Experimental Investigation of Thermo-Hydraulic Characteristics of Two-Phase Flow of FC72 in Microchannel Heat Sinks
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Advances in electronics such as chip level integration and die stacking have led to a bottleneck in further development since dissipation of the resulting high heat fluxes continues to be a challenge. Due to the tremendous potential, microelectronics also finds applications in a wide range of industries such as automobiles and aircrafts, along with high power computing which places additional emphasis on thermal management of electronics. Ongoing research in the field of flow boiling to meet the rising demands has resulted in the evolution of potential cooling technologies such as microchannel heat sinks. Although microchannel heat sinks offer numerous advantages, a number of roadblocks impede its transformation to a commercial heat dissipation device. These include flow instabilities and lack of theoretical models which poses a great difficulty in the prediction of hydraulic and thermal characteristics. In an effort to understand and characterize the flow boiling in these micro-structures, experiments were conducted using 19 parallel, surface enhanced microchannels with a hydraulic diameter of 253-microns. The working fluid used is a perfluorocarbon (C6F14), also known as FC72. The surface of microchannels was enhanced by the fabrication of re-entrant cavities which served as vapor trapping sites. Heaters were integrated with the test section to simulate the heat from a chip. The study was performed at mass fluxes ranging from 500-2000 kg/m2-s and inlet subcooling up to 20°C. The performance of the microchannel heat sink was measured in terms of heat transfer coefficient under a wide range of operating conditions characterized by the system variables - heat flux, mass flux and inlet subcooling. The effect of these variables on the Onset of Nucleate Boiling (ONB) and nucleation hysteresis was also studied. Analysis was also carried to compare the obtained heat transfer coefficient with those predicted by empirical models available in the literature and a nucleate boiling correlation has also been proposed, which correlates most of the experimental data within ±15%. The high heat transfer coefficient observed in flow boiling in microchannels is often accompanied by high pressure drop due to the phase-change taking place in the channels. The pressure drop has to be contained within acceptable levels, as this directly translates into pumping power required to operate the system. The study conducted encompasses the experimental analysis of the aforementioned parametric effects on the pressure drop. The data were also compared with the predictions of conventional homogeneous and separated flow pressure drop correlations. Flow instabilities were observed under certain subcooled boiling conditions although these were mitigated in saturated conditions by the presence of 20-micron sized cavities incorporated into the base of each microchannel. The influence of system variables on the flow instabilities was studied and the flow instabilities were classified as high and low frequency oscillations based on pressure transducer signals and high speed videos. Investigations were also done to demarcate the stable and unstable flows and hence identify the stable regimes of operation. However, to completely eliminate the instabilities, it is important to identify the underlying mechanisms. Two possible causative mechanisms are channel-to-channel interaction and the effect of compressible volumes at channel exit and inlet. In order to isolate these causes, a study of a single silicon microchannel was conducted using flow boiling experiments in a single microchannel test section of height 347 microns and width ranging from 100-400 microns. The base of the microchannel was augmented with either two or six re-entrant cavities. The results include the parametric effects of inlet subcooling, mass flux, heat flux and microchannel size on the pressure drop and comparisons with multichannel configurations. Data acquired from pressure transducers and high speed visualization showed that the pressure drop oscillations in the subcooled boiling regime observed earlier in a multichannel configuration, were not observed in the subcooled regime of single channel of width 100 microns, indicating that instabilities reduce with reduced interactions between the channels. However, very slight oscillations were observed under saturated conditions although it did not affect the pressure drop.