Experimental investigation and new void-fraction calculation method for gas–liquid two-phase flows in vertical downward pipe

Abstract

The void fraction is a key parameter of the two-phase flow in vertical downward pipes, particularly for calculating the mixture density, mixture velocity, mixture viscosity, heat transfer coefficient, and pressure gradient of the flow. Studies on two-phase flows have focused primarily on vertical upward and horizontal pipe flows, while vertical downward pipe flows have been largely neglected. Compared with the flow characteristics of the gas–liquid two-phase flow in upward pipes, those of a downward pipe flow are more complex under the interaction of the gravity, buoyancy, and inertial forces. In this study, a detailed experimental analysis of the void fraction of the gas–liquid two-phase flow in a vertical downward pipe was conducted with a pipe having an inner diameter of 20 mm. A total of 171 void-fraction data points were measured using the quick-closing valve method, and the performances of 12 commonly used correlations that do not take into consideration the flow pattern were assessed based on experimental data points. Errors were obtained on using some of the correlations for calculating the void fraction of the gas–liquid two-phase.flow in a vertical downward pipe at a low liquid superficial velocity. These models will regularly produce errors in the calculation of the void fraction, mainly because this problem is outside the scope of their application. In addition, a new model that takes into consideration the flow pattern was established based on the drift flux model; this new model overcomes the deficiencies of the conventional correlations in the calculation of the void fraction of the gas–liquid two-phase flow in vertical downward pipes at a low liquid superficial velocity. The reliability of the new method and 12 conventional correlations was assessed using 243 published data points. The mean relative errors of these correlations ranged from −21.65% to 8.65%, and the average absolute errors ranged from 8.57% to 23.17%. The results of the proposed method fit the experimental data in the literature better than those of the 12 existing correlations; the average relative error was −2.80%, and the average absolute error was 6.49%. The proposed model thus improves the prediction of void fractions of the gas–liquid two-phase flow in downward pipes.