Boiling experiments under steady state and controlled transient conditions

Systematic experiments are presented to study the pool boiling heat transfer characteristics under steady-state and transient conditions. A specific control concept enables transient experiments with constant heater temperature change from the steady state limit up to 50 K/s along the entire boiling curve. Throughout all phases of this joint project, the requirements on the experimental infrastructure for reproducible boiling experiments is adressed by the integration of theoretical (RWTH-Aachen) and experimental (TU-Berlin) investigations. In the early phase, these are concerned with heater design as well as the development and optimization of the control concept. The experiments are carried out with FC-72 boiling on top of a horizontally positioned copper heater with a diameter of 18.2 mm and 5 mm thickness. By solving a one-dimensional inverse heat conduction problem the temperature measurements by the sheathed thermocouples are used to obtain transient boiling curves. In all boiling regimes the heat flux increases with increasing wall temperature slopes, e.g. CHF for a 50 K/s heating transient is about 400% higher than in the steady-state case. Transient cooling leads to lower transferred heat fluxes. Very close to the boiling surface, temperature fluctuations as a result of evaporation and convective boiling phenomena are measured by six microthermocouples. Furthermore an optical glass-fiber probe of 10 #mu#m tip diameter is used to measure void fraction and vapor contact frequency in the steady-state case very close to the heater surface. (orig.)Available from TIB Hannover: RR 8872(2000,22) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman