Study of dividing two-phase flow in a vertical rectangular mini-channel with a horizontal branch

Abstract

Recently, the flow and pressure distributions at the junctions between the header to the parallel channels are becoming of interest in predicting heat transfer performance of compact heat exchangers. In many cases, flow rates through the channels are not uniform and occasionally there is no flow through some of them. Especially with a two-phase flow, the situation becomes even more complicated since the phase separation occurs at the dividing junctions. The header-channels configuration of the compact heat exchangers can be simulated as an accumulation of T-junctions with rectangular cross sections. Therefore, to provide a basic information on the two-phase flow distribution and the pressure drop at header-channel junctions, a series of experiments were conducted with a small, vertical dividing T-junctions with rectangular cross sections. Three different aspect ratios (1, 0.5, 0.125) for the branch were tested. For small T-junctions, the channel orientation is insignificant due to the predominance of the surface tension effect over the gravitational effect. The smaller fraction of the liquid flows through the branch with the smaller aspect ratio of the branch (i.e., with the smaller branch height). Shoam et al.’s model was modified to represent the separation of the liquid fraction for the present size range ; the proposed model represents the experimental data within a deviation of ±20%. At the same time, the pressure distribution at the T-junction was experimented. The pressure rises between the inlet and the run, while drops between the inlet and the branch. As the branch aspect ratio decreases, the pressure drop between the inlet and the branch increases, while the pressure rise between the main and the run almost remains unchanged. Four different pressure-drop models were tested with the measured data ; among them, the separated flow model proposed by Fouda and Rhodes (1974) turned out to be the best-estimate correlation.