An experimental and numerical investigation of mixed convection in channels and the application of extended surfaces for heat transfer enhancement

James R Maughan, Purdue University

Abstract

Experiments were performed to determine secondary flow development and Nusselt number distributions for laminar mixed convection in the thermal entry region of a parallel plate channel heated uniformly from below. Flow visualization showed the onset of secondary flow on the heater surface and revealed the developing longitudinal plumes and vortices. Longitudinal distributions of the local Nusselt number initially followed forced convection. Subsequent mixing associated with the development of secondary flow caused Nusselt numbers to rise to an initial maximum before decreasing slightly and assuming a fully developed value. Simultaneous flow visualization and heat transfer measurements showed that noticeable secondary flow development preceeded any appreciable increase in heat transfer. Data were scaled with z$\sp\*$ and Ra$\sp\*$ to delineate locations for the onset of secondary flow, onset of enhancement, and the maximum Nusselt number, and a correlation was developed to predict fully developed conditions. In addition, numerical analysis was used to predict flow acceleration and heat transfer enhancement in a stable, two-dimensional boundary layer and computations were verified by comparison to experimental data for similar conditions. Additional computations were performed to investigate fully developed laminar mixed convection in a channel with longitudinal fins. In general, the effect of the additional surface area was largely offset by a reduction in the strength of the buoyancy-driven secondary flow, resulting in only a modest increase in performance. Experiments for fully developed flow between a heated isothermal bottom plate and a cooled upper plate were also performed. Fins mounted on the bottom surface greatly increased heat transfer for forced convection, but had a much smaller effect on fully developed mixed convection. In some instances, the fins reduced heat transfer. Further experiments concentrated on measuring and enhancing heat transfer from a heated upper surface, which is largely unaffected by mixed convection. Perforated ribs mounted on the top surface effectively enhanced heat transfer, while vortex generators had only a slight effect beyond that of mixed convection alone. This smaller effect was attributed to the correspondence between the flow driven by the vortex generators and the buoyancy driven vortices.

Degree

Ph.D.

Advisors

Incropera, Purdue University.

Subject Area

Mechanical engineering

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