Author Topic: How the heat transfer coefficient is computed in Fluent?  (Read 10557 times)

Offline william

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How the heat transfer coefficient is computed in Fluent?
« on: April 25, 2012, 10:52:21 AM »
Just wanted to share some information on how the heat transfer coefficient is computed in Fluent:

The heat transfer coefficient is a characteristic of the flow. It is used to measure the ability of a flow to convect energy from walls. HTC=qwall/(Twall-Tref)

For forced convection flows, HTC is traditionally conceived to be a function of velocity (flow rate), fluid properties, and geometry. i.e. NU =HTC/KL = NU(Re,Pr); it is not thought in terms of wall boundary conditions. This is true only if Tref is a bulk (or "mixing cup", or mass-averaged) fluid temperature. For constant properties, by this definition, HTC becomes independent of thermal field.

There are at least 3 methods in calculating HTC in Fluent. The first two uses heat flux to get HTC, which requires a converged thermal field. In these two methods, HTC is determined by measuring the affect of it in the thermal field - by measuring heat flux. The 3rd method does not require to run the thermal field.

1. HTC=qwall/(Twall-Tref)

Tref is defined by the user in the Report->Reference Values. This HTC is a Fluent variable; it can directly be selected under 'Wall Fluxes'. This cannot be used if the bulk temperature changes along the flow direction, which gives it a limited usage. For example, it can't be used for flow inside a heated duct or a pipe, because the bulk temperature changes along the pipe. In these cases, this HTC becomes a fictitious value; it may be good only for that reference temperature. If it is going to be applied as boundary condition for another simulation or FEA analysis, it may only be used with that reference temperature - a fixed thermal boundary condition.

But it can be used for flow over a flat-plate, where the reference temperature far away from the plate remains unchanged in the flow direction. (For flat
plate, the bulk temperature turns out to be temperature at infinity.)

2. HTC = qwall/(Twall-Tcell)
In this definition, Tcell is the adjacent cell temperature. This definition is
much better than a fixed reference temperature for most complicated geometries.
In most cases, if wall functions are used and Y+ is obeyed, the adjacent cell
temperature becomes close to the bulk temperature. (Note that when using
standard wall functions, the Y+ at adjacent cell, ideally, should be between 30
and 60, mostly depending on the application.)

This definition cannot be applied for two-layer model where the first node is
too close to the wall. In this case the adjacent cell temperature will be much
higher than the should-be-used bulk temperature. This will over-predict HTC.

In practice, one may apply a constant wall temperature - value that is close
to actual value, and a fluid inlet temperature. Converge the thermal and the
flow field and then extract this HTC.

Two ways to get this HTC in Fluent:

This HTC can be exported into RADTHERM by typing in the text command:

Enter the filename.
Enter desired output surfaces' names, one at a time.
Hit Enter to exit out.

*Note* For double sided walls, you only need to select one of a
wall-shadow pair. The heat transfer info will be written
for both sides of it.

The output file is in Patran format and contains Packet 16, 17, and 18
16: V_x, V_y, V_z on both sides of the wall.
17: Heat Transfer Coefficients based on T_cell.
18: T_cell

HTC = ('Total Surface Heat Flux' - 'Radiation Heat Flux')/('Wall Temperature
(outer surface)' - 'Static Temperature'). You can perform contour plot of this
CFF without the node values. Without the node value 'Static Temperature' will
grab the adjacent cell temperature.

Note that in order to use q"/(Twall - Tcell), there has to be sufficient flux through the wall.

3. Another method to obtain HTC is to get it directly from wall functions. Note that this method can be used even if there is no flux through the wall!

For segregated solver, incompressible flow, and Ystar > Ystar_T:
Tstar=(Twall-Tcell)*Density * cp * Cmu^.25 * kcell^.5/qwall = Prt * [1/k *
ln (E * ystar) + P]

HTC = qwall/(Twall-Tcell) = Density * cp * Cmu^.25 * kP^.5 / {Prt * [1/k *
ln (E * ystar) + P]}

For segregated solver, incompressible flow, and Ystar>Ystar_T:
Tstar=(Twall-Tcell)*Density * cp * Cmu^.25 * kcell^.5/qwall = Pr * ystar

HTC = Density * cp * Cmu^.25 * kP^.5 / (Pr * ystar)

where P = (pi/4)/sin(pi/4) * (A/k)^.5 * (Pr/Prt - 1) * (Prt/Pr) ^.25

How to get this:

In Fluent6.1.x:
File -> Export
-select RADTHERM under 'File Type'
-select the walls under 'Surfaces'
-select 'Wall Function' under 'Heat Transfer Coef.'
-click on Write

You can also write a UDF to do this.