Numerical Study on R32 Flow Condensation in Horizontally Oriented Tubes with U-Bends

In this paper, the flow condensation heat transfer characteristics of R32 in a horizontally oriented tube with a horizontal U-bend (HUB tube) and a horizontally oriented tube with a vertical U-bend (VUB tube) were numerically investigated. The volume of fluid (VOF) multiphase model with the mass transfer assumption by Lee is adopted to simulate the flow condensation behavior of R32, which are validated by well-known empirical correlations. The influence of structural parameters, mass flux and vapor quality on the heat transfer coefficient (HTC) and liquid film distribution is investigated. Meanwhile, the local liquid film thickness (LLFT) and local heat transfer coefficient (LHTC) are also depicted. The phase transition at the U-bend section is captured. Results show that the U-bend has remarkable disturbance on the flow pattern and LHTC due to the effect of centrifugal force where the LLFT is changed, inducing strong secondary flow. The LHTC is increased by a maximum of 8.47% and 11.86% in VUB tube and HUB tube, respectively, when compared to the case in a horizontally straight oriented tube at the same operating conditions.