Numerical simulation of three-dimensional motion, molecular transport, and chemical reactions is reported for gaseous nC6H14 autoignition in an ASTM-E659 apparatus, a 500 mL round-bottom, heated flask filled with hot air and the top open to the atmosphere. The injected fuel is heavier than air, initially flows to the bottom of the flask and diffuses up over time as a stratified layer. Inflow of cold air from the open end of the flask penetrates downward and mixes with the flask contents. The inflow and convection due to the heated flask are observed to create a vortical motion. Chemical reactions were modeled using a reduced kinetic mechanism suitable for low-temperature oxidation of nC6H14. Reaction begins slowly and more substantially near the walls where the local gas temperature is hottest. A ring-like ignition kernel forms followed by a localized two-stage ignition event in a rich fuel/air mixture. Critical reaction pathways were identified for both ignition stages. The resultant flame propagation, which is inhibited towards the bottom by lack of oxygen molecules in this region of the flask, convects upwards, chasing unburnt fuel expelled from the flask due to gas expansion from the ignition event itself. Implications are discussed for refinement of autoignition testing.

In this study, experimental measurements and three-dimensional numerical simulations were used to characterize the non-reactive gas motion and mixing inside the flask of our laboratories’ Autoignition Temperature (AIT) facility based on the ASTM-E659 standard. Downward plumes of cool air enter through the open top of the flask, through the neck, and into the spherical vessel. This creates a single dominant toroidal vortex that transports fluid upwards along the walls and downward along the center line. Increasing the height of the flask holder caused the average temperature to increase and the magnitude/frequency of fluctuations to decrease. Studies of fuel-air mixing of ethene (C2H4), n-hexane (nC6H14), and n-dodecane (nC12H26) found that the lighter fuels more readily diffuse into air whereas the heavier fuels are more strongly influenced by buoyancy effects and take longer to mix. Further, in some cases the mixing time will be comparable to the time to ignition and long ignition times may result in significant quantities of fuel molecules escaping from the open top of the flask.

Balanced detection is demonstrated to be a simple method for achieving shot-noise–limited measurements with a focused laser differential interferometer (FLDI). Both polarization components of the signal beam are separately detected, and differential amplification of the two signals cancels common-mode laser-intensity noise. An ultrasonic source is used to provide a reference signal to compare measurements made by a conventional FLDI with those from the modified instrument. Balanced detection is shown to improve the signal-to-noise ratio by 30 dB and demonstrated to achieve shot-noise–limited measurements.

NASA's Artemis missions plan for a long-term lunar habitat and thriving economy through a continuous human presence. The supporting infrastructure, however, is at risk of failure due to the surrounding lunar environment. Hazardous lunar regolith dust ejecta generated by rocket landing and launch events will damage nearby infrastructure, putting astronauts' lives at risk. Traditional landing pad designs can help, but require large amounts of material and labor to construct. PILLARS, the Plume-deployed Inflatable for Landing and Launch Abrasive Regolith Shielding, offers a lightweight solution for dust ejecta mitigation as a temporary alternative to traditional landing pads. As a rocket lands or launches on the lunar surface, the force of the rocket plume inflates this large toroidal membrane to shield the Artemis base from regolith ejecta, allowing for early Artemis development to proceed unimpeded by environmental hazards. System verification and testing for PILLARS, as a part of the 2024 NASA BIG Idea Challenge, have proven PILLARS to be a viable technology to mitigate this risk.

Improvements on ASTM-E659 apparatus are used to investigate autoignition (AIT) of a Synthetic Paraffinic Kerosene (SPK). The apparatus injection system has been automated, and the temperature acquisition system has been improved to reduce variability due to human factors. The SPK was compared with a Jet A standard, POSF4658. The two fuels have a similar range of combustion behaviors but the SPK shows a lower AIT and lower effective activation energy than Jet A. A statistical analysis is proposed to quantify the likelihood of ignition for a range of injected fuel volumes and types of ignition events. We observe that luminous ignition (Mode I) and non-luminous cool flame (Mode III) both result in a vigorous reaction and comparable peak temperatures. This highlights the importance of using the temperature signal to detect ignition instead of relying only on flame visualization. Surveys of the temperature distribution inside the hot vessel demonstrate that a single point measurement is not sufficient to characterize the temperature and that subtle changes in the assembly of the apparatus can significantly alter the temperature distribution and the measured AIT.

We examine two instances of transverse wave collision in a mixture of methane and oxygen diluted with 45% nitrogen. This mixture is classified near-limit and manifests a highly unstable structure. Simultaneous imaging of schlieren and broadband chemiluminescence was performed at a rate of 5 MHz to study the spatially- and temporally-coupled interactions between gas dynamic and chemical kinetic processes. Distinct modes of combustion are observed following transverse wave collision in each case. An unsteady reaction zone model is applied to understand the reactivity of the lead shock using measured values of the shock speed, acceleration, and curvature. In one case, an explosion behind the front immediately follows the creation of the new high-speed shock, which is an example of what is typically observed in detonation with regular cellular structure. Results of the model show that the high-speed shock directly initiates reaction before decaying and becoming non-reactive. In the other case, a reactive gas jet follows the high-speed shock, and no explosion occurs. The high-speed shock is unable to support reaction due to a combination of rapid deceleration and low shock speed. Analysis of the chemiluminescence field indicates that the reaction within the forward jet is supported by turbulent vortex structures and fresh reactants are provided along the shear layers between the high-speed shock and transverse wave. In each case a reverse jet is also observed that makes contact with unreacted gas pockets behind the front. The role of these jetting structures on wave propagation is discussed.

Normal detonation reflection generates a shock wave that exhibits complicated dynamics as it propagates through the incident detonation and post-detonation flow. Ideal models have historically neglected the influence of a finite detonation thickness on the reflected shock due to its small size relative to laboratory scales. However, one-dimensional numerical simulations show that the reflected shock accelerates to a large shock speed not predicted by ideal theory as it propagates through the incident detonation. Analysis with a derived shock-change equation identifies the principal role of the highly nonuniform upstream flow on producing the large shock acceleration. Simulations of detonation reflection show how a finite detonation thickness affects the entire trajectory of the reflected shock.

The occurrence of reactive transverse waves in a narrow channel are studied in stoichiometric mixtures of methane and oxygen with 40-45% nitrogen dilution. Schlieren and CH* chemiluminescence measurements are obtained at MHz rates to study dynamic features of the detonation. Observations are made regarding the different ways that reactive and detonative transverse waves appear. A specific instance of transverse detonation is quantitatively analyzed and it is found that the shocked gas upstream of the transverse detonation builds up until consumed by the transverse detonation. The speeds of the transverse detonation and leading high-speed shock, itself a detonation, are measured to be moving at U_CJ and 1.2U_CJ, respectively.

We report an experimental study of ignition of flammable mixtures by highly unexpanded, supersonic hot jets. The high-pressure, hot-gas reservoir supplying the jet is created by impacting a projectile on a plunger to rapidly compress and ignite a rich n-hexane/air mixture, resulting in a peak reservoir pressure of more than 20 MPa. A locking mechanism was used to prevent the plunger from rebounding and the jet was created by rupturing a diaphragm covering a nozzle with an exit diameter between 0.25 and 1 mm. The jet development and ignition processes in the main chamber filled with hexane-air mixture were visualized using high-speed schlieren and OH * chemiluminescence imaging. The ignition threshold was determined as a function of com-position in the jet and main chamber, the nozzle diameter, and the initial pressure in the main chamber. Unlike the case of subsonic jets in which ignition occurs at the shear layer near the nozzle exit, ignition of combus-tion in the main chamber was found to take place downstream of the Mach disk terminating the supersonic expansion and within the turbulent mixing region created by the startup of the supersonic jet. The results are interpreted using a constant-pressure, well-stirred reactor model simulating the mixing between the hot jet and cold ambient gas. The critical conditions for ignition are determined by the competition between energy release due to chemical reactions initiated by the hot jet gas and cooling due to mixing with the cold chamber atmosphere. The critical value (maximum for which ignition occurs) of the mixing rate was computed using a detailed chemical reaction model and found to be a useful qualitative guide to our observations. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Hydrocarbon fueled detonations are imaged in a narrow channel with simultaneous schlieren and broadband chemiluminescence at 5 MHz. Mixtures of stoichiometric methane and oxygen are diluted with various levels of nitrogen and argon to alter the detonation stability. Ethane is added in controlled amounts to methane, oxygen, nitrogen mixtures to simulate the effects of high-order hydrocarbons present in natural gas. Sixteen unique mixtures are characterized by performing statistical analysis on data extracted from the images. The leading shock front of the schlieren images is detected and the normal velocity is calculated at all points along the front. Probability distribution functions of the lead shock speed are generated for all cases and the moments of distribution are computed. A strong correlation is found between mixture instability parameters and the variance and skewness of the probability distribution; mixtures with greater instability have larger skewness and variance. This suggests a quantitative alternative to soot foil analysis for experimentally characterizing the extent of detonation instability. The schlieren and chemiluminescence images are used to define an effective chemical length scale as the distance between the shock front and maximum intensity location along the chemiluminescence front. Joint probability distribution functions of shock speed and chemical length scale enable statistical characterization of coupling between the leading shock and following reaction zone. For more stable, argon dilute mixtures, it is found that the joint distributions follow the trend of the quasi-steady reaction zone. For unstable, nitrogen diluted mixtures, the distribution only follows the quasi-steady solution during high-speed portions of the front. The addition of ethane is shown to have a stabilizing effect on the detonation, consistent with computed instability parameters. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

We present experimental observations of the density field and reaction structure of methane and natural gas detonation waves propagating in a narrow channel. Simultaneous time-resolved schlieren and CH* chemiluminescence images are used to describe the structure of the unstable front. Nitrogen dilution concentration is varied, and the effect of increasing dilution is to increase the instability level and cell size and decrease the chemiluminescence intensity. Comparison is made between methane- and natural gas-fueled detonations. The effect of the higher hydrocarbons present in natural gas, primarily ethane, is to increase the fine-scale structure of the detonation front and create a more continuous reaction front. Utilizing the simultaneous images, observations are made about the formation and dissipation of the material separated across the shear layer behind the front.

In this work, thermal ignition of laminar external natural convection flows by vertical cylinders is investigated. Effects of cylinder size are studied, with cylinder height ranging from 12.7 to 25.4 cm, and cylinder surface areas varying from 25 to 200 cm(2). The minimum ignition threshold for a stoichiometric hexane-air mixture is 1019 K for a cylinder 25.4 cm long and 200 cm2 in surface area. The ignition threshold is found to have a weak dependence on surface area, in contrast to historical data. The ignition results are consistent with ignition temperature decreasing inversely with the logarithm of surface height. Experiments are performed with multi-component, heavy-hydrocarbon fuels including POSF-4658 Jet A and two surrogate fuels, Aachen and JI, as well as hexane. The setup is heated for multi-component fuel testing. POSF-4658 Jet A has an ignition temperature of 971 K at an ambient temperature of 333 K. All fuels investigated were found to have ignition thresholds within 38 K of the POSF-4658 Jet A ignition threshold. JI is found to be the most appropriate surrogate for POSF-4658 Jet A. (C) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Multi MHz schlieren measurements are performed of methane-oxygen 2D planar detonations diluted with between 20% and 33% nitrogen or argon. The 5 MHz temporal resolution provided by these images allows for key structures to be traced as they evolve. An edge tracking algorithm is developed and applied in order to compute the velocity of the leading front. A detailed velocity map is presented that shows the cellular structure of each detonation, similar to a soot foil, but also illustrates the dynamic evolution of the structure. Finally, kernel density estimates of the probability density function of front velocity and the first four moments are used to statistically describe the two-dimensional structure of unstable methane detonations.

An experimental study of the low-temperature and low-pressure autoignition of Jet A and surrogate fuels was conducted using the ASTM-E659 standardized test method. Two surrogate fuels (Aachen and JI mixtures), their individual components and two batches (POSF-4658 and POSF-10325) of standardized Jet A were tested using the ASTM-E659 method for a range of fuel concentrations and temperatures. The ignition behaviors were categorized into four distinct ignition modes. The individual hydrocarbon components had a wide range of ignition behaviors and minimum autoignition temperatures (AIT) values depending on the molecular structure. The two Jet A batches showed similar ignition behavior with measured AITs of 229 +/- 3 degrees C and 225 +/- 3 degrees C respectively. Both surrogates exhibited similar ignition behavior to Jet A with comparable AITs of 219 +/- 3.1 degrees C (Aachen) and 228 +/- 3 degrees C (JI) with the JI mixture proving to be a more suitable surrogate to Jet A in the lowtemperature thermal ignition regime.

The potential of water hammer events for igniting hydrogen-oxygen mixtures was examined in an experimental study. Compression waves simulating water-hammer events were created by projectile impact on a piston in a water-filled pipe terminated by a test section filled with gas. Triangular wave forms with peak pressures up to 50 MPa propagated through the piping system and compressed the gas in the test section. Experiments were carried out with both air and hydrogen-oxygen gas mixtures using high-speed video of the transparent test section, dynamic pressure and spectroscopic measurements to examine the motion of the water-gas interface and determine ignition thresholds. The impulsive acceleration of the water-gas interface and deceleration created by the compression of the gas resulted in Richtmyer-Meshkov and Rayleigh-Taylor instabilities that grew to create large distortions of the initially planar and horizontal water-gas interface. The gas layer was compressed in volume by up to a factor of 50 and the gas pressures increased to as high as 20 MPa within 2 to 4 ms. The distortion of the water surface during compression resulted in a significant increase in interfacial area and ultimately, creation of a two-phase mixture of water and compressed gas. Some ignition events were observed, but the dispersion and mixing of water with the gas almost completely suppressed the pressure rise during the ignition transient. Only by eliminating the instability of the water interface with a solid disk between the water and gas were we able to observe consistent ignition with significant pressure rises associated with the combustion.

An experimental study of thermal ignition of stoichiometric n-hexane and air mixtures by vertical cylinders is performed. Five cylinders were tested with surface areas ranging from 50 to 200 cm2. Limited variation of igntion temperature with surface area was observed with ignition temperatures from 1019 to 1117 K; this is approximately 200 to 500 K higher than previous studies with similar surface areas. Quantitative temperature fields from the experiments were extracted via interferometry and the profile of the thermal layer agreed well with temperature results from numerical similarity solutions to the boundary layer equations. The ignition results are compared to relevant literature and the importance of a change in surface size versus a change in flow type is discussed.

A numerical study of a transiently (uniformly/non-uniformly) heated cylindrical reactor was performed using a computationally inexpensive one-step model capable of capturing the experimentally observed transition behavior from slow to fast reaction. The methodology used to find the kinetic parameters of the simplified model was described in detail. A parametric study using a control volume (0-D) thermal ignition model provided transition maps due to changes in heating rate, initial pressure and composition. Two-dimensional reactive Navier-Stokes equations were used to examine the fluid mechanics and chemical reaction leading to slow or fast consumption of the mixture. During uniform heating, a dynamic buoyancy flow is induced in which the mixture rises along the walls and turns at the centerline creating two well defined vortical structures. Once significant chemical heat release is generated, the flow reverses. During non-uniform heating, the flow field is composed of two large vortices in the center of the vessel, and two sets of smaller vortices trapped at the top and bottom of the reactor. Depending on the heating rate, and irrespective of the mode of heating, the mixture undergoes either slow oxidation or ignition whereby a flame that propagates from the top of the vessel consumes the mixture.

Gas injection into supersonic flow past a 5 degrees half-angle cone is studied with three injected gases: helium, nitrogen, and RC318. Experiments are performed in a Mach 4 Ludwieg tube with nitrogen as the free stream gas. The injector section is shaped to admit a "tuned" injection rate where the displacement created by injection counteracts the effects created by the injector geometry. A high-speed schlieren imaging system with a framing rate of 290 kHz is used to study the instability in the region of flow downstream of injection, referred to as the injection layer. Measurements of wavelength, convective speed, and frequency of the instability waves were made. The stability characteristics of the injection layer are found to be very similar to those of a shear layer. The findings of this work suggest that shear layer modes should be a primary concern for future stability analyses of supersonic flow with injection.

The dynamics of ignition of premixed hydrogen–air from a hot glow plug were investigated in a combined experimental and numerical study. Surface temperatures during heating and at ignition were obtained from 2-color pyrometry, gas temperatures were measured by high-speed Mach–Zehnder interferometry, and far-field effects were captured by high-speed schlieren imaging. Numerical simulations considered detailed chemical kinetics and differential diffusion effects. In addition to the known cyclic (puffing) combustion phenomenon, singular ignition events (single puff) were observed near the lean flammability limit. Detailed analysis of the results of our numerical simulations reveal the existence of multiple combustion transients within the thermal boundary layer following the initial ignition event and, at late times, sustained chemical reaction within a thermal plume above the glow plug. The results have significant implications for ignition from hot surfaces within near-flammability limit mixtures, at the edge of plumes resulting from accidental release of hydrogen, or within the containments of nuclear power plants during severe accidents.