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Particle Deposition and Reentrainment
Published in Ko Higashitani, Hisao Makino, Shuji Matsusaka, Powder Technology Handbook, 2019
Manabu Shimada, Shuji Matsusaka, Hiroaki Masuda
The transport of particles suspended in a turbulent flow field is enhanced by turbulent eddies in the fluid. Figure 2.8.2 shows the representative experimental results for turbulent deposition in a circular pipe.10 The ordinate and abscissa of the figure correspond to the dimensionless deposition velocity vd+ (= vd/u*, u*, friction velocity) and the dimensionless relaxation time of particles τ+ (=ρfρpDp2u*2CC/(18μ2)), respectively. Turbulent deposition is approximately divided into the following three regimes based on the magnitude of τ+ that indicates the effect of particle inertia. For τ+ < 0.2, the effect of inertia should be small, and thus, particles follow the fluid motion. When 0.2 <τ+ < 20, the deposition velocity depends strongly on τ+ as the motion of particles tends to deviate from that of fluid near the wall as a result of the inertial force. The deposition velocity for τ+ > 20 is only slightly dependent on τ+ as the inertia-induced motion of particles tends to prevail over the entire region of the fluid. The three empirical equations shown in the figure10 are practically useful as a rough estimation as vd+ is highly correlated to τ+. In addition to the aforementioned equations, several theoretical or semi-empirical models were proposed to explain the measured results for turbulent flows.11–17 In a recent notable model based on the Eulerian method, a term representing particle transport by turbophoresis (i.e. transport induced by the interaction of particle inertia and the gradient of turbulent intensity) is added to the convection–diffusion equation of particles (Equation 2.8.1). This model was used to quantitatively analyze particle deposition in turbulent pipe flow enhanced by wall roughness, thermophoresis, and electrostatic forces.18
Deposition of non-spherical particles on indoor surfaces: Modification of diffusion coefficient
Published in Aerosol Science and Technology, 2022
The conditions of non-spherical particle deposited on the side wall can be found in Figure 4(a)–(d). Under this circumstance, the gravitational settling was no longer a mechanism that affected the particle deposition process. For large particles, turbophoresis replaced gravitational settling and played an important role in particle deposition. As shown in Figure 4(a), the condition of diffusion-controlled region was similar to that of deposition on the floor, and 10–25% deviation of deposition velocity was observed. However, with the increment in particle diameter, the effect of turbophoresis and diffusion on particle deposition was opposite. Because the gravitational settling had no impact in this situation, the aerodynamic diameter could not prevent the divergence brought about particle shape. The deviation even increased and exceeded 100% when the particle diameter was closed to 10 As shown in Figure 4(b)–(d), with the decrease in friction velocity, the turbulent intensity reduced, consequently reducing the impact of turbophoresis. The discrepancy of deposition velocity between spherical and non-spherical particles reduced as well.
Preconceptual Design of Multifunctional Gas-Cooled Cartridge Loop for the Versatile Test Reactor—Part I
Published in Nuclear Science and Engineering, 2022
Piyush Sabharwall, Kevan Weaver, N. K. Anand, Chris Ellis, Xiaodong Sun, Di Chen, Hangbok Choi, Rich Christensen, Brian M. Fronk, Joshua Gess, Yassin Hassan, Igor Jovanovic, Annalisa Manera, Victor Petrov, Rodolfo Vaghetto, Silvino Balderrama-Prieto, Adam J. Burak, Milos Burger, Alberto Cardenas-Melgar, Londrea Garrett, Genevieve L. Gaudin, Daniel Orea, Reynaldo Chavez, Byunghee Choi, Noah Sutton, Ken William Ssennyimba, Josh Young
The Lagrangian particle tracking method could also be used to calculate the deposition distribution at the wall. Figure 19 shows the positions of the particles deposited on each wall (bottom, left, top, and right surfaces) when 100 000 particles were injected into the channel (left side of the graph), with a particle average size of 37.5 µm to simulate large particles and vary the relaxation time τ variable in Eq. (8). Separate tests of liquid particles were used for smaller-diameter particles between 1 and 10 µm. The Reynolds number based on the average velocity was approximately 6000. Most of the particles were deposited on the bottom surface, implying that gravity settling (gravitational term) was the most dominant in this τ+ range (τ+ > 10−3). However, in this region, the turbulence effects also impacted the particle deposition. Although some particles were deposited on the left and right walls, most of the particles were deposited on the bottom wall. If simulations were conducted without considering the turbophoresis force, all of the particles would be deposited on the bottom wall. Turbulence deposition was estimated to be the result of a combination of turbophoresis and the drag force. Particles were trapped in the secondary vortices generated at channel corners, increasing the probability that particles would remain in the proximity of the wall. The turbulence effects and secondary vortices increased the particle deposition velocity. This explained the deposition patterns observed at the bottom wall.
Numerical simulation of aerosol dynamics in an impinging jet with microdroplet coalescence
Published in Aerosol Science and Technology, 2020
Jiandong Wu, Jiyun Xu, Hao Wang
Turbophoresis is a possible reason for the second deposition peak. In the region near the second deposition peak, the turbulence intensity near the wall is lower than the turbulence intensity away from the wall, as the Figure 5d shows. Micro-droplets could drift from regions high turbulence intensity to regions with lower turbulence near the wall according to the turbophoresis theory. Caporaloni et al. (1975) proposed the turbophoresis theory and then Reeks (1983) reexamined later to explain the existence of the net discrete phase flux toward the wall. Marchioli and Soldati (2002) stated that in the case of ejection, particles are concentrated under the low-speed streaks and accumulate in the near-wall region with lower turbulence intensity. In the current simulation, the turbophoresis effect is calculated by the discrete random walk model.