Thesis: M.N. Tasker

Doctor of Philosophy, University of Oxford, Hilary Term 1987

The Effect of Heat Transfer on the Dispersion of Cold Dense Gases

Summary

This thesis describes an investigation of the importance of heat transfer on the dispersion of cold dense gas clouds. The scaling requirements for modelling this process in a wind tunnel are outlined and simple wind tunnel experiments indicated that the rate of heat transfer from the ground did affect the temperature of a continuous plume of cold dense nitrogen vapour, but did nor alter the shape of the plumes.

Mathematical simulations using a 'box-type' model showed that convection induced mixing could significantly dilute a cold dense gas cloud assuming an entrainment coefficient (B) of 2.5. Simulations were not performed using lower values of B.

A rig was built to measure convective entrainment rates into a one metre deep cloud of nitrogen, having a bulk temperature of 170K and heat fluxes of up to 1100 W/m were applied. It was found that when the heater temperature exceeded ambient temperature (high heat fluxes) the convective entrainment velocity (Ue) could be described by the formula Ue/W* = 0.15/Ri* but at low heat fluxes, no convective entrainment occurred. It is concluded that convective entrainment may only affect the dispersion of a cold dense gas cloud, if the gas is buoyant at ambient temperature i.e. for gases such as methane and ammonia.

(no thesis available)