Effect of surface radiation on internal flows with non-isothermal boundary conditions.

摘要

The effects of surface radiation on convective heat transfer to a transparent gas flowing inside channels with non-isothermal boundary conditions are investigated numerically. The introduction of radiative heat transfer, between axial locations on the duct walls and the inlet and exit areas, leads to a set of nonlinear integro-differential equations. These equations replace the standard energy constraints boundary conditions on the convective field equations. A mathematical model has been developed to describe the flow and heat transfer in the entry region of the channels. This model has been derived and formulated in a general manner to describe forced, mixed and free convection between two parallel plates and in concentric annuli. An iterative-marching technique is developed to solve numerically the resulting group of partial and integro-differential equations.;The effect of surface radiation on the developing laminar forced convection flow between two plates is investigated. The effect of the controlling parameters on wall temperatures, fluid temperature profiles, and Nusslet number are illustrated. The values of the radiation number at which surface radiation can engender symmetric heating (and hence maximum average Nusslet number on the heated wall and maximum reduction in the maximum heated wall temperature are achieved) are obtained.;The effects of surface radiation on the developing laminar forced convection flow in concentric annuli are numerically investigated. To conduct this investigation, four shape factors not available in the literature have been derived and validated. The effects of the controlling parameters on wall temperature and Nusslet number are illustrated.;The effects of surface radiation on the developing laminar mixed convection flow between two vertical parallel plates are numerically investigated. The effect of the surface radiation on wall temperatures, fluid temperature profiles, location of the channel height at which the buoyancy forces balance the viscous forces, the location of the onset of pressure build up, the location of the onset of flow reversal, average friction factor, and Nusslet number are illustrated. The values of the emissivity at which surface radiation engenders minimum pumping power requirements are obtained.;The obtained numerical solutions have been compared with their corresponding available experimental, numerical and analytical solutions (if any). These comparisons provided a mean of validation of the present scheme and its method of solution.;The effects of surface radiation on the developing laminar mixed convection flow in vertical concentric annuli are numerically investigated. The results reveal the significant effect of surface radiation on the velocity profiles, fluid pressure, friction factor, temperature profiles, walls temperatures, and Nusslet number.;The radiation effect on the developing laminar natural convection flow between two parallel plates and in a vertical concentric annulus is numerically investigated. It was found that the maximum wall temperature decreases with the presence of surface radiation.