Cyclonic separation is a widely used separation technique in the process, oil and gas, and cement industries. The mechanism of separation occurring within the cyclone is known as classification, whereby fine and coarse, as well as lighter and heavier particles, are separated according to their terminal velocity in the fluid. The cyclone works by inducing spiral rotation in the primary phase, which imposes an enhanced radial acceleration on suspended particulates due to the centrifugal force. Conventional cylindrical cyclone devices consist of a conically shaped vessel, two outlets (underflow and overflow), both on the axis of symmetry, and a feed inlet connected to a cylindrical section. The underflow is situated at the apex of the cone. The overflow is an inner tube which passes through the top of the cyclone. The portion of the overflow pipe that extends into the body of the cyclone is a short, removable section known as the vortex finder. The vortex finder prevents short-circuiting of the feed directly into the overflow.
Due to the imposed swirl, larger particles migrate radially to the outer wall and then spiral down to the underflow. Smaller particles migrate more slowly and are captured in an upward spiral in the center of the cyclone and leave through the top via the vortex finder. Liquid cyclones, commonly known as hydrocyclones, are open to the atmosphere, which adds additional complexity. Due to the low pressure at the cyclone axis, a backflow of gas can occur, forming a gas core. Experimental work has shown that in the central core region under the vortex finder, the tangential velocity increases sharply with increasing radial distance; outside the central core, the tangential velocity decreases with increasing radius.
Simulate the motion of air in a cyclone separator and understand the physics of swirling/rotating flows. For the purpose of this example, the focus will only be on the motion of the air and no suspended particles will be included in the simulation.
Download the Mesh file needed for setting up the simulation and the associated Case & Data files here. Follow the instructions below to set up this simulation in Ansys Fluent starting with the Mesh file. In case you face any issues setting up or running the simulation, please refer to the corresponding initial and final Case and Data files.
Let us now analyze the simulation and understand the physics of rotating air flow in the cyclone separator.