TED-AJ03-311 HEAT TRANSFER AND PRESSURE DROP CHARACTERISTICS OF R134a CONDENSING IN A MULTI-PORT EXTRUDED TUBE :

In the present study, the local characteristics of pressure drop and heat transfer are investigated experimentally for the condensation of pure refrigerant R134a in four kinds of multi-port extruded aluminum tubes of abut 1 mm in hydraulic diameter. Two tubes, named type A and type B, consists of plane rectangular channels, while remaining two tubes, named type C and type D, consist of rectangular channels with straight micro-fins. In the experiments for types A and B, the local pressure drop is measured at an interval of 191 mm and the local heat transfer coefficient is measured in every subsection of 75mm in effective cooling length using heat flux sensors. In the experiment for types C and D, the total pressure drop of 955 mm in length and the local heat transfer coefficient in every subsection of 75mm in effective cooling length are measured. Experimental ranges are as follows : the mass velocity of G=100-700kg/m^2s and vapor quality of x=1.0-0.0 at a constant inlet pressure of 1.7MPa. In the cases of tubes without micro-fins, the trend of the present experimental data of frictional pressure drop is the same as the Mishima-Hibiki correlation. However, correlations of Chisholm-Laird, Soliman et al. and Haraguchi et al. overpredict the experimental data. This result suggests that the effect of tube diameter should be included in the correlation of frictional pressure drop. The present experimental data except for cases of low mass velocity are relatively in good agreement with the heat transfer correlation of Moser et al., although it is proposed for the in-tube forced convective condensation based on the data of relatively large diameter tubes. It is inferred from this result that the effect of gravitational acceleration should be considered in the case of low mass velocity. The present experimental data were also compared with the heat transfer correlation of Haraguchi et al., in which both effects of the forced convection and the free convection are taken into account. However, the agreement between the experiment and the prediction is not so good. This reason may be caused mainly by the estimation of the forced convection term in their correlation. Considering the effects of surface tension and kinematic viscosity, new correlation of frictional pressure drop is developed for tubes without micro-fins, based on the Mishima-Hibiki correlation. New correlation of heat transfer coefficient is also developed modifying the effect of diameter in the correlation of Haraguchi et al. Experimental data for total pressure drop in tubes with micro-fins agree with the prediction using new correlation of frictional pressure drop proposed in the present study. This means that the micro-fin effect is taken into account by using hydraulic diameter. Experimental data for local heat transfer coefficient in tubes with micro-fins also agree with prediction using new correlation of heat transfer coefficient proposed in the present study. This means that the heat transfer enhancement effect of micro-fins is mainly due to the enlargement of heat transfer area.[figure]