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HTC Calculations in a Helical Coil

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Old   February 3, 2015, 09:00
Default HTC Calculations in a Helical Coil
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Thakar
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I have performed hand calculations of HTC and Outlet Temperature in a Helical coil using equations from Heat Atlas(Gnielinski) and have obtained HTC:4739 W/m2k and T_outlet:52C.I am now trying to validate these results with ANSYS CFX.I have read all the posts related to this query and yet left with some questions.

My V_inlet:0.5m/s,T_inlet:10 C,T_wall:100 C,Tbulk for HTC:304K with Laminar flow as the Reynold number is less than the critical in the hand calculations.

My results are varying hugely with Inflation layers and its Growth rate(Eg: 2794 W/m2K and 37 C for 6 layers inflation to 3498 W/m2K and 45 C for 10 layers and 1,4 growth rate for both) and unable to get close to the results abtained from hand calculations.I find these results surprising becase it a laminar flow with Reynolds number:6800.Am I missing anything else?Thanks in advance

Sorry for so many questions...I am a beginner with CFD and CFX.
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Old   February 3, 2015, 13:13
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HTC calculations are based on a temperature difference between the wall and a reference value.

What reference value are you using for CFX ? Read output file for warning message regarding this issue.

How does the reference value used for CFX compares with the used in the hand calculations ?

HTC calculations (by hand or any software) are useless w/o the complementary definition of the reference temperature (named bulk temperature, free stream, etc). The only quantity that is unchanged is the heat flux since it should always be computed as:

heat flux = HTC_for_Tref * (T_wall - T_ref)..

In CFX,

heat_flux = HTC_for_WAT * (T_wall - Wall Adjacent Temperature)

Hope the above helps,
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Old   February 4, 2015, 05:46
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I have accidentally double posted the same question.

From the previous posts reg this topic I understood that by entering an expert parameter(tbulk for HTC) I can force CFX to take a Temp_ref for calculating HTC otherwise it might take just take the wall Adj temperature.I have chosen my tbulk for HTC as the average of Inlet-Outlet that I have obtained from my hand calculations.Therefore T_Delta is (T_wall-Tbulk for HTC).So my HTC will change only with Heat Flux now as T_Delta is constant.

For my hand calculations (Iterations with approx outlet) I use T_Delta as ((T_wall-T_inlet)-(T_wall-T_approx outlet))/ln((T_wall-T_inlet)/(T_wall-T_approx outlet)).

Please correct me If I am making a mistake.
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