Boiling flow of R141b in vertical and inclined Serpentine Tubes

A series of visual experiments are conducted to study the two-phase flow of refrigerant R141b in vertical and declined serpentine tubes of inner diameter 6 mm at inlet velocities of 0.1–0.2 m s−1 and wall heat fluxes of 4687–22064 W m−2. Under certain conditions, stagnant vapor slugs of length longer than the straight section are formed in the descent sections. A VOF (volume of fluid) model with phase change is developed to simulate the flow and heat transfer using the commercial CFD software Fluent. The simulated flow patterns agree well with the experimental observations. The formation of a stagnant slug is discussed based on the simulation results. It is found that a liquid film is formed on one side of the stagnant slug in the vertical layout, constituting a smooth flow path for each phase; while the film is twisted in the inclined layout because of intensive interactions between the two phases, causing a larger secondary flow and more efficient heat transfer. Flow in the descent straight sections turns more difficult in the vertical layout due to a stronger resistance of buoyancy imposed by the stagnant slugs, resulting in a larger pressure drop relative to that in the inclined layout.