Experimental and numerical study of a gas cyclone with a central filter

This paper studies a novel gas cyclone with a cylindrical filter face installed in the center from the vortex finder to the bottom hopper.The experimental resul...     

Experimental study on filtration performance of the moving bed granular filter with axial flow

The filtration performance of the moving bed granular filter with axial flow (MBGF-AF) is investigated through a large cold experiment. The effect of different operation parameters on the filtration performance (collection efficiency, pressure drop) of the axial-flow moving bed filter is investigated in combination with the dust deposition effect and the mechanism of trapping dust by the capturing particles. The results show that the collection efficiency of MBGF-AF is enhanced by decreasing the superficial gas velocity, increasing the inlet dust concentration properly, or decreasing the moving velocity of the capturing particles. A model covering the above operation parameters is established to calculate the collection efficiency of the moving bed granular filter. It is used in a wide range of operating parameters for the MBGFs.     

Effect of inlet volute wrap angle on the flow field and performance of double inlet gas cyclones

In response to the divergent understanding of double inlet cyclone performance in the literature, the effect of inlet volute wrap angle on the performance and flow field of double inlet cyclone separator was studied by Computational Fluid Dynamic (CFD) method. The results showed that the inlet volute wrap angle can affect the comparison results of the single and double inlet gas cyclones with the same total inlet cross-sectional area and velocity. 0° and 90° volute double inlet improved the efficiency mainly by separating particles below 10 μm, while 180° volute double inlet had no separation advantage for any particles, so the symmetrical double inlet does not always improve the efficiency, and the appropriate inlet volute wrap angle should be selected according to the actual situation, otherwise, the expected performance requirements of the symmetrical double inlet cyclone cannot be achieved. Compared with the flow field, it is found that the inlet volute wrap angle changed the tangential velocity of the symmetrical double inlet cyclone separator, thus changing the performance.     

Peristaltic flow and heat transfer in a vertical porous annulus, with long wave approximation

In this paper, we study the interaction of peristalsis with heat transfer for the flow of a viscous fluid in a vertical porous annular region between two concentric tubes. Long wavelength approximation (that is, the wavelength of the peristaltic wave is large in comparison with the radius of the tube) is used to linearise the governing equations. Using the perturbation method, the solutions are obtained for the velocity and the temperature fields. Also, the closed form expressions are derived for the pressure-flow relationship and the heat transfer at the wall. The effect of pressure drop on flux is observed to be almost negligible for peristaltic waves of large amplitude; however, the mean flux is found to increase by 10-12% as the free convection parameter increases from 1 to 2. Also, the heat transfer at the wall is affected significantly by the amplitude of the peristaltic wave. This warrants further study on the effects of peristalsis on the flow and heat transfer characteristics.     

A Semi-empirical Correlation for Pressure Drop in Packed Beds of Spherical Particles

The effect of a confining wall on the pressure drop of fluid flow through packed beds of spherical particles with small bed-to-particle diameter ratios was investigated to develop an improved pressure drop correlation. The dependency of pressure loss on both wall friction and increased porosity near the wall was accounted for by using a theoretical approach. A semi-empirical model was created based upon the capillary-orifice model, which included a wall correction factor for the inertial pressure loss. In this model, packed beds were treated as a bundle of capillary tubes whose orifice diameter in the core region was different from that of the wall region. Using this model, a new pressure drop correlation was obtained, based on the Ergun equation and applicable for a wide range of Reynolds numbers (10⁻²–10³). The proposed correlation was compared with previous correlations, as well as with experimental data. This correlation showed close agreement with the experimental data for both low- and high-Reynolds number regimes and for a wide range of bed-to-particle diameter ratios. The ratio of the pressure drop in finite packing to that in homogeneous packing was then calculated. This ratio clearly shows how the wall effect depends on the Reynolds number and the bed-to-particle diameter ratio.     

Experimental and numerical study of pressure wave refrigerator performance

A new compact pressure wave refrigerator has been designed and manufactured at the Shock Wave Research Center, Institute of Fluid Science, Tohoku University. The performance of this device was investigated for combinations of major operational parameters, such as the rotational speed of gas distributor, the length of expansion tubes, the input gas pressure. The maximum temperature decrease of 20 K has been measured. Some theoretical consideration to the efficiency of the pressure wave refrigerator and two-dimensional numerical simulations were carried out in order to understand the wave interactions that take place inside the device.Received: 26 May 2003, Accepted: 12 August 2003, Published online: 14 October 2003 Correspondence to: T. Saito     

Experimental and numerical investigations of pressure drop in a rectangular duct with modified power law fluids

Numerical solutions are presented for fully developed laminar flow for a modified power law fluid (MPL) in a rectangular duct. The solutions are applicable to pseudoplastic fluids over a wide shear rate range from Newtonian behavior at low shear rates, through a transition region, to power law behavior at higher shear rates. The analysis identified a dimensionless shear rate parameter which, for a given set of operating conditions, specifies where in the shear rate range a particular system is operating, i.e. in the Newtonian, transition, or power law regions. The numerical results of the friction factor times Reynolds number for the Newtonian and power law region are compared with previously published results showing agreement within 0.05% in the Newtonian region, and 0.9% and 5.1% in the power law region. Rheological flow curves were measured for three CMC-7H4 solutions and were found to be well represented by the MPL constitutive equation. The friction factor times Reynolds number values were measured in the transition region for which previous measurements were unavailable. Good agreement was found between experiment and calculation thus confirming the validity of the analysis.     

Filtration characteristics of a binary multi-layer granular bed filter based on CFD-DEM coupling simulation

The coupled CFD-DEM method with the JKR (Johnson-Kendall-Roberts) model for describing the contact adhesion of dust to filter particles (FPs) is used to simulate the distribution pattern of dust particle deposition in the granular bed filter (GBF) with multi-layer media. The minimum inlet flow velocity must meet the requirement that the contact probability between dust and FPs is in the high contact probability region. The air flow forms vortices on the leeward side of the FPs and changes abruptly at the intersection of different particle size FPs layers. Dust particles form large deposits at the intersection of the first and second layers and the different particle size filter layers. Dual element multilayer GBF can further optimize the bed structure by interlacing filter layers with different particle sizes. Compared with single particle size multi-layer GBF, the bed pressure drop is reduced by 40.24%–50.65% and the dust removal efficiency is increased by 21.93%–55.09%.     

Forming and breaking the ceiling of inlet gas velocity regarding to separation efficiency of cyclone

The maximum-efficiency inlet velocity (MEIV) is a ceiling of inlet gas velocity that defines separation efficiency during cyclone design and operation. Experiment and computational fluid dynamics (CFD) simulation exhibited that an apex cone at the dust outlet can break the ceiling and improve the separation efficiency. The phenomenon is closely related to the effect of excessive high inlet gas velocity on the back-mixing escape of fine particles, which is the final result of back mixing, entrainment by the rapid upward airflow, and secondary separation of the inner vortex. In the center of the inner vortex, the airflow rotates slowly and moves rapidly upward. This elevator type of airflow delivers re-entrained particles to the vortex finder. A higher inlet gas velocity accelerates the elevator, causing more entrained particles to escape. This explains the decrease in efficiency at an excessively high inlet gas velocity. When an apex cone is installed at the dust outlet, the back-mixing is significantly weakened because the vortex core is bounded to the center of separator, while the transport effect of rapid upward airflow is weakened by the decrease in axial velocity in the center. Therefore, particle escape is weakened even at excessive high inlet gas velocities. Instead, the centrifugal effect is enhanced because of increased tangential velocity of the gas and particles. Consequently, the ceiling of inlet gas velocity is broken.     

Experimental study of flow and heat transfer in rib-roughened rectangular channels

Secondary flow patterns, pressure drop and heat transfer in rib-roughened rectangular channels have been investigated. The aspect ratio of the channels is 1–8, and ribs are attached to the wide channel walls in order to set up swirling motions. The geometries tested consist of channels having cross ribs, parallel ribs, cross V-ribs, parallel V-ribs, and multiple V-ribs (Swirl Flow Tube). The flow patterns were investigated using smoke wire visualization and LDV measurements. The smoke wire experiments have been performed at Re=1100 and the LDV measurements at Re=3000 at periodic fully developed conditions. The heat transfer and pressure drop are described by j and f factors for Reynolds numbers from 500 to 15 000. The distributions of axial mean velocity and turbulent fluctuations are strongly influenced by the secondary flows. Large mean velocities and small fluctuations are found in regions where the secondary flow is directed towards a surface, while small mean velocities and large fluctuations are found in regions where the secondary flow is directed away from a surface. The Swirl Flow Tube provides a significant increase in the j factor at Reynolds numbers from 1000 to 2000, but unfortunately also an increase in the f factor. At higher Reynolds numbers, the j and f factors of the Swirl Flow Tube are of the same order of magnitude as for the other rib-roughened channels. It is found that the flow direction in a channel with parallel V-ribs has important influence on the j/f ratio. At Reynolds numbers above 4000, this channel provides the highest j/f ratio if the V-ribs are pointing upstream; while it provides the lowest j/f ratio of all rib configurations, if the V-ribs are pointing downstream.     

Two-dimensional interaction of a wetting front with an impervious layer: Analytical and numerical solutions

Wetting-front movement can be impaired whenever the flow region includes boundaries such as the soil surface, seepage faces, planes of symmetry, or actual layers that are effectively impermeable, such as heavy clays or coarse materials below the water-entry pressure. An approximate analytical solution for interaction of flow from a line source with a parallel plane, impervious layer is derived. The solution ignores gravity and assumes a particular diffusivity that is related to the constant flow rate. It is exact until interaction begins, and provides an accurate approximation for short times thereafter. It can therefore be used to test the accuracy of numerical solutions of the flow equation, which can then be used with confidence for later times when the analytical approximation breaks down, for instance because gravity is ignored. A finite difference solution was tested in this way for both gradual and steep wetting fronts. Agreement between the two solutions was excellent for the gradual front, with the analytical approximation only slightly in error at later times. Numerical errors at the steep front were much greater; an accurate solution needed a finer spatial grid and a restart from the exact analytical values at the beginning of the interaction. The analytical approximation, though not as accurate as for the gradual front, was still good.     

A numerical method for the determination of slip characteristics between the layers of a two-layer slurry flow

The observed phenomenon of slip between the layers of a flowing slurry is modelled mathematically by a finite-element-based numerical technique. This technique enables us to quantify variables such as the slip velocity and shear stress distribution at the interlayer boundary and the pressure drop within the flow domain.     

Numerical simulation of non-Darcian flow through a porous medium

This work reports on fluid flow in a fluid-saturated porous medium, accounting for the boundary and inertial effects in the momentum equation. The flow is simulated by Brinkman-Forchheimer-extended Darcy formulation (DFB), using MAC (Marker And Cell) and Chorin pressure iteration method. The method is validated by comparison with analytic results. The effect of Reynolds number, Darcy number, porosity and viscosity ratio on velocity is investigated. As a result, it is found that Darcy number has a decisive influence on pressure as well as velocity, and the effect of viscosity ratio on velocity is very strong given the Darcy number. Additional key findings include unreasonable choice of effective viscosity can involve loss of important physical information.     

Experimental investigation of the interaction of shock waves with textiles

Experimental studies have been performed to investigate the pressure amplification experienced behind a textile when exposed to a shock wave. Three textiles with different masses and air permeabilities were studied. Mach numbers for tests varied between 1.23 and 1.55. The distance between the back wall and the textile was varied between 3 and 15 mm. It was found that in most cases the presence of the textile led to a pressure amplification at the back wall. This amplification was dependent on the textile, Mach number and distance from the back wall. The processes causing the pressure amplification were identified by analysing pressure traces and contact shadowgraphs. It was found that when the incident wave impinges on the textile, a part is reflected upstream and a part is transmitted through the textile. The transmitted portion reflects back and forth in the gap between the textile and the back wall leading to a back wall pressure trace with a stepped profile. In addition, the textile moves towards the back wall causing compression waves to propagate towards the back wall. The combination of the stepped profile and the compression waves cause the pressure amplification. The contribution of each mechanism depends on the textile properties. Approximate wave diagrams have been constructed. Tests involving multiple layers of textiles are also discussed. Received 17 October 2000 / Accepted 2 February 2001     

Numerical study of short-circuiting flow and particles in a gas cyclone

Short-circuiting flow is an important secondary flow in gas cyclones, which has a negative impact on the separation performance. To improve the understanding of the short-circuiting flow and guide the optimization of gas cyclones, this paper presents a numerical study of a cyclone using computational fluid dynamics. Based on the steady flow field, three methods were adopted to investigate the formation mechanism and characteristics of the short-circuiting flow and particles. The temporal variation of the tracer species concentration distribution reveals that the formation mechanism of the short-circuiting flow is the squeeze between the airflows entering the annular space of the gas cyclone at different times. The short-circuiting flow region, distinguished through the spatial distribution of the moments of age, is characterized by a small mean age and a large coefficient of variation. The proportion of the short-circuiting particles increases with the increase of the inlet velocity only for small particles. But with the increase of particle size, the proportion of the short-circuiting particles decreases faster at higher inlet velocities, resulting in significant differences in collection efficiency curves.     

Temperature dependent mechanical behaviour of PVDF: Experiments and numerical modelling

The mechanical behaviour of Polyvinylidene Fluoride (PVDF) is analysed. To this end, tensile tests are performed on both smooth and notched specimens, for several values of the notch radius in order to set specific values of the stress triaxiality ratio in the net section. Tests were performed at various temperatures and at various strain rates. Experimental data together with fracture surface examinations by SEM allow the dependence of deformation and void growth processes on strain rate and temperature to be investigated. This experimental work was carried out in order to test the mechanics of porous media model. For each investigated temperature, constitutive relations take both porosity and strain rate sensitivity into account. The model is proposed for deformation leading to crazing. The material coefficients are optimised by imposing a continuous dependence on temperature.     

Combined Forced and Free Convective Flow in a Vertical Porous Channel: The Effects of Viscous Dissipation and Pressure Work

Buoyant flow is analysed for a vertical fluid saturated porous layer bounded by an isothermal plane and an isoflux plane in the case of a fully developed flow with a parallel velocity field. The effects of viscous dissipation and pressure work are taken into account in the framework of the Oberbeck–Boussinesq approximation scheme and of the Darcy flow model. Momentum and energy balances are combined in a dimensionless nonlinear ordinary differential equation solved numerically by a Runge–Kutta method. Both cases of upward pressure force (upward driven flows) and of downward pressure force (downward driven flows) are examined. The thermal behaviour for upward driven flows and downward driven flows is quite different. For upward driven flows, the combined effects of viscous dissipation and pressure work may produce a net cooling of the fluid even in the case of a positive heat input from the isoflux wall. For downward driven flows, viscous dissipation and pressure work yield a net heating of the fluid. A general reflection on the roles played by the effects of viscous dissipation and pressure work with respect to the Oberbeck–Boussinesq approximation is proposed.     

Flow and heat transfer performance of a mini-channel radiator with cylinder disturbed flow

Mini-channel heat sinks have relatively low Nusselt number due to small Reynolds number. For heat transfer enhancement purpose, a mini-channel radiator with cylinder disturbed flow was proposed. The disturbed flow was created by a circular cylinder placed horizontally in front of channels entrance. The performance of heat transfer and pressure drop with/without disturbed flow was studied experimentally. It was found that the friction factor of mini-channel flow was larger than that of the macro-channel flow due to larger surface roughness, and the pressure drop caused by cylinder disturbed flow was less than 5%. It also concluded that the average Nusselt number increases with augment of Reynolds and Prandtl number. The Nusselt number correlations as the function of the Reynolds and Prandtl number were given for evaluation the heat removal performance of similar heat radiators. There is an inflexion point in the empirical formulas when the channel length equals to the thermal entrance length. For the mini-channels heat radiators with disturbed flow, the inflexion Reynolds number is larger than that of without disturbed flow. Due to the flow pulsing caused by circular cylinder placed in front of channels entrance, the thermal entrance length increases. On the other hand, for both mini-channels with or without disturbed flow, the thermal resistance increases with the decrease of pressure drop.     

The effect of porosity on shock interaction with a rigid, porous barrier

This work investigates the pressure amplification experienced behind a rigid, porous barrier that is exposed to a planar shock. Numerical simulations are performed in two dimensions using the full Navier–Stokes equations for a M = 1.3 incoming shock wave. An array of cylinders is positioned at some distance from a solid wall and the shock wave is allowed to propagate past the barrier and reflect off the wall. Pressure at the wall is recorded and the flowfield is examined using numerical schlieren images. This work is intended to provide insight into the interaction of a shock wave with a cloth barrier shielding a solid boundary, and therefore the Reynolds number is small (i.e., Re = 500 to 2000). Additionally, the effect of porosity of the barrier is examined. While the pressure plots display no distinct trend based on Reynolds number, the porosity has a marked effect on the flowfield structure and endwall pressure, with the pressure increasing as porosity decreases until a maximum value is reached.