Particle motion in flows: Contributions to airborne aerosol measurement.

摘要

Accurate, in situ measurement of airborne particles is critical in understanding the global atmosphere. An airborne instrument is designed to sample and measure the aerodynamic diameter of particles in the size range of 1-30 mum. Knowledge of the aerodynamic size of a particle includes the impact of unknown variables such as shape, size and density of a particle. Particles of the same aerodynamic diameter have the same trajectory and settling velocity in air, regardless of their actual size, density or shape. Knowing the time a particle resides in air can improve the climate models substantially.;The developed instrument consists of a low turbulence inlet and a laser-Doppler velocimeter to sample and measure the super-micron particles all in one unit. Air enters the inlet at true air speed of the aircraft, which is a relatively high speed in the range of 100-200 ms-1. The low turbulence inlet reduces the air speed to 5-10 ms-1 in a short distance without generation of additional turbulence. It uses boundary layer suction through a porous diffuser to remove a substantial amount of air that is responsible for turbulence generation and particle loss in deposition to the inlet walls. Reduction of turbulence makes it possible to model and solve the flows inside the inlets and calculate the particle trajectories using computational fluid dynamic (CFD) tools.;A new technique is developed to determine the size distribution of particles based on the motion of particles and measurement of the particles' velocity. Velocimetric measurement of particle size is achieved by slowing the velocity of the flow that is carrying the particles and measuring the velocity of the particles in response to this change. Larger particles have higher tendency to continue at a higher speed, while smaller particles tend to follow the air stream lines. A laser Doppler velocimeter (LDV) measures the velocity of the particles as they pass through the viewing volume of the velocimeter. The aerodynamic diameter of the particles and their concentration is determined from the velocity measurements.;Accurate determination of particle size distribution at the location of measuring sensor does not guarantee the accuracy of the ambient size distribution. The modification of number of particle due to particle loss, enhancement, and particles bouncing off the leading edge of the inlet is considered to accurately measure the ambient size distribution of particles.;This research work also focuses on quantifying the effects of particle enhancement in the low turbulence inlet used in ACE-Asia sampling inlet and the deposition of particles in transport through the bends of ACE-Asia and NOAA inlets. The effect of force of gravity on deposition efficiency of particles has also been analyzed.