Effect of contraction on the convective boiling heat transfer of microchannel heat sinks

The present study examines the effect of contraction before straight fin microchannels in the cold plate by using refrigerant R-134a as a working fluid. The experiments compared two test sections with the same geometry and dimension, but one has contraction before the microchannels (CBM), and another one has no contraction before the microchannels (NCBM); the mass flux ranges from 25 to 425 kg/m2⋅s while the heat flux spans from 60 to 300 kW/m2. The inlet flow conditions can be either a fully liquid inlet or with vapor qualities (from 0.1 to 0.45). Improved flow distribution is found with the introduction of the contraction. Based on the mass flux and irrespective of the inlet conditions, the test results indicated that the performance of the CBM shows a superior heat transfer coefficient (HTC) by about 3 ∼ 30 % than that of NCBM. Nonetheless, the pressure drop of CBM is lower than NCBM when the inlet is fully liquid. Conversely, the trend of pressure drop is reversed with a vapor-quality inlet. For the fully liquid inlet, the CBM contains a much lower pressure drop by 22 % compared to NCBM at a low mass flux of 36.2 kg/m2⋅s. However, the reduced pressure drop has narrowed to 3 % at a mass flux of 402.8 kg/m2⋅s. Flow visualizations were performed, and it was found that the CBM is free from flow reversal, and the fluctuations in temperature or pressure drop are much less pronounced. By contrast, the NCBM design shows flow reversal and sluggish bubble phenomena.