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Steven F. Son
School of Mechanical Engineering, Purdue University
Title: Tailoring the Ignition and Combustion of Aluminum in Propellants
Aluminum is used widely in solid propellants, and other energetic materials, because it yields significant impulse performance gains and improved stability at reasonable cost and good storage life. Generally, particle size and morphology are all that can be changed. Nanoscale metal fuels do offer potential advantages, including faster burning rates and more complete combustion. Motivated by this, replacing micrometer-scale aluminum in energetic materials with nanoscale aluminum has been studied for more than two decades, including studies involving aluminium-water propellants that will be reviewed in this talk. However, simply substituting micrometer-scale particles with nanoscale particles in solid propellants can lead to drawbacks including poor rheology and mechanical properties. Nanoscale aluminum also has a relatively thick oxide layer that reduces the performance. Indeed, examples of fielded propellants containing nanoscale aluminum are not currently found and may likely never be. A key question is how one can obtain the advantages of nanoscale fuels without the drawbacks. Recently, efforts have focused on micrometer-scale aluminum particles with an intraparticle nanoscale structure. The ideal solution may be a micrometer-sized composite particle that has significantly lower ignition temperature and, when ignited, produces much smaller particles/droplets. In multiphase liquid combustion, “micro-explosion” can occur for some miscible liquids and also in emulsions. A key requirement for this to occur is for one of the constituents to be more volatile than the others. For miscible liquids, a disparity between the liquid-phase mass and thermal diffusion is necessary in establishing droplet dynamics. Emulsions, such as fuel oils and water, can also yield self-atomization, or micro-explosion dynamics. In an analogy to liquid fuels, metal alloys are similar to miscible liquid fuels in that the atoms or molecules are mixed intimately to form a single phase, eutectic, or solid solution. Likewise, inclusions of more volatile materials within a metal fuel (e.g., a polymer in aluminum) are the equivalent to an emulsion where multiple phases are intertwined together, but not atomically or molecularly mixed. The intertwining of phases in a metal can be achieved using milling processes, which can result in lower particle ignition temperatures and dispersive dynamics. This method is also inexpensive and readily scalable. Another approach to achieve a similar outcome, which has also been pursued, could be the direct bottom-up fabrication of nanoscale fuel particles held together with a more volatile binder. In this talk I will review our efforts to tailor the ignition and combustion of aluminum in propellants. This has applications to other metals and other combustion systems. I will also discuss new directions that could allow even more tailoring of the combustion, including additive manufacturing applied to solid propellant fabrication.
Department of Mechanical Engineering, University British Columbia
Title: Thermodynamic and Optical Combustion Characterization of Natural Gas Fuelling Strategies for Compression Ignition Engines
In light of the significant North American natural gas (NG) reserves, NG is considered an attractive alternative fuel for internal combustion engines. Relative to combustion of diesel or gasoline, NG combustion results in lower CO2 emissions, and potentially lower NOx and particulate emissions. Beyond this, NG is often viewed as a transitional fuel, as technologies suitable for use with NG may be well suited for renewable natural gas (RNG), which is of particular interest for heavy-duty and large-bore engine applications, where alternatives such as electrification are not as attractive for de-carbonization. While NG is an attractive alternative fuel, it is nonetheless plagued by significant methane emissions, which can account for a significant increase in greenhouse gas emissions. In the case of spark ignited engines, NG requires high ignition energies, particularly for lean operation, resulting in limited spark plug life. For heavy duty applications, compression ignition NG fuelling strategies are preferred due to their higher efficiencies and improved ignition robustness, relative to spark ignited approaches.
In this work, two NG compression ignition fuelling approaches are considered. In both approaches, the combustion of a second fuel, such as conventional diesel, is used for the ignition of the NG. In “dual-fuel” strategies, a premixed air-NG mixture is compressed and ignited near top dead center by the combustion of a small, direct-injected diesel “pilot”. Depending on the relative quantities of air, premixed NG, and direct-injected diesel, as well as the diesel pilot injection pressure, the fuel conversion efficiency and emissions can vary significantly. High Pressure Direct Injection (HPDI) of NG provides an alternative dual fuel strategy with the potential for significantly lower methane emissions and higher efficiency. During HPDI operation, NG is injected directly into the combustion chamber when the piston is near top dead center. Similar to premixed dual fuel combustion, a small diesel pilot combustion event is used to ignite the NG, resulting in a mixing controlled combustion process. Due to the non-premixed nature of HPDI combustion, higher compression ratios and fuel conversion efficiencies are possible, as are lower methane emissions.
A convertible thermodynamic (i.e., metal) and optical single cylinder engine is used here to characterize the combustion and emissions performance of premixed dual fuel and HPDI combustion strategies. Of particular interest is the effect of fuelling parameters on the combustion mode, which was observed to include (partially) premixed flame propagation, multipoint autoignition, and/or mixing controlled combustion, depending on the fuelling approach. For premixed dual-fuel combustion, regimes of flame propagation and non-flame-propagation were identified on the basis of heat release rate characteristics and high-speed OH* chemiluminescence imaging. The fuelling conditions resulting in these combustion modes are identified using an equivalence ratio – diesel substitution rate map. In addition, the sensitivity of the reaction zone evolution (ignition, propagation, and growth) to the pilot injection pressure is demonstrated through the use of OH* chemiluminescence and visible luminosity imaging.
A preliminary characterization of HPDI combustion was also carried out to identify the influences of injection parameters (injection pressure, relative NG and diesel injection timings) on the combustion process. Due to the large size and high-pressure fuel supply requirements of the HPDI injector, a new cylinder head was developed to accommodate the HPDI injector, and additionally included accesses for in-cylinder diagnostics. Here, this access was used for pyrometric and infrared absorption probes to characterize the local in-cylinder particulate matter concentration and temperature, and methane concentration, respectively. Through this preliminary characterization, high-speed OH* and natural luminosity imaging were used to provide the first in-cylinder optical characterization of HPDI NG combustion.
Matei Ioan Petru Radulescu
Department of Mechanical Engineering, University of Ottawa
Title: The detonation paradox: the influence of diffusive processes in controlling the burning rate in detonations
Traditionally, detonation waves have been assumed as being governed by the reactive inviscid Euler equations. Transport phenomena, which govern the propagation of low-speed reactive waves (flames) are neglected. In recent years, with the growth in the numerical resources available, Euler simulations have been possible with high resolution. These results have demonstrated a fundamental paradox when compared to experiments. While experiments have demonstrated that more unstable detonations can be initiated easier and are more difficult to fail, the predictions based on the Euler equations have shown clearly the opposite trend. The paradox is not unlike the well-known d’Alembert paradox in fluid mechanics, whereby inviscid potential flow fails to capture the experimentally observed drag forces on an object.
My talk will highlight the physics of unstable turbulent detonation waves for which diffusive phenomena (amplified by turbulence) control the burning rate of gases inside the reaction zone structure of detonations. Due to the inherent cellular instability of detonations, the stronger shocks provide auto-ignition events, while the weaker shocks give rise to unburned pockets. These are surrounded by reactive gas in what resembles a distributed regime of turbulent combustion. These pockets burn out by surface turbulent flames.
I review detailed experiments of the detonation structure, which reveal the main mechanisms promoting turbulent mixing: Kelvin-Helmholtz on shear layers, Richtmyer-Meshkov from shock-flame interactions and internal strong jet formation associated with shock reflections.
The paradox identified points to the necessity of modeling not only the detonation instability, which provides the driving mechanism of turbulence, but also the dissipation of turbulence, which controls the reaction rate of approximately half the gas processed by the detonation front.
Given the unique character of detonation waves, which require a proper description of turbulent flame propagation by transport, auto-ignition phenomena and gas dynamics, typical tools used in LES of turbulent combustion (e.g., flamelet or thickened flame approaches) are not possible. I highlight our recently demonstrated computational methodology for detonations based on the Linear Eddy Model for turbulent combustion and its extensions to compressible flow in the context of Large Eddy Simulations.