An interview with Paul Dunwoody, P.Eng.
Paul Dunwoody is a mechanical engineer at CWA with expertise in Discrete Element Method and Computational Fluid Dynamics modeling. He has worked with materials handling equipment at marine terminals and mines in Canada and the USA, performing 3D design and DEM analysis on complex transfer chutes and feed hoppers. Paul has also performed detailed mechanical design on numerous conveyors and feeders.
In the interview below, Paul explains how mechanical engineers use this specialized simulation analysis to solve real world problems and optimize the performance of materials handling systems in the ports and terminals, mining, construction materials, and wood products sectors.
What is Discreet Element Method and Computational Fluid Dynamics modeling and how is this analysis used?
PAUL: Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD) modeling are two types of numerical methods that allow us to use computers to simulate what is going on in the real world.
DEM modeling is a technique used to simulate the movement and effect of numerous, small, solid particles such those found in the bulk materials handling industries. These particles could be anything from coal and potash to rocks or wood fiber. Basically, any solid material made up of particles.
Computational Fluid Dynamics (CFD) modeling uses numeric analysis and data structures to solve problems involving fluid and air flows, simulating their movement around objects. CFD is often used for modeling air flows around buildings, airplanes, and bridges or for modeling water flows around ships or through pipes and valves. So, this type of modeling is used in a variety of industries.
What kinds of projects typically utilize this type of analysis?
PAUL: At CWA we use DEM modeling for analysis of material flows through transfer chutes and hoppers before they are built, allowing us to create designs with optimal performance without having to build them. We also use this type of analysis to improve the performance of existing transfer chutes. We replicate the given problem or issue on the computer so that we can design a solution for it.
We generally do not use CFD on its own, but rather, combine it with DEM analysis to model air flow through a transfer chute. This is known as CFD-DEM coupling. Adding the CFD airflow computations to the DEM particle flow simulation provides a picture of how the dust particles flow inside the transfer chute. We do this to try and reduce or eliminate dust that gets generated as materials flow through a transfer point.
Performing a coupled CFD-DEM simulation is a very involved process and is not typically performed except in rare instances where dust control is of paramount concern. An example would be the handling of lead or some other potentially hazardous materials that require careful and safe handling due to health and safety considerations. In these types of situations our clients require a very thorough analysis of the dust flow and are thus willing to spend the engineering effort. But in most cases, we rely on DEM to reduce dust generation and that generally provides a very good result.
Aside from addressing the issue of dust management, we frequently use DEM modeling by itself to design transfer chutes, reduce impact angles, and to make sure that the material is being loaded centrally on the receiving conveyor, which provides the client with better belt tracking and helps to reduce product spillage and waste.
We also look to reduce wear on the walls of the chute and to reduce particle breakage. These are all issues that we address with DEM modeling, which enables us to optimize a conveyor system’s chute transfers during the design phase.
DEM modeling can also be used to protect against material degradation which is very important to some clients, such as when they are selling a product based on the particle sizes. This would include products like potash, where buyers of the product require a particular size, or certain agricultural products such as grain, where the integrity of the product needs to be maintained. These are all cases where the client is looking to protect the materials against degradation.
Additionally, reducing the amount of wear on the chute walls results in savings to the clients by reducing both expenditures on equipment, such as liner replacements, and the need for downtime to their operations for equipment repair and replacement.
Can DEM Modeling be used at both greenfield and brownfield facilities?
PAUL: At brownfield sites, a client might notice a problem with a transfer chute and need to fix or optimize the equipment within their system that is creating the problem. In such cases, we would initially perform a DEM simulation of the existing chute, validate the model, and ensure that it is accurately reproducing what is happening in real life. We would then modify the computer model to check whether various solutions would work.
What inputs do you use to create the computer model?
PAUL: We begin by making a 3D model of the chute using the existing drawings and/or field measurements. Then, to model how each of the materials behave, we rely on a database of the materials that we have successfully modeled in the past. So, that is basically our starting point.
We then visit the facility to witness the operations and take videos of the material flowing through the transfer chute to evaluate how the system is performing. We open the inspection access points and insert GoPro cameras to capture what is going on inside the transfer chute. We get as many videos of the material flow as possible from many different angles.
We then use the information and videos gathered at site to tweak the parameters of the computer simulation to match it as closely as possible to the videos that were taken. The parameters that we are interested in are usually the wall friction factor, which represents how the material slides against the walls, the internal friction factor, which represents how the material slides against itself, as well as the cohesion factors, which represent how sticky the material is to itself and to the walls.
CWA has been performing this type of modeling on materials handling chute transfers for a long time now and we have gained a great deal of experience and knowledge about the behavior and flow of the various materials used at our client’s facilities, such as coal, potash, grains, mineral concentrates, and the like.
As I mentioned earlier, our database of material flow behavior is quite extensive. Because of this experience we can design, for example, a new potash conveyor system with confidence, knowing that the transfer chutes that we design will operate efficiently once they are built, even if a client cannot supply us with a sample of potash or doesn’t have an operating chute for us to use to validate our simulation.
Similarly, if we are designing a greenfield system, we have enough historical information in our database to design a highly efficient operating system.
What are some of the latest developments in the field of DEM modeling and where do you see this technology going in the future?
PAUL: Our simulations are increasingly using more and finer particles, giving us a closer match to what is happening in the real world. The capability to create these more detailed simulations is where the advancements are coming from – being able to refine the particle sizes in larger, more complex models. As a result, our simulations have become increasingly accurate and now take less time to complete, which makes DEM an even more attractive tool for solving real world problems.