LFA2.1.pdf Laser Ignition Conference © OSA 2017 Application of Laser Induced Ignition Interferometry and Plasma Spectroscopy (LI3PS) to Spray Ignition Laurent Zimmer Laboratoire EM2C, CNRS, CentraleSupélec, Université Paris-Saclay, Grande Voie des Vignes, 92295 Châtenay-Malabry, France Corresponding author: laurent.zimmer@cnrs.fr Abstract: Different measurement techniques are developed to help understanding the ignition phenomena of practical aeronautical combustion chamber. They allow having instantaneous information on the local equivalence ratio within each sparks. Using Laser Ignition, it is also possible to have direct measurements of droplets interacting with the laser pulse and measure their position in 3D, as well as their actual size. Using high-speed approach with Mie scattering images, it is possible to understand the dynamics of the spray under an initial flame kernel development. Using simultaneously all those techniques can provide useful information on conditionally resolved events. OCIS: (300.6365) Spectroscopy, laser induced breakdown ; (100.6890) Three-dimensional image processing ; (010.7060) Turbulence. 1. Introduction Current aeronautical operating conditions, relying on lean combustion to fulfill the emissions target, require the use of novel strategy for ignition. Igniting an aeronautical chamber relies on several mechanisms: from the initial energy deposition to the first flame kernel creation before its convection towards the injection system for its global stabilisation. Looking at the complete dynamics of the system requires having a good knowledge of different quantities. This is easily done with numerical simulations ([1]). The main drawback of numerical simulations is that the comparison with real experiments is not easy, especially as it would require running a huge number of test cases to cover all the possible dynamics. Therefore, experimental approaches are usually used to characterize the complete success of the ignition, based on statistical approaches ([2]). However, the different quantities required to fully understand the faith of each attempt are usually not provided and only their mean, usually uncorrelated, quantities are provided. Among others, laser ignition has several advantages over conventional spark plugs. It can be focused in the middle of the chamber to create a spark that would not suffer from heat losses at the electrodes. Furthermore, using advanced optical system; it is possible to have multi-sparks [3] and also to have a perfect control of the timing, within nanosecond accuracy. It is possible hence to synchronize optical measurements in order to obtain instantaneous measurements, before, within and after the spark. Three different techniques are presented below and their availability to be used for Laser Ignition is discussed. 2. Laser Induced Interformetry. When investigating ignition of sprays, knowing the positions and sizes of droplets along the laser beam and around the laser induced plasma is mandatory to predict the faith of the plasma formation and the evolution of the flame kernel. This requirement leads to the need for a three-dimensional measurement technique that could be synchronised with the laser pulse. Using Mie scattering [4] theory, it is possible to relate the images of interferences within each droplet to their physical size, if reflection and first order refraction intensity are similar in intensity. This technique, called Interferometric Particle Imaging, or Interferometric Laser Imaging for Droplet Sizing required on the elastic scattering. Usually, a nano-second laser pulse is used and images are taken with Couple Charged Device camera, for which the exposure time is not an issue. It becomes more complicate when using the same laser for both Induced Ignition and measurement. As the plasma is created only after its irradiance overcomes the breakdown threshold, the fist 1 to 2 nanoseconds of the laser pulse leads only to Mie scattering effects (see Figure 2). Therefore, having a camera that can be exposed as short as 1 nanosecond is required to detect the droplets within the laser beam. Together with a precise synchronisation, the camera is equipped with an interferential filter centred around the wavelength of the laser (usually 532 nm). In Figure 2, an ICCD with a 512x512 pixel resolution is used to measure all signal within a 14 mm long volume. The reference for the horizontal axis is the focal point of the system. One can clearly notice that a droplet with 4 interference fringes can be detected ([5]). Its apparent size is linked to its position with respect to the focal plane of the camera. If the density of droplets becomes an issue for the imaging, advanced imaging techniques can be used, so as to compress most of the signal into linear information ([6]).