Investigation of the flow boiling performance in mini channel with micro pin fin

Flow boiling in micro pin fin arrays is an effective way to solve the heat dissipation problem. However, studies on the flow boiling performance of pin fin arrays with different structural parameters are still insufficient, especially for the application of long channels for practical evaporators in aircraft environmental control systems. In this paper, the flow boiling performance of R134a was experimentally investigated in a 300 mm long mini channel with micro pin fin arrays. The mass fluxes, heat fluxes, inlet saturation pressures and vapor qualities were in ranges of 200 -500 kg/(m 2 s), 15–30 kW/m 2 , 500–700 kPa and 0.05–0.90, respectively. The local heat transfer coefficient and pressure drop of staggered diamond pin fin arrays with different channel widths of 1.0, 1.2 and 1.4 mm were comprehensively studied. Subsequently, the performance analysis of the above channels was carried out. Results indicate that the nucleate boiling and convective boiling together govern the two-phase flow boiling heat transfer in the pin fin array. In particular, the increase in channel width delays the transition to convective boiling dominated flow. The local heat transfer coefficient increases with mass flux. However, in the high vapor quality region, the flow disturbance caused by a large mass flux may suppress the heat transfer, as high-speed gas destroys the continuous thin liquid film on the walls. Furthermore, the influence of saturation pressure on the heat transfer coefficient is reversed with the increase in vapor quality. Additionally, the performance analysis indicated better performance of the pin fin array at low vapor quality regions and small mass flux. Significantly, the best performance was obtained in pin fin array with a channel width of 1.4 mm, due to a drastic increase in two-phase pressure drop compared to heat transfer. Finally, a new correlation with the consideration of pin fin array structure parameters was proposed with an overall mean absolute deviation of 16.2%. This new correlation is proved to have an obvious accuracy improvement in predicting the flow boiling local heat transfer coefficient of R134a in a mini channel with pin fin arrays. The present work can provide critical information for the design optimization of a mini channel two-phase heat exchanger with pin fin arrays.