Heat transfer coefficient - what is waht
 

Hello

I am solving heat flow problem between two rotating cylindrical surfaces (part of my master thesis). The gap between them is more less 0.075 (depends on case) .

As far as i know this heat flow can be modeled in analitic way by miechiejew equation (Nusslet number is function of Reynolds; Prandtl_fluid; Prandtl_wall). The Heat transfer coefficient obtained on paper is arouns 850 W/m^2/K.

In Fluent i obtain Surface heat transfer coefficient around 68 W/m^2/K. But the Wall Func. Heat transfer coefficient is 1530 W/m^2/K.

My sense tells me that is the result of different aproach of calculating heat transfer coefficient by Fluent. I tried to find anwser in help but i did'n manage to find it. Does anybody can redirect me to source where is the description how heat transfer coefficient is calculated and what is Wall Func. Heat transfer coefficient . Thank you in advice.

PS: details about solver settings: Standart k-epsilon model with viscous heating with standard wall functions.

Regards Stan

Re: Heat transfer coefficient - what is waht
 

Fluent directly calculates heat flux on the boundaries. In Fluent, heat transfer coefficient (h) is a derived quantity using reference temperature, adjacent fluid temperature and heat flux. In the actual scenario, the reference temperature should be the wall temperature. Therefore, the values reported by fluent will be an indicative values. To get the actual h values you need to write a UDF according to your need.

Re: Heat transfer coefficient - what is waht
 

Thank you very much. Now i understand how h is calculated. According tp help this is the way of calculating it in laminar flow. For turbulent flow it uses wall functions which are beyond my borders of understanding, at this momemnt.

I do not understand at all second part of your post. Anyway my question was touching the difference between two values available for postprocessing: Wall Fluxes\Surface heat transfer coefficient Wall Fluxes\Wall Func. heat transfer coefficient

Additionaly I wolud like to know how they are calculated (my thesis supervisor wishes that because difference between analitical model and numerical are to big and he do not buys that this is caused by material data).

Hi,
I have identical problem. How to find actual h values by UDF?

Best regards,
doodek

The h values reported by the fluent are wrong. It has something to do with the reference temperature which can be different for different models. Ask fluent engineer. To get the right h value you need to write a udf.

h = heat_flux / (T(z) - Tbulk(z))

Tbulk = SUM mu*cp*rho*Tf dA / SUM mu*cp*dA

Hi, following the above discussion, therefore is Fluent overestimating the h value, or underestimating? Because I have h value (surface heat transfer coefficient) of about 400, and it is natural convection

Can you describe what you are modeling, such as geometry details? Also the 400 value, is that a constant h value across your fluid domain?

It is a natural convection in an enclosure (with heating element at the center). The heating element is a square, with heat transfer coefficients at the top, left, and right surface are about 150-400 W/m^2.K.
My question is...normally surface heat transfer coefficient in Fluent will be difference by how much from reality (as in using UDF)?

Your natural convection is very high. Such high convection is only found in micro and nano channels. As far as the difference between fluent h and real h is concerned, there is no specific number. If you raise or lower your reference temperature, the difference that you are asking will change. Thats why I suggest you make planes across your fluid domain and use the formula above to get Tbulk and then h = q/T(z) - Tbulk.

Hi, I am new to UDF. So, in order to write these functions..
"h = heat_flux / (T(z) - Tbulk(z))

Tbulk = SUM mu*cp*rho*Tf dA / SUM mu*cp*dA"

how to I define these variables "mu, cp, rho"? and is this equation applicable for 2D problem?

Thank you.

Hi, everyone.

I need help,
How to get the value of convection coefficient [h] from Fluent directly ?

Thank you,

I also need help for that.
 

Hi

I am using a heat transfer problem where there is hot water flowing above a sphere...And have to find the heat transfer coefficient from fluent which has to be used for other calculations...kindly help me...

Elina,

You should modify 'reference temperature' under 'Reference values' tab in Fluent. As 'reference temperature' you can use other value, which is commonly used as a bulk temperature for your case. Fluent uses this temperature to calculate heat transfer coefficient based on heat flux through a cell boundary and temperature difference between wall temperature and the 'reference temperature'. Then, you can calculate area-weighted average of heat transfer coefficient on a sphere surface.

Hope it will help.

Regards,
Marcin

Effect of velocity on h
 

Hi everybody,
I have a question:
When simulating a simple forced convection in Fluent (heated wall, velocity inlet, pressure outlet). I want to know if the convection heat transfer I assign to the wall in the BC is automatically going to grow because of the velocity of the fluid or what I assign in the BC is already the forced convection heat transfer itself ?
Thanks a lot in advance :)

Loffy

Hello zomayabssa,
Have you defined an Heat flux or an Heat Transfer Coefficient?.

Regards

Hi Bionico,
Thanks for your interest. Actually Yes I added a heat flux generation and fixed a convection coefficient of 20 w/m2K. What I wanted to know is if this coefficient is supposed to grow with the velocity inlet or it's gonna be fixed. I am not able to check that because I don't know what the "wall func. heat transfer coefficient" in fluent takes into consideration.
Any idea?
Thanks a lot

Good morning,
Wall function heat transfer coefficient takes into account the temperature of the cell next to the wall (the first): this method works well only with certain values of Y* (Y_star)

Regards

Thanks a lot Flavio. What about the velocity? do you think that Fluent increase the "h" value you define consedering the velocity imposed as a BC ?
thanks a lot in advance :)

Well, it depends on the type of boundary condition:

1) if you fix the Heat Transfer Coefficient then it won't change during the simulation, because it's a boundary condition!

2) If you fix the Heat Flux, instead, "h" will change of course, but it depends on how you calculate it (with reference temperature or bulk temperature...).

Regards

Bulk Temp
 

Hi

What do you mean the Bulk Temp.?? is it inlet fluid temp??


thanks