طراحی و شبیه‌سازی یک حسگر فوتوآکوستیک برای سنجش آلاینده‌های حاصل از احتراق

Considering the importance of measuring the concentration of pollutant gases resulting from combustion, in this research, a photoacoustic sensor for measuring the concentration of pollutant gases is designed, analyzed and simulated. Photoacoustic sensors are based on the excitation of molecules due to light absorption and sound production in their decay stage. In this research, a photoacoustic sensor is designed to simultaneously measure the concentration of four polluting gases CO, NO, CO2 and SO2. This sensor has two buffer chambers to minimize the effect of background sounds and four resonant cylindrical cells to maximize the amplitude of the sound wave and the generated signal. An infrared light source with a radiation spectrum of 2-12 μm and a MEMS microphone are considered for the designed sensor. To transfer the light beam from the source to the four cells, two light reflective surfaces with a layer of gold have been used, and four lenses have been used to focus the light in the cells. Also, to control the spectrum of incoming radiation to the cells and match them to the absorption spectrum of the desired gases, four intermediate optical filters have been used. All optical elements were simulated in TracePro optical analysis software, the results of which show a 15% drop in the power of the radiation source after passing through the optical elements. Finally, using the existing theories, the resonance frequency for the first longitudinal mode was 1190 Hz and the quality factor of the cells was 61, which is a relatively suitable value. Also, the amount of audio and electronic signals produced in each cell were also calculated based on existing relationships. It was observed that the produced signals directly depend on the absorption coefficient of the target gases. In the present study, the lowest produced signal was obtained for NO gas and the highest signal was obtained for CO2 gas. The important parameters of the designed sensor were also calculated using COMSOL Multiphysics finite element software, which showed the resonance frequency as 1050 Hz and the quality factor r as 51. These results showed a 10% difference in cell resonance frequency and a 16% difference in the quality factor obtained from theoretical and numerical methods. The results of the software analysis also determined the best point for installing the microphone. In the end, the effect of temperature changes on resonance frequency was also investigated.