Measurement of transient liquid film and its effect on flow boiling heat transfer in non-circular microchannels

Micro liquid film plays an important role during flow boiling in microchannels. Current studies on flow boiling in microchannels have mainly focused on circular microchannels. However, microchannels with sharp corners, such as square and rectangular microchannels, are more common in practice. In the present study, flow boiling experiments were performed in square microchannels using ethanol as the working fluid. The mass flux was 26.71 kg (m⁻² s⁻¹) and heat flux ranged from 10 to 80 kW m⁻². The transient liquid film thickness during flow boiling in square microchannels was measured using a laser confocal displacement meter. The pressure drop between inlet and outlet of the microchannel and the wall temperature were measured synchronously. Experimental results demonstrated that heat transfer performance depended on liquid film thickness and the thickness in square microchannels was smaller than in circular microchannels under the same capillary number. The variation of liquid film thickness during flow boiling in square microchannels was analyzed and a theoretical model was proposed for predicting liquid film thickness. According to variation in wall temperature and liquid film thickness, the flow boiling process in square microchannels was divided into liquid slug, elongated bubble, and dryout zones. The heat transfer mechanism of different zones was analyzed. In addition, the microchannel pressure drop fluctuation and backflow phenomenon were analyzed.