By Dr. Shinichi Ookawara, Assistant Professor, Department of Chemical Engineering, Tokyo Institute of Technology, Tokyo, Japan

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Images of traditional chemical engineering plants often include such things as large mixing tanks, cyclone separators, and distillation columns. Chemical engineering "plants" are also being developed at the other end of the size spectrum, particularly in Japan. There, a growing interest is afoot to develop microreactors, micromixers, and microseparators for chemical and biochemical processes.

Dr. Shinichi Ookawara

Dr. Shinichi Ookawara, an assistant professor at the Tokyo Institute of Technology, has developed a novel approach to separating and classifying particles in a dilute fluid-particle mixture. The principle takes advantage of the properties of laminar flow patterns found in curved microchannels. The separator is made of a simple curved channel, which is split at one end. A mixture containing fluid and particles (with a range of sizes) is pumped into one end of the channel and leaves through one of the two exit passageways at the other end. Dr. Ookawara's experiments have shown that the larger particles are concentrated in the outermost exit channel, while the smaller particles empty through the innermost channel. The efficiency of the device is related to many parameters, such as the flow rate, particle size distribution, and channel geometry. In practice, many microchannels can be stacked together in parallel to increase the throughput. Dr. Ookawara has been using FLUENT 6 to help understand the flow phenomena in a typical microseparator, and to optimize the geometry and the operating conditions involved.

Schematic of a micro-channel separator

Flow in a curved channel has characteristic secondary flow patterns called Dean vortices. Because of the centrifugal force acting on it, the fluid moves radially outward in the center of the channel, where there is little resistance to flow. It returns to the inner channel wall through two circulation loops in the top and bottom halves of the channel. In a particle-laden fluid, the Dean vortices, coupled with the centrifugal force acting on the particles, can be used as a separation mechanism. Large particles migrate to the outer wall of the channel, while small particles remain entrained in the secondary flow field. At the end of the curved section of the channel, separate passageways can be used to extract fluid containing either large or small particles.

FLUENT 6 simulation of a lateral slice through a curved channel illustrating the prediction for Dean vortices

The small size of the channel means that experiments to determine the flow patterns within the tube are not practical. Using FLUENT, Dr. Ookawara's research group has been able to calculate how changes in both the shape of the channel and flow rates influence the strength of the secondary vortices thus, the separation efficiency. Simulations using using the Eulerian granular multiphase model have been performed to study particle separation. The researchers have also used FLUENT to optimize the channel cross sectional area and improve the shape of the exit split, which are other important parameters in the separator design.

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