Cyclones flow field

There is also a large body of papers which can be characterized as concerned with intensification of the microfiltration process. These are mostly concerned with remedies for fouling, with flux enhancement or reduction of concentration polarization (e.g. by changes in the membrane surface characteristics, by pre-treatment of the feed, by feed flow management, cyclonic flow fields or by using fluidized beds). 

It makes sense to compare the implications (in terms of simulation times) of using FV vs. LB in simulating turbulent-flow fields in process devices. Hoekstra (2000) demonstrated the numerical implications of applying different numerical schemes in an industrial application. He compared the outcome of his RANS simulation for a gas cyclone with that of a LES carried out by Derksen and Van den Akker (2000). Table I presents a number of numerical features of the two types of simulations. 

Hoekstra, A. J., Gas flow field and collection efficiency of cyclone separators , Ph.D. Thesis, Delft University of Technology, Delft, Netherlands (2000). 

There have been several studies in which the flow patterns within the body of the cyclone separator have been modelled using a Computational Fluid Dynamics (CFD) technique. A recent example is that of Slack et a/. 54 in which the computed three-dimensional flow fields have been plotted and compared with the results of experimental studies in which a backscatter laser Doppler anemometry system was used to measure flowfields. Agreement between the computed and experimental results was very good. When using very fine grid meshes, the existence of time-dependent vortices was identified. These had the potentiality of adversely affecting the separation efficiency, as well as leading to increased erosion at the walls. 

Cyclone mixers give a rotational flow field [109], The corresponding formation of vortex patterns is another way of laminating and focusing streams. It is hoped that by folding of the vortices, thinning of the lamellae can be achieved, with an increase of residence time in the mixer [131]. A full rotation should halve the lamellae width. Hence the optimization parameter may be to have as many rotations as possible. 

Figure 1.114 Schematic of the cyclone mixer design (left) and streamline indicating the rotational flow field in the cyclone micro mixer [109] (by courtesy of Kluwer Academic Publishers).

The cyclone, or inertial separation method, is a common industrial approach for segregating a dispersed phase from a continuous medium based upon the difference in density between the phases. The concept takes advantage of the velocity lag which occurs for dense particles with respect to a lower density medium when both phases are subject to an accelerating flow field, such as within a rotating vortex. The larger the acceleration, the smaller the particle which fails to follow the continuous phase streamlines and will migrate to the outer wall of the cyclone for collection. 

Iso-surfaces can be constructed by connecting all points in the flow domain that have exactly the same value of a given variable. These surfaces can be colored by the contours of any other variable. Iso-surfaces are useful to visualize certain flow-field features, such as the central vortex in the cyclone separator shown in Fig. 4. Contour plots can be used to show the local values of scalar variables. A cross section of the flow domain is created and colored by the local value of the variable of interest, e.g., temperature. A color scale is used indicating how each color corresponds to a certain value of the variable. Such plots can be used to quickly see the variation of important variables throughout the domain. An example of a contour plot is found in Fig. 5, which is described in the example in the following section. 

Another separator with a comparable task is the cyclone. In the cyclone, a flow field allows forces to act on particles in the vapour flow. This force presses the particles against the container jacket and hold them back. The flow conditions make the cyclone a compact device, and the shape is a good indicator of its effectiveness. The cyclone is designed for an application window, so its use is limited to processes that can be precisely specified. 

The RSM solves transport equations for all Reynolds stresses and the dissipation rate e and therefore does not rely on the isotropic turbulent viscosity m,. This makes the RSM suitable to predict even swirling flows, however, the major drawback of this model is the computational effort needed to solve its equations. For 3D simulations, seven additional transport equations must be solved (six for the Reynolds stresses and one for e). However, the RSM is highly recommended if the expected flow field is characterized by anisotropy in the Reynolds stresses as is the case with swirling flows, e.g., cyclones or spray drying with tangential inlet ducts. Crowe (1980) emphasized that the k-e model is not suitable for swirl flow problems. 

Currently the widely used turbulence models are standard K-s model, RNG K-e model and the Reynolds stress model (RSM). Standard K-s model is based on isotropic turbulence model, its simulation result error of separator flow field is large (Shan Yongbo, 2005). RNG K-s model has improved with a standard K-s model, but there are still larger defects. To improve the cyclone vortex field strength prediction results a greater extent, algebraic stress turbulence model based... 

The following conclusions can be get by simulation and analysis of the gas-liquid cyclone separator internal flow field with CFD software ... 

Cyclone collectors are simple units with a complex flow field that even today baffles our understanding. The cyclones work best for the collection of coarse materials. Cyclones are a source of high wear and breakage of the material conveyed. Friable materials should not be employed in cyclone arrangements. See Chapter 22 for more details. 

It is not easy to study the particle flow pattern experimentally. In order to give an impression of the flow of a particle through a cyclone, we can resort to CFD simulations. Figure 3.1.4 shows a series of particle trajectories. The particles are injected at different radial positions along the inlet in a precalculated gas flow field. The swirling motion is not shown. 

Fig. 3.1.4. Particle tracks in a cyclone by computational fluid dynamics. The swirl components are not shown. On the left 10 tracks are calculated from the mean flow field on the right 5 particle tracks are shown where the response to the turbulent motion of the gas is taken into account. Conditions cyclone diameter 20 cm, gas inlet velocity 15 m/s, gas at SATP ( standard ambient temperature and pressure , 25 C and 1 atm), particle density 2730 kg/m ...

In CFD, the equations governing the flow of the gas the Navier-Stokes equations, are written in a finite difference form, and solved with the aid of a computer on a grid of points spanning the body of the separator. The particles can either be treated as a sort of second fluid in the cyclone, or as individual particles, which can be tracked in the precalculated gas flow field. 

In this approach, the particle equation of motion is solved, mostly in a precalculated gas flow field. The particle position and velocity are calculated after successive short time intervals, and in this way the particle is tracked through the cyclone or swirl tube. 

We mentioned that one assumption in Derksen (2003) was one-way coupling between the gas and the solid phase. In Derksen et al. (2006), this assumption is relaxed, and the effect of the solids on the gas flow pattern, i.e. two-way coupling, is taken into account in simulations of the flow in the same cyclone geometry as in Derksen (2003). In order to do this without having to trace prohivitively many particles, each particle that is traced is considered to represent a whole assembly of like particles, the action of which on the gas flow Is fed back Into the gas equations to determine the effect of the particles on the gas flow field. 

Since the flow in a cyclone or swirl tube is complex, the direction of the axial and radial velocity components are not known in advance. This has to be borne in mind when considering how to measure the flow field. 

Hoekstra A J 2000 Gas Flow Field and Collection Efficiency of Cyclone Separators PhD thesis Delft University of Technology. 

Derksen (2003), Shalahy et al (2008), and De Souza et al (2012) tracked point particles in a gas flow field obtained by means of LESs with the view of studying separation performance of gas—solid cyclones, while Derksen et al... 

Cyclones work by using centrifugal force to increase the gravity field experienced by the solids. They then settle to the wall under the influence of their increased weight. Settling is improved as the path the solids traverse under centrifugal flow is increased. This pam is... 

The fine particle airstream from the cyclone was sampled by two total filters in parallel. A Millipore Fluoropore 47 mm diameter Teflon filter with a 1 pm pore size was used for the first seven samples. Subsequent samples were obtained with a 0.4 pm pore size 47 mm Nuclepore polycarbonate filter because particle absorption measurements and elemental analysis by particle induced X-ray emission (PIXE) were easier and more accurate using the Nuclepore filters. In parallel with the Nuclepore filter, a TWOMASS tape sampler collected aerosol using a Pallflex Tissuequartz tape. The aerosol deposit area was 9.62 cm on the Nuclepore and Millipore filters and 0.317 cm on the Tissuequartz tape. The flow rate was 16-20 1pm through the Nuclepore and Millipore filters and 10 1pm through the Tissuequartz tape. Each Millipore or Nuclepore filter was placed in a labeled plastic container immediately after collected, sealed with Parafilm, enclosed in a ziplock bag, and placed in a refrigerator in the trailer. The tape in the TWOMASS sampler was advanced between samples. The tape sample was removed about once every 8-10 weeks and stored similarly to the Nuclepore filters. The TWOMASS was cleaned at that time. All samples were stored in an ice chest during the return trip to Caltech. Field blanks were handled identically to the samples. Of approximately 100 filter samples collected in 1979, 61 were selected for analysis. The 61 were chosen to span the variation in bjp and to obtain representative seasonal and diurnal samples. Sample times varied from 6 to 72 hours, with an average of 20.1 hours. 

The Reynolds stress model requires the solution of transport equations for each of the Reynolds stress components as well as for dissipation transport without the necessity to calculate an isotropic turbulent viscosity field. The Reynolds stress turbulence model yield an accurate prediction on swirl flow pattern, axial velocity, tangential velocity and pressure drop on cyclone simulation [7,6,13,10],... 

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