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Nusselt number calculation in Fluent

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Old   September 7, 2012, 19:17
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Negin Nazarian
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well...

Im trying to simulate a matrix of wall-mounted cubes (representing buildings in urban area) and I have a streamwise periodic boundary condition.

As to account for effect of solar load in urban temperature distribution, Im using solar calculator with Do radiation model. Im also accounting for buoyancy and used boussinesq approximation.

The most annoying problem that I have is with conduction heat transfer. Fluent dont have the option to show this flux but I used custom field function and the value is relatively small.
....
So technically, Im interested in having all heat transfer terms separately.
I believe the value of heat transfer flux in fluent is accounting for radiative and convective terms and the heat transfer coefficient is not only convective.

anyway, tnx for the help.

Cheers
Negin
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Old   September 8, 2012, 12:18
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Mohammad Zakerzadeh
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Negin...

You are right... it computes all types of heat transfer.
I suggest you to have a look at Fluent User Guide... it somehow show you what is the used equation in the background, basically I mean this section:
31.4 Alphabetical Listing of Field Variables and Their Definitions in Fluent 6.2.
...
I hope it works, it helped me when I was struggling with the definition of y+ in a turbulent case and Nusselt number when I wanted to show it to the TA!

Mohammad
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Old   September 9, 2012, 15:41
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( i make the 2d model of shell and tube heat exchanger and mesh it but when select heat exchanger model in fluent it give error that primary and auxiliary regions are not overlapping ..... so plz help me what i do .... i'm new to use ansys plz give reply on samiullahh09me08.muet@gmail.com
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Old   March 26, 2013, 23:26
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Ravi Sharma
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Hi Mohammad,
I am trying to simulate for the Nusselt number for a phase change material in a 2-d square cavity (left is hot wall and right one is the cold). can you please help me how to do that..
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Old   March 27, 2013, 01:38
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Mohammad Zakerzadeh
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Quote:
Originally Posted by ravipvb View Post
Hi Mohammad,
I am trying to simulate for the Nusselt number for a phase change material in a 2-d square cavity (left is hot wall and right one is the cold). can you please help me how to do that..
Of course, I have not worked FLUENT for a long time, but I think I remember somethings ..
Could you explain more about your case and definition of the Nusselt number there (how it is defined) ?
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Old   March 27, 2013, 01:46
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Ravi Sharma
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Thanks Mohammad for reply.
My case is like..its a 2-D square cavity filled with water and then I am diffusing some percentage of copper nanoparticles in it. I want to plot the values of Nusselt number against the various values of Gr number (1e3-1e5) and solid fraction of nanoparticles. In Ansys Fluent how to do that, I don't know.
Thanks
Ravi
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Old   May 29, 2014, 06:03
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in my case ,im modelling parallel pipe flow heat exchanger ,i need to plot local nusselt number vs reynolds number and need to find average nusselt number ? as you said , i can get surface heat transfer coefficient and multiply with L/K , but ive got different answer than stated by surface nusselt number (in surface integrals ) ?? help me out
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Old   May 31, 2014, 05:20
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shenyan
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Quote:
Originally Posted by Sharadkumar Yeri
;151656
Dear all, I am simulating flow and heat transfer in a rectangular duct with some kind of roughness.I need to calculate Nusselt number at the heating surface.The heat transfer is forced convection. I tried to calculate Nusselt number using Report>Surface integrals>Surface nusselt number

But the Nusselt number I am getting is absurd.Is there any way to calculate Nusselt number?

Thanks with regards

Sharad
you can take mean temprature of fluid and the wall tem. in this manual:report>>area-weighted average>>temprature and select those faces& interiors,then calculate h from:q"/(Tw-Tf) & ofter that calculate Nusselt No.
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Old   August 24, 2014, 05:00
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Hi
i want to calculate average nusselt number around the sphere
I red thermal conductivity and others properties of water in 293.15 k
And surface temperature of sphere is 400 k
I used RNG k-E method to solved this problem in ansys fluent 15 my problem condition is steady
Wall is symmetry . Outlet is presser outlet .inlet is by 293.15 temperature and sphere defined as wall by 400k constant temperature
Reference length value is set as particle diameter
My question is what is temperature reference value?
Please help me
When I set it in 293.15
My computational nusselt number for Reynolds between 0.6617 to 661.7 is very accurate but in 6617 a little non accurate and in high Reynolds its non accurate
What's my mistake?
What's the accurate temperature reference value?
Please help me and if it possible completely explain my mistake
Thanks a lot
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Old   September 1, 2014, 09:04
Default Calculating nusselt numbers if fluent
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faraz
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hi buddies
i have modeled a square box in fluent for natural convection at different Ra number,a square that have 4 walls that 2 of them are adiabatic(top and bottom wall) and one of them has high temp(left wall) and the other is cold(right wall),i have solved it and all of my contours are correct,that shows my solution is correct,I have problem for calculating surface nusselt nember,the surface nusselt number that fluent shows is incorrect,can any body help me to calculate the correct nusselt number for surface?

best regards
Thanks

Last edited by faraz92; September 21, 2014 at 17:20.
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Old   November 23, 2014, 02:18
Default Average nusselt number calculation in fluent
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Reli John
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Hi

My problem is to calculate the average Nusselt number and friction factor in fluent for a solar air heater with smooth wall rectangular duct in which heat flux is applied on one side of the duct, whereas the other 3 sides are insulated ..The problem was solved in 2-d.and the flow is fully developed..I calculated the heat transfer coefficient using h=qw(Tw-Tm), where qw is the heat flux applied, Tw is the average wall temperature, Tm is the average of mean bulk temperature calculated at different locations..But my results are not matching with the Dittus-Boelter-equation....

Can anybody reply what is the correct way to calculate the average Nu???

Thanks with regards

Reli John
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Old   November 23, 2014, 05:18
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Negin Nazarian
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Hey just checking.
Is there a typo that u missed the division sign? "/"
H=qw/(tw-tm)
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Old   November 23, 2014, 06:32
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Reli John
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The division sign is included...it was a typing error
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Old   November 23, 2014, 06:51
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Negin Nazarian
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Can you explain your problem and settings in more details?
What are the boundary conditions/models used?
How do you calculate Tm? and so fort!
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Old   November 23, 2014, 07:39
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Reli John
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To simulate the flow, I considered a 2-d rectangular domain...At one side of the domain(i.e. at the top), is placed an Aluminium plate of 0.5 mm thickness...To calculate Tm, I constructed two vertical lines, one at the inlet and the other at the outlet..den calculated the mass weighted temperature(from surface report) at each of these two lines, then took the average of these two to calculate the Tm.....To calculate Tw, I plotted Temp vs x at the interface of fluid(air) and solid(Al plate), wrote down the values in an excel file and calculated the average of these values to obtain Tw.....But still I am getting average Nusselt number value much higher than that of Dittus-Boelter eqn......
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Old   November 23, 2014, 07:41
Default Nusselt number calculation in fluent
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The boundary calculations are velocity-inlet with temp 300K at inlet, pressure outlet at the outlet, constant heat flux at the top of Al plate, coupled boundary condition at the interface of air and Al, and the remaining boundaries are insulated....
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Old   June 5, 2015, 05:08
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DENG Weriping
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Quote:
Originally Posted by Rakesh Jha
;151721
Sharad I dont know in what way you are trying but here is what I would do: (i)Create a line (iso-surface of grid at the mid of the wall) (ii) Plot-> XY -> Wall fluxes -> Total surface heat flux (assuming radiation is not on) as Y-co-ordinate and channel length as x-co-ordinate. (q). write this!

(iii) craete another line at the centerline of the channel. (iv) Tcenterline, extract teperature vs channel length as in step (ii). (Write this!)

(v) What wall thermal Bc you have, if it is const heat flux then extract wall temperature vs channel length (you can use the same line this time which was created for heat flux). if the wall Bc const wall temp then no need to do this step.

(vi) Now obtain h = q/(Twall-Tcenterline). then Nu = h*D/k

(vii) if you have fluid with varying properties 9say incomressible ideal gas) then be careful of talking that into count wh8ile doing step (vi).

I think Nu calculated in this manner can be comparable to literature (I guess your case matches closely with Dittus-Boelter correlation).

Please note that your centerline temperature is chaning with the cordintae along the channel length. so fixing the reference temperature (in fluent) may not serve the purpose. All the best!
ur way is right? the heat flux along a line is really right?
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