Carbon dioxide local heat transfer coefficients during flow boiling in a horizontal circular smooth tube

Carbon dioxide is gaining renewed interest as an environmentally safe refrigerant. In order to improve the energy efficiency of R744 systems, an accurate knowledge of heat transfer coefficients is fundamental. In this paper experimental heat transfer coefficients during flow boiling of R744 in a smooth, horizontal, circular, 6.00 mm inner diameter tube are presented. We obtained 217 experimental points in 18 operating conditions commonly encountered in dry-expansion evaporators investigating the effect of the mass flux within the range from 200 to 349 kg/m2 s, the saturation temperature within the range from −7.8 to 5.8 °C, the heat flux within the range from 10.0 to 20.6 kW/m2 and the vapor quality within the range from 0.02 to 0.98. An interpretation of the experimental trends based on the local circumferential distribution of heat transfer coefficients, the flow regimes and the thermophysical properties is proposed. Besides the measured data are compared with those predicted by the Cheng et al. [L. Cheng, G. Ribatski, J.R. Thome, New prediction methods for CO2 evaporation inside tubes: Part II – An updated general flow boiling heat transfer model based on flow patterns, International Journal of Heat and Mass Transfer 51 (2008) 125–135] and Yoon et al. [R. Yun, Y. Kim, M.S. Kim, Y. Choi, Boiling heat transfer and dryout phenomenon of CO2 in a horizontal smooth tube, International Journal of Heat and Mass Transfer 46 (2003) 2353–2361] correlations to determine the best predictive method for the tested operating conditions