Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.
When there is a forced flow over a body that has a surface
temperature that is different from the temperature of the
undisturbed forced flow the buoyancy forces that arise due to
the density differences associated with the temperature
differences in the flow can have a significant effect on the flow
and consequently on the heat transfer rate from the body. Flows
such as these are termed mixed- or combined natural and forced
convective flows. The present paper reports on the study of
mixed convective flow over a thin vertical flat plate which has
a uniform surface heat flux for conditions under which
transition from laminar to turbulent flow occurs. Attention has
been restricted to the case where the buoyancy forces act in the
opposite direction to the forced flow, i.e., to the case of
opposing mixed convective flow. Most existing studies of this
type of situation have assumed that steady laminar flow exists.
Laminar, transitional, and turbulent flow situations have been
considered in the present study and the development of
unsteady flow has been allowed for. The forced flow has been
assumed to be steady and the Boussinesq approach has been
used. The solution has been obtained by numerically solving
the governing equations subject to the boundary conditions
using the commercial CFD solver, ANSYS FLUENT©. The kepsilon
turbulence model with the full effect of buoyancy
forces accounted for and with standard wall functions has been
used in obtaining the solutions. The heat transfer rate from the
surface of the plate has been expressed in terms of the mean
Nusselt number based on the overall plate length and the
difference between the overall mean plate temperature and the
undisturbed fluid temperature. This Nusselt number depends on
the values of the heat flux Rayleigh number based on the plate
length, the Reynolds number based on the value of the forced
velocity ahead of the plate and on the plate length, and the
Prandtl number. Results have been obtained for a Prandtl
number of 0.74, i.e., essentially for the value for air. The
conditions under which the flow can be assumed to be purely
forced convective and under which the flow can be assumed to
be purely natural convective have been investigated.
