Flow boiling of water in a micro pin-fin heat sink at sub-atmospheric pressure

Flow boiling of deionized water in a silicon-based micro pin-fin heat sink with sub-atmospheric outlet pressure are experimentally studied in this work. Experiments are conducted with a constant mass flow rate of 4 ml/min, a controlled outlet pressure ranging from 20 to 60 kPa, and effective heat flux up to 580 kW/m². The inlet temperature is controlled at 20 K below the saturation temperature corresponding to the outlet pressure. The effects of sub-atmospheric outlet pressure on the flow boiling characteristics are investigated and discussed. It is found that lower sub-atmospheric outlet pressure leads to higher pressure drop due to the significant reduction in vapor phase density. When the outlet thermodynamic quality is about 0.27, the pressure drop is about 14.87 kPa at P_out = 20 kPa, while it is only 8.50 kPa at P_out = 60 kPa. In the meantime, the average heat transfer coefficient at P_out = 60 kPa is approximately twice that at P_out = 20 kPa. Besides, a decreasing tendency of wall temperature along the flow direction is observed, which is caused by the decreasing tendency of saturation temperature along the micro pin-fin heat sink. Under lower outlet pressure condition, the heat transfer performance degradation occurs earlier. The flow boiling visualization results show that the rapid expansion behavior of upstream bubbles and the evaporation of the thin liquid film in the downstream area are the main flow boiling behaviors in micro pin-fin heat sink under outlet sub-atmospheric pressure. The expansion of upstream bubbles is an important driving force for the replenishment of downstream liquid.