[Gargioni]

Dear friends,

I have simulated the flow in a simple geometry (like 2 paralel walls - "2,5D" case).
My BC are (for laminar flow):

In - low velocity and fixed temperature;
Wall - no slip and fixed temperature;
Out - Openning with static pressure.

In CFX-Post:
I have created a line in wall position, through the length.
When I plot the variable (Wall Transfer Coefficien), the value found isn't the value hoped. 1) Why this happen?

But when I take the Heat Flow "q" (in same line created) and temperature average of section "Tav" (with formulas in own CFX-Post)... I can obtain "h" correctly: q /(Tw-Tav) = h

2) Why "q" is correct, but "h" isn't?

3) How can I obtain "h" directly?

4) I have seen thread about that. Some users has talked about reference temperature.. But I don't understand that...
Someone can explain it for me? and how can I change this reference temperature?

I'm starting to study the CFD world...

Thanks in advance everyone.

[ghorrocks]

I would start by working through this FAQ:

http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

[Gargioni]

First of all, thanks for answer.
I liked the link! There are a lot of good things over there!

but... don't worry... I have done all steps, mainly for to learn how the software works.

My results aren't inaccurate. I'm using: RMS = 10^-09 and highresolution for advection and couple velocity-pressure. The simulation doesn't need great adjustments.. So highresolution scheme is good.

I have problem just with Heat Transfer Coefficient, even the "q" seem is right.

The manual doesn't let clear about "h". There said some thing about: "... using an external heat transfer coefficient, hc...".

As am I getting the value of "q" correct, while the value of "h" seem being wrong? How can I obtain the truly value of "h"?

*I did the same simulation on FLUENT software and I obtained the "h" correct there!

[ghorrocks]

There is more to accuracy than convergence tolerance and differencing scheme.

h is referenced to a ambient temperature. By default CFX uses a function of the local fluid temperature which is usually quite different to the engineering definition (which is usually inlet temperature or far field temperature).

To get h as engineers understand it have a look in the output file. There is a discussion in there about HTC reference temperatures and how to define your own temperature.

[Gargioni]

"There is more to accuracy than convergence tolerance and differencing scheme." I said just for let clear that I don't have problems with accuracy.

May you help me to find this thread about HTC?

Thanks in advance =]

[ghorrocks]

It is in the output file. Have a look in your simulation. It is not a thread, but there are plenty of threads in the forum which have discussed the issue.

[Gargioni]

I changed the expert parameter tbulk for htc. And I think CFX considers the tbulk value like constant

How I said:
I have simulated the flow in a simple geometry, like 2 paralel walls - "2,5D" case. I have created a line in wall position, through the length, for captures the "h", however the value of "tbulk" is variable along the length.

What can I do when the tbulk value isn't constant?

[ghorrocks]

If you have set the tbulk parameter then tbulk will be constant and the value you define, otherwise it will depend on local flow conditions.

You can easily extract h for a constant tbulk anyway. The local tbulk temperature used is an available variable, so between that and the wall heat transfer variable you can calculate h based on any tbulk you like.

[Gargioni]

I solved my problem... doing a CEL routine.

[TrII4d]

maybe u can give a short summary of the steps which u made to help me?

[Gargioni]

!$Tw=298.15;
!$Zposi = 0.4;
!$TBulkZ = $Zposi*20;
!$a=0;

!for ($i=1; $i<89; $i++) {
!$a=$a+0.005;
!$Xposi[$i] = $a;
!}


!for ($i=1; $i<89; $i++) {
PLANE: Teste.$i
Option = XY Plane
Z = $Xposi[$i]
Colour Mode = Variable
Colour Variable = Temperature
Range = Local
END

!$Tbulk[$i] = massFlowAve("Temperature","Teste.$i");
!$q[$i] = massFlowAve("Wall Heat Flux","Teste.$i");
!$h[$i]=($q[$i]/($Tw-$Tbulk[$i]));
!}


!$b=0;
!for ($i=1; $i<41; $i++) {
!$b=$b+0.0001875;
!$Yposi[$i] = $b;
!}


!for ($i=1; $i<41; $i++) {
POINT: Point.$i
Option = XYZ
Point = 0.0, $Yposi[$i], $Zposi
Colour Mode = Variable
Colour Variable = Temperature
Range = Global
END
!$T[$i] = maxVal("Temperature","Point.$i");
!$Tadm[$i] = ($Tw-$T[$i])/($Tw-$Tbulk[$TBulkZ])
!}


$Tbulk[1] $q[1] $h[1] $Xposi[1]
$Tbulk[2] $q[2] $h[2] $Xposi[2]
$Tbulk[3] $q[3] $h[3] $Xposi[3]
$Tbulk[4] $q[4] $h[4] $Xposi[4]
$Tbulk[5] $q[5] $h[5] $Xposi[5]
$Tbulk[6] $q[6] $h[6] $Xposi[6]
$Tbulk[7] $q[7] $h[7] $Xposi[7]
$Tbulk[8] $q[8] $h[8] $Xposi[8]
$Tbulk[9] $q[9] $h[9] $Xposi[9]
$Tbulk[10] $q[10] $h[10] $Xposi[10]
$Tbulk[11] $q[11] $h[11] $Xposi[11]
$Tbulk[12] $q[12] $h[12] $Xposi[12]
$Tbulk[13] $q[13] $h[13] $Xposi[13]
$Tbulk[14] $q[14] $h[14] $Xposi[14]
$Tbulk[15] $q[15] $h[15] $Xposi[15]
$Tbulk[16] $q[16] $h[16] $Xposi[16]
$Tbulk[17] $q[17] $h[17] $Xposi[17]
$Tbulk[18] $q[18] $h[18] $Xposi[18]
$Tbulk[19] $q[19] $h[19] $Xposi[19]
$Tbulk[20] $q[20] $h[20] $Xposi[20]
$Tbulk[21] $q[21] $h[21] $Xposi[21]
$Tbulk[22] $q[22] $h[22] $Xposi[22]
$Tbulk[23] $q[23] $h[23] $Xposi[23]
$Tbulk[24] $q[24] $h[24] $Xposi[24]
$Tbulk[25] $q[25] $h[25] $Xposi[25]
$Tbulk[26] $q[26] $h[26] $Xposi[26]
$Tbulk[27] $q[27] $h[27] $Xposi[27]
$Tbulk[28] $q[28] $h[28] $Xposi[28]
$Tbulk[29] $q[29] $h[29] $Xposi[29]
$Tbulk[30] $q[30] $h[30] $Xposi[30]
$Tbulk[31] $q[31] $h[31] $Xposi[31]
$Tbulk[32] $q[32] $h[32] $Xposi[32]
$Tbulk[33] $q[33] $h[33] $Xposi[33]
$Tbulk[34] $q[34] $h[34] $Xposi[34]
$Tbulk[35] $q[35] $h[35] $Xposi[35]
$Tbulk[36] $q[36] $h[36] $Xposi[36]
$Tbulk[37] $q[37] $h[37] $Xposi[37]
$Tbulk[38] $q[38] $h[38] $Xposi[38]
$Tbulk[39] $q[39] $h[39] $Xposi[39]
$Tbulk[40] $q[40] $h[40] $Xposi[40]
$Tbulk[41] $q[41] $h[41] $Xposi[41]
$Tbulk[42] $q[42] $h[42] $Xposi[42]
$Tbulk[43] $q[43] $h[43] $Xposi[43]
$Tbulk[44] $q[44] $h[44] $Xposi[44]
$Tbulk[45] $q[45] $h[45] $Xposi[45]
$Tbulk[46] $q[46] $h[46] $Xposi[46]
$Tbulk[47] $q[47] $h[47] $Xposi[47]
$Tbulk[48] $q[48] $h[48] $Xposi[48]
$Tbulk[49] $q[49] $h[49] $Xposi[49]
$Tbulk[50] $q[50] $h[50] $Xposi[50]
$Tbulk[51] $q[51] $h[51] $Xposi[51]
$Tbulk[52] $q[52] $h[52] $Xposi[52]
$Tbulk[53] $q[53] $h[53] $Xposi[53]
$Tbulk[54] $q[54] $h[54] $Xposi[54]
$Tbulk[55] $q[55] $h[55] $Xposi[55]
$Tbulk[56] $q[56] $h[56] $Xposi[56]
$Tbulk[57] $q[57] $h[57] $Xposi[57]
$Tbulk[58] $q[58] $h[58] $Xposi[58]
$Tbulk[59] $q[59] $h[59] $Xposi[59]
$Tbulk[60] $q[60] $h[60] $Xposi[60]
$Tbulk[61] $q[61] $h[61] $Xposi[61]
$Tbulk[62] $q[62] $h[62] $Xposi[62]
$Tbulk[63] $q[63] $h[63] $Xposi[63]
$Tbulk[64] $q[64] $h[64] $Xposi[64]
$Tbulk[65] $q[65] $h[65] $Xposi[65]
$Tbulk[66] $q[66] $h[66] $Xposi[66]
$Tbulk[67] $q[67] $h[67] $Xposi[67]
$Tbulk[68] $q[68] $h[68] $Xposi[68]
$Tbulk[69] $q[69] $h[69] $Xposi[69]
$Tbulk[70] $q[70] $h[70] $Xposi[70]
$Tbulk[71] $q[71] $h[71] $Xposi[71]
$Tbulk[72] $q[72] $h[72] $Xposi[72]
$Tbulk[73] $q[73] $h[73] $Xposi[73]
$Tbulk[74] $q[74] $h[74] $Xposi[74]
$Tbulk[75] $q[75] $h[75] $Xposi[75]
$Tbulk[76] $q[76] $h[76] $Xposi[76]
$Tbulk[77] $q[77] $h[77] $Xposi[77]
$Tbulk[78] $q[78] $h[78] $Xposi[78]
$Tbulk[79] $q[79] $h[79] $Xposi[79]
$Tbulk[80] $q[80] $h[80] $Xposi[80]
$Tbulk[81] $q[81] $h[81] $Xposi[81]
$Tbulk[82] $q[82] $h[82] $Xposi[82]
$Tbulk[83] $q[83] $h[83] $Xposi[83]
$Tbulk[84] $q[84] $h[84] $Xposi[84]
$Tbulk[85] $q[85] $h[85] $Xposi[85]
$Tbulk[86] $q[86] $h[86] $Xposi[86]
$Tbulk[87] $q[87] $h[87] $Xposi[87]
$Tbulk[88] $q[88] $h[88] $Xposi[88]
$Tbulk[89] $q[89] $h[89] $Xposi[89]
$Tbulk[90] $q[90] $h[90] $Xposi[90]
$Tbulk[91] $q[91] $h[91] $Xposi[91]
$Tbulk[92] $q[92] $h[92] $Xposi[92]
$Tbulk[93] $q[93] $h[93] $Xposi[93]
$Tbulk[94] $q[94] $h[94] $Xposi[94]
$Tbulk[95] $q[95] $h[95] $Xposi[95]
$Tbulk[96] $q[96] $h[96] $Xposi[96]
$Tbulk[97] $q[97] $h[97] $Xposi[97]
$Tbulk[98] $q[98] $h[98] $Xposi[98]
$Tbulk[99] $q[99] $h[99] $Xposi[99]
$Tbulk[100] $q[100] $h[100] $Xposi[100]

$Yposi[1] $Tadm[1]
$Yposi[2] $Tadm[2]
$Yposi[3] $Tadm[3]
$Yposi[4] $Tadm[4]
$Yposi[5] $Tadm[5]
$Yposi[6] $Tadm[6]
$Yposi[7] $Tadm[7]
$Yposi[8] $Tadm[8]
$Yposi[9] $Tadm[9]
$Yposi[10] $Tadm[10]
$Yposi[11] $Tadm[11]
$Yposi[12] $Tadm[12]
$Yposi[13] $Tadm[13]
$Yposi[14] $Tadm[14]
$Yposi[15] $Tadm[15]
$Yposi[16] $Tadm[16]
$Yposi[17] $Tadm[17]
$Yposi[18] $Tadm[18]
$Yposi[19] $Tadm[19]
$Yposi[20] $Tadm[20]
$Yposi[21] $Tadm[21]
$Yposi[22] $Tadm[22]
$Yposi[23] $Tadm[23]
$Yposi[24] $Tadm[24]
$Yposi[25] $Tadm[25]
$Yposi[26] $Tadm[26]
$Yposi[27] $Tadm[27]
$Yposi[28] $Tadm[28]
$Yposi[29] $Tadm[29]
$Yposi[30] $Tadm[30]
$Yposi[31] $Tadm[31]
$Yposi[32] $Tadm[32]
$Yposi[33] $Tadm[33]
$Yposi[34] $Tadm[34]
$Yposi[35] $Tadm[35]
$Yposi[36] $Tadm[36]
$Yposi[37] $Tadm[37]
$Yposi[38] $Tadm[38]
$Yposi[39] $Tadm[39]
$Yposi[40] $Tadm[40]

[gaza]

Originally Posted by Gargioni

!$Tw=298.15;
!$Zposi = 0.4;
!$TBulkZ = $Zposi*20;
!$a=0;

!for ($i=1; $i<89; $i++) {
!$a=$a+0.005;
!$Xposi[$i] = $a;
!}


!for ($i=1; $i<89; $i++) {
PLANE: Teste.$i
Option = XY Plane
Z = $Xposi[$i]
Colour Mode = Variable
Colour Variable = Temperature
Range = Local
END

!$Tbulk[$i] = massFlowAve("Temperature","Teste.$i");
!$q[$i] = massFlowAve("Wall Heat Flux","Teste.$i");
!$h[$i]=($q[$i]/($Tw-$Tbulk[$i]));
!}


!$b=0;
!for ($i=1; $i<41; $i++) {
!$b=$b+0.0001875;
!$Yposi[$i] = $b;
!}


!for ($i=1; $i<41; $i++) {
POINT: Point.$i
Option = XYZ
Point = 0.0, $Yposi[$i], $Zposi
Colour Mode = Variable
Colour Variable = Temperature
Range = Global
END
!$T[$i] = maxVal("Temperature","Point.$i");
!$Tadm[$i] = ($Tw-$T[$i])/($Tw-$Tbulk[$TBulkZ])
!}


$Tbulk[1] $q[1] $h[1] $Xposi[1]
$Tbulk[2] $q[2] $h[2] $Xposi[2]
$Tbulk[3] $q[3] $h[3] $Xposi[3]
$Tbulk[4] $q[4] $h[4] $Xposi[4]
$Tbulk[5] $q[5] $h[5] $Xposi[5]
$Tbulk[6] $q[6] $h[6] $Xposi[6]
$Tbulk[7] $q[7] $h[7] $Xposi[7]
$Tbulk[8] $q[8] $h[8] $Xposi[8]
$Tbulk[9] $q[9] $h[9] $Xposi[9]
$Tbulk[10] $q[10] $h[10] $Xposi[10]
$Tbulk[11] $q[11] $h[11] $Xposi[11]
$Tbulk[12] $q[12] $h[12] $Xposi[12]
$Tbulk[13] $q[13] $h[13] $Xposi[13]
$Tbulk[14] $q[14] $h[14] $Xposi[14]
$Tbulk[15] $q[15] $h[15] $Xposi[15]
$Tbulk[16] $q[16] $h[16] $Xposi[16]
$Tbulk[17] $q[17] $h[17] $Xposi[17]
$Tbulk[18] $q[18] $h[18] $Xposi[18]
$Tbulk[19] $q[19] $h[19] $Xposi[19]
$Tbulk[20] $q[20] $h[20] $Xposi[20]
$Tbulk[21] $q[21] $h[21] $Xposi[21]
$Tbulk[22] $q[22] $h[22] $Xposi[22]
$Tbulk[23] $q[23] $h[23] $Xposi[23]
$Tbulk[24] $q[24] $h[24] $Xposi[24]
$Tbulk[25] $q[25] $h[25] $Xposi[25]
$Tbulk[26] $q[26] $h[26] $Xposi[26]
$Tbulk[27] $q[27] $h[27] $Xposi[27]
$Tbulk[28] $q[28] $h[28] $Xposi[28]
$Tbulk[29] $q[29] $h[29] $Xposi[29]
$Tbulk[30] $q[30] $h[30] $Xposi[30]
$Tbulk[31] $q[31] $h[31] $Xposi[31]
$Tbulk[32] $q[32] $h[32] $Xposi[32]
$Tbulk[33] $q[33] $h[33] $Xposi[33]
$Tbulk[34] $q[34] $h[34] $Xposi[34]
$Tbulk[35] $q[35] $h[35] $Xposi[35]
$Tbulk[36] $q[36] $h[36] $Xposi[36]
$Tbulk[37] $q[37] $h[37] $Xposi[37]
$Tbulk[38] $q[38] $h[38] $Xposi[38]
$Tbulk[39] $q[39] $h[39] $Xposi[39]
$Tbulk[40] $q[40] $h[40] $Xposi[40]
$Tbulk[41] $q[41] $h[41] $Xposi[41]
$Tbulk[42] $q[42] $h[42] $Xposi[42]
$Tbulk[43] $q[43] $h[43] $Xposi[43]
$Tbulk[44] $q[44] $h[44] $Xposi[44]
$Tbulk[45] $q[45] $h[45] $Xposi[45]
$Tbulk[46] $q[46] $h[46] $Xposi[46]
$Tbulk[47] $q[47] $h[47] $Xposi[47]
$Tbulk[48] $q[48] $h[48] $Xposi[48]
$Tbulk[49] $q[49] $h[49] $Xposi[49]
$Tbulk[50] $q[50] $h[50] $Xposi[50]
$Tbulk[51] $q[51] $h[51] $Xposi[51]
$Tbulk[52] $q[52] $h[52] $Xposi[52]
$Tbulk[53] $q[53] $h[53] $Xposi[53]
$Tbulk[54] $q[54] $h[54] $Xposi[54]
$Tbulk[55] $q[55] $h[55] $Xposi[55]
$Tbulk[56] $q[56] $h[56] $Xposi[56]
$Tbulk[57] $q[57] $h[57] $Xposi[57]
$Tbulk[58] $q[58] $h[58] $Xposi[58]
$Tbulk[59] $q[59] $h[59] $Xposi[59]
$Tbulk[60] $q[60] $h[60] $Xposi[60]
$Tbulk[61] $q[61] $h[61] $Xposi[61]
$Tbulk[62] $q[62] $h[62] $Xposi[62]
$Tbulk[63] $q[63] $h[63] $Xposi[63]
$Tbulk[64] $q[64] $h[64] $Xposi[64]
$Tbulk[65] $q[65] $h[65] $Xposi[65]
$Tbulk[66] $q[66] $h[66] $Xposi[66]
$Tbulk[67] $q[67] $h[67] $Xposi[67]
$Tbulk[68] $q[68] $h[68] $Xposi[68]
$Tbulk[69] $q[69] $h[69] $Xposi[69]
$Tbulk[70] $q[70] $h[70] $Xposi[70]
$Tbulk[71] $q[71] $h[71] $Xposi[71]
$Tbulk[72] $q[72] $h[72] $Xposi[72]
$Tbulk[73] $q[73] $h[73] $Xposi[73]
$Tbulk[74] $q[74] $h[74] $Xposi[74]
$Tbulk[75] $q[75] $h[75] $Xposi[75]
$Tbulk[76] $q[76] $h[76] $Xposi[76]
$Tbulk[77] $q[77] $h[77] $Xposi[77]
$Tbulk[78] $q[78] $h[78] $Xposi[78]
$Tbulk[79] $q[79] $h[79] $Xposi[79]
$Tbulk[80] $q[80] $h[80] $Xposi[80]
$Tbulk[81] $q[81] $h[81] $Xposi[81]
$Tbulk[82] $q[82] $h[82] $Xposi[82]
$Tbulk[83] $q[83] $h[83] $Xposi[83]
$Tbulk[84] $q[84] $h[84] $Xposi[84]
$Tbulk[85] $q[85] $h[85] $Xposi[85]
$Tbulk[86] $q[86] $h[86] $Xposi[86]
$Tbulk[87] $q[87] $h[87] $Xposi[87]
$Tbulk[88] $q[88] $h[88] $Xposi[88]
$Tbulk[89] $q[89] $h[89] $Xposi[89]
$Tbulk[90] $q[90] $h[90] $Xposi[90]
$Tbulk[91] $q[91] $h[91] $Xposi[91]
$Tbulk[92] $q[92] $h[92] $Xposi[92]
$Tbulk[93] $q[93] $h[93] $Xposi[93]
$Tbulk[94] $q[94] $h[94] $Xposi[94]
$Tbulk[95] $q[95] $h[95] $Xposi[95]
$Tbulk[96] $q[96] $h[96] $Xposi[96]
$Tbulk[97] $q[97] $h[97] $Xposi[97]
$Tbulk[98] $q[98] $h[98] $Xposi[98]
$Tbulk[99] $q[99] $h[99] $Xposi[99]
$Tbulk[100] $q[100] $h[100] $Xposi[100]

$Yposi[1] $Tadm[1]
$Yposi[2] $Tadm[2]
$Yposi[3] $Tadm[3]
$Yposi[4] $Tadm[4]
$Yposi[5] $Tadm[5]
$Yposi[6] $Tadm[6]
$Yposi[7] $Tadm[7]
$Yposi[8] $Tadm[8]
$Yposi[9] $Tadm[9]
$Yposi[10] $Tadm[10]
$Yposi[11] $Tadm[11]
$Yposi[12] $Tadm[12]
$Yposi[13] $Tadm[13]
$Yposi[14] $Tadm[14]
$Yposi[15] $Tadm[15]
$Yposi[16] $Tadm[16]
$Yposi[17] $Tadm[17]
$Yposi[18] $Tadm[18]
$Yposi[19] $Tadm[19]
$Yposi[20] $Tadm[20]
$Yposi[21] $Tadm[21]
$Yposi[22] $Tadm[22]
$Yposi[23] $Tadm[23]
$Yposi[24] $Tadm[24]
$Yposi[25] $Tadm[25]
$Yposi[26] $Tadm[26]
$Yposi[27] $Tadm[27]
$Yposi[28] $Tadm[28]
$Yposi[29] $Tadm[29]
$Yposi[30] $Tadm[30]
$Yposi[31] $Tadm[31]
$Yposi[32] $Tadm[32]
$Yposi[33] $Tadm[33]
$Yposi[34] $Tadm[34]
$Yposi[35] $Tadm[35]
$Yposi[36] $Tadm[36]
$Yposi[37] $Tadm[37]
$Yposi[38] $Tadm[38]
$Yposi[39] $Tadm[39]
$Yposi[40] $Tadm[40]

Hi

How did you implement this code into CFX?

[gaza]

Ok I found that it is in Reference guide in Power Syntax chapter