Numerical investigation on condensation heat transfer and pressure drop characteristics of R134a in horizontal flattened tubes

The condensation performance of R134a in horizontal round tubes and flattened tubes was numerically investigated. Three round tubes with different hydraulic diameters (d_h = 3.25, 4.57 and 6.5 mm) were deformed into flattened tubes with the aspect ratio (AR) equals to 2, 4 and 6, respectively. The liquid film thickness combined with the interface line was shown in detail to illustrate the effect of shear stress, surface tension and gravity. The heat transfer coefficients increased with the vapor quality and aspect ratio, and the enhancement was magnified at higher vapor quality and mass velocity. The smaller the original round tube diameter was, the higher the enhancement of thermal performance in flattened tubes was. The average film thickness of flattened tubes with AR = 2, 4, 6 was respectively 35%, 50% and 60% thinner than that of round tube. The relative role of surface tension and gravity determined the local thermal performance in flattened tubes. The main thermal resistance was located in the arc. The gravity effect was almost negligible in the straight part but still played an important role in the arc of flattened tubes. The heat exchanging process mainly occurred near the intersection of straight part and arc and this location tends to move toward the straight side for smaller flattened tubes because of the reinforcement of the surface tension effect. The pressure gradient also increased with the vapor quality, aspect ratio and mass velocity but decreased with the increasing hydraulic diameter. The existing correlations almost tend to underestimate both the heat transfer coefficients and pressure gradients. Two improved correlations of condensation heat transfer and pressure gradient in flattened tube were developed for practical applications.