Utilizing CFD to Optimize the Air Flow Channel for a Particle Counter Sensor
Sin Ming Loo
The Hartman Systems Integration Laboratory at Boise State University is collaborating with the National Institutes of Health to create home air quality monitor for use in the National Children’s Study. The monitor consists of sensors that continually measure various aspects of air quality, providing NIH researchers with data needed for research into the effects of environmental factors on children’s health. Of great importance is the measurement of airborne particulate matter. However, the commercially available particle counters are not suitable for in-home use due to their size, cost, or maintenance requirements. To meet the needs of the National Children’s Study, a custom particle counter was designed and built at BSU to provide real-time measurements of particulate concentration in two different size categories.
The particle counter is a visible laser-based device for detecting airborne particles of sizes 0.3 μm and larger. The particle counter establishes an airflow using a DC fan to pull air across a photodiode sensor. A focused laser beam is passed through the air flow channel immediately above the photodiode, illuminating any passing particles. As particles intersect the laser beam they scatter laser light, some of which impacts the photodiode to generate an electrical pulse. The electrical pulse is amplified and then sampled with an analog to digital convertor. A microcontroller monitors the digitized signal for pulse amplitude and duration, comparing with calibrated values to determine particle size. The device currently differentiates particles into two size ranges of 0.3 - 2.5 μm and larger than 2.5 μm.
In order for the particle counter to make accurate measurements, an airflow channel must be designed around the sensor that allows for a consistent, smooth flow of air. Using the ANSYS computer software suite FLUENT, Computational Fluid Dynamics (CFD) analyses are being conducted to optimize this flow channel. Due to the size constraints of the monitor, the volumetric flow rate of the fan guiding the air flow, and the small size of the particle counter sensor, a turbulent flow is inevitable through a uniform channel. Using an inflation mesh algorithm provided by the software package, the air channel is designed to minimize the boundary layer thickness which forms inside the channel. The airflow within the boundary layer is difficult to predict and inconsistent, which could have negative effects on the sensor’s resolution.