Convection problems in a greenhouse
 

Hi,

I'm currently trying to simulate the effects of two exhaust fans in greenhouse. It seemed ok until I decided to try an extreme test which I set the free stream temperature in 600 ºC, the initial temperature inside the greenhouse in 35 ºC and the pads that are responsible to refrigerate the environment are set in 15 ºC. The fans have a mass flow around 11 kg/s. In the end of the transient simulation (100 s) the internal temperature is the same as the pad's temperature, as you can see in the picture.

I have already tried:
1- Incompressible fluid approximation
2- Boussinesq approximation
3- Set the free stream temperature in 600 ºC (Convection thermal condition)
4- Set the walls temperature in 600 ºC (Temperature thermal condition)

Do you have any idea about what am I doing wrong?
Thank you.

Free Stream Temperature
 

What is meant by free-stream temperature? Is the temperature you are using for a wall boundary with convection condition or do you mean temperature of the incoming air? Essentially, both should be same since the air outside is the one being sucked in by the fans.

The free stream temperature is the one outside the greenhouse.
I've uploaded a picture with the boundary conditions I've used.

Thermal Field
 

What is the temperature at the inlet? It should be same as free stream temperature used for wall boundary. If it is not, then, the results are as expected.

The inlet temperature comes from the water passing through the pads and it's lower than the free stream temp. I've started the simulation with a temperature of 600 ºC inside and outside the greenhouse now, and the pads are set in 25 ºC. In the end of the simulation the temperature inside is the same as pads' temperature, and I imagine that at this circumstances it would be impossible to let the temperature thaat lower than the temperature outside. Is it possible that the time I'm using for the simulation is to short (200 s)?

Inlet Temperature
 

I am sorry, but you have not provided answer to a simpler question; what is the temperature at the inlet? Because a whole lot depends on that.

The pads are the inlet, the temperature is 15 ºC.

Temperature
 

If the inlet temperature is 15, then most of the temperature will be 15. Only very close to the walls, temperature might be slightly higher, but not much because HTC on the outside walls is too small.

Hmm, I got it. I've tried another situation in which I set the wall temperature in 600 ºC and in this situation is not necessary to provide the HTC, only the surface temperature (wall temperature) and I still have the same answer. Is it supposed to be this way?

Because it seems to me that with a high temperature on the walls, the inside temperature is most likely to be closer to 600 ºC.

Physics
 

Temperature does not do anything on its own. Its the mechanisms of heat transfer that do the job. I suppose you are not considering radiation as of now. So, lets look at the other two modes, convection and conduction. If the flow velocities are very high, then convection rate would be high, implying the heat coming in from the walls will be carried away by the moving fluid. So, if you check the temperature at the outlet, it would be slightly higher than incoming one. Since there is no radiation, and most of the flow is going from inlet towards outlet without much recirculation, it is only conduction that is responsible for transporting thermal energy from the walls to the interior. The conduction depends on turbulence, if the flow is turbulent, else only on the thermal conductivity. If the flow is turbulent, you should observe some thermal boundary layer adjacent to the walls.

Reduce the flow rate to a very small value since 11 kg/s could be large value for flow rate of air. Set it to 1 or lower and you will see the effects. However, actual simulation should be done with whatever is the flow rate of the fans used; do check for their accuracy though because 11 kg/s is very high.

Ok, I got the first part. About the flow rate I'm using the information provided by the fan supplier as you can see in the picture. The static pressure ir around 12 Pa. Converting m³/h in m³/s and multiplying by the air density we have something around 11 kg/s, am I right?

Attachment 78558

Flow Rate
 

If the flow rate is correct, then so is the thermal contours, assuming you have included the required models, such as, turbulence.

The turbulance model was missing. I am using know the K-epsilon to simulate that extreme situation.
As a matter of fact, my simulation is quite different and the real values are:
Free stream temperature: 34 ºC
Inlet temperature: 25 - 27 ºC
Temperature inside the greenhouse is 35 ºC (starting value)

Can you advise me another turbulence model or this one that I'm using is ok for this situation? Is it ok to use the default values provided by the software?

Turbulenc
 

Realizable is alright. Use actual conditions. Default values are good.