Experimental characterisation of the thermohydraulic performance of a micro-pin fin evaporator: Heat transfer, flow visualization and modelling

The global tendency towards miniaturization driven by the microelectronics industry is pushing system density and packaging towards unprecedented values of thermal design power, with a dramatic reduction of the surface area of the devices. Vertical integration, which consists of single dies stacked and connected by means of Through-Silicon-Vias (TSVs), is a promising architecture, but it is hampered by the requirement of finding innovative and more effective cooling technology for dissipating the greater amount of heat. On-chip interlayer two-phase cooling represents a very attractive long-term solution for this problem. However, the design of a two-phase-based cooling system and the ability to predict its performance depend on both the availability of high-accuracy experimental data and appropriate models. The focus of the present study is thus to provide a broad experimental database to describe the physical processes associated with two-phase flow and characterise the thermal and hydraulic performances of a new micro-pin fin evaporator, in order to assess its feasibility for the realization of a two-phase-based cooling technology. The micro-evaporator tested here has a heated area of 1cm^2, consisting of 66 rows of cylindrical in-line micro-pin fins with a diameter, height and pitch of respectively 50 micron, 100 micron and 91.7 micron. Channel entrances with and without inlet restrictions are tested in order to demonstrate the beneficial stabilizing effect of placing an extra row of micro-pin fins with a larger diameter of 100 micron at the beginning of the flow passage area. Insights on the local trends and magnitudes of the heat transfer coefficient and pressure drops were gained over a wide range of test conditions with mass flux varying from 500 kg/(m^2s) to 2500 kg/(m^2s), heat flux ranging from 20 W/cm^2 to 48 W/cm^2, and constant outlet saturation temperatures of 25 °C, 30.5 °C and 35 °C. The tested four refrigerants were R236fa, R245fa, R1234ze(E) and R134a.

Based on the experimental observations, the heat transfer coefficient trend enhances significantly with the applied heat and the vapor quality, while mass flux exhibits a slightly minor influence. These results evidence the mutual influence between the thermal performance of the micro-evaporator and the flow pattern development during its operation, which has been further investigated by synchronizing the infrared temperature measurements and the high speed flow visualization. Moreover, new insights on the nucleation process and on the interface dynamics were gained by applying image processing and time-strip technique to the high-speed camera videos.

Finally, a new heat transfer prediction method that focused on the physical understanding and predictive capability of flow boiling in between micro-pin fins was developed and compared with the obtained experimental database (7219 local measurements). The new model incorporates the complex flow geometry and the characteristics of the flow regimes occurring along the micro-evaporator, and it successfully predicted the experimental data.