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QUANTIFYING THE EFFECTS OF THE MIXING PROCESS IN FABRICATED DILUTION SYSTEMS ON PARTICULATE EMISSION MEASUREMENTS VIA AN INTEGRATED EXPERIMENTAL AND MODELING APPROACH

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Impact/Purpose:

The main objective of the proposed study is to investigate a key uncertainty in PM emissions measurement by examining the following questions: How do the mixing processes in the current constant volume sampler (CVS) systems differ from that in the real-world conditions? How do the mixing processes in the different CVS systems differ from each other? How does the mixing process interact with aerosol dynamics that affect PM measurements in the CVS systems and PM transformation in the atmosphere?

Description:

Mixture properties vs Aerodynamic properties
 
Considering a number of parameters influencing particulate emission measurements, we first categorize them into two groups based on their characteristics, i.e., to mixture properties and the aerodynamic properties, as described in Table 1.
 
 
 
  • The tunnel configuration in Group I (“Aerodynamics”) is usually designed ahead of the measurements and remains fixed during the measurements, while the operating conditions can vary during the experiment depending on the purpose of the study.
  • For Group II (“Mixture properties”), the properties of exhaust (e.g., temperature, sulfuric acid concentration) depends on the types of fuel, engines, after-treatment devices, and their operating conditions during the experiment, while the properties of dilution gas (e.g., relative humidity and temperature) can be varied during the experiment depending on the purpose of the study.
Even though some qualitative relationships have been developed on how Group II parameters, i.e., “Mixture properties”, affect the measured particle size distributions, such knowledge is seriously lacking with regard to the effects of Group I parameters, i.e., “Aerodynamic properties”, currently only characterized by dilution ratios. At the same time, there is strong experimental evidence demonstrating the critical role of aerodynamic properties. To be consistent with the original proposal, we also refer to the aerodynamic properties as the mixing process. The main objective of this project was to investigate how different mixing processes in the dilution systems affect particulate emission measurements. 
 
Approach
 
The physical nature of the evolution of particle size distributions inside a dilution sampling system is coupled turbulent mixing and aerosol dynamics process. The current lack of knowledge on the roles of the mixing process (i.e., aerodynamic properties) can be attributed to the fact that it is extremely difficult to quantify experimentally the rapid mixing process taking place within a few seconds. Therefore, we implemented an integrated experimental and modeling approach. In short, we developed an advanced turbulent reacting flow model named CTAG that is capable of capturing the coupled turbulent mixing and aerosol dynamical processes, and utilized this model to analyze the existing experimental data comparing particle size distribution measurements using different dilution systems. The adoption of this approach is critical to the success of this project.
 
The major research activities are described as follows.
  • First, we developed and evaluated a new turbulent reacting flow model named CTAG, whose advanced capabilities enabled us to conduct analyses that have never been done before. The role of CTAG is crucial in the overall project.
  • Then, we applied the CTAG model to analyze the data collected from an intercomparison experiment of two laboratory dilution systems.
  • Next, we applied the CTAG model to analyze the data collected from a study designed to compare on-road chasing and laboratory dilution measurements.
  • Finally, we summarized the results from those analyses, and present a framework to characterize the effects of the mixing processes on particulate emission measurements.
The Development of the CTAG model
 
The Comprehensive Turbulent Aerosol Dynamics and Gas Chemistry (CTAG) model is designed to simulate transport and transformation of multiple air pollutants.
  • As the physical natures of atmospheric dilution (on and near roadways) and artificial dilution (in dilution sampling systems) are the same, CTAG is also designed to be applicable for both on-road/near-road applications and dilution sampling applications, which is a major advantage in comparing on-road chasing and laboratory dilution sampling methods.
  • For the on-road/near applications, CTAG explicitly couples the major turbulent mixing processes, i.e., vehicle-induced turbulence (VIT), road-induced turbulence (RIT) and atmospheric boundary layer turbulence with gas-phase chemistry and aerosol dynamics. CTAG’s transport model is referred to as CFD-VIT-RIT.
  • Another unique capability of the CTAG model is the implementation of a presumed finite-mode probability-distribution function (PDF) method used to couple the turbulent mixing process with the aerosol dynamics, and capture the micro-mixing effects on aerosol dynamics. This capability enabled us to quantify, for the first time in the research community, dilution rates in diluting sampling systems, which is a key component in the framework to characterize aerodynamic properties.
The development of the CTAG model has undergone extensive evaluations by comparing the modeling results against the respective field measurements, including:
  • VIT behind a moving van
  • Evolution of particle size distributions in the wake of a diesel car
  • Sensitivity studies to analyze the relative roles of VIT, sulfuric acid induced nucleation, condensation of organic compounds and presence of soot-mode particles in capturing the dynamics of exhaust plumes as well as their implications in vehicle emission controls.
Analyses of turbulent flow fields and aerosol dynamics of diesel exhaust: Comparison between two laboratory dilution sampling systems
 
We employ the CTAG model to investigate the effects of aerodynamic properties on a set of particulate emission measurements comparing two dilution tunnels, i.e., a T-mixing lab dilution tunnel and a portable field dilution tunnel with a type of coaxial mixing. The turbulent flow fields and aerosol dynamics of particles are simulated inside two dilution tunnels. Particle size distributions under various dilution conditions predicted by CTAG are evaluated against the experimental data. It is found that in the area adjacent to the injection of exhaust, turbulence plays a crucial role in mixing the exhaust with the dilution air, and the strength of nucleation dominates the level of particle number concentrations. Further downstream, nucleation terminates and the growth of particles by condensation and coagulation continues.
 
The CTAG model allowed us to quantify the dilution rates using the scalar dissipation rate, representing the rate at which the diesel exhaust and dilution air are brought together at the molecular level. Then we arrived at several important findings:
  • At the same dilution ratio, higher dilution rates typically lead to higher nucleation rate and nucleation-model particle concentrations.
  • The T-mixing lab tunnel tends to favor the nucleation due to a larger dilution rate of the exhaust than the coaxial mixing field tunnel.
  • Therefore, dilution rate is a critical parameter to characterize aerodynamic properties, in addition to dilution ratios.
Analyses of turbulent flow fields and aerosol dynamics of diesel exhaust:
 
Comparison between on-road and laboratory dilution sampling systems The main task for this part of the project was to compare laboratory emission measurements (i.e., using fabricated dilution systems) and on-road chasing measurement (i.e., using atmospheric dilution) of vehicle emissions through numerical simulations and analysis of experimental data. This is a key step in explaining how the mixing processes in the fabricated dilution systems differ from those in the real-world conditions. First, we conducted numerical simulations, the first of its kind in the research community, to represent the chasing measurements, and the simulation results are compared favorably with the experimental results. Second, we developed techniques to simulate the operations of two widely used fabricated dilution systems, i.e., porous and ejector dilutors, both of which were deployed in the laboratory measurements.

 

Record Details:

Record Type:PROJECT( ABSTRACT )
Start Date:05/01/2010
Completion Date:04/30/2013
Record ID: 251053