Flow Boiling Pressure Drop and Heat Transfer of Refrigerants in Multi-microchannel Evaporators under Steady and Transient States

Multi-microchannel evaporators used for the cooling of high heat flux electronics have been of interest to both industry and academia for more than a decade. Such interest has sparked a large number of research studies on the flow boiling pressure drop and heat transfer in multi-microchannel evaporators. However, there are still several aspects that need to be addressed in order to better understand the complicated flow boiling process taking place in such micro-evaporators. Firstly, the mechanism governing flow boiling heat transfer in microchannels is arguable; secondly, the availability of fine-resolution local heat transfer data is very limited; thirdly, over-simplified heat conduction models are used in the literature to reduce such local heat transfer data; finally, rare attention has been taken on the thermal behavior of such micro-evaporators under transient status. Inspired by the forgoing aspects, an extensive experimental program has been conducted to study the flow boiling pressure drop and heat transfer of refrigerants in multi-microchannel evaporators under steady and transient status. For the steady-state tests, three fluids ( R245fa, R236fa and R1233zd(E)) were tested in two multi-microchannel evaporators. The silicon microchannel evaporators were 10 mm long and 10 mm wide, having 67 parallel channels, each 100 $\times$ 100 $\mu$m$^2$, separated by a fin with a thickness of 50 $\mu m$. Two types of micro-orifices (25 and 50 $\mu$m$^2$ in width) were placed at the entrance of each channel to stabilize the two-phase flow and to obtain good flow distribution. The test section backside temperatures were measured by a self-calibrated infrared (IR) camera. The operating conditions for stable flow boiling tests were: mass fluxes from 1250 to 2750 kg m$^{-2}$s$^{-1}$, heat fluxes from 20 to 64 W cm$^{-2}$, inlet subcoolings of 5.5, 10 and 15 K, and nominal outlet saturation temperatures of 31.5, 35 and 40 $^{\circ}\mathrm{C}$. The resulting maximum exit vapor quality at the outlet manifold was 0.51. The steady-state experimental data were reduced by solving a 3D inverse heat conduction problem to obtain the local heat transfer coefficients on a pixel-by-pixel basis. The required fluid temperature in the subcooled region was calculated from the local energy balance, while that in the saturated flow boiling region came from the general pressure drop model proposed in this manuscript based on the present data base. According to the present data base of fine-resolution local heat transfer coefficients, a new flow pattern based prediction model was developed here starting from the subcooled region all the way through the annular flow regime. This new flow pattern based model predicted the total local heat transfer database (1,941,538 local points) well with a MAE of 14.2% and with 90.1% of the data predicted within $\pm$30%. It successfully tracks the experimental trends without any jumps in predictions when changing flow patterns. For the transient tests, an extensive experimental study was conducted to investigate the base temperature response of multi-microchannel evaporators under transient heat loads, including cold startups and periodic step variations in heat flux using two different test sections and two coolants (R236fa and R245fa) for a wide variety of test conditions. In addition, a transient flow boiling test under a heat flux disturbance was performed, and a new method of solving the transient 3D inverse heat conduction problem was proposed to obtain the local transient flow boiling heat transfer coefficients.