Research

Research Objectives and Summary

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Our research goals are to explore new knowledge and to develop new combustion technologies and functional nano-materials to enable renewable fuels, advanced propulsion, efficient energy conversion, and bio-imaging. Our research areas include following thrusts:

  • Extreme Combustion (High pressure, ultra-lean, high speed...)

  • Flame Chemistry and Fuels

  • Non-Equilibrium Plasma Assisted Combustion

  • Micro-scale Combustion

  • Multi-scale Modeling

  • Advanced Diagnostics

  • Functional Nano-materials

Research Projects        

1. 1. Extreme Combustion and Fuels

  • The flammability limit and near or sub-limit combustion

    Advanced engines and propulsion systems often work at extreme conditions near or below the flammability limit to improved energy efficiency and reduce emission.  The flammability limit is governed by fuel chemistry and flame radiation. Due to the preferential diffusion effect, fuels with smaller Lewis numbers can burn below the flammability limit if stretched. The general flammability limit diagram is given by the G-curve (see the figure on the right for CH4/air). E is the the fundamental flammability limit of methane, B is the sublimit of stretched methane flame. That is, a streched flame can burn leaner than unstretched flames. This was observed in microgravity counterflow flame experiment.

  • The minimum ignition energy and critical flame radius

Ignition and ignition to flame transition in engines are governed by the so called minimum ignition energy (MIE).   Despite research in several decades,  the physics behind MIE was not understood. Recently, by introducing radiation and flame stretch, we discovered that MIE is governed by the critical flame initiation radius, which is a function of pressure, Lewis number, and fuel activation energy (see Figure on the right). These criterions are experimentally measured and provide guidance for engine design. This critical flame initiation radius was measured for liquid fuels.

The critical flame radius vs. Lewis number

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  • Alternative fuels, Surrogate fuels, Radical Index

    Energy sustainability and climate change present a urgent call for renewable fuels. These new fuels such as synfuels as well as solar fuels and biomass derived alcohol and biodiesel have hundreds of different molecular structures and distinctive combustion and emission characteristics. It is important to understand the combustion characteristics of fuels with different molecular structures and develop a surrogate fuel model with a few selected components to mimic the real fuel performance. Our research focus on the measurements of flame properties (e.g. flame speeds, ignition and extinction limit, flame structure, and speciation) by using counterflow flames, high pressure spherical bomb, and jet stirred reactor; the development of generic correlation between flame extinction and molecular structure by introducing Radical Index;  Transport weighted enthalpy (TWE), and the development of generic surrogate fuel models to mimic real fuels, and validation of kinetic mechanism of jet fuel, methyl esters (biodiesel), and  alcohols.

Correlation between extinction limit and radical index (Ri) and transport weighted enthalpy (TWE) for different fuels

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Measured and predicted hydrogen burning rates

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  • Cool Flames and Low Temperature Fuel Oxidation

Cool flame has been  regarded later as a key process for engine knock, motivating extensive studies on large hydrocarbon low temperature chemistries. However, establishing a stable cool flame is extremely challenging.  A novel method to establish self-sustaining cool diffusion flames with well-defined boundary conditions has been experimentally demonstrated by using ozone and plasma.  For more, pls read,

[1] Won, et al., Self-Sustaining n-Heptane Cool Diffusion Flames Activated by Ozone, Proceedings of Combustion Institute, 35, Accepted, 2014

[2]Sun, W.et al. http://dx.doi.org/10.1016/j.combustflame.2014.01.028

Comparison of a hot and a cool diffusion        n-heptane flame

  • Turbulence Combustion with Low Temperature Chemistry

Combustion in practical gas turbine engines, after-burners, and internal combustion engines are governed by igntion, and turbulent premixed and partially premixed flames at elevated temperature and pressure at high Reynolds numbers. However, little is known about the role of low temperature fuel chemistry on turbulent combustion and propagation speeds. The results suggest that contrary to the previous studies, the turbulent flame regimes and burning velocities for fuels with low temperature chemistry can have multiple values and may not be uniquely defined at elevated temperatures.

A new high temperature, high Reynolds number, Reactor Assisted Turbulent Slot (RATS) burner has been developed to investigate turbulent flame regimes and burning rates for large hydrocarbon transportation fuels, which exhibit strong low temperature chemistry behavior. It is found for the first time that for n-heptane/air mixtures there are four unique turbulent flame regimes. Moreover, low temperature ignition can induce flame flashback.

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HO2 diagnostics using mid-IR Faraday Rotational Spectroscopy (FRS)

2. Non-Equilibrium Plasma Assisted Combustion

The development of scramjet engines for supersonic propulsion requires ignition and complete combustion to be achieved within one millisecond. Non-equilibrium plasma provides a new opportunity to enhance ignition. However, despite of many technological developments and claims in the literature, the kinetic mechanism of plasma based combustion enhancement was not well understood. Our research has been focused on the understanding of kinetic and transport role of electronically excited species (singlet delta, long lifetime catalytic species, and active radicals generated by gliding arc, microwave, and nanosecond discharge on ignition and flame extinction.

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กค  3. Microscale Energy Conversion

Microscale energy conversion has a broad application on micro-power generation, surveillance propulsion, hydrogen synthesis, chemical sensors, and solid oxide fuel cells. On such a small scale, the surface to volume ratio significantly increases and the thermal inertia time of the reactor structure becomes comparable to the thermal diffusion time of the reactants. As such, increased heat loss, flame-structure coupling and radical quenching on the surfaces lead to rich and new flame dynamics. We have been developing a number of theoretical models and conducted experiments to describe the impact of heat recirculation and radical quenching on mesoscale combustion. The theory predicted new flame regimes such as weak flame, normal flame, as well as flame streets.  Read more

The methane-air diffusion flame street formed in a mesoscale channel

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กค  4. Functional Nano-materials

The sharp and up-converting properties of rare earth doped nanophosphors (e.g. NaYF4:Ln3+) provide new opportunities for temperature imaging, bioimaging, counterfeit security, solar cells and LEDs. However, optical quenching and monodisperse synthesis are the great challenges. We developed new flame based and high temperature in-solution based down-conversion and up-conversion nanophosphor synthesis methods to obtain monodispersed nanocrystals from 5 nm to 250 nm. We also developed a kinetic model to control the particle size monotonically by using a single parameter of lanthanide ion to Na ratio. By conducting dynamic luminescence time decay measurements, nonlinear luminescence power dependence analysis, and Raman scattering at different crystal sizes, we proposed that it is the surface quenching but the quantum confinement responsible for the luminescence decrease with the decrease of particle size. By using the temperature dependency of luminescence, we developed a simultaneous particle velocimetry and thermo(PIVT) method to measure temperature and velocity in reactive flows at the same time. By collaborating with Wole Soboyejo in MAE, Robert Prud'homme in Chemical Engineering, and Robert Austin in Physics, we successfully demonstrated the singlet oxygen production of biofunctionalized nanoparticles and cancer killing ability using near infrared photodynamic therapy.

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