Pool Spreading and Vaporization Models

Immediately after release, the liquid hydrogen spreads out on the ground. It will spread until it meets an artificial boundary such as a dyke, until it reaches a minimum depth at which no longer spreads, or until the evaporation rate is equal to the release rate so that the amount of liquid in the pool is no longer increasing. Also immediately after release, the liquid hydrogen starts to boil off as it absorbs heat from the atmosphere, the ground and possibly from the sun. Mass is also lost from the pool when wind removes the evaporated material from the surface of the pool so that hydrogen evaporates in order to restore the partial vapour pressure. The Fay equations for instantaneous radial spill are given below [17]:

"4rm2

3gVe

where rm - max radius of the spread/pool radius (in m)

tm - time to max radius spread (s)

m - max rate of vaporization of liquid spread (kg/s)

Vo - total volume of liquid spilled (m3)

Ve g rate of liquid pool without burning (=0.00194 m/s)

gravitation constant (9.8 m/s ) For a continuous radial spill at a fixed location used the equation dV/

0.25

If hydrogen is released from its containment as a liquid, vaporization must occur before a vapor cloud is formed. The rate at which vaporization takes place determines the formation of a vapor cloud. Immediately after release the heat boil-off is taken from the ground. The model given by TNO(1979) [17] gives the evaporation rate as:

where, mg

T Is

Ta Hv

Tb as

is evaporation rate from the ground (kg/s); is time(s)

is surface coefficient of heat conduction (W/mK) is ambient temperature (K) ; is heat of evaporation of LH2 (J/kg); is saturation temperature of LH2 (K) is surface thermal diffusity (m2/s)

Both Is and as are properties of the surface and typical values as shown in the Table E-1:

Values for soil heat conductivity and thermal diffusity [ 183]

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