Heat and mass transfer modeling of R-245fa and R1233zd(E) with concurrent boiling and convective evaporation in falling film applications

Abstract

Abstract Design of some thermal industrial equipment like Heating Ventilation and Air Conditioning falling film evaporators and desalination units requires knowing discrete portion of convective evaporation and boiling. While most models are based on sole dominant evaporation, one small variation in design parameters can increase boiling part and threaten the model. Heat transfer simulation of such equipment is dependent on mass transfer modeling. Therefore, this study uses different mass transfer models and numerically investigates two laminar falling film flows around a horizontal tube heating insomuch that leads to sole evaporation and evaporation with boiling. The multi-phase volume of fluid model is used and the performance of mass transfer models is investigated for heat transfer coefficient, film thickness, bubble generation and liquid superheating. Results show that for 240 mL*min−1 of R-245fa at saturation pressure of 123.8 kPa at wall and saturation temperature difference of below 7 K, main contribution of heat transfer is due to evaporation which is accurately modeled by Tanasawa and Lee phase change models. With increasing wall heat flux, boiling portion signifies. Accordingly, Tanasawa model is not able to simulate boiling part but Lee model predicts total heat transfer coefficient with 7% error. After validating the numerical model, the model has been used for R1233zd(E) as a low Global Warming Potential refrigerant and transition to boiling presence of the falling flow is investigated. Comparison of the results represents governing evaporation up to 7 K of wall and saturation temperature difference of R1233zd(E) at similar saturation temperature of R-245fa.