Abstract
With microprocessor performance increasing, the power generation from a microprocessor chip is expected to exceed 180 W/cm2 and the limits of current air-cooling technology will be reached, i.e., forced air heat sinks have become significantly larger with more expensive and noisier. Therefore, there is a need to address the thermal challenge of high-heat-flux for next generation of power electronics. Flow boiling in microchannels, considered as one of the most promising technologies, has the advantages of highest heat fluxes, lowest pumping powers, and the highest efficiency. This study explores experimentally the flow boiling stability, channel-to-channel interactions and convective boiling heat transfer in 10 parallel diverging microchannels with/without ANS. Three types of diverging microchannel heat sinks (named type-1, type-2, and type-3) were designed. Each microchannel had a mean hydraulic diameter of 120 □m. Water and FC-72 was used as the working fluid with different mass fluxes, based on the mean cross section area, ranging from 99 kg/m2s to 999 kg/m2s. Type-1 system did not contain any ANS, whereas type-2 system contained ANS distributed uniformly along the downstream half of the channel and type 3 system contained ANS distributed uniformly along the entire channel. The ANS are laser-etched pits on the bottom wall of the channel and have a mouth diameter of 24 μm, as indicated by the heterogeneous nucleation theory. Flow visualization shows that slug and annular flow is the dominant two-phase flow pattern. It may imply the dominant heat transfer mechanism may be convective boiling. During CHF, the dryout of annular liquid film appears near the outlet region with frequent rewetting of liquid film with slug bubble or rewetting of liquid column on the dryout surface, while wavy annular flow is the dominant flow pattern. Moreover, correlations for boiling heat transfer coefficient and the CHF are developed and reviewed, respectively. The proposed correlations for boiling heat transfer coefficient show excellent agreement with the experimental data of the present study. Furthermore, the CHF correlation of Bowers and Mudawar can predict the present CHF data very well with the overall MAE of about 16%. Under boiling condition, a significant improvement in stabilizing the flow boiling, suppressing flow reversals, enhancing heat transfer performance can be obtained by using diverging microchannel heat sinks with ANS. Among three types of microchannels, type-3 system shows the best boiling heat transfer performance. This particular design can be regarded as a highly stable and high-heat-flux microchannel heat sink.