School of Mechanical Engineering Seminar
Monday, February 21, 2022 at 14:00
Wolfson Building of Mechanical Engineering, Room 206
Avishai Oved
Master’s student of Dr. Herman Haustein
Re-examination of submerged jet theory and its applicability to heat transfer
For Impinging jets (IJ) arrays, to date only empirical correlations have been developed, which are limited to specific liquids and narrow ranges of operation. In addition, none of these correlations account directly for the velocity profile, which is now known to have a strong influence on heat transfer (HT) distribution. This study examines and adapts recently developed single-jet theory to an unlimited non-interacting array in the submerged configuration.
This comprehensive study aims to unify the novel IJ theory recently developed by our group, which is reviewed hereafter. First, by analytically deriving an improved and more physically correct semi-analytical solution for the jet flight. Second, the study will examine the existing limits of this theory, caused by the occurrence of unwanted jet-edge vortices and excessive jet-wall interaction due to excessive-proximity using CFD numerical simulations. And finally, by the development of a predictive tool for jet flight and impingement, implemented in a Matlab GUI, which integrates the new complex theory in a package for even inexperienced HT engineers. This calculator tool is provided with nominal jet parameters and predicts all relevant flow and HT distributions, such as velocity profile, wall pressure and wall HT coefficient. In a future version this tool will be extended to arrays with spent liquid extraction between the jets.
In the first part, a major improvement in the jet velocity profile prediction is found which now allows examination of key jet parameters, not dealt with previously in a theoretical manner. In the second part, the jet instability is found to depend both on the issuing velocity profile and on the inertial level (Reynolds number), the latter parameter is critical for vortex roll-up an extends classic jet instability theory. Furthermore, a new characteristic scale was established and a self-similar regime found for the excessive nozzle-to-plate proximity case. In other words, flow and heat transfer distributions were found to be indifferent to the nozzle proximity in the range 0.4 < z/zw < 2. Below this, constriction and backpressure become apparent, breaking the theory validity. These findings, are of engineering importance for consistent jet operation under various proximities, and was successfully predicted by the mentioned theory.
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