Experimental investigation of flow boiling heat transfer in microchannels with inlet two-phase flow effects

•Introduced inlet vapor to reduce along-length heat transfer non-uniformity.•Enhanced bubble generation and coalescence at the inlet.•The along-length heat transfer coefficient exhibits three distinct trends.•Improved overall heat transfer performance with reduced temperature non-uniformity. Microchannel heat exchangers show great potential in cooling electronic devices due to their high heat transfer efficiency and compact design. However, the non-uniform distribution of along-length heat transfer coefficients often results in localized overheating or overcooling, significantly affecting heat transfer performance and wall temperature uniformity. In this study, visualization experiments are conducted to measure the along-length and overall heat transfer coefficients by regulating the inlet vapor quality to improve temperature uniformity. The results show that increasing preheater power leads to three distinct patterns of along-length heat transfer coefficient distribution under constant mass flux and evaporator load: checkmark-shaped, U-shaped, and inverse checkmark-shaped. Bubble coalescence and generation enhance the local heat transfer coefficient at the inlet. The overall heat transfer coefficient with the two-phase inlet effect is up to 2.25 times higher than that of a single-phase inlet. Within the mass flux range of 89 kg/m²/s to 537 kg/m²/s, the overall heat transfer coefficient exhibits four distinct behaviors: slow rise (SR), sustained development (SD), decline after sustained development (DASD), and rapid decline (RD). Introducing vapor at the evaporator inlet facilitates stabilizing the vapor-liquid interface and improving wall temperature uniformity. These findings provide a comprehensive understanding of heat transfer characteristics in microchannel heat exchangers and offer valuable insights for optimizing their thermal performance.