New modeling approaches for liquid holdup and pressure drop in vertical downward gas-liquid two-phase flow

The impact of the buoyancy force direction in the different hydrodynamic phenomena occurring, as well as the few numbers of studies carried out, explain the existing complexity in modeling of liquid holdup and pressure drop in vertical downward gas-liquid flow. In this paper, we tackle this challenge through an experimental investigation, a parametric analysis and proposition of new predictive correlations. First a series of experiments were carried out using air-water mixture and 30 mm ID pipe. The experimental results demonstrate the difficulty to model the two parameters using the traditional approaches. Then, using both the present results and data collected from the literature, we propose a new approach that employs liquid-to-gas superficial velocities and input liquid fraction to correlate liquid holdup. Analysis of frictional pressure drop results using the Lockhart-Martinelli approach demonstrates the need to consider a fourth term. This term is originally correlated by means of an analogy with the buoyancy force acting on the Taylor bubbles. The assessment analysis clearly shows that both proposed correlations perform better than existing ones for a large number of data obtained under different flow regimes, pipe diameters and operating conditions. With average absolute relative error values lower than 25 %, both correlations demonstrate their ability to satisfactorily predict data that were not used for their development.