Design and optimization of silencers in the subway air conditioning system

Dissertation for receiving a master's degree in technical engineering

in the field of mechanical engineering

Manufacturing and production orientation

Abstract

The growth and expansion of cities and the development of city life require infrastructure It is up-to-date and expandable. Transportation systems are among the basic infrastructures in cities, among which the creation and development of subway networks is of undeniable importance due to the high passenger carrying capacity, high movement speed, compliance with environmental standards and lack of pollution. One of the basic discussions in the creation and expansion of subway networks is the issue of air conditioning in tunnels and subway stations. In these systems, the task of moving and passing the air flow through the dug channels is usually the responsibility of the fans, and the passage of air through these channels is often associated with the creation of sound, which, if this sound is not controlled, noise pollution becomes a major problem. In addition, the different environmental conditions of different climates have caused many of the proposed materials and methods to be ineffective with changing conditions. Therefore, trying to localize the knowledge of voice recorders is considered as the main goal in this research. Therefore, in the upcoming research, while fully familiarizing with the issue of noise pollution in underground transportation systems, all kinds of noise control and reduction methods will be discussed and investigated, and also by measuring and applying reverse engineering methods, the silencer design used in a channel along with four other proposed designs was designed by Solidwork software and the flow through the silencers was simulated by Gambit and Fluent software. It was also introduced to those who want more research in this field by presenting the results of each design and a general comparison between the results of an optimal and a new design (taking into account all the design parameters).

Key words: Metro, air conditioning, sound pollution, sound insulation, layout, simulation.

Sound pollution

Metro systems improve the quality of the urban environment by reducing transportation pressure, reducing noise and improving air quality. Although subway stations usually have significant acoustic issues and problems in their interior spaces, they should provide both passengers and the people working in them with a calm and clean psychological environment with their proper design.

Research on noise control and acoustic design of subway stations is especially important from the point of view of sound intelligibility. In emergency situations, it is necessary to have a public alarm system to warn about the danger and also to convey other information. Voice intelligibility should meet an accepted minimum standard in such conditions. Another research on noise control is related to the fact that low-frequency noise in subway stations causes hearing problems for employees and has harmful effects on their mental health in the long term.

This is in a situation where other design decisions in stations make it difficult to control noise and reduce the level of voice intelligibility. For example, the use of hard and sound-reflecting materials in the internal surfaces of stations to create sufficient endurance, fire resistance, maintenance, and also comply with some health issues, including design issues, which make noise control difficult. Usually, the internal surface materials used in the stations are not chosen from the beginning according to the acoustic quality, but what is important about them are the safety issues. The use of suitable materials with different properties of sound absorption and scattering plays a significant role in the final performance. In addition, the total surface of these materials, which are installed in the right places, play an important role in the success of acoustic design. 1-2 Acoustics of subway stations: In general, the acoustic problems of subway stations can be divided into two parts: noise produced by internal and external factors. Among the most important internal factors are the noise caused by the movement of the train, brakes, noise of passengers and so on.Among the external factors, we can mention the noise outside the stations, such as the noise of cars, fans and other facilities that are directed into the station by air conditioning systems.

Recent studies of subway stations deal with general design parameters. Altay has examined the issues related to the movement of passengers, their safety, as well as their mental and physical health issues, and introduces noise control as the most important factor in creating a satisfying atmosphere. Durmisevic has also studied the issues related to lighting, fire, air conditioning, routing in stations and psychological issues, but he has not paid attention to acoustic features. A comprehensive study by Durmisevic and Sariyildiz (Durmisevic and Sariyildiz) examined the parameters related to the comfort and comfort of passengers in underground spaces and also paid attention to acoustic issues.

Metro stations are long covered spaces that, due to the variety and type of equipment in them, have different acoustic characteristics than normal spaces. Therefore, classical acoustic theories are not suitable for their analysis. Sound prediction studies can be classified into two broad groups: studies based on computer modeling methods and studies on scale models. 1-2-1 Computer modeling Computer modeling deals with the estimation of sound reverberation time and sound intelligibility. Kang has studied the basic features of sound distribution and reflection in long spaces. Lam and Li also studied the characteristics of reflection time and presented a theoretical model for long rectangular spaces. A ray-tracing computer model is presented by Yang and Shield to predict sound intelligibility in stations with a rectangular cross-section. Comprehension and characteristics of sound transmission have been studied by Shuo-xian and Yue-zhe using a visual method. Carman has presented a theoretical model for predicting train noise in stations and its effect on sound intelligibility. 1-2-2 Study on scale models The study on scale models is limited. Recently, Kang has used scale models with regard to the ability to understand the sound of several speakers in a station. Another study by him investigates how train sound is emitted in the station.

The volume, shape of the space and the materials used on the surface determine the acoustics of subway stations. As mentioned, surface materials are not initially selected based on acoustic characteristics, but different criteria related to safety issues are important in their selection. The geometry of the station along with the reflectivity of tiles or concrete walls and ceilings leads to a long reflection time which has an adverse effect on the sound intelligibility. On the other hand, accurate design and proper architecture improves sound intelligibility and reduces noise pollution.

According to Kang, absorbers should be properly installed in stations in order to improve sound intelligibility. In order to achieve a suitable sound level reduction, the absorbers should be installed equally in one section. Crockett and Pyke showed that for an equal number of absorbers, when installed on the ceiling and side walls, instead of just on the ceiling, the initial sound attenuation time will be shorter and the sound pressure level attenuation will be greater. Irvine and Richards (Irvine and Richards) studied the use of false ceilings above the platforms, and the results of this study indicate that the false ceiling system made of mineral or glass fibers that are embedded in the panels in compliance with fire safety issues, can significantly contribute to the absorption of noise [1]. but the lack of studies on the reduction of external noises is quite evident