Combined forced and buoyancy-driven convection between heated vertical parallel plate

Combined forced and buoyancy driven convection between vertical parallel plates is frequently encountered in industry applications. Despite the simple geometry, interaction between the external driving mechanism and buoyancy causes complexity to the flow and heat transfer as well as unknown conditions at the boundary. A numerical simulation has been made in this study that overcomes the difficulties arising from such interaction.;A unified picture emerges from physical analysis of flow and heat transport that helps explain some conceptual contradictions of the commonly accepted theory. In this research, a 'natural Reynolds number' is proposed which provides a division for the buoyancy-aided and buoyancy-opposed fluid flow, and also an indication of the inertia force transferred from buoyancy. Depending upon the imposed Reynolds number and the Rayleigh number, the fluid flows in vertical channels are then categorized into buoyancy-aided, buoyancy-opposed upward and downward flows. A numerical algorithm is developed that suits the physical conditions encountered in all these situations, including the unknown velocities and pressure at the boundary. The three-dimensional, transient formulation is the SIMPLE type incorporating the QUICK scheme that deals with the possible flow reversal in the channel.;The buoyancy-induced flow is studied first and the natural Reynolds number is correlated to the Rayleigh number and the aspect ratio of the channel. An extension from the existing results of convective heat transfer is made. By using the natural Reynolds number obtained, the buoyancy-aided and the buoyancy-opposed flows are classified, and the three situations involved in the combined forced and natural flows are then calculated. Moreover, the newly discovered special hydrodynamic and heat transfer behavior of the vertical channel due to the buoyancy-induced updraft is discussed. Since the three-dimensional experimental data are not available, the present numerical results at the center vertical cross section are compared with the existing two-dimensional data to benchmark the formulation. It is found that the flow and temperature distributions are steady and two-dimensional in a buoyancy-aided flow and a strong downward flow. In a buoyancy-opposed or a weak downward flow, the flow becomes unsteady and three-dimensional. All the above cases of mixed convection are examined through a wide range of parameters. The numerical results with distributions of velocity, temperature and heat transfer reveal some new phenomena in the finite length passage.;Based on the numerical results, correlations for mixed convective heat transfer are obtained under different flow and heating conditions. It is found that the natural Reynolds number and the heat transfer coefficient for the buoyancy-induced flow are very important in these correlations. Based on the correlations obtained, an easy method for evaluating the mixed convective heat transfer coefficient is proposed for engineering applications.