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-   -   Nusselt number calculation in Fluent (http://www.cfd-online.com/Forums/fluent/48553-nusselt-number-calculation-fluent.html)

 Sharadkumar Yeri June 20, 2008 10:15

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

 red lemon June 20, 2008 10:25

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).

 Sharadkumar Yeri June 20, 2008 10:43

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

 red lemon June 20, 2008 11:45

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.

 Sharadkumar Yeri June 20, 2008 12:04

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

 red lemon June 20, 2008 12:18

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).

 Sharadkumar Yeri June 20, 2008 13:34

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

 red lemon June 20, 2008 13:45

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.

 Sharadkumar Yeri June 20, 2008 15:01

Re: Nusselt number calculation in Fluent

Thanks a lot for your so valuable and quick response.

Thanks with regards

 Rakesh Jha June 23, 2008 06:24

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!

 red lemon June 23, 2008 14:26

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.

 Sharadkumar Yeri June 23, 2008 14:29

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.

With regards

 moh1367 February 14, 2010 05:09

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?

Thanks alot
.

 hassan79 August 8, 2010 06:27

hello

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

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

 hassan79 August 8, 2010 06:32

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

Quote:
 Originally Posted by hassan79 (Post 270699) hi r u used turbulent folw ? and r u used k omeg or k epslon ?

 mskamran October 14, 2010 06:48

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

 abc August 28, 2012 01:46

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

 moh1367 August 28, 2012 02:06

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

Quote:
 Originally Posted by abc (Post 379018) 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

 nenazarian September 7, 2012 16:45

Quote:
 Originally Posted by moh1367 (Post 379023) 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.

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.

Cheers
Negin

 moh1367 September 7, 2012 17:14

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.