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Infrared radiation and gas absorption

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Old   September 16, 2020, 03:49
Default Infrared radiation and gas absorption
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Ananya
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Hello,


I am trying to model the following scenario:
I have a silicon volume filled with CO2. Also, I have an IR emitter (which contains a heater, that heats up and emit IR radiation). The silicon has IR transmissivity of about 60%. So I would assume 60% IR intensity in the volume. Once the IR is in the volume, I want to model -
  1. IR absorption (and subsequent relaxation) by CO2 over time
  2. Other parameter such as temperature and pressure change in the volume corresponding to this absorption and relaxation.
For the beginning, I plan to use uncomplicated P1 radiation model. My queries are:
  1. Can I simulate just the volume w/ "IR source" at the bottom face of the volume without an actual IR source as in the figure below? I know I can provide temperature for the bottom face, but the case will then deviate from the original scenario and I will have temperature effects in addition to the radiation. Is there any other way to define an IR source for a surface?
  2. In real life the gases don't just absorb, but they relax as well (release of that absorbed energy) after a certain relaxation time. In ANSYS, I know that we can model the absorption by the gas. But if we run transient simulation, do we also see the relaxation? Is the relaxation time for the gases defined for the gases in ANSYS?






Fig: A rough model of the scenario explained above


Thanks
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Old   September 20, 2020, 11:14
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Quote:
Originally Posted by fluent_noob View Post
  1. Can I simulate just the volume w/ "IR source" at the bottom face of the volume without an actual IR source as in the figure below? I know I can provide temperature for the bottom face, but the case will then deviate from the original scenario and I will have temperature effects in addition to the radiation. Is there any other way to define an IR source for a surface?

Hi, I can't answer your second question but for your first one, you could just add a simple heat flux boundary condition. You can be more detailed by filling up the wall thickness box as well as defining the material properties correctly.

You can also attempt to define the heat flux absorption and internal emissivity of the material as a function of material temperature in Fluent. I believe this would give the most accurate results.
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Old   September 21, 2020, 06:11
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  • Thank you for the reply. I specify heat flux/temperature at the bottom wall, then the effect is not because of external "radiation", but now thermal energy will be involved, which is not the case. So for eg. IR radiation through the wall because of an external thermal source is different from defining heater temperature at the wall itself, because then the temperature of the wall will also change, and not just the emitted radiation.
  • I came across "External Radiation Temperature" under radiation section of thermal boundary conditions. According to some discussions like this, I believe I can define "External emissivity" and "External radiation temperature" corresponding to the IR source (i.e. heater in my case), directly at the bottom wall to replicate the effect of the radiation through the wall. What do you think?
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Old   September 22, 2020, 11:20
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Originally Posted by fluent_noob View Post
  • Thank you for the reply. I specify heat flux/temperature at the bottom wall, then the effect is not because of external "radiation", but now thermal energy will be involved, which is not the case. So for eg. IR radiation through the wall because of an external thermal source is different from defining heater temperature at the wall itself, because then the temperature of the wall will also change, and not just the emitted radiation.
I see, but you may want to consider the absorption % of infrared of the material itself, which causes the temperature of the wall to change. Since you mentioned in the problem diagram that the transmissivity of the silicone is 60%, then I assume that this absorption % is an important criteria. Then this will constitute your external thermal energy source.

I suppose it really depends on how detailed you really wanna go. If you wanna model the absorption thoroughly and transiently, then you may need to model the silicone walls and have a linear heat input source term as you travel through the silicone while having your 60% remaining IR radiation applied into the fluid domain.

Quote:
Originally Posted by fluent_noob View Post
  • I came across "External Radiation Temperature" under radiation section of thermal boundary conditions. According to some discussions like this, I believe I can define "External emissivity" and "External radiation temperature" corresponding to the IR source (i.e. heater in my case), directly at the bottom wall to replicate the effect of the radiation through the wall. What do you think?
Yes these settings are good. I've looked through the thread. Essentially, Fluent does not model the emission of heat from the walls due to the absorption of IR if you use the "semi-transparent" BC, you will need to input that yourself. I'm not quite sure about the "opaque" BC on the other hand.

I'm also not quite sure exactly what "Diffuse fraction" is supposed to mean. An explanation can be found here but I don't understand it either.

here
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Old   September 23, 2020, 02:35
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Yes, material absorption is an important parameter and it can be defined with the help of material properties.

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Originally Posted by srsel6 View Post

Yes these settings are good. I've looked through the thread. Essentially, Fluent does not model the emission of heat from the walls due to the absorption of IR if you use the "semi-transparent" BC, you will need to input that yourself. I'm not quite sure about the "opaque" BC on the other hand.

I'm also not quite sure exactly what "Diffuse fraction" is supposed to mean. An explanation can be found here but I don't understand it either.

here

Using the "semi-transparent" BC, one will have the absorbed radiation on the other side of the wall (in this case, towards the inner gas volume), if one defines appropriate material properties (i.e. absorption coefficient, scattering coefficient etc.) and wall boundary conditions (i.e. external emissivity, external radiation temperature, internal emissivity and material thickness etc.)


Regarding "Diffuse fraction", I went through the thread you have mentioned. Additionally, one can also have a look here for a more clear explanation. From my understanding, it's used on the case of solar radiation modelling, to account for diffuse radiation. Otherwise, it can be set to 0. Check here

Last edited by fluent_noob; September 23, 2020 at 05:16.
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Old   September 23, 2020, 08:08
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I see, then there is no need to apply a source term / heat flux to the wall then.


So the diffuse fraction is the fraction of energy that is reflected away from the surface, while the specular fraction is the fraction of energy that is refracted in the material and transmitted through.
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Old   September 23, 2020, 08:15
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Quote:
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I see, then there is no need to apply a source term / heat flux to the wall then.
Yes, the "external radiation temperature" and the "external emissivity" serve as the "source term".


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So the diffuse fraction is the fraction of energy that is reflected away from the surface, while the specular fraction is the fraction of energy that is refracted in the material and transmitted through.

That seems to be the case
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Old   September 24, 2020, 08:19
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Quote:
Originally Posted by fluent_noob View Post
That seems to be the case

Apparently, I was incorrect. After some more reading (Lec-5 Radiation Heat Transfer, Heat Transfer modelling using ANSYS Fluent), I came across the concept that the fundamental difference between both diffuse and specular reflections is their nature. Please see the attached images from the above mentioned text, to get a more clear idea of types of surfaces, in addition to different reflections.
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Old   September 25, 2020, 10:19
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Quote:
Originally Posted by fluent_noob View Post
Apparently, I was incorrect. After some more reading (Lec-5 Radiation Heat Transfer, Heat Transfer modelling using ANSYS Fluent), I came across the concept that the fundamental difference between both diffuse and specular reflections is their nature. Please see the attached images from the above mentioned text, to get a more clear idea of types of surfaces, in addition to different reflections.

I see, thanks for the update and clarification. I learned a lot from this thread even though you were the one asking for help hahaha
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