Nusselt number calculation in Fluent
 

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

Re: Nusselt number calculation in Fluent
 

Use a custom field function calling h for use in hD/k. h can be surface based or wall function based.

I suspect default h values for Nu call surface based h and use the reference temperature in the reference values panel so you may need to only adjust this. It may also by default use the reference length for D in the calc of Nu which may also need adjusting from the default (defined in same panel as the ref temperature).

Re: Nusselt number calculation in Fluent
 

Hi Red Lemon You mean to say,if I need to calculate average Nu of the duct,note down Area weighted average of Surface heat transfer coefficient of the face of which I need Nu and just multiply by D/k ?

Thanks

Re: Nusselt number calculation in Fluent
 

Well I was meaning so you could plot Nu over the surface as a contour but if you want a single value as an average for the wall zone then just take the average h value from report surface integral and multiply by D/k but make sure reference values are correct for h(avg) calculation. D is the hydraulic diameter for internal flows but use L for external.

Re: Nusselt number calculation in Fluent
 

Hi Red Lemon Thanks for the information.I have not changed any reference value.Its default setting.Do I need to change reference values and run the model again?

Thanks

Re: Nusselt number calculation in Fluent
 

no, reference values are just for post processing so if you plot htc's using one reference value then change the reference value then plot again the results will be different as its just a post calc operation and no extra iterations are required.

You need to select an appropriate reference length scale (for Nu) and reference temperature (for h).

Re: Nusselt number calculation in Fluent
 

Hi Thanks for you quick response. You mean to say, in Reference values panel,I need to substitute values for

Area= Surface area of heat transfer for which Nu to be calculated

Length=Hydraulic diameter

Temperature= Temperature of air at inlet

Velocity= free stream velocity

Since I have not used Reference values, I have these doubt. Your help is highly appreciated.

Thanks with regards

Sharad

Re: Nusselt number calculation in Fluent
 

Correct. Be careful that for reference temp you could use inlet temp, outlet temp, bulk mean fluid temp, near wall fluid temp etc so htc values are quite arbitrary and will give different values depending on what ref temp you use. This is why I prefer to plot fluxes (Q) instead.

Use the same reference temp as used in the calculation of h for your experiements if you have xp data. I think you only need the first 3. reference velocity is used for lift and drag.

Re: Nusselt number calculation in Fluent
 

Thanks a lot for your so valuable and quick response.

Thanks with regards

Sharadkumar

Re: Nusselt number calculation in Fluent
 

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!

Re: Nusselt number calculation in Fluent
 

I think Sharad is looking for an avg surface Nu value rather along a line mid wall though this process is good to demonstrate decaying htc or Nu along the surface in the streamwise direction as it accounts for varying gas temp as well as wall temperature. As I mentioned, you need to concur with the method for which the data was reduced for the experiment. Note in this case Tref=Tcentreline but Tref could be near wall gas temp or bulk fluid mean temp etc.

Re: Nusselt number calculation in Fluent
 

Hi Rakesh Thanks for the reply. I have done almost same as you have mentioned. My flow is incompressible and with constant heat flux.I needed to shown variation of Nu along the length of plate. So I created a line at the mid plane at the heated surface. This will have temperature values. I calculated mean temperature of fluid in the duct. In FLUENT is reference value, I changed area of HT, Length as Hyd dia. and temperature as mean temperature of the fluid. Then I can use this Nu from Fluent for my analysis.I also needed avarage Nu for the duct for particular flow rate.So this methods served both purpose.

As you have mentioned in step (iv), use of center line temperature for calculation of Nu. I guess this does not suit to my problem.

Comments are highly appreciated.

With regards

Sharad

Hi guys!

My case is very simple , its laminar flow in a pipe with constant wall temperature. I take the length as long as the flow can reach to fully thermally and Hydrodunamically developed. When I get the Nusselt Number from the xyplot/Surface Nusselt Number it can seemed that the Nu begins from a large number(as predicted by theory) and go toward zero at the end of pipe (the theoric value is 3.66). Can anybody help me?
I can send my case to you for more information.

Thanks alot
.

hello
 

hi
r u used turbulent folw ? and r u used k omeg or k epslon ?

hello
 

i want simulate heat transfer in sudden expansion pipe and calculate surface nusselt numbr

turbulent flow .
can u help me wich model to my case k omega sst or k epslon relizable

Hello bro,

I am trying to find NUSSELT NUMBER around a cylinder. I have constant heat flux at the cylinder surface.

I am doing XY-Plots>Wall Flux>Surface Nusselt Number to find the nusselt number.
My results are not good.

Can you please advise me how to do this? or any good reference to do this? Your help would really appreciated!

Thanks

heat transfer coefficient error
 

Hey, i was doing a problem in fluent on a bifurcating t-junction with a heated bottom plate of the side branch. On doing heat transfer upon this geometry i am facing a problem that the results for surface heat transfer coefficient plot for heated plate which i get under wall fluxes is coming out to be different from that when i calculate it like this:
h=q/(Tw-Tf)
where q is total surface heat flux
Tw is the heated wall temperature
Tf is the fluid temperature

q was also taken from the plot of total surface heat flux for heated plate under wall fluxes.flow is incompressible.

plzz help mee out

Fluent (at least up to version 6.3. I have worked on) has a bug on heat transfer post processing calculation.
In Nusselt number calculation we usually use T*m (instead of Tf... it refers to what? No idea!), which is the average temperature weighted by mass flow rate (c.f. Incropera book for example), but fluent just uses some constant values defined in the reference value (in the report tab).
I think it is better to define some function to calculate T*m in Fluent and then calculate and define additional function for Nusselt calculation based on it.
Also it is better to take a look at the reference values in Fluent, maybe it helps.

Good luck
Mohammad

Hi Mohammad

I found your previous post to be very useful since I myself am suffering from heat transfer coef and Nu# calculation.

My question right now is about Total surface heat flux:
does it account for all Radiation/Convection/conduction?

I am accounting for all of these in my case but Im confused on how to check the heat balance using fluent values.

I would appreciate your help.

Cheers
Negin

Hi Negin...

Happy to hear that it helps...
Regarding your question, I am not quite sure I got what you exactly mean by heat balancing in Fluent, it is better if you can explain more about your case, its BCs and the values given by Fluent in post processing.
(And also the reference values.... have you checked them?)

About total heat transfer I suppose that it should consider all effects as you assumed.

Mohammad

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

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

( 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

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) ?

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

srini
 

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

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.

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

Calculating nusselt numbers if fluent
 

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

Average nusselt number calculation in fluent
 

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

Hey just checking.
Is there a typo that u missed the division sign? "/"
H=qw/(tw-tm)

The division sign is included...it was a typing error

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!

Nusselt number calculation in fluent
 

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......

Nusselt number calculation in fluent
 

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....

ur way is right? the heat flux along a line is really right?

nusselt number calculation
 

how to calculate avergae nusselt in transient natural convection for different time step

how to determine nusselt number in fluent
 

i am doing cfd analysis of shell and tube heat exchanger with boundary condition of mass flow rate and inlet and outlet temp of respective hot and cold fluid with outlet temp of hot gas unknown.I havnt apply any wall flux. Now when i am checking for surface nusselt number am getting absurd value as high as 712 .can anyone help me how to get the actual theoritical value of 20 and so..

My nusselt number becomes zero all over the domain. What should I do?!
The case is a 2D u-pipe with a constant heat flux from walls. The temperature seems to be OK i guess but Nu=0 for every node.