Flow and heat transfer characteristics of microchannel flow boiling enhancement with channel configurations

Abstract

The heat dissipation problem encountered in the process of the development of new electronic equipment and mechanical components towards miniaturization has become the bottleneck of continuous progress in the field of mechanical and electronic control engineering, and the flow boiling heat transfer technology in microchannel may provide the effective cooling guarantee for this purpose. Given the reasonable operating temperature ranges of devices and the diversity of coolant evaporation temperature and pressure choices, refrigerants have become an important option, especially for zeotropic mixtures, which can not only modulate the properties of pure components, but also present the unique phase change heat transfer characteristics with great application prospects. In the present study, the flow boiling heat transfer research of R134a/R245fa zeotropic mixtures as well as their pure compartments in parallel multi-microchannel with continuous and segmented channel configurations were experimentally studied. Each microchannel test section consisted of seven parallel channel passages with the same total length of 110 mm. The cross-sectional area of each channel passage was 2 mm × 1mm (width×height) with the slab width of 0.5 mm. What difference from continuous microchannel was that 10 mm-long interconnected area was arranged every 30 mm along the channel length in segmented one. The experiments were carried out at the same inlet saturation temperature of 26 °C under the conditions comprising the heat flux range of 20− 350 kW/m2 and mass flux range of 300− 400 kg/(m2 s), respectively. In order to obtain more accurate data matching relationship, the analysis of heat transfer performance focused on the region near the outlet of the test sections. It’s worth noting that the multi-microchannel should be treated as fin effects during the heat transfer data reduction. The ribbed slabs could be considered as the rectangular straight fins either for continuous or segmented multi-microchannel. The results showed that compared with continuous microchannel, the heat transfer performance was slightly enhanced for pure components while slightly suppressed for zeotropic mixtures in segmented microchannel at lower effective heat flux. With the increase of effective heat flux, the heat transfer suppression effect of zeotropic mixtures was gradually weakened. The interconnected area was helpful to delay the occurrence of the transfer deterioration and improve the flow boiling CHF regardless of pure or zeotropic refrigerants. Besides, the flow boiling pressure drop in segmented microchannel was obviously reduced for zeotropic mixtures and their pure components. The smaller the liquid-vapor density ratio of test refrigerants, the higher the reduction of pressure drop with the help of interconnected area. For zeotropic mixtures, the flow boiling pressure drop was smaller at lower effective heat flux and the increasing trend of pressure drop on heat flow is slower.