## Fluids

#### study state thermal analysis in convection media using fluent

• tcs engineer
Subscriber

Dear members,

I require help to setup my problem step by step.

Description : a electronic card is placed (conduction on mounting is neglected) in a room with air temperature 25c, and 2 components mounted on card (FPGA IC)  Dissipate heat 0.5W and 0.02 W. closed room air convection heat coefficient considered 8W/m2k.

I have set the respected material properties. in cell zone condition i specified fluid for room and solid for electronic package.

I am exactly facing problem with boundary condition. I have created virtually fluid domain like a room.

1) In boundary condition I have made all 6 walls Pressure Inlet. (Your suggestion are required please comment)

2)My solution is keep on iterating more than 1000 iteration. still not showing solutions converged. can you please tell what is the reason to take more iteration.

3)another approach i have made all wall to Mixed boundary condition. even though their no converged solution.

I have set the temperature plot for my components i could see graph looks stable but their is very small change in temperature,

residual set for 1e-6

can any one tell is it because of convection problem ?

I am happy to take your suggestion.

Regards,

VIJAY

• Karthik R

Hello Vijay,

Since you are modeling natural convection in a closed room, you might want to change the boundary conditions from pressure inlet to walls. You might also want to make sure that gravity is turned on. Please be careful about the operating density, temperature, and pressure values. You might want to refer to the Fluent Users Guide for more assistance on what values to use for the density models you are choosing.

You can use this example tutorial to set-up your model.

The first tutorial contains both radiation and natural convection. You might want to avoid following the steps used to set-up the radiation model.

Also, I am moving this post to the 'Fluid Dynamics' thread in hope that your question gets more visibility there.

Thank you.

Best Regards,

Karthik

• tcs engineer
Subscriber

Dear Kremella

Hopefully i have done like video 1,

but i have taken radiation model is DO. and in material selection density of air taken as in-compressible ideal gas as i understood this (PV=mRT).

and in operating condition i specified the operating P, T. and gravity.

but 1) i could not understand the operating density, i have checked the guide and i have read the theory. but i couldn't understand well.

if you know what will be my operating density could you please tell me and give some explanation on it. (does it calculate itself because i treated air as in-compressible idea gas density in material selection?)

problem description is : electronic card keeping in aluminium casing like above picture and casing is treated as mixed boundary condition. with convection coefficient 8w/m2k. total heat dissipation on card is 0.57W.

2) if my radiation model is not recommend please comment.

3) when i check the flux report including casing and electronic package its showing -0.57W if possible please make me understand i am confused with this (because i was expecting total heat transfer net =0)

Regards,

VIJAY BR

• Karthik R

Hello,

1. you might want to characterize the importance of radiation compared to natural convection. Once you decide that radiation modeling is important, you might want to pick a model.

2. Selecting operating density depends on your density model you pick. Please refer to section 12.2.4.5 in the ANSYS Users guide if you want to learn more about operating density.

3. About your reports question, is your input power 0.57 W? Can you post a screenshot of your total heat transfer rate report?

Thank you.

Best,

Karthik

• tcs engineer
Subscriber

results after 2000 iteration

FPGA 1 0.5W

FPGA2 0W

PROCE 0.02W

Total heat dissipation  is 0.52W

if i check the net flux for without casing its almost 0 its correct.

when i check the net flux report for casing its showing 0.573,  so (0.52-0.573=0.053 unknown heat added to the casing wall)

can any one explain why its showing like this in reality the total heat source is 0.52W, it much show -0.52,

please explain me if any one know how does it get 0.053W heat.

my mesh is good, i don't think so its numerical error,

its simple problem not a complex

Regards,

vijay b r

• tcs engineer
Subscriber

results after 2000 iteration

FPGA 1 0.5W

FPGA2 0W

PROCE 0.02W

Total heat dissipation  is 0.52W

if i check the net flux for without casing its almost 0 its correct.

when i check the net flux report for casing its showing 0.573,  so (0.52-0.573=0.053 unknown heat added to the casing wall)

can any one explain why its showing like this in reality the total heat source is 0.52W, it much show -0.52,

please explain me if any one know how does it get 0.053W heat.

my mesh is good, i don't think so its numerical error,

its simple problem not a complex

Regards,

vijay b r

• tcs engineer
Subscriber

Dear karthik,

Please check if you could able to help with this problem,

Regards,

VIJAY B R

• Rob
Ansys Employee

It's likely that you've got some transients in the flow: how well converged are the residuals.  This could explain the missing energy. Similarly how good is the near wall mesh (y+ values).

Re density. If the domain is sealed you'll need to use Ideal Gas as you need to have some way for the gas to compress as it gets warm.

• tcs engineer
Subscriber

Dear sir,

thank you for time and reply,

i would like to put my converges if you can understand please comment on it,

its laminar flow and s-s radiation model,

solution methods used coupled and psudo transient and wrap face selected.

i have considered incomprehensible ideal gas,

• tcs engineer
Subscriber

y+ will be consider when the flow is turbulent,  if i am wrong please explain me how does y+ related to my problem,