Two-phase frictional pressure drops in U-bends and contiguous straight tubes for different refrigerants, orientations, tube, and bend diameters: Part 1. Experimental results (RP-1444)

Abstract

Over the years, much research has been conducted on two-phase flows in straight tubes because these tend, in length, to be predominant in two-phase systems. However, two-phase heat exchangers can also have singularity elements, where the most common type is typically a U-bend. The importance of these elements on the design of a system is that their frictional pressure gradients are much higher than those occurring in straight tubes. Therefore, in this study, pressure drop measurements were made for U-bends and their contiguous straight tubes with two-phase refrigerants. The experiments were made with R410A and R134a at two saturation temperatures (5°C and 10°C [41°F and 50°F]) flowing at three mass fluxes (150, 300, and 500 kg s−1m−2 [110.6·103, 221.2·103, and 368.7·103 lb h−1 ft−2]) inside five different test sections with three different internal diameters (13.4, 10.7, and 7.8 mm [0.527, 0.421, and 0.307 in.]) and five different bend diameters (66.1, 54.8, 38.1, 31.7, and 24.8 mm [2.602, 2.157, 1.5, 1.248, and 0.976 in.]) in three different orientations (horizontal, vertical upflow, and vertical downflow) with vapor qualities ranging from 0.05 to 0.95. The first part of this article focuses on the experimental pressure drop results, while the second part presents a new flow pattern based frictional pressure drop model for U-bends and an updated version of the straight tube frictional pressure drop model of Moreno Quibén and Thome (2007b).