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•Indoor ventilation grill cannot well reduce the peak concentration.•UPE system can effectively decrease the particle concentration in subway station.•UPE system can decrease the ...particle concentration by more than 50 percent.
Thermal comfort has been the main target of the ventilation in subway systems. However, pollutant concentration and aerosol dispersion could be the leading health issues in underground metro stations. This study numerically simulated a train movement inside a subway system using the Dynamic Mesh Technique for a 3-D computational domain consisting of four stations and connecting tunnels. The effects of both the ventilation system and the train-induced fluid flow inside the subway system were investigated. Then, the particle generation and dispersion due to train braking are considered, and the impact of the ventilation system on reducing the particle concentration inside the station was investigated. It is shown that the airflow inside the subway system is entirely affected by the piston effect. The airflow generated by the train movement is much higher than that generated by the operation of the ventilation system when only one train passes through the tunnel. The results show that the ventilation system, consisting of the supply and exhaust fans inside the tunnel and supply grilles inside the platform, can reduce the particle concentration by half, except for the platform beside the stopped train when the train enters the station and during half of the train stop time. The other design concept demonstrates that the under-platform exhaust system considerably reduces the concentration of the particles released by the train braking system on the trackside platform.
•Particle size distributions of nucleation mode change remarkably along exhaust system.•Particle number concentration reduces more significantly with higher accumulation mode.•The constructed ...coagulation model can well catch the measured particle variation.•The coagulation history was provided to explain the particle evolution.
In this article, Brownian coagulation of particles in the exhaust system of a gasoline engine were investigated by measuring particle size distribution (PSD) at different sampling points and Monte Carlo simulation. The measurement results show that the particle size distributions of nucleation mode (NM) change remarkably after the exhaust gas passes through the exhaust plenum chamber. The constructed coagulation model based on the classic coagulation theory can well catch the measured PSDs, particle number (PN) concentrations, and count median diameters (CMDs) along the exhaust system under both the in-cylinder direct injection and port fuel injection modes, indicating that the particle process in the exhaust system is dominated by coagulation. The NM PN concentration decreases, and the NM CMD increases in the downstream exhaust gas, and the changes are more significant at higher accumulation mode (AM) PN concentration conditions. The coagulation history provided by modelling shows that coagulation mainly occurs between two NM particles (NM-NM) or one NM and one AM particles (NM-AM). At higher AM PN concentration conditions, the NM-AM coagulation dominates and quickly eliminates NM particles, resulting in more significant NM particle reduction. The PN concentration variation as a function of time indicates that PN concentration decreases quickly at first and then the decreasing rate slows down.
Exhaust ventilation system with one central fan and multiple terminals has been widely used for the heat and contaminant removal in building environment. Conventional design without pressure ...balancing leads to uneven distribution of exhaust airflow rate among the multiple outlets. Existed balancing methods usually uses dampers (constant-air-volume valve or regulating valve), tapered duct, or varied inlet area. However, these methods result in higher fan energy consumption, or complicated construction and on-site commissioning. In this paper, a flow-guide device was developed for adjusting the pressure distribution of duct branches. This new device is integrated with the interflow Tee-junction and does not need any commissioning or regulating. The resistance performance of the device responding to the structural parameter was derived using the CFD simulation and experiment. The negative direct resistance featured by the device was found to effectively benefit exhaust at the outlets farther away from the central fan. The ductwork hydraulic model based on the Bernoulli's law of airflow and the fitted resistance correlations were further proposed to fulfill the parametric design. Finally, full-scale test was carried out for a central exhaust system installed with the flow-guide devices referring to a factory workshop with 30 heat and contaminant sources. Compared to the system without the devices, the total rate of the system increased by 25%. Discrepancy of exhaust rate decreased by 78% and uneven degree decreased by 82%, which well meets the engineering balancing requirement. Meanwhile, total resistance of the system reduced 23.8% owing to the negative loss the devices bring.
•The device can act as regulating valve through adjusting its structural parameter.•Uniform exhaust can be realized by the devices in central air exhaust system.•The novel device conditionally brings negative pressure loss along the main duct.•Total loss of system reduced 23.8% due to the negative loss the devices bring.
Vehicle exhaust pipe is an excellent heat source and its integration with thermoelectric generators for thermo-electrical conversion has been recently attracting much attention. The methods of ...increasing power output include the parameter optimizations of the thermoelectric module material, thermoelectric module geometrical shape and exhaust gas property. Because of the coupling nature of these parameters, however, there is a lack of lateral comparison providing with the idea of which parameter gives the largest effect and which parameter combination gives the best power output performance. For this purpose, a novel hexagonal shaped housing has been developed for an engine exhaust thermoelectric generator unit to contain thermoelectric modules with higher temperature difference. The concept of hexagonal shaped housing provides a larger contact area which will increase the temperature differences applied on the hot and cold sides of the thermoelectric module. The thermoelectric generator unit performance has also been improved by the concept of the cross flow which will enlarge the high temperature difference area. Built with these two new concepts, an innovative engine exhaust thermoelectric generator unit has been designed, fabricated and tested.
The performance optimization of the thermoelectric generator unit has been conducted based on the validated simulation model. The identified key parameters include the number of thermocouples, geometric shape/curvature of the thermoelectric module, thickness of the thermocouples, thermoelectric module material, number of layers of thermoelectric modules, exhaust gas temperature and the thickness of the ceramic substrate. It suggests that four parameters need to be focused for optimization, which are the number of thermocouples, exhaust gas temperature, thermoelectric material and geometric shape/curvature. In order to reduce the complication brought by modifying other parameters, further, Optimized Systems 1, 2 and 3 with different optimized parameters are compared. It is found that a good trade-off between achieving high power output and avoiding unnecessary technical complexities and related cost increase can be reached for Optimized System 3 with 152 thermocouples and 480 °C exhaust gas temperature.
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•The concepts of hexagonal shape and cross flow are applied in the TE generator.•The prototype generates up to 23 V on a vehicle engine with series connection.•Lateral comparison includes the TE material property, TE geometry and temperature.•Optimized systems with the easy implementation are proposed and compared.
Abstract
In order to further improve accuracy and stability of detection of combustion coal fallout propensity of cigarettes, author of the paper adopted computational fluid dynamics (CFD) technology ...for a three-dimensional numerical simulation of exhaust system of detection instrument, aiming to study characteristics of flow field near cigarettes. Moreover, a simulation model of eight-channel exhaust enclosure was established, obtaining vector diagram for flow velocity of flow field, velocity contour diagram, and pressure distribution cloud diagram. According to findings, flow field of eight channels is evenly distributed, with slow flow velocity around the instrument but furious inside channels. The wind velocity of cigarette monitoring channel is stable at about 200mm/s specified as per standard. However, there is significant change in pressure and flow velocity at the corners of channels, causing local turbulence. In experiments, average wind velocity of 8 monitoring channels was measured, and simulation results were compared with experiment data. Eventually, a conclusion is drawn that simulation result at cigarette monitoring channels changes consistently with the experimental data, with small errors as a whole. Therefore, the designed exhaust system complies with regulations on wind velocity stipulated by YC/T558-2018 Cigarettes—Determination of Combustion Coal Fallout Propensity of Burning Cigarettes. In a word, this paper is hoped to provide technical support for analogue simulation of exhaust system of cigarette detection instrument, and improve detection accuracy.
Scrubbers are widely used for air pollution treatment in semiconductor. Efficiency of waste gas treatment must be carefully maintained for environmental and health issue. However, due to the complex ...types of exhaust system and the needs of using chemicals with different characteristics at the same time in the semiconductor process, many conditions will occur in the operation of the scrubber tower and the processing efficiency will be reduced. This paper aims at the common problems such as growth of bacteria, unstable pH, white smoke, and inaccurate monitoring of exhaust gas flow rate. Based on the investigation, all the analysis, results and solutions are provided as practical reference for other semiconductor companies.
In order to understand the coupled effect of the nacelle and exhaust system and to improve their overall performance, we studied the aerodynamic performance and the flow characteristics of the high ...bypass ratio turbofan nacelle and exhaust system by numerical simulation. The geometric parameters of a nacelle and exhaust system (e.g., the contraction ratio of the cowl afterbody and the fan nozzle exit angle) were investigated to evaluate their influence on the overall performance of the nacelle and exhaust system. The related flow mechanism was explored as well. The results show that the flow field of the nacelle and exhaust system under the mid-cruise condition exhibits characteristics of transonic flow. A stagnation zone exits at the nacelle lip and there is a velocity peak at the nacelle forebody. There exist a number of complex flow phenomena (such as shockwave, expansion wave, shear flow and shock wave-boundary layer interaction) in the downstream of the fan nozzle exit plane. The magnitude of the fan nozzle thrust or the intake ram drag is much higher than that of the additional drag, the nacelle drag or the core nozzle thrust. And for the nacelle drag, the friction drag of the cowl is in the same order of magnitude as the pressure drag of the cowl, the core cowl and the plug. But it is much larger than the friction drag of the core cowl and the plug. The effective thrust increases by 4.7% as the contraction ratio of the cowl afterbody increases; and it increases by 2.4% as the fan nozzle exit angle increases. The expansion degree of the fanjet flow, the shock wave strength and location, and the existence of the flow separation or second shock wave are influenced by the contraction ratio of the cowl afterbody and the fan nozzle exit angle. These phenomena have effects on the pressure distribution of the core cowl and the surrounding fanjet flow velocity, and hence they further affect the nacelle drag. The increase in the fan nozzle exit angle can noticeably reduce the thrust of the fan nozzle.
Constantly tightening vehicle emission standards leads to the creation of new technologies for reducing the content of toxic substances in exhaust gases while maintaining the highest possible engine ...performance. At the moment, however, it is clear that with regard to future emission standards, despite the technologically highly refined system of catalytic converters, the reduction of emissions with the help of catalytic converters alone will be insufficient. That is why a variable exhaust manifold/system was developed and patented, which has a significant impact on the reduction of vehicle emissions. This innovation enables three working modes of the engine, depending on the demand and output power requirements. The presented article presents the complex architecture of the proposed system, including the description and simulation of individual operating modes. The first working mode is the most ecological and economical mode, but it has a relatively lower performance. The second working mode is characterized by the production of higher power, and the third mode can be used at high engine loads. 17 refs.
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