[Jammy]

My problem is based on a condenser. Steamis condensing on the outer surface of a thin-walled circular tube of diameter D = 50mm and length L = 6m maintains a uniform outer surface temperature of 100C. Water flows through the tube at a rate of 0.25 kg/s, and its inlet and outlet temperatures are Ti = 15C and To= 57C. What is the average convection coefficient associated with the water flow? Please suggest how to to find the avg heat transfer coeff. in fluent.

[Jammy]

Please help me...if some one has any clue...

[h0rst]

you can calculate it in fluent postprocessing

[KaLium]

Why do you need to find it from fluent? You can calculate it with pen and paper.

q = hc A dT

[LuckyTran]

As KaLium says, you already have the means to calculate the heat transfer coefficient from a simple energy balance. This isn't a Fluent problem because you know your outlet temperatures.

But suppose you didn't know your outlet temperatures, then you would just simulate it in Fluent (mesh it, setup the model, set the boundary conditions, make an initial guess, iterate, wait until it converges, then get htc).

[Jammy]

Respected Sir,

Thank you for your reply. I am a beginner in Heat transfer through Fluent. Actually the problem which I stated earlier is a problem from the book you mentioned in your last reply. The solution is there in the book but I want to do that using Fluent in order to see if the results match or not.

I have attached the image of geometry. Boundary conditions are as follows:

Inlet: Mass flow inlet

Outlet: Pressure Outlet

Wall: At constant wall temperature of 373 K and having zero thickness

Water as the fluid.

The calculated heat transfer coefficient in the book is 755 W/m2 K but I am getting an area weighted average of Surface heat transfer coefficient value of 1696 W/m2 K in Fluent. I have set the reference value temperature equal to LMTD.

Please tell me where am I making the mistake. Also the temperature at outlet is 373 K in Fluent which different from that given in the book i.e., 330 K. Why is this difference? This is very very important for me to proceed further as I need a validation to show.

Please reply.

[Jammy]

The name of the book is actually "Fundamentals of Heat and Mass Transfer"

[LuckyTran]

First I recommend you double check your settings and make sure you didn't make any super stupid mistakes, like forgetting to change the default properties of air to water. Then I recommend to run your case on several grids. Your grid should be ~ 1 million cell mark or more for this L/D of 120. Things like y+ are pretty important, are you able to achieve y+ < 100 or so at least?

These problems look deceptively simple. What makes them simple is that you somehow already know the outlet temperature and can therefore rely on other means (conservation of energy) to determine the heat transfer coefficient.

In CFD you do not know your heat transfer coefficient and must solve for it. That means you must be able to solve the relevant physics involved, in this case that means correctly resolving the boundary layer such that you get the correct heat transfer coefficient at the wall.

[Jammy]

Lucky Tran Sir,

Thank you for your reply. I have cross-checked my solution and do not find any mistake. However one of the CFD member has helped me and finally I have got the solution for the problem using axisymmetric concept.

Y+ values are not clear to me. Can you tell me how to get y+ values numerically in Fluent or Gambit?

Regards.

[LuckyTran]

If you have solved it then what do you need y+ for?

The y+ at wall adjacent cells is a variable called yplus in Fluent. You can't get y+ in Gambit because y+ is a flow property, not a mesh property.