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Experimental Characterization of Electrical Discharges and Formation of the Ignition Kernel. Application to the Study of Performances of Aeronautical Igniters

Abstract : Spark ignition systems are generally defined by the electrical energy input used to operate them. However, the physical characteristic that directly affects the ignition process is the energy deposit supplied to the fluid by the system. This work focuses on the development of two proposed methodologies for the characterization of the thermal energy deposit of electrical discharges produced by different ignition systems, and their implementation through a parametric study. An experimental device is developed for this purpose, using simultaneously a non-optical and an optical technique. The experimental techniques are first validated in a reference configuration: a pair of pin-to-pin electrodes with an automobile-type inductive ignition system.Constant volume calorimetry measures a thermal energy deposit supplied to the fluid via the pressure rise inside a reduced volume chamber. The ratio between thermal energy deposit and electrical energy supply represents the efficiency of energy transfer, which is between 15 and 40% for the reference configuration. Energy deposit and efficiency are higher as pressure and inter-electrode gap increase. Tests with an inert propane-nitrogen mixture show that energy deposit is greater in the presence of fuel than in clean air.SBOS (Speckle-based Background-Oriented Schlieren) is an optical method that quantifies changes in the optical index generated by the phenomenon under study. This technique has been adapted to the spatial and temporal specificity of an electrical discharge. Image-processing procedure has been developed to obtain density, temperature and local energy fields at the time of image acquisition. The volume of the hot kernel produced by the plasma and the energy deposit are deduced from it. These properties are measured at different times during the evolution of the kernel. Temperatures in the hot kernel reach higher values (up to 1400 K) at longer inter-electrode gaps and higher pressures, or in the presence of gaseous fuel. Energy deposit measurements performed by SBOS are in good agreement with calorimetry results.Finally, both methodologies are adapted to the study of different ignition systems. Two igniters were tested: an innovative multi-filament radiofrequency discharge igniter and a capacitive helicopter engine igniter. For the latter, the energy deposit is measured for different initial pressures and gas mixtures to simulate the actual engine conditions. The electrical energy input is 2 J, the electrical energy measured at the electrodes is 625 mJ and finally the thermal energy deposited in the gas is about 85 mJ. The estimated efficiency of 14% is not very pressure dependent. The SBOS technique is used to estimate the temperature in the hot kernel at the first moments of discharge (around 3700 K) and the thermal energy deposit, which is in good agreement with the calorimetric measurement.
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Carlos Javier Benito Parejo. Experimental Characterization of Electrical Discharges and Formation of the Ignition Kernel. Application to the Study of Performances of Aeronautical Igniters. Other. ISAE-ENSMA Ecole Nationale Supérieure de Mécanique et d'Aérotechique - Poitiers, 2019. English. ⟨NNT : 2019ESMA0021⟩. ⟨tel-02860055⟩

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