An experimental investigation of the limiting critical quality and its impact on CHF in horizontal and vertical tubes cooled with water and R-134a

An experimental study of the critical heat flux (CHF) and the associated limiting critical quality phenomenon has been conducted recently for horizontal and vertical tubes cooled with water and the refrigerant R-134a. The objective of this study is to compare the CHF and the associated limiting critical quality data obtained in horizontal and vertical tubes, in order to assess the effect of orientation on the CHF and the limiting critical quality. The investigated ranges of flow parameters in water (the R-134a equivalent values are given in brackets and have been transformed using the CHF fluid-to-fluid modeling relationships) are outlet pressure of 8 (1.31) 9 (1.48) 10 (1.67) 11(1.84) and 12 (2.03) MPa; mass fluxes from 500 (350) to 4300 (3000) kg/m2s; and critical qualities from -0.1 to +0.9. A comparison of the data for a horizontal tube with those for a vertical tube shows a strong orientation effect on CHF and limiting critical quality for mass fluxes below 4300 kg/m2s in water (or below 3000 kg/m2s in R-134a). The orientation effect decreases with increasing mass flux. In general, at similar flow conditions, the limiting critical quality values in a horizontal tube are lower than those in a vertical tube. This phenomenon is affected by the flow parameters and the thermophysical properties of the working fluid (i.e., mainly the densities of liquid and vapor). A drop in CHF within the limiting critical quality region in a horizontal tube can be less significant than that in a vertical tube. These differences seem to be mainly due to the effect of the buoyancy force. (author)