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Plasma, Combustion and Fluid imaging (PCFi) Laboratory
Omid Askari, Associate Professor


High-Pressure Combustion Characteristics and Chemical Kinetics Development

This image depicts laminar flame propagation and the thermodynamic model to measure the flame speed which is an important parameter for developing the chemical kinetics.

The fundamental combustion characteristics such as laminar burning speeds , onset of auto-ignition and ignition delay time at engine relevant conditions are necessary to develop, validate and to improve the predictive capabilities of computational tools and chemical kinetics models. These fundamental combustion parameters can be obtained through well-defined experimental systems (i.e., Constant Volume Combustion Chamber and Rapid Compression Machine) along with state-of-the-art theoretical models (i.e., in-house differential-based multi-shell model). The measured combustion properties in conjunction with rigorous mathematical methods can be used to extract the chemical kinetics pathways (i.e., Reaction Mechanism Generation) of each fuel over a wide range of operating conditions to mimic the real chemical behavior of fuels. Moreover, the development of new alternative fuels is achieving greater market penetration every day. To facilitate the application of alternative fuels in advanced combustion devices and to help industrial partners and national labs to perform a technical assessment on fuel options and to expand the stable operating conditions for advanced engines, it is very important to perform fundamental investigations on combustion characteristics of clean alternative fuels (i.e., oxygenated biofuels and hydrogen containing fuels: NH3, H2 blends, etc) at engine-relevant conditions.

High-Pressure Injection and Partial-Premixed Combustion


Due to increasing concern over energy shortages and the advent of strict environmental regulations, researchers in combustion and engine development have become motivated to discover novel ways to improve fuel economy, reduce pollutant emissions and greenhouse gases. One of the significant methods to achieve these goals is lean combustion. The most important problem in lean combustion for most hydrocarbon fuels (e.g., CH4, syngas, GTL, JP-8, etc) is the low burning speed which leads to flame extinction, large cycle variation, loss in power, increase in fuel consumption and unburned hydrocarbon emissions. One effective method to solve this problem is to enrich the region near the spark plug to initiate the flame with greater speed. This can be accomplished by high-pressure fuel direct-injection to create a stratified mixture just before the ignition which leads to a different combustion regime called partial- premixed combustion. Development of this lean combustion method requires research on many parameters including: spark delay time, stratification ratio, turbulence intensity, injection pressure, chamber pressure, chamber temperature, exhaust gas recirculation addition, hydrogen addition and different equivalence ratios to study flame propagation and turbulent-chemistry interaction of lean partially premixed combustion. In addition, macroscopic and microscopic spray characteristics of high-pressure injection such as penetration length, cone angle, penetration velocity, droplet size and velocity distribution, evaporation and mixing are strong functions of fuel physical properties as well as fuel evaporation and distillation characteristics. Fuel distribution affects ignition, burn-rate, particulate matter, temperature field and all key parameters (e.g., combustion phasing, combustion duration, etc.) that directly impact efficiency of combustion devices. Therefore, a complete knowledge of spray parameters is essential in developing of advanced combustion systems.

Flame Morphology and Instability Analysis

In this image the structure of a spherical propagating flame is shown. At one-point which is corresponding to instability point the cell formation starts to occur and flame becomes fully cellular.

Besides the laminar burning speed and ignition delay time, which are among the important thermo-physical combustion characteristics, the initiation and propagation of cellular instabilities such as cracking and wrinkling over the flame surface should be investigated in detail to achieve more efficient combustion devices. The two most significant instabilities that occur during the flame propagation result from hydrodynamic and thermo-diffusive effects. These instabilities distort the flame structure through the cell formation. Hydrodynamic instability is caused by the density variation across the flame while thermo-diffusive instability is due to the inequality of thermal diffusion from the flame and mass diffusion towards the flame and is distinguished by Lewis number. The instability analysis is performed through optical diagnostics systems in conjunction with chemical kinetic analysis using CANTERA. As a part of this work, the effect of diluent addition, equivalence ratio pressure and temperature on flame morphology and instability are extensively investigated. The impact of cell formation on burning speed and total flame front area is also investigated in terms of a newly developed parameter, called cellularity factor.

Ignition and Flame Kernel Formation at Ultra-High Pressures

Ignition and flame kernel formation is shown in this figure. Spark ignition forms thermal plasma which is combination of ions and electrons.

The next generation of advanced combustion devices are being developed to operate under ultra-high-pressure conditions to improve the combustion efficiency and to reduce the pollutant emissions. However, at such extreme conditions, flame tends to become unstable and measurement of fundamental properties becomes challenging. The laminar burning speed (LBS) is among those properties, as it is required for the validation of kinetic models and the modeling of turbulent combustion. One potential method to resolve this issue and achieve LBS measurement at very high pressures (i.e., 20-100 atm ) is focusing on ignition affected region. The flame kernel in this region is more resistant to perturbations and remains smooth due to the high stretch rates (i.e., small radii). The complication with this region is that, the kernel growth rate does not only depend on the chemical reaction but also on other terms such as energy discharge, as well as radiative and conductive energy losses. None of these terms has been adequately assessed, due to the generation of ionized gas (i.e., plasma). The research in the area aims to fill this broad knowledge gap via combined experimental and modeling studies focused with three specific goals: (1) using a well-defined and well-controlled high-pressure experimental configuration; (2) developing a self-consistent theoretical framework to explain the influence of energy discharge on the plasma formation and initial flame propagation; and (3) modifying an available high-fidelity direct numerical simulation (DNS) code to account for the evolution of the plasma kernel and the ignition process. On the experimental side, we utilize high-speed imaging of the plasma kernel propagation in conjunction with laser diagnostics (i.e, schlieren, PLIF, OES) for a time- and space-resolved investigation. The plasma properties are calculated using statistical thermodynamics .

Plasma-Assisted Combustion/Ignition in Turbulent Jet Flame

This image shows Plasma-Assisted Combustion in a Jet Flame. Low-temperature plasma will stabilize the flame through kinetic and transport enhancement pathways.

There has been recently a growing interest in the use of methane as a strong candidate for both interplanetary and descent/ascent propulsion solutions. The higher boiling point and higher density of methane compared with hydrogen, makes its storage tank lighter, cheaper and smaller. Methane is abundant in the outer solar system and can be harvested from Mars, Titan, Jupiter and many other planets and therefore, it can be used in reusable rocket engines. However, there are still some technological challenges in methane engines development path. Among those challenges, ignition reliability and flame instability are of great importance. To take advantage of methane in the next generation propulsion devices, an external stabilization system is required. The above challenges can be addressed by integrating high-frequency  repetitive nanosecond pulsed (HFRNP) discharge into the injector design. In this work, the flame stabilization in a coaxial injector has been studied. It was shown that the HFRNP discharge has the capability to reduce the liftoff height by 60% and flame length by 75%. The numerical modeling indicated that this behavior is due to generation of key radicals and excited species in the plasma region. However, the pressure and temperature in a real engine is significantly higher which resulted in autoignited jet flame. The stabilization of autoignited jet flame under high fuel dilution (i.e., MILD combustion) becomes challenging which affect the engine performance and prevent the safe operation of advanced vehicles. Current research involves plasma-enhanced MILD combustion under high pressures.

Alternative and Sustainable Fuels


The need for finding an alternative to oil-based fuels is higher now than ever before due to environmental impact and supply security. Synthetic-gas (syngas) is an alternative gaseous fuel obtained from feed stocks such as biomass, natural gas, and coal. With further chemical process of syngas through Fisher-Tropsch (F-T) synthesis, synthetic liquid fuels, called gas-to-liquid (GTL), such as diesel and jet-fuels will be produced. Recently, the interest on synthetic fuels as a viable alternative fuel has gained traction as these fuels do not warrant any major modifications to the existing fuel injection/combustor system and also offer cleaner combustion characteristics due to the near absence of sulfur and less aromatic content. Renewable fuels are materializing to offset petroleum use, but in order to be considered sustainable, we must consider their impact on society as a whole. New fuel sources, including ammonia, hydrogen containing fuels, synthesized fuels (e.g., GTL, CTL) and oxygenated biofuels (e.g., ethanol, butanol, anisole) are chemically different than traditional petroleum based fuels. This translates to fundamentally different combustion characteristic and reaction chemistry occurring within a combustion device, leading to different combustion heat release, efficiencies and exhaust emissions and therefore they need to be fully explored.

Other Research Topics


High-Pressure Ultra-Lean (HPUL) Combustion using Combo-Ignition System

The HPUL combustion is the most promising method to improve combustion efficiency and reducing emissions. However, it has not yet attained the desirable market penetration due to an ignition instability. In this work we aim to combine thermal and non-thermal plasmas and investigate the transition process into a sustainable flame kernel.

CO2 Conversion into Fuel using Non-Thermal Plasma

CO2 conversion into value-added chemicals and fuels is considered one of the great challenges of the 21st century. Due to the limitations of traditional approaches, several novel technologies are being developed. One promising approach, which has received little attention to date, is high-frequency nanosecond plasma discharge due to its unique capability in generating plasma with high electron density, low electron energy, low translational temperature, and very short stimulation time. These characteristics will promote vibrational excitation which is known as the most energy-efficient means for decomposition of CO2 molecules.

Plasma Enhanced Energy Conversion of Renewable Fuels

The transition to renewable energies is dependent on increasing the capacity for energy storage. Chemical storage in the form of ammonia is a promising solution at which the stored energy is recovered through combustion. This research examines the effect of low-temperature plasma on ammonia synthesis, controlling the ignition and emissions.

Stabilization and Scaling Analysis in Hypersonic Vehicles

There has been recently a growing interest in the development of innovative propulsion systems for future reusable hypersonic air vehicles (HAVs). These vehicles are expected to operate over a full range of speeds between Mach 0 and Mach 5+ which leads to flame instability (e.g., blow out) and ignition issues and prevent the safe operation of HAVs. To promote the flame stability and ignition reliability, non-equilibrium plasma has been proposed as an innovative and promising technology. The current study of partially-premixed regime in hypersonic flow, particularly under plasma discharge, is very limited, lacking detailed time- and space-resolved data (likewise premixed and non-premixed regimes). To fill this gap, a deep understanding of underlying physicochemical processes controlling flame dynamic, ignition characteristics and scaling parameters in conjunction with advanced laser spectroscopy is required.

Ducted Fuel Injection for High-Efficiency Clean Engines

Ducted Fuel Injection (DFI) is a new technology in which the fuel is injected through a small duct to enhance air-fuel mixing. The DFI is capable of reducing the PM, UHC and CO while maintaining comparable level of NOx emissions and thermal efficiency than conventional diesel. This indicates that DFI has a great potential to achieve high efficiency clean engines since it breaks traditional NOx-PM trade-off characteristics. Despite an unquestionable improvement in CO, UHC and PM emissions, the mechanisms of combustion enhancement are still elusive. This lack of understanding has plagued wider adoption of this technology in many advanced combustion devices.