Thermodynamics Definitions

Thermodynamics MTX 220 Definitions

Thermodynamics System Control Volume Control Mass Phase State Properties

Intensive property Extensive property Thermal Equilibrium Mechanical Equilibrium Phase Equilibrium Chemical Equilibrium Thermodynamic Equilibrium Process Cycle Isothermal Process Isobaric Process Isochoric Process

Thermodynamics is the science that deals with the relationship of heat and mechanical energy and conversion of one into the other A system is a specifically identified fixed mass of material separated from its surroundings by a real or imaginary boundary. A control volume is a region is space separated from its surroundings by a real or imaginary boundary, the control surface, across which mass may enter/exit. The control surface is closed to the flow of mass. Therefore a control mass contains the same amount of matter at all times. Quantity of matter that is homogeneous throughout. A thermodynamic state of a system is defined by the values af all the system thermodynamic properties. Defined as a quantifiable macroscopic characteristics of a system, or Any quantity that depends on the state of the system, and is independent of the path by which the system arrived at the given state. Independent of mass. (pressure, temperature, density) Depends on how much of the substance is present or the size of the system under consideration. (mass, volume, total energy) Temperature is the same throughout the system. No change in pressure at any point of the system with time. The amount of substance in any one phase (vapour, liquid and solid) may not change with time. Chemical composition doesn’t change with time, ie no chemical reactions. Thermal, Mechanical, Phase and Chemical equilibrium. The path of successive states through which system passes A thermodynamic cycle consists of a sequence of processes in which the working fluid returns to its original state A constant temperature process. A constant pressure process. A constant volume process.

Quasi-static/quasi Equilibrium process Specific Volume Density Pressure Equality of Temperature The Zeroth Law of Thermodynamics Ice Point

A process that occurs sufficiently slow such that departures from thermodynamic equilibrium are nagebly small. All the states the system passes through may be considered equilibrium states. The volume per unit mass. The mass per unit volume. Pressure is defined as the normal component of force per unit area. When two bodies are in thermal communication, and no change in any observable property occurs. If two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other. A mixture of ice and water that is in equilibrium with air saturated with vapour at 1 atm.

Chapter 3 Steam point A Pure Substance

Saturation Temperature Saturated Liquid Saturated Vapour Superheated Vapour

A mixture of liquid water and water vapour (with no air) in equilibrium at 1 atm. A pure substance is identified as one that has a homogeneous and invariable chemical composition. It may exist in more than one phase but the chemical composition is the same in all phases. The temperature at which vaporization takes place at a given pressure (called the saturation pressure). A liquid about to vaporize. Substance exist as liquid at the saturation temperature and pressure. A liquid that is just about to condense. Substance exist as vapour at the saturation temperature. Vapour that is not about to condense. Substance exists as a vapour at a temperature higher than the saturated temp.

P < Psat @ T Sub cooled Liquid

,

T > Tsat @ P

Temperature of the liquid is lower than the saturation temperature for the existing pressure.

T < Tsat @ P Compressed Liquid

The pressure is greater than the saturation pressure for a given temperature.

P > Psat @ T Quality Allotropic Transformation

When a substance exists as part liquid and part vapour. It is defined as the ratio of the mass of the vapour to the total mass. A transition from one solid phase to another solid phase.

Chapter 4 Work

Heat

Adiabatic Process Modes of Heat Transfer • Conduction • •

Convection Radiation

Work is the energy transfer associated with a force acting through a distance. Work is done by a system if the sole effect on the surroundings (everything external to the system) could be the raising of a weight. • Work done by a system is positive – Boundary work output • Work done on a system is negative – Boundary work input • Work is a path function The form of energy that is transferred across the boundary of a system at a given temperature to another system (or the surroundings) at a lower temperature by virtue of the temperature difference between the two systems. • Heat transferred to a system is positive • Heat transferred from a system is negative. A process in which there is no heat transfer (Q=0) Conduction is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interaction between the particles. Takes place when a medium is flowing. The transfer of energy due to the emission of electromagnetic waves/photons.

Chapter 5 First Law of Thermodynamics

It states that during any cycle a system (control mass) undergoes, the cyclic integral of the heat Is proportional to the cyclic integral of the work. 

Ñ ∫ δQ = Ñ ∫ δW Sensible Energy Latent Energy Chemical Energy Nuclear Energy

The portion of the internal energy of a system associated with the kinetic energies of the molecules. The portion of internal energy associated with the phase of a system. Internal energy associated with the atomic bonds in a molecule. Energy associated with the strong bonds within the nucleus of the atom itself.

Chapter 6 Conservation of Energy Specific Heat Steady State Assumptions for a Steady-State process

Examples of SteadyState Processes • Heat Exchanger •

Nozzle

•

Diffuser

•

Throttle

•

Turbine

•

Compressor & Pump

Transient Process Assumptions for a Transient Model

The net change of the energy of the control mass is always equal to the net transfer of energy across the boundary as heat and work. The energy required to raise the temperature of a unit mass of a substance by one degree. A steady-state has no storage effects, with all properties constant with time, and constitutes the majority of all flow-type devices. 1. The control volume does not move relative to the coordinate frame. 2. The state of the mass at each point in the control volume does not vary with time. 3. As for the mass that flows across the control surface, the mass flux and the state of this mass at each discrete area of flow on the control surface do not vary with time. The rate at which heat and work cross the control surface remain constant.

It is a simple fluid flowing through a pipe or system of pipes, where heat is transferred to or from the fluid. A nozzle is a steady-stae device whose purpose is to create a high-velocity fluid stream at the expense of the fluid’s pressure. It is a device constructed to decelerate a high-velocity fluid in a manner that results in an increase in pressure of the fluid. It occurs when a fluid flowing in a line suddenly encounters a restriction in the flow passage. It is a rotary steady-state machine whose purpose is to produce shaft work (power, on a rate basis) at the expense of the pressure of the working fluid. The purpose of a steady-state compressor (gas) or pump (liquid) is the same: to increase the pressure of a fluid by putting in shaft work. Change in mass (storage) such as filling or emptying of a container. 1. The control volume remains constant relative to the coordinate frame. 2. The state of the mass within the control volume may change with time, but at any instant of time the state is uniform throughout the entire control volume (or over several identifiable regions that make up the entire control volume) 3. The state of the mass crossing each of the areas of flow on the control surface is constant with time although the

mass flow rates may be time varying.

Chapter 7 Heat Engine

The Second Law Of Thermodynamics • Kelvin-Planck (Heat Engine) •

Clausius (Refrigerator/Hea t Pump) Reversible Process Irreversible Process The Carnot Cycle

Defined as a device that operates in a thermodynamic cycle and does a certain amount of net positive work through the transfer of heat from a high-temperature body to a low temperature body.

It is impossible to construct a device that will operate in a cycle and produce no effect other than the raising of a weight and the exchange of heat with a single reservoir. It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a cooler body to a hotter body. Defined as a process that once having taken place can be reversed and in so doing leave no change in either system or surroundings. The initial process because it could not be reversed without leaving a change in the surroundings. If every process in the cycle is reversible, the cycle is also reversible; and if the cycle is reversed, the heat engine becomes a refrigerator. This is the most efficient cycle that can operate between two constant-temperature reservoirs. • 1-2 Isothermal Heat Addition in at QH TH •

2-3 Adiabatic Expansion Process

goes down T

•

3-4 Isothermal Heat Rejection

out at QL

•

TL

4-1 Adiabatic Compression Process

goes up. T

•

Proposition 1

•

Proposition 1

It is impossible to construct an engine that operates between two given reservoirs and is more efficient than a reversible engine operating between the two same reservoirs. All engines that operate on the Carnot cycle between two given constant-temperature reservoirs have the same efficiency.

Chapter 8 Isentropic Process Standard Entropy

Constant entropy process. Entropy remains constant in a reversible adiabatic process. Integrate the results of the calculations of statistical

thermodynamics from reference temperature

to any other

T0 temperature

.

T

Chapter 9 Steady State Process Fluid Incompressible

No change with time of the entropy per mass unit at any point within the control volume. v = constant

Typical Steady-Flow Devices Device Aftercooler

Purpose Cool a flow after a compressor.

Boiler

Bring substances to a vapour state

Given

Assumption

w=0

P = const

w=0

P = const

Condenser

Take q out to bring substance to liquid state

Combustor

Burn fuel; acts like heat transfer in

Compressor

Bring a substance to higher pressure

Deaerator

Remove gases dissolved in liquids

Dehumidifier

Remove water from air

Desuperheater

Add liquid water to superheated vapour steam to make it saturated vapour

Diffuser

Convert KE energy to higher P

Economizer

Low-T, low-P heat exchanger

Evaporator

Bring a substance to a vapour state

Expander Fan/Blower

Similar to a turbine, but may have a q Move a substance, typically air

Feedwater Heater

Heat liquid water with another flow

Flash Evaporator

Generate vapour by expansion (throttling)

Heat Engine

A device that converts part of heat into work

Heat Exchanger

Transfer heat from one medium to another

Heat Pump

A device moving a Q from

w=0

P = const

w=0

P = const

w in

q=0

w=0

P = const

-

to

Tlow

w=0

P = const

w=0

q=0

w=0

P = const

w=0

P = const

-

-

w in, KE up

q = 0, P = C

w=0

P = const

w=0

q=0

q in, w out

w=0 ,

Thigh

Heater

requires a work input, refrigerator Heat a substance

Humidifier

Add water to air-mixture

Intercooler

Heat exchanger between compressor stages

Nozzle

Create KE; P drops. Measure flow rate

Mixing Chamber

Mix two or more flows

Pump

Same as compressor but handles liquid

Reactor

Allow reaction between two or more substances

P = const

w in

-

P = const -

w=0

P = const

w=0

P = const

w=0

P = const

w=0

q=0

w=0

q=0

w in, P up

q=0

w=0

q = 0, P = C

Regenerator

Usually a heat exchanger to recover energy

w=0

P = const

Steam Generator

Same as boiler, heat liquid water to superheated vapour

w=0

P = const

Supercharger

A compressor driven by engine shaft work to drive air into an automotive engine A heat exchange that brings up Tsat over

Superheater

T Turbine

Create shaft work from high P flow

Turbocharger

A compressor driven by an exhaust flow turbine to charge air into an engine

Valve /Throttle

Control flow by restriction; P drops

w in

-

w=0

P = const

w out

q=0

W&turbine = W&C .V .

w=0

-

q=0