Evaporation heat transfer and pressure drop of refrigerant R-134a in a small pipe

An experiment was carried out to investigate the characteristics of the evaporation heat transfer and pressure drop for refrigerant R-134a flowing in a horizontal small circular pipe having an inside diameter of 2.0 mm. The data are useful in designing more compact and effective evaporators for various refrigeration and air conditioning systems. The effects of the imposed wall heat flux, mass flux, vapor quality and saturation temperature of R-134a on the measured evaporation heat transfer and pressure drop were examined in detail. When compared with the data for larger pipes (D_i ≥ 8.0 mm) reported in the literature, the evaporation heat transfer coefficient for the small pipe considered here is about 30–80% higher for most situations. Moreover, we noted that in the small pipe the evaporation heat transfer coefficient is higher at a higher imposed wall heat flux except in the high vapor quality region, at a higher saturation temperature, and at a higher mass flux when the imposed heat flux is low. In addition, the measured pressure drop is higher for increases in the mass flux and imposed wall heat flux. Based on the present data, empirical correlations were proposed for the evaporation heat transfer coefficients and friction factors.