Target mass flow rate

cathie · July 27, 2023, 10:43

Hello all,

I am new here and this is my first post !

I have a CFD model of fluid domain which consists of different types of heat exchangers. To simplify the model we want to impose a boundary condition at the exit of every heat exchangers with the calculated mass flow rate and temperature.

I will also define the velocity of the air and its temperature at the inlet of this fluid domain which is cubic and atleast 5 times the size of the CFD model. And in the outlet of this domain will be zero pressure.

I tried with defining outlet_pressure with "Target flow rate" on the exits of heat exchangers. When energy equation is activated I see that only with this boundary condition I can define the temperature also in these outlets from heat exchangers.

My question is what will be the pressure on this surfaces? Or is there any other option to impose mass flow rate and temperature in the boundaries inside the fluid domain?

My objective is to find the variation of temperature in this fluid domain under the different wind conditions along with the hot air that exit from the heat exchangers.

Thanks in advance for your help.

LuckyTran · July 27, 2023, 18:06

The target massflow rate option iterates the outlet pressure until the set massflowrate is achieved.

You cannot specify outlet temperature, physics doesn't work that way. If you want a desired outlet temperature, you need to adjust your inlet conditions until the desired outlet temperature is achieved.

marwane cherkaoui · July 28, 2023, 05:03

Hello, if I have understood your question correctly, you need to define the surfaces on which you want to apply the boundary conditions (the definition is done in the meshing section). For the pressure, you can see the results in the solution part (contour).

cathie · July 28, 2023, 05:35

I could also refine my question as "is it possible to define temperature or mass flow rate in an interface surface" (internal boundary condition as per Fluent)?

cathie · July 28, 2023, 05:38

Thanks for your reply. I understand this option more clearly now. Clearly it wont be applicable for my model. I dont want to model all the physics of the cooling tower, which will increase the computational cost. Thats y I want to impose only the mass flow rate and the temperature at the exit of cooling tower. Will this be possible?

marwane cherkaoui · July 28, 2023, 06:51

For the interior interface, I don't think you can set conditions; you can only set inlet, outlet, and wall conditions; otherwise, you can change the mixture of the interior fluid in the material part. For the solution, I don't think you can do just one part of the simulation, but you can visualize one of the parts of your body. I hope this can help you find a solution to your problem.

LuckyTran · July 28, 2023, 10:11

I think there is some confusion.

Inlet or outlet is with respect to the CFD domain. Is the outlet of your heat exchanger a domain inlet or a domain outlet? It matters. You can specify massflow and temperature at an inlet using the massflow BC.

cathie · July 28, 2023, 11:23

The outlet of the heat exchanger will neither be the inlet nor the outlet of the fluid domain.

I just want to impose some mass flow rate in a surface inside this domain (this surface will be the exit of the cooling tower inside this fluid domain).

The air that enters the cooling tower and exits will have the mass flow rate and temperature I will define at the exit.

This is simply to avoid modelling all the physics inside the cooling tower since it will not be the purpose of my study to see whats happening inside the cooling tower but simply how the hot air that will exit will affect the air temperature around the cooling tower.

Will this be possible?

Or how can I simplify the model?

Because I have atleast 20 of such cooling towers inside this fluid domain and definitely the computational cost will be huge if model the entire cooling tower.

July 27, 2023, 10:43	Target mass flow rate	#1
cathie New Member Catherine Join Date: Jul 2023 Posts: 4 Rep Power: 3	Hello all, I am new here and this is my first post ! I have a CFD model of fluid domain which consists of different types of heat exchangers. To simplify the model we want to impose a boundary condition at the exit of every heat exchangers with the calculated mass flow rate and temperature. I will also define the velocity of the air and its temperature at the inlet of this fluid domain which is cubic and atleast 5 times the size of the CFD model. And in the outlet of this domain will be zero pressure. I tried with defining outlet_pressure with "Target flow rate" on the exits of heat exchangers. When energy equation is activated I see that only with this boundary condition I can define the temperature also in these outlets from heat exchangers. My question is what will be the pressure on this surfaces? Or is there any other option to impose mass flow rate and temperature in the boundaries inside the fluid domain? My objective is to find the variation of temperature in this fluid domain under the different wind conditions along with the hot air that exit from the heat exchangers. Thanks in advance for your help.

July 27, 2023, 18:06		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,753 Rep Power: 66	The target massflow rate option iterates the outlet pressure until the set massflowrate is achieved. You cannot specify outlet temperature, physics doesn't work that way. If you want a desired outlet temperature, you need to adjust your inlet conditions until the desired outlet temperature is achieved. cathie likes this.

July 28, 2023, 05:35	define mass flow rate in an interface surface	#4
cathie New Member Catherine Join Date: Jul 2023 Posts: 4 Rep Power: 3	I could also refine my question as "is it possible to define temperature or mass flow rate in an interface surface" (internal boundary condition as per Fluent)?

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July 28, 2023, 05:03		#3
marwane cherkaoui New Member marwane cherkaoui Join Date: May 2023 Posts: 7 Rep Power: 3	Hello, if I have understood your question correctly, you need to define the surfaces on which you want to apply the boundary conditions (the definition is done in the meshing section). For the pressure, you can see the results in the solution part (contour).

July 28, 2023, 05:38		#5
cathie New Member Catherine Join Date: Jul 2023 Posts: 4 Rep Power: 3	Thanks for your reply. I understand this option more clearly now. Clearly it wont be applicable for my model. I dont want to model all the physics of the cooling tower, which will increase the computational cost. Thats y I want to impose only the mass flow rate and the temperature at the exit of cooling tower. Will this be possible?

July 28, 2023, 06:51		#6
marwane cherkaoui New Member marwane cherkaoui Join Date: May 2023 Posts: 7 Rep Power: 3	For the interior interface, I don't think you can set conditions; you can only set inlet, outlet, and wall conditions; otherwise, you can change the mixture of the interior fluid in the material part. For the solution, I don't think you can do just one part of the simulation, but you can visualize one of the parts of your body. I hope this can help you find a solution to your problem.

July 28, 2023, 10:11		#7
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,753 Rep Power: 66	I think there is some confusion. Inlet or outlet is with respect to the CFD domain. Is the outlet of your heat exchanger a domain inlet or a domain outlet? It matters. You can specify massflow and temperature at an inlet using the massflow BC.

July 28, 2023, 11:23		#8
cathie New Member Catherine Join Date: Jul 2023 Posts: 4 Rep Power: 3	The outlet of the heat exchanger will neither be the inlet nor the outlet of the fluid domain. I just want to impose some mass flow rate in a surface inside this domain (this surface will be the exit of the cooling tower inside this fluid domain). The air that enters the cooling tower and exits will have the mass flow rate and temperature I will define at the exit. This is simply to avoid modelling all the physics inside the cooling tower since it will not be the purpose of my study to see whats happening inside the cooling tower but simply how the hot air that will exit will affect the air temperature around the cooling tower. Will this be possible? Or how can I simplify the model? Because I have atleast 20 of such cooling towers inside this fluid domain and definitely the computational cost will be huge if model the entire cooling tower.