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Laminar burning speed calculation at high pressures is challenging because of unstable surface conditions at large flame kernel diameters. It is therefore desired to take these measurements at small dimensions (i.e., during and immediately after the ignition discharge process) when the flame kernel is smooth and stable. Taking accurate measurements at these sizes is challenging because the kernel growth rate does not only depend on the chemical reaction but also on other phenomena such as energy discharge, as well as radiative and conductive energy losses. The effect of these events has not been adequately assessed, due to the generation of ionized gas (i.e., plasma). In order to better understand the effect of the ignition plasma in this work, spark ignition in air for 1–5 atm of pressure is studied. Understanding the ignition event and modeling its behavior is important to capture accurate combustion measurements at pressures pertinent to the advanced high-pressure engines and technologies. The relationship between the electrical energy supplied to the spark and the thermal energy dissipated within a gas mixture has been studied. This work relates the electrical discharge power to the volume of the ignition kernel measured via schlieren imagery. Voltage and current data are also captured as the input to a thermodynamic model which is used to predict the volume versus time data of the spark event. The model, which utilizes measured electrical power, thermodynamic properties of ionized air, and radiation losses in air show agreement with the experimental kernel measurements in terms of overall shape of the volume data within the measured kernel uncertainty. With these results and further experimental validation the present model is considered to represent the relationship between the electrical spark power and the measured ignition kernel volume. Future work will be done to determine inaccuracies present in the arc discharge regime as well as the effectiveness of the model in combustible media.
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Structure and Measurement of Atmospheric and High-Pressure Ignition Plasma
Experiments are conducted to understand atmospheric spark ignition process in more detail. The research done relates the electrical energy dissipated across the spark gap to the measured schlieren ignition volume. The result is the supplied electrical thermal energy. The study provides insight into the structure of plasma and the mechanisms which convert electrical power into heat. The research is done to support laminar burning speed calculations to increase accuracy and extend diagnostic techniques to conditions otherwise immeasurable. Typically, plasma measurements are taken via a Langmuir probe. However, for the automotive ignition plasma, this measurement technique is challenging because of the transient nature, high pressure, and temperatures involved. Therefore, several alternative techniques will be used in order to find the potential distribution of the plasma and unveil the structure of the plasma more specifically the cathode fall. Three different voltage measurements are taken in order to capture the cathode fall of the plasma. One method simply measures the potential using a high voltage probe. This method may be inaccurate because of the presence of charged ions, however, these results are compared to non-intrusive measurements where voltage data is extrapolated over various gaps sizes to zero length. It is generally agreed that the desired measurement for this work, the cathode fall, remain constant and depends on the composition of the gas and the electrode. Therefore, changing the system input power and the gap will only change the voltage drop across the bulk plasma. The linear change in voltage potential through variation of testing parameters like gap length can then be extrapolated to zero length of the bulk plasma or minimum energy value which should be equal the value of cathode fall and bulk plasma potential respectively. It was found that after excluding systemic losses such as electrical resistance and ignition coil inefficiencies, the primary loss within the plasma gap is the potential drop across the cathode sheath. Excluding the loss in the cathode fall results in a measured electrical data that is responsible for thermal discharge. In order to highlight the findings, electrical discharge energy is compared to the volume of the heated gas kernel in atmospheric air. Removal of the cathode fall data will show that the energy is proportional to the volume of heated gas whereas, before the change in energy dissipation between glow and arc plasmas prevented this relationship from being visible. The data and methods discussed in this research provides the means to determine the thermal energy of ignitions and sparks even when the spark is inaccessible or obscured. Further work will be done utilizing the power measurement found in this work in a model to predict the affected thermal spark volume. It is also proposed that further validation of the proposed measured electrical thermal energy should be compared to the energy measured with a calorimeter to determine any other inefficiencies in the plasma discharge process. Additionally, the experimentation done observes the cathode fall of only glow plasma, Additional work should be done to find the cathode fall of arc plasma.
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- Award ID(s):
- 2137585
- PAR ID:
- 10356793
- Date Published:
- Journal Name:
- ASME 2021 International Mechanical Engineering Congress and Exposition
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1115/IMECE2021-73138
