ICONE11-36549 AN EXPERIMENTAL INVESTIGATION OF CHF ENHANCEMENT IN HORIZONTAL AND VERTICAL TUBES COOLED WITH WATER AND R-134A

Abstract

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/m^2s, 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.