Abstract
The behavior of two-phase flow and corresponding flow regimes in helical tubes significantly differ when compared to two-phase flows in straight tubes due to centrifugal and torsion effects. In order to gain physical insight and gather data for validating computational models, a large number of experiments were performed on a helical coil experimental setup operated with a mixture of water and air. The experimental data were used to assess the predictive capabilities of current two-phase Computational Fluid Dynamics (CFD) models based on the Volume of Fluid (VOF) approach. In the present paper, a comparison of the CFD simulation results with the high-resolution experimental data is discussed, with special emphasis on two-phase pressure drops and void fraction distributions. It is shown that the CFD VOF model is able to correctly capture the occurrence of five flow regimes observed in the experiments, namely bubbly flow, plug flow, slug flow, slug-annular flow, and annular flow. However, a good quantitative agreement for pressure drops and void fraction distributions is found in slug flow and slug-annular flow regimes only.
Highlights
Because of their superior heat transfer performance when compared to straight pipes and the compactness of the cylindrical geometry, helical coils are widely used in the food industry, steam generators, chemical processing, and medical equipment (Fsadni et al, 2016)
The two-phase pressure drops for the last half turn upstream of the helical coil outlet were measured, so in the simulations, the pressure drop across the same section was monitored for comparison with the experimental data
The simulation results show that by employing proper boundary conditions, initial conditions, mesh and time step, the air-water flow regime as well as the pressure drop can be predicted by Computational Fluid Dynamics (CFD) models reasonably well
Summary
Because of their superior heat transfer performance when compared to straight pipes and the compactness of the cylindrical geometry, helical coils are widely used in the food industry, steam generators, chemical processing, and medical equipment (Fsadni et al, 2016). El-Genk and Schriener (2017) published a literature review on convection heat transfer and pressure losses for singlephase flows in toroidal and helically coiled tubes. They collected 2,410 pressure losses data and 193 Nusselt number data and summarized the effect of the dimensions, geometric parameters, flowrates, and the fluids’ properties on the critical Reynolds number, friction factor, and Nusselt number. Experiments on two-phase flow in vertical helical coils have been performed in the past (Kasturi and Stepanek, 1972; Xin et al, 1996; Mandal and Das, 2003; Zhu et al, 2017) to investigate pressure losses, void fraction and flow regimes and corresponding empirical correlations have been proposed
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