ABSTRACT

Critical heat flux condition of forced convection boiling inside an uniformly heated tube usually takes place at the tube exit end but it is sometimes detected at an upstream location under high mass velocity at high system pressure. To make clear the characteristics of this anomalous CHF phenomenon, systematic experiments were conducted by employing Freon-115 as a test fluid for wide experimental ranges of system pressure, mass velocity, heated tube length and inlet subcooling, using three kinds of test tube which have the same inner diameter but different inner surface roughness. Spatial and temporal wall temperature excursion at and post CHF was investigated in detail to detect correctly the upstream CHF. From the experiments, the followings are found: (l)upstream CHF is an autonomous phenomenon; (2)upstream CHF condition is likely to occur in a rough tube; (3)upstream CHF is likely to be detected at low inlet subcooling at high pressure under high mass velocity; (4)upstream CHF is higher than regular downstream CHF for the same inlet subcooling. CHF data are correlated as a function of local quality χc and it is shown that the data of upstream CHF branch from an unique CHF-χc relation.