CFD Online Discussion Forums

CFD Online Discussion Forums (http://www.cfd-online.com/Forums/)
-   CFX (http://www.cfd-online.com/Forums/cfx/)
-   -   Heat transfer CHT in ansys CFX (http://www.cfd-online.com/Forums/cfx/122068-heat-transfer-cht-ansys-cfx.html)

shaheen August 11, 2013 23:47

Heat transfer CHT in ansys CFX
 
Hi,

I have modelled a pipe in which oil is flowing at 85 deg C with 1m/s speed.There is a heater cable on the pipe to heat the oil as it flows.The heater provides 100 W/m output.The pipe and heater cable is insulated with additional layer of rockwool.

I have created seperate domains to represent fluid and solids and also verified the domain interface properties.

The problem is the oil is not heated and temperature at inlet and outlet is same.But in actual system the oil gets heated to 120 deg C.

Please can any one share their thoughts on this.

ghorrocks August 12, 2013 02:47

Have you put interfaces to connect the solid and fluid domains? Also if you have multiple solid domains?

How have you modelled this? Can you show a picture which shows how you have modelled the heating cable.

shaheen August 12, 2013 04:22

3 Attachment(s)
Hi Glenn,

I have put interfaces to connect the solid and fluid domains with heat transfer option set to conservative heat flux.

Attachment 24313

Attachment 24314

Attachment 24315

I have attached images of the geometry for your viewing

ghorrocks August 12, 2013 06:21

You will also need solid to solid interfaces. Did you put them in too?

JuPa August 12, 2013 16:38

You also need to enable conservative interface flux at the interfaces.

shaheen August 12, 2013 23:30

I have put solid to solid and fluid to solid interfaces and enabled conservative heat flux between them.

ghorrocks August 12, 2013 23:32

Can you post an image of the temperature distribution you do get (from the post processor) and your CCL?

shaheen August 12, 2013 23:48

2 Attachment(s)
The temperature distribution of oil.
Attachment 24342

The ccl file output
Attachment 24343

ghorrocks August 13, 2013 00:00

So it looks like heat is being transferred to the water, just less than you think is correct.

From your CCL I see you have a large pipe flowing water at 1m/s. This is quite a lot of water. And your heat is 100W/m. That is not much heat. My kettle at home is 2.4kW and it takes a couple of minutes to boil 0.5 litres of water. So it does not surprise me that 100W/m does not heat things up much.

Are you sure you have your geometry, flow rate and heat input correct? You really need to do some back of the envelope calculations to check you are in the right direction.

For instance, a 0.5m diamater pipe at 1m/s is 0.196m^3/s or 196 kg/s. At 4.2 kJ/kgK that works out to be 1e-4 K temperature difference averaged across the flow. That looks about what you got with CFX :). So I think the simulation is accurately showing what you asked it to model.

shaheen August 13, 2013 00:08

Thanks for the nice explanation :).I will recheck every input and get back asap.

selvam2487 August 13, 2013 03:29

heat flow calculation
 
[For instance, a 0.5m diamater pipe at 1m/s is 0.196m^3/s or 196 kg/s. At 4.2 kJ/kgK that works out to be 1e-4 K temperature difference averaged across the flow. That looks about what you got with CFX :). So I think the simulation is accurately showing what you asked it to model. ]

Dear Glenn,

Can you please tell me which formula you used to calculate the 1e-4 K temperature difference across the flow. I am interested in knowing how you included the power input value of 100 W/m in the calculation.

Regards,
Karthick

ghorrocks August 13, 2013 05:51

Your thing looks about 1m long so at 100W/m that is 100W. Then you have a power input, a mass flow rate, a specific heat and that defines the temperature rise.

shaheen August 14, 2013 00:01

Yes.In the actual system the pipe is long continous and oil is maintained above 85 deg and at intermediate lengths there is a supporting structure to support the pipe which is also partially insulated.At the support due to ambient conditions the oil temp drops.

The heater cable has output of 100 W/m and in operating condition the heater temperature is 92 deg.:)


All times are GMT -4. The time now is 02:55.