Journal of Nanotechnology

Volume 2018, Article ID 7025458, 10 pages

https://doi.org/10.1155/2018/7025458

## Experimental Views of Tran-Bend Particle Deposition in Turbulent Flow with Nanoscale Effect

^{1}The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China^{2}National-Provincial Joint Engineering Research Center of High Temperature Materials and Lining Technology, Wuhan University of Science and Technology, Wuhan 430081, China^{3}Department of Mechanical Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong

Correspondence should be addressed to Ke Sun; moc.liamg@noos.ladnar

Received 1 February 2018; Accepted 5 May 2018; Published 10 June 2018

Academic Editor: Martin Seipenbusch

Copyright © 2018 Kun Zhou et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This paper presented experimental views of nano- and microaerosol distribution and deposition in turbulent tran-bend flows. These views included the particle flow measurement and particle depositions through individual bends, bifurcation bends, and those behind bends. Selected experiments were summarized and compared according to the gas flow, the bend geometry, and the particle flow properties. Based on recent studies, the influencing factors of environmental humidity, particle and surface properties, nanoparticle formation, coagulation, or evolution phenomena were discussed, and then research suggestions were given for future research and applications. It is specially mentioned that the new particle formation and nanoparticle growth affect its deposition under environmental contaminant conditions; nanoscale particle dynamics and transport have a growing trend on attracting the research and industry attentions.

#### 1. Introduction

The living environment is filled with suspended aerosol particles such as nanoparticles and PM2.5 (airborne particles with an aerodynamic diameter less than or equal to 2.5 *μ*m) [1, 2]. These nano- and microparticles will commonly flow into enclosed places like window cracks, sampling pipes, or ventilation ducts [3]. Generally, these enclosed places own certain bends or curved ducts, which play a significant role of changing the air and particle flow directions [4, 5].

Accurate measurements of the background gas flow properties, particle properties, particle concentrations, and curved flow line geometries can accurately predict particle flow distribution, deposition, and accumulation status [6, 7]. These kinds of measurements include but not limit to gas flow velocity, wall surface roughness, particle size distribution and evolution, deposition amount, concentration distribution and evolution, and so on [8].

Hence, this article aims to review the experimental bend nano- and microparticle flow, distribution, and deposition; to analyse influencing factors; to summarize recent findings; and to give future research and application recommendations. To summarize, particle distribution and deposition in a single bend, behind a bend, and through bifurcation bends are reviewed. Selected or potential influencing parameters are discussed, including the Dean number, curvature ratio, nonspherical particle, particle evolution, particle surface effect, roughness, and environment humidity.

#### 2. Basic Definitions

Previous studies of aerosol flow in bends focus mainly on averaged deposition and penetration. The basic aerosol deposition or loss efficiency can be expressed as follows:where is the particle penetration ratio or efficiency, and are, respectively, the mean particle mass/number concentration at the cross sections of bend exit and entrance for a steady or periodic measurement condition. Apparently, is reversely proportional to , which is determined by the concentration status and related measurement technique. The commonly adopted averaged concentration is a convenient statistic parameter of the particle distribution description and flow status. Meantime, on the view of the statistic theory, there are also fluctuating components varying with the measurement location and the time slot. The concentration changes with the time is generally controlled by the measurement method. While for location-dependent changes, there exist particle concentration distributions, which will affect the local deposition velocity, related particle deposition distribution, and accumulation status.

Particle deposition velocity has the definition as below:where stands for a deposition flux onto a specific surface and denotes a mean particle concentration near a surface. An effective approach to interpret the particle deposition is to build up the relationship between the dimensionless deposition velocity and the dimensionless relaxation time . The former parameter is defined as follows:where is the airflow friction velocity, which is determined by the averaged or bulk flow line velocity and the friction condition of the flow lines. The background flow velocity could be easily measured by the modern apparatus. From this equation, is found also to be influenced by the friction condition of the flow lines, for example, the wall surface roughness.

The dimensionless relaxation time mentioned above is determined aswhere represents the eddy lifetime, which could be computed by the background flow properties. means the particle relaxation time defined by the following equation:where stands for Cunningham slip correction factor for particles; and are, respectively, particle density and diameter; and means the gas dynamic viscosity. Cunningham coefficient caused by slippage is determined by the Knudsen number Kn, which is defined as the ratio between the mean free length of the air molecules and particle diameter [9]. These particle parameters can be determined from the preknown or measured background flow and particle properties.

For diffusion dominated nanoparticles, Schmidt number is a crucial parameter to describe the effect of viscosity and diffusivity. It can be expressed aswhere is gas kinematic viscosity; is particle diffusivity; means the Boltzmann constant; and stands for the temperature [10].

Along with the particle deposition expressions, the penetration efficiency is usually depicted against the particle Stokes number or particle aerodynamic diameter . can be formulated aswhere is the average air speed in the flow line like pipe, channel, or duct; and is the hydraulic diameter of the flow line, given bywhere is the cross-sectional area of the flow line and is the perimeter of a cross section.

For bend particle flows, additionally, the Reynolds number () in flow lines is determined aswhere stands for background gas density. Based on the flow line Reynolds number, the bend Dean number can be calculated bywhere means the curved flow line curvature ratio. It is computed by (*r*_{1} *+* *r*_{2})/*D*, where the parameters *r*_{1} and *r*_{2} are inner and outside radii of the curved flow line wall, respectively. Both the Dean number and Reynolds number are useful to nondimensionally depict the bend particle flow and deposition phenomena, and they can be determined by accurate measurement of the bend geometry, background gas property, and flow velocity.

#### 3. Particle Flow Measurement through Bends

Some experiments of particle flow focused on the velocity investigation of gas and particle phases [11–13]. Kliafas and Holt studied the average radial and streamwise velocities and related turbulent stresses in a 90° vertical to horizontal bend with a square section [11]. They adopted the laser Doppler velocimetry (LDV) and analysed the effects of Reynolds number, mass ratio, microparticle diameter, particle-wall collision, and bend deflection angles. Later, Yang and Kuan conducted similar turbulent experiments of dilute (<1% by mass loading) microsphere particle (77 *μ*m in average) flow through a 90° horizontal to vertical bend with square section by using 2D LDV [13]. Both average and fluctuating velocities of the air and particle phases were obtained under a Reynolds number of 102,000 (bulk velocity 10 m/s). Obvious air-particle separation was observed around the bend outer wall and so was the slip velocity. The fluctuating velocities of particle flow were found to be higher than those of airflow at the inlet of the bend.

#### 4. Particle Deposition through Bends

##### 4.1. Aerosol Deposition in Individual Bends

Particle deposition in pipe or duct bend sections is of potential significance, but this behavior has not been fully studied by experimental methods under turbulent flow conditions. There are limited but growing experimental investigations on bend aerosol deposition, especially in recent years as shown in Tables 1 and 2 [10, 14–24]. The scarcity of experimental work during earlier years might be attributed to the reason that both the particle deposition and distribution are comprehensive and can be varied by many factors even in straight pipe/channel/ducts [25–28].