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Disperse multiphase flows are very common for processes in mechanical and thermal process technology (e.g. gas-particle or gas-droplet flows, coal combustion, pneumatical conveying, erosion phenomena). Furthermore processes for the separation of solid particles from gases or fluids and for the classification and particle size analysis are an important field of interest in process technology.
The paper deals with the experimental and numerical investigation of the particle precipitation in a special class of cyclone separators, the so called symmetrical double cyclone separator (see figure). This type of cyclones was developed by the Gesellschaft für Luft- und Umwelttechnik mbH, Eckernförde/Germany (LUT ltd.). By using this type of cyclone the secondary flow along the lid, which is observed in standard cyclones, can be avoided. In the case of a secondary flow along the lid small particles can move directly from the inlet to the clean gas exit bypassing the main vortex flow in the conical part of the cyclone. The diagonal secondary flow induced by the walls of the conical parts of the symmetrical double cyclone leads to enrichment of the particle phase along the walls. The secondary flow is led to the walls of the sedimentation chamber by special shielding or guiding equipment attached at the lower end of the conical part of the cyclone to the outer diameter of the vortex finder tubes. In this flow region along the walls of the sedimentation chamber also smaller particles are able to agglomerate and to sedimentate as larger agglomerates in the sedimentation chamber.
In comparison with conventional cyclone separators and other kinds of special cyclones better particle precipitation can be achieved with this special type of a symmetrical double cyclone separators. This means that the cut-off particle diameter d_50 for the particle precipitation rate T (d_P) (with T (d_50) = 0.5) can be significantly decreased. For a diameter of the separation chamber of the cyclone of 40mm a cut-off particle diameter d_50 of less than 100 nanometers has been measured. Even for diameters of the separation chamber of about 200-250mm values for the cut-off particle diameter d_50 less than 1 micrometer could be measured (Schneider98, Bachmann96, Wieck97).
Experiments described in this paper were carried out at the Flensburg University of Applied Sciences. In a series of experiments a symmetrical double cyclone separator with a diameter of the separation chamber of 230 mm has been investigated. Particles have been dispersed with the RBG 1000 and the particle size measurements have been carried out using the particle sizer PCS 2000, both made by the PALLAS GmbH, Karlsruhe/Germany. For the particle phase calcium carbonate particles were used, produced under the trading name OMYACARB 2--GU by OMYA GmbH, Köln/Germany. So the particle phase used in the experiments for investigation of particle precipitation in the symmetrical double cyclone separator can be characterized by the mean particle diameter d_50,3 of the distribution sum Q_3 (d_P) of about 2.5 micrometers which corresponds to an aerodynamic diameter of about 4.1 micrometers.
The experimental results for the particle precipitation rates in the symmetrical double cyclone have been compared with numerical predictions for three different variations of the cyclone geometry. For these numerical simulations a 3-dimensional Lagrangian approach developed by Frank et al. (Frank92, Frank96b, Frank97c, Frank98a) was used. The numerical method is based on the modified Navier-Stokes solver FAN-3D (Peric92a, Peric92b) which is able to calculate 3-dimensional, steady, incompressible flows in complex geometries using non-orthogonal, boundary fitted, block-structured numerical grids. Due to the complex flow geometry of the investigated cyclone separators numerical grids with up to 95 different grid blocks and about 350.000 grid cells had to be designed for the numerical calculations of the gas-particle flow.
The disperse phase is treated by the Lagrangian approach where a large number of particle trajectories is calculated throughout the flow domain. For the formulation of particles equation of motion a small density ratio rho_F / rho_P is assumed. So the drag force, the lift force due to fluid shear (Saffman force), the pressure force, the gravitational and added mass force are taken into account (Frank97c, Frank98a). Particle precipitation rates were obtained from the calculation of about 10.000 particle trajectories with a particle diameter distribution in the range of d_P = 0.6 - 20.0 micrometers and by analyzing the number of particles reaching the particle hopper vs. the number of particles reaching the clean gas exit.
The numerical investigations for the precipitation of limestone particles (rho_P=2700 kg/m^3) were carried out for three different geometrical configurations of the symmetrical double cyclone using a constant gas inlet velocity of u_F = 25 m/s. In a first numerical simulation the influence of the gap width between the apex cone and the inner cyclone wall on the particle precipitation rate has been investigated. In a second numerical experiment the spiral inflow into the cyclone main body has been changed to a tangential inflow.
The numerical flow simulations confirm the expected main vortex flow structure known from cyclone theory and from experimental observations. The numerical predictions especially confirm the substantial contribution for particle precipitation of the recirculation of a certain gas volume flow rate from the cyclone main body through the gap at the apex cone into the particle hopper. The obtained numerical results for the particle precipitation rates are in good agreement with the experimentally predicted precipitation rates.
Title
Contents
Different types of experimentally investigated symmetrical double cyclone separators - I
Different types of experimentally investigated symmetrical double cyclone separators - II
Functional diagram of symmetrical double cyclone separators
Scheme of experimental test rig at FH Flensburg
Technical data of experimental investigations
Separation performance of cyclones ZA and ZS - experimental investigations
Comparison of a standard cyclone and symmetrical double cyclone ZT
The Eulerian-Lagrangian approach MISTRAL / PartFlow-3D - I
The Eulerian-Lagrangian approach MISTRAL / PartFlow-3D - II
Equations of motion of the fluid phase
The 3-dimensional equations of motion for the disperse phase - I
Equations of motion of the disperse phase - II
Particle-wall interaction
Models for erosion prediction in gas-particle flows
Separation rate T(x) and distribution functions q(x) and Q(x)
Numerical prediction of the particle separation rate T(X)
Comparison of experimentally predicted separation rates for ZS18, ZS30 and ZT30
Comparison of numerically predicted separation rates for ZS18, ZS30 and ZT30 - I
Comparison of numerically predicted separation rates for ZS18, ZS30 and ZT30 - II
Particle separation in symmetrical double cyclones
Particle separation rates for symmetrical double cyclones of different size
Numerical mesh for the double cyclone with spiral inflow ZS30
Fluid flow field in the double cyclone ZS30 (x-z-plane)
Detail of the fluid flow field for ZS30 in the vicinity of the apex cone (x-z-plane)
Fluid flow field in the double cyclone ZS30 (y-z-plane)
Detail of the fluid flow field for ZS30 in the vicinity of the apex cone (y-z-plane)
Particle trajectories in ZS18 - I
Particle trajectories in ZS18 - II
Particle trajectories in ZS18 - III
Particle separation in symmetrical double cyclones ZS18 and ZS30 - I
Particle separation in symmetrical double cyclones ZS18 and ZS30 - II
Numerical mesh for the double cyclone with spiral inflow ZT30
Fluid flow field in the double cyclone ZT30 (x-z-plane)
Detail of the fluid flow field for ZT30 in the vicinity of the apex cone (x-z-plane)
Fluid flow field in the double cyclone ZT30 (y-z-plane)
Detail of the fluid flow field for ZT30 in the vicinity of the apex cone (y-z-plane)
Particle trajectories in ZT30 - I
Particle trajectories in ZT30 - II
Particle trajectories in ZT30 - III
Distribution of the mean particle diameter for the ZT30 cyclone
Wall erosion in ZT30 cyclone (y-z-plane)
Wall erosion in ZT30 cyclone (x-z-plane)
Particle number density distribution in ZT30 cyclone
Particle separation in symmetrical double cyclones ZS18 and ZS30 - I
Particle separation in symmetrical double cyclones ZS18 and ZS30 - II
Parallel computation of gas-particle flow in cyclone separators
Conclusions