Measuring the Size and Speed of Particles and Drops

Physicist Walter Schäfer and his colleagues at the Univ. of Applied Sciences in Darmstadt have developed at time-based technique for measuring particle size distribution and speed. (Picture: Wühr)

The characterization of drops and particles in a stream is the key element of many process engineering tasks, such as drop measurement during spray drying.

“Existing methods for measuring speed and size are however unsuitable for non-transparent particles and drops of suspensions and emulsions,” explained physicist Walter Schäfer. By using new analysis and validation algorithms, he and his colleagues at the Department of Fluid Mechanics and Aerodynamics at the University of Applied Sciences in Darmstadt have been able to improve the familiar time shift technique.

As a result of their work, these measurements can now be performed reliably in technical systems with minimal equipment requirements. They have developed an optical configuration which is substantially simpler and smaller.

The new version of the time shift method can be used with both the forward and backward scatter technique, making it possible to take measurements when access to the process is restricted. The measurement system can easily be adapted to handle special tasks, for example in milk powder or hair spray production.

The technique can be used to measure the speed and size of particles as well as drops. In many instances, the refractive index of the particle can be measured. This makes it possible to distinguish between different phases. Users have an inline measurement system which offers the advantage of simplicity and flexible positioning and can be used for process monitoring and optimization. Prototypes are already in use, and the researchers have applied for a patent covering the key aspects of the new technology.

They claim that in terms of measurement range and accuracy for transparent particles, the new method is comparable to existing optical techniques. Funding for the research carried out by Schäfer and his team is provided by the German Research Foundation (DFG) and Postgraduate Program 1114 among others.