Comparison of CHF measurements in R-134a cooled tubes and the water CHF look-up table

An exhaustive experimental study of the critical heat flux (CHF) in R-134a-cooled tubes has been recently completed. The objective of this study was to provide a consistent set of CHF data to be used as a reference for future separate effect studies. The investigated range of flow parameters in R-134a was: outlet pressure 0.96–2.39 MPa (equivalent to 6–14 MPa in water), mass flux 500–3000 kg m−2 s−1 (equivalent to 700–4300 kg m−2 s−1 in water), and critical quality −0.1 to +0.9. To extend the range of critical qualities, different heated lengths (0.45–2 m) and two-phase flow at inlet to the test section were used. Scaling laws were applied to convert the standard CHF look-up table from water to R-134a equivalent conditions. The comparison of the data with the water-based look-up table showed that for high mass fluxes (G = 1400–4300 kg m−2 s−1 in water equivalent), the look-up table provides an excellent CHF prediction for R-134a-cooled tubes. The differences were mainly noticeable for the limiting critical quality range and for low mass flux. A discussion of the observed limiting critical quality phenomenon is also included.