Study on fin-induced pseudo-turbulent flow boiling characteristics of micro-gap evaporators

Abstract

Micro-gap offers high performance heat sinks for micro-scale devices, while reduces prevailing two-phase problems such as higher pressure drop, flow boiling instabilities, flow reversal and hot spot generation events. Micro-gap heat sinks incorporated with micro-fins and operated under critical heat flux (CHF) condition may substantially elevate rate of evaporative cooling. Turbulence generation in micro-gap evaporator is a challenging but highly beneficial task as it improves heat transfer coefficient significantly. The obstacle arises from confined domain, which restricts the flow to be scattered. Internal micro-fins not only enhance heat transfer rate but also promote turbulence by inducing supplementary agitation similar to vortex generators. However, larger pumping power requirement due to higher pressure drop is a significant drawback of using micro-fins in micro-gap evaporators. Heat transfer rate, pressure drop and turbulence kinetic energy generation by micro-fins are influenced by number of factors such as fin profile, dimensions and operating conditions. Characterization of micro-gap evaporator based on parametric study may provide an insight on provoking heat transfer rate and turbulence effect and reducing pumping power requirement. This study embarks on conceptualizing the longitudinal fin-induced pseudo-turbulent flow boiling in micro-gap evaporator and optimizing operating conditions to reduce total thermal resistance of the heat sink and pressure drop during coolant flow. Both numerical and experimental investigations have been carried out for refrigerant-134a working fluid. In numerical simulation, results for micro-fins of rectangular and triangular profiles have been compared. Simulation results demonstrate that micro-fins significantly improve the Nusselt number and turbulence kinetic energy generation in micro-gap evaporator. A test rig has been developed for experimental validation of numerical results, which facilitates a micro-gap with 30mm×30mm footprint area, 2 mm base thickness and 1 mm gap height. The micro-gap accommodates 48 internal micro-fins of rectangular profile with 0.3mm×0.3mm dimensions. The heater power is varied from 1-8 kW/m2 and flow rate is controlled by operating compressor between 20-50 Hz frequency. In addition, a pre-heater is installed at the compressor outlet, whose temperature varied from 93-159°C. Experimental data show a good agreement with numerical results. Transient flow boiling features have been illustrated. In comparison to plain micro-gap, rectangular micro-fins enhance Nusselt number almost two-times higher than triangular fins. Optimized values of heat flux, pumping power and inlet void fraction have been prescribed from response surface plot, which can reduce total thermal resistance by 56.3% and pressure drop by 87.2%. Results show that evaporator wall temperature, condenser outlet temperature, vapour flow rate at evaporator outlet and pressures at different locations fluctuate in transient state due to quasi-periodic dry out and surface rewetting observed at inlet manifold of the test section. Heat absorption rate escalated by 128.6% and condenser heat loss increased by 93.8% for frequency increment from 20 Hz to 30 Hz. Finally, it is concluded that flow boiling with low evaporation rate is highly efficient for cooling. Another major contribution of this research is to develop an empirical quadratic model for dimensionless heat flux as a function of Biot number and Reynolds number calculated from vapour velocity at evaporator outlet. The correlation signifies the effect of evaporation rate on thermal resistance of the evaporator. However, the major limitations of this study are inadequate test facility to measure void fraction and use of low power heater, which is incapable of achieving critical heat flux.