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附录；Numericalstudyonparticle；headforastreetvacuumswee；BofuWu*,JinlaiMen,JieChe；SchoolofMechanicalEnglis；ABSTRACT；Thepurposeofthispaperist；Keywords:Pickuphead;Part；1.Introduction；Cu附录Numerical study on particle removal performance of pickuphead for a street vacuum sweeperBofu Wu *, Jinlai Men, Jie ChenSchool of Mechanical English Shanghai Jiao TongUniversity，Shanghai,200240,ChinaABSTRACTThe purpose of this paper is to investigate the particle removal performance of pickup head for a street vacuum sweeper numerically. An integrated 3D numerical model was constructed based on particle suction process in computational fluid dynamics (CFD) software. The airflow through the pickup head was treated as a continuum, while particles were modeled as dispersed phase. The Reynolds stress model (RSM) and discrete particle model (DPM) were chosen in order to predict the air and particles flow accurately. The numerical simulation results show that the sweeper-traveling speed and the pressure drop across the pickup head have great effects on the particle removal performance. The removal efficiency of particles increases with the lower sweeper-traveling speed or the higher pressure drop, and small size particles have higher grade efficiency than that of large size particles under the same operating conditions. Moreover, the removal mass flow rate of particles increases with the higher sweeper-traveling speed. Therefore, a trade-off should be considered among high removal efficiency, low energy consumption, and high removal mass flow rate. Through the numerical simulation, the effectiveness of street vacuum sweeper for removing particles from road surface is evaluated, and an optimal operating condition is obtained. Besides, more information is generated to better understand the particle suction process of the pickup head.Keywords: Pickup head; Particle removal performance ;CFD ;Sweeper-traveling speed Pressure drop1.IntroductionCurrently, there is a widespread concern over the pollution of particle matter. Dust and silt are the major sources of particle matter pollution, the removal of which therefore attracts considerable attention 1. Street sweeping is typically practiced toremove the accumulation of dust and silt from road surface to improve aesthetics, public healthy, and storm water quality, so it is considered as an effective pollutant control practice for many local authorities 2,3. Pickup head is the key component of street vacuum sweeper, which is designed to pick up particles efficiently from road surface and send them to dust collection hopper smoothly. The particle removal performance of the pickup head is the most important index for a street vacuum sweeper.Many researches have been performed on estimating the particle removal performance of street sweeping. For example, a study by Chang et al. 4 evaluated the effectiveness of street sweeping and washing for controlling ambient total suspended particles by experiments, which indicated that the street sweeping and washing process was effective at removing dust and silt from urban roads. However, some researchers such as Vaze and Chiew 5 considered that the contribution of street sweeping to environmental quality was not very clear, and may have an adverse impact because street sweepers did not pick up smaller size particles effectively. Kang and Stenstorm 6 studied the street sweeping effectiveness as a stormwater management practice by using statistical power analysis. They pointed out that the effect of street sweeping should not be underestimated because some previous researches were based on insufficient data. Therefore, new methods were needed to evaluate the street sweeping effectiveness.As the particle removal performance for street vacuum sweeper varies based on sweeping technology, operating conditions, sweeping frequency, street dirt loading and particle size distribution 7, it is necessary to develop a repeatable and reliable method to calculate the particle removal performance of pickup head for a street vacuum sweeper. In order to evaluate the particle removal performance of pickup head, engineers generally concentrate on two parameters, the sweeper-traveling speed and the pressure drop across the pickup head. Their influences on the particle removal efficiency and the particle removal mass flow rate directly relate to the performance of the street vacuum sweeper. Chen et al. 8 investigated the influence of sweeper structure and sweeper-traveling speed on the particle removal performance by experiments. They found that the wing plate of pickup head and the sweeper-traveling speed had great influence on the critical pickup velocity of particles. Meanwhile, they analyzed the relationship of the particle pickup velocities and the airflow rates.With the rapid development of the computer technology, the computational fluid dynamics (CFD) has been successfully adopted to study various industrial pneumatic conveying processes 9,10. Although many works have studied the particle removal performance by experiments over the past decades, few works considered the particle removal performance of pickup head using CFD. Xu et al. 11 conducted a 3D numerical simulation on the flow field of pickup head and gas system for a highway mechanical sweeper. They analyzed the interaction between the pickup system and the filter system, and optimized the combined system based on numerical results. Zeng et al. 12 employed the CFD technology to simulate the flow field of pickup head. The study showed that the performance of street sweeping was improved significantly by changing the structure of pickup head, and the numerical simulation proved that the CFD was an effective tool to calculate the flow field characteristics of pickup head. However, these works did not account for the influence of particles on the flow field during the investigation, so the particle removal efficiency and the particle removal mass flow rate cannot be obtained. More importantly, influences of the sweeper-traveling speed and the pressure drop, which are the most important parameters, should not be neglected while modeling the performance of pickup head.Gas cyclone is the device that utilizes centrifugal force to separate particles from carrier gas, whereas pickup head for a street vacuum sweeper is the device that employs vacuum pressure to suck up particles from road surface. Therefore, the particle removal performance of pickup head will be also calculated by using the CFD approach, which has been validated by the successful investigation of the particle collection performance of gas cyclones 13,14.The aim of the present study is to investigate the particle removal performance of pickup head under various sweeper-traveling speeds and pressure drop using commercial CFD program Fluent 6.3. The numerical CFD model is verified by sweeping field experiments. Based on the numerical results, the balance among the particle removal efficiency, the pressure drop across the pickup head and the removal mass flow rate is considered, and an optimal operating condition is obtained.2.Sweeping field experimentsA street vacuum sweeper with an engine power of 57 kw was used in this work. It used an elaborately designed vacuum system for dusts pickup. Fig. 1 showed that the street vacuum sweeper was picking up sand particles from experimental road surface by using a pickup head. As street vacuum sweeper moves forward, the centrifugal fan mounted on the sweeper establishes a vacuum in the dust collection hopper. Particles lay at rest on the road surface initiate to roll, slide and bounce as certain airflow velocity is reached, and then they are vacuumed through the four narrow slots (front, rear, left and right) into the pickup head. After entering the pickup head, most particles are picked up by the airflow and then sent from outlet into the dust collection hopper through a vacuum hose, but some will escape from the rear narrow slot to the outside of the pickup head. The schematic diagram of the pickup head used in the sweeping field experiments is shown in Fig. 2, and its main dimensions are given in Table 1.Fig. 1. Picking u