This paper describes a novel laser diagnostic and its demonstration in a practical aero-propulsion engine (General Electric J85). The diagnostic technique, named hyperspectral tomography (HT), enables simultaneous 2-dimensional (2D) imaging of temperature and water-vapor concentration at 225 spatial grid points with a temporal response up to 50 kHz. To our knowledge, this is the first time that such sensing capabilities have been reported. This paper introduces the principles of the HT techniques, reports its operation and application in a J85 engine, and discusses its perspective for the study of high-speed reactive flows.

50-kHz-rate 2D imaging of temperature and H2O concentration at the exhaust plane of a J85 engine using hyperspectral tomography.

This overview presents examples of applications of high frame rate imaging diagnostics in fundamental and applied combustion research. Progress in the performance of high frame rate digital cameras and high repetition rate lasers enabled the development of a range of new imaging diagnostics for measurements of velocities, concentrations and temperatures. Camera frame rates and storage capacities are now adequate to resolve and follow time scales spanning six orders of magnitude while camera chip size limitations restrict the spatial dynamic range to about three orders of magnitude. High-speed imaging studies of mixing processes, flame stabilization, ignition and extinction and the coupling of acoustic and chemical processes in turbulent flames and internal combustion engines have produced a wealth of new understanding, contributing also to the development of predictive models. Future progress in designing and operating cleaner and more efficient combustion device hinges on our ability to push operating conditions to leaner mixtures and often to higher pressures. There, small variations in boundary conditions, e.g. flow patterns or the formation of a fuel spray, might lead to combustion failures that can range from acoustic noise in a jet flame, a misfire in an automobile engine, to lean-blow-out of an aircraft gas turbine engine; from nuisance to catastrophe. High-speed imaging in turbulent flames and internal combustion engines allowed capturing and identifying detrimental conditions that might be rare in occurrence and defining in leading to failure. The examples presented in this review illustrate the status of diagnostic capabilities, show sample results, and examine some future directions.

High speed imaging in fundamental and applied combustion research

Recent advances in ultra-high repetition rate (100?kHz and above) laser diagnostics for fluid dynamic measurements are reviewed. The development of the pulse burst laser system, which enabled several of these advances, is described. The pulse burst laser system produces high repetition rate output by slicing the output of a low power continuous wave laser and passing the resulting burst of pulses through a series of pulsed Nd:YAG amplifiers. Several systems have been built with output approaching 1.0 J/pulse over bursts of up to 100 pulses generated at between 50 and 1000?kHz. Combined with the capabilities of several types of commercially available high-speed cameras, these systems have been used to make a wide variety of high repetition rate and 3D flow measurements. Several examples of various high repetition rate laser diagnostics are described, including flow visualization, filtered Rayleigh scattering, planar Doppler velocimetry, particle image velocimetry, planar laser induced fluorescence, molecular tagging velocimetry and 3D flow visualization.

https://iopscience.iop.org/article/10.1088/0957-0233/24/1/012002/pdf

Review of ultra-high repetition rate laser diagnostics for fluid dynamic measurements

Premixed flames subjected to extreme turbulence: Some questions and recent answers

Nitric oxide planar laser-induced fluorescence (NO PLIF) imaging at repetition rates as high as 1MHz is demonstrated in the NASA Langley 31 in. Mach 10 hypersonic wind tunnel. Approximately 200 time-correlated image sequences of between 10 and 20 individual frames were obtained over eight days of wind tunnel testing spanning two entries in March and September of 2009. The image sequences presented were obtained from the boundary layer of a 20° flat plate model, in which transition was induced using a variety of different shaped protuberances, including a cylinder and a triangle. The high-speed image sequences captured a variety of laminar and transitional flow phenomena, ranging from mostly laminar flow, typically at a lower Reynolds number and/or in the near wall region of the model, to highly transitional flow in which the temporal evolution and progression of characteristic streak instabilities and/or corkscrew-shaped vortices could be clearly identified.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1183&context=me_pubs

MHz-rate nitric oxide planar laser-induced fluorescence imaging in a Mach 10 hypersonic wind tunnel

Nitric-oxide planar laser-induced fluorescence was used to perform velocity measurements in hypersonic flows by generatingmultiple tagged lines thatfluoresce as they convect downstream.Determination of axial velocitywasmade by application of a cross-correlation analysis of the horizontal shift of individual tagged lines. A single interline, progressive scan-intensified charge-coupled device camera was used to obtain two sequential images of the nitricoxide molecules that had been tagged by the laser. The charge-coupled device allowed for submicrosecond acquisition of both images, resulting in submicrosecond temporal resolution as well as submillimeter spatial resolution (0.5 mm horizontal, 0.7 mm vertical). Quantification of systematic errors, the contribution of gating/ exposure duration errors, and the influence of collision rate on temporal uncertainty were made. This velocity measurement technique has been demonstrated for two hypersonic flow experiments: 1) a reaction control system jet on an Orion crew exploration vehicle wind-tunnel model and 2) a 10 deg half-angle wedge with a 2-mm-tall 4-mmwide cylindrical boundary-layer trip. Mean-velocity uncertainties below 30 m=s (2.7% of the measured average velocity) and single-shot uncertainties below 100 m=s (9.7%of themeasured average velocity) have been obtained in regions with optimal signal intensities using this technique.

Velocity Profile Measurements in Hypersonic Flows Using Sequentially Imaged Fluorescence-Based Molecular Tagging

Nitric oxide (NO) planar laser-induced fluorescence (PLIF) has been use to investigate the hypersonic flow over a flat plate with and without a 2-mm (0.08-in) radius hemispherical trip. In the absence of the trip, for all angles of attack and two different Reynolds numbers, the flow was observed to be laminar and mostly steady. Boundary layer thicknesses based on the observed PLIF intensity were measured and compared with a CFD computation, showing agreement. The PLIF boundary layer thickness remained constant while the NO flowrate was varied by a factor of 3, indicating non-perturbative seeding of NO. With the hemispherical trip in place, the flow was observed to be laminar but unsteady at the shallowest angle of attack and lowest Reynolds number and appeared vigorously turbulent at the steepest angle of attack and highest Reynolds number. Laminar corkscrew-shaped vortices oriented in the streamwise direction were frequently observed to transition the flow to more turbulent structures.

/pdf/no-plif-study-of-hypersonic-transition-over-a-discrete-1e3wnz2f2p.pdf

NO PLIF Study of Hypersonic Transition Over a Discrete Hemispherical Roughness Element

Accurate interface normal and curvature estimates on three-dimensional unstructured non-convex polyhedral meshes

In two separate test entries, advanced laser-based instrumentation has been developed and applied to visualize the hypersonic flow over cylindrical protrusions on a flat plate. Upstream of these trips, trace quantities of nitric oxide (NO) were seeded into the boundary layer. The protuberances were sized to force laminar-to-turbulent boundary layer transition. In the first test, a 10-Hz nitric oxide planar laser-induced fluorescence (NO PLIF) flow visualization system was used to provide wide-field-of-view, high-resolution images of the flowfield. The images had sub-microsecond time resolution. However these images, obtained with a time separation of 0.1 sec, were uncorrelated with each other. Fluorescent oil-flow visualizations were also obtained during this test. In the second experiment, a laser and camera system capable of acquiring NO PLIF measurements at 1 million frames per second (1 MHz) was used. This system had lower spatial resolution, and a smaller field of view, but the images were time correlated so that the development of the flow structures could be observed in time.

/pdf/high-speed-plif-imaging-of-hypersonic-transition-over-6zbnrxg3dl.pdf

Christopher B. Ivey

Papers

MHz-rate nitric oxide planar laser-induced fluorescence imaging in a Mach 10 hypersonic wind tunnel

Velocity Profile Measurements in Hypersonic Flows Using Sequentially Imaged Fluorescence-Based Molecular Tagging

NO PLIF Study of Hypersonic Transition Over a Discrete Hemispherical Roughness Element

Accurate interface normal and curvature estimates on three-dimensional unstructured non-convex polyhedral meshes

High-Speed PLIF Imaging of Hypersonic Transition over Discrete Cylindrical Roughness