in the free stream (far from the plate) to zero at the plate. Within this boundary layer, the flow is initially laminar but can proceed to turbulence once the Reynolds Number Re of the flow is sufficiently high. The transition from laminar to turbulent for flow over a flat plate occurs in the range,
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Two-dimensional flow analysis over a flat plate serves well to illustrate several key concepts in forced convection heat transfer.
The viscosity of the fluid requires that the fluid have zero velocity at the plate's surface. As a result a boundary layer exists where the fluid velocity changes from |
| 3 x 105 < Rex < 3 x 106 , |  |
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| For the current problem, we consider that the plate is heated starting at a point x = x0 and continuing downstream. Furthermore we assume that the plate is maintained at constant temperature Tw, making this problem isothermal. We are interested only in laminar flow, so it is assumed that the plate length L is sufficiently short such that turbulent flow is never triggered (i.e. Rex < 3 x 105). |
Nusselt Number Calculation
An analysis of the fluid flow over the plate that considers conservation of momentum and energy, including the effects of viscosity, temperature, and heat entering the fluid from the plate, results in the following equation for the Nusselt Number Nu as a function of x,
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Heat Transfer Calculation
| The Nusselt Number equation can be used to calculate the heat transfer coefficient h via, |
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| which can then be used to calculate the heat convected away by the fluid via Newton's Law of cooling, |
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| Since fluid properties (such as viscosity, diffusivity, etc.) can vary significantly with temperature, there can be some ambiguity as to which temperature one should use to select property values. The recommended approach is the use the average of the wall and free-stream temperatures, defined as the film temperature Tf, |
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| This problem is executed in a calculator. |
