External Flow Forced Convection Free Convection 8.pdfconvection in enclosures have been written by...
Transcript of External Flow Forced Convection Free Convection 8.pdfconvection in enclosures have been written by...
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Forced Convection Free Convection
External Flow
Internal Flow
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We focus on free convection problems in which the density gradient is due to a temperature gradient and the body force is gravitational. However, the presence of a fluid density gradient in a gravitational field does not ensure the existence of free convection currents.
Buoyancy is due to the combined presence of a fluid density gradient and a body force that is proportional to density.
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A plate at uniform temperature Twall immersed in a stagnant fluid at Tfluid.
Tfluid > Twall
Two assumptions are kept for the following analysis:
1) The boundary layer is thin enough to accommodate the simplifications made in forced convection.
2) The Prandtl number is approximately 1.
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Recall that the Reynolds number provides a measure of the ratio of the inertial to viscous forces acting on a fluid element. In contrast, the Grashof number is a measure of the ratio of the buoyancy forces to the viscous forces acting on the fluid.
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In this correlation, the thermal properties should all be evaluated at the film temperature, Tf = (Tfluid + Twall)/2, except for �, which is to be evaluated at Tfluid if the fluid is a gas.
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SPHERE
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Uniform heat flux
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Combined Forced and Free convection
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Combined Forced and Free convection