Numerical and Analytical Modelling for Droplet Evaporation Resulting in Convective Flow Cooling Verified by Experimentation
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The temperature of a gaseous convective stream can be reduced by injecting a colder liquid into the stream. The extent to which this gas temperature can be reduced depends on the amount of liquid being injected into the gas flow domain, rate of energy transfer (heat) from the gas phase to the liquid resulting in evaporation and the length of the domain itself referred to in this study as a “mixer”. This study proposes the use of a non-dimensional Damkohler number that relates the liquid residence time and the liquid life time which could serve as a yardstick to determine a proposed mixer’s efficiency. In order to facilitate injecting the right amount of liquid capable of complete evaporation by the mixer exit, a suitable venturi geometry can be optionally included which facilitates pulling liquid based on the developed difference between the liquid static pressure and throat gas static pressure. Comparison of multiphase pressure and temperature variations along a mixer for different liquid-gas systems typically used in aerospace applications revealed a considerable difference between one another. The developed analytical model is then compared for accuracy against experimental and numerical results which precisely predict non-isotropic flow phenomenon like droplet breakup and coalescence that cannot be modelled in the quasi 1-D analytical model. Comparison in the temperature profile between the numerical and analytical models with experimental data obtained using suitable thermocouples positioned along the length of mixer are within reasonable agreement but revealed an unexpected result of non-isotropic temperature reduction which is limited close to the axis of the mixer. The principle reason for this effect is due to the formation of a liquid injection boundary caused by limitations in the diffusion process occurring along the radial direction.