Hi everyone,
I hope you all are doing well.
Can anybody in the forum explain what adiabatic wall temperature and heat transfer coefficients are? More importantly, how to estimate it using CFD (say, ANSYS Fluent).
I am dealing with transonic flows past airfoil geometries with heat transfer. Some literature (links are given below) suggests the use of adiabatic wall temperature and heat transfer coefficients for high-speed flows.
I am aware that the heat transfer coefficients are estimated using Newton's law of cooling. But, the choice of reference temperature/bulk fluid temperature can be confusing at times.
Your suggestions are appreciated.
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Hi.
when we say "adiabatic" it means there isn't any heat transfer. So, I think you did a mistake by choosing the adiabatic wall to your boundary conditions.
In ordinary boundary conditions, you can estimate the heat transfer coefficient by calculating the temperature gradient in each section. The temperature gradient is obtained by delta T per delta x in CFD.
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I agree with Tohid. Perhaps you mean "isothermal" wall. For adiabatic conditions, the heat transfer rate and coefficient are zero by definition.
Hi, Hariharan Kallath
The adiabatic wall that is mean not heat transfer through it. Thus the coefficient of heat transfers equal to zero. I accepted the answer of Tohid Adibi and Dudley J Benton .
Thanks
Dear all,
I have the similar question. I want to simulate transient turbulent convective heat transfer in corrugated tube. however, I find the inlet and outlet is too close to the corrugated tube. I am afraid the backflow or wake could affect the simulation result. Hence, I lengthen the inlet section and outlet section (10 * Internal diameter of tube) to make sure that thermal and hydrodinamics flow is fully developped. Heat is applied only on corrugated tube, but I put wall of the extension into Adiabatic condition. Because I want to see the effect heat transfer only in corrugated side. Is my extension methods is correct?
Thank you
Best regards
Hi,
Adiabatic heat transfer coefficient is the convective heat transfer which is based on adiabatic wall temperature. But what is adiabatic wall temperature?
Consider Couette flow which is the flow between two parallel plates with very small spacing. The lower wall is stationary while the upper one is moving. Temperature of the lower wall is T1 and the upper wall is T2. It is assumed that T1 is higher than T2. So, it is expected that heat is transferred from the lower wall (T1) to the upper wall T2. Moreover, temperature varies linearly between two plates.
However, it can be observed that due to viscous heat dissipation (the amount of work which changes to heat due to fluid viscosity) the intermediate fluid layers temperatures are higher than both walls temperature and even for Br>2 heat transfer direction is reversed. It means that heat is transferred from Fluid to both walls!
(Note: Br or Brinkman number is mu*u^2/(K*(T1-T2)) and mu is fluid viscosity and u is upper wall velocity).
To calculate heat transfer from the lower wall it is still true to say that q”=h(T1-Tref) in which Tref is a reference temperature. But what would be this reference temperature? Is it mean temperature? or is it mixing temperature? The answer is No! it must be adiabatic wall temperature (Tad). So: q”=h(T1-Tad).
Adiabatic wall temperature is the temperature that the wall would attain if q” was zero or wall would be adiabatic.
Unfortunately, Researchgate platform limits me to explain more or type formulas freely. However, you can find more explanation by Googleing the keyword: “Couette flow” and “adiabatic wall temperature”
Hi all,
Thank you for trying to help me out. I appreciate it.
First of all, I agree that some thermal boundary condition has to be specified (not adiabatic) for the estimation of heat transfer coefficient.
But, the literature (which I quoted above) says that when the 'adiabatic wall temperature' is used as the reference temperature in Newton's law of cooling (Q=h*A*dT), we get 'adiabatic heat transfer coefficient'. It is suggested as an invariant descriptor of the forced convection in high-speed flows. Please see the screenshot taken from the literature. I doubt that the word 'adiabatic' is misleading in this context.
I hope the above explanation is making sense. I wish to have some discussion on these aspects.
Hossain Nemati, your answer seems to be something that I am seeking. I am interested in transonic flows past heated surfaces. Can you please tell me how these values can be estimated using CFD simulations?
Thank you all.
It is an airfoil. I suppose we may conveniently take it as a flat plate if needed.
Mr. Hariharan Kallath
greeting
i believe in opinion of mr. Tohid Adibi , adiabatic means there is no heat transfer, so by boundary condition
1-by temperature(constant temperature)
2- by enthalpy ( constant enthalpy)
therefore there is no heat transfer
while the heat transfer coefficient depending on the difference in temperature in single phase heat transfer so if there is no difference in temperature then no heat transfer coefficient
while heat transfer coefficient in tow phase flow ( change in phase) if there is no heat transfer so there is no change in phase and the latent heat of vaporization or condensation equal to zero
in CFD adiabatically procedure represented by BC
also represent by equal in temperature in governing equation -equation of energy that presented by temperature.
while in two phase adiabatically procedure represent by giving zero to the source term in equation of energy that presented by enthalpy.
Simply change the B.C. to adiabatic. Then calculate the temperature. It is the adiabatic wall temperature. Return back the wall B.C. and solve the problem.In estimation of convective coefficient, use adiabatic wall temperature instead of wall temperature.
Hint: Adiabatic wall temperature is applicable in high velocity problems. Be careful to consider viscous heat dissipation terms.
In dealing with high-speed flows, you need to accommodate for the flow stagnation in the boundary layer. Per definition, (stagnation enthalpy)=c_p*T_freestream+(u_freestream^2)/2, leading to a stagnation temperature of T'=T_freestream+(u^2)/(2*c_p). T' is often referred to as the adiabatic wall temperature, as this is the temperature to be used in Newtons law of cooling.
For fluids with Pr different from unity, one should also consider the diffusion of heat in the boundary layer, reducing T' somewhat. Typically, T'=T_freestream+k*(u_freestream^2)/(2*c_p), where (for gases), k=sqrt(Pr). For more information, consult an advanced textbook on heat transfer, e.g. Kays/Crawford: Convective Heat and Mass Transfer.
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