Grantee Research Project Results
Final Report: Quantifying the Effects of the Mixing Process in Fabricated Dilution Systems on Particulate Emission Measurements via an Integrated Experimental and Modeling Approach
EPA Grant Number: R834561Title: Quantifying the Effects of the Mixing Process in Fabricated Dilution Systems on Particulate Emission Measurements via an Integrated Experimental and Modeling Approach
Investigators: Zhang, Ke Max
Institution: Cornell University
EPA Project Officer: Chung, Serena
Project Period: May 1, 2010 through April 30, 2013 (Extended to October 30, 2013)
Project Amount: $250,000
RFA: Novel Approaches to Improving Air Pollution Emissions Information (2009) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Objective:
Laboratory particulate emission measurements using fabricated dilution systems are essential to almost all emission testing procedures and major combustion sources (e.g., diesel engines, gas turbines, biomass stoves). The current dilution-based samplings have many known limitations and those limitations are pronounced for measuring the semi-volatile composition and the ultrafine range (<100 nm) of the particulate emissions.
The main objective of this study is to investigate a key uncertainty in PM emissions measurement by examining the following questions:
- How do the mixing processes in the fabricated dilution systems differ from those in the real-world conditions?
- How do the mixing processes in the different fabricated dilution systems differ from each other?
- How does the mixing process interact with aerosol dynamics that affect PM measurements in the fabricated systems and PM transformation in the atmosphere?
Summary/Accomplishments (Outputs/Outcomes):
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.
Group I. Aerodynamics | Fixed parameter: Tunnel configuration | Mixing type of the dilution tunnel: T-mixing dilution tunnel, coaxial mixing dilution tunnel, performeated tube diluter (Dekati Ltd.), ejector diluter (Dekati Ltd), rotating disk diluter (Matter Engineering Inc.), etc. |
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The mixing enhancer: fan shape plate, crifice plate, baffle, etc. | ||
Variable parameter: Operating Condition | Dilution ratio (DR) at the end of the dilution tunnel | |
Residence time inside the distribution tunnel | ||
Group II: Mixing properties | Properties of engine exhaust: temperature, water content, sulfuric acid concentrations, OC concentation and composition, size, distribution of the primary soot-mode particles, etc. | |
Properties of dilution gas: temperature of dilution gas, relative humidity (RH), particle size distribution, OC concentration and composition, type of dilition gas, (e.g., pure nitrogen or air), etc. |
- 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.
Conclusions:
- For the first time in the research community, we have achieved a mechanistic understanding on the effects of aerodynamics properties on particulate emission measurements and we have developed a tool to quantify them. Our integrated experimental and modeling approach is key to those achievements.
- For fabricated dilution sampling systems under small dilution ratios (e.g., <100), the maximum dilution rate is generally a good indicator for nucleation-mode concentrations, because the conditions are usually dilution rate-limited. Typical fabricated dilution systems operate at this regime, where higher maximum dilution rate corresponds to higher particle number concentrations.
- For atmospheric dilution that typically results in large dilution ratios (e.g., ~1000), both dilution ratio and dilution rate have to be considered to compare nucleation-mode concentrations.
- In general, dilution rate and dilution ratios are both needed to be quantify to characterize the aerodynamic properties in fabricated dilution sampling systems and atmospheric dilution sampling systems.
- Dilution rate profiles of any dilution sampling systems can be reliably predicted by CTAG using sampling system geometric configurations, exhaust and dilution temperature and air flow rates.
- Future work should be directed at further characterizing conditions with large dilution ratios and developing isopleth contours for dilution ratios and dilution rates, for aerodynamic properties and mixture properties. Those contours can be used to compare the emission measurement results from different sampling systems.
- Our study indicates that numerical simulation tools can be utilized to develop strategies to reduce the uncertainties associated with dilution samplings of emission sources, and to develop new dilution sampling systems that can minimize those uncertainties.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 5 publications | 2 publications in selected types | All 2 journal articles |
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Type | Citation | ||
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Wang YJ, Zhang KM. Coupled turbulence and aerosol dynamics modeling of vehicle exhaust plumes using the CTAG model. Atmospheric Environment 2012;59:284-293. |
R834561 (2011) R834561 (2012) R834561 (Final) |
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Wang YJ, Yang B, Lipsky EM, Robinson AL, Zhang KM. Analyses of turbulent flow fields and aerosol dynamics of diesel engine exhaust inside two dilution sampling tunnels using the CTAG model. Environmental Science & Technology 2013;47(2):889-898. |
R834561 (2011) R834561 (2012) R834561 (Final) R834554 (Final) |
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Supplemental Keywords:
Air quality, climate change, emissions, computational fluid dynamics, CFD
Relevant Websites:
Analyses of turbulent flow fields and aerosol dynamics of diesel engine exhaust inside dilution sampling tunnels using the CTAG model Exit
Progress 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.