Joseph D. Miller

A Detailed Investigation of Bluff Body Stabilized Flames

Abstract : Reduced Order Models (ROMs) and Computational Fluid Dynamics (CFD) codes are tools used to predict the extinction of flames behind bluff bodies. Accurate prediction of these models and codes is predicated on their validation with experimental data. This paper describes detailed experiments to obtain validation data for bluff body stabilized flames over a wide range of conditions. Included are non-reacting data from CFD and LDV, lean blowout and high speed images for three different flame holders. In our previous paper (Kiel 2006) it was asserted that the large vortices were a major driver of extinction. Those assertions are further supported here. It is concluded that the vortex dynamics and not geometry is the dominant mechanism for bluff body flame extinction. This conclusion is supported by the lean blowout data, by the high speed images and reference data from NACA.

MHz-Rate NO PLIF Imaging in a Mach 10 Hypersonic Wind Tunnel

NO PLIF imaging at repetition rates as high as 1 MHz is demonstrated in the NASA Langley 31 inch Mach 10 hypersonic wind tunnel. Approximately two hundred time correlated image sequences, of between ten and twenty individual frames, were obtained over eight days of wind tunnel testing spanning two entries in March and September of 2009. The majority of the image sequences were obtained from the boundary layer of a 20° flat plate model, in which transition was induced using a variety of cylindrical and triangular shaped protuberances. The high speed image sequences captured a variety of laminar and transitional flow phenomena, ranging from mostly laminar flow, typically at 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. A series of image sequences were also obtained from a 20° compression ramp at a 10° angle of attack in which the temporal dynamics of the characteristic separated flow was captured in a time correlated manner.

Development of a diode-pumped 100-ms quasi-continuous burst-mode laser for high-speed combustion diagnostics

An all-diode-pumped quasi-continuous burst-mode laser with burst period of 100 ms and pulse energy up to 225 mJ at 1064 nm has been developed. The high-speed laser incorporates a fiber amplifier and six diode-pumped amplifiers in a transportable 2.5-foot × 4-foot system. The repetition rate can be varied from 10–100 kHz resulting in pulse sequences of 1,000–10,000 pulses, a nearly ten-fold increase in record length over current state-of-the-art systems. Individual-pulse shaping allows temporal tailoring of the burst envelope, reducing pulse-to-pulse standard deviation two-fold and producing nearly flat-top bursts. Finally, the generation of pulse pairs with 1-μs spacing is enabled by acousto-optic modulation, allowing straightforward implementation of particle-image velocimetry with a single laser.

Stereoscopic Planar Laser-Induced Fluorescence Imaging at 500 kHz

A new measurement technique for obtaining time- and spatially-resolved image sequences in hypersonic flows is developed. Nitric-oxide planar laser-induced fluorescence (NO PLIF) has previously been used to investigate transition from laminar to turbulent flow in hypersonic boundary layers using both planar and volumetric imaging capabilities. Low flow rates of NO were typically seeded into the flow, minimally perturbing the flow. The volumetric imaging was performed at a measurement rate of 10 Hz using a thick planar laser sheet that excited NO fluorescence. The fluorescence was captured by a pair of cameras having slightly different views of the flow. Subsequent stereoscopic reconstruction of these images allowed the three-dimensional flow structures to be viewed. In the current paper, this approach has been extended to 50,000 times higher repetition rates. A laser operating at 500 kHz excites the seeded NO molecules, and a camera, synchronized with the laser and fitted with a beam-splitting assembly, acquires two separate images of the flow. The resulting stereoscopic images provide three-dimensional flow visualizations at 500 kHz for the first time. The 200 ns exposure time in each frame is fast enough to freeze the flow while the 500 kHz repetition rate is fast enough to time-resolve changes in the flow being studied. This method is applied to visualize the evolving hypersonic flow structures that propagate downstream of a discrete protuberance attached to a flat plate. The technique was demonstrated in the NASA Langley Research Centers 31-Inch Mach 10 Air Tunnel facility. Different tunnel Reynolds number conditions, NO flow rates and two different cylindrical protuberance heights were investigated. The location of the onset of flow unsteadiness, an indicator of transition, was observed to move downstream during the tunnel runs, coinciding with an increase in the model temperature.

High-Speed Multi-Line OH Planar Laser-Induced Fluorescence in Unsteady Flames

We describe the further development and application of a dual-injection-seeded optical parametric oscillator and associated kHz-rate burst-mode pump laser for high-speed OH planar laser-induced fluorescence in unsteady flames. A detailed overview of the novel double-pulse configuration of the burst-mode laser is given, indicating the potential for use in systems where both high energy and high repetition rate are necessary. Dual-wavelength injection-seeded operation of the optical parametric oscillator is demonstrated and preliminary OH PLIF line switching in a well-characterized natural-gas/air laminar diffusion flame is presented. Applications to high-speed nearly-instantaneous background capture for the elimination of laser scatter and background contamination from polycyclic aromatic hydrocarbons in fuel-rich hydrocarbon flames are also discussed and preliminary data are shown. Lastly, the potential for high-speed two-line planar OH thermometry is described including advantages of utilizing the burst-mode laser and multi-wavelength OPO for simplifying high-speed experimental systems.

Strategies for Single-Laser-Shot Femtosecond Pure- Rotational CARS Thermometry

We discuss recent experiments and modeling for the chirped-probe-pulse generation of single-laser-shot femtosecond pure-rotational CARS/CSRS spectra from room-temperature gases. A pure-rotational Raman coherence is impulsively generated using near-transformlimited femtosecond pump/Stokes excitation, and the coherence is probed by stretching a nominally 100-fs near-transform-limited probe beam to approximately 1.7 ps via the refractive-index dispersion in a 30-cm long flint-glass rod. The linearly chirped probe spectrum and phase beat against the time-dependent Raman polarization to generate complex spectra. Chirped-probe-pulse rotational CARS/CSRS offers an interesting alternative to hybrid fs/ps rotational CARS, in which a band-limited pulse of limited energy is used, because all of the available probe pulse energy can be retained in a chirped-probepulse experiment. Our early chirped-probe spectra are presented and the details of our initial model calculations are provided. The temperature sensitivity of the chirped-probe results is illustrated using calculated spectra.

High-speed 2D Raman imaging at elevated pressures

Two-dimensional (2D) Raman scattering at 10xa0kHz in non-reacting flow mixtures is demonstrated by employing a burst-mode laser with a long-duration pulse of about 70xa0ns and pulse energy of about 750xa0mJ at 532xa0nm. To avoid optical breakdown, the pulse width of the laser was varied in the range of 10-1000xa0ns. The effects of pulse shape, pulse energy, and harmonic conversion on 2D measurements are also studied. The applications of high-speed, single-shot, 2D imaging of CH4 and H2 jets in N2 at elevated pressures are demonstrated. In addition, the scalar dissipation rate of CH4 in N2 at 20xa0bar is determined, and multi-dimensional, multi-species, high-speed imaging of flows at elevated pressures is demonstrated.

Evaluation of Hybrid fs/ps coherent anti-Stokes Raman scattering temperature and pressure sensitivity at combustor relevant conditions

Vibrational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (hybrid CARS) spectroscopy is evaluated for the simultaneous measurement of temperature and pressure through the use of a single probe pulse and multiple probe pulses at various time delays. The relevant temperature and pressure parameter space is explored with the goal of determining feasibility of simultaneous pressure and temperature measurements with simplified CARS probe schemes. The sensitivity of spectral features to pressure and temperature changes is evaluated quantitatively from 1-60 atm and 300-3000 K. This study establishes a framework for experimental design and testing under relevant engine conditions.

Vibrational/Rotational Hybrid fs/ps Coherent Anti-Stokes Raman Scattering for Combustion Analysis

A method for simultaneous vibrational and rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) spectroscopy is presented for multispecies detection and improved temperature sensitivity over a wide range of flame conditions. N 2 /CH 4 vibrational and N 2 /O 2 /H 2 rotational Raman coherences are excited simultaneously using fs pump/Stokes pulses at 675/799 nm and 799/799 nm, respectively, and a fourth narrowband ps pulse at 799 nm is used to probe all coherence states at a time delay that minimizes non-resonant background and the effects of collisions. The strength of these transitions is concentration dependent, while the distribution among observed transitions can be related to temperature through the Boltzmann distribution of energy states. The multiplexed signal covers the entire range of temperatures observed in combustion reactants and products (298‐2300 K) with one measurement and can detect four species (N 2 , O 2 , CH 4 , H 2 ) using broadband excitation. This approach is discussed in the context of the feasibility for simultaneous, high-speed, interference-free measurements of key combustion species and temperature in flames with widely varying conditions.

Hybrid fs/ps CARS Spectroscopy for Single-Shot kHz-Rate Thermometry in High-Temperature Flames

This work expands on previous studies to utilize hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) in high-temperature flames for background-free thermometry. The goal of the current work is to quantify the precision and accuracy of the temperature measurements over an expanded temperature range from 1250 – 2400 K, while also quantifying the influence of nonresonant background on the measurements. Temporal suppression of the nonresonant background by three orders of magnitude while retaining up to 20% of the time-zero signal is demonstrated while the background-free single-shot spectra show a signal-to-noise ratio of ~ 200:1 at ~2400 K. The accuracy and precision of the singleshot temperature measurements are investigated over an expanded range of 1250 – 2400 K to address hybrid fs/ps CARS applicability to lower temperature flames. The resulting accuracy and precision of the measurements is better than 10% and 3.5%, respectively, at all conditions. Finally, the effect of the nonresonant background on the accuracy and precision of the temperature measurement is quantified at ~2400 K, and the need for nonresonant-free detection is discussed.