Experimental investigation of heat transfer during flow condensation of HC-R600a based nano-refrigerant inside a horizontal U-shaped tube

This experimental study investigates the effect of MWCNT (multi-walled carbon nanotube) nanoparticles on condensation heat transfer characteristics of R600a/oil mixture inside a horizontal U-shaped tube. The experiments are organized in three parts: (i) pure refrigerant flow, (ii) refrigerant-oil mixture flow with oil mass fraction of 1%, and (iii) R600a/oil/MWCNT nano-refrigerant flow with nanoparticle mass fractions of 0.1, 0.2, and 0.3%. The condenser consists of two 135 cm straight sections and a U-bend with the inner diameter of 8.7 mm and the curve ratio of 21 installed in a 6 cm polyethylene shell. The tests are carried out in a wide range of operating conditions, including condensation pressure of 510-630 kPa, mass velocity of 140, 187, 233, and 280 kg/m(2)s, and vapor quality of 0.04-0.75. The results show that at low vapor quality, adding lubricating oil into the refrigerant improves the heat transfer coefficient of the mixture in comparison to the pure refrigerant, while this trend is reversed at medium and high vapor quality due to the lubricating oil impact on the condensation liquid film. In addition, the nano-refrigerant flow heat transfer coefficient increases with mass velocity while there is a critical vapor quality in which the local coefficient becomes maximum. Based on the results, using 0.3% of MWCNT causes maximum local heat transfer enhancement up to 68.2%, in comparison to the pure refrigerant. Heat transfer enhancement by adding nanoparticles is more effective at low and intermediate vapor quality.