CFD simulation on hydrodynamic and thermal behavior of elliptical condensing tubes with an improved mass transfer model

Abstract

Comprehensive two-phase CFD simulations were conducted to investigate the hydrodynamic and thermal performance of the elliptical condensing tubes. An improved mass transfer model was developed to account for the condensation phase change. The model was validated to be in good agreement with the analytical solution as well as the experimental data. The transient drop-off behavior of elliptical tubes was first reported. The drop frequency increased with the eccentricity, except for the case of E-0.8, which configuration showed a unique geometric shape whose frequency was 10% lower than E-0.6, therefore it should be avoided in condenser design. The surface tension affected the local heat transfer more in the bottom flooding regions, but had little effect on the average heat transfer. Both external and internal heat transfer were evaluated for overall thermal performance, suggesting that the elliptical tube with an eccentricity beyond 0.92 is not preferred to be adopted unless the pump power of the internal flow is not a concern. Although increasing eccentricity allows the tube to achieve higher condensation heat transfer, it is still a restrictive factor for the overall performance. Therefore, future work will consider additional improvements such as adding low-fins or 3-D roughness to the smooth elliptical tube.