TAILIEUCHUNG - Heat Transfer Handbook part 56

Heat Transfer Handbook part 56. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | 542 NATURAL CONVECTION g Gravity field 00 Poo surface Figure Natural convection boundary layer flow over a semi-infinite horizontal surface with the heated surface facing upward. y x 5v n Gt i Figure shows the computed velocity and temperature profiles for flow over a heated horizontal surface facing upward or a cooled surface facing downward. For a heated surface facing downward or a cooled surface facing upward a boundary layer type of flow is not obtained for a fluid that expands on heating. This is because the fluid does not flow away from the surface due to buoyancy. The local Nusselt number for horizontal surfaces is given by Pera and Gebhart 1972 for both the isothermal and the uniform-heat-flux surface conditions. The Nusselt number was found to be approximately proportional to Pr1 4 over the Pr range to 100. The expression given for an isothermal surface is Nux 5 Pr1 4 k and that for a uniform-flux surface NuM is NuXi - 5 Pr1 4 It can be shown by integrating over the surface that for the isothermal surface the average Nusselt number is 3 times the value of the local Nusselt number at x L. Therefore the natural convection heat transfer from inclined surfaces can be treated in terms of small inclinations from the vertical and horizontal positions detailed results on which are available. For intermediate values of y an interpolation between these two regimes may be used to determine the resulting heat transfer rate. Numerical methods such as the finite difference method can also be used to solve the governing equations to obtain the flow and temperature distributions and the heat transfer rate. This regime has not received as much attention as the horizontal and EXTERNAL LAMINAR NATURAL CONVECTION FLOW IN OTHER CIRCUMSTANCES 543 0 1 2 3 4 5 6 7 Figure Calculated a velocity and b temperature distribution in natural convection boundary layer flow over a horizontal surface with a uniform heat flux.

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