Numerical investigation of the pool film boiling of water and R134a over a horizontal surface at near-critical conditions

Saturated pool film boiling over a flat horizontal surface is investigated numerically for water and refrigerant R134a at near-critical conditions for wall superheats (Delta T-Sup) of 2 K, 5 K, 8 K, 10 K, 15 K, and 20 K. The flow is considered to be laminar and incompressible. The governing equations are solved using a finite volume method with a collocated grid arrangement. For capturing the interface in two-phase boiling flows, a Coupled Level Set and Volume of Fluid (CLSVOF) with a multidirectional advection algorithm is used. Both single-mode and multimode boiling models are used for the numerical investigation to understand the effect of computational domain sizes on flow and heat transfer characteristics. In the case of water, the evolution of interface morphology shows the formation of a discrete periodic bubble release cycle occurring at lower Jacob numbers, Ja(v) <= 10.2 (Delta T-Sup <= 8 K), and the generation of jets of stable vapor film columns occurs at higher Ja(v) >= 12.7 (Delta T-Sup >= 10 K). In the case of R134a, for all the Ja(v) values considered in this study (0.163 <= Ja(v) <= 1.63), the formation of a discrete periodic bubble release is observed. The results show that multimode boiling model should be used to understand the flow characteristics better. The magnitude of average Nusselt number obtained from the multimode film boiling model is lower than that of the single-mode film boiling model. The Nusselt numbers obtained from the present numerical studies are also compared with the available semiempirical correlations.