Experimental study of R600a/oil/MWCNT nano-refrigerant condensing flow inside micro-fin tubes

The present experimental study aims to investigate condensation heat transfer characteristics of R600a/oil/multi-walled carbon nanotube (MWCNT) nano-refrigerant flow inside a horizontal micro-fin tube. The test condenser is a horizontal double-pipe heat exchanger consists of a copper micro-fin tube with 14.18 mm inner diameter which is installed in a polyethylene shell with a diameter of 6 cm. The experiments cover a wide range of parameters, including mass velocity of 54 to 90 kg/m(2) s, vapor quality of 0.03 to 0.76, and condensation pressure of 550 to 700 kPa. The investigations are conducted in three parts, including the characteristics of pure R600a condensing flow inside micro-fin tubes, the effect of adding lubricating oil with concentration of 1%, and the impact of adding nano-oil with nanoparticles compactness of 0.1, 0.2, and 0.3%. Comparison of the experimental data for the pure refrigerant flow with three well-known correlations shows the maximum deviation of +/- 20% which validates the test procedure. Based on the results, R600a/oil heat transfer coefficient is higher than pure R600a in low and medium vapor qualities, while at high qualities the pure R600a shows higher coefficient due to the impact of lubricating oil on the condensate liquid film thermal resistance. In addition, adding nanoparticles is more effective at low and medium vapor qualities and high mass velocity. The results showed that using MWCNT nanoparticles with the compactness of 0.3% increases the heat transfer coefficient up to 74.8% in comparison to the pure refrigerant.