Numerical simulation of evaporating wavy falling liquid films in laminar gas streams. (1st December 2022)

• Record Type: Journal Article
• Authors: Karmakar, Avijit; Acharya, Sumanta
• Is Part Of: International journal of heat and mass transfer. Volume 198(2022)
• Publication Date: 2022-12-01
• Subjects: Falling film evaporation -- Solitary and sinusoidal waves -- Co-current and counter-current gas flow -- Sherwood number -- Gas vortices; Heat -- Transmission -- Periodicals; Mass transfer -- Periodicals; Chaleur -- Transmission -- Périodiques; Transfert de masse -- Périodiques; Electronic journals; 621.4022
• DOI: 10.1016/j.ijheatmasstransfer.2022.123426
• Languages: English
• ISSNs: 0017-9310

Abstract

Highlights: Evaporating wavy falling liquid films interact with different co-current and counter-current gas flow rates. Liquid film waviness and gas flow direction/rates influence mass transfer (Sherwood number) at the liquid-gas interface. Wave trough locations encounter increment in Sherwood number at lowest dimensionless gas flow rates of Qg = ±50. Highest dimensionless gas flow rate conditions of Qg = ±800 contain vortices decreasing local Sh values. Correlations are proposed for predicting Sherwood number under gas flow effects. Abstract: Numerical simulations are performed to investigate the interfacial heat and mass transfer from evaporating wavy falling liquid films in interaction with laminar gas streams. The OpenFOAM solver has been used to conduct the simulations where the liquid-gas interface is resolved using the Volume of Fluid method of solving for three phases, i.e., liquid, vapor, and air. The configuration considered is a falling liquid (water) film on a heated vertical plate with a confined laminar moist air (gas) flow that is either (a) co-current or (b) counter-current to the downward liquid flow. The evaporation at the liquid-gas interface is driven by the interfacial gradient of the vapor mass fraction. Interfacial waves are triggered using a monochromatic forcing disturbance that leads to sinusoidal or solitary waves forming at the liquid-gas interface under respective forcing frequencies. The numerical model is validated with the available experimental data. The results show nearly a 15% enhancement in time-averaged Sherwood number (Sh) due to film waviness (sinusoidal or solitary) at the lower volumetric gas flow rate, Qg = +50 (co-current) and Qg = −50 (counter-current). This enhancement in the Sh for both the waves further increases by 11% with Qg = +800 and 196% with Qg = −800. A 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 at these locations. However, with Qg = +800, although the overall Sh increases, vortices appear at the wave trough locations, leading to a corresponding decrease in the local Sh values. Correlations are proposed for predicting Sh under co-current and counter-current gas flow effects.