surfaces become cooler as velocity increases. The Pinned Surface becomes coolest at any given air velocity.

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Experiment 8: Heat Transfer Coefficient and Nusselt Number Free Convection

Tout-Tin Against Distance across Duct Free Convection Flat Plate 9 8 7

Tout-Tin (°C)

6 5 4 3 2 1 0 0

10

20

30

40

50

60

70

80

Distance (mm)

Figure 32 Typical Results Free Convection

Tout from an average of 73 positions (1 mm to 74 mm) gives: 26.2°C Average Tin = 22.1 Average Ts = 96.8 Power was actually 19.9 Watts

Tm = (26.2-22.1)/ln [(96.8-22.1)/(96.8-26.2)] Tm = 4.1/ln (74.7/70.6) Tm = 4.1/0.05645 = 72.63 hc = 19.9/(0.0112 x 72.63) hc = 19.9/0.813 = 24.48 Nu = 24.48 x 0.106/0.026 Nu = 2.56/0.026 = 99.8

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Forced Convection

Tout-Tin Against Distance Across Duct Flat Plate Forced Convection 3.5 3

Tout-Tin (°C)

2.5 2 1.5 1 0.5 0 -0.5

0

10

20

30

40

50

60

70

80

Distance across duct (mm)

Figure 33 Typical Results Forced Convection

Tout from an average of 73 positions (1 mm to 74 mm) gives: 23.0°C Average Tin = 22.2 Average Ts = 74.5 Power was 20 Watts

Tm = (23.0 - 22.2)/ln [(74.5-22.2)/(74.5-23.0)] Tm = 0.8/ln (52.3/51.5) Tm = 0.8/0.0154 = 51.95 hc = 20/(0.0112 x 51.95) hc = 20/0.58= 34.48 Nu = 34.48 x 0.106/0.026 Nu = 3.66/0.026 = 140.6 The results show that the free convection gives lower values of heat transfer coefficient and Nusselt number than in forced convection. The numbers generally agree with those shown in the theory. Note that due to slight differences in the thermocouples and the temperature profile across the duct, a lower temperature may be seen at the outlet than at the inlet. However, this should be less than 0.5 degree difference.

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Experiment 9: Free Convection from Horizontal and Vertical Surfaces

Finned Plate Power = 30 W

Heat Transfer Position

T1 or T3

T2

Ambient Temperature (Probe) Tin (°C)

Heat Transfer Surface Temperature Ts (°C)

Ts-Tin (°C)

Vertical (fins upwards)

22.2

87.7

65.5

Horizontal (fins horizontal)

22.3

93.9

71.6

Table 11 Typical Results The results should show that the Finned Surface becomes hotter for the same power input when it is in the horizontal position. This shows that in horizontal position, it does not transfer its heat to the air as effectively as when it is vertical or flat.

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Useful Textbooks Basic Engineering Thermodynamics By Rayner Joel Published by Longman ISBN 0-582-25629-1

Engineering Thermodynamics By G.F.C Rogers and Y.R Mayhew Published by Longman ISBN 0-582-02704-7

Heat Transfer By J.P Holman Published by McGraw Hill ISBN 978-0-07-352936-3

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Maintenance, Spare Parts and Customer Care Maintenance General •

Regularly check all parts of the equipment for damage, renew if necessary.

•

When not in use, store the equipment in a dry dust free area, preferably covered with a plastic sheet.

•

If the equipment becomes dirty, wipe the surfaces with a damp, clean cloth. Do not use abrasive cleaners.

•

Regularly check all fixings and fastenings for tightness; adjust where necessary.

NOTE

Renew faulty or damaged parts with an equivalent item of the same type or rating.

Electrical Only a qualified person may carry out electrical maintenance. WARNING

• • • •

Obey these procedures:

Assume the apparatus is energised until it is known to be isolated from the electrical supply. Use insulated tools where there are possible electrical hazards. Confirm that the apparatus earth circuit is complete. Identify the cause of a blown fuse before renewing.

To renew a broken fuse • • • •

Isolate the apparatus from the electrical supply. Renew the fuse. Reconnect the apparatus to the electrical supply and switch on. If the apparatus fails again, contact TecQuipment Ltd or the agent for advice.

NOTE

Renew faulty or damaged parts or detachable cables with an equivalent item of the same type or rating.

Fuse Location The fuse is to the back of the unit, next to the mains supply connection. Use a screwdriver with a small flat blade to remove the fuse carrier.

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Spare Parts Check the Packing Contents List to see what spare parts we send with the apparatus. If technical help or spares are needed, please contact the local TecQuipment Agent, or contact TecQuipment direct. When asking for spares, please tell us: • • • • • • •

Contact Name The full name and address of the college, company or institution Contact email address The TecQuipment product name and product reference The TecQuipment part number (if known) The serial number The year it was bought (if known)

Please give us as much detail as possible about the parts needed and check the details carefully before contacting us. If the product is out of warranty, TecQuipment will advise the price of the spare parts.

Customer Care We hope our products and manuals are liked. If there are any questions, please contact our Customer Care department: Telephone: +44 115 9722611 Fax: +44 115 973 1520 email: [email protected] For information about all TecQuipment Products and Services, visit: www.tecquipment.com

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