Mechanisms of micro liquid film heat transfer during flow boiling in non-circular microchannels part I: Measurement and theoretical model of transient film thickness

Liquid film thickness is a dominant feature for understanding boiling heat transfer mechanism in microscale slug flow. Flow boiling in circular microchannels has been extensively studied. Microchannels with non-circular cross-section are more common in industrial applications, but there have been few studies on such complex cross-sections. In the present study, the transient liquid film thickness during flow boiling in non-circular microchannels was experimentally investigated by a laser confocal displacement meter. Non-circular tubes with inner dimension of 0.39 x 0.39, 0.5 x 0.5, 0.6 x 0.6, 0.7 x 0.7 and 0.3 x 0.8 mm2 2 were used for the test section, and water and ethanol were used as working fluids. The variation of liquid film thickness under adiabatic condition in non-circular microchannels was analyzed and an empirical correlation was proposed for predicting initial liquid film thickness. On this basis, a new theoretical model for liquid film thickness variation under flow boiling in non-circular microchannels was developed, considering the effects of evaporation, shear force and transversal flow.