An experimental investigation of CHF enhancement in horizontal and vertical tubes cooled with water and R-134A

An experimental study of the effect of flow obstacles on critical heat flux (CHF) has been conducted recently in horizontal and vertical tubes cooled with water and the refrigerant R-134a. The objective of this study is to compare the CHF experimental data obtained in horizontal and vertical tubes (with upward flow) cooled with water and R-134a, in order to assess the effects of orientation and fluid type on CHF enhancement. The investigated ranges of flow parameters in water (the R-134a equivalent values are given in brackets and have been transformed using CHF fluid-to-fluid modeling relationships) are an outlet pressure of 10 (1.67) MPa, mass fluxes from 1000 (705) to 6515 (4605) kg/m2s, and critical qualities from -0.1 to +0.4. A comparison of the CHF enhancement data for a horizontal tube with those for a vertical tube shows a strong orientation effect on CHF enhancement for R-134a within the investigated range. In general, at equivalent flow conditions, CHF enhancement in water and R-134a flowing in vertical tubes is about the same at high mass fluxes. However, at lower mass fluxes, CHF enhancement in water is lower than that in R-134a. On the other hand, in horizontal tubes and at equivalent flow conditions, CHF enhancement in water is noticeably higher than that in R-134a at high mass fluxes. However, at lower mass fluxes, this trend changes to the opposite (i.e., at higher positive qualities, CHF enhancement in R-134a can be higher than that in water). Also, the limiting critical quality region can be observed in a R-134a-cooled horizontal CHF-enhanced tube, while it is not observed in the enhanced tube cooled by water. (author)