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Laminar flame speeds at high pressure are difficult to experimentally observe. Typically, high-pressure flame diagnostic is accomplished through observation of a spherically expanding flame in a constant volume chamber. However, flame instabilities at large radius and ignition effects at small radius limit measurable pressure range for Laminar flame speeds. Advanced combustion devices which operate at high pressures to improve capabilities and efficiencies require high quality laminar flame speed to aid in simulations and development. To expand the measurable range of flame data for high pressure flame measurement, this study proposes the further investigation of the ignition source and the incorporation of ignition diagnostics into the traditional flame speed analysis. To achieve this goal in future research, experimental spark propagation is observed and described in detail with the goal of improved experimental control and understanding. Parameters such as pressure, composition, electrode geometry and surface quality are discussed and the implication it has on the observed schlieren kernel propagation. Careful preparation of the ignition source can lead to improved surface and shape for future flame measurements. It is also shown that the thermal energy dissipated into the gas during discharge can be captured experimentally through measurement of the spark voltage, current and plasma sheath voltage drop. With the experimentally captured thermal energy, a simple energy balance can be used to describe the size of the observed ignition kernel for radius larger than 0.3 mm (following breakdown). The measurement of the thermal energy is described utilizing two experimental methods to find the discharge voltage of the nonequilibrium region at the boundary of the electrodes.
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High Pressure Spherically Expanding Laminar Flame Speed Measurement with Plasma Affected Data
Measurements of a propagating flame are crucial to the understanding and verification of important flame phenomena. Specifically, laminar, unstretched flame speed is employed to describe complex flame behavior such as turbulence or stability. This data is necessary because advanced and more efficient combustion devices operate at these high pressures where this data is less reliable from the onset of flame instabilities. This research aims to develop a new flame speed measurement technique as an improvement over the traditional method. The analysis utilizes a constant pressure technique where the velocity of a spherical flame is visually measured. Flame propagation of stoichiometric flame from 1-6 atm are examined at radius up to 20mm at 300K. The novel analysis method incorporates flame data which is ignition affected to improve the traditional constant pressure methods. This allows for with the inclusion of ultra-small, highly stretched flame radii (0-10 mm). Electrical measurements of the ignition are utilized with a thermodynamic model to predict the velocity and temperature which result from plasma formation. This predicted temperature is used describe the influence ignition has on the flame velocity and is compared to the traditional adiabatic flame propagation over the radius observed. The extrapolated laminar burning speed measurement is found for both traditional and novel methods where the novel method has the benefit of additional, previously unused, flame propagation at the high stretch regime. Additionally, practical information about the plasma morphology, crucial to the experimental application of this method is also discussed.
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- Award ID(s):
- 2137585
- PAR ID:
- 10430094
- Date Published:
- Journal Name:
- AIAA SciTech
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The next generation of advanced combustion devices is being developed to operate under ultra-high-pressure conditions. However, under such extreme conditions, flame tends to become unstable and measurement of fundamental properties such as the laminar flame speed becomes challenging. One potential method to resolve this issue is measuring the ignition-affected region during spherically expanding flame experiments. The flame in this region is more resistant to perturbations and remains smooth due to the high stretch rates (i.e. small radii). Stable flame propagation allows for improved flame measurement, however, the experimentally observed kernel propagation is a function of both inflammation and ignition plasma. Therefore, the goal of the present study is to better understand the plasma formation and propagation during the ignition process, which would allow for reliable laminar flame speed measurements. To accomplish this goal, thermal plasma operating at high pressures is studied with emphasis on the spark energy effects on the formation of the ignition kernel. The thermal effect of the plasma is experimentally observed using a high-speed Schlieren imaging system. The energy dissipated within the plasma is measured with the use of voltage and current probes with a measurement of plasma sheath voltage drop as an input to numerical modeling. The measured kernel propagation rate is used to assess the accuracy of the model. The experiments and modeling are conducted in dry air at 1, 3, and 5 atm as well as in CH 4 -N 2 mixtures at 1 atm, and kernel radius, temperature, and mass are reported. The voltage-drop (as a non-thermal loss) is measured to be approximately 330 ± 5 V (dry air at 1 atm) for glow plasma with a large dependency on pressure, gas composition, electrode surface quality, electrode geometry, electrode shape, and current density. The same loss within the arc plasma is measured to be 15 ± 5 V, however the arc phase loss which agrees with arc propagation is significantly higher (∼45 V) which suggest additional unaccounted for phenomena occurring during the arc phase. With these losses, the modeling results are shown to predict the final kernel radius within 10%–20% of the observed kernel size. The difference found between the modeling and experimental results is determined to be a result of assuming that the primary loss mechanism (voltage drop across sheath formation) remains constant for the duration of glow discharge. The discrepancy for arc discharge is discussed with several potential sources, however, additional studies are required to better understand how the arc formation affects the kernel propagation.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.2514/6.2023-2050
