Digital holographic interferometry applied to the investigation of ignition process

We use the digital holographic interferometry (DHI) technique to display the early ignition process for a butane-air mixture flame. Because such an event occurs in a short time (few milliseconds), a fast CCD camera is used to study the event. As more detail is required for monitoring the temporal evolution of the process, less light coming from the combustion is captured by the CCD camera, resulting in a deficient and underexposed image. Therefore, the CCD's direct observation of the combustion process is limited (down to 1000 frames per second). To overcome this drawback, we propose the use of DHI along with a high power laser in order to supply enough light to increase the speed capture, thus improving the visualization of the phenomenon in the initial moments. An experimental optical setup based on DHI is used to obtain a large sequence of phase maps that allows us to observe two transitory stages in the ignition process: a first explosion which slightly emits visible light, and a second stage induced by variations in temperature when the flame is emerging. While the last stage can be directly monitored by the CCD camera, the first stage is hardly detected by direct observation, and DHI clearly evidences this process. Furthermore, our method can be easily adapted for visualizing other types of fast processes.