REFLUX CONDENSATION AND OPERATING LIMITS OF THE TWO-PHASE CLOSED THERMOSYPHON (WAVINESS, NON-CONDENSABLES, FLOODING)

The performance characteristics of the two-phase closed thermosyphon are investigated experimentally and analytically by considering countercurrent vapor-liquid flows in a closed tube. The system is basically a gravity-assisted wickless heat pipe. The present work focuses on two important characteristics: the reflux condensation flow phenomena and the operating limits of the system. First, reflux condensation is studied. The condensation heat transfer coefficients along the condenser wall are locally measured. The results indicate that Nusselt's solution for film condensation cannot interpret satisfactorily the observed trend. Further improvements are made to consider the effects of interfacial shear, waviness and non-condensable gas on condensation. The vapor shear retards the condensate flow and thus increases the film thickness, which results in lower heat transfer coefficients than those calculated from Nusselt theory. Modified Fanning friction factors which account for the augmentation of interfacial shear through phase change are used to evaluate the reduction of heat transfer by vapor shear. On the other hand, the waves appearing on the interface can enhance heat transfer rates. Such enhancement is determined by solving numerically the nonlinear equation for the wavy interface. When non-condensable gases are present in the system, they will accumulate at the condenser end forming a gas barrier to the vapor and shut off that portion. A two-dimensional model is developed to include both axial and radial diffusion of gas mass. This two-dimensional analysis indicated the inadequacy of the common one-dimensional diffuse-front model in considering only axial diffusion of gas in most physical systems. The second part of this work concerns the operating limits of the closed thermosyphon. A series of experiments were made on the dry-out of the tube surface. Interesting oscillation patterns of distinctive physical characteristics were observed. Various characteristic regimes for operating limits are illustrated in a regime map.