New Perspectives on Turbulent Combustion: Multi-Parameter High-Speed Planar Laser Diagnostics

Over the past three decades laser combustion diagnostics have guided an improved understanding of turbulent combustion processes. Until recently, this was based on statistically independent sampling using sampling rates much slower than typical integral time-scales of turbulent flames. Recent developments in laser and camera technology enabled an increase in sampling rates by more than three orders of magnitudes. Using these new instruments for particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) at high sampling rates (high-speed diagnostics) allowed the resolution of integral time-scales of turbulent flames. This statistically dependent sampling is increasingly used to temporally track transients in turbulent combustion, such as flame extinction, ignition, flashback and cycle-to-cycle variations in IC engines. The simultaneous application of flow and scalar field measurements makes insights into these transients possible that were not when using statistically independent sampling with low data acquisition rates. Conditioning on distinct flame features with high-speed diagnostics enables the inclusion of time as an additional dimension. This paper reviews the emerging field of multi-parameter, high-speed, planar laser diagnostics in combustion applications. The benefit of high data acquisition rates in turbulent combustion applications is discussed in detail as well as requirements and constraints imposed by the time-scales of the investigated phenomenon are addressed. Recent developments in laser and detector hardware are highlighted, as these are the limiting factors of the sampling rate. Finally, multi-parameter high-speed measurements in combustion are summarized, with a few examples discussed in more detail.