Numerical study on particle removal performance of pickup head for a street vacuum sweeper

Bofu Wu , Jinlai Men, Jie Chen

School of Mechanical English Shanghai Jiao TongUniversity，Shanghai,200240,China

ABSTRACT

The 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 drop

1.Introduction

Currently, 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 to remove 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

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道路吸尘清扫车吸嘴的颗粒去除性能的数值模拟研究

Bofu Wu *, Jinlai Men, Jie Chen

机械工程学院，上海交通大学，上海，200240，中国

摘 要

本文的目的是探讨道路吸尘清扫车吸嘴的颗粒去除性能数值。基于粒子吸气过程的计算流体力学软件(CFD)，构建了一个集成的三维数值模型。气流通过吸嘴被视为连续时，颗粒仍作为分散的模型。选择雷诺兹应力模型（RSM）和离散颗粒模型（DPM）以准确预测空气和颗粒流。数值模拟结果表明，清扫车的行驶速度和吸嘴的压降对颗粒去除性能产生很大的影响。去除效率在清扫车行驶速度较低或更高的压力降时会增加，小尺寸颗粒比大尺寸颗粒，在相同的操作条件下相比，具有更高的去除效率。此外，去除颗粒的质量流率会随着清扫车的行驶速度增加而增加。因此，我们应该选择在除效率高，能耗低，和高去除质量流率之间找到一个平衡。通过数值模拟，对街道吸尘清扫车的道路表面除去颗粒的有效性进行评估，并获得最佳操作条件。此外，获取更多的信息以便更好地了解吸嘴颗粒的抽吸过程。

关键词：吸嘴;颗粒去除性能; CFD;清扫车行驶速度的压力降

1 引言

目前，社会上普遍的关注颗粒物的污染。灰尘和泥沙作为颗粒污染的主要来源物，因此，有效的去除掉它们成为了人们关注的焦点[1]。扫街，通常用来除去路面上积累的灰尘和淤泥以提高路面美观，保障公共健康以及改善雨水的质量，因此它被许多地方当局认为是一种有效的污染控制工作[2] [3] 。吸嘴是道路吸尘清扫车的关键组成部分，它的设计主要用来从地面有效地吸取灰尘等颗粒并输送到集尘箱中。吸嘴的颗粒去除性能是一辆道路吸尘清扫车的最重要指标。

许多研究已经进行估算颗粒去除的性能。例如，Chang等[4]通过实验里控制环境中的总悬浮量来评估街道清扫和清洗的有效性，这表明街道清扫和清洗过程在从城市道路去除灰尘和淤泥中是有效的。然而，一些研究人员，如Vaze 和Chiew [5]认为清扫街道对于环境质量的贡献还不是很清楚，此外可能会产生不利的影响，因为清扫并没有有效地除去较小尺寸的颗粒。Kang和Stenstorm [6]通过雨水管理实践来研究清扫道路的的有效性，他们使用了统计功率分析的方法。他们指出，扫街的有效性不应该被低估，因为以前的一些研究是基于数据不足的基础。因此，需要新的方法来评估街道清扫的有效性。

由于街道吸尘清扫车的颗粒去除性能是基于清扫技术、操作条件、扫频、街头污垢负荷和粒度分布[7]，因此有必要开发一种可重复的和可靠的方法来计算道路吸尘清扫车吸嘴的颗粒去除性能。为了评价吸嘴的颗粒除去性能，工程师们一般关注于两个参数，清扫车的行进速度和吸嘴的压力降。以上两个参数对于颗粒去除率和去除颗粒质量流率的影响，直接关联到道路吸尘清扫车的性能。Chen等人[8]在实验中研究了清扫器结构和清扫行进速度对颗粒的除去性能的影响。他们发现，吸嘴的翼板和清扫车行驶速度对于颗粒的临界吸取速度有很大影响。同时，他们对于颗粒拾取速度和气流速率的关系进行了分析。

随着计算机技术的飞速发展，计算流体动力学（CFD）已成功地应用到研究各种工业气力的输送过程中[9] [10] 。虽然在过去的几十年里许多实验研究了颗粒去除性能，但是很少实验采用CFD研究吸嘴的颗粒去除性能。Xu等人[11]对公路机械清扫车的吸嘴和气体系统的流场进行了三维数值模拟。他们分析了拾取系统和过滤系统之间的相互作用，在数值计算结果的基础上优化组合的系统。Zeng等[12]采用CFD技术模拟吸嘴的流场。研究表明，清扫性能能显著提高通过改变吸嘴的结构，数值模拟证明CFD是一种有效的工具来计算吸嘴的流场特性。然而，这些研究并没有考虑到实验中流场中颗粒的影响，因此不能得到微粒去除效率和去除微粒的质量流率。更重要的是，当设计吸嘴的性能时，不应该被忽视清扫车行驶速度和压降这两个重要的参数的影响。

旋风分离器利用离心力从载体气中分离颗粒，而道路吸尘车的吸嘴采用真空负压从路面吸入颗粒。因此，吸嘴的颗粒去除性能也将使用CFD方法来计算，而且这种方法已经在旋风分离器中的颗粒收集过程中被成功验证[13]和[14] 。

本研究的目的是使用商业CFD program Fluent 6.3调查吸嘴在各种清扫车行驶速度和压降的颗粒去除性能。CFD数值模型通过清扫场的试验进行了验证。在数值模拟结果的基础上，考虑到颗粒去除效率，吸嘴的压力降和去除颗粒的质量流率之间的平衡，选择最佳的操作条件。

2 清扫现场实验

用于这项实验的街道吸尘清扫车的发动机功率是57千瓦。它使用一个精心设计的真空系统来吸取粉尘。图1显示吸尘清扫车通过使用吸嘴吸取实验路面的沙粒。道路吸尘清扫车向前移动，安装在清扫车上的离心风机在集尘箱中建立了类真空环境。达到一定的气流速度时，颗粒在路面上开始滚动、滑动和反弹，然后它们通过窄缝（前，后，左，右）吸取到吸嘴中。进入吸嘴后，大多数颗粒被被气流带动，然后通过真空软管进入集尘料斗，但有些人会从后方的狭槽逸出到吸嘴的外部。清扫场试验中使用的吸嘴的示意图如图2所示，它的主要尺寸在表1中给出。

图1 场地实验中从路面拾取颗粒灰尘

图2 吸嘴的示意图

在现场实验中，安全因素是至关重要的，因为这将是危险的，一旦测量阻止了交通。因此，选择了没有交通的道路，如图1中所示。在路面上，一个2.0米times;1.8米采样路面被漆成白色。砂均匀地分布的采样表面上，作为试验用的微粒的密度为2500公斤/米3，质量负荷设定为0.1公斤/米2。砂的粒度分布Yacute; eth;质量可以使用松香Rammler分布来定义

d p 是颗粒的直径，d是颗粒平均直径，n是传播参数。

从砂数据计算，颗粒平均直径和传播参数分别是81微米和5.95。砂颗粒的累积粒径分布绘制在图3中。

图3 砂颗粒的累积粒径分布

清扫场试验在清扫车的行驶速度是6、7、8、9、10、11、12、13和14公里每小时进行，通过测量在清扫前后从采样表面收集的的泥沙的质量得到的微粒去除效率。气流速度和压降采用热丝测速仪和压力计测量。

3 物理模型和生成网络

为了模拟上述颗粒吸入过程中，在本研究中进行了一些假设，图4中示出。

1）吸嘴以一个恒定的行驶速度在清扫过程中向前移动。

2）在四个窄槽的周围，存在着外部空气领域，它的压力是大气压。

3）通过吸嘴的气流被认为是不可压缩的，动荡的，稳定的。

4）颗粒假设在前窄槽前被吸取到吸嘴中。

图4 吸嘴的物理模型

网格生成在CFD中提高数值模拟的精度和效率起着至关重要的作用，所以网格类型和网格方案的选择是至关重要的。在这项研究中，吸嘴段除了颈部外具有规则的形状。

因此，为了获得高品质的网格，将几何模型分解成几个部分是必要的。颈部由于几何形状复杂连通四面体网格。对于其他部分，设计成带着桶形状的六面体模型。在这项研究中的计算网格约274157个网格。图5示出物理模型的表面网格。

图5 CFD表面网格的物理模型

4 数学模型

4.1 控制方程

通过吸嘴的气流通过求解一组控制方程的数值计算。考虑到稳定和不可压缩气流通过吸嘴，Reynolds- Averaged Navier–Stokes方程可以写成

ui和gi分别是沿坐标 x 的气流速度和重力加速度，rho;为空气密度，P 为压力，mu;是粘度，tau;ij = minus;rho; P u′i u′j是雷诺应力，代表湍流波动的影响。

4.2 雷诺应力模型（RSM）

由于具有高的速度，并成功模拟颗粒抽吸过程中的吸嘴的气流湍流，它准确地描述出了动荡的行为。湍流模型的选择取决于物理模型。与kminus;ε模型相比，它被广泛应用于工业流量计算中，RSM更准确因为RSM考虑到了流线曲率，漩涡，旋转，应变速率以严格的方式的迅速的变化的影响，因此在复杂流动中它具有更大的可能性给出准确的预测[15]。许多研究表明，在复杂流场的计算模型中RSM可以比kminus;ε提供更好的精度[16] 和 [17] 。

在RSM中的涡流粘度的方法将被丢弃，雷诺应力输运方程用于描述雷诺应力的影响。然后雷诺应力用于取得封闭的雷诺平均动量方程 [18] 。对于稳定，无旋，不可压缩气流，雷诺应力输运方程采用以下形式

Cij, DTij, DLij, Pij, ϕij和εij 分别是对流项、扩散项、分子扩散越长期、生产长期压力、压力应变项、粘性耗散率。

在式（4）中，可以通过求解输运方程获得的紊流动能k和耗散率ε

mu;t是湍流粘度，在这个模型中使用的常量sigma;k=0.82, Cmu;=0.09, sigma;ε=1.0, C1ε=1.44, C2ε=1.92 [19]。

4.3颗粒的运动

欧拉-拉格朗日方法预测吸嘴中气固两相的流场。气相中被视为一个统一体，通过求解Reynolds-Averaged Navier–Stokes方程如上所述，而固相通过跟踪在连续

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