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By Tim Olson, Krebs Engineers, Tucson, Arizona
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Hydrocyclones use the principle of centrifugal separation to remove or classify solid particles from a fluid based on size, shape, and density. A mixture of fluid and particles is fed tangentially into the upper or larger diameter part of the hydrocyclone. The resulting rotational flow forces the larger and denser particulate matter to the wall of the device, after which it exits through the bottom or apex of the cone. The cleaned liquid and fine particles exit through an overflow outlet on top after passing through the vortex finder, a pipe that penetrates into the main cyclone chamber from above. Hydrocyclones are simple in design, but the swirling multiphase flow represents a challenging problem for numerical analysis. Nonetheless, CFD simulations of hydrocyclones have met with considerable success in recent years.
A clear model of a gMAX cyclone
Rather than invest in an annual CFD license and in-house CFD analysts, Krebs worked with Enductive Solutions to develop a custom tool that could be used to enable design engineers without a CFD background to carry out hydrocyclone simulations. The collaboration began as a technical consulting project where Enductive engineers gained an understanding of Krebs’ design processes and the key parameters that affect hydrocyclone performance. Based on the lessons learned, they developed a template-style tool that could be used for cyclone simulations for a wide range of geometries and operating conditions.
Some gMAX inlet heads
The tool consists of a custom interface that automates the setup, solution and postprocessing steps in GAMBIT and FLUENT. The user enters parameters like length, diameter, and flowrate into a series of panels, and a geometry and hybrid mesh are created automatically in GAMBIT. The topology of the mesh is very important in highly swirling flows, so the tool embeds the most appropriate meshing best practices. It then sets up the multiphase problem in FLUENT, using the Reynolds stress turbulence model. The solution is performed on FLUENT’s remote simulation facility (RSF). Since the physics of hydrocyclone flows are so demanding, the tool follows a solution strategy that was developed for this class of problems, taking into account periodic flow variations that are known to develop in hydrocyclones. Once the problem converges, the tool creates a predefined set of contour plots to display the results. All of these steps are controlled by the custom interface at a cost of about $300 per run.
The tool was completed at about the same time that a new advanced hydrocyclone design called the gMAX was being developed. The basic idea behind this design was to make a finer cut with a large cyclone by using two cone angles, the first steep and the second shallow. The two sections work together to keep the tangential velocities as high as possible throughout the length of the unit. Engineers first modeled the old design and compared the simulation results with laboratory tests in order to evaluate its accuracy. The flow-splits and pressure drops from laboratory tests were very close to the CFD predictions for two different vortex finder sizes. The new gMAX design was then modeled under a variety of operating conditions, and the results also compared favorably with lab tests.
A sample panel showing how the Enductive tool works
At this stage in the gMAX development process, the separation efficiency was high but excessive wear was occurring at the inlet. In general, it is difficult to measure comparative wear rates on installed cyclones in the field. The length of time required is prohibitive because the component may last over a year before failure. Inspection of worn inlet liners from the early designs showed evidence of a “hot spot” or location where the rubber liner had worn through, whereas the rest of the liner had not. In lab tests, Krebs was able to define the high wear areas by coating the inside of the inlet with multiple layers of different colored paint. The paint layers are quickly worn away in areas of the inlet that are likely to experience high erosion. The CFD model could predict the wear location, and by keeping track of the number and size of particles that collided with each area of the inlet, it could also calculate the erosion rate. In comparison with the paint tests, the correct prediction of the wear location and wear rate constituted another validation of the CFD methodology.
Contours of tangential velocity for the Krebs cyclone (left), and contours of air volume fraction, showing the central air core (right)
Engineers conceived of another inlet design with the goal of eliminating the turbulence in this area in hopes of extending the service life of the hydrocyclone. The new inlet features a refinement of the involute-style inlet that has been a hallmark of Krebs Engineers hydrocyclones for 50 years. In the new design, the partition between the in-bound slurry and the separation chamber is extended significantly to further classify the solids before introducing them into the main body of the cyclone. The design provides an area for the inbound slurry to gradually mix with the incoming fluids.
Velocity vectors in the central plane, colored by velocity magnitude.
The CFD model predicted, and physical testing confirmed that the new design for the inlet would result in substantially lower erosion levels. In fact, the new inlet lasted twice as long as the old design. The CFD study also provided substantial information on the performance of the alternative inlet designs that led to valuable insights. For example, particle tracks showed the paths of both a relatively large and a relatively small particle that report to the underflow and overflow respectively. The larger particle was spun out towards the outer wall after entering the internal high tangential velocity area. The smaller particle, following the predominant flow path, successfully traversed this area and exited with the overflow stream. Both particles had a residence time of about 1.5 seconds. With the old inlet design, small and large particles experienced a sharp change in direction as they entered the open cylindrical section of the hydrocyclone. With the new design, the CFD results showed smooth particle trajectories from the inlet to the main body. Other Krebs cyclones have also benefited from CFD analysis carried out by their design engineers. Overall, the results have demonstrated the many benefits that can be realized from template-based CFD simulation. _ A sample panel showing how the Enductive tool works Contours of tangential velocity for the Krebs cyclone (left), and contours of air volume fraction, showing the central air core (right) Velocity vectors in the central plane, colored by velocity magnitude.
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