A numerical analysis is performed to investigate the onset of convective instability and latent heat transfer, in connection with the vaporization of a liquid-water film, for laminar forced convection in the thermal entrance region of horizontal rectangular ducts. Major dimensionless groups identified are aspect ratio gamma, effective Rayleigh number Ra+, Prandtl number Pr, and Schmidt number Sc. The effects of the changes of bottom wall temperature T(w), relative humidity of air phi, gamma, and Ra+ on the local Nusselt number Nu(z). Sherwood number Sh(z), and the onset point of convective instability are examined in detail. To understand the effects of the selection of different criteria on the onset point, two typical definitions are chosen: a 2 percent departure of Nu(z) from that of Graetz theory and the local minimum Nu(z). Results show that the onset point, under the effects of the combined buoyancy forces of thermal and mass diffusion, is rather advanced compared with that under the effect of thermal buoyancy force only. Results also show that the convective instability is affected by changes of phi and T(w) for fixed gamma. Additionally, the influence of water evaporation along the wetted wall on the laminar mixed convection heat transfer in this study is also presented.
