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COMPUTATIONAL MODELLING OF FALLING FILM FLOW AND HEAT TRANSFER OVER HORIZONTAL TUBES
Title
COMPUTATIONAL MODELLING OF FALLING FILM FLOW AND HEAT TRANSFER OVER HORIZONTAL TUBES
Creator
Karmakar, Avijit
Date
2021
Description
In this study, numerical simulations based on the volume of fluid (VOF) method are conducted to investigate the hydrodynamic behavior,...
Show moreIn this study, numerical simulations based on the volume of fluid (VOF) method are conducted to investigate the hydrodynamic behavior, sensible heat transfer behavior, and tube surface wettability effects for a falling film over heated horizontal tubes encountered in falling film heat exchangers. The Reynolds number ranges from 15 - 210, covering the droplet, jet (inline and staggered), and sheet flow modes. To consider evaporation under liquid film waviness and gas (vapor and air) flow effects, a simplified case was studied for the wavy liquid film over a heated vertical surface with the surrounding gas flowing in either co-current or counter-current direction. The OpenFOAM CFD solver has been used to conduct the numerical simulations.For hydrodynamics, the liquid film thickness and interface velocity variation for all the flow modes are presented. In droplet mode, the movement of the liquid waves formed by the drop impact causes an over 350% change in film thickness. A dimple around the jet impingement region in the steady inline jet mode is formed with a relative change in film thickness by 40%. The base of the impinging jets possess ripples of wavelengths 0.3-1.0 times the capillary length. For the steady staggered jet mode, the neighboring jets interact to develop crest and stable segments with film thickness ratio of 1.7. Finally, for the sheet mode, interfacial waves are seen to travel along the tube periphery with amplitudes of about 20% of the nominal film thickness. A set of correlations have been presented to predict film thickness and interfacial velocity with RMSE = 0.2 for 80% of the data.The local Nusselt number (Nu) distribution depends on the flow features in each mode. In the droplet mode, the Nu value varies significantly as the droplet impinges and the remnant liquid-bridge retracts (peak instantaneous Nu = 6), followed by wave propagation with peak Nu = 0.25. For the jet modes, the local maximum in Nu occurs off-center to the impingement location with peak Nu = 3.1 for the inline jet mode and Nu = 2.7 for the staggered jet mode, while for other locations, Nu varies as inversely proportional to film thickness. Substantial variations in the Nu value are also recorded in the middle of the two impinging jets with Nu = 0.95 in the inline jet mode, and Nu = 0.60 in the crest region of the staggered jet mode. In the sheet mode, the Nu varies with the thickness of the traversing liquid waves. Lower Nu values were recorded beneath the crest location of the liquid waves, which increases (1.4 - 11.6%) abruptly at the advancing fronts of the waves. The temperature distribution in the liquid film in each of the modes was examined to evaluate the mechanism of heat transfer process. This study also compares the Nu distribution with the available analytical heat transfer models.The tube surface wettability results present the liquid film thickness, the wetted areas, and the Nusselt number (Nu) over the tube surface. The resistance imposed by the increasing contact angles inhibits the extent of the liquid spreading over the tube surface, and this, in turn, influences film thickness and wetted areas. A significant decrement in the heat transfer rate from the tube surfaces was observed as the equilibrium contact angle increased from 2 to 175 degrees. The local distributions of the Nu over the tube surface are strongly influenced by the flow recirculation in the liquid bulk.Finally, for wavy film evaporation under gas flow effects, the results show a 15% and 16% enhancement in time-averaged Sherwood number (Sh) due to film waviness (sinusoidal and solitary) with gas flow rate, Qg = +50 and Qg = -50, respectively. This enhancement in the Sh for both the waves further increases by 11% with Qg =+800 and by 196% with Qg = -800. Closer examination of the mass transfer process over a wave demonstrates that with Qg = +50, the concentration of the gas side streamlines at the trough locations of the wave leads to higher values of Sh than the rest of the locations. However, with Qg = +800, although the overall Sh increases, vortices appear at the wave trough locations, leading to decreased local Sh values than the surrounding locations.
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