Direct Computational Simulations for Condensation Inside Tubes and Channels

The paper presents accurate numerical solutions of the full 2D governing equations for steady and unsteady laminar/laminar internal condensing flows of pure vapor (R-113 and FC-72) inside a vertical tube and a channel. The film condensation is on the inside wall of a tube or one of the walls of a channel (the lower wall in case of a downward sloping channel). The new geometry in this paper is the cylindrical in-tube geometry with axisymmetric flows (vertical 1g or 0g flows). The new results encompass both the cylindrical and the earlier studied channel geometry. Exit condition specifications are again found to be important. The computations are able to predict whether or not a steady flow exists under a natural exit condition (selected from a range of choices available at the exit). If natural steady/quasi-steady flows exist — as is shown to be the case for gravity dominated or strong shear dominated condensate flows — the computations are able to predict both the natural exit condition and the associated condensate flow’s point of transition from stable to unstable behavior. Compared to gravity driven, shear driven cases (zero gravity or horizontal cases) tend to destabilize easier and generally have much larger pressure drops, much slower wave speeds, much larger role of surface tension, and much narrower flow regime boundaries within which the vapor flow can be modelled incompressible. It is found that only in gravity driven cases, be it vertical in-tube or inclined channel geometry, interfacial waves are able to cause a concurrent enhancement in heat transfer rates along with an enhancement in interfacial shear. Also it is found that this enhancement is significant if the condensing surface noise is in resonance with the intrinsic waves.