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 over a wide pressure range from 1.10 MPa to 3.98 MPa, corresponding to reduced pressures between 0.27 and 0.98. The mass flux was varied between 300 kg/m²s and 2000 kg/m²s, while the local critical vapor quality ranged from highly subcooled conditions − 3.43 up to 0.69. The uniformly heated tube had an inner diameter of 10 mm and heated lengths of 1 m and 3 m. The extensive experimental matrix resulted in a total of 500 CHF data points. At the onset of the boiling crisis, the wall temperature increases rapidly due to the significantly lower heat transfer capability of the vapor phase compared to the liquid phase. Under high subcritical pressure conditions, specifically at reduced pressures of above 0.8, this temperature excursion became less pronounced, particularly under dryout conditions. At the same time, the critical vapor quality marking the onset of the boiling crisis shifted toward lower values and the CHF decreased significantly as the system pressure approached the critical pressure. ... mehrIn experiments performed at reduced pressures of 0.95 and 0.98, CHF was in some cases observed upstream of the outlet of the heated section, a phenomenon referred to as upstream-CHF. The experimental results show that the occurrence of upstream-CHF is strongly influenced by inlet subcooling and mass flux. Based on the measured wall temperature distributions, a possible explanation for this behavior is the onset of homogeneous nucleation at a local wall temperature maximum along the heated surface. The results provide a comprehensive CHF dataset for R-134a over a wide range of operating conditions close to the critical pressure and contribute to an improved understanding of CHF behavior and the upstream-CHF phenomenon under near-critical pressure conditions.