06 Convective Heat Transfer over a Flat Plate

06 Convective Heat Transfer over a Flat Plate - Experiment...

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Experiment Six Heat Transfer Experiment Convective Heat Transfer over a Flat Plate Instructor: Professor Chie Gau - 33 -
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I. Objective The objective of this experiment is to obtain local heat transfer distribution over a heated flat plate that is placed in a wind tunnel supplying uniform air flow, with measurements of temperatures and heat flux along the plate. This will make students familiar with not only measurement techniques of temperatures and heat flux, but also analysis of the experimental data. Comparison of the heat transfer data with the theoretical analysis in the text book is made in order to confirm the theory of boundary heat transfer and gain a better understanding of heat transfer and heat transfer enhancement over a roughened surface. II. Experimental Principle For uniform air flowing over a heated plate, the flow velocity varies drastically in the region close to the wall due to viscosity of the fluid. The region where flow velocity varied drastically is called boundary layer or momentum boundary layer. In addition, the heat transfer from the wall can cause large variation of temperature for flow in the wall region. The region where large variation of temperature occurs is called thermal boundary layer. Both boundary layers are different, which develop separately from the leading edge downstream as shown below: Both boundary layers grow thicker as fluid moves downstream, and the degree of variation of both the velocity and the temperature inside the boundary layer is inversely proportional to the thickness of the boundary layer. On the other hand, the local heat transfer or the local convective heat transfer coefficient, h, varies in proportional to the degree of variation of - 34 -
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temperature inside the boundary layer. Therefore, it is expected that the convective heat transfer coefficient, h, varies inversely in proportional to the boundary layer thickness and the heat transfer coefficient varies as follows: At a critical value of Re x 10 5 ( Re x U x L ) , the laminar boundary layer becomes unstable and gradually transformed into turbulent boundary layer. Due to large increase in momentum transport in direction perpendicular to the wall, large increase in the wall heat transfer and both the momentum and the thermal boundary layer thickness occurs. At the same time, fluctuations in both velocity and temperature inside the boundary layer occur.
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