## Fluids

• ananyas
Subscriber

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?

• jibrahim
Ansys Employee
Hello Ananya,n1- You can do that in multiple ways:nA-You can use mixed bc where you can specify the radiation at the bc.nB-You could apply a heat flux at the boundary directly.nC-You can use Solar radiation model to consider radiation from outer sources.n2- I am not completely sure what you mean but I guess you are talking about gas absorption and scattering. They are considered in steady as well as transient simulation and can be specified in the material panel.nBest Regards,nJohnn
• ananyas
Subscriber
Hello John,nThank you for the reply.n Regarding the point 1B in your reply, if 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. nI 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?nRegarding the second point, I figured out I can define the absorption and relaxation as the function of the source term. For eg. I can have a sinusoid or a square wave form of the source term, which will correspond to the absorption and relaxation time in the system.nBest Regards,nAnanyann