Microjet array impingement heat transfer crossflow effects in single-phase and flow boiling

Abstract

Microjet array performance steadily decreased with increasing crossflow. Baseline heat transfer coefficients were reduced by 20-40% at a critical jet-to-channel mass flux ratio of 1. For greater ratios, the single-phase and flow boiling array impingement heat transfer asymptotically approached performance characteristics associated with microchannel flow. Microchannel mass flow ranged 380<G_ch<10500 kg/m2-s and corresponded to simultaneously developing flow conditions. Single-phase microchannel heat transfer ranged 15<Nu_ch<115 for 450<Re_ch<8340. Microchannel flow boiling enhanced the single-phase heat transfer rates by 22% and increased CHF as much as 88% depending on the flow conditions. Increased mass flow and subcooling improved ONB by lowering superheat and increasing the heat flux. The peak microchannel flow performance dissipated 160W/cm2 at ΔT_sat=30ºC with a heat transfer coefficient of 22750W/m2-K.

The design and performance of liquid microjet array impingement in dissipating high heat fluxes for areas greater than 1cm^2 are of significant interest for microelectronics cooling applications. The fundamental flow dynamics and heat transfer characteristics of large-scale microjet arrays were represented and investigated experimentally by cooling a smaller 1mm2 area in controlled crossflow conditions to simulate the developing outflow of a larger array. Circular 100µm microjets in staggered and inline array patterns were microfabricated with a standoff ratio of two jet diameters, and jet-to-heater area ratios from 6.5% to 16.4%. Single-phase and flow boiling microjet array impingement with crossflow was investigated using HFE-7000, with jet-to-channel mass flux ratios from 0.059 to 7.67.

The jet mass flow ranged 725<G_j<21150kg/m2-s (162<Re_D<4920) with inlet subcooling from 15<ΔT_sub<35ºC (T_sat=58ºC). The single-phase array impingement heat transfer in minimum crossflow corresponded well to correlations from the literature (13<Nu_D<100). Performance increased with increasing area ratio and mass flow, and dissipated a maximum 450W/cm2 with a peak heat transfer coefficient of 76200W/m2-K. The single-phase heat transfer performance was not significantly affected by the onset of boiling. Increased vapor generation decreased heat transfer rates to 52600W/m2-K leading to CHF at 490W/cm2.