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Trouble patching domain in CFX 11.0

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Old   October 9, 2008, 04:16
Default Trouble patching domain in CFX 11.0
  #1
Jimmy
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Dear All,

I have a premixed combustion simulation i am running. I have done the cold flow and i want to switch to the hot flow. I need to patch only the combustor and so to do this, i need to put some products (H2O and CO2) into the domain inside the combustor since i am using the eddy dissipation model. How can i do this in CFX? I am thinking of using "Expression" but when i type the following Temp=1800[K]*step((x-0.8)/1[m])+300[K]*step((0.8-x)/1[m], it is not running. My combustor axis is along the x axis and my combustor length is 0.8m and my x=0 point is at the entry to the combustor.

When i run the simulation, i keep getting the following error messages:

"Error processing expression 'Temperature'. Inconsistent dimensions on each side of "-" operator at position 16.

Dimension on left: 'm' Dimension on right: '<Dimensionless>' Error processing expression: Temperature = Temp.

Any help on how to do this?

Thanks.

Jimmy
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Old   October 9, 2008, 04:37
Default Re: Trouble patching domain in CFX 11.0
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rohit
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I do not have much knowledge of combustion but, as far as the your expression it need to be slightly modified as 1800[K]*step((x-0.8[m])/1[m])+300[K]*step((0.8[m]-x)/1[m]

you need to have same dimensions on the both side of any operator.

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Old   October 10, 2008, 03:41
Default Re: Trouble patching domain in CFX 11.0
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Jimmy
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Hi and thanks for the help. I made the modification and it seems to work. but i am not clear about how this step function works in CFX. Maybe i can explain what i want to do so you can see whether i did the right thing in the expression. My geometry is such that i have the origin at the entry to my combustion chamber and so my combustor is in the positive x direction and mixing chamber with the fuel and air inlets in the negative x direction. The combustor length is 0.8m. So what i want is set the temperature everywhere inside the combustion to 1800K and to set the temperature in the mixing zone to 300K. Any idea whether what i have is right?

The reason i have to do it this is because i only want burning inside the combustor and so if i leave the initialization temperature to "automatic", there will be burning in my entire domain.

Any suggestion will be appreciated.

Thanks.

Jimmy
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Old   October 10, 2008, 05:25
Default Re: Trouble patching domain in CFX 11.0
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rohit
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Dear Jimmy, the problem with your equation is for the value of x= 0 -> 0.8, the first step function always gives a zero value

i have modified your equation little bit 1500[K]*(step((0.8001[m]-x)/1[m])*step(x))+300[K]

you can try this and let me know if it works, but there is a slight problem that at x=0 the output will be 1050K, but for the rest of the domain i hope it works as per requiremnt

you can just think a bit more and can avoid the problem at x=0
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