# defining heat transfer coefficient (HTC)

 The heat transfer coefficient is a characteristic of the flow. It is used tomeasure the ability of a flow to convect energy from walls. HTC=qwall/(Twall-Tref)For forced convection flows, HTC is traditionally conceived to bea 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. Thisis true only if Tref is a bulk (or "mixing cup", or mass-averaged) fluidtemperature. For constant properties, by this definition, HTC becomesindependent of thermal field.There are at least 3 methods in calculating HTC in Fluent. The first twouses heat flux to get HTC, which requires a converged thermal field. In thesetwo methods, HTC is determined by measuring the affect of it in the thermalfield - by measuring heat flux. The 3rd method does not require to run thethermal field.1. HTC=qwall/(Twall-Tref)Tref is defined by the user in the Report->Reference Values. This HTC is aFluent variable; it can directly be selected under 'Wall Fluxes'. This cannot beused if the bulk temperature changes along the flow direction, which gives it alimited usage. For example, it can't be used for flow inside a heated duct or apipe, because the bulk temperature changes along the pipe. In these cases, thisHTC becomes a fictitious value; it may be good only for that referencetemperature. If it is going to be applied as boundary condition for anothersimulation 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 temperaturefar away from the plate remains unchanged in the flow direction. (For flatplate, 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 ismuch better than a fixed reference temperature for most complicated geometries.In most cases, if wall functions are used and Y+ is obeyed, the adjacent celltemperature becomes close to the bulk temperature. (Note that when usingstandard wall functions, the Y+ at adjacent cell, ideally, should be between 30and 60, mostly depending on the application.)(see http://www.fluentusers.com/fluent6/doc/ori/html/ug/node433.htmhttp://www.fluentusers.com/fluent6/doc/ori/html/ug/node433.htm )This definition cannot be applied for two-layer model where the first node istoo close to the wall. In this case the adjacent cell temperature will be muchhigher than the should-be-used bulk temperature. This will over-predict HTC.In practice, one may apply a constant wall temperature - value that is closeto actual value, and a fluid inlet temperature. Converge the thermal and theflow field and then extract this HTC.Two ways to get this HTC in Fluent:A. This HTC can be exported into RADTHERM by typing in the text command:(ti-write-radtherm)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 information. 16: V_x, V_y, V_z on both sides of the wall. 17: Heat Transfer Coefficients based on T_cell. 18: T_cellB. You can also make a Custom Field Function (CFF) of the following existingFluent Field variables(see http://www.fluentusers.com/fluent6/doc/ori/html/ug/node966.htmhttp://www.fluentusers.com/fluent6/doc/ori/html/ug/node966.htm )(see http://www.fluentusers.com/fluent6/doc/ori/html/ug/node965.htmhttp://www.fluentusers.com/fluent6/doc/ori/html/ug/node965.htm )HTC = ('Total Surface Heat Flux' - 'Radiation Heat Flux')/('Wall Temperature(outer surface)' - 'Static Temperature'). You can perform contour plot of thisCFF without the node values. Without the node value 'Static Temperature' willgrab 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!(see http://www.fluentusers.com/fluent6/doc/ori/html/ug/node430.htmhttp://www.fluentusers.com/fluent6/doc/ori/html/ug/node430.htm )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 * ystarHTC = Density * cp * Cmu^.25 * kP^.5 / (Pr * ystar)where P = (pi/4)/sin(pi/4) * (A/k)^.5 * (Pr/Prt - 1) * (Prt/Pr) ^.25How 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-etcYou can also write a UDF to do this.

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