Experimental investigation on two-phase flow parameters and patterns of gas-liquid flow in small-diameter helically coiled tubes

The helically coiled tube (HCT) heat exchangers, recognized for the high heat transfer coefficients and excellent thermal expansibility, are crucial devices with wide-ranging engineering applications. The unique combination of gravity and centrifugal force yields a distinctive distribution and transformation of flow patterns, emphasizing the need for precise measurement and understanding of the two-phase parameters within the HCT. This paper presented an electrical conductivity probe measurement system designed to detect two-phase flow patterns and parameters in gas-liquid flow within small-diameter HCTs, specifically with inner diameters of 6 mm and 10 mm. The influence of superficial gas and liquid velocities on flow patterns in HCTs was investigated, resulting in four distinct classifications: bubble flow, plug flow, slug flow, and annular-slug flow. Correspondingly, the voltage signals from the conductivity probes manifested as descending pulse waves, descending narrow rectangular waves, descending wide rectangular waves, and ascending pulse waves. The variations in two-phase flow parameters were analyzed, revealing that the probability density distribution of bubble chord lengths conformed to the Lorentzian peak function. Then, quantitative criteria for flow pattern classification within the HCTs were developed using primarily voltage signals and supplemented by high-speed camera imagery. The effect of the inner diameter of the HCT on the flow pattern transition boundaries was discussed. Finally, the applicability of the existing correlations for the flow transition boundaries was verified based on experimental data within small-diameter HCTs.This study provides a crucial reference basis for subsequent studies on bubble dynamics and heat transfer mechanisms within HCTs.