Numerical Validation of a Cryogenic Thermo-Fluid Mixing Model
Bowman, Sean William
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Thermal management of a hot convective gaseous flow can be achieved by the injection of a cold liquid stream. This cooling effect is constrained by the amount of liquid injected, the rate of evaporation of the liquid, and the length of the domain. In this study, such a domain is referred to as a “mixer”. The mixer used to facilitate this process is designed with a venturi geometry, such that the throat of the pipe creates a drop in the static pressure of the gaseous crossflow. Liquid coolant can thereby be passively entrained from a central pipe (plenum) by the static pressure difference established at the venturi throat between the crossflow gas and the liquid. This study considers the results of experimental tests performed for this setup, and a numerical model is established and validated against both the experimental data and a previously developed analytical model . To represent the complete experimental design more accurately and to provide additional insight to the applications of the mixer, an additional geometry including a 90-degree inlet bend is examined. At low momentum ratios between the gaseous crossflow and the entrained liquid a substantially non-uniform droplet distribution is found. This results in gas streamlines near the walls of the mixer being devoid of liquid droplets, which leads to pronounced radial thermal stratification. The driving factor of this phenomena is the formation of a liquid injection boundary caused by the limitations of mass and thermal diffusion of the liquid in the radial direction. This non-isotropic temperature reduction near the mixer axis presents in both the straight pipe and bent inlet pipe cases, however an unexpected difference in radial droplet penetration is found for the 90-degree inlet bend pipe. The variation in radial droplet penetration and the influence that this difference has on the comparison to experimentally measured temperatures in the mixer is discussed. Non-uniform velocity at the inlet of the straight venturi pipe section caused by flow separation and stagnation in the bend influences the radial spreading of liquid droplets in a plane orthogonal to the direction of the bend, at the cost of a diminished spreading in the same plane as the bend.