Heat transfer coefficient, pressure drop, and flow patterns of R1234ze(E) evaporating in microchannel tube

摘要

This paper presents heat transfer coefficient and pressure gradient of R1234ze(E) in a tube of 0.643 mm. In addition, visualization sections are added for evaluation of flow patterns. Both heat transfer coefficient and pressure gradient are presented against real saturation pressure, while flow pattern captures at exit of data points are presented in the same plot (Fig. 8). Experiments are conducted on a 24-port microchannel tube with a hydraulic diameter of 0.643 mm. The single-phase friction factor shows fRe = 64 in laminar and underestimation of Churchill prediction in transition (1700 Re 3000). It seems that Gorenflo and Kenning (2010) predicts boiling HTC in a nucleate boiling dominated part very well since the prediction is close to HTC measurement at quality close to 0.1. The pressure gradient of R1234ze(E) is slightly higher than R134a, and the heat transfer coefficient is almost the same at the same condition. The pressure gradient increases with quality and mass flux but decreases with reduction of saturation pressure. R1234ze(E) heat transfer coefficient increases when heat or mass flux rises, decreases when saturation temperature increases. Heat transfer coefficient of R1234ze(E) first increases when quality increases due to the enhancement of convective effects and then drops at moderate quality due to the reduction of nucleate boiling and dry-out. The local maximal value of HTC increases from 3.65 to 3.95 and the quality shifts from 0.55 to 0.7 as mass flux increases from 100 to 200 kg-m(-2) s(-1). Supported by the captures of flow patterns, HTC reaches local maximal with turbulence at G = 200 kg-m(-2) s(-1). Comparing measurements to the existing models these two models are recommended: Mishima and Hibiki (1996) has an MAE (Mean Absolute Error) of 11.7% for pressure gradient and Bertsch et al. (2009) has an MAE of 25.1% for heat transfer. (C) 2019 Elsevier Ltd. All rights reserved.