An experimental investigation of rewetting phenomena at low flow rates using round tubes and pressurised freon-12

Abstract

Experiments have been performed to determine heat transfer characteristics occurring during the transition from post-dryout to rewetted wall conditions. The experimental technique employed was one in which the heating power to a high thermal capacity test section at the exit of a long stainless steel pre-heater tube was rapidly reduced. Three copper test sections with internal diameters of 8.5, 16.6 and 21.3 mm were investigated. Limited ranges of coolant pressures, qualities (i.e. mass fractions) and mass fluxes were investigated for each tube diameter. Analysis of experimental data was performed by matching observed thermal responses with ones predicted by a two-dimensional finite element transient heat transfer code. Comparison of the results with those from previous experimental investigations of the dryout process indicates that the post-dryout heat transfer characteristics of the dryout and rewetting processes are significantly different at low flow rates. For test conditions in which the inlet quality to the high thermal capacity test section was high, thermal responses and associated surface heat fluxes were found to be complexly dependent on quality and mass flow rate, whether the wetted front was near or well upstream of the test section. At low inlet qualities to the test section, however, the influence of quality and mass flow rate on thermal characteristics appeared to be less significant. Pressure effects were found to be small.