A Circumferential Slot Virtual Impactor
Abstract:A virtual impactor aerosol concentrator has been developed that uses circumferential slots for acceleration of aerosol particles and for collection of the coarse fraction. This allows for accurate and economical machining of small slot widths, which leads to low-pressure losses for the separation process. One important application of the device is in the concentration of bioaerosols, especially for military field applications where minimization of power consumption is necessary. A prototype configuration of the circumferential slot virtual impactor (CSVI), which was designed using numerical methods, was constructed and tested. The device has a curvilinear slit nozzle with a diameter of 150.3 mm (5.918 in), which provides a total slot length of 472 mm. Its slot width was 0.499 mm (0.0197 in). According to Loo and Cork, for circular-jet virtual impactors the misalignment between the axis of the acceleration jet and the receiver nozzle will cause an increase in wall losses of about 1.6% for each 1% of misalignment. Measurements were made of the nozzle dimensions in the critical region of the CSVI that showed 1.8% relative misalignment. When this prototype was operated at a flowrate of 122 l/min and a flow fraction (minor air flowrate/total air-flowrate) of 10%, the cutpoint was 2.2 μm aerodynamic diameter and the corresponding cutpoint Stokes number was 0.58. The collection efficiency was greater than 72% for particle sizes larger than twice the cutpoint, up to the largest particle size tested (≈10 μm aerodynamic diameter). The peak collection efficiency was greater than 95%. For virtual impactors, a critical performance parameter is the loss of particulate matter to the inner walls of the system. For the prototype system, where numerical methods had been used to generate designs that reduced wall losses, the losses at the cutpoint size of 2.2 μm aerodynamic diameter, are approximately 3%. For an operational condition of a total flowrate of 122 l/min and a coarse particle flow fraction of 10%, the pressure drop across the major flow stream (fine particle stream) was 63 Pa (0.25 in of water), with an ideal power consumption of 0.14 watts.
Document Type: Research Article
Affiliations: Aerosol Technology Laboratory, Department of Mechanical Engineering, Texas A&M University, College Station, Texas
Publication date: 2004-07-01