Properties Of Cryogenic Fluids

CRYOGENIC ENGINEERING 2

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Properties of Cryogenic Fluids Saturated Liquid Property

Unit

LHe3

LHe4

Liquid Hydroge n

Normal Boiling point

K

3.19

4.214

20.27

27.09

77.36

Critical temperature

K

3.32

5.2

33.2

44.4

126.1

MPa

0.117

0.229

1.315

2.65

3.39

K

-

-

13.9

24.53

63.2

Density

Kg/m 3

58.9

124.8

70.79

1206

807.3

Latent Heat

KJ/Kg

8.49

20.9

443

85.9

199.3

Critical Pressure Triple point

Liquid Neon

LN2

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Properties of Cryogenic Fluids Saturated Liquid Property

Unit

Liquid Air

Liquid Argon

LOX

Liquid methane

Normal Boiling point

K

78.8

87.28

90.18

111.7

Critical temperature

K

133

150.7

154.6

190.7

MPa

3.92

4.89

5.08

4.64

K

-

83.8

54.4

88.7

Density

Kg/m3

874

1394

1141

424.1

Latent Heat

KJ/Kg

205

161.9

213

511.5

Critical Pressure Triple point

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Cryogenic Fluids-Liquid Nitrogen (LN2) • • • • •

Boils at 77.36 K and freezes at 63.2 K. Resembles water in appearance - 807 kg/m3 at 1 atm. Exists in 2 stable isotopes - N14 & N15. Produced by distillation of air. Used to provide an inert atmosphere in chemical and metallurgical industries. • It is also used as a liquid to provide refrigeration. • Food preservation, blood, cells preservation. • High temperature superconductivity. 4

Cryogenic Fluids-Liquid Oxygen (LOX) • • • • • • •

Blue in color – due to long chains of O4. Boils at 90.18 K and freezes at 54.4 K at 1 atm. Has a density of 1141kg/m3. O2 is slightly magnetic (paramagnetic) High chemical reactivity exists in 3 stable isotopes - O16, O17, O18 Produced by distillation of air 5

Cryogenic Fluids-Liquid Argon • • • • •

It is a colorless, inert and non toxic gas. It boils at 87.3 K and freezes at 83.8 K. It has a density of 1394 kg/m3. Exists in 3 stable isotopes – Ar36, Ar38, Ar40. The property of inertness of argon is used to purge moulds in casting industry. • It is used in Argon-oxygen decarburization (AOD) process in stainless steel industry. • It offers inert atmosphere for welding stainless steel, aluminum, titanium etc. 6

Cryogenic Fluids-Liquid Air • It has a boiling point of 78.9 K and 874 kg/m3 as density. • Liquid air was earlier used as pre-coolant for low temperature applications. • Liquid air is primarily used in production of pure nitrogen, oxygen, and rare gases.

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Cryogenic Fluids-Liquid Neon • It is a clear, colorless liquid with boiling point at 27.1 K. • Neon is commonly used in neon advertising. • Liquid neon is commercially used as cryogenic refrigerant. • It is compact, inert and less expensive as compared to liquid helium.

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Cryogenic Fluids-Liquid Methane • • • •

Clear, colorless It boils at 111.7 K. Density 424.1 kg/m³ It can be used as rocket fuel.

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Cryogenic Fluids-Liquid Fluorine • Light yellow liquid. • Normal boiling point 85.24K, freezing point 53.5K (yellow solid, at 45.6K transforms to white solid) • Most dense cryogenic liquid 1507 kg/m³. • Highly toxic, sharp pungent odor.

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Liquid Hydrogen • Hydrogen exists in diatomic form as H2. • It boils at 20.27 K and freezes at 13.95 K. • It has a density of only 70.79 kg/m3 (one of the lightest). • It has a latent heat of 443 kJ/kg 11

Hydrogen • Exists in 2 stable isotopes – hydrogen, deuterium and tritium. • Tritium is radioactive and is unstable.

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Hydrogen • Hydrogen exists in two molecular forms – Ortho and Para. • A H2 molecule has 2 protons and 2 electrons. • The distinction between the two forms of hydrogen is the direction of the spin of protons. 13

Hydrogen • The two protons possess a spin which gives the angular momentum. • If the nuclear spins are in same direction for both the protons, it is Ortho Hydrogen. • If the nuclear spins are in opposite direction for both the protons, it is Para Hydrogen. 14

Hydrogen

Ortho- Hydrogen

Para- Hydrogen 15

Hydrogen • With the decrease in the temperature, the Ortho hydrogen is converted to the Para hydrogen. • At 300 K Ortho and Para hydrogen exists in the ratio 3:1 • At 20 K 99.821% of the Hydrogen will be in Para form 16

Hydrogen • Para form is a low energy form and therefore heat is liberated during conversion. • Conversion of ortho to para form of hydrogen is an exothermic reaction. • This conversion is a very slow process. 17

Hydrogen • During liquefaction, the heat of conversion causes evaporation of 70% of hydrogen originally liquefied. • In order to make this conversion faster, catalysts are added. 18

Deuterium • Deuterium atom has one proton and one neutron. Two Deuterium atoms make up one D2 which is called as Heavy Hydrogen. • Similar to hydrogen, it also exists in two molecular forms – Ortho and Para. • Normal deuterium exists in ratio of 2/3 Ortho and 1/3 Para. 19

Deuterium • As temperature decreases, Para D2 gets converted to Ortho D2. • At 20 K 98 % of D2 will be in Ortho form • Most of the physical properties of Hydrogen and Deuterium mildly depend on Ortho – Para Composition. 20

Hydrogen-Uses • Cryogenic

engines are powered by propellants like liquid hydrogen. • It is being considered as fuel for automobiles. • Cryocoolers working on a closed cycle use hydrogen as working fluid. 21

Helium • In the year 1908, K. Onnes at Leiden liquefied Helium using Helium gas obtained by heating Monazite sand from India. • Helium exists in two isotopes- He4 and He3. • The percentage of He3 is 1.3 x 10-4 %. So mostly it is He4 22

Helium • Liquid Helium is inert, odorless, colorless and exists in monatomic state. • It boils at 4.2 K. • It has a density of 124.8 kg/m3. • Critical temperature and pressures are 5.25 K and 0.227 Mpa • It has a latent heat of 20.28 kJ/kg. 23

Helium Phase Diagram • It has no triple point. • Saturated liquid Helium must be compressed to 25.3 bar to solidify. (no freezing point at 1 atm)

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Helium Phase Diagram • As Liquid Helium is further cooled below a particular temperature a new liquid phase, LHe –II, emerges out. • The two different liquids are called as LHe – I and LHe – II. 25

Helium Phase Diagram • These liquid phases are distinguished on the basis of viscosity as follows. • LHe–I : Normal fluid • LHe–II : Super fluid

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Helium Phase Diagram • This phase separation line is called as Lambda Line. • The point of intersection of phase separation line with saturated liquid line is called as Lambda Point (T=2.171 K at 5.073kPa).

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Helium Phase Diagram • LHe – II is called as super fluid because it exhibits properties like zero viscosity and large thermal conductivity. • This fluid expands on cooling. • The variation of specific heat in Liquid Helium is abrupt and posses a discontinuity at the lambda point.

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Helium Phase Diagram-Variation of specific heat

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Helium Phase Diagram • The point is called as lambda point because shape of the curve resembles the Greek letter λ • There is no energy involved in lambda transition. 30

Helium Phase Diagram • Also, heat transfer in super fluid helium (LHe – II) is very special. When the pressure above LHe - I is reduced by pumping, the fluid boils vigorously. • During pumping, the temperature of liquid decreases and a part of the liquid is boiled away. • When T < λ (LHe-1 to LHe-II) point temperature, the apparent boiling of the fluid stops . 31

Helium Phase Diagram • Liquid becomes very clear and quiet, even though it is vaporizing rapidly. • Thermal Conductivity of He – II is so large (86500 W/m-K), that the vapor bubbles do not have time to form within the body of the fluid before the heat is quickly conducted to the surface. • Thermal Conductivity of He – I is 0.024 32

Super Fluid • LHe – II, called as super fluid, exhibits properties like zero viscosity and large thermal conductivity. • At lambda point liquid composition is 100% normal fluid. • At absolute zero liquid composition is 100% superfluid • It flows through narrow slits and channels very rapidly.

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Super Fluid • The peculiar properties of Liquid Helium – II give rise to interesting thermal and mechanical effects as listed below. • Thermomechanical Effect • Mechanocaloric Effect • Fountain Effect • Rollin Film Effect

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Thermomechanical Effect • Consider a flask filled with super fluid helium (LHe – II) and a heating coil placed inside a differential container as shown in the figure. • When the heat is applied to the fluid in the inner container, the concentration of normal fluid increases. 35

Thermomechanical Effect • The Super fluid component tends to move towards this region to equalize the concentration. • Super fluid being less viscous, can flow rapidly through the narrow channel. • Normal fluid being more viscous, its flow is impeded by the channel resistance

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Thermomechanical Effect • As a result, due to the induced pressure difference, a pressure head called as Thermo Mechanical Pressure Head is developed. • Because of thermal action, one mechanical head is getting generated or the flow of fluid is happening from outside container to inside container through a thin little channel, and this is what we call as thermomechanical effect • This head is proportionate to the temperature rise of the fluid. 37

Mechanocaloric Effect • The apparatus consists of a round flask filled with a fine powder and Super fluid Helium (LHe – II). The flask has an opening at the bottom. • The Super fluid Helium (LHe - II) being less viscous flows through the fine powder easily.

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Mechanocaloric Effect • As a result, the concentration of normal fluid increases above the powder. • Hence, the temperature increases inside the flask, which is sensed by resistance thermometer. • Higher temperature is created at this point and lower temperature below this flask and this is what we call as mechanocaloric effect. 39

Fountain Effect • The U-tube is filled with a fine powder and is immersed in Super fluid Helium (LHe – II) bath. • When heat is added to the powder, the concentration of normal fluid increases due to rise in the temperature

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Fountain Effect • As a result, the Super fluid rushes in, to equalize the concentration. • Normal fluid, being more viscous cannot flow through the fine powder. • The inflow of super fluid builds up with time and finally squirts out (25 to 30 cm) through the fine capillary opening at the top. 41

Rollin Effect

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Rollin Effect • The Liquid Helium – II exhibits a property of clinging to the walls of the container called as Creeping effect. • The thickness of the film is in the order of 30 nm. • Consider a test tube filled with Liquid Helium – II as shown in the figure.

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Rollin Effect • When the test tube is lowered into the Liquid Helium - II bath, the Rollin film clings to the tube and gradually fills the tube. • On the other hand, if the tube is raised above the bath level, it empties out slowly. • In these films, the capillary forces dominate the gravity and viscous forces. 44

Rollin Effect • The enclosure or the container has to be designed properly otherwise Helium – II creeps to the warmer side through valves and openings and will evaporate. 45

Second Sound • Due to difference in concentrations of normal helium and super fluid in LHe 4, there exists a temperature gradient. This gradient causes oscillations of Normal fluid and Super fluid which are called as Second sound. • The velocity of Second sound varies from zero at lambda point to 239 m/s at near 0 K. 46

Second Sound • Second sound consists of temperature waves or oscillations in temperature rather than pressure waves in ordinary sound. • If only the super fluid component in Second sound oscillates, then it is called as Third sound. • The velocity of propagation of Third sound is around 0.5 m/s. 47