Cryogenic FuelLiquid hydrogen

Introduction to Cryogenics:

• Cryogenics may be defined as the branch of physics which deals with the production of very low temperature and their effect on matter.

• . The word “cryo” is derived from a Greek word “kruos” which means cold.

• For storage of liquids at cryogenic temperatures Dewar flasks prove useful

The cooling process:

• By compressing the gas, the gas is cooled releasing heat and later allowed to expand producing ultra low temperatures.

• Cooling can also be obtained by magnetocaloric effect: it is a magneto-thermodynamic phenomenon in which a change in temperature of a suitable material is caused by exposing the material to a changing magnetic field.

Uses:

• Cryogenic fuels

• Liquid nitrogen: used in cryotherapy, as a coolant, cryonic preservation

• Nuclear magnetic resonance spectroscopy: NMR is one of the most common methods to determine the physical and chemical properties of atoms by detecting the radio frequency absorbed and subsequent relaxation of nuclei in a magnetic field.

• Frozen food: Cryogenic gases are used in transportation of large masses of frozen food.

• Blood banking: Certain rare blood groups are stored at low temperatures, such as −165 °C.

Liquid hydrogen:

• Hydrogen was liquefied by James Dewar in 1898 by using regenerative cooling and his invention, the vacuum flask.

• Hydrogen (H2 ) is found in gaseous form in the atmosphere.

• To exist as a liquid, H2 must be cooled below hydrogen's critical point of 33 K. However, for hydrogen to be in a full liquid state without evaporating at atmospheric pressure, it needs to be cooled to 20.28 K.

• One common method of obtaining liquid hydrogen involves a compressor resembling a jet engine in both appearance and principle. 

Properties of liquid hydrogen:

• The byproduct of its combustion with oxygen alone is water vapor (although if its combustion is with oxygen and nitrogen it can form toxic chemicals), which can be cooled with some of the liquid hydrogen.

• Since water is considered harmless to the environment, an engine burning it can be considered "zero emissions." 

•  Liquid hydrogen also has a much higher specific energy than gasoline, natural gas, or diesel.

•  It has a remarkably low volumetric energy density, many fold lower.

Liquefaction of hydrogen:(Regenerative cooling)

• The basic principle is based on joule thompsons effect.

Steps:

1. Pure hydrogen is compressed to a high pressure and its temperature rises

2. Then it is passed through two heat exchangers.

3. Now it is combined to pass with a tank containing liquid nitrogen and then again one final heat exchanger.

4. Now we have gaseous Hydrogen which is it a low temperature but high pressure

Uses of liquid Hydrogen:

• It is a common liquid rocket fuel for rocket applications.

• Liquid hydrogen can be used as the fuel storage in an internal combustion engineor fuel cell.

•  Various submarines (Type 212 submarine, Type 214 submarine) and concept hydrogen vehicles have been built using this form of hydrogen

• Liquid hydrogen is also used to cool neutrons to be used in neutron scattering. 

Liquid hydrogen as a Rocket fuel:

• In most liquid-propellant rocket engines, a fuel and an oxidizer (for example, gasoline and liquid oxygen) are pumped into a combustion chamber.

• There they burn to create a high-pressure and high-velocity stream of hot gases.

• These gases flow through a nozzle that accelerates them further (5,000 to 10,000 mph exit velocities being typical), and then they leave the engine giving the required thrust.

• One of the big problems in a liquid-propellant rocket engine is cooling the combustion chamber and nozzle, so the cryogenic liquids are first circulated around the super-heated parts to cool them.

• Many early rocket theorists believed that hydrogen would be a marvelous propellant, since it gives the highest specific impulse. 

• It is also considered the cleanest when used with a liquid oxygen oxidizer because the only by-product is water.

Drawbacks of using liquid hydrogen as a rocket fuel:• Because of the low temperatures of cryogenic propellants, they are

difficult to store over long periods of time.

•  liquid hydrogen has a very low density (0.071 g/ml) and, therefore, requires a storage volume many times greater than other fuels.

• Despite these drawbacks, the high efficiency of liquid oxygen/liquid hydrogen makes these problems worth coping with when reaction time and storability are not too critical.

•  Liquid hydrogen delivers a specific impulse about 30%-40% higher than most other rocket fuels.