Numerical model and analysis of heat transfer during microjets array impingement

Abstract

In this paper heat and fluid flow characteristics during impingement of an array of microjets was numerically investigated. The numerical model which was based on the compressible steady-state Navier-Stokes equations and SST k-ω turbulence model was developed. Then an array of 8 × 8 microjets which impinged on a hot surface was analyzed. The influence of both a ratio of a distance between a nozzle and hot plate (H/d) as well as a microjet diameter-based Reynolds number (Red) on both the local and average temperature and heat transfer coefficient (HTC) on the surface of the hot plate were numerically studied. During simulations the ratio of the distance between the nozzle and hot plate to the microjet diameter was H/d = 3.125, 25 and 50, while the microjet diameter-based Reynolds number was equal to Red = 690, 1100 and 1510. It was found that the H/d ratio and Red significantly influenced flow patterns in the gap between the nozzle and hot plate as well as the temperature and HTC on the surface of the hot plate. With increase of the H/d ratio a more uniform distributions of the plate temperature and HTC were observed. Moreover, differences between extreme values of these parameters decreased. The optimal ratio of the distance between the nozzle and hot plate to the microjet diameter was calculated to be H/d = 25. Increasing or decreasing of this ratio resulted in the higher average temperatures and lower average values of the HTC on the hot plate. For the small value of the H/d ratio (i.e., H/d = 3.125) the influence of the microjet flow on adjacent microjets was vestigial. The area of the hot plate which was influenced by impinging microjets was small and degradation of the microjet velocity profile was negligible, i.e., high-speed microjets impinged the plate. Air from the microjet flowed above the hot plate and left the zone through spaces between adjacent microjets. For higher values of the H/d ratio influence of the microjet flow on adjacent microjets was observed. The microjet left the nozzle and greatly expanded, while its velocity profile was degraded. The area of the hot plate which was influenced by impinging microjets was large and the microjet velocity was low. Due to significant enlargement of the size of the microjet, it interacted with adjacent microjets, which resulted in the lost of their axisymmetric shape. Moreover, air from the microjet flowed above the hot plate and affected adjacent microjets. The rise in the Red intensified mentioned effects as well as heat transfer intensity on the hot plate. Moreover, the increase of the Red resulted in the decrease of the average temperature of the heater surface as well as the increase of the average HTC regardless the height of the microjet. Calculated average hot plate temperatures were also compered with experimentally measured temperatures and the good matching was found. However, much slower degradation of microjet profiles was observed in experiments than predicted numerically.