4D imaging of fast flow dynamics: From challenging dream to reality

Abstract

The design and operation of high-performance combustion systems to meet current and future propulsion requirements face overarching technical challenges from our finite understanding of turbulent combustion. Propulsion systems will continue to operate with flames stabilized at high Reynolds numbers in complex burner geometries, such as swirl-stabilized flames in the main combustor of gas-turbine engines. High-Reynolds-number turbulent combustion, however, is inherently a “four-dimensional” (4D) — three dimensional (3D) in space and dynamic in time—phenomenon that accesses a wide range of both length and time scales. These length scales range from meters to microns with temporal dynamics occurring at frequencies from ∼100 Hz—associated with thermoacoustic instabilities—to 100 kHz or higher, as in the case of emerging detonation-engine technologies and other advanced propulsion and reacting-flow systems. Moreover, understanding the complex interplay between underlying turbulent fluid dynamics and combustion chemistry requires multi-dimensional diagnostics that allow simultaneous measurement of multiple physicochemical parameters, such as temperature and chemical-species concentrations. Unfortunately, combustion diagnostics have not traditionally offered kHz data rates for measurements in the 3D spatial domain, as is required to resolve the spatio-temporal dynamics of turbulent combustion processes. Recent developments in burst-mode laser technology, however, are now paving the way toward 4D measurement capabilities at dynamic rates of hundreds of kHz and potentially into the MHz regime. This paper will focus on the recent developments of lasers and imaging systems that portend unprecedented transformative approaches for characterizing the three-dimensional evolution of highly complex phenomena in reacting and non-reacting flows. Such phenomena, which control the performances of various propulsion systems, are inherently difficult to study because they evolve at ultrafast time scales and span an extremely wide range of temperatures and pressures, often under optically dense conditions. They include, but are not limited to, turbulence–chemistry interactions in gas-turbine, rocket, and pressure-gain combustion systems; detonation physics and dense particle aerodynamics in explosive munitions; and shock/boundary-layer interactions in other propulsion systems.