Experimental study on CHF of R134a flow boiling in a horizontal helically-coiled tube near the critical pressure

Critical heat flux (CHF) experiments were conducted in a uniformly heated horizontal helically-coiled tube to investigate the CHF characteristics at near the critical pressures with R134a as the working fluid. The experiments were performed over a wide range of parameters: pressure from 2.85 to 4.03 MPa, i.e., reduced pressures (Pr) ranging from 0.70 to 0.99, mass flux from 250 to 2100 kg m−2 s−1, inlet subcooling from 45 to 160 kJ kg−1 and heat flux from 20 to 450 kW m−2. Two types of CHF phenomenon which are dry-out (DO) and departure from nucleate boiling (DNB) have occurred under the experimental conditions. The DO happens at the lower pressures while the DNB happens at the higher pressures. The wall temperature shows a very different characteristic for the two types of CHF. Under the DO conditions, wall temperature gets a sudden rise firstly at the inner-side of the outlet cross-section and temperature at the other side remains on a lower level, while under the DNB conditions, wall temperature around the outlet cross-section jumps almost simultaneously. When the pressure is very close to the critical point, there is a constant pressure region where the wall temperature rises gently and the CHF no longer exists. The effects of the mass flux and inlet subcooling on CHF have also been discussed. Based on the experiment, a correlation applied for the pressure region close to the critical pressure was proposed to estimate the CHF in the horizontal helically-coiled tube.