Study On The Performance Optimization Of The Air Supply System And CO₂ Diffusion Inside Subway Car

The good air distribution is essential for maintaining the good thermal comfort conditions and fresh air quality inside the subway cars. To ensure uniform air supply through the air duct is a prerequisite to maintain good airflow distribution inside the subway cars. On the premise of uniform air supply through the air duct, that how to choose the velocity of supplying air, the number of the supplying air outlets of the main air duct and the ratio of the supply air volume of the main air duct and the flat air duct will directly affect the performance of air supply system of the subway car, thereby affect air distribution inside the subway car. Meanwhile, the knowing of pollutant CO2 diffusion inside the subway car will also provide important guidance for improving airflow inside the subway car. Therefore, the paper will start with duct structure optimization and further study the performance optimization of supply air system and the diffusion law of pollutant CO2 in the subway car.In order to facilitate quantifying the uniformity of supply air through lots of air outlets of supply air duct in the subway car, a correlation measuring the uniformity of air distribution is proposed based on area-weighted average velocities and mass-weighted average velocities in this paper. Since area-weighted average velocity and mass-weighted average velocity at the supply air outlets can be computed directly in numerical simulation, the correlation can be used to analyze velocity distribution uniformity of the simulation results more conveniently. To test the proposed approach to quantifying airflow distribution uniformity, the correlation is applied in two studies-one based on assumed velocity distribution, another based on the simulated velocity distribution- in comparison with two existing formulae from prior studies. The results show that the evaluation results of the proposed correlation are in complete agreement with that of the two formulae from prior studies in the overall trend.The supplying air through a certain independent air supply duct in the subway car is simulated and tested, the simulation results are compared with the experimental results according to the velocity of supply air and the supply air volume through air outlets of the air duct, the results verify the reliabilty of the numerical simulation that is used to the optimization of air duct structure in the subway car.A new idea on the optimization of air duct structure—guide plates in the main air duct will be replaced with perforated plates, the baffles are used in the flat duct and its width is the same with the width of supply air outlet of air duct—is used in a air duct of the subway car and its results on the optimization are analyzed by simulation optimization, the results found that the perforated plate set within the main duct can effectively balance the pressure within various sections of air duct and make the static pressure within each section of air duct relatively balanced, which provides a good prerequisite for ensuring the uniformity of supply air through each air outlet. The baffle set in the flat duct can effectively improve supply air direction through each air outlet in the flat duct, at the same time, supply air volume through each air outlet can be good adjusted and the uniformity of supply air through each air outlet can be improved by adjusting the height of the baffle and making the second half of the roof of flat duct tilt to the end of flat duct.In order to understand the effect of simultaneously changing of air supply velocity, the number of the supplying air outlets of the main air dudct and the ratio of the supply air volume of the main air duct and the flat air duct on the air distribution performance inside the subway car, the three factors with the three levels seperately are arranged 15 different combinations of three factors by central composite design. The air distribution performance of 15 schemes are obtained by the numerical simulation. Based on the response surface methodology,15 sets of data are analyzed by the commercial statistical software MINITAB, air distribution performance model predicted is obtained. Analysis of variance on the predicted model is implemented, the results show that the supply air velocity has greatest impact on the performance index of air distribution in the subway car. Moreover, the interaction effect of supply air velocity and the number of the supplying air outlets of the main air duct on the air distribution performance index is more obvious. When the supply air velocity is constant, the interaction effect of the number of the supplying air outlets of the main air duct and the ratio of the supply air volume of the main air duct and the flat air duct has a certain influence on the air distribution performance index. Meanwhile, according to the response surface model of the air distribution performance index obtained, the designer of air supply system can easily choose the best combination of the design variables affecting the performance of air supply system to obtain the maximize air distribution performance index value.Finally, the paper study the effect of changing the location of passengers on pollutant diffusion inside the subway car when the subway car is at full strength. According to the standing way adopted possibly by passengers in the subway car, the four schemes are set, the dispersion of pollutant CO2 at the 1.7m height from the floor in the standing room and at the 1.1m height from the floor in sitting areas inside the subway car is studied on the four schemes by numerical simulation, respectively. The results show that at the 1.7m height from the floor, under the mode of supplying air and returning air from the top of subway car and exhausting air from the top of both sides of subway car, the pollutant CO2 concentration in the region away from the return air inlet and near the end of subway car is significantly lower than that in the region near the return air inlet and the inner end of subway car. the pollutant CO2 concentration in the region near the return air inlet and at the 1.1m height of the passengers sitting of both sides in the subway car is overall higher than that in the region away from the return air inlet and at the 1.1m height of the passengers sitting of both sides in the subway car. The changing of the passenger location have not significantly influence on the overall distribution trend of the dispersion of pollutant CO2, but have some impact on the dispersion of local pollutant CO2 in the subway car. The study provides the theoretical base for arranging measurement points to monitor contamination content and improving airflow in the subway car.