The Law Group has been conducting research on the combustion chemistry of alternate fuels and their combustion characteristics within the high-pressure environment of internal combustion engines. The researchers are also interested in the explosion hazards resulting from hydrogen leakage due to the rupture of high-pressure storage tanks.
Outreach in China
Chung Law visited China for two weeks in the fall, and gave a total of six lectures and seminars on various topics on fuels, energy, and the environment. These include the plenary lecture entitled “The role of combustion in climate change and energy sustainability,” at the annual Combustion Conference held in Xi’an, the plenary lecture entitled “From atomic to cosmic: a panoramic view of combustion,” at the 7th Asian-Pacific International Conference on Combustion and Energy Utilization, held in Beijing, and a seminar on “Clean and efficient combustion for transportation: research agenda and recent progress,” at the BP Center at Tsinghua University, of which he is a guest professor. While at Tsinghua he also visited the newly established Laboratory on Low Carbon Energy, and discussed possible collaborations. Furthermore, in his role as a past president and a director of the International Combustion Institute, he conducted a site visit on behalf of the 33rd International Combustion Symposium, which will be held at Tsinghua in 2010. In addition, he visited the Chinese Academy of Sciences, at which he is a guest professor, the Beijing University of Aeronautics and Astronautics, where he received appointment as an honorary professor, and the University of Science and Technology of China, where he received appointment as a guest professor. Throughout this trip he discussed broadly with Chinese colleagues the establishment of an infrastructure for fundamental combustion research in China to support the country’s technological needs for energy sustainability and climate issues.
Combustion of Methyl Decanoate – A Surrogate Biodiesel Fuel
Methyl decanoate (n-C3H7C(=O)OCH3), abbreviated as MD, is a large methyl ester that can be used as a surrogate for biodiesel. In an experimental and computational study, the combustion of MD was investigated in nonpremixed, nonuniform flows. Experiments were performed employing the counterflow configuration with a fuel stream made up of vaporized MD and nitrogen, and an oxidizer stream of air. The mass fraction of fuel in the fuel stream was measured as a function of the strain rate at extinction, and critical conditions of ignition were measured in terms of the temperature of the oxidizer stream as a function of the strain rate. A detailed mechanism of 8555 elementary reactions and 3036 species had been developed previously to describe combustion of MD. Since it is not possible to use this detailed mechanism to simulate the counterflow flames because the number of species and reactions is too large to employ current flame codes and computer resources, a skeletal mechanism was deduced from this detailed mechanism using the “directed relation graph” method. This skeletal mechanism has only 713 elementary reactions and 125 species. Critical conditions of extinction and ignition were calculated using this skeletal mechanism and they were found to agree well with experimental data. In general, the MD mechanism provides a realistic kinetic tool for simulation of biodiesel fuels.
Self-Acceleration and Fractal Propagation of Flames
In previous studies the group has conclusively demonstrated that the propagation of a flame in a combustible medium could be accompanied by the development of fine-scale wrinkles over the flame surface. The propensity for such a destabilized mode of propagation is further enhanced in high-pressure environments characteristic of internal combustion engines used for transportation. The presence of wrinkles increases the total flame surface area and as such would increase the flame propagation rate. Furthermore, the continuous generation of the wrinkles implies the increase in the flame speed can be accelerative, and could lead to the eventual transition of the flame propagation mode from being laminar to turbulent and finally to detonative. The transition to detonation is the crucial factor in the onset of explosions either due to the rupture of high-pressure hydrogen storage tanks or as a mechanism for engine knock.
Work this year conclusively demonstrated that such an accelerative mode of flame propagation is indeed possible. Furthermore, if the instantaneous flame radius R(t) at time t is expressed as t α , then it was found not only that α>1, which indicates self acceleration, but also that α attains a constant value of 4/3, which implies that the flame propagation is self-similar, with a fractal dimension of 2.25.
Noting that the fractal dimension for turbulent flame propagation is 2.33, this result therefore suggests the interesting possibility that a propagating laminar flame, which is deterministic in nature, can transition to a turbulent flame, which is probabilistic in nature. Furthermore, the existence of self-acceleration also implies the possibility of transition to detonation. An interesting offshoot of this study is its potential application to the astrophysical phenomenon of supernovae. Here the reactions are nuclear in nature, although it is believed that it is the transition to detonation of the nuclear flame that leads to the star’s extraordinarily rapid rate of attainment of explosion.
Collaboration with Ford Colleagues on Engine Simulations
The collaboration with Ford researchers (James Yi) on engine simulation has continued. The Law Group’s contribution has been the development of reduced-order oxidation mechanisms of engine fuels that are needed in the computer codes for large-scale engine simulations. The challenge here is that the oxidation mechanisms of engine fuels are extremely complex, being described by hundreds to thousands of reacting species and thousand to tens of thousands of reactions. Consequently, the associated computational burden is simply too large to make computations practical, even with anticipated advances in computer hardware and algorithms. This in turn implies that engine combustion cannot be simulated with realistic chemistry.
In order to circumvent this difficulty, the researchers have developed a suite of mathematical algorithms that allows the systematic and accurate reduction of these mechanisms to a level that is amenable to computational simulation. During the past year they have reduced the mechanisms for n-heptane and iso-octane, which are the constituents of surrogate gasoline fuels. They have supplied these mechanisms to their Ford colleagues, who are now implementing them in their engine codes.