Aerosol particle detection and determination finds important applications in the commercial, military and aerospace sectors. Monitoring of clean room environments, and spacecraft integration and check out facilities are some of the most important aplications. In the early days test filters were examined with a microscope to determine the number and size of particles that were being removed from air. Today, most of the commercially available clean room airborne particle counters work on a light scattering principle. They are referred to as Optical Particle Counter or OPC. Essentially, they utilize a very bright laser light source to illuminate the particles. The burst of light energy is converted into a pulse of electrical energy. By measuring the height of the signal and counting the number of pulses the sizes and quantities of particles could thus be determined. The microscope and the OPC techniques have their limitations. The microscope technique is a post contamination assessment technique and the OPC is costly, hard to maintain, lack in counting efficiency and is not mobile. This experimental study demonstrates a novel and inexpensive particle detection technique which is based on the acoustic signature of airborne particles as they are accelerated through an acoustic transducer. The transducer consists of an inlet converging nozzle, a capillary tube and an expansion section. If the air is laden with particles, as the flow accelerates through the inlet, the particles cannot follow the large change in velocity due to their inertia. Vortices are generated as air flows over the particles prior to entering the capillary. These vortices are believed to generate sound, which is amplified by the transducer acting as an organ pipe. This sound emission if measured contains frequencies that are harmonics of the natural frequency of the transducer's air column. Results show how the frequency content of the acoustic signature relates to the fundamental frequency of the transducer's air column. The transducer is able to detect micron sized particles ( 5 to 50 micron) and the sound intensity is a function of the flowrate but not of particle size. This study also shows the ability of the transducer to determine particle concentration as low as few parts per liter (ppl) and compare the data with that obtained from a commercially available aerodynamic particle sizer.
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Master of Science in Mechanical Engineering (M.S.M.E.)
College of Engineering and Computer Science
Mechanical, Materials and Aerospace Engineering
Length of Campus-only Access
Masters Thesis (Open Access)
Haddad, George, "Automatic Particle Counting Using An Acoustic Transducer" (2005). Electronic Theses and Dissertations. 330.