Development of a Thermal-Vacuum Chamber for Extending Calibration of Optical Particle Detectors to Stratospheric Conditions

Abstract

Hypersonic flight dynamics are complex and certain important issues, such as what can trigger laminar hypersonic flows to become turbulent, are not well understood; thus new advances in science need to be made before aircraft can be designed that can safely and routinely fly at many times the speed of sound [1]. A team of undergraduate and graduate student researchers in the Aerospace Engineering and Mechanics (AEM) Department, working on a recently-funded MURI grant [2], are using optical particle detectors flown on weather balloon missions to characterize the particulate content of the stratosphere. Data collected will be used in computer simulations that study the onset of turbulence in hypersonic flows. The purpose of this project is to build and test a thermal/vac chamber to assist in the calibration of optical particle detectors such as the modest-cost Alphasense OPC-N2 detector [3] and the much-higher-cost LOAC detector [4] for use on stratospheric balloon flights. The University of Minnesota is one of the three main institutional players in the MURI grant and our role is to characterize particulate content in the stratosphere at altitudes from 80,000 to 120,000 feet – the altitude range at which hypersonic vehicles fly. Particle detectors are characterized by the manufacturer at STP conditions, yet the MURI project requires understanding detector output in the low-temperature, low-pressure conditions of the stratosphere. The MURI team is working with the Particle Calibration Lab (PCL) in the Mechanical Engineering (ME) Department to help calibrate the Alphasense particle detectors at STP in various airflow environments. However the current PCL calibration set-up is unable to duplicate the extreme temperature and pressure conditions encountered on stratospheric balloon flights; hence the need for a way to extend STP calibration results to stratospheric conditions.