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Old   June 13, 2017, 10:31
Unhappy Nu in CFX
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Ali karimi
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hey guys!

I wanna simulate a heat exchanger with internal and external flow, water flows inside pipe and air outside of it.

my problem is I don't know how to calculate Nu number. My reference paper used functions that attached for calculating Nu number
please someone help how to calculate Nu in CFX.

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Old   June 13, 2017, 12:18
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Your question is very general.

Which type of Nu number are you interested in ? What is the purpose/goal of computing the Nu number at all ?

There are several variants of the Nu number, and each has a different formula. Are you interested in the local Nu number or the mean/average Nu to compute total heat transfer rates ?

For example, an energy balance for inner flow implies (neglecting kinetic energy for now)

Q_wall = Mass Flow * (Static Enthalpy_outlet - Static Enthalpy_inlet)

For an average Heat Transfer Coefficient

HTC_average * Area Pipes * Appropriate_Delta_T = Q_wall

Then,

Nu_average = Q_wall * Diameter/ (Area Pipes * Appropriate_Delta_T)

Still unanswered questions; therefore, more information specific to your setup is required.

Summary: it is not trivial to answer w/o specifics.

Hope the above helps somehow
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Old   June 13, 2017, 13:48
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Quote:
Originally Posted by Opaque View Post
Your question is very general.

Which type of Nu number are you interested in ? What is the purpose/goal of computing the Nu number at all ?

There are several variants of the Nu number, and each has a different formula. Are you interested in the local Nu number or the mean/average Nu to compute total heat transfer rates ?

For example, an energy balance for inner flow implies (neglecting kinetic energy for now)

Q_wall = Mass Flow * (Static Enthalpy_outlet - Static Enthalpy_inlet)

For an average Heat Transfer Coefficient

HTC_average * Area Pipes * Appropriate_Delta_T = Q_wall

Then,

Nu_average = Q_wall * Diameter/ (Area Pipes * Appropriate_Delta_T)

Still unanswered questions; therefore, more information specific to your setup is required.

Summary: it is not trivial to answer w/o specifics.

Hope the above helps somehow
Thanks buddy, I think I should compare average Nu number with my reference paper, the equation is in the pic I attached :


So I should enable HTC in expert parameters , my inlet temp is 40c , what value should I set for reference temp in HTC?

IMG_20170613_190649.jpg

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Old   June 13, 2017, 16:16
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Check the FAQs
https://www.cfd-online.com/Wiki/Ansy...ficient_in_CFX
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