Simultaneous CH Research Articles

Investigation of combustion mode conversion driven by fuel flow variation in a cavity-based scramjet

This work aimed to investigate combustion mode conversions by rapid variation of the fuel flow rate in a cavity-based scramjet combustor. The experiments were carried out on a direct-connected facility with an inflow condition of Mach number 2.52, a total pressure of 1.35 MPa, and a total temperature of 1650 K. The fuel injector consisted of two injection ports: fuel was continuously injected from one port while the other controlled the fuel flow for mode conversions by switching it on or off. Simultaneous schlieren and CH* imaging techniques were used to characterize the dynamics of combustion mode conversions. It was recognized that the combustion modes characterized by different flow field structures and heat release distributions can be classified into three types: the shear-layer mode, the transition mode, and the jet-wake mode. During the combustion mode conversion, the mixing region of the transverse jet and air became thicker with the increase in fuel flow rate, and the gradient of the flow field density and the flame area increased, making the flame more likely to propagate upstream. The combustion suppression induced by rapid fuel addition was observed at low equivalence ratios. It was speculated that the weak heat supply was insufficient to provide adequate heat for the rapid ignition of the added fuel. Furthermore, it was found that the flame-flow matching process with frequent flame propagation upstream occurred during the combustion mode conversion. This process was attributed to the mismatch between the increasing heat release and the original flow field structure.

Role of volatile secondary char on the combustion behavior of cellulose-based hydrochars

Hydrothermal carbonization (HTC) can transform a wide range of biomass into renewable biofuels. Hydrochar, the solid fuel resulting from HTC, has a similar energy density to low-rank coals. Despite an extensive body of literature detailing the thermogravimetric analysis (TGA) of hydrochar under slow pyrolysis and oxidation, there remains a limited understanding of hydrochar's behavior in realistic combustion settings. In this work, we integrate combustion experiments and TGA to understand the combustion characteristics of a model hydrochar fuel. Cellulose-based hydrochars produced at different temperatures along with solvent-extracted chars are tested in a Hencken burner across varying temperatures and oxygen concentrations in the oxidizer gas. Simultaneous CH* chemiluminescence, particle image velocimetry, and two-color pyrometry are used to measure ignition delay time and identify homogeneous/heterogeneous ignition mechanisms and combustion phases. Incorporating TGA with combustion results shows that the ignition mode and combustion processes are strong functions of surrounding gas temperature and oxygen mole fraction. The level of carbonization of the hydrochar dictates the ignition delay time and combustion modes. Further, the presence of a tar-like secondary char on the as-carbonized hydrochars leads to more rapid ignition due to high volatility.

Production and transport of turbulent kinetic energy from premixed flames subjected to mean pressure gradients

The influence of mean favorable pressure gradients on the production and transport of turbulent kinetic energy is experimentally investigated in a premixed bluff-body combustor. The combustor wall geometry is modified into converging, straight, or diverging configurations to experimentally alter the mean pressure gradient in the reacting flow field. For each configuration, turbulent kinetic energy production and transport dynamics are characterized with high-speed particle image velocimetry and simultaneous CH* chemiluminescence imaging. For all cases, the premixed reaction increases the turbulent kinetic energy and integral length scales in the flow. However, increasing the magnitude of the mean favorable pressure gradient increases the turbulent kinetic energy produced by the flame. The source of increased turbulent fluctuations from reactants to products is first investigated with a steady flow energy analysis, and is accompanied with a Reynolds decomposed investigation of the kinetic energy in the flow. As the mean pressure gradient increases, the turbulent kinetic energy in the flow increases due to stronger magnitudes of flame-generated turbulence in the form of baroclinic torque. The evolution of the flame-generated turbulence is examined by decomposing the transport equation of turbulent kinetic energy. Increasing the mean favorable pressure gradient augments the largest magnitude transport terms, and the budget of turbulent kinetic energy is primarily driven by pressure gradient work and viscous dissipation. The results here differ from previous direct numerical simulation (DNS) studies, and should ultimately motivate additional research to aid the development of computational models which can better predict turbulent flame-flow dynamics in confined environments with mean pressure gradients.

Conformations and structures of 1,4-pentadien-3-ol and its water complex characterized by rotational spectroscopy

The 1,4-pentadien-3-ol and its monohydrate have been characterized by microwave spectroscopy in combination with theoretical computations. Experiments and ab initio calculations revealed that the 1,4-pentadien-3-ol monomer prefers a configuration with one vinyl being syn to the hydroxyl oxygen and the hydroxyl hydrogen toward the skew arranged vinyl, which therefore makes possible simultaneous CH···O and OH···π interactions. The observed monohydrate corresponds to the global minimum predicted theoretically, which is stabilized through a primary OH···Ow hydrogen bond together with a much weaker OwH···π hydrogen bond. The NCI analyses, NBO calculation and SAPT method were applied to further elucidate the characteristics of hydrogen bonds in the 1,4-pentadien-3-ol···water complex.

The Influence of Pressure on Flame-Flow Characteristics of a Reacting Jet in Crossflow

Abstract This work experimentally investigates the effects of elevated combustor pressures on the characteristics of a lean premixed reacting methane/air jet injected into a lean vitiated crossflow using a 12.7 mm axial jet. Experiments were conducted in an axially staged combustor, which implements a reacting jet in crossflow (RJIC) configuration and operates over a pressure range of 1−5 atmospheres. Simultaneous CH* chemiluminescence and particle image velocimetry (PIV) are used to study the flow field and flame behavior. The results show that the reacting jet trajectory exhibits greater penetration with elevated pressure, which is a novel finding compared to available data in the literature. However, the flame liftoff point and ignition delay time both decreased with elevated pressure, which was attributed to decreased vorticity along the flame boundary which corresponds to increased Damköhler numbers (Da). Emissions measurements confirm the NOx increase with pressure as reported in the literature for single-stage gas turbine combustors. Concurrently, emission measurements for the staged configuration show the strong NOx benefit of the RJIC system: the data prove a reduction of global outlet emission levels at elevated pressure with the axially staged configuration. The axial emission reduction was attributed to the decreasing liftoff at elevated pressure levels. Hence, the research emphasizes that the flame and emission characteristics are coupled; they are not only dependent on the geometric parameters and momentum flux ratios but are also a function of pressure.

Energy deposition characteristics of a pulsed detonation igniter through geometric manipulation

This study investigates the transient energy deposition mechanisms of a detonation wave undergoing diffraction through a geometric area expansion and contraction. A subcritical detonation diffracts across a sudden expansion. The two expansion ratios studied resulted in two different modes of re-initiation: transverse wall reflections for 2.5 times the incoming diameter (D1) and reflection from an aft wall for 5D1. Re-initiation forms a supercritical detonation generating amplified pressures up to 2.5 times the Chapman-Jouguet pressure (PCJ). For the 2.5D1 expansion, amplification occurs between 6 < x/D1 < 12, attenuates to 1.5PCJ between 12 < x/D1 < 28, and decreases back to PCJ at the exit contraction. For the 5D1 expansion, re-initiation occurs at the downstream wall and thus the pressure measured at the exit contraction is approximately 2PCJ. Placement of an exit contraction within 5D1 of the peak pressure enables an amplified detonation to be extracted at the exhaust. It is also shown that a supercritical detonation does not achieve further amplification because the cell count is greater than the critical threshold, preventing diffraction and re-initiation. Simultaneous high-speed CH* chemiluminescence and schlieren imaging illustrate the amplification mechanisms of the decoupling and re-initiation process. Simultaneous pressure and wave speed measurements coincide with the optical diagnostics in terms of the decoupling and re-initiation dynamics

The effects of turbulence and pressure gradients on vorticity transport in premixed bluff-body flames

The current study experimentally evaluates the effects of turbulence and pressure gradients on vorticity transport in premixed bluff-body flames. The flow field pressure gradient is modified using a variable-geometry test section. In tandem, a custom-designed turbulence generator is used to modulate the turbulence intensity in the test section. The objective of the combined pressure gradient and turbulence tailoring is to alter the balance between the individual terms of the vorticity transport equation and ultimately identify the subsequent impacts on the overall behavior of the flame and flow field. Simultaneous high-speed particle image velocimetry and CH* chemiluminescence are employed to measure the turbulent reacting flow field. The vorticity dynamics are analyzed using a mean, conditionally filtered, fluid element tracking method to calculate individual terms of the vorticity transport equation as fluid elements evolve across the flame front. The analysis shows that increasing the pressure gradient increases the reacting vorticity production for all vorticity terms. In contrast, the overall vorticity budget, or relative contribution of each vorticity term, is much more sensitive to turbulence effects. At low turbulence conditions, vorticity is dominated by baroclinic torque, regardless of the pressure gradient. As the turbulence intensity is increased, the exothermic effects of baroclinicity and dilatation are diminished, and the reacting flow field approaches that of a non-reacting turbulent flow. The relative balance of vorticity transport terms is shown to influence the overall flow field, as well as the structure and evolution of the flame front.

Critical region in the spatiotemporal dynamics of a turbulent thermoacoustic system and smart passive control

We develop a passive control strategy for suppressing thermoacoustic instability in a bluff-body stabilized premixed turbulent combustor. When the equivalence ratio is varied, there is a transition from combustion noise to thermoacoustic instability via intermittency in the combustor. We perform simultaneous acoustic pressure, 2D-PIV, and CH * chemiluminescence measurements to capture the pressure fluctuations, the velocity field, and the heat release rate (HRR) field during the transition. We measure the spatial distribution of the amplitude of turbulent velocity at the acoustic frequency, time-averaged vorticity, time-averaged HRR, and Rayleigh index and identify various regions of significance. We implement a passive control strategy by targeting these regions with a steady injection of secondary micro-jet of air to optimize the injection location and determine the critical region. Targeting the critical region with secondary air leads to greater than 20 dB suppression of the dominant thermoacoustic mode. We observe that the coherent structure forming from the shear layer following the dump plane gets suppressed, leading to an incoherent spatial distribution of HRR fluctuations. We find that the turbulent velocity amplitude correctly identifies the critical region for optimized passive control during thermoacoustic instability. In contrast, the Rayleigh index identifies the region of the most significant acoustic driving; however, it does not identify the region most sensitive to control. Finally, we extend our analysis by determining the spatial distribution of the Hurst exponent measured from the turbulent velocity field. We show that the Hurst exponent identifies the critical region during thermoacoustic instability and intermittency, unlike the other physical measures. Thus, we develop a smart passive control method by combining the need for finding critical regions in the combustor with the predictive capabilities of the Hurst exponent.

CH and NO planar laser-induced fluorescence and Rayleigh-scattering in turbulent flames using a multimode optical parametric oscillator.

An optical parametric oscillator (OPO) is developed and characterized for the simultaneous generation of ultraviolet (UV) and near-UV nanosecond laser pulses for the single-shot Rayleigh scattering and planar laser-induced-fluorescence (PLIF) imaging of methylidyne (CH) and nitric oxide (NO) in turbulent flames. The OPO is pumped by a multichannel, 8-pulse Nd:YAG laser cluster that produces up to 225 mJ/pulse at 355 nm with pulse spacing of 100 µs. The pulsed OPO has a conversion efficiency of 9.6% to the signal wavelength of ∼430nm when pumped by the multimode laser. Second harmonic conversion of the signal, with 3.8% efficiency, is used for the electronic excitation of the A-X (1,0) band of NO at ∼215nm, while the residual signal at 430 nm is used for direct excitation of the A-X (0,0) band of the CH radical and elastic Rayleigh scattering. The section of the OPO signal wavelength for simultaneous CH and NO PLIF imaging is performed with consideration of the pulse energy, interference from the reactant and product species, and the fluorescence signal intensity. The excitation wavelengths of 430.7 nm and 215.35 nm are studied in a laminar, premixed CH4-H2-NH3-air flame. Single-shot CH and NO PLIF and Rayleigh scatter imaging is demonstrated in a turbulent CH4-H2-NH3 diffusion flame using a high-speed intensified CMOS camera. Analysis of the complementary Rayleigh scattering and CH and NO PLIF enables identification and quantification of the high-temperature flame layers, the combustion product zones, and the fuel-jet core. Considerations for extension to simultaneous, 10-kHz-rate acquisition are discussed.

Turbulent flame-vortex dynamics of bluff-body premixed flames

Abstract To better understand the coupled dynamics of turbulence and premixed flames, this study experimentally investigates the effects of turbulence on reacting vorticity dynamics. In this investigation, a bluff-body stabilized, turbulent, premixed flame is analyzed in a high-speed combustor. A custom-designed turbulence generator is used to vary the inflow turbulence conditions and understand the subsequent impacts on the flame-flow field. The turbulent reacting flow-field is measured using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. A Lagrangian tracking method is implemented to study flame-vortex dynamics. Lagrangian fluid elements are tagged from the experimental data and are tracked as they convect through the flame, from reactants to products. The components of the vorticity transport equation are decomposed along the Lagrangian trajectories to determine their relative balance under various turbulence conditions. The results confirm that altering the turbulence intensity influences flame-vortex interactions and the relative balance of the vorticity components. At high turbulence intensities, the baroclinic torque and dilatation decrease, while the magnitude of viscous diffusion increases, and vortex stretching dominates the reacting flow field. These results suggest that increasing the turbulence intensity alters the dominant physical processes in the reacting domain as the combustion-driven phenomena become less significant.

An efficient Pd-catalyzed intramolecular cyclization reaction followed by formation of benzimidazole derivatives: Synthesis of novel quinolin-fused benzo[d] azeto[1,2-a]benzimidazole analogues

Novel quinolin-fused benzo[d]azeto[1,2-a]benzimidazole analogues have been synthesized from o-phenylene diamine and various substituted 2-chloro-5-(2-methylquinoline-4-yl) benzaldehyde through Pd-mediated simultaneous CH–NH and CH–CX bond activation via intramolecular cyclization reaction followed by formation of benzimidazole derivatives. Developed two step, one pot route for quinoline fused four membered azeto[1,2-a] benzimidazole derivatives through Pd-catalyzed cross coupling reaction. The synthesis was carried out by using Pd2(dba)3 as a catalyst, Xphos as a ligand, Cs2CO3 as a base and toluene as a solvent and developed its mechanistic pathway.

Ignition and combustion enhancement in a cavity-based supersonic combustor by a multi-channel gliding arc plasma

A multi-channel gliding arc (MCGA) plasma was used to enhance ignition and combustion of ethylene fuel in a cavity-based scramjet combustor with an inflow speed of Ma = 2.92. Simultaneously electro-optical diagnostics were conducted to show characteristics of the MCGA-enhanced ignition and combustion, as well as simultaneous schlieren and CH* emission images. The voltage and current measurements demonstrate that the average power of the MCGA declines from 1824 W of an ignition state to 1196 W of a stable combustion state. Several flame kernels produced simultaneously by the power spikes of the MCGA can be merged to enhance the ignition, resulting in a faster speed of flame propagation (48% reduction in the flame propagation time) and a shorter ignition time (61% reduction in the ignition time) compared to the ignition by a single-channel gliding arc. The enhancement effect of the MCGA on combustion is more significant with an increasing global equivalence ratio. The wall pressure of the scramjet combustor in the presence of the MCGA at a global equivalence ratio of 0.22 is comparable to the pressure at a global equivalence ratio of 0.32 in the absence of the MCGA. The main reasons for the significant combustion enhancement may be the re-ignition of the cavity flame by the MCGA plasma, adding extra heat and reactive species to the flame in the shear layer.

Mechanisms of flame extinction and lean blowout of bluff body stabilized flames

Lean flame blowout is investigated experimentally within a high-speed combustor to analyze the temporal extinction dynamics of turbulent premixed bluff body stabilized flames. The lean blowout process is induced through fuel flow reduction and captured temporally using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. The evolution of the flame structure, flow field, and the resulting flame strain rate are analyzed throughout extinction to distinguish the physical mechanisms of blowout. The flame–vortex dynamics are found to be the main driving mechanism of flame extinction; namely, a reduction of the flame-generated vorticity coupled with an increase in the downstream shear layer vorticity. The vorticity dynamics are linked to hydrodynamic instabilities that vary as a function of the decreasing equivalence ratio. A frequency analysis is performed to characterize the dynamic changes of the hydrodynamic instability modes during flame extinction. Various bluff body inflow velocity regimes are investigated to further characterize the extinction instability modes. Both equivalence ratio and flow-driven instabilities are captured through a universal definition of the Strouhal number for the reacting bluff body flow. Finally, a Karlovitz number-based criterion is developed to consistently predict the onset of global extinction for the different inflow velocity regimes.

Simultaneous high speed PIV and CH PLIF using R-branch excitation in the C2Σ+-X2Π (0,0) band

Simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) utilizing R-branch transitions in the C-X (0,0) band were performed at a 10-kHz repetition-rate in a turbulent premixed flame. The CH lines at 310.690 nm (from the R-branch of the C-X band) used here have greater efficiency than A-X and B-X transitions, which allows for high-framerate imaging with low laser pulse energy. Most importantly, the simultaneous imaging of both CH PLIF and PIV is enabled by the use of a custom edge filter, which blocks scattering at the laser wavelength (below ∼311 nm) while efficiently transmitting fluorescence at longer wavelengths. The Hi-Pilot Bunsen burner operated with a turbulent Reynolds number of 7900 was used to demonstrate simultaneous PIV and CH PLIF utilizing this filtered detection scheme. Instances where pockets of products were observed well upstream of the mean flame brush are found to be the result of out-of-plane motion of the flame sheet. Such instances can lead to ambiguous results when interpreting the thickness of reaction layers. However, the temporally resolved nature of the present diagnostics facilitate the identification and proper treatment of such situations. The strategy demonstrated here can yield important information in the study of turbulent flames by providing temporally resolved flame dynamics in terms of flame sheet visualization and velocity fields.

Simultaneous TR-SPIV and CH* chemiluminescence during precursor to LBO in a lean premixed swirl dump combustor

Blowout process in premixed swirl dump combustors is known to have temporary partial extinction followed by re-ignition events as precursors. This re-ignition process is investigated using high-speed CH* chemiluminescence and simultaneous TR-SPIV. It was found that during the extinction phase, the flame split into two zones, causing fresh mixture to enter the inner recirculation zone. The sudden loss of heat release causes the flow field to change such that the stagnation point moves further downstream, causing high negative velocity paths in the flow. The flame that was convected downstream, now uses these negative velocity paths to consume the fresh mixture that entered the IRZ. This is the re-ignition phase of the precursor event.

Pressure-gradient tailoring effects on the turbulent flame-vortex dynamics of bluff-body premixed flames

This paper explores the effects of pressure-gradient tailoring on the turbulent flame and vorticity generation mechanisms of premixed flames. A turbulent premixed flame stabilized by a bluff-body in a high-speed combustor is used for the investigation. The combustor pressure gradient is altered using a variable-geometry test section. The turbulent flame-flow field is measured and characterized using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. A Lagrangian tracking technique is applied to analyze the details of the flame-vortex interactions from the experimental data. Lagrangian fluid elements are tracked as they evolve across the flame. The vorticity mechanisms are decomposed along the Lagrangian trajectories to determine their relative balance under various pressure gradient conditions. It is demonstrated that the induced pressure-gradient affects the relative magnitudes of combustion-generated dilatation and baroclinic torque, as well as the vortex stretching. An increase in the magnitudes of the vorticity mechanisms is shown with the largest gain in baroclinicity for the augmented pressure gradient relative to the attenuated.

Ignition processes and modes excited by laser-induced plasma in a cavity-based supersonic combustor

The ignition processes in an ethylene-fueled supersonic combustor excited by laser-induced plasma (LIP) were investigated experimentally under the condition of inflow Ma number of 2.92. The LIP excitation was implemented near the center floor of a flameholding cavity. Optical measurements, including simultaneous CH∗/OH∗ chemiluminescence imaging and Schlieren photography, were used to investigate the ignition processes. It is found that the CH∗ and OH∗ initiated by the LIP are mainly in the region between the cavity front wall and the LIP excitation site. The CH∗ and OH∗ are quenched rapidly, in particular at a low fueling rate. After a short delay, the distribution of CH∗ and OH∗ appears in the region between the rear wall of the cavity and the LIP excitation site, showing the onset of ignition therein. A stable flame was established in the shear layer between the downstream part of the cavity and the outer supersonic flow. It is concluded that the ignition processes excited by the LIP can be divided into a LIP initiation regime and a transient ignition reaction regime. Both the fueling rate and the LIP energy significantly affect the cavity ignition processes. Increasing the fueling rate or the laser energy can shorten the ignition processes in the cavity. A weak ignition mode and an intense ignition mode are postulated to explain the combustion behavior of the ignition processes in the cavity-based supersonic combustor.

Characterization of ignition transient processes in kerosene-fueled model scramjet engine by dual-pulse laser-induced plasma

Dual-pulse laser-induced plasma ignition of kerosene in cavity at model scramjet engine is studied. The simulated flight condition is Ma 6 at 30 km, and the isolator entrance has a Mach number of 2.92, a total pressure of 2.6 MPa and a stagnation temperature of 1650 K. Two independent laser pulses at 532 nm with a pulse width of 10 ns, a diameter of 12 mm and a maximum energy of 300 mJ are focused into cavity for ignition. The flame structure and propagation during transient ignition processes are captured by simultaneous CH* and OH* chemiluminescence imaging. The entire ignition process of kerosene can be divided into five stages, which are referred as turbulent dissipation stage, quasi-stable state, combustion enhancement stage, reverting stage and combustion stabilization stage. A local closed loop of propagations of the burning mixtures from the shear layer into the recirculation zone of cavity is revealed, which the large-scale eddy in the shear layer plays a key role. The enhancement of mass exchange between shear layer and the recirculation zone of cavity could promote the flame propagation process and enhance the ignition capability as well as extend the ignition limits. A cavity shear-layer stabilized combustion of kerosene is established in the supersonic flow roughly 3.3 ms after the laser pulse. Chemical reactions mainly occur in the shear layer and the near-wall zone downstream of the cavity. The distribution of OH* is thicker than CH* at stable combustion condition.

On the Flame-generated Vorticity Dynamics of Bluff-body-stabilized Premixed Flames

This investigation considers the dynamics of flame-generated vorticity for a premixed, submerged bluff-body stabilized flame. Experimentation characterizes the far-field region in particular with a level of detail not previously afforded to this type of flow. Simultaneous particle imaging velocimetry (PIV), Mie scattering and CH ∗ chemiluminescence are used to obtain velocity fields and flame location. Mean static pressure measurements at the combustion chamber wall capture the pressure field. Analysis of the flame fronts in relation to the mean velocity and vorticity fields provides useful insight into the interaction of the flame and the flow. The unique nature of the velocity and vorticity fields and their effect on downstream flame structures are explained by the baroclinic torque generation of vorticity. The coupling that exists among pressure, heat release, and baroclinic generation is acknowledged and will influence strategies for control of the baroclinic mechanism.

Direct synthesis of acetic acid by simultaneous co-activation of methane and CO2 over Cu-exchanged ZSM-5 catalysts

The ultimate goal of this study is the development of a new strategy to exploit methane and carbon dioxide resources more efficiently and cleanly. Due to the high stabilities of both methane and CO2, simultaneous CH bond activation of methane and CO2 is one of the toughest challenges in catalysis. Formation of acetic acid was achieved under the continuous flow micro-reactor system by the concurrent feed of methane and carbon dioxide over Cu loaded M+-ZSM-5 catalysts (M=Li+, Na+, K+ and Ca++). The formation of acetic acid was observed in the temperature regime of 425–525°C with a co-feed of CO2 and CH4, under low space velocity of 360mlh−1 based on methane. Compared to Cu0-H-ZSM-5, the M+-ZSM-5 catalyst showed remarkable increment in the formation rate of acetic acid, and the catalytic efficiency was in order K > Na > Ca > Li. The obtained results indicated that the M+ assists in enriching surface active CO2 in the form of carbonates and subsequently reacted with the homolytic activated CH over Cu0-H-ZSM-5. Gathering Cu0 and M+, the Cu0-K-ZSM-5 catalyst showed the highest formation rate of acetic acid (395μ mole gcat−1h−1) at steady state, for 10h time-on-stream at 500°C.