Grantee Research Project Results
1999 Progress Report: Modeling Collision Efficiencies for Coalescence of Small Drops and Particles
EPA Grant Number: R827115Title: Modeling Collision Efficiencies for Coalescence of Small Drops and Particles
Investigators: Koch, Donald L.
Institution: Cornell University
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1998 through September 30, 2001
Project Period Covered by this Report: October 1, 1998 through September 30, 1999
Project Amount: $305,155
RFA: Exploratory Research - Physics (1998) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Land and Waste Management , Air , Safer Chemicals
Objective:
The objective of this research project is to determine the rate of coagulation of small particles and drops driven by Brownian motion, turbulence, and differential sedimentation. Particular attention will be given to the influence of noncontinuum gas flow between colliding particles on the collision efficiency.Progress Summary:
A prerequisite for modeling the collision efficiency in aerosol systems is determining the resistance produced by the noncontinuum gas to the relative motion of colliding particles. In our previous work, we had predicted the resistance for the case of two rigid spheres when the mean-free path of the gas is much smaller than the particles, using a lubrication analysis for the noncontinuum flow in the thin gap between the particles. During 1999, we considered the opposite limit where the mean-free path is much larger than the particle size. An integral equation governing the flux of gas molecules exchanged between the aerosol particles was derived and solved, and the drag force on the particles was derived.When small drops collide, their interfaces deform due to the pressure induced by the flow in the gas. The drops may coalesce or bounce depending on whether the gas is forced out of the gap between the drops before the kinetic energy of their relative motion is transformed into energy of surface deformation. We have developed a program to simulate the noncontinuum gas flow in the gap between two colliding drops as well as the resulting surface deformation. This program currently is being used to map out the region of parameter space for which bounces and coalescence events occur.
To validate the predictions for the bounce/coalescence transition, we have constructed an experimental apparatus in which a drop is propelled toward a gas-liquid interface in a chamber in which the gas pressure can be controlled. Pressures of 0.2 to 5 atmospheres can be attained. Drops with well controlled size and velocity are produced from a thin capillary using a piezoelectric device. The collision is visualized using a high-speed video camera. The apparatus has been constructed and tested. Quantitative experimental results will be obtained in the next year of the project.
In addition to studying the detailed two-particle interactions that lead to coalescence, we also are developing methods to determine the net rate of coalescence or coagulation in an aerosol. Previous theories of turbulence-induced coagulation have assumed that the particles are well mixed so that the particle number density may be assumed independent of spatial position. However, turbulent intermittency implies that the strain rate driving coagulation events varies over the integral (macroscopic) length scale of the flow. This leads to variations in the rate of coagulation, which can induce fluctuations in particle number density. The resulting negative correlation between particle number density and turbulent shear rate decreases the net rate of coagulation. In addition, particles in high shear regions are larger and grow faster than those in low shear regions, so that mixing limitations tend to broaden the particle size distribution. These effects are important when the product of the Reynolds number based on the Taylor microscale and the particle volume fraction is greater than about 0.1. Thus, the effects are likely to be more important in geophysical flows where the Reynolds number is very high than they are in laboratory experiments or numerical simulations. We have developed a statistical model of this phenomenon that will allow researchers to quantify the effects of intermittency on the rate of coagulation.
Future Activities:
During the next year, we plan to obtain quantitative experimental measurements and theoretical predictions for the drop size, gas pressure, and impact velocity corresponding to the transition from coalesence to bouncing. We also will develop a simulation method for determining the hydrodynamic interactions between two particles when the particle size and interparticle separation are comparable with the mean-free path of the gas.Journal Articles on this Report : 1 Displayed | Download in RIS Format
| Other project views: | All 7 publications | 4 publications in selected types | All 4 journal articles |
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Gopinath A, Koch DL. Hydrodynamic interactions between two equal spheres in a highly rarefied gas. Physics of Fluids 1999;11(9):2772-2787. |
R827115 (1999) R827115 (Final) |
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Supplemental Keywords:
air, atmosphere, precipitation, particulates, physics, mathematics, modeling., RFA, Scientific Discipline, Air, Waste, particulate matter, Environmental Chemistry, Physics, Atmospheric Sciences, Ecology and Ecosystems, Engineering, Chemistry, & Physics, Incineration/Combustion, collision efficiency models, pollution control technologies, air pollution modeling system, air pollution, Brownian motion, pollutant transport, differential sedimentation, emission controls, atmospheric transport, coalescence, particle surface interactions, incineration, pollution dispersion models, particle collision models, atmospheric models, particle collision, particle transportProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.