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What are the conventions for Heat Flux at surfaces?

Altair Forum User

Heat Transfer into the fluid domain is positive - when referring to OSI Heat Flux or ORI Heat Flux. However, heat transfer into the domain for OSI Convective Temperature Flux is negative - because of the U dot N term in the integral. (See the chart in the acuTrans section of the Programs Reference Manual.)

The total heat flux across a surface is the sum of OSI Convective Temperature Flux, OSI Heat Flux, radiation surface heat flux, solar radiation surface heat flux. In order to perform an energy balance across a control volume, all of these components must be summed. It is not possible to break out conduction and convection portions of the heat flux. (Note that Convective Temperature Flux is not the convection heat transfer at the wall - in fact the Convective Temperature Flux should be nearly zero at the wall - because it is due to the flow - and there should be zero flow at the wall.)

When you sum the various types of heat flux for all of the bounding surfaces, the sum should be nearly zero - depending on convergence.

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Altair Forum User

Heat Transfer into the fluid domain is positive - when referring to OSI Heat Flux or ORI Heat Flux. However, heat transfer into the domain for OSI Convective Temperature Flux is negative - because of the U dot N term in the integral. (See the chart in the acuTrans section of the Programs Reference Manual.)

The total heat flux across a surface is the sum of OSI Convective Temperature Flux, OSI Heat Flux, radiation surface heat flux, solar radiation surface heat flux. In order to perform an energy balance across a control volume, all of these components must be summed. It is not possible to break out conduction and convection portions of the heat flux. (Note that Convective Temperature Flux is not the convection heat transfer at the wall - in fact the Convective Temperature Flux should be nearly zero at the wall - because it is due to the flow - and there should be zero flow at the wall.)

When you sum the various types of heat flux for all of the bounding surfaces, the sum should be nearly zero - depending on convergence.

Hi tried a demo problem ( AcuSolve Training - Tut 2 - CHT - heating of fluid flowing through a heated pipe - outer pipe srf is at higher temperature) and the results says the exact opposite :

1) Heat transfer into the fluid domain (as well as solid) is negative!

Elaborated as follows:

A solid and a fluid always have two interfaces in between - coincident and exactly same overlapping surface meshes - one interface is attached to the fluid volume ( call this fluid_int) the other to solid volume (solid_int) - so heat is transferred from solid to fluid volume

so If I plot the 'heat flux' in acuProbe on 'solid_int' its +ve and on 'fluid_int' is negative

Hence the (1) above!

acupro

Indeed, the above statement does appear to be incorrect. In the Programs Reference Manual, the integrated heat flux (OSI) definition has the q-dot-n term, where n is the outward-pointing normal to the surface. Thus, for acuProbe and acuTrans, heat flux into the parent volume would indeed have a negative sign. This is also in acuFieldView if one looks at nodal heat flux values - negative values for heat flux into the parent volume. However, the boundary condition does hold to the other convention - heat flux applied to a surface is positive into the parent volume.