Heat Transfer Coefficient
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• Heat flux from the surface defined by Newton’s law of cooling: where h is convective heat transfer coefficient. It is used as a measure of the ability of a flow to convect energy from viscous walls. ) )( ( ) ( " ∞ - = T T x h x q w ∞ u y ) ( " x q w T © 2008 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary convect energy from viscous walls. • Generally, for forced convection, h should be independent of thermal BC only if the reference temperature is taken as the local bulk fluid temperature.
description
HTC Description
Transcript of Heat Transfer Coefficient
• Heat flux from the surface defined by Newton’s law of cooling:
where h is convective heat transfer coefficient. It is used as a measure of the ability of a flow to convect energy from viscous walls.
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))(()(" ∞−= TTxhxq w
∞uy
)(" xq
wT
© 2008 ANSYS, Inc. All rights reserved. 1 ANSYS, Inc. Proprietary
convect energy from viscous walls.• Generally, for forced convection, h should be
independent of thermal BC only if the reference temperature is taken as the local bulk fluid temperature.
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• Two ways to extract HTC from CFD1. Flux-based, h=q”(Tw-Tref)
– Using local bulk temperature as Tref :– Most “correct”, but calculating bulk temperature is not easy for complex
geometry. Even for simple pipe flow, we need to do some coding!– Using fixed reference temperature: Not correct everywhere.
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– Using fixed reference temperature: Not correct everywhere.– Using adjacent cell as a reference temperature. Note: This can be done using Custom Field Function of (Total Heat Surface Heat Flux - Radiation Heat Flux)/ (Wall Temperature (Outer Surface) - Static Temperature)and plotting without node values.– This requires running the energy equation.
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2. Based on wall functions– This requires only to run turbulence equations.– It would be equivalent to using the adjacent cell as reference
temperature in the flux-based. It should give exactly the same hno matter what wall functions you use!
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( ) ( )( ) ( )
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• Generally, for accurate h, we want TP to be close to the bulk temperature. This can be achieved only if y+ is large. This is the best we can do for a complicated geometry.
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PT
bulkT
wTPT
bulkT
wT
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• The Fluent field variable, Surface Heat Transfer Coe., under Wall Fluxes, is:
h=q”total(Tw-Tref)q”total=qrad+qconvection
– T is taken from Report � Reference values.
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– Tref is taken from Report � Reference values.– It is an effective HTC.
• The wall function based heat transfer coefficient is also listed under Wall Fluxes.
