Showing papers by "Terrence R. Meyer published in 2023"

KTP Optical Parametric Oscillator for Extended Duration High Repetition Rate NO Planar Laser Induced Fluorescence

Abstract: Non-intrusive, high repetition rate flow visualization is imperative in understanding the flow physics of high-speed, turbulent, and reacting flow environments. Planar laser induced fluorescence (PLIF) is a useful technique that allows visualization of a single species in a planar slice of a flow with high signal to noise ratio. This provides higher spatial resolution over chemiluminescence. PLIF above repetition rates of 50 kHz can be performed with a series of crystals and mirrors oriented in a cavity and sum frequency mixing to make up an optical parametric oscillator (OPO). Previous work has used beta barium borate (BBO) crystals pumped by a 355 nm laser to excite species such as OH, NO, CH, and krypton. This work utilizes potassium titanyl phosphate (KTP) non-linear crystals and coated cavity optics pumped by a 532 nm laser to generate deep UV to excite multiple combustion relates species such as NO, OH, NO v2, CO, O, and CH. The benefits include higher repetition rates, higher damage threshold of optics, longer burst durations, and lower build cost. The KTP OPO is applied to perform NO and OH PLIF in an optically accessible rotating detonation engine combustor. This allows the injected gas and detonation dynamic response to be visualized.

Single-shot time-resolved thermometry of atmospheric-pressure nanosecond pulsed plasma discharges

Abstract: We describe the development and characterization of an OES-thermometry technique using a streak camera. Temperature measurements were performed in air and N2 atmospheric- pressure discharges. Temperatures were obtained in a single spark discharge between pin-to- pin electrodes with a temporal resolution of ~160 ps. Streak-sweeps provide single-shot and continuous recording of spectral signatures. We describe the unique tradeoffs between spatial, temporal and spectral resolution in applying a streak camera for OES-thermometry. Preliminary temperature measurements are presented which highlight the unique capabilities of the streak-spectroscopy method.

Lifetime-based Phosphor Thermometry via X-ray Excitation

Abstract: Phosphor thermometry has become an established remote sensing technique for acquiring the temperature of surfaces and gas-phase flows. Often, phosphors are excited by a light source (typically emitting in the UV region), and their temperature-sensitive emission is captured. Temperature can be inferred from shifts in the emission spectra or the radiative decay lifetime during relaxation. While recent work has shown that the emission of several phosphors remains thermographic during x-ray excitation, the radiative decay lifetime was not investigated. The focus of the present study is to characterize the lifetime decay of the phosphor Gd2O2S:Tb for temperature sensitivity after excitation from a pulsed x-ray source. These results are compared to the lifetime decays found for this phosphor when excited using a pulsed UV laser. Results show that the lifetime of this phosphor exhibits comparable sensitivity to temperature between both excitation sources for a temperature range between 21 °C to 140 °C in increments of 20 °C. This work introduces a novel method of thermometry for researchers to implement when employing x-rays for diagnostics.

Effects of Nitrogen Dilution on Detonation Wave Physics in a H2-Air Non-premixed Rotating Detonation Combustors

Abstract: The effects of nitrogen dilution on rotating detonation wave physics are investigated in a hydrogen/nitrogen-air rotating detonation engine (RDE) as a precursor to the potential use of reformed ammonia (NH3) as a carbon-free fuel. Chemiluminescence images are collected within the combustion channel at rate of 100 kHz to qualitatively evaluate changes in detonation wave structure, while pressure transducer measurements sampled at 2.5 MHz are used to quantify wave speeds and the local pressure rise as N2 is added to the H2 fuel stream. As the percent volume of N2 in the fuel mixture is increased from 0% to 25%, the wave speed drops substantially from 80 to 65%, respectively, of the Chapman-Jouguet (C-J) velocity. Phase-averaged pressure profiles for a detonation wave cycle and chemiluminescence images show that the primary reaction zone shifts axially downstream towards the exhaust due to the effects of N2 on fuel-oxidizer mixing and as a thermal inhibitor. Nitrogen dilution also results in a more diffuse detonation wave structure with an extended post-reaction zone that is unable to support trailing detonation waves within the azimuthally reflected shock system. These results are attributed to the non-premixed nature of the RDE, in which the detonation wave propagates through a wide range of equivalence ratios and, therefore, a wide range of local dilution ratios and chemical time scales. This investigation highlights the challenges associated with operating a non-premixed rotating detonation engine using a substantial fraction of reformed ammonia as a carbon-free fuel. While the feasibility and physics of rotating detonation is investigated for H2/N2-air compositions representing fully reformed ammonia, further investigation is required to characterize global performance metrics.

Development of kHz-rate CO Laser-Induced Fluorescence in High Speed Flows

Abstract: We report on the development of a custom optical parametric oscillator (OPO) pumped by a burst-mode laser to study flow and combustion parameters in high-speed flows. Current work focuses on an architecture that enables the excitation of carbon monoxide (CO) species via burst-mode two-photon laser induced fluorescence (TP-LIF). The OPO design enables narrowband (0.5 cm\textsuperscript{-1}) output without the use of an external seed laser, providing tunability to reach electronic transitions of CO. High conversion efficiency is critical for detecting CO near 230.1 nm for studies of boundary layer characteristics for a range of surface conditions. The OPO design is described, along with preliminary data confirming its operation and potential for species detection. Further data on the performance of the fluorescence imaging system will be acquired for a variety of pressures of interest in high-speed flows, followed by application to the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at the Aero Sciences Lab at Purdue University.

Variably Premixed Rotating Detonation Engine for Evaluation of Detonation Cycle Dynamics

Abstract: A variably premixed rotating detonation engine using gaseous hydrogen and air reactants is introduced to enable investigation of key cycle processes while varying the homogeneity of the reactant inlet conditions. Two chamber configurations are investigated, the first with reactants filling the entire span of a straight annular channel and the second with a slightly larger channel and a backward-facing step. The first configuration permits both premixed and non-premixed fuel injection, enabling mixing quality modulation. The second configuration is operated only at fully premixed conditions. Operating modes and detonation wave speeds are characterized using exhaust-plume imaging, while the chamber heat release field is captured by transverse imaging through a transparent outer body. Tests using the first configuration were characterized by unstable counterpropagating modes with low detonation wave speeds regardless of the state of premixing, while the second configuration rendered single-wave behavior with wave speeds up to 86% of the Chapman–Jouguet velocity. Comparisons with a simple computational fluid dynamics model of the second configuration indicate that reactant preheating significantly influences the detonation wave topology, highlighting the potential utility of the test platform for isolating key physics associated with the effects of reactant premixing, preheating, and chamber geometry on rotating detonation engine operation.

Time resolved visualization of liquid jet interaction with H2-air rotating detonations using MHz rate diesel PLIF

Abstract: The characterization of the dynamic response of liquid jets to transient detonation wave passage is critical for optimization and modeling of liquid fueled rotating detonation combustors. In this work, a rotating detonation combustor (RDC) is operated on hydrogen and air to sustain stable detonation waves that acts as a detonation driver and interact in a one-way coupled manner with a single liquid fuel jet that propagates into the combustion chamber with cycle periods of ~ 250 μs. Diesel is used as a fuel surrogate with higher aromatic compounds to enable fluorescence excitation, using the 355 nm third-harmonic output of a burst-mode Nd:YAG laser, imaged at repetition rates up to 1 MHz. By optimizing the technique to accommodate orders of magnitude variations in the fuel density throughout the injection process, the PLIF data enable quantitative measurements including the refill time, the relative recovery between liquid and gaseous jets and jet trajectory across various momentum flux ratio. As the passage of the detonation wave imparts significant changes in the momentum flux ratio, the qualitative liquid break-up process and spatial distribution varies significantly in time. As the injection system recovers ~ 70% of the cycle period and return to a quasi-steady position and allow comparisons with theoretical jet trajectories. These data, enabled by ultra-high-speed PLIF imaging, represent some of the first detailed measurements for quantifying the dynamic response and recovery of liquid jets exposed to periodic detonations in an operating RDC

Joint Temperature and Velocity Statistics in High-speed Flows Using Simultaneous CARS Thermometry and FLEET Velocimetry

Abstract: Temperature and velocity are important parameters for the characterization of high-speed flows. Precise experimental measurements of these two quantities are needed to provide joint statistics that are important for developing accurate heat transfer models in computational fluid dynamics (CFD) simulations. Hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (CARS) provides non-intrusive thermometry with high precision and accuracy. Femtosecond laser electronic excitation tagging (FLEET) allows for velocimetry measurements without tracer species or particles. This work evaluates the feasibility of achieving high precision in each of the two quantities to enable the accurate measurements of the joint statistics in the fluctuations of temperature and velocity. These two techniques are performed simultaneously in the freestream of a high-speed wind tunnel operating with nitrogen at Mach 3 and higher. Additional measurements are collected within the boundary layer to evaluate the change in precision due to turbulence. The accuracy and precision of the measurements are evaluated, and strategies for improving the measurement of joint statistics are discussed. These data are used to evaluate the feasibility of performing such measurements at rates up to 1 kHz for the study of turbulent convective heat transfer in high-speed flows.