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April 1, 2014, 14:29 
CFX Monte Carlo Radiation

#1 
New Member
Orry Shamash
Join Date: Apr 2014
Posts: 4
Rep Power: 3 
I am currently modeling a parabolic solar trough using ANSYS CFX 14.5 using the Monte Carlo model in oredr to utilize the directional radiation flux feature.
The directional radiation is defined using two points for axis definition and cylindrical components to concentrate the radiation to a focal line. The radiation needs to pass through the air (Air Ideal gas) domain and a solid quartz tube (absorption coefficient 0, refractive index of 1 and scattering coefficient of 0). the air and quartz tube are set to surface to surface and the working fluid (which i would like to absorb the radiation) is set to participating media. For some reason, The radiation is not heating up the working fluid as it passes through the trough. Does this have to do with the thermal radiation control in the advanced options of the Solver Control. Any help would be greatly appreciated!! 

April 3, 2014, 01:55 

#2 
Senior Member
Bharath kumar
Join Date: Apr 2009
Posts: 157
Rep Power: 8 
1) Do you have enough heat source for the radiation to heat?
2) Have you enabled thermal energy or total energy model? 3) Post some screen shots on pre set up. 

April 3, 2014, 02:59 

#3 
Member
Join Date: Apr 2010
Location: Pisa / Italy
Posts: 57
Rep Power: 7 
Hi,
the reason air is not heated is that absorpion is 0, so the radiation simply pass through it. This is true, I suppose, also for the quartz tube. Furthermore: Do you think a 0 refractive index for quartz is correct? This can highly affect your solution 

April 3, 2014, 21:42 

#4 
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Orry Shamash
Join Date: Apr 2014
Posts: 4
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"Hi,
the reason air is not heated is that absorption is 0, so the radiation simply pass through it. This is true, I suppose, also for the quartz tube. Furthermore: Do you think a 0 refractive index for quartz is correct? This can highly affect your solution" The absorption coefficient for the air and quartz tube ONLY are set to 0 in^1, while the working fluid's absorption coefficient is set to 1 in^1. The 0 refractive index is certainly not an appropriate approximation, I'm glad you point that out because a refractive index of 1 means the light does not change direction as it passes through the medium. I should set the refractive index to 1 to be safe, and 1.3 or 1.4 to be more accurate right? I'll see if this works 

April 3, 2014, 21:45 

#5 
New Member
Orry Shamash
Join Date: Apr 2014
Posts: 4
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"1) Do you have enough heat source for the radiation to heat?
2) Have you enabled thermal energy or total energy model? 3) Post some screen shots on pre set up." 1) I have a 1000 W/m^2 radiative heat flux directly upon the center of the tube, I imagine that should be sufficient for the photons within the Monte Carlo model to maintain above "threshold" values, no? 2)I have enabled thermal energy without viscous damping. I will change this to total energy and see if that changes anything, thanks for the help! 3) I'll post the CFXPre screenshots once the license becomes available as there is a simulation currently ongoing Thanks for your reply! 

April 4, 2014, 02:14 

#6 
Member
Join Date: Apr 2010
Location: Pisa / Italy
Posts: 57
Rep Power: 7 
Hi,
For my expereince the Absorption coefficient have to be calibrated respect and "average lenght" of your problem. Maybe 39.4 m^1 isn't enough ? I usually use the Beer  Lambert law to evaluate that ( http://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law ) If radiation pass through your working fluid for a small distance maybe you have to increase it. Another suggestion could be to insert also the abs.coeff. for the quartz so that It can heat the fluid through conduction 

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