Mixed Convection Heat and Mass Transfer with Phase Change and Flow Reversal in Channels

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 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 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 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 30C and inlet liquid film temperature was taken equal to 30C or 35C. 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 www.intechopen.com Mass Transfer in Multiphase Systems and its Applications 180 at a constant temperature in a humid air stream. They considered forced A boundary layer type model was adopted. The authors stated that the liquid film-gas interface has approximately the same temperature as the plate in the case of forced convection. Yan & Soong (1995) considered turbulent heat and mass transfer convection over a wetted inclined plate. The liquid film flow is turbulent and waves less. Their results pointed out that the plate and the gas-liquid interface temperatures are reduced consequently to the increase of the plate's inclination angle, the inlet film thickness or the air velocity. The plate is insulated or heated by a constant heat flux. The main result of this study was that the enthalpy diffusion term in the energy equation does not influence the film temperature. This term represents the effect of the species diffusion on enthalpy of the humid air mixture. Volchkov et al. ( The authors aim to establish the validity of the heat-mass transfer analogy. The steam in the humid airflow may condense on the plate whose temperature is lower than that of the airflow. The authors made a major simplification by neglecting the effect of the condensate film. This assumption is justified by the experimental data in the literature which indicate that measurements of the liquid film-air interface temperature in humid-air flows show that it is close to the saturation temperature corresponding to the vapour concentration at the plate. This point is addressed hereafter. Recently, Maurya et al. ( Their results show that the evaporation process begins only after the growing thermal boundary layer reaches the interface. Heat and mass transfer in a vertical heated tube with a liquid film falling on its inside walls was treated by They considered co-current downward flows of liquid water and humid air. The airflow is turbulent while the liquid one is laminar and without surface waves. The authors concluded that better cooling of the liquid film is obtained for higher heat flux or lower inlet liquid flow. Their results show that large film evaporation rates are obtained for larger tube wall temperature. Convective instability of heat and mass transfer for laminar forced convection in the thermal entrance region of a horizontal rectangular channel has been examined by The rectangular channel is thermally insulated except for the bottom wall. The latter is maintained at a constant temperature and covered by a thin liquid water film. The thickness of this film is neglected, thus it is treated as a boundary condition for heat and mass transfer. Inlet air temperature was fixed at 20C. The effects of changes of bottom wall temperature, relative humidity of air at the entrance and the channel aspect ratio are examined. The results show that the convective instability is affected by changes in inlet air relative humidity and temperature and also the channel aspect ratio. Huang et al. ( Two of the duct walls were wetted by a thin liquid water film and maintained at different constant temperatures. The other walls are insulated. Air entered the duct with a constant temperature lower than that of the walls. The authors established that vapour condensation occurred on the wall with lower temperature. They used a boundary-layer approximation model to derive a www.intechopen.com

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