An experimental investigation on the evaporation and condensation heat transfer of air-cooled multi-port flat heat pipes

Abstract

An experimental investigation was conducted to evaluate the evaporation as well as condensation heat transfer of air-cooled multi-port flat heat pipes (AMFHPs), designed for extreme working conditions of power battery thermal management in electric vehicles. The experimentally obtained evaporation as well as condensation heat transfer coefficients (HTCs) under various working conditions were compared and analyzed. It was found that, both the orders of the intensity of heat transfer mode in the liquid pool and the condensation Nuc were R134a > ammonia > acetone. The orders of the evaporation HTCs referring to ammonia and R134a AMFHPs with different filling ratios (FRs) were 50 > 30 > 100 > 70 and 30 > 50 > 70 > 100%, while those of Nuc were 50 > 30 > 70 > 100 and 30–50–70 > 100%, respectively. In most cases, evaporation HTC increased with the rise of heating power as well as the decrease of cooling air velocity, while the condensation Nuc decreased with the increasing in the heating power as well as the air velocity slightly. The measured evaporation HTCs were compared with some predictive correlations of pool boiling and pool boiling-thin film evaporation expressions for the evaporator section at different FRs, and the condensation HTCs were also evaluated by several available filmwise condensation models. Results showed that Cooper correlation for pool boiling revealed quite a good agreement with measured results for the AMFHPs of 100% R134a, 70% ammonia and 100% ammonia, and Rohsenow correlation predicted the condensation HTCs of R134a AMFHPs with FR<100% accurately. However, for ammonia, the prediction of the evaporation HTCs in the case of FR < 70% was difficult, and Nusselt theory significantly overestimated the condensation HTCs of all the ammonia samples.