The CFD studies on the influence of un-wetted area on the heat transfer performance of the horizontal tube falling film evaporation

Abstract

This paper discusses about the effect of un-wetted area of tube on the heat transfer performance of horizontal tube falling film evaporation. A 2D CFD model was developed to perform simulations and investigate the output and validated them with published data available in the literature. In the present study the VOF method is used to track the boundary of the liquid vapour from the contours of volume fraction. Effect of varying tube wall temperature or wall super heat (6 to 11?C) on un-wetted area, heat transfer co-efficients and mass transfer co-efficients of the circular tube were obtained from the simulation model and the results were analysed and reasons were identified and discussed here. The threshold value of wall super heat above which phase change occurs between liquid film and tube surface is identified as 6?C. Also it is noted that mass transfer rate increases and then decreases with increase of wall super heat and heat transfer co-efficient showed declining trend.

Recent Advances in CFD Simulations of Multiphase Flow Processes with Phase Change

“…It was found that a thicker wavy film was formed over the tube as a result of the upward air streams. Balaji et al confirmed that the heat transfer coefficient declined at the local dryout in a horizontal tube evaporator. Additionally, the mass transfer coefficient increased with wall superheat increased from 6 to 9 K, and then keep almost unchanged with the increase of superheat.…”

“…The Tanasawa model is a simplified version of the Schrage model, which assumes that the mass flux is linearly related to the temperature jump when the difference between the interface temperature and saturation temperature is tiny . However, the determination of key empirical coefficients ( r or w ) in Hertz–Knudsen relations can be challenging as they are not constant in different phase change processes and are varying with the configuration, surface roughness, local superheat, saturation temperature, and dryout. , There are still other well-known phase change models, such as Rohsenow, Hardt and Wondra, De Schepper et al, Ganapathy et al, and the energy jump model . These correlations are commonly used in the gas–liquid phase change with a large-scale interface.…”

“…Consequently, the drying processes become essential for the preservation, transportation, and usage of these materials. Moreover, particle coating and granulation are often intertwined with wetting and drying processes, as the liquid solvent can help the solute to wrap around a particle surface homogeneously. Evaporation, condensation, and crystallization processes can efficiently separate the solute from the solvent, which is attractive for wastewater treatment, desalination, dehumidification, and the production of specialty chemicals and pharmaceuticals It is anticipated that solidification and melting will play a pivotal role in the forthcoming industrial revolution concerning manufacturing and materials self-healing …”

“…Evaporation, condensation, and crystallization processes can efficiently separate the solute from the solvent, which is attractive for wastewater treatment, desalination, dehumidification, and the production of specialty chemicals and pharmaceuticals …”