Energy coupling mechanism for pulse detonation ignition of a scramjet cavity

The effect of detonation decoupling on the ignition capability of a pulse detonation igniter in a scramjet cavity is investigated. It was observed that a strongly coupled detonation is required for igniting a fueled cavity and the ignition capability rapidly decays for a weak or slightly decoupled detonation. The pulse detonation igniter pressure and wave speed were measured at subatmospheric pressure to characterize the pressure and fill characteristics dependence on backpressure. Temperatures measurements using 100 kHz H₂O absorption measurements showed an increase in peak temperature for critically filled conditions but a decrease in bulk fluid temperature with decreasing fill time. Simultaneous schlieren and chemiluminescence measurements demonstrate that a fully coupled detonation entrains greater quantities of high-temperature and reacting fluid in the vortex formed on the edges of the detonation plume, enhancing the mixing and spreading of products into the cavity. This shedding of high-temperature intermediate species is the primary mechanism governing successful ignition in the scramjet cavity.     

Numerical study of self-sustained oblique detonation in a non-uniform mixture

Self-sustained oblique detonation behind a spherical projectile formed in a non-uniform H₂/O₂/Ar mixture was numerically investigated. A hypersonic combustible mixture flow around a 4.76 mm diameter body was modeled to be flowing at 2500 m/s and 100 kPa. The concentration gradient was prescribed applying the Gaussian distribution to hydrogen concentration. Axisymmetric Euler equations including a detailed kinetics of 9 species and 27 elementary reactions were solved with an explicit 2nd-order time integration scheme combined with point implicit method for chemical reaction. Oblique detonation was always obtained when the mixture on the centerline was stoichiometric, as it is for a uniform mixture, and a broader range of equivalence ratio could sustain oblique detonation far from the sphere. Local detonation angle was revealed to reasonably match Chapman-Jouguet analytical solutions with a minor difference attributed to curvature, less reactive composition, and the concentration gradient. Also, a strongly fuel-rich region encountered decoupling of the shock-flame, in which an abrupt deflection of the shock front appeared. These decoupling phenomena can be attributed to a slower kinetics of a less reactive mixture. All of interesting findings in this study will also benefit understanding of various form of detonation in non-uniform mixture taking advantage of the analogy between them.     

One-electron energies in the presence of electron-phonon interactions

Decoupling of an electron-ion interacting system is considered and the pseudopotential method of Phillips and Kleinman is extended to include the effect of electron-phonon interactions. It is shown that the effective potential experienced by a decoupled electron is the instantaneous pseudopotential averaged over all possible phonon states. The averaged pseudopotential is not only periodic but also temperature-dependent. After decoupling, a simple procedure for calculating single-electron excitation energies resulting from the coupling term is given and the result is formally in agreement with that obtained by Green's function method.     

Ignition limit and shock-to-detonation transition mode of n-heptane/air mixture in high-speed wedge flows

In this work, oblique detonation of n-heptane/air mixture in high-speed wedge flows is simulated by solving the reactive Euler equations with a two-dimensional (2D) configuration. This is a first attempt to model complicated hydrocarbon fuel oblique detonation waves (ODWs) with a detailed chemistry (44 species and 112 reactions). Effects of freestream equivalence ratios and velocities are considered, and the abrupt and smooth transition from oblique shock to detonation are predicted. Ignition limit, ODW characteristics, and predictability of the transition mode are discussed. Firstly, homogeneous constant-volume ignition calculations are performed for both fuel-lean and stoichiometric mixtures. The results show that the ignition delay generally increases with the wedge angle. However, a negative wedge angle dependence is observed, due to the negative temperature coefficient effects. The wedge angle range for successful ignition of n-heptane/air mixtures decreases when the wedge length is reduced. From two-dimensional simulations of stationary ODWs, the initiation length generally decreases with the freestream equivalence ratio, but the transition length exhibits weakly non-monotonic dependence. Smooth ODW typically occurs for lean conditions (equivalence ratio < 0.4). The interactions between shock/compression waves and chemical reaction inside the induction zone are also studied with the chemical explosive mode analysis. Moreover, the predictability of the shock-to-detonation transition mode is explored through quantifying the relation between ignition delay and chemical excitation time. It is demonstrated that the ignition delay (the elapsed time of the heat release rate, HRR, reaches the maximum) increases, but the excitation time (the time duration from the instant of 5% maximum HRR to that of the maximum) decreases with the freestream equivalence ratio for the three studied oncoming flow velocities. Smaller excitation time corresponds to stronger pressure waves from the ignition location behind the oblique shock. When the ratio of excitation time to ignition delay is high (e.g., > 0.5 for n-C₇H₁₆, > 0.3 for C₂H₂ and > 0.2 for H₂, based on the existing data compilation in this work), smooth transition is more likely to occur.     

Detailed and simplified kinetic models of n-dodecane oxidation: The role of fuel cracking in aliphatic hydrocarbon combustion

A detailed kinetic model is proposed for the combustion of normal alkanes up to n-dodecane above 850 K. The model was validated against experimental data, including fuel pyrolysis in plug flow and jet-stirred reactors, laminar flame speeds, and ignition delay times behind reflected shock waves, with n-dodecane being the emphasis. Analysis of the computational results reveal that for a wide range of combustion conditions, the kinetics of fuel cracking to form smaller molecular fragments is fast and may be decoupled from the oxidation kinetics of the fragments. Subsequently, a simplified model containing a minimal set of 4 species and 20 reaction steps was developed to predict the fuel pyrolysis rate and product distribution. Combined with the base C₁-C₄ model, the simplified model predicts fuel pyrolysis rate and product distribution, laminar flame speeds, and ignition delays as close as the detailed reaction model.     

High resolution numerical simulation of the structure of 2-D gaseous detonations

Two dimensional numerical simulation of the structure of gaseous detonation is investigated by utilizing the single step Arrhenius kinetic reaction mechanism in both high and low activation energy mixtures, characterized by their irregular and regular detonation structure, respectively. All the computations are performed on a small Beowulf cluster with six nodes. The dependency of the structure on the grid resolution is performed and it is found that, resolution of more than 300 cells per hrl is required to demonstrate the role of hydrodynamic instabilities, (KH and RM instabilities) in detonation propagation in irregular structures, while due to the absence of fine-scale structures, resolution of 50 cells per hrl, gives the physical structure of detonation with regular structures. Results show that the transverse waves in irregular structure are significantly stronger than the transverse wave in regular structure detonation, which can enhance the burning rate of the unburned pockets behind the shock front. Results for resolution of 600 cells per hrl illustrate that, in addition to the primary mode, the interaction of large vortices with the shock front provides secondary modes in the structure which leads to the irregularity of the structure in high activation energy mixture. In contrast with the results obtained for regular structure, which no unburned gas pockets and vortices observed behind the front, the results for irregular structure reveal that most portions of the gases, escape from shock compression and create large unburned gas pockets behind the both weak section of the Mach stem and the incident wave, which will burn eventually by the turbulent mixing due to the vortices associated with hydrodynamic instabilities. Therefore, the ignition mechanism in irregular structure is due to the both shock compression and by turbulent mixing associated with hydrodynamic instabilities, while the shock compression yields the ignition mechanism in regular structure detonation.     

n-Dodecane oxidation at high-pressures: Measurements of ignition delay times and OH concentration time-histories

Ignition delay times and OH concentration time-histories were measured during n-dodecane oxidation behind reflected shocks waves using a heated, high-pressure shock tube. Measurements were made over temperatures of 727-1422 K, pressures of 15-34 atm, and equivalence ratios of 0.5 and 1.0. Ignition delay times were measured using side-wall pressure and OH^∗ emission diagnostics, and OH concentration time-histories were measured using narrow-linewidth ring-dye laser absorption near the R-branchhead of the OH A-X (0, 0) system at 306.47 nm. Shock tube measurements were compared to model predictions of four current n-dodecane oxidation detailed mechanisms, and the differences, particularly in the low-temperature negative-temperature-coefficient (NTC) region where the influence of non-ideal facility effects can be significant, are discussed. To our knowledge, the current measurements provide the first gas-phase shock tube ignition delay times (at pressures above 13 atm) and quantitative OH concentration time-histories for n-dodecane oxidation under practical engine conditions, and hence provide benchmark validation targets for refinement of jet fuel detailed kinetic modeling, since n-dodecane is widely used as the principal representative for n-alkanes in jet fuel surrogates.     

Spatio-temporal evolution of cavity ignition in supersonic flow

Successful ignition in the recirculating flow of a scramjet flame holder can be highly dependent upon the location of energy deposition because of the spatial variation of fuel concentration and flow properties. The current work experimentally investigated ignition processes when energy was deposited (～100 mJ) via a spark discharge at four locations in the base of a cavity or by laser-induced breakdown in a Mach 2 flow with a stagnation temperature and pressure of 590 K and 483 kPa, respectively. The cavity was directly fueled with ethylene injection. The time dependent heat release was imaged at 40,000 frames per second and fuel concentration and distribution measurements were taken in the cavity prior to ignition. The average fuel concentration at the lean and rich ignition limits near the energy deposition locations measured 4.4–9.3% (Φ= 0.75 to 1.47). Energy deposition near the cavity step resulted in near immediate ignition kernel development and rapid achievement of self-sustained flame propagation in the front of the cavity, often faster than the bulk recirculation time of the cavity, leading to a spike in heat release. Energy deposition away from the cavity step region led to competition between local flow velocity, fuel concentration, and flame propagation rates. Ignition kernels formed along the floor of the cavity towards the closeout ramp and were rapidly advected towards the cavity step region before flame propagation could ensue. The fastest and most robust ignition events for all fueling cases showed rapid spanwise flame propagation near the cavity step.     

Decoupling potentials for the three-body final state Schrödinger equation of knockout reactions

In the conventional distorted wave impulse approximation (DWIA) approach the three-body final state of a knockout reaction is decoupled by assuming a plane wave form for the coupling term. The influence of this decoupling approximation on the analyses of cluster knockout reactions has been investigated for a test case where the exact solution is obtainable. A proper treatment of the coupling term causes large oscillations in the effective distorting optical potentials for the decoupled Schrödinger equation. These decoupling potentials depend strongly not only on the partial wave angular momentum,l but also on their azimuthal projection,m.     

Unstable combustion induced by oblique shock waves at the non-attaching condition of the oblique detonation wave

The instability of oblique shock wave (OSW) induced combustion is examined for a wedge with a flow turning angle greater than the maximum attach angle of the oblique detonation wave (ODW), where archival results rarely exist for this case in previous literatures. Numerical simulations were carried out for wedges of different length scales to account for the ratio of the chemical and fluid dynamic time scales. The results reveal three different regimes of combustion. (1) No ignition or decoupled combustion was observed if a fluid dynamic time is shorter than a chemical time behind an OSW. (2) Oscillatory combustion was observed behind an OSW if a fluid dynamic time is longer than a chemical time behind an OSW and the fluid dynamic time is shorter than the chemical time behind a normal shock wave (NSW) at the same Mach number. (3) Detached bow shock-induced combustion (or detached overdriven detonation wave) was observed if a fluid dynamic time is longer than a chemical time behind a NSW. Since no ignition or decoupled combustion occurs as a very slow reaction and the detached wave occurs as an infinitely fast reaction, the finite rate chemistry is considered to be the key for the oscillating combustion induced by an OSW over a wedge of a finite length with a flow turning angle greater than the maximum attach angle for an ODW. Since this case has not been previously reported, grid independency was tested intensively to account for the interaction between the shock and reaction waves and to determine the critical time scale where the oscillating combustion can be observed.     

Phase amplitude decoupling in modulated structures

We examine under what conditions phase and amplitude modulations are decoupled in a modulated structure, within a Landau Ginzburg approach. For the general case of a pth order commensurability, the condition for phase amplitude decoupling depends on the leading anharmonic terms in the Landau Ginzburg free energy. For a 2Mth order leading anharmonic term, the case p = 2M is a border line case. For p > 2M, phase and amplitude are decoupled near the disordered-incommensurate transition temperature, while they are not for p < 2M. In the latter case decoupling occurs at lower temperatures. We discuss the case of deuterated thiourea, which exhibits a commensurate pinning for p = 9 at T = 191 K.     

Effect of ozone addition and ozonolysis reaction on the detonation properties of C2H4/O2/Ar mixtures

Ozone is one of the strongest oxidizers and can be used to enhance detonation. Detonation enhancement by ozone addition is usually attributed to the ozone decomposition reaction which produces reactive atomic oxygen and thereby accelerates the chain branching reaction. Recently, ozonolysis reaction has been found to be another mechanism to enhance combustion for unsaturated hydrocarbons at low temperatures. In this study, the effects of ozone addition and ozonolysis reaction on steady detonation structure and transient detonation initiation and propagation processes in C₂H₄/O₂/O₃/Ar mixtures are examined through simulations considering detailed chemistry. Specifically, the homogeneous ignition process, the ZND detonation structure, the transient direct detonation initiation, and pulsating instability of one-dimensional detonation propagation are investigated. It is found that the homogenous ignition process consists of two stages and the first stage is caused by ozonolysis reactions which consume O₃ and produces CH₂O as well as H and OH radicals. The ozonolysis reaction and ozone decomposition reaction can both reduce the induction length though they have little influence on the Chapman–Jouguet (CJ) detonation speed. The supercritical, critical and subcritical regimes for direct detonation initiation are identified by continuously decreasing the initiation energy or changing the amount of ozone addition. It is found that direct detonation initiation becomes easier at larger amount of ozone addition and/or larger reaction progress variable. This is interpreted based on the change of the induction length of the ZND detonation structure. Furthermore, it is demonstrated that the ozonolysis reaction can reduce pulsating instability and make the one-dimensional detonation propagation more stable. This is mainly due to the reduction in activation energy caused by ozone addition and/or ozonolysis reaction. This work shows that both ozone decomposition reaction and ozonolysis reaction can enhance detonation for unsaturated hydrocarbon fuels.     

Decoupling multimode vibrational relaxations in multi-component gas mixtures: Analysis of sound relaxational absorption spectra

Decoupling the complicated vibrational-vibrational (V-V) coupling of a multimode vibrational relaxation remains a challenge for analyzing the sound relaxational absorption in multi-component gas mixtures. In our previous work [Acta Phys. Sin. 61 174301 (2012)], an analytical model to predict the sound absorption from vibrational relaxation in a gas medium is proposed. In this paper, we develop the model to decouple the V-V coupled energy to each vibrational- translational deexcitation path, and analyze how the multimode relaxations form the peaks of sound absorption spectra in gas mixtures. We prove that a multimode relaxation is the sum of its decoupled single-relaxation processes, and only the decoupled process with a significant isochoric-molar-heat can be observed as an absorption peak. The decoupling model clarifies the essential processes behind the peaks in spectra arising from the multimode relaxations in multi-component gas mixtures. The simulation validates the proposed decoupling model.     

Shock tube measurements of ignition delay times and OH time-histories in dimethyl ether oxidation

Ignition delay times and OH concentration time-histories were measured in DME/O₂/Ar mixtures behind reflected shock waves. Initial reflected shock conditions covered temperatures (T₅) from 1175 to 1900 K, pressures (P₅) from 1.6 to 6.6 bar, and equivalence ratios (?) from 0.5 to 3.0. Ignition delay times were measured by collecting OH^∗ emission near 307 nm, while OH time-histories were measured using laser absorption of the R₁(5) line of the A-X(0,0) transition at 306.7 nm. The ignition delay times extended the available experimental database of DME to a greater range of equivalence ratios and pressures. Measured ignition delay times were compared to simulations based on DME oxidation mechanisms by Fischer et al. [7] and Zhao et al. [9]. Both mechanisms predict the magnitude of ignition delay times well. OH time-histories were also compared to simulations based on both mechanisms. Despite predicting ignition delay times well, neither mechanism agrees with the measured OH time-histories. OH Sensitivity analysis was applied and the reactions DME ↔ CH₃O + CH₃ and H + O₂ ↔ OH + O were found to be most important. Previous measurements of DME ↔ CH₃O + CH₃ are not available above 1220 K, so the rate was directly measured in this work using the OH diagnostic. The rate expression k[1/s] =  1.61 × 10⁷⁹T^−18.4 exp(−58600/T), valid at pressures near 1.5 bar, was inferred based on previous pyrolysis measurements and the current study. This rate accurately describes a broad range of experimental work at temperatures from 680 to 1750 K, but is most accurate near the temperature range of the study, 1350-1750 K. When this rate is used in both the Fischer et al. and Zhao et al. mechanisms, agreement between measured OH and the model predictions is significantly improved at all temperatures.     

Direct ignition and S-curve transition by in situ nano-second pulsed discharge in methane/oxygen/helium counterflow flame

A well-defined plasma assisted combustion system with novel in situ discharge in a counterflow diffusion flame was developed to study the direct coupling kinetic effect of non-equilibrium plasma on flame ignition and extinction. A uniform discharge was generated between the burner nozzles by placing porous metal electrodes at the nozzle exits. The ignition and extinction characteristics of CH₄/O₂/He diffusion flames were investigated by measuring excited OH¹ and OH PLIF, at constant strain rates and O₂ mole fraction on the oxidizer side while changing the fuel mole fraction. It was found that ignition and extinction occurred with an abrupt change of OH¹ emission intensity at lower O₂ mole fraction, indicating the existence of the conventional ignition-extinction S-curve. However, at a higher O₂ mole fraction, it was found that the in situ discharge could significantly modify the characteristics of ignition and extinction and create a new monotonic and fully stretched ignition S-curve. The transition from the conventional S-curves to a new stretched ignition curve indicated clearly that the active species generated by the plasma could change the chemical kinetic pathways of fuel oxidation at low temperature, thus resulting in the transition of flame stabilization mechanism from extinction-controlled to ignition-controlled regimes. The temperature and OH radical distributions were measured experimentally by the Rayleigh scattering technique and PLIF technique, respectively, and were compared with modeling. The results showed that the local maximum temperature in the reaction zone, where the ignition occurred, could be as low as 900 K. The chemical kinetic model for the plasma–flame interaction has been developed based on the assumption of constant electric field strength in the bulk plasma region. The reaction pathways analysis further revealed that atomic oxygen generated by the discharge was critical to controlling the radical production and promoting the chain branching effect in the reaction zone for low temperature ignition enhancement.     

Two-dimensional numerical simulation on galloping detonation in a narrow channel

The numerical simulations of the two-dimensional galloping detonation performed by using two-dimensional full Navier–Stokes simulations with a detailed chemistry model are presented. The detonation in a narrow channel with d = 5 mm, which is approximately twice the half-reaction length of hydrogen, shows a feature of galloping detonation with two initiations during its propagation under the laminar flow assumption. The distance between these two initiations is approximately 1300 mm, which causes the induction time behind the leading shock wave. As the channel width increases, the galloping feature diminishes. The detonation propagates approximately 4% lower than D_CJ for d = 10 and 15 mm. By increasing the channel width, the strength of the detonation increases, as shown in the maximum pressure histories. The effects of turbulence behind the detonation show that the galloping feature disappears, although its propagation velocity becomes 0.9 D_CJ. The strength of the detonation becomes significantly weak compared with the detonation propagating in the wide channel widths, and this feature is similar to the laminar assumption. The trend of the velocity deficits in the NS simulations agrees fairly well with the trend of the modified ZND calculations with η = 0.25.     

Reconstruction of randomly under-sampled spectra for in vivo 13C magnetic resonance spectroscopy

PurposeOver the past decade, many techniques have been developed to reduce radiofrequency (RF) power deposition associated with proton decoupling in in vivo Carbon-13 (¹³C) magnetic resonance spectroscopy (MRS). In this work we propose a new strategy that uses data under-sampling to achieve reduction in RF power deposition.Materials and methodsEssentially, proton decoupling is required only during randomly selected segments of data acquisition. By taking advantage of the sparse spectral pattern of the carboxylic/amide region of in vivo ¹³C spectra of brain, we developed an iterative algorithm to reconstruct spectra from randomly under-sampled data. Fully sampled data were used as references. Reconstructed spectra were compared with the fully sampled references and evaluated using residuals and relative signal intensity errors.ResultsNumerical simulations and in vivo experiments at 7 Tesla demonstrated that this novel decoupling and data processing strategy can effectively reduce decoupling power deposition by greater than 30%.ConclusionThis study proposes and evaluates a novel approach to acquire ¹³C data with reduced proton decoupling power deposition and reconstruct in vivo ¹³C spectra of carboxylic/amide metabolite signals using randomly under-sampled data. Because proton decoupling is not needed over a significant portion of data acquisition, this novel approach can effectively reduce the required decoupling power and thus SAR. It opens the possibility of performing in vivo ¹³C experiments of human brain at very high magnetic fields.     

Numerical study on three-dimensional C-J detonation waves: detailed propagating mechanism and existence of OH radical

An unsteady three-dimensional simulation is performed for a hydrogen/air C–J detonation in a rectangular tube, where a detailed chemical reaction model is used to reveal the C–J detonation structure. In this simulation, detailed propagating detonation structures for a diagonal mode are described in three-dimensions. The detonation front structures, the line of triple points, and the strong explosions at the corners of the rectangular tube are revealed by using a three-dimensional numerical visualization. From the spatial isosurface profiles of H₂ mass fraction, it is confirmed that the triple point lines have a role of “shutter” to generate unburned gas pockets and become of a ring shape behind the detonation front due to its explosion. The explosion process and its influence on an induction delay are observed by visualizing the spatial isosurface profiles of OH mass fraction. Moreover, a high “peninsula-shaped” OH mass fraction area, which has been experimentally reported, is reproduced on the side wall of the rectangular tube.     

空气中激光支持爆轰波实验及理论分析

为了研究激光击穿空气产生的等离子体爆轰波形成机制和传播规律,利用高能量CO2激光器产生强激光,进行了空气中产生激光支持等离子体爆轰波实验。实验中:设置了诱导靶板,用于诱发和定位空气中的激光支持爆轰波;以激光器升压过程球隙放电产生的光信号作为触发源,触发高时间分辨率(纳秒级)的高速相机,记录了激光支持爆轰波的成长和传播全过程。分析了激光支持爆轰波的形成机理和传播规律。采用C-J爆轰理论,计算了激光支持爆轰波的压力和温度。研究结果表明:激光支持等离子体爆轰波形成初期,等离子体爆轰波发光体为球形;随着时间增加,等离子体爆轰波发光体的形状类似流星,且头部为等离子体前沿吸收层,亮度较高,而尾部等离子体温度较低,亮度较弱。等离子体爆轰波高速向激光源的方向移动,爆轰波速度高达18 km/s,温度约为107K。随着激光强度的减弱,爆轰波速度迅速按指数规律衰减,当爆轰波吸收的激光能量不能有效支持爆轰波传播时,爆轰波转变为冲击波。     

A modified PANS model for computations of unsteady turbulence cavitating flows

A modification to the PANS (partially averaged Navier-Stokes) model is proposed to simulate unsteady cavitating flows. In the model, the parameter f _k is modified to vary as a function of the ratios between the water density and the mixture density in the local flows. The objective of this study is to validate the modified model and further understand the interaction between turbulence and cavitation around a Clark-Y hydrofoil. The comparisons between the numerical and experiment results show that the modified model can be improved to predict the cavity evolution, vortex shedding frequency and the lift force fluctuating in time fairly well, as it can effectively modulate the eddy viscosity in the cavitating region and various levels of physical turbulent fluctuations are resolved. In addition, from the computational results, it is proved that cavitation phenomenon physically influences the turbulent level, especially by the vortex shedding behaviors. Also, the mean u-velocity profiles demonstrate that the attached cavity thickness can alter the local turbulent shear layer.