IGNITION OF A SAFRAN'S HELICOPTER ENGINE WITH A COMPACT NANOSECOND LASER SYSTEM
Résumé
We report on the ignition of a Safran's helicopter engine using a compact laser system. The laser system delivers nanosecond infrared pulses at a high repetition rate which energy can be up to 40 mJ. The first tests were run on a Safran Helicopter Engines experimental bench operating at room temperature and ambient pressure with jet A-1 fuel. We studied in detail the impact of the laser pulse energy and the spark position on the engine ignition domain. An optical focusing system fixed on the engine casing enables to focus the laser pulse at different positions in the combustion chamber. The focused laser beam generates a plasma in the combustion chamber, which ignites the engine. Our experimental data show that the laser spark position impacts the pulse energy required to ignite the engine. At the optimal position, we very reliably ignite the engine with a pulse energy of only 3 mJ. To understand this behavior, similar experiments were run on the MERCATO experimental bench at ONERA (the French aerospace lab). This bench faithfully reproduces the geometry and the flow field of the Safran engine's combustion chamber. Using a Mie scattering imaging technique, we studied the kerosene spray generated by the start injector. Our data indicates that depending on the laser spark position in the combustion chamber, the kerosene droplets scatter more or less the laser light. This impacts the pulse energy and its spot size at the focal point and influences the laser ignition probability. In addition, ignition success requires a minimum density of kerosene droplets. Hence, the optimal laser spark position is a compromise between the droplet density at the focal point and the laser beam scattering induced by droplets before reaching that position.
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