Learning Forum

[saichowdary3303]

Hi everyone,

I am working on simulation of Photoacoustic effect in fluent.

In my model, I have a closed detection chamber filled entirely with 100% CO2 (please see attached pictures).

The absorption of CO2 varies with respect to wavelength. I have downloaded the absorption coefficient data from the HITRAN on web database, covering a wavelength range from 4.18 to 4.35 µm.

Due to the significant variation and numerous data points of the absorption coefficient across this wavelength range, it is not practical to include every data point in the simulation. To simplify, I summed all the absorption coefficients over the wavelength range. I planned to use a single interval with one summed absorption coefficient in Fluent.

In respect to heat source, Radiation from the IR emitter is incident on the sapphire window. For this, I have data of emitted power w.r.t wavelength from IR emitter manufacturer. So I selected a wavelength range from 4.18 to 4.35 mum and used the summation to calculate Direct irradiation entering sapphire opening area.

I am unsure if this approach is correct. Please advise if there is a more accurate method to implement this.

With this model setup, I could see the following results, although pressure generated is not as much as expected, but I could see the variations in expected direction.

Further, during testing, I made slight changes to the wavelength interval, which led to significant changes in the output parameters.

I am uncertain about the relationship between the wavelength intervals and the resulting variations. Could you please provide information on this??

Additionally, I would appreciate any guidance on how to simplify non-gray radiation absorption by CO2 gas or on the discretization of wavelength intervals, absorption coefficients, or power data.

Let me know if you require further information.

Thank you for your time.

Note: Density of CO2 set to ideal gas

[Rohan Gulavani]

Hello,

What I understood is , you are adding both emmitted power across all wavelength as well as you are adding aborption coefficient as well, this lead to very high heat is absorbed in surface. So, What I suggest is take average of abosrption coefficient across all wavelength while you can sum emitted power across wavelength this will give you closer abosorbed value if not accurate. Whatever absoption coeffcient you used is looks odd. please check the value (Remember its unit is in 1/m). You can use "gray band" in material properties panel instead of constant. this facilitate you to use multiple bands of absorption coefficient. 

To understand how bands are affecting non gray raditions, suggest to refer Fluent Theory guide. So briefly, The behavior in each band is assumed gray and the radiative transfer equation (RTE) is integrated over each wavelength interval