Concentric tubes heat exchanger simulation

eliastorca · June 22, 2015, 10:53

Hi everyone,

I'm trying to simulate in Fluent a concentric tube countercurrent heat exchanger and I'm experiencing some troubles.
The heat exchanger is a 10 meters length vertical tube with 10mm as internal diameter and 20mm as external diameter. The fluid which flows through the interior tube is water liquid at 298K and atmospheric pressure with a mass flow rate of 1495 kg/m^2s and through the exterior tube is air at 473K and in reverse direction with a mass flow rate of 2360kg/m^2s.
First of all I define the model without taking into account the boiling phenomenon and everything performs in a good way. But when I introduce the boiling phenomenon (because water is going to boil at 373K so I activate the Eulerian boiling RPI model and I define water vapor as secondary phase) it is not possible to match each fluid with each zone (as known, water through the interior tube and air though the exterior tube) as for each cell zone the only possibility is to play with each of the phases (mixture, liquid or vapor).
I match each fluid to each cell zone by applying 3 phases at the Eulerian boiling model (air is the 3rd phase) and doing a patch after initializing the case but, however, the model stops due to divergence problems whenever the 2nd phase volume fraction starts to appear.
I don't know how to solve this problem so I want to ask you if any of you have tried previously this type of problem and how have you make it. I also want to ask you if Fluent is the correct software to solve this problem or I have to try with CFX or any other one.

Thanks in advance for your help

mariam.sara · July 2, 2015, 04:28

Hello what model you apply in fluent to model your problem? why you not use Heat Exchanger model available in FLUENT models?

mariam

eliastorca · July 2, 2015, 04:54

Hello Mariam,

First of all, thank you for your reply.
I'm applying Eulerian boiling RPI model. I don't use the Heat Exchanger model because I think that this model is too general for my case, because I want to see the properties (temperature, pressure, heat transfer coefficient) change of each fluid at each location (both for the primary and the secondary fluid)
I can tell you also that without using the heat exchanger model I'm achieving now some good results doing a patch of the secondary fluid and attaching it to the external tube, so I think that I'm now in the correct way.

mariam.sara · July 2, 2015, 05:01

Hello eliastorca I am really just start to work with heat exchanger simulations and I am confused to what model to use? do I need to use Heat Exchanger model or other model? I have finned tube cross flow heat exchanger?

eliastorca · July 2, 2015, 05:06

Hello Mariam

I have never used the heat exchanger model because in my case is a simpler heat exchanger (only two concentric tubes).
But if you have finned tube I think that it is a better option to model it with the heat exchanger model. You should look at the Fluent help of the heat exchanger model

Elias

June 22, 2015, 10:53	Concentric tubes heat exchanger simulation	#1
eliastorca New Member Elias Join Date: May 2012 Posts: 10 Rep Power: 14	Hi everyone, I'm trying to simulate in Fluent a concentric tube countercurrent heat exchanger and I'm experiencing some troubles. The heat exchanger is a 10 meters length vertical tube with 10mm as internal diameter and 20mm as external diameter. The fluid which flows through the interior tube is water liquid at 298K and atmospheric pressure with a mass flow rate of 1495 kg/m^2s and through the exterior tube is air at 473K and in reverse direction with a mass flow rate of 2360kg/m^2s. First of all I define the model without taking into account the boiling phenomenon and everything performs in a good way. But when I introduce the boiling phenomenon (because water is going to boil at 373K so I activate the Eulerian boiling RPI model and I define water vapor as secondary phase) it is not possible to match each fluid with each zone (as known, water through the interior tube and air though the exterior tube) as for each cell zone the only possibility is to play with each of the phases (mixture, liquid or vapor). I match each fluid to each cell zone by applying 3 phases at the Eulerian boiling model (air is the 3rd phase) and doing a patch after initializing the case but, however, the model stops due to divergence problems whenever the 2nd phase volume fraction starts to appear. I don't know how to solve this problem so I want to ask you if any of you have tried previously this type of problem and how have you make it. I also want to ask you if Fluent is the correct software to solve this problem or I have to try with CFX or any other one. Thanks in advance for your help

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July 2, 2015, 04:28		#2
mariam.sara Senior Member Join Date: Jan 2011 Posts: 339 Rep Power: 16	Hello what model you apply in fluent to model your problem? why you not use Heat Exchanger model available in FLUENT models? mariam

July 2, 2015, 04:54		#3
eliastorca New Member Elias Join Date: May 2012 Posts: 10 Rep Power: 14	Hello Mariam, First of all, thank you for your reply. I'm applying Eulerian boiling RPI model. I don't use the Heat Exchanger model because I think that this model is too general for my case, because I want to see the properties (temperature, pressure, heat transfer coefficient) change of each fluid at each location (both for the primary and the secondary fluid) I can tell you also that without using the heat exchanger model I'm achieving now some good results doing a patch of the secondary fluid and attaching it to the external tube, so I think that I'm now in the correct way.

July 2, 2015, 05:01		#4
mariam.sara Senior Member Join Date: Jan 2011 Posts: 339 Rep Power: 16	Hello eliastorca I am really just start to work with heat exchanger simulations and I am confused to what model to use? do I need to use Heat Exchanger model or other model? I have finned tube cross flow heat exchanger?

July 2, 2015, 05:06		#5
eliastorca New Member Elias Join Date: May 2012 Posts: 10 Rep Power: 14	Hello Mariam I have never used the heat exchanger model because in my case is a simpler heat exchanger (only two concentric tubes). But if you have finned tube I think that it is a better option to model it with the heat exchanger model. You should look at the Fluent help of the heat exchanger model Elias