In this study, we have experimentally measured the heat transfer coefficient and pressure drop for the R-134a and R-407C evaporation in the small circular tubes (Di=0.83 & 2.0 mm). The influences of the refrigerant mass flux, heat flux, saturated temperature and vapor quality on the evaporation heat transfer and frictional pressure drop have been investigated in detail. The major results obtained here can be briefly summarized as follows.
(1). Both the R-134a and R-407C evaporation heat transfer coefficients and frictional pressure drops increase significantly with the mass flux and vapor quality except at low mass fluxes. A rise in the imposed heat flux results in a significant increase in the evaporation heat transfer coefficient for both R-134a and R-407C except at low heat fluxes. Nevertheless the influence of the imposed heat flux on the frictional pressure drop is rather slight. The increase in the saturated temperature of R-134a & R-407C results in a smaller increase in the evaporation heat transfer coefficient when compared that due to a rise in the mass flux. But the trend in the frictional pressure drop is opposite. The decrease in the R-134a & R-407C saturated temperature results in a noticeable increase in the frictional pressure drop. It is worth to note that for R-134a evaporation in the smaller tubes with Di=0.83 mm partial dryout of the refrigerant can occur at high vapor quality, causing the evaporation heat transfer coefficient to decline at increasing quality. We also note that R-407C has higher latent heat of vaporization, higher thermal conductivity and lower viscosity than R-134a. Thus the evaporation heat transfer coefficient for R-407C is somewhat higher than that for R-134a. In the mean time the R-407C frictional pressure drop is
significantly lower.
(2). For the refrigerant vapor quality ranging from 0.2 to 0.8 the void fraction in the tube is high and heat transfer in the refrigerant flow is considered to be dominated by the vaporization of thin liquid film at the vapor-liquid interface. Therefore the refrigerant mass flux, saturated temperature, vapor quality and imposed heat flux all exhibit pronounced effects on the evaporation heat transfer coefficient in both the 2.0-mm and 0.83-mm tubes.
(3). Empirical correlations for the heat transfer coefficient and friction factor for the R-134a and R-407C evaporation in 0.83-mm and 2.0-mm small tubes were provided.
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