Research On The Nanoparticles Nucleation And Coagulation Processes In Twin-jet Exhaust Plume Of A Vehicle

PM_2.5 is a major component of air pollution in urban areas. Study shows that nanoparticles are able to penetrate into lung where they may enter interstitial tissue, causing severe respiratory inflammation and acute pulmonary toxicity. Automobile emission is a major source of air pollution. The sulfur in diesel fuel is oxidized to SO_X, that leads to the sulfuric acid and sulfates inthe PM_2.5.The purposes of this thesis are to study the flow structure and the formation of pollutantnanoparticles in a vehicular exhaust twin-jet plume with the nucleation and coagulation equations and to a better understand of the particle number concentration and size distribution. The thesis can be divided into fore parts.In the first part, experiments have been carried out with a hot-wire anemometer to investigate the flow field of the twin-jets. The velocities have been measured to assist in understanding the change in the flow. The effects of Reynolds number and the spacing between two nozzles on the flow field along the flow and lateral directions were examined. The conclusions are drawn as follows: In the flow direction, the twin-jets are clearly separated near the nozzle exits. Away from this region, the two jets interact, and then the two jets mix and merge to appear as a single jet. The velocity and turbulent energy profiles are fairly symmetrical about the center line of two jets for various Reynolds numbers and the spacing between two nozzles. The velocities between two jets change along the lateral direction more quickly than that of previous studies. The interference between two jets increases as Reynolds number increasing. The stronger the interference is, the larger the Reynolds shear stress and turbulent energy are.. The interference between two jets increases as the spacing between two nozzles decreasing. Furthermore, the width of the twin-jets spreads linearly downstream and grows with B. The merging length of two jets can be increased either by reducing B or increasing Reynolds number.In the second part, large eddy simulation method has been used to calculate the parallel twin-jet flow coupled with nucleation and coagulation equations. The nanoparticle number concentration distribution and size distribution produced by the pure nucleation and coagulation are shown in the paper . The binary homogeneous nucleation will produce many nanoparticle and it takes place mainly in the interval of the twin jets region and the circumambience of the jets. Coagulation modifies the particle size distribution since it reduces the overall number of particles and increases the mean size of them resulting from forces. Sulfur content effects on the nanoparticle number concentration distribution. The nucleation process will generate higher number concentration and smaller size of nanoparticles as sulfur content increases. High relative humidity and jet Reynolds number will increase the nucleation rate, which results in more nanoparticles.In the third part, large eddy simulation method has been used to calculate the flow and obtain the detailed flow structures of the impinging twin-jet for various conditions. The nucleation of sulphuric acid/water system and subsequent coagulation processes are simulated.The jets are clearly separated around the exits of nozzle, but significantly altered at the interacting side with their inner cone layers collided with each other at the middle plane between the two jets. After impinging on the plane, they are separated from the plane and forced to turn upward. There exist free jet region, adherent plane jet region and stagnation region in the flow. The interference between two jets increases with the decreasing of the space between two jets and the distance from the exit of nozzle to plane. The region of maximal particles size appears around the region of free jet. The number concentration decreases significantly after coagulation. The maximal number concentration produced by both nucleation and coagulation occurs in the region near the plane. The significant difference for various spaces between two jets is the number concentration and size distributions in the interface region of two jets. For the case with larger space, more nonoparticles are produced by nucleation and coagulation. There is also a wider size distribution for the case with larger space.The longer the distance from the exit of nozzle to plane is, the lower the number concentration is, and the fewer particles distribute near the plane. Increasing the distance from nozzle to plane is benefit to reduce the formation of nanoparticles. Though the number concentration is low for the case with longer distance from nozzle to plane, the size of particle in the region near the plane is large.In the forth part, large eddy simulation method has been used to calculate the flow and obtain the detailed flow structures of the moving car for various conditions. The maximal number concentration produced by both nucleation and coagulation occurs in 1m region behind the car. The longer the distance from the exhaust exits, the fewer particles distribute. The size of particleproduced by pure nucleation is larger on the edge of the gas-to-nanoparticle conversion regionThe nanoparticles number concentration and size distribution will be effected by the flowstructure which induced by the changing of the ratio of the ambient wind velocity to the exhaust gas velocity. Because a larger amount of H₂SO₄ vapor is diluted by high-ambient wind speed than by a low-ambient wind speed, the higher ambient wind speed is benefit to reduce the formation of nanoparticles and increase the particle size.