Experimental Study on the Heat Transfer Performance of a Zeotropic Refrigerant Mixture in Horizontal Tubes

In the present study, the condensation and evaporation heat transfer performance of a zeotropic refrigerant mixture (HFC32/HFC125/HFC134a) inside horizontal tubes with smooth, grooved or cross·grooved inner surfaces was investigated experimentally. The cross-gTOoved tube showed the highest heat transfer coefficient for both condensation and evaporation with the zeotropic mixture. Its heat transfer coefficient was nearly three times as high as that of the smooth tube, and 20 to 40% higher than that of the grooved tube. Thus, the cross·grooved tube appears suitable for practical use in heat exchangers using zeotropic mixtures. l.INTRODUCTION In accordance with the regulations introduced at the 7th meeting of the parties to the Montreal Protocol, there will be a complete phase·out of HCFC22 by the end of 2020. Ternary zeotropic (i.e., non-azeotropic) mixtures are among the probable substitutes for HCFC22 and their heat transfer performance has been investigated by several researchers [1-3]. Goto et. al. have performed experiments on a binary zeotropic mixture (HFC32/HFC125) and reported that the condensation and evaporation heat transfer coefficients of zeotropic mixtures are both lower than those of HCFC22 regardless of the tube diameter. Ebisu & Torikoshi [2) investigated the heat transfer characteristics of binary (HFC32/HFC134a) and ternary (HFC32/HFC125/BFC134a) refrigerant mixtures with various compositions. They showed that the minimum heat transfer coefficient was attained at 25 to 30 wt% of HFC32 in the binary mixture, and that the heat transfer coefficients for ternary mixtures were lower than the values inteipOlated linearly between the results for HFC32/HFC134a (30/70 wt%) and HFC125. Zhang et. al. [3) theoretically predicted the heat transfer coefficients of the ternary mixture HFC32/HFC125/HFC134a (23/25/52 wt%) inside a smooth tube, and their results agreed with the experimental data within ±30%. Uchida et. al. [4] showed that the condensation heat transfer coefficients of the binary zeotropic mixture HFC32/HFC134a (30/70 wt%) are lower than those of pure refrigerants, especially in internally grooved tubes as compared with smooth ones. The decrease with the grooved tube was as much as 50%, while the smooth tube showed only a moderate drop (30% ). All of these experimental and theoretical results demonstrate the low heat transfer performance of zeotropic refrigerant mixtures, and it appears that special treatment will be required to attain an adequate heat transfer performance if zeotropic refrigerant mixtures are to be used in practical heat exchangers. The condensation and evaporation heat transfer performance of a zeotropic refrigerant mixture HFC32/HFC125/HFC134a (30/10/60 wt%) inside horizontal tubes with smooth, grooved or cross·grooved inner surfaces was