Terrence R Meyer | Search NASA

NASA NTRS2022

Simultaneous 100-kHz Acetone Planar Laser-Induced Fluorescence and OH* Chemiluminescence in a Linear Non-Premixed Detonation Channel

Reactant mixing and combustion are investigated in an optically accessible, self-excited linear detonation combustor. The mixing field is captured using 100 kHz planar laser-induced fluorescence (PLIF) imaging of acetone as a tracer in the fuel supply, while 100 kHz chemiluminescence imaging of excited-state hydroxyl (OH*) radicals simultaneously resolves the evolution of the detonation wave. Time sequences are acquired over multiple detonation cycles in each test, with acetone-PLIF images collected along multiple orthogonal planes to reveal the complex three-dimensional topography of the fuel distribution. The instantaneous and phase averaged acetone-PLIF images enable measurement of key fuel injection characteristics, such as the injector recovery time, fuel jet velocity, and refill height for a range of operating conditions. Instantaneous and phase-averaged measurements of acetone-PLIF with the time-coincident OH* chemiluminescence images also reveal a number key features, such as fuel stratification and weak detonation in the injector near field, incomplete combustion and deflagration behind the detonation wave, vitiation and deflagration of reactants ahead of the detonation wave, and fuel and oxidizer recovery time mismatch leading to combustion inefficiency. These measurements significantly enhance the ability to obtain detailed information on the intracycle and intercycle spatiotemporal evolution of the reactant refill process and its coupled effects on the detonation wave structure and propagation.

Propulsion↗

NASA NTRSDate not supplied

Design and Operability of a Variably Premixed Rotating Detonation Engine for the Evaluation of Mixing Effects

The design of a variably premixed rotating detonation engine (RDE) is presented with preliminary experimental results. Premixed operation decouples the mixing process from the detonation cycle, enabling direct comparison with premixed simulations. A porous medium, formed into a ring of 6.45 mm radial thickness, serves as the premixture injector, arresting potential flash-back events. Two chamber geometries are tested, the first aimed at eliminating recirculation zones and comprised of a 7.62 mm wide channel, the width of which the porous injector occupies 85%. The second chamber geometry features a 10.7 mm channel width and a 3.08 mm wide backward-facing step on the internal diameter of the premixture injector. Transverse optical access is incorporated into the design using a transparent outer body. Tests were conducted at mixing conditions ranging from non-premixed to fully premixed with gaseous hydrogen and air reactants in the narrow-channel configuration. Engine operating modes and detonation wave speeds were characterized using aft-end high-speed chemiluminescence imaging, while detonation wave topology and heat release in the refill zone are captured by transverse imaging. A two-wave counterpropagating mode was most commonly observed in the narrow-channel configuration with wave speeds ranging from 49-67% of the theoretical Chapman-Jouguet (CJ) velocity. Fully premixed operation in the backward-facing step configuration rendered wave speeds up to 86% of CJ velocity, consistent with previous non-premixed results using the same channel width and a backward-facing step. These results demonstrate successful premixed RDE operation with high wave speeds and highlight the role of product recirculation zones in stabilizing the detonation cycle.

detonation↗

NASA NTRSDate not supplied

Investigation of Liquid Fuel Injection Processes in a Rotating Detonation Combustor using Megahertz Planar Laser-Induced Fluorescence Imaging

Direct imaging of liquid fuel injection is performed within an optically accessible rotating detonation combustor (RDC) using planar laser-induced fluorescence (PLIF) to spatio-temporally resolve the highly dynamic spray characteristics at rates up to 1 MHz. The RDC is operated on air and hydrogen to sustain a stable, cyclically propagating detonation wave with cycle periods of up to ∼250 μs. One of the hydrogen fuel injection sites is replaced with a liquid jet to introduce a diesel spray as a fuel surrogate to enable tracer-free fluorescence excitation using the 355 nm third-harmonic output of a burst-mode Nd:YAG laser. The time-resolved PLIF measurements reveal the evolution of the highly unsteady spray, including the dwell period after the arrival of the detonation wave, the jet recovery, breakup and entrainment into the supersonic crossflow of air, propagation into the detonation chamber, and interaction with the detonation wave. Following passage of the detonation wave, the spray characteristics reveal significant changes in the momentum flux ratio between the liquid and air streams, altering the jet trajectory and temporarily halting fuel delivery to the detonation channel. This temporary cessation of fuel spray injection into the combustor is quantified, along with the fill rate as a function of time. As the injection system recovers, the fuel spray eventually returns to a quasi-steady position prior to the arrival of the detonation wave, allowing qualitative comparisons with theoretical jet trajectories for a range of air mass-flux conditions. These data, enabled by ultra-high-speed PLIF imaging, represent some of the first detailed measurements for characterizing and quantifying the interactions of liquid jets and detonations in an operating RDC.

Propulsion↗

NASA NTRSDate not supplied

Effect of Injection Dynamics on Wave Propagation in a Linear Detonation Combustor

Injector response to self-excited and sustained detonation wave propagation in an optically-accessible nonpremixed natural gas-oxygen linear detonation combustor is investigated. 100 kHz measurements of planar laser induced fluorescence (PLIF) of trace quantities of acetone injected into the fuel supply and simultaneous OH∗ chemiluminescence measurements were used to correlate the effect on injection dynamics on detonation wave structure and propagation. Measurements over a range of equivalence ratios (0.61-1.48) revealed wave propagation through regions of high fuel stratification in the axial and transverse direction of reactant injection. Consequently, the propagation of the detonation wave along the channel is through a flow-field with significant variation in fuel mass fraction, reactivity, and sound speeds. At lean conditions, wave propagation frequency is generally lower (∼6 kHz) resulting in longer time between wave passages for mixing and pre-heat resulting in steeper detonation wave-fronts anchored near the injector exit plane. Conversely, at rich equivalence ratios the wave frequency is higher (∼10 kHz) resulting in shorter intra-cycle period for reactant fill and mixing resulting waves lifted from the injection plane that are more compact. Higher chemiluminescence intensity in the post-wave region with deflagrative combustion is observed in these cases. Phase averaged representation of the acetone-PLIF measurements indicate that the fraction of time between wave passages available for reactant fill, mixing and pre-heat is consistent across all equivalence ratios.

Propulsion↗

NASA NTRSDate not supplied

Effect of Injection Dynamics on Detonation Wave Propagation in a Linear Detonation Combustor

Injector response to self-excited and sustained detonation wave propagation in an optically-accessible nonpremixed natural gas-oxygen linear detonation combustor is investigated. 100 kHz measurements of planar laser induced fluorescence (PLIF) of trace quantities of acetone injected into the fuel supply and simultaneous OH∗ chemiluminescence measurements were used to correlate the effect on injection dynamics on detonation wave structure and propagation. Measurements over a range of equivalence ratios (0.61-1.48) revealed wave propagation through regions of high fuel stratification in the axial and transverse direction of reactant injection. Consequently, the propagation of the detonation wave along the channel is through a flow-field with significant variation in fuel mass fraction, reactivity, and sound speeds. At lean conditions, wave propagation frequency is generally lower (∼6 kHz) resulting in longer time between wave passages for mixing and pre-heat resulting in steeper detonation wave-fronts anchored near the injector exit plane. Conversely, at rich equivalence ratios the wave frequency is higher (∼10 kHz) resulting in shorter intra-cycle period for reactant fill and mixing resulting waves lifted from the injection plane that are more compact. Higher chemiluminescence intensity in the post-wave region with deflagrative combustion is observed in these cases. Phase averaged representation of the acetone-PLIF measurements indicate that the fraction of time between wave passages available for reactant fill, mixing and pre-heat is consistent across all equivalence ratios.

Propulsion↗

NASA NTRSDate not supplied

Fiber-Based Multi-Resolution Imaging and Multidimensional X-ray Diagnostics for Ejecta Dynamics in Plume Surface Interactions

The interaction of a rocket exhaust plume with a particulate-laden surface creates a complex, multiphase flow field that can destabilize the vehicle and damage nearby equipment. This study investigates two methods for capturing the particulate dynamics of plume-surface interaction (PSI): optical fiber-based multi-resolution Mie scattering and multi-dimensional X-ray radiography. Mie scattering was used to track PSI-interacted particles, trace their paths, and measure velocities. While effective for the jet periphery and early PSI stages, the technique becomes limited as the scattering cross-section increases over time due to particle displacement from the soil bed, causing the core to become optically dense and appear as a luminous, opaque region. To address this, X-ray radiography was explored as a complementary method for visualizing the optically dense core. PSI experiments were conducted with both reacting and non-reacting jets to evaluate these approaches across a range of optical and flow parameters. The results demonstrated the capability of the fiber-based multi-resolution imaging system to capture simultaneous fields of view at varying magnifications (1x, 2x, 4x), and the ability of X-ray imaging to penetrate the optically dense plume, revealing flow structures that would otherwise be obscured in scattering-based methods. Data were collected for various PSI parameters, including three different heights above the surface, to analyze the ejecta properties and the plume’s temporal evolution. These results provide the first imaging strategy capable of resolving flow structures over a wide spatial dynamic range while also offering the first visualization of the optically dense core.

Ejecta↗

NASA NTRSDate not supplied

High-Speed Visualization of Spray Breakup and Velocity Estimation in a Rotating Detonation Combustor Using Laser Induced Fluorescence

The underlying physical processes that liquid fuel sprays undergo in rotating detonation engines, particularly breakup and combustion, are not well understood nor are the coupling effects between the detonation and the spray. In this work, a single liquid jet of liquid fuel is inserted into a hydrogen-air RDC in order to study repetitive-one way coupled detonation interactions. High speed laser induced fluorescence of Jet-A at 300 kHz is used to visualize the dynamic processes inside the RDC for up to 10 ms per test. The employed imaging schemes allowed for unique insight into the spreading of liquid fuel in the azimuthal-axial directions via volumetric laser illumination. The Jet-A velocity field was estimated using OpenOpticalFlow and the impulse events observed colloquially agree with prior work on this test platform.

Instrumentation↗