Flow boiling near the critical heat flux

An experimental investigation of the flow boiling of water at atmospheric pressure was undertaken, including a high—speed cine photographic study of the flow structure near the Critical Heat Flux (CHF). Experimental tests from single-phase forced convection to burnout were conducted at different flow velocities and inlet subcoolings for water flowing upwards through a vertical channel of rectangular cross—section electrically heated on one wall with a glass window forming the opposite wall. The test surfaces were stainless steel strips of constant dimensions, except that wall thickness ranged from 0.08 mm to 0.20 mm. Quantitative measurements of the bubble parameters for the same heating surface under the same operating conditions with varying levels of heat flux (70p to 95p of CHF) were carried out. A nucleation site deactivation/reactivation process was observed with increasing heat flux. A proposed site deactivation mechanism explained this behaviour. A nucleate boiling heat transfer model was proposed for the fully— developed nucleate boiling region, with allowance made for the overlapping areas of bubble influence. It compared favourably with the experimental data. The effect of wall thickness on CHF was investigated: increases in CHF as between the 0.08 mm and the 0.20 mm wall thickness ranging from 38p to 57p were observed. An empirical expression for CHF, including wall thickness as a parameter was developed, correlating the experimental data to within 15p and indicating a limiting value for wall thickness affecting CHF. The flow regimes near burnout were identified as bubbly and slug, these being independent of wall thickness. Other models proposed for the CHF mechanism were tested against the detailed experimental observations at high subcoolings. They were found to be inconsistent with the experimental evidence. A possible alternative for the CHF mechanism points towards stabilisation/ growth of a vapour patch following bubble coalescence as a most likely cause for burnout.