Is Nusselt number a constant?
 

hi,everyone:All books on heat and mass transfer tell me the Nusselt number of the fully-developed laminar flow in a pipes is a constant;if the wall temperature is constant,the Nu=3.66;if the heat flux is constant,the Nu will be 4.36. firstly, I use the the periodic flow model to simulate the flow ,and the Nusselt is NOT a constant!when the wall temperture or the heat flux rise, the Nu will enlarge!!! who can tell me what should I do ? Anyone did this?

thanks

zhenhai

Re: Is Nusselt number a constant?
 

Hi Zhenhai

I would think that the Nu is primarily dependant on whether the flow is forced or free. If it is free, the Nu is highly dependant on the temperature difference, but if it is strongly forced, the Nu is almost constant (and given from the flow conditions).

If you want to make an analytically estimation, I would find a correlation that fits your conditions as good as possible, and the carry out the calculation.

If you want to do cfd calculations, why do you want to estimate the Nu. Can't you calculate it from the given case?

Cheers Christian

Re: Is Nusselt number a constant?
 

Cheers Christian

first of all,thank you very much!

I want to simulate some kind of heat transfer,and I want to find out the correlation between the Nu,Re and Pr.Just as you mentioned in the topic,the the Nusselt number will be constant when the condition is forced flow.But when I change the wall temperature or the heat flux with the same construction and fluid,the Nu will change,the condition is laminar flow.

So I use the simplest case,a pipe with laminar flow[forced flow],to ensure that the Nu is constant.But the result let me down, the Nusselt number will change and is Not the analysis result when I change the wall temperature or the heat flux through the wall.I have no idea about it.

Can u test it on your computer? You can email me : metry@21cn.com

Zhenhai

Re: Is Nusselt number a constant?
 

If you want to do heat transfer properly in Fluent, make sure you have the correct y+ values in combination with the chosen near-wall treatment. If your y+ values are too large or too small the heat transfer prediction will become very inaccurate.

Re: Is Nusselt number a constant?
 

yes, I do agree with Mr.Erwin

it is necessary to make sure that you choose the correct y+ in combination with near-wall boundary condition. I am simulating a fully developed turbulent flow through a pipe with a periodic length. I assume that the obtained values of Nusselt Number, Pressure Loss etc, from Fluent are said to be reliable with in a range of Y+ 30 to 60. You can also check it with in this range.

regards madasu.

Re: Is Nusselt number a constant?
 

Thank all of you,I will check the y+ value.

Re: Is Nusselt number a constant?
 

Fluent calculates heat transfer coefficient and Nusselt number based on reference values that you enter under Report->Reference Values... I have made the mistake before of not changing these values, and seen results that look terribly wrong.

Re: Is Nusselt number a constant?
 

Hi,Mr. keith: when I change the reference values,the nusselt will change,but what's the reference values?what's the correct values of those? the bulk value of the fluid ?

Re: Is Nusselt number a constant?
 

Hi,Mr. madasu

the y+ is available when the turbulent model is used.But now I am simulating the laminar flow,what should I do?

zhhuang

Re: Is Nusselt number a constant?
 

Hi,Mr.madasu

I changed the y+, the min is 30 and the max is 60 but the result is not constant,I get the Nusselt number by select report>surface integral...>the surface nusselt.Is that right?

Re: Is Nusselt number a constant?
 

...keith, Exactly right.....i had the same experience....

in the begining, I didnot change the reference values accordingly and I found very strange resulting values...i think, zhhuang, you can now check it once.

In my case, I am simulating turbulent flow through a Pipe at a temperature of 1273 k,.......... so I use this value at the Temperature Cell and.................. I have replaced the Length cell with the value of the diameter of the Pipe Geometry. I Hope it is not confusing for you.......... And I left all the other cells in the Reference Values menu as default and at the end I selected the refence zone as Fluid. And it worked out and I got good results.

try for your case...

regards madasu

Re: Is Nusselt number a constant?
 

thank you very much,madasu: I will check this,can you mail me ,my E-mail: metry@21cn.com

regards zhhuang

Re: Is Nusselt number a constant?
 

hi madasu: 1/ I checked the reference values [I fill the cell just like you, the wall temperature as the referance temperture and the diameter as the length,but the result is not right ],can you mail me your case? 2/when I use the constant wall temperuature as the boundary condition of wall,Fluent will display this hint "temperature limited to 1.000000e+00 in 103500 cells on zone 2 Done",what should I do?

regards zhhhuang

Re: Is Nusselt number a constant?
 

Dear sir I have a problem with this title. If it's possible for you please send me your received responses. Very thanks for your attention to my request. With the best regards. Mahdi saniee nezhad

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
.

Please refer to my post in this thread, if you still have this problem:

http://www.cfd-online.com/Forums/cd-...inar-flow.html