Numerical simulation of falling film flow hydrodynamics over round horizontal tubes

The performance of a falling film heat exchanger depends significantly on the flow characteristics and the associated heat transfer in the liquid film flowing over its horizontal tube surfaces. In this study, three-dimensional numerical simulations based on the Volume of Fluid method are conducted to investigate the hydrodynamic behavior of a falling film over horizontal round tubes with diameter and tube-to-tube spacings each of 16 mm. Using a mixture of water and ethylene glycol, the simulations comprise a range of flow rates (Re = 15 to 210) that covers all basic intertube flow modes i.e., droplet, jet (in-line and staggered), and sheet mode. InterFoam, a two-phase incompressible flow solver in OpenFOAM, has been used to simulate the flow field. The computational model agrees well with the available experimental data. The numerical results present the variation of the liquid film thickness and the film interface velocity over the tube surface for all the flow modes. In the droplet mode, the impact of the droplet on the tube produces liquid waves that interact with the neighboring waves and are seen to travel on the tube surface downstream of the impingement region and the region between the impingement sites. The movement of these waves causes a significant change in the film thickness over the tube surface, quantitatively, by more than 350% during a short interval of the droplet mode cycle. After the droplet impact, the liquid bridge formed in between the tubes undergoes necking and finally bifurcates under the Raleigh-Plateau instability mechanism at a location of spacing-to-tube-diameter ratio of 0.8 from the top surface of the impinging tube and produces satellite droplets in between the tubes. In the steady in-line jet mode, the liquid from the neighboring jets do not interact with each other, and the bulk liquid flows downstream of the impingement region. A dimple around the jet impingement region is formed where the film thickness changes by around 40%. The base of the impinging jets was seen to possess ripples due to the Raleigh-Plateau instability with wavelengths ranging from 0.3 - 1.0 times the capillary length scale. For the steady staggered jet mode, the liquid jet impinges on the tube surface, which then spreads rapidly and interacts with the adjacent jet liquid to develop crest and stable segments along the tube length direction. The ratio of the film thickness between the crest and the stable segments was found to be 1.7. Finally, for the sheet mode, due to the formation of surface waves in the liquid sheet, interfacial waves are seen to travel along the tube periphery with amplitudes of about +/- 20% of the nominal film thickness. These flow mode characteristics alter the film thickness and flow velocity distribution over the tube surface and are expected to play an essential role in the surface heat transfer behavior over round horizontal tubes. A set of correlations have been presented to predict the film thickness and interfacial velocity over the tube surface for each of the studied modes of flow with reasonable accuracy. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study utilized three-dimensional numerical simulations to investigate the hydrodynamic behavior of a falling film over horizontal round tubes with diameter and tube-to-tube spacings each of 16 mm. The results showed the variation of the liquid film thickness and interface velocity over the tube surface for different flow modes.