Experimental Investigation on Critical Heat Flux and Upstream-CHF with R-134a at high Pressures

An experimental study of the critical heat flux (CHF) in R-134a was performed in a vertical round tube with upward flow. The experiments were conducted in a wide pressure range from 1.10 MPa up to 3.98 MPa, corresponding to a reduced pressure of 0.27 to 0.98, respectively. The mass flux was varied between 300 kg/m²s and 2000 kg/m²s, while the local critical vapor quality ranged from − 3.43 to 0.69. The uniformly heated tube had an inner diameter of 10 mm and a heated length of 3000 mm and 1000 mm. The extensive experimental matrix resulted in a total of 500 CHF data points. At the onset of the boiling crisis, the wall temperature suddenly rises due to the poor heat transfer capability of the vapor phase compared to liquid. The temperature jump was less dramatic in the high subcritical pressure range, especially under dryout conditions. But the critical vapor quality, which marks the beginning of the boiling crisis, shifted to lower values and the CHF value decreased significantly with increasing pressure, especially near the critical pressure. During the experiments at reduced pressures of 0.95 and 0.98, the so-called upstream-CHF as well as the influence of the inlet subcooling and mass flux on its occurrence could be investigated. A possible explanation for the upstream-CHF phenomenon is the onset of homogeneous nucleation at a local wall temperature maximum, where the upstream-CHF point is located.