High Wall Roughness Research Articles

Experimental and numerical investigation for the geometrical parameters effect on the labyrinth-seal flow characteristics of fast reactor fuel assembly

The labyrinth seal is widely used in the fast reactor fuel assembly to control leakage flow rate, adjust pressure drop and cool assembly outer wall. In the current investigation, a new non-rotating labyrinth seal with right-angle trapezoidal throttle tooth is designed and manufactured, and its flow characteristics are experimentally and numerically investigated. Reynolds number from 61,512 to 81,261 is considered and experiment data were generated from hydraulic experiment conducted at constant water temperature (84 °C) in the labyrinth-seal test section. Numerical results with different turbulence model, wall-normal grid spacing, inlet turbulence intensity and stream-wise grid length are compared with experiment results to demonstrate the effectiveness of numerical simulation. Geometrical parameters have a significant effect on labyrinth-seal flow characteristics. Increasing throttle-tooth number, spacing and throttle-tooth thickness can improve the labyrinth-seal sealing performance, while increasing gap width influences the flow velocity limitation and increases the leakage. Labyrinth-seal resistance coefficient is validated to be independent of Reynolds number (61,512–81,261). It is deduced that the fully rough regime still exists in an annular-tube turbulence flow with high wall roughness. The energy dissipation caused by throttling effect, jet, vortex and secondary transverse turbulence is the main sealing mechanism. Moreover, a leakage-coefficient empirical correlation is obtained by regression analysis of experiment data, which can be used for the structure design of sodium-cooled fast reactor and preliminary estimation of leakage flow rate in similar non-rotating seals.

Prediction of horizontal gas–solid flows under different gravitational fields

Abstract In this paper the performance of horizontal pneumatic conveying under different gravity environments is evaluated. An Euler–Lagrange approach validated versus ground experiments is employed to predict the relevant particle variables such as particle mass flux, mean conveying and fluctuating velocities in terrestrial, lunar and micro-gravity conditions. Gravity reduced computations predict a reduction in the global particle–wall collision frequency. Also, in the case of low wall roughness and small particle mass loading, reduction of gravity acceleration implies an increase of particle–wall collision frequency with the upper wall of the channel affecting greatly the particle mass flux profile. In the case of high wall roughness and/or high particle-to-fluid mass loading (i.e., around 1.0) particle conveying characteristics are similar in the three gravity conditions evaluated. This is due to the fact that both, wall roughness and inter-particle collisions reduce gravitational settling. However, the influence of gravity on the additional pressure loss along the channel due to the conveying of the particles is much reduced.

Four-way coupled simulations of the gas–particle flow in a diffuser

In this work, a numerical investigation on the gas–particle flow in a vertical diffuser is carried out. This study was motivated by the experimental work of by Kale and Eaton (1984), who noticed that the fully attached flow in a diffuser in the freeboard region of a particle bed would become detached if no particles were present. It was concluded at the time that this effect was not caused by the high inlet turbulence levels, but rather by the particles. With the goal to better understand the interactions between the particles and the fluid in a diffuser, simulations of the particle-laden gas flow in a vertical conical diffuser are run using the Euler/Lagrange approach. The model, which includes particle-to-particle collisions, the particle influence on the gas phase and wall roughness effects, is first validated based on experimental results from a horizontal channel and a vertical diffuser for both the continuous and dispersed phases at different mass loadings. Investigations on the effects of particles at different mass loadings and wall roughness on the diffuser flow are then carried out. It has been found that, even at moderate mass loadings, particles can significantly affect the diffuser flow pattern, and actually reattach the otherwise separated flow under some conditions. It has also been found that wall roughness plays a very important role in homogenizing the particle distribution at the diffuser section. The resulting more uniform concentration and velocity profiles can then reenergize the separated boundary layer and reattach it to the wall. The mechanism for the flow reattachment owing to the high wall roughness is investigated and an explanation is proposed.

Numerical calculation of pneumatic conveying in horizontal channels and pipes: Detailed analysis of conveying behaviour

The present study deals with pneumatic conveying of spherical particles in horizontal ducts, a 6 m long rectangular cross-section horizontal channel and a circular pipe, from a numerical perspective. The three-dimensional numerical computations were performed by the Euler–Lagrange approach in connection with the k– ε and a Reynolds Stress turbulence model accounting for full two-way coupling. For the calculation of the particle motion all relevant forces (i.e. drag, slip-shear and slip-rotational lift and gravity), inter-particle collisions and particle-rough wall collisions were considered. For all considered cases an average air velocity of 20 m/s was selected. Calculations are carried out for spherical glass beads with a diameter of 130 μm at a mass loading of 1.0 (kg particles/kg air). Additionally, different wall roughnesses are considered. The agreement of the computations with experiments was found to be satisfactory for mean and fluctuating velocities of both phases as well as for the normalised particle mass flux in the case of the channel flow. The main part of this contribution is related to a detailed analysis on the differences between pneumatic conveying properties in the rectangular channel and the circular pipe. For that the influence of wall roughness and the degree of coupling (i.e. two- or four-way) was analysed by visualising the cross-sectional distributions of air and particle properties. The observed focussing effect has a remarkable influence on particle concentration distribution in the pipe cross-section and the wall collision frequency over the circumference. Distinct differences in the velocity profiles of both phases (mean velocities and fluctuating components), as well as the concentration profiles, for channel and pipe flow are identified. The particle fluctuating velocities in the pipe are higher than in the channel for all situations, yielding mostly higher wall collision frequencies. As a consequence, in all the considered cases, the pressure drop in the pipe is larger than in the channel, especially for high wall roughness.

Numerical Simulation of Roughness Effects Inside a Brick-lined Cyclone Separator

An industrial cyclone used for separating dust from a hot gas mixture was studied by means of numerical simulation. Because of the high gas temperatures (about 1075 K), the inner surfaces of the cyclone separator and the inlet pipe must be lined with refractory bricks, causing high wall roughness. The virtual wall model was used to simulate this roughness, and a new model extension accounting for joints between the bricks is proposed and validated experimentally. As the cyclone inlet mass load was relatively low (0.01 kg/kg), the Eulerian-Lagrangian approach was used for the simulations. The continuous phase conservation equations were solved in the Eulerian reference frame on a fixed grid, whereas the discrete phase properties were determined by computing particle trajectories through the computational domain in a Lagrangian reference frame. Besides inlet particle diameters ranging from 1 μm to 100 μm, different particle materials with varying particle densities had to be considered. The measurements and the Muschelknautz analytical model were compared to the numerical simulations regarding pressure loss, particle spectra at the outlet, and the fractional separation efficiency. The Muschelknautz model exhibited some difficulties with the kind of wall roughness considered here (especially joints between bricks). Moreover, this method was not designed to handle multiple different particle densities simultaneously. Nevertheless, comparisons show good agreement between numerical simulations, measurements and theoretical predictions.

Wall material and roughness effects on transmittable shear stresses of magnetorheological fluids in plate–plate magnetorheometry

A direct comparison of plate–plate magnetorheometry results for nonmagnetic (titanium/brass) and ferromagnetic plates is presented, using a modified Anton Paar magnetocell MRD180/1T. Necessary corrections to derive the true flux density in the magnetorheological fluid (MRF) from the online Hall probe reading and to account for the gap opening effect caused by normal forces on shear stress and flux density are addressed. The measured shear stress versus magnetic flux density characteristics agree in the low flux density regime <0.1 T but yield distinctly higher transmittable shear stresses for ferromagnetic plates at elevated flux densities (49% increase at 1 T for 90% by weight carbonyl iron powder (CIP) and 84% for 85% by weight CIP). Remarkably, the normal force, if corrected for its magnetostatic part, remains independent of the type of plates up to about 0.6 T. We address the role of normal forces, of magnetic interactions between CIP and wall, as well as the role of wall roughness in a solid body friction model. A systematic variation of wall properties and materials was achieved by introducing both a modular rotor and stator, which ease the variation of the walls in contact to the MRF. The transmittable shear stress of nonmagnetic plates (e.g., brass) may be increased up to the level of ferromagnetic disks by a higher wall roughness or by grooves. No shear stress increase is obtained for grooves in ferromagnetic plates, which is explained by the different local flux density modulation at the grooves for ferromagnetic compared to nonmagnetic plates. Finally, we address the effect of ferromagnetic and nonmagnetic coatings on brass and steel disks, and show that, e.g., a layer of CIP on brass efficiently increases the transmittable shear stress.

Chaotic flows in microchannels: a lattice Boltzmann study

Roughness effects on lubricant flows are investigated via 2D lattice Boltzmann simulations. At a Reynolds numbers of order 1000 a transition from laminar to unsteady flow is observed by an increase of the roughness height from about 10% to about 25% of the channel width. At lower Reynolds numbers (where the flow is laminar in both channels), the transition is observed when increasing the wall roughness further. In other words, the critical Reynolds number for the transition from laminar toward unsteady flow decreases at higher wall roughness. Wall roughness may, therefore, qualitatively change the flow properties in confined geometry. Due to the ubiquitous presence of the wall roughness, the phenomenon is relevant in all cases where relatively high Reynolds number flow occur in strongly confined channels such as lubricant flow during the deformation of solid surfaces. For a fixed Reynolds number and channel geometry, time and spatial dependence of the velocity field and fluctuating quantities obey the scaling behavior as expected from the structure of the Navier–Stokes (NS) equations. This underlines the physical significance of the observed transition. As a possible application, wall roughness may be used in order to enhance mixing efficiency at a given Reynolds number.