Simultaneous Effects of Flattening Tube and Adding Nanoparticles on Boiling Heat Transfer

Abstract

An empirical investigation is performed on boiling heat transfer characteristics of R600a refrigerant flow inside horizontal flattened tubes. Round copper tubes of 8.7 mm inner diameter are deformed into flattened shapes with different internal heights of 6.9, 5.5, and 3.4 mm as the test sections. Effects of different parameters such as mass flux, vapor quality, and internal tube height on the heat transfer coefficient are studied. It is shown that flattening the tube causes significant heat transfer enhancement. The maximum augmentation ratio of 163% is obtained for the flattened tube with the maximum aspect ratio of 3.56. A new correlation is developed based on the obtained experimental data to predict the heat transfer coefficient in flattened tubes; 90% of the acquired data are predicted within. Consequently, the tested flattened tube with maximum heat transfer rate is selected to study the effects of nanoparticles. Experiments are conducted for three different working fluid types including 1) pure refrigerant (R600a); 2) refrigerant/lubricant (R600a/oil); and 3) nanorefrigerant: refrigerant/lubricant/nanoparticles (R600a/oil/CuO). Polyolester oil is used as the lubricant in the two latter cases. In addition, nanorefrigerants (R600a/oil/CuO) are prepared by dispersing CuO nanoparticles with different mass fractions of 0.5, 1, and 1.5% in the baseline mixture (R600a/oil). Thereby, maximum heat transfer enhancement of 79% is achieved compared to the pure refrigerant case. The results of this study provide the platform for designing compact efficient heat exchangers/heat sinks for the emerging thermal engineering applications under high thermal load conditions.