Nu in CFX

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 June 13, 2017, 10:31 Nu in CFX #1 New Member   Ali karimi Join Date: May 2017 Posts: 11 Rep Power: 2 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. __________________ Life is fluid...

 June 13, 2017, 12:18 #2 Senior Member   Join Date: Jun 2009 Posts: 701 Rep Power: 15 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 Ali_91en likes this.

June 13, 2017, 13:48
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Ali karimi
Join Date: May 2017
Posts: 11
Rep Power: 2
Quote:
 Originally Posted by Opaque 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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 June 13, 2017, 16:16 #4 Senior Member   Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 694 Rep Power: 13 Check the FAQs https://www.cfd-online.com/Wiki/Ansy...ficient_in_CFX Ali_91en likes this.

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