High repetition rate planar laser induced fluorescence of OH in a turbulent non-premixed flame

Abstract

High speed planar laser induced fluorescence (PLIF) imaging of the OH radical is presented in a turbulent nonpremixed methane-air flame. Sequences of up to eight successive PLIF images could be resolved sequentially at several kHz repetition rates. Turbulent flow phenomena such as vortex formation and air entrainment in the flow could be visualised and their development tracked in real time. Turbulence/chemistry interactions such as local extinction and re-ignition and their temporal evolution can be studied with the presented system. PACS: 47.27.-i; 42.79.-e The interactions between flow phenomena and chemistry taking place in turbulent combustion processes pose a severe challenge to modern combustion research [1]. Although much insight into these processes is gained from statistical, time averaged data [2], time and spatially resolved measurements of relevant scalars are needed for a better understanding of the evolution of turbulent reactive structures. High speed Mie scattering [3] has been used to this end but it requires seeding of foreign species into the flow. Schlieren videography [4] has also been applied but is a line of sight technique. Planar laser induced fluorescence (PLIF) of OH, on the other hand, has been shown to be a valuable tool to study flame structures [5] since OH is one of the most important chemical intermediates occurring in combustion systems of interest. Its local concentration is a complex function of molecular transport processes and the prevailing flow field characteristics. Sequential imaging of OH thus gives direct clues on the evolution of turbulence-chemistry interactions, but this is a very complex experimental task: The repetition rates of the laser and detection systems must be matched to the characteristic times of the flow (10’s to 100’s of kHz are required for many flames of interest). Multiple high power laser pulses in the UV spectral region are required for OH PLIF excitation and the detector needs good sensitivity as well as time resolution ∗ To whom correspondence should be addressed to yield useful data. Sequential PLIF of OH has been performed in the past [6–8] but technology available at the time has either limited temporal and spatial resolutions of these measurements, or the number of sequential images that could be captured. In this paper we report on the first measurement of up to eight sequential PLIF OH images in a turbulent nonpremixed methane air flame which were recorded at several kHz repetition rates giving “cinematographic” insight into turbulent reactive flow phenomena.