Experimental Investigation of Post-Dryout Heat Transfer with R-134a at High Pressures

An experimental study of post-dryout (PDO) heat transfer in the coolant R-134a was performed in a vertical round tube with upward flow. Experiments were conducted at high pressures from 28.4 bars up to 39.8 bars, corresponding to a reduced pressure of 0.7 to 0.98, respectively. Mass flux was varied in the interval of 300 to 1500 kg/m²∙s, and overall equilibrium vapor quality was between −1.88 and 4.89. Depending on the settings of the experimental parameters, the heat flux was varied from around 11 to 100 kW/m². The uniformly heated tube had an inside diameter of 10 mm and a heated length of 3000 mm.

In total, more than 10 000 PDO data points were obtained. In the PDO region, the wall temperature distributions had similar behavior across the pressure range. At the occurrence of dryout, the wall temperature suddenly increased until it reached a peak. For higher mass flux, the wall temperature decreased after reaching the peak, followed by a second temperature increase with a lower slope. For lower mass flux, the wall temperature kept increasing after the dryout point. The temperature peak after the dryout point was smaller at higher pressure, while this effect was even stronger near the critical pressure. Likewise, the vapor quality corresponding to the first peak shifted to even lower values with increasing pressure. Furthermore, it was found that at increasing pressure and at increasing mass flux, the dryout location and the total temperature distribution shifted toward lower vapor qualities. In addition, several PDO correlations were assessed, and a new correlation for high-pressure conditions was developed and compared with the results of the existing ones.