Influence of two-phase flow characteristics on critical heat flux in low pressure

In the present study, the critical heat flux (CHF) was measured over wide quality range from subcooled boiling to annular-mist flow at the atmospheric pressure. At a low pressure around the atmospheric pressure, the two-phase flow pattern and void fraction changes easily along the heated length and the flow becomes more complex due to the low density of gas phase. By using Pyrex glass tube as a test channel installed with a heater rod at the centerline. the two-phase flow situation was observed visually. The flow pattern was kept nearly constant over the length of test section due to the low power input into the fluid. Therefore, the characteristics of CHF could be investigated at each flow patterns of bubbly, slug, annular and annular-mist flow. In the subcooled boiling region of bubbly how, the CHF decreased with increase of quality and was less sensitive to flow rate. In the slug flow region, the CHF showed a minimum value. With more increase of quality in the annular flow, the CHF increased and reached a peak value at a certain quality depending on flow rate. The peak of CHF occurred almost at a constant vapor mass velocity. In the annular-mist flow region, the CHF decreased with increase of quality. In the region, the effect of heated length on the CHF was systematically measured and validity of an analytical model of CHF considering dryout of liquid was investigated. (C) 1999 Elsevier Science Inc. All rights reserved.