R134a/R125混合工质水平管外凝结换热

Condensation heat transfer of mixed R134a/R125 outside horizontal tubes

对纯工质R134a以及R134a/R125在三种不同组成比例下的混合工质,在光管和相同肋密度的二维及三维强化管外进行凝结换热试验研究。结果表明:R134a在光管外凝结表面传热系数与Nusselt数理论值的相对偏差均在±10%以内,R134a在光管及强化管外凝结表面传热系数变化趋势与Nusselt数理论解相一致。与纯R134a相比,含R125的混合工质管外凝结表面传热系数均所有下降;对于光管,含R125的混合工质管外凝结表面传热系数随壁面温差的增大而下降,但对于强化管,含6%及以上的R125混合工质,其凝结表面传热系数随壁面温差的增大而增大,有接近纯R134a凝结表面传热系数的趋势,表明混合工质凝结换热热阻分布与纯工质有较大差异。相同组分的工质,三维强化管凝结表面传热系数均高于二维强化管,二维强化管亦明显高于光管,在壁面温差为8K时,强化管HT-3D、HT-2D相对于光管的传热强化倍率分别为9.83和7.85。

Experimental studies of film condensation of R134aand mixed R134a/R125at three different concentrations have been conducted on three tubes,one is smooth tube,the others are two-dimensional and three-dimensional enhanced tubes at the same fin density.The results indicate that the predicted condensation heat transfer coefficients of R134aon smooth tube from Nusselt theory agree with the experimental data within ±10percent.The condensation heat transfer coefficients of pure R134aare consistent with Nusselt theoretical trend for smooth and enhanced tubes.Compared with pure R134a,the condensation heat transfer coefficients of mixed R134a/R125 are all decreased.For smooth tube, condensation heat transfer coefficient of mixed refrigerants decreased with the increase of temperature difference.But for enhanced tubes,condensation heat transfer coefficients increased with the increase of temperature difference,which is close to pure R134aat high temperature difference,the mixed refrigerant including 6 percent and more R125.Indicating that condensation heat transfer of mixed refrigerants is quite different with pure refrigerants.Condensation heat transfer coefficient of three-dimensional enhanced tube is higher than that of the two-dimensional tubes,and two-dimensional tube is significantly higher than that of smooth tube at the same working fluid.The condensation heat transfer enhancement factor of HT-3D, HT-2D are 9.83 and 7.85,respectively, which compared with smooth tube at the temperature difference 8K.