Background: Exhaust noise is known to be a major pollutant in the environment and workplaces due to the development of industry and transportation. Exhaust noise can be reduced to normal levels by mufflers or silencers. A reactive muffler efficiently dampens noise at low frequencies by reflecting sound waves. Therefore, muffler design is of great importance in exhaust noise reduction. Transmission loss (TL) is an essential characteristic of mufflers, demonstrating their acoustical properties. Any acoustical appliance is selected based on its damping performance and reliability. Predicting TL through experimentation is different from theoretical calculations. Methods: In the present study, a double-expansion chamber muffler was designed as a reflective muffler on a laboratory scale by equations. Next, TL was evaluated by an impedance tube applying a 4-microphone technique to determine the acoustical performance of the designed muffler. Results: Findings revealed that the TL of the muffler at 312 Hz frequency obtained 27.5 dB agreement with the required TL of the muffler of 25 dB. In addition, the TL of the muffler against frequency attenuates noise in broadband frequencies. Conclusions: These results indicated that the built muffler provides desired TL for exhaust chambers. Therefore, equations can be used as a precise method for muffler design. Furthermore, multi-expansion chamber mufflers are useful for reducing noise at a wide range of frequencies.
Air pollution is a major health problem in developing countries and has adverse effects on human health and the environment. Non-thermal plasma is an effective air pollution treatment technology. In this research, the performance of a dielectric barrier discharge (DBD) plasma reactor packed with glass and ceramic pellets was evaluated in the removal of SO 2 as a major air pollutant from air in ambient temperature. The response surface methodology was used to evaluate the effect of three key parameters (concentration of gas, gas flow rate, and voltage) as well as their simultaneous effects and interactions on the SO 2 removal process. Reduced cubic models were derived to predict the SO 2 removal efficiency (RE) and energy yield (EY). Analysis of variance results showed that the packed-bed reactors (PBRs) studied were more energy efficient and had a high SO 2 RE which was at least four times more than that of the non-packed reactor. Moreover, the results showed that the performance of ceramic pellets was better than that of glass pellets in PBRs. This may be due to the porous surface of ceramic pellets which allows the formation of microdischarges in the fine cavities of a porous surface when placed in a plasma discharge zone. The maximum SO 2 RE and EY were obtained at 94% and 0.81 g kWh −1 , respectively under the optimal conditions of a concentration of gas of 750 ppm, a gas flow rate of 2 l min −1 , and a voltage of 18 kV, which were achieved by the DBD plasma packed with ceramic pellets. Finally, the results of the model's predictions and the experiments showed good agreement.
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