CHARACTERISTICS OF TRANSITION BOILING AND THERMAL OSCILLATION IN AN UPFLOW CONVECTIVE BOILING SYSTEM

The characteristics of transition boiling and thermal oscillation in a single-channel forced-convection upflow system were experimentally studied. The working fluid, R-12, flows vertically upward inside the test channel of inside diameter 2.85 cm and outside diameter 3.37 cm. The test section is 339 cm long and is heated by the Joule effect. Temperature fluctuations were measured in the tube wall in the transition boiling region under both hydrodynamically stable and unstable conditions. Without exit restriction, the system was hydrodynamically stable, and the usual irregular wall temperature fluctuation in the transition boiling region was observed; with an exit restriction, the system was found to be hydrodynamically unstable, and sustained temperature fluctuations with greater magnitudes and much larger periods were observed. Wall temperature fluctuations of this type are termed thermal oscillation. During thermal oscillation the hydrodynamic instabilities were also measured. The system pressure oscillations were measured at the inlet and the void fraction oscillations were measured at the exit with a capacitance void meter. The mechanism of thermal oscillation is given, and the effects of heater wall capacitance and axial conduction on the processes of transition boiling and thermal oscillations are studied. Typical graphs of instantaneous heat flux to the fluid versus time as well as typical recordings of wall temperature oscillations are presented. The instantaneous heat flux to the fluid versus the wall temperature is also plotted, and a limit cycle is produced. The critical heat flux (CHF) values were determined under both hydrodynamically stable and unstable conditions and are compared on maps of CHF versus exit quality at different mass fluxes with constant system pressure. This study shows that the hydrodynamically unstable system has substantially lower CHF values. Thermal oscillation has been observed over the following parameter ranges: mass flux, 300-3000 kg/(m2 s); heat flux, 25-80 kW/m2; inlet quality, -0.2 to +0.6; system pressure, 10 bar.