Studies on critical heat flux in flow boiling at near critical pressures

Experimental studies on critical heat flux (CHF) have been conducted in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length at different reduced pressures ranging from 0.24 to 0.99 with R-134a as the working fluid. The onset of CHF was determined by the sudden rise in the wall temperature of the electrically heated tube. Experiments were performed over a wide range of parameters: mass flux values from 200 to 2000 kg/m2 s, pressure from 10 to 39.7 bars and heat flux from 2 to 80 kW/m2 and exit quality from 0.17 to 0.94. The results show considerably lower critical heat flux at high pressures. Well known CHF prediction methods, such as the look-up table and correlations of earlier workers show poor agreement at high pressures. A new correlation has been proposed to estimate the CHF in uniformly heated vertical tubes up to the critical pressure and over a wide range of parameters.