Flow pattern identification for internally threaded tubes based on pressure drop fluctuations

Abstract

Internally threaded tubes, renowned for their exceptional heat transfer capabilities, are extensively used in heat exchangers. However, directly identifying two-phase flow patterns within internally threaded tubes poses a formidable challenge. To address this, this study conducted two-phase flow experiments with R600a within an internally threaded tube, encompassing a range of mass fluxes spanning from 40 to 130 kg m−2 s−1. The experimental observations unveiled three distinct flow patterns: plug, slug, and annular flow. To validate the accuracy of these observed flow patterns, they were compared with an empirical flow pattern map. Meanwhile, pressure drop signals corresponding to various flow patterns were recorded under different experimental conditions using a high-frequency data acquisition card. The experimental findings indicated that the pressure drop signals fluctuate because of the difference in gas-liquid phase compressibility, while the fluctuation period and flow pattern change simultaneously with the flow rate. Specifically, plug flow exhibited the longest pressure drop fluctuation periods and the sharpest probability density function (PDF) curve. In contrast, annular flow exhibited the shortest fluctuation periods and the least sharp PDF curve. Based on the distinctive characteristics of the PDF curve, a novel parameter, K, was introduced primarily derived from the sharpness of the curve. This led to the formulation of innovative transition criteria, which achieved a prediction accuracy of 86.99% across all datasets in this study. This work furnishes a straightforward methodology for distinguishing flow patterns. The advantages of the proposed criteria include the requirement of measuring only pressure drop fluctuations to identify flow patterns without direct observation.