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 st udied. The working fluid, R-12, flows vertically upward inside the tes t 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. Temper ature fluctuations were measured in the tube wall in the transition bo iling region under both hydrodynamically stable and unstable condition s. Without exit restriction, the system was hydrodynamically stable, a nd 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 fluct uations with greater magnitudes and much larger periods were observed. Wall temperature fluctuations of this type are termed thermal oscilla tion. During thermal oscillation the hydrodynamic instabilities were a lso measured. The system pressure oscillations were measured at the in let and the void fraction oscillations were measured at the exit with a capacitance void meter. The mechanism of thermal oscillation is give n, and the effects of heater wall capacitance and axial conduction on the processes of transition boiling and thermal oscillations are studi ed. Typical graphs of instantaneous heat flux to the fluid versus time as well as typical recordings of wall temperature oscillations are pr esented. The instantaneous heat flux to the fluid versus the wall temp erature is also plotted, and a limit cycle is produced. The critical h eat flux (CHF) values were determined under both hydrodynamically stab le 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 substantiall y lower CHF values. Thermal oscillation has been observed over the fol lowing 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.