Numerical study on the influence of fine particle deposition characteristics on wall roughness

Highlights

• The fine particles deposition mechanism on a rib-roughened surface is studied.

• The vortex structure is the main factor of small particle deposition enhancement.

• The upwind has a strong interception effect on the particle deposition.

• Wall surface energy has a significant effect on the deposition of large particles.

Abstract

This paper presents a study of the characteristics of a 1–50 μm particle deposition in a rib-roughened channel using a combined computational fluid dynamics-discrete element method (CFD-DEM). The effects of particle size, roughness element shape and surface energy on the deposition ratio are defined. Notably, The deposition rate of small particles (0.1<τ_p^＋<1) is greatly improved, the particle size of medium particle size (1<τ_p^＋<10) is small, However, the large particles (τ_p^＋>10) have a lower deposition rate due to larger mass and inertia; Due to the different angles between the windward and leeward surfaces of rough elements with different shapes and the direction of airflow, the velocity fluctuation of the flow field in the duct and the eddies near the rough elements can make obvious changes. The deposition rate of a sharp-angle rough structure was the highest, while that of a triangular rough structure was the lowest. And the increase of surface energy the particles deposition rate increase.

Introduction

Particulate matter is ubiquitous in nature and in engineering applications. Smoke, dust, fog, haze and other fine particles are often suspended in the air to pollute the environment. The International Organization for Standardization GB/T15604-2008 specifies that solid suspensions with a particle size of less than 75 μm are defined as dust. Dust particles (d_p < 75 μm) in urban environment the main pollution factors [1]. Several modern-day activities produce these fine particles, including pulverized coal combustion in power plants, automobile exhaust gas and building flue gas emissions. Their combined emission make the dust particles in the atmosphere increase sharply, causing serious harm to urban air quality and affecting human health [2]. When airflow carries particles through a wall, particles will collide with the wall, roll, slide and jump up and down due to the gravity of particles and the interaction between airflow and particle, but they eventually deposit on the wall. The fine particles are more likely to deposit on the wall because of their greater adhesion tendencies. It is found that the arrangement of rib elements in a channel can help improve the performance of a particle removal device [3,4]. The main reason is that a rough structure not only affects the air flow near the wall, but also has a strong interception effect on the incoming particles. However, rough structures of different shapes have different effects on airflow and particle interception. Therefore, it is of great significance to study the effect of rough elements with different shapes on the wall deposition of particles in the channel for cleaning and particle removal.

Due to environmental and industrial production needs, a comprehensive understanding of particle deposition is becoming increasingly important. At present, most scholars use experimental and numerical simulation methods to study the deposition of particulate matter in channels. Papavergos and Hedley [5] as well as Reeks [6], and Wood [7] proposed an empirical formula for three subsidence zones (diffusion zone, diffusion collision zone and inertial buffer zone) in the horizontal channel turbulent flow. Sippola [8] compares the above empirical formulas with Liu & Agarwal's [9] experimental data, which shows the Wood empirical formula to be more in line with the experimental results. Sommerfeld et al. [10] carried out an experimental analysis of a gas particle flow in narrow channels from two aspects of particle velocity distribution and pressure loss. They found that the roughness makes the particle distribution in the channel more uniform. The effect of wall roughness on particle concentration distribution decreases with the increase of particle size. Deshmukh [11] used high-speed particle tracking velocimetry and found that small particles are easier to disperse in channels than large particles. With the increase of mass load ratio, particles are easier to accumulate at the bottom of the pipeline. Barth et al. [12] recorded the turbulent flow field between periodic ribs using a three-dimensional particle image velocimetry (PIV) system. The structure of intercostal granular deposits was measured by a laser distance sensor. The accumulation of granular layers varies linearly with time. Particle impact, turbulent dispersion and gravity settlement have some effects on the thickness distribution of multi-layer deposits. Subsequently, Lecrivain et al. [13,14] studied the multilayer deposition process of aerosol particles in turbulent channels by numerical simulation. Zhao et al. [15] proposed an improved Eulerian model, which was used to predict the rate of particle deposition on the rough wall in a fully developed turbulent flow. They found that the dimensionless deposition velocity increases with the roughness. Milici [16] considered the elastic rebound of particles on a rough wave wall. The influence of particles on a turbulent flow field was analyzed. The results show that the particles near the wall surface flow to the flow area at a higher velocity. Matsusaka et al. [17] found that the deposition and re-entrainment of turbulent aerosol particles can form granular deposits layers. Among them, micron-sized particles tend to form zonal and film-like deposits. Submicron particles only form a film-like deposit layer. The thickness of the sedimentary layer is analyzed theoretically. Lo Iacono et al. [18] used large eddy simulation (LES) and Lagrange particle tracking techniques. The dynamic behavior differences of spherical and cylindrical particles in single-surface ribbed channel flow were studied. The results show that spherical particles mainly concentrate on the front surface of ribs, while cylindrical particles are not as likely to stick. At present, there are few studies on coarse wall particle deposition, and most scholars use clear large-scale obstacles to approximately replace the surface roughness. Lai et al. [19,20] used neutron activation to label particles. The aerosol particle deposition in fully developed turbulent flow fields with duplicate ribs on the surface was experimentally investigated. The spatial distribution of aerosol particles on a rib surface was monitored. It was found that repeated ribs on the surface resulted in an increase in pressure. In addition, particle resuspension and rebound reduce particle deposition on rib surface. Recently, H Lu and L Lu [21,22] combined their discrete particle model (DPM) with a Reynolds stress model (RSM), using a numerical simulation method. The particle deposition in a two-dimensional rectangular pipe with a rough structure was simulated numerically. At the same time, the effects of height-span ratio between a rough structure and the layout of a rough structure on particle deposition were studied. The height-span ratio study shows that the particle deposition on a rough wall is higher than that on a smooth wall. Dritselis et al. [23] found that the increase of a particle deposition coefficient on the rough surface is closely related to the mechanism of direct inertial collision and interception. The number of particles deposited on the back surface of the square column was very small. This shows that an increase in the effective deposition area does not necessarily mean an increase particle removal. In recent years, the discrete element method (DEM) has been widely used in the study of gas-solid two-phase flow. The process of particle deposition belongs to the process of adhesive contact between particles. The adhesion between particles and walls is mainly due to the force between particles or between particles and walls. Adsorption between particles and adherent substrates is achieved by force. Afkhami et al. [24] uses large eddy simulation and the discrete element method. The turbulent particle flows such as particle diffusion and agglomeration in the channel were studied, and results showed that the surface energy of particles is positively correlated with the agglomeration rate. Zhang [25] describes a particle trajectory and collision process by the discrete element method. They found that the collision between particles in the near wall region enhances the diffusion of particles along a vertical direction. At the same time, the resuspension rate of particles near the channel or chute bottom has a great influence. Li Shuiqing et al. [26] used the Johnson, Kendall and Roberts (JKR) model to simulate the deposition and aggregation of particles in gas-solid dilute phase flow. The deposition process of particles on fibers was analyzed. The results show that micron-sized particles are easier to deposit near the central streamline of the pipeline.

However, the effect of adhesion force on particle deposition was rarely considered in previous studies. In the contact process between particles and the wall surface, particles are deposited or detached from the wall due to van der Waals adhesion force and the interaction between airflow and particles. Notably, the van der Waals adhesion force is larger than the force of gravity with a decrease in particle size through the analysis of force on particles with a diameter of 1–50 μm, which makes it necessary to analyze the influence of adhesion force on particle deposition. At the same time, in previous studies, it was generally considered that the particles were considered to be deposited after contact with the wall surface. Therefore, the rebound effect between particles, particles and walls was considered.

Therefore, it is necessary to conduct in-depth research on particle deposition. It not only reveals the inherent laws of natural phenomena (such as the deposition of dust particles in the respiratory system)and industrial applications (such as ash deposition on the surface of heat exchangers and photovoltaic modules). At the same time, it is of great significance to the motion of micron particles under the action of multi-field. In this paper, the discrete element method (DEM) is coupled with computational fluid dynamics (CFD). Based on the JKR contact theory of the van der Waals adhesion force, the deposition of micron-sized particles on a rough surface was studied. The purpose of this paper is to discuss the effect of different rough structures on particle deposition and obtain the distribution of particles on rough walls.