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This paper presents a numerical modeling of steam condensation in a vertical channel in the presence of a noncondensable gas (air) during forced convection. A laminar vapor steam-air mixture flows downwards through a rectangular channel with a constant temperature and concentration and a uniform velocity profile at the inlet. The model includes the axial diffusion terms. From numerical modeling and for the different inlet parameters and aspect ratios values, the condensed mass flow rate has been evaluated. The obtained results show that the increase in the inlet humidity, the inlet Reynolds number, and the inlet temperature, as well as the aspect ratio, promote condensation. It has been found that at a given axial position, the mass flow rate increases with the above-mentioned parameters and it reduces through the channel.

The aim of the present study is to investigate numerically the effect of inlet air humidity on an upward airflow in a humidifier intended for a humidification–dehumidification desalination system. A vertical parallel-plate channel constitutes the humidifier. One of the plates is wetted by a liquid water film and maintained at a constant temperature, while the other is dry and thermally insulated. The airflow enters the channel with constant temperature, humidity and velocity. The results show that the increase of air humidity at the channel entrance affects seriously the performances of the humidifier as it induces condensation of the water vapour on the walls. On the other hand, it was stated that the humidifier works well for low inlet humidity.

Due to its widespread applications, heat and mass transfer in an air stream with liquid film evaporation or condensation in open channels has received considerable attention in the literature. This kind of flows is present in many natural and engineering processes, such as human transpiration, desalination, film cooling, liquid film evaporator, cooling of microelectronic equipments and air conditioning. Since the original theory for flow of a mixture of vapour and a non-condensable gas by Nusselt (1916) and its extension by Minkowycz & Sparrow (1966), many theoretical and experimental studies have been published in the literature. These studies deal with different geometric configurations such as a flat plate, parallel-plate channel and rectangular or circular-section ducts. Heat and mass transfer convection over a flat plate wetted by a liquid film has been investigated by Vachon (1979). He performed an analytical and experimental study of the evaporation of a liquid film streaming along a porous flat plate into a naturally driven airflow. This author established correlations for Nusselt and Sherwood numbers in connection with a combined Grashof number. Ben Nasrallah & Arnaud (1985) investigated theoretically film evaporation in buoyancy driven airflow over a vertical plate heated with a variable heat flux. The solutions of the governing equation have been obtained by means of semi-analytical and finite difference methods. The authors present their results in term of expressions of the wall temperature and mass fraction as well as the local Nusselt and Sherwood numbers. A numerical and experimental analysis has been carried out by Tsay et al. (1990) to explore the detailed heat transfer characteristics for a falling liquid film along a vertical insulated flat plate. Free stream air temperature was set at 30°C and inlet liquid film temperature was taken equal to 30°C or 35°C. The results show that latent heat transfer connected with vaporization is the main cause for cooling of the liquid film. The authors affirm that when the inlet liquid temperature is equal to the ambient one, latent heat transfer due to the film vaporization initiates heat transfer in the film and gas flow. Aguanoun et al (1994; 1998) studied numerically the evaporation of a falling film on an inclined plate heated

International audience; The objective of the present study is to investigate experimentally the effect of film evaporation on mixed convective heat and mass transfer in a vertical rectangular channel. Two parallel channel walls are wetted by a water film and heated by a constant heat flux, while the other walls are dry and thermally insulated. The liquid film temperature, the evaporated flow rate, the upward airflow temperature and humidity are measured. Wide ranges of inlet airflow velocity, heat flux and liquid film flow rate are considered. Cases of laminar and turbulent airflow are considered. The experimental results show that evaporation takes place on the majority of the surface of the two walls and, in some cases, evaporative cooling occurs especially for small heating flux and large air velocities. Moreover, a numerical study is carried out in the case of a laminar flow and its results are compared to experimental ones. These comparisons lead to a good agreement with respect to liquid film and airflow temperatures in addition to the evaporated flow rate.