Multi-fluid compressor efficiency

Zurent · April 17, 2017, 12:51

Hello everybody,

I have successfully simulated with Ansys CFX a fluid flow around a compressor and I would like to calculate the isentropic efficiency. The ideal gas law was implemented for the simulations.

I have already calculated this value for the case where water vapor was the operating fluid. The efficiency was easily available after checking the efficiency output case in the Output control section. Now I am trying to simulate the case of a different operating fluid, this time composed by a mixture of 4 different gases (ideal gas model). The problem is that I end up in CFD Post with 4 different isentropic efficiencies and what I am interested in is the global efficiency.

I have gone through a few tutorials, guides and best practices (as suggested in this post: Efficiency calculation) but I could not come to a solution of the problem.

I tried adding "Isentropic Compression Efficiency" in the extra output variables but every time I add these extra variables they are nowhere to be found in CFD Post. I also tried to add “Total Temperature in Stn Frame” and it could not be found in CFD Post, only the 4 values for the separated gases were available.

I made another attempt, after going through ANSYS CFX Reference Guide, and declared a CEL expression using the short-variable name for the efficiency "icompeff" (massFlowAve(icompeff)@OUTBlock OUTFLOW). Unfortunately, the expression cannot be calculated in CFD Post and the following message is displayed: “ERROR. The following unrecognized name was referenced: icompeff”.

Finally, I tried with the Turbo macro but, to calculate the Gas Compressor Performance, the Total Temperature in Stn Frame is necessary and since I failed to add it into the extra output variables this macro cannot be used.

I am sorry for the long post, I tried to be as precise as I could. I will be grateful for any help you can provide.

Opaque · April 17, 2017, 18:41

Would mind posting the section for your mixture material ?

Zurent · April 18, 2017, 03:33

Hi,

Thanks for finding the time to read the post. I am not sure I understand what you mean by "section of the mixture material", is it the section where the fluids were defined in CFX Pre? If that were the case, the attached images show the definition for the first fluid as well as the volume fraction definition. All the other fluids are defined in the same way.

turbo · April 18, 2017, 18:53

Since you are dealing with a mixture gas, there is only a single isentropic efficiency of the mixture gas.

Zurent · April 19, 2017, 04:39

Hello,

A single (global) isentropic efficiency is what I would like to estimate but it is not what I get. Only the 4 separate isentropic efficiencies are calculated.

Opaque · April 19, 2017, 08:23

If you are trying to model the mixture of 4 gases, it seems you have setup the case as a multiphase model with 4 fluids, and not as a multicomponent model.

If you set it up as multicomponent, you will get what you need, and the simulation should be what you are expecting.

Zurent · April 27, 2017, 14:38

Hi everybody,

I am sorry for posting this reply after a week. I just wanted to say that modelling as multicomponent is probably the solution I was looking for. This way, instead of defining 4 different fluids, I created a new material with the properties I desired. As a result the global efficiency is calculated.

Many thanks to those who found the time to help me.

April 17, 2017, 12:51	Multi-fluid compressor efficiency	#1
Zurent New Member Join Date: Oct 2016 Location: Switzerland Posts: 4 Rep Power: 9	Hello everybody, I have successfully simulated with Ansys CFX a fluid flow around a compressor and I would like to calculate the isentropic efficiency. The ideal gas law was implemented for the simulations. I have already calculated this value for the case where water vapor was the operating fluid. The efficiency was easily available after checking the efficiency output case in the Output control section. Now I am trying to simulate the case of a different operating fluid, this time composed by a mixture of 4 different gases (ideal gas model). The problem is that I end up in CFD Post with 4 different isentropic efficiencies and what I am interested in is the global efficiency. I have gone through a few tutorials, guides and best practices (as suggested in this post: Efficiency calculation) but I could not come to a solution of the problem. I tried adding "Isentropic Compression Efficiency" in the extra output variables but every time I add these extra variables they are nowhere to be found in CFD Post. I also tried to add “Total Temperature in Stn Frame” and it could not be found in CFD Post, only the 4 values for the separated gases were available. I made another attempt, after going through ANSYS CFX Reference Guide, and declared a CEL expression using the short-variable name for the efficiency "icompeff" (massFlowAve(icompeff)@OUTBlock OUTFLOW). Unfortunately, the expression cannot be calculated in CFD Post and the following message is displayed: “ERROR. The following unrecognized name was referenced: icompeff”. Finally, I tried with the Turbo macro but, to calculate the Gas Compressor Performance, the Total Temperature in Stn Frame is necessary and since I failed to add it into the extra output variables this macro cannot be used. I am sorry for the long post, I tried to be as precise as I could. I will be grateful for any help you can provide. Last edited by Zurent; April 18, 2017 at 03:21.

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April 17, 2017, 18:41		#2
Opaque Senior Member Join Date: Jun 2009 Posts: 1,803 Rep Power: 32	Would mind posting the section for your mixture material ?

April 18, 2017, 18:53		#4
turbo Senior Member Join Date: Jun 2009 Posts: 174 Rep Power: 16	Since you are dealing with a mixture gas, there is only a single isentropic efficiency of the mixture gas.

April 19, 2017, 04:39		#5
Zurent New Member Join Date: Oct 2016 Location: Switzerland Posts: 4 Rep Power: 9	Hello, A single (global) isentropic efficiency is what I would like to estimate but it is not what I get. Only the 4 separate isentropic efficiencies are calculated.

April 19, 2017, 08:23		#6
Opaque Senior Member Join Date: Jun 2009 Posts: 1,803 Rep Power: 32	If you are trying to model the mixture of 4 gases, it seems you have setup the case as a multiphase model with 4 fluids, and not as a multicomponent model. If you set it up as multicomponent, you will get what you need, and the simulation should be what you are expecting.

April 27, 2017, 14:38		#7
Zurent New Member Join Date: Oct 2016 Location: Switzerland Posts: 4 Rep Power: 9	Hi everybody, I am sorry for posting this reply after a week. I just wanted to say that modelling as multicomponent is probably the solution I was looking for. This way, instead of defining 4 different fluids, I created a new material with the properties I desired. As a result the global efficiency is calculated. Many thanks to those who found the time to help me.