Transient Ignition Process Research Articles

Kinetics and hydrothermal combustion characteristics of ethanol in supercritical water

Supercritical hydrothermal combustion is a novel, clean, and efficient combustion method. In this work, a mechanism-based kinetic model of ethanol appropriate for high-pressure hydrothermal environment was developed. Combined with theoretical analysis and continuous ignition experiments, the transient ignition process, ignition temperature, and extinction temperature were discussed. A turbulent combustion model coupling detailed kinetics was constructed to analyze the co-flow diffusion hydrothermal flame. It was found that the ethanol kinetic model can well predict the reaction process, critical ignition temperatures, extinction temperatures, and diffusion combustion process of hydrothermal combustion. The ignition temperatures of 2.40–5.72 wt% ethanol ranged between 500–390 °C, and OH was a significant ignition indicator. As an auxiliary fuel, ethanol is superior to methanol. During co-flow hydrothermal combustion, the interfacial reaction between fuel and oxidant in jet core area had an important influence, and the local high-temperature flame was mainly distributed near the downstream of fuel jet.

Forced ignition of cool, warm and hot flames in a laminar non-premixed counterflow of DME versus air

Recently, there has been great interest in the ignition of cool and warm flames (controlled by low-temperature chemistry, LTC, and intermediate-temperature chemistry, ITC, respectively) as they may have significant influence on the ignition and subsequent propagation of hot flame. However, it is still unclear how the flow affects the ignition of the cool/warm flame and their transition to the hot flame in a non-premixed fuel/oxidizer system. In this study, we conduct 2D simulations on the forced ignition of cool, warm and hot flames in a laminar, axisymmetric, non-premixed counterflow of dimethyl ether (DME) versus air. The objective is to assess the effects of flow on the ignition of cool, warm, and/or hot flames and the transition among them. Different ignition energies and strain rates are considered. It is found that the transient ignition process is mainly controlled by the extinction/transition limits and the minimum ignition energies of cool, warm and hot flames. When the strain rate is lower than the transition limits of cool and warm flames, all three flame types can be directly initiated but they eventually evolve into a hot flame. At intermediate strain rate between the transient limit and the extinction limit, quasi-steady cool and warm flames can be obtained without further transition to a hot flame. When the strain rate exceeds the extinction limit of cool and warm flames, only the hot flame can be ignited. An ignition regime diagram in terms of ignition energy and strain rate is proposed, in which different regimes for cool, warm and hot flames as well as transition among them are identified. Furthermore, the ignition of cool and warm flames in a non-premixed counterflow is shown to be very sensitive to ignition position as well as strain rate and ignition energy.

Flame propagation patterns and local flame features of an annular combustor with multiple centrally staged swirling burners

In this paper, the transient ignition process of an annular combustor with 16 centrally staged swirling burners is experimentally investigated to study the mechanism of burner–burner flame propagation. The flame propagation patterns are studied by high-speed imaging. Three typical patterns of the burner–burner flame propagation are identified: the kindled-swirling pattern, entrained-swirling pattern, and sweeping pattern. The patterns are featured with different flame paths of motion. For fixed flow rates, the paths of motion are mainly determined by the overall equivalence ratio Φ. Furthermore, during the burner–burner flame propagation, the effect of the flow field on the local flame fronts is analyzed by Mie scattering and particle image velocimetry (PIV) methods. The PIV results show that the flame paths of motion are greatly influenced by the flow structure of the annular combustor. The optical diagnosis of the flame–flow interaction provides new insights into the ignition dynamics of the centrally staged annular combustor.

Experimental Study on Transient Ignition Characteristics of Acoustic Excited Methane Jet Diffusion Flames

The ignition process of fuel plays an important role in the flame development and emission characteristics, which has attracted intensive attention in the combustion field. However, the transient ignition process for jet flames under acoustic excitation is rarely reported. In the current study, the effect of external acoustic excitation with different frequencies on the ignition process of methane jet diffusion flames has been studied experimentally using high-speed color and schlieren imaging systems. The fuel nozzle used in the experiment features a concentric ring structure, with fuel in the middle and air around it. The acoustic excitation was added to the air side through the loudspeaker, and the frequency of the acoustic excitation was set as 10 Hz, 30 Hz, 50 Hz and 100 Hz, respectively, while a case without external excitation was used as the control group. It is found that the periodic vortex structure propagates downstream in the flow field after acoustic excitation is added, which leads to an uneven velocity distribution in the flow field and the appearance of a local high-speed zone. The acoustic excitation of 30 Hz and 50 Hz can reduce the probability of successful ignition, which is mainly because the acoustic wave propagates in the flow field and causes drastic velocity changes near the ignition position. For the case of 100 Hz, the acoustic perturbation is confined in a small region near the nozzle exit, while the flow field velocity is slightly higher than the case without acoustic excitation.

Experimental investigation on the flame propagation pattern of a staged partially premixed annular combustor

The transient ignition process of a staged partially premixed low-emission annular combustor was investigated experimentally in this paper. This study focused on the mechanism of injector-to-injector flame propagation. For various overall equivalence ratios and bulk velocities, a side visualization of the spreading flame captured by 10 kHz high-speed imaging technology during the light-round process illustrated that flame propagation from injector to injector presents three typical modes: spanwise-upstream, archlike-entrainment, and spanwise-entrainment, instead of following a purely azimuthal direction. The detailed processes of flame propagation with the various modes were analysed based on the flow field and spray patterns. Three schematic diagrams for the injector-to-injector flame propagation modes were established. The injector-to-injector flame propagation mode is dependent on the bulk velocity and equivalence ratio, which have a critical impact on the flow field and spray pattern as well as the subsequent flame morphology of the dome.

An Optimization and Parametric Study of a Schlieren Motion Estimation Method

Schlieren imaging is a widely used technique for flow visualization in turbulence and combustion investigations due to its high sensitivity, flexibility and easiness in use. With the development of digital imaging and image processing techniques, it is possible to retrieve velocity measurements using time-resolved schlieren imaging sequences. In this paper, an optimization and parametric study has been conducted on a newly proposed schlieren motion estimation (SME) algorithm, based on the high speed schlieren images of a jet flow and the transient ignition process of impinging flames. The SME algorithm is optimized using a graduated non-convexity (GNC) computing scheme, which employs a three stage strategy by linearly combining a convex quadratic function and a slightly non-convex generalized Charbonnier function. The Euler–Lagrange equations have been derived, while the penalty function was separated so that penalty functions can be changed conveniently. Parametric investigations have been conducted to discuss the influence of weight parameters, while the suitable ranges have been obtained after intensive calculations. Comprehensive comparisons have been made between the SME and GNC-SME methods, which indicates that the GNC scheme can preserve the boundary well and avoid local divergence and over-smoothness at the same time. The suitable weight parameter range is also broadened by using the GNC technique. The better robustness of GNC-SME method makes it more adaptive to various applications.

Experimental investigation of transient ignition dynamics of hydrogen enriched methane diffusion impinging flames

The ignition characteristics of hydrogen enriched methane diffusion impinging flames are investigated experimentally. Keeping the total fuel flow rate constant, the hydrogen volume percentage is set at 40% and 60% respectively, while a pure methane flame is also involved for comparison. Two traditional optical diagnostics, high speed colour and schlieren imaging techniques have been used to tackle the fast transient ignition process with advanced digital image processing algorithms. The weak blue flame during the ignition process, which is hardly seen on the original high speed images, has been enhanced selectively to reveal the complete flame structure. The flow field velocity during the ignition process has been estimated using an optimised schlieren motion estimation method. The relative velocity between the flame and flow velocities has also been resolved and analysed. The results indicate that the flow field velocity increases with increasing hydrogen percentage. The maximum velocity detected in the test cases can be as high as over 3.5 m/s when the hydrogen volume percentage is at 60%. The techniques used in the current study are simple to set up and convenient to use, which have shown enormous potential to be applied in the experimental investigations into more complex combustion configurations.

Detailed measurements of transient two-stage ignition and combustion processes in high-pressure spray flames using simultaneous high-speed formaldehyde PLIF and schlieren imaging

This study investigates the low- and high-temperature ignition and combustion processes in a high-pressure spray flame of n-dodecane using simultaneous 50-kHz formaldehyde (HCHO) planar laser-induced fluorescence (PLIF) and 100-kHz schlieren imaging. The PLIF measurements were facilitated through the use of a pulse-burst-mode Nd:YAG laser, producing a 355-nm pulse-train with 300 pulses at 70 mJ/pulse, separated by 20-µs, in a 6-ms burst. The high-speed HCHO PLIF signal was imaged using a non-intensified CMOS camera with dynamic background emission correction. The acquisition rate of this HCHO PLIF diagnostic is unique to the research community, and when combined with high-speed schlieren imaging, provides unprecedented opportunity for analysis of the spatiotemporal evolution of fuel jet penetration and low- and high-temperature ignition processes relevant to internal combustion engine conditions. The present experiments are conducted in the Sandia constant-volume preburn vessel equipped with a new Spray A injector. The influences of ambient conditions are examined on the ignition delay times of the two-stage ignition events, HCHO structures, and lift-off length values. Consistent with past studies of traditional Spray A flames, the formation of HCHO is first observed in the jet peripheries where the equivalence ratio (Φ) is expected to be leaner and hotter and then grows in size and in intensity downstream into the jet core where Φ is expected to be richer and colder. The measurements demonstrate that the formation and propagation of HCHO from the leaner to richer region leads to high-temperature ignition events, supporting the identification of a phenomenon coined “cool-flame wave propagation” during the transient ignition process. Subsequent high-temperature ignition is found to consume the previously formed HCHO in the jet head, while the formation of HCHO persists in the fuel-rich zone near the flame base over the entire combustion period.

Ignition of dimethyl ether/air mixtures by hot particles: Impact of low temperature chemical reactions

Understanding and characterizing ignition of flammable mixtures by hot particles is important for assessing and reducing the risk of accidental ignition and explosion in industry and aviation. Recently, many studies have been conducted for ignition of gaseous mixtures by hot particles. However, the effects of low-temperature chemistry (LTC) on ignition by hot particles received little attention. LTC plays an important role in the ignition of most hydrocarbon fuels and may induce cool flames. The present study aims to numerically assess the effects of LTC on ignition by the hot particles. We consider the transient ignition processes induced by a hot spherical particle in quiescent and flowing stoichiometric dimethyl ether/air mixtures. 1D and 2D simulations, respectively, are conducted for the ignition process by hot-particles in quiescent and flowing mixtures. A detailed kinetic model including both LTC and high-temperature chemistry (HTC) is used in simulations. The results exhibit a premixed cool flame to be first initiated by the hot particle. Then a double-flame structure with both premixed cool and hot flames is observed at certain conditions. At zero or low inlet flow velocities, the hot flame catches up and merges with the leading cool flame. At high inlet flow velocities, the hot flame cannot be initiated due to the short residence time and large convective loss of heat and radicals. Comparing the results with and without considering LTC confirms that LTC accelerates substantially ignition via HTC in a certain range of hot particle temperatures. The mechanism of ignition promotion by LTC is interpreted by analyzing the radical pool produced by the LTC and HTC surrounding the hot particle. Moreover, the influence of inlet flow velocity on ignition by hot particles is assessed. Non-monotonic change of ignition delay time with flow velocity is observed and discussed.

Model for transient n-heptane droplet ignition at elevated pressure

The present work deals with transient droplet evaporation and ignition process of an isolated n-heptane droplet at high pressure. The vapor–liquid equilibrium has been described by Peng–Robinson equation of state. In present numerical model, the effects due to elevated pressure, inert species mixing in the liquid phase and thermo-physical properties are considered as variable. The evaporation model has been comprehensively validated with experimental results available in the literature. For the present investigation, the ambient pressure has been varied from $$5$$ to $$80\;{\text{bar}}$$ and at constant ambient temperature of $$1000\;{\text{K}}$$ . The range of diameter was varied from $$0.5$$ to $$2.0\;{\text{mm}}$$ . The ignition delay was found to be strong functions of temperature, pressure and diameter. There is monotonic decrease in the ignition time due to increasing ambient temperature and pressure. At each pressure, there existed a minimum diameter below which ignition does not takes place and this minimum diameter decreases with increase in pressure.

Investigations of autoignition and propagation of supersonic ethylene flames stabilized by a cavity

Two analysis methods for time scale and energy balance relevant to flame ignition and stabilization in cavity-stabilized flames are developed. The interaction time of hot product in the recirculation zone of the cavity with the surrounding unburned mixture and the reaction induction time of the mixture are estimated in the time scale method. The energy release from chemical reactions and the energy loss due to species exchange in the recirculation zone are included in the energy balance method. The autoignition and propagation of supersonic ethylene flames in a model supersonic combustor with a cavity is investigated first using highly resolved large eddy simulation. The evolutions of the two time scales are then calculated in the ignition process of the supersonic ethylene flames. It is found that the time scale theory is well valid in the flame propagation and stabilization stages. The rates of energy generation and loss are then analyzed in the cavity. It is found that initially the local energy generation rate is relatively small, resulting in slow net energy accumulation in the cavity. Then the energy generation increases due to the intermittent flame propagation in the cavity, whereas the energy loss oscillates consistently since the burned gas leaves the cavity. Also, energy generation and loss are generally balanced in the cavity and all tend to zero after the flame is globally stabilized. The two methods present the characteristic time scales and energy balancing during the transient ignition process for the first time.

Ignition of a sequential combustor: Evidence of flame propagation in the autoignitable mixture

Ignition of the second stage in a lab-scale sequential combustor is investigated experimentally. A fuel mixing section between jet-in-cross-flow injection and the second stage chamber allows the fuel and vitiated, hot cross-flow to partially mix upstream of the main heat release zone. The focus of the present work is on the transient ignition process leading to a stable flame in the second stage. High-speed OH-PLIF as well as OH chemiluminescence imaging is applied to obtain complementary planar and line-of-sight integrated information on the ignition. We find experimental evidence for the co-existence of two regimes dominating the chamber ignition, i.e. autoignition and flame propagation. As the mass flow of the dilution air injected downstream of the first stage is increased (i.e. mixing temperatures in the fuel mixing section are decreased), we transition from an autoignition to a flame propagation dominated regime. Hysteresis in the ignition behavior is observed indicating that the first stage in a sequential combustor may be operated at leaner conditions than required for ignition of the second stage. The time traces of integral heat release obtained simultaneously with a photomultiplier tube show distinct features depending on the dominating regime, which is important for high-pressure testing with limited optical access.

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.

Spatially and temporally resolved regression rate measurements for the combustion of paraffin wax for hybrid rocket motor applications

Spatially and temporally resolved regression rate measurements were obtained for paraffin wax combustion in a two-dimensional slab-type rocket motor configuration using an optically accessible chamber. The use of high-intensity backlighting and a one-dimensional gray moment edge detection allowed for sub-pixel accuracy for the regression rate of the fuel-grain surface. At higher oxidizer (gaseous oxygen) mass fluxes approximately 50% of the fuel grain surface experienced a relatively uniform regression rate (±15%) and an increasing trend with streamwise position. For lower mass fluxes more scatter was observed and the average regression rate decreased slightly along the length of the grain. The spatially averaged regression rate was found to be constant after approximately two seconds of an ignition transient. A novel method of removing the ignition wires at the slab's leading edge to avoid artificial flow obstruction was demonstrated. Visualization of the entire transient ignition process as well as flame propagation along the slab length was obtained. The streamwise pressure gradient of the gaseous oxidizer was controlled by modifying the combustor cross-sectional area. Favorable pressure gradients were correlated to an amplification of the regression rate. Increased convective heat transfer associated with a low shape factor typical of a favorable pressure gradient boundary layer was proposed as the mechanism responsible for the increased regression rate. Flame progression rate was measured for various flow conditions and was shown to increase with increasing axial mass flux.

Experimental investigation on laser-induced plasma ignition of hydrocarbon fuel in scramjet engine at takeover flight conditions

Laser-induced plasma ignition of an ethylene fuelled cavity is successfully conducted in a model scramjet engine combustor with dual cavities. The simulated flight condition corresponds to takeover flight Mach 4, with isolator entrance Mach number of 2.1, the total pressure of 0.65 MPa and stagnation temperature of 947 K. Ethylene is injected 35 mm upstream of cavity flameholder from four orifices with 2-mm-diameter. The 1064 nm laser beam, from a Q-switched Nd:YAG laser source running at 10 Hz and 940 mJ per pulse, is focused into cavity for ignition. High speed photography is used to capture the transient ignition process. The laser-induced gas breakdown, flame kernel generation and propagation are all recorded and ensuing stable supersonic combustion is established in cavity. The highly ionized plasma zone is almost round at starting, and then the surface of the flame kernel is wrinkled severely in 150 μs after the laser pulse due to the strong turbulence flow in cavity. The flame kernel is found rotating anti-clockwise and gradually moves upstream as the entrainment of circulation flow in cavity. The flame is stabilized at the corner of the cavity for about 200 μs, and then spreads from leading edge to trailing edge via the under part of shear layer to fully fill the entire cavity. The corner recirculation zone of cavity is of great importance for flame spreading. Eventually, a cavity shear-layer stabilized combustion is established in the supersonic flow roughly 2.9 ms after the laser pulse. Both the temporal evolution of normalized chemiluminescence intensity and normalized flame area show that the entire ignition process can be divided into four stages, which are referred as turbulent dissipation stage, combustion enhancement stage, reverting stage and combustion stabilization stage. The results show promising potentials of laser induced plasma for ignition in real scramjets.

Experimental Research of The Pulse II Ignition Impact Process of Double Pulse Motor

Based on the pulse II transient ignition process of double pulse motor, in this paper, the cooling gas impact experiment platform was used to study the transient impact of the pulse II ignition pressure peak on the double pulse motor. The cooling gas impact experiment platform of double pulse motor and two different types of test motor were designed and manufactured. The test was carried out under different conditions such as the pressure of gas storage tank and the number of interstage hole. The motion law of the plugs in the pulse I combustion chamber are found out by high speed imaging technology. The results show that the blowout pattern and the motion law of the plug under the pressure (0.5MPa, 1MPa, 1.5MPa) of gas storage tank.

Investigation of transient ignition process in a cavity based scramjet combustor using combined ethylene injectors

Large Eddy Simulation (LES) and experiment were employed to investigate the transient ignition and flame propagation process in a rearwall-expansion cavity scramjet combustor using combined fuel injection schemes. The compressible supersonic solver and three ethylene combustion mechanisms were first validated against experimental data and results show in reasonably good agreement. Fuel injection scheme combining transverse and direct injectors in the cavity provides a benefit mixture distribution and could achieve a successful ignition. Four stages are illustrated in detail from both experiment and LES. After forced ignition in the cavity, initial flame kernel propagates upstream towards the cavity front edge and ignites the mixture, which acts as a continuous pilot flame, and then propagates downstream along the cavity shear layer rapidly to the combustor exit. Cavity shear layer flame stabilization mode can be concluded from the heat release rate and local high temperature distribution during the combustion process.

On the influence of modelling choices on combustion in narrow channels

This paper examines the effect of modelling choices on the numerical simulation of premixed methane/air combustion in narrow channels. Knowledge on standard and well-accepted numerical methods in literature are collected in a cohesive document. The less well-established modelling choices have been thoroughly evaluated and discussed. A systematic method of computing the grid convergence index (GCI) has been presented for refining the computational grid. Two types of inflow boundary conditions have been tested and compared in terms of their wave-damping characteristics. The effect of different reaction schemes on simulation results have been examined and an appropriate mechanism (DRM-19) has been selected. Various types of ignition strategies to initiate the flame have been tested and compared. The transient ignition process which has not been discussed extensively in existing literature has been quantitatively described in this paper.

Investigation of transient ignition processes in a model scramjet pilot cavity using simultaneous 100 kHz formaldehyde planar laser-induced fluorescence and CH* chemiluminescence imaging

Ignition processes in scramjet pilot cavities are highly transient events that are influenced by factors including freestream Mach number and inlet geometry, turbulence intensity, cavity geometry, ignition source, and fueling composition and flow rate. In particular, the location of the flame kernel and associated propagation rate of the flame front throughout the cavity can significantly influence the end state of the ignition process. In this work formaldehyde (CH2O) was used as a flame marker to track ignition progress in a plane throughout the span-wise width of the cavity, while chemiluminescence imaging provided path-integrated flame location along the span-wise and axial directions. Planar laser-induced fluorescence (PLIF) excitation utilized the 355nm frequency-tripled output of an Nd:YAG burst-mode laser operating at 50–100kHz over 10ms with available pulse energy up to 80mJ. Simultaneous CH* chemiluminescence imaging from the top of the cavity was obtained with a high-speed complementary metal-oxide semiconductor camera. A freestream Mach number of 2 with ethylene fuel rates from 55–90 standard liters per minute were examined along with two different ignition sources: a spark discharge and pulse detonator. The resulting formaldehyde PLIF and chemiluminescence images indicate a strong correlation between fueling rate and the delay between the onset of ignition and stable combustion. More importantly, the span-wise propagation rate and structure of the flame front is highly dependent on the fueling rate, burning region, and ignition source.

Effects of pre-filled gas and spraying condition on ignition of oxygen/kerosene spray combustion

The transient characteristics of kerosene/oxygen spray were investigated using a windowed rocket combustor. In order to investigate the effects of ignition conditions, viz. aerodynamic Weber number and the oxygen fraction of pre-filled gas, on GOx/kerosene ignition, the ignition process was visualized directly using the shadowgraph technique. In addition, the dynamic pressure and global OH* chemiluminescence were measured simultaneously. The transient ignition process was significantly changed by the type of pre-filled gas (nitrogen or oxygen). The nitrogen-filled ignition condition resulted in smooth ignition, while the oxygen-filled ignition condition induced two-step ignition following the sequence of ignition with high-pressure peak, temporary extinction, and re-ignition. The ignition process was less affected by the spraying condition (Wea), except that the change of the spraying condition influenced ignition to steady state transition, such as a combustion chamber pressure overshoot. It was revealed that the oxygen-filled ignition condition caused drastic energy generation at the moment of ignition, producing a high-pressure peak and causing reverse flow of combustion gas through the gaseous oxidizer injector. This resulted in temporary extinction of the initial flame followed by re-ignition. Once the two-step ignition phenomenon occurred, the combustion chamber pressure did not increase to its steady state value during the entire run time (1.5s). As the oxygen fraction in the pre-filled gas increased because of the increase in oxygen pre-injection time, the ignition delay time decreased and the peak value of global OH* intensity increased exponentially. When the oxygen pre-injection time became more than 14ms, the ignition process transformed from smooth ignition to two-step ignition, and the peak value of global OH* intensity began increasing, the ignition process was converted from the smooth ignition to the two step ignition.