Numerical Simulation Of Single Phase And Boiling Microjet Impingement

Dissertation (Ph.D.) University of Alaska Fairbanks, 2008This work presents results from the numerical simulation of single phase and boiling microjets primarily for high density electronics cooling. For the single phase microjets, numerical simulation results for the flow fields and heat transfer characteristics in a laminar, confined microjet (76 mum in diameter) impingement arrangement are presented. The parameters varied included the jet Reynolds Number, the fluid Prandtl Number and the ratio of the nozzle-to-plate distance to the jet diameter. Primary and secondary recirculation zones were observed in the stagnation region and the radial outflow region which had a significant impact on the local Nusselt Number distribution on the heated surface. The location and the displacement of the primary and secondary recirculation zones are of particular importance and are associated with secondary peaks in the Nusselt Number similar to those observed for turbulent jet impingement in larger conventional jets. Numerical simulation results are presented for boiling microjet impingement in a confined arrangement. The Rensselaer Polytechnic Institute (RPI) model was modified for laminar flow boiling for simulating these types of flows. The model primarily proposes three different heat transfer components, the single phase heat transfer, the quenching heat transfer and the evaporative heat transfer. The model was first validated with experimental results from the literature and then extended to study the effects of liquid subcooling, microjet Reynolds Number based on the nozzle inlet, and heat flux levels. The simulation results were in good agreement with results from comparable experiments in the literature. The average wall temperature increases as the applied wall heat flux is increased. The slopes of the temperature curves in the radial direction flatten out at higher heat fluxes and lower levels of subcooling indicating the effectiveness of boiling heat transfer. For the cases considered in this study, the single phase heat transfer component dominates the other two modes of heat transfer The liquid velocity profile has a considerable impact on the vapor bubble nucleation, vapor drag and the bubble departure diameter. Lower levels of subcooling are associated with boiling inception and more vigorous boiling in the vicinity of the stagnation zone rather than those with higher levels of subcooling. The degree of subcooling emerged as the single largest factor controlling the lateral temperature rise in an electronic chip cooled by a single, confined impinging microjet. Increases in the jet inlet Reynolds Number for the same heat flux and subcooling levels increased the dominance of forced convection heat transfer over the boiling heat transfer. Lower Reynolds Number flows are marked by partial nucleate boiling in contrast to higher Reynolds Number flows marked by forced convection boiling. For all the cases considered in this work, the single phase heat transfer component dominated the other two modes of heat transfer. The evaporative mode dominates the quenching heat transfer mode, an observation that is markedly different from those observed for turbulent evaporative jets found in the literature