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Stable hydrogen is in the form of a diatomic molecule. The most common form is H2, although there are two other isotopes of hydrogen; these are deuterium and tritium, which is radioactive. Liquid hydrogen is used extensively as a fuel in rocketry and more recently in commercial transportation. H2 has two different nuclear spin states: ortho-hydrogen (spin 1 state) and para-hydrogen (spin 0 state). Near room temperature, equilibrium hydrogen is 75% ortho and 25% para. However, at low temperatures, around the normal boiling point of 20.3 K, hydrogen is nearly all in the para-hydrogen state. The conversion process from ortho to para hydrogen is exothermic and generates about 700 kJ/kg. It is also quite slow unless a catalyst in employed to facilitate the conversion process. Thus, for long term storage of LH2, the ortho-para conversion is usually carried out as the LH2 is produced at a centralized plant. The critical point for H2 is Tc = 33 K at pc = 1.3 MPa, making it one of the coldest liquids known. It also has a very low density r = 70.8 kg/m3 at the NBP. The triple point where hydrogen begins to solidify under saturated vapor pressure is TTP = 13.8 K. The latent heat of LH2 is hfg = 445 kJ/kg at the NBP. The transport properties of LH2 are quite similar to those of other fluids with the exception of the extremely low temperature and density. Heat transfer and pressure drop in forced flow LH2 generally correlates with classical treatments. While using LH2, it is important to use proper safety techniques. Hydrogen is extremely flammable as with a small ignition energy it will combine with the oxygen is air with a strong exothermic reaction.
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