Annular flow simulations of refrigerants flowing in a small diameter channel

Annular flow is commonly encountered in many components, such as condensers and evaporators in the HVAC sector. The increasing adoption of small channels further emphasizes the need to comprehend the dynamics of this phenomenon, which is strictly related to the heat transfer performance of the heat exchangers. Berto et al. [1] experimentally measured the liquid film thickness of R245fa and R134a in a 3.4 mm circular channel during film condensation. They observed that the heat transfer coefficient is correlated to the liquid film thickness, but a clear relationship between these parameters is not well established yet. CFD simulations can represent a valid tool to address this issue. In the present work, a novel numerical framework for investigating annular flow in small channels is presented. Simulations were performed with the open-source code OpenFOAM, using an in-house developed transient solver based on the Volume of Fluid (VoF) solver interIsoFoam. The numerical setup used for the annular flow simulations is reported to Fig.1a). The domain is 2D axisymmetric where liquid and vapor are supplied at the inlet with a guess value of the interface position. The inlet velocity of each phase is then estimated based on the reference mass flow rate and vapor quality. The results showed that with the advancement in the longitudinal direction, the flow field and waves characteristics develop, and that the main statistical indicators of the liquid film are independent from the guess position of the interface at the inlet. An adaptive mesh refinement (AMR) model has been added to the solver to better capture the interface position and save computational time. Fig. 1b) reports a picture when highlighting the AMR that is activated during the passage of a liquid wave. The simulations were realized considering the geometry presented in [1] and using R245fa and R134a at mass flux between 50 and 150 kg m-2s-1 for various vapor qualities at 40 and 30 °C saturation temperature respectively. Turbulence modelling (k-ω SST) was included to the solver, with the addition of the source term related to the turbulence damping at the interface proposed by Fan and Anglart [2]. The so-called B parameter of the damping model was found to remarkably influence the statistical behavior of the liquid film thickness. The effect of B was shown for all the tested operative conditions and fluids, and a new method to set its values based on the liquid Reynold number is proposed.