Flowing boiling heat transfer characteristics test for microchannels with different surface energy in refrigeration system

Abstract: With the development of many applications in modern industrial system and equipment, the ability to dissipate a large amount of heat from small surface area has been becoming increasingly urgent. Flow boiling in microchannels provides unique advantages when applied in micro-module equipment. Therefore, the microchannel heat exchangers have a broad industrial and market prospects for their high heat transfer coefficients and appreciable reduction in weight and volume. Flow boiling heat transfer in microchannels becomes one of popular researching hot spots. But the researches about flow boiling in microchannels with different surface energy are still lacked and further researches are needed. In this study, the current experiments fabricated 3 different modules with the identical sizes of 240 mm × 400 mm × 7.5 mm and with the different surface energy. Flow boiling experiments were conducted with the refrigerant R141b in a test module containing 22 microchannels which were 2 mm wide and 1 mm deep. During the experiments, the heat flux imposed on the aluminum substrate varied from 9 to 26 kW/m2 and the mass flux flowing into the channel varied from 50 to 583 kg/(m2·s). The effects of the microchannels with different energy on boiling heat transfer under a certain pressure were experimentally investigated with different heat flux and mass flux. The results showed that the thermodynamic equilibrium quality increased fairly linearly with axial variation, with the slope inversely proportional to mass flow rate. The two-phase boiling heat transfer came earlier when mass flow rate was smaller and the increase of mass flow rate was beneficial to boiling heat transfer, but leading to the increasing of the length of the subcooled region. Heat transfer coefficient was relatively stable in the saturated boiling region of the microchannels and had a tendency to decrease along the flow direction due to the rise of gas phase proportion. Compared to the microchannels with the surface energy of 84.16 mN/m, the heat transfer coefficient of the microchannels with the surface energy of 21.12 and 62.99 mN/m increased by 18.42% and 9.28%, respectively. The augment of surface energy of microchannels was attributed to the boiling heat transfer coefficient. One reason was that the microchannels with higher surface energy had better wettability, which increased the liquid layer thickness and promoted dry patch rewetting, thus avoiding local dryout at heat transfer surface. Another was that there was more active nucleation in the microchannels with high surface energy, which made bubbles coalesce laterally during the growth phase before they departed from the surface. Three popular correlations were compared, and the correlation of Lazarek showed the best predictive capability with the mean absolute error (MAE) of 21.2%. In order to further improve the accuracy, the Jacobi number and thermodynamic equilibrium quality were introduced. The new correlation based on the correlation of Lazarek was obtained by fitting, which showed good predictions and was evidenced by an overall MAE of 9.76%. This research is beneficial to the parameter optimization of the predictive tool of the two-phase heat transfer coefficient and also provides the reference for the design of the microchannel heat exchangers.