Ignition Distance Research Articles

Experimental investigation of flame propagation characteristics in the in-line crimped-ribbon flame arrester

An experimental system that consisted of gas mixing equipment, a sensor detection system, a data acquisition device, and an electric spark ignition device was set up to investigate fuel/air deflagration flame propagation and quenching processes through a crimped-ribbon flame arrester in an enclosed horizontal pipe. Deflagration suppression experiments showed that when the concentration of flammable gas was close to the stoichiometric ratio, the evolution processes of explosion pressure for the propane-air and ethylene-air premixed gases in the pipe diameter (DN32–DN400) were similar and could be divided into four stages: isobaric combustion, slow pressure rise, quick pressure rise, and pressure oscillation. However, the explosion duration of the hydrogen-air premixed gas was relatively short, and the peak explosion pressure was high. The pressure rose quickly after the isobaric combustion stage. Therefore, the process can be divided into three stages in the pipe diameter (DN15–DN150). Deflagration speed results indicated that the propane-air flame speed initially increased and eventually decreased along with increases in the pipe diameter (DN32–DN400); however, the ethylene-air flame speed gradually increased with the increase of the pipe diameter (DN80–DN400). No notable pattern of change in the hydrogen-air flame speed was observed in the pipe diameter (DN15–DN150). The maximum propane-air flame speed occurred at 5% concentration. The maximum flame speed for ethylene-air and hydrogen-air happened when the mixture was close to stoichiometric ratio. Under the conditions of the same size of experimental tube configuration and the same ignition distance but different pipe lengths, or the same pipe length but different ignition distances, experimental results showed that the flame arrester successfully stopped the flames at high flame speed and low explosion pressure, but failed at low flame speed and high explosion pressure.

Effects of tertiary air damper opening on flow, combustion and hopper near-wall temperature of a 600 MWe down-fired boiler with improved multiple-injection multiple-staging technology

In consideration of increasing the tertiary air damper opening of a 600 MWe down-fired boiler with prior multiple-injection multiple-staging technology facilitated the coal burnout, while largely increasing the NOx emissions. Additionally, the flame kernel was greatly moved downwards, thus causing significant temperature variations in the hopper near-wall region and the water wall in the lower furnace was vulnerable to overheating. This work concentrated on the comprehensively improved multiple-injection multiple-staging technology, both 1:20 scale cold aerodynamic tests and industrial experiments were conducted to examine the effects of tertiary air damper opening on flow, combustion, NOx emissions and especially the hopper near-wall temperatures. The aerodynamic tests indicated that, on increasing the tertiary air damper opening from 40 to 70%, all the flow fields exhibit good symmetry. The tertiary air flows downwards along the hopper near-wall region, with a maximum near-wall dimensionless vertical velocity significantly increasing from 0.48 to 0.66, and accordingly, the dimensionless depth of downward airflow increases from 0.744 to 0.846. The industrial experimental results showed that, upon introducing more tertiary air, the ignition distance of fuel-rich coal/air flow shortens from 1.25 to 0.87 m. The coal burnout is enhanced, carbon in fly ash drops from 6.90 to 6.15%, while the NOx emissions slightly increase from 593 to 641 mg/m3 at 6% O2. On reducing the measuring height of hopper near-wall temperature from 9.1 to 3.3 m, the average heating rate increases from 0.44 to 0.63 °C/mm. The increased tertiary air damper opening presents an increasingly obvious cooling effect on the hopper near-wall region, with the temperature reductions around 50 °C, which is conductive to protect the water wall in the lower furnace from overheating.

Homogeneous ignition during fuel-rich H2/O2/N2 combustion in platinum-coated channels at elevated pressures

The hetero-/homogeneous combustion of fuel-rich H2/O2/N2 mixtures (equivalence ratios φ = 2.5-6.5) was investigated experimentally and numerically in a platinum-coated channel at pressures p = 1-14bar. One-dimensional Raman measurements of major gas-phase species concentrations over the catalyst boundary layer assessed the heterogeneous combustion processes, while planar laser induced fluorescence (LIF) of OH at pressures below ∼5bar and of hot-O2 at pressures above ∼5bar (wherein OH-LIF was not applicable) determined the onset of homogeneous ignition. Simulations were carried out using a 2-D code with detailed hetero-/homogeneous chemical reaction schemes and transport. Both Raman measurements and numerical simulations attested a transport-limited catalytic conversion of the deficient O2 reactant over the gas-phase induction zones. The agreement between measured and predicted homogeneous ignition distances was better than 12%, thus establishing the aptness of the employed hetero-/homogeneous chemical reaction mechanisms. Analytical homogeneous ignition criteria revealed that the catalytic reaction pathway introduced a scaling factor 1/p to the homogeneous ignition distances. This outcome, in conjunction with the intricate pressure dependence of the gaseous ignition chemistry of hydrogen, yielded shorter homogeneous ignition distances at 14bar compared to 1bar. The practical implication for gas turbine burners utilizing the catalytic-rich/gaseous-lean combustion concept was that the high operating pressures of such systems promoted the onset of homogeneous ignition within the catalytic module. Sensitivity analysis has finally identified the key catalytic and gaseous reactions affecting homogeneous ignition.

Experimental investigation on non-premixed methane/air combustion in Y-shaped meso-scale combustors with/without fibrous porous media

Diffusion methane-air flames in Y-shaped meso-scale combustors of three different diameters (i.e., d=4, 5 and 6mm) were experimentally studied. The impact of adding fibrous porous media on mixture ignitability, flame stability and flammable range were systematically investigated. Without porous media, the mixture can only be ignited at positions farther than a certain distance, which was defined as ignition distance in this paper. The ignition distance increases with an increasing inlet velocity and decreases with a decreasing channel diameter. The flame cannot be stabilized and propagates downstream with an inclination angle. The propagation velocity increases with the increase of inlet velocity or with the decrease of channel diameter. With the addition of porous media, the mixture can be ignited near the splitter due to improved methane/air mixing. Moreover, the flames become more stable and stationary flames were observed under lower velocities in the combustors of d=5mm and 6mm with fibrous porous media. Furthermore, the application of porous media can significantly expand the flammable range. In summary, the combustion performances can be notably improved by the application of fibrous porous media.

Large-activation-energy analysis of gaseous reacting flow in pipes

This paper analyzes the exothermic reaction of an initially cold gaseous mixture flowing with a moderately large Reynolds number along a cylindrical pipe with constant wall temperature. An overall irreversible reaction with an Arrhenius rate having a large activation energy is used for the chemistry description. The flow is chemically frozen in the cold entrance region, where the velocity evolves towards the Poiseuille profile as the gas temperature increases towards the wall value, ushering in a reaction stage during which the rate of heat transfer from the wall changes from positive to negative. The subsequent downstream evolution of the flow depends critically on the competition between the heat released by the chemical reaction and the heat-conduction losses to the wall, as measured by the Damköhler number δ, first introduced by Frank-Kamenetskii in his seminal analysis of thermal explosions in cylindrical vessels. For values of δ below the critical value δ=2 corresponding to the quasi-steady explosion limit, heat losses to the wall keep the gas temperature close to the wall value, so that the chemical reaction occurs slowly along the pipe in a flameless mode, which is analyzed to give an implicit expression for the streamwise reactant distribution. By way of contrast, for δ > 2 the slow reaction rates occur only in an initial ignition region, which ends abruptly when very large reaction rates cause a temperature runaway, or thermal explosion, at a well-defined location, whose computation must account for the temperature found at the end of the entrance region. The predictions of the large-activation-energy analyses, including ignition distances for δ > 2 and flameless reactant consumption rates for δ ≤ 2, show good agreement with numerical computations of the reactive pipe flow for finite values of the activation energy.

Anthracite combustion characteristics and NOx formation of a 300 MWe down-fired boiler with swirl burners at different loads after the implementation of a new combustion system

A new combustion system has been applied to a 300MWe down-fired boiler with swirl burners to reduce NOx emissions. The unit provided the introduction of overfire air (OFA) and a decrease in the flow area of the inner and outer secondary air ducts of the swirl burners. Industrial experiments on the retrofitted boiler were performed at different loads. Full-scale measurements of the flue gas temperature distribution in the burner outlet region, the furnace temperature distributions measured by a pyrometer and the local mean gas species concentrations in the region near the sidewall were made at loads of 180, 250, and 300MWe. The results show that the ignition distance increased with decreasing load, especially as the load decreased from 250MWe to 180MWe. At three different loads, the retrofitted coal/air flow could be all ignited in time at a distance in the range of 0.6–1.4m from the burner outlet. Compared with the original combustion system, the ignition distance of the coal/air flow was significantly reduced at a load of 300MWe. In addition, at a load of 300MWe, the temperature of the boiler hopper was much higher than that at loads of 180MWe and 250MWe. Compared with the original combustion system, the upper furnace temperature decreased slowly with increasing measurement height at a load of 300MWe after the retrofit. Measurements of the O2 and CO concentrations in the region near the sidewall indicate that the fullness degree of the coal flame in the furnace at different loads was different. After the retrofit, the reheat steam temperatures reached the design temperature of 541°C at loads of 180MWe and 250MWe, and the average reheat steam temperatures increased by approximately 13°C. Compared with the original combustion system, a significant NOx reduction (more than 40%) at different loads was achieved without increasing the levels of unburnt carbon in the fly ash.

Ignition and combustion characteristics of n-pentanol–diesel blends in a constant volume chamber

Pentanol is considered as one of the most promising alternative biofuels due to its excellent physicochemical properties. The objective of this work was to compare the ignition and combustion characteristics of different n-pentanol/diesel blends in an optical constant volume combustion chamber. The tested fuels included 20% (D80P20) and 40% (D60P40) of n-pentanol blended with diesel in volume, and pure diesel (D100). Broadband chemiluminescence technique was used to measure the timing and location of spray ignition. A high-speed CCD camera with two ND8 dimmer lenses was used to capture the incandescence radiated from the soot particles during combustion. A wide range of experimental conditions was investigated. The ambient temperature ranged from 800K to 1200K and the oxygen concentration ranging from 10% to 21%, covering both the conventional and low temperature combustion regimes. The results show that pure diesel has shorter ignition delay and distance comparing to pentanol blends. A larger blending proportion of pentanol D60P40 advances the ignition phase more than the D80P20 in low oxygen concentration conditions. Due to the fuel-borne oxygen and the dilution effect, the natural flame luminosity is reduced significantly with the increase of pentanol ratio in most conditions except under the intermediate temperature region of 1000K. In that condition, the shorter ignition delay and flame lift-off length of pentanol blends cause a slightly increase in the natural flame luminosity. The natural flame luminosity images showed that the oxygen-contained structure of pentanol could accelerate soot oxidation under all conditions. This indicates that pentanol blends could decrease final soot emissions in internal combustion engines.

Ignition and flame characteristics of cryogenic hydrogen releases

In this work, under-expanded cryogenic hydrogen jets were investigated experimentally for their ignition and flame characteristics. The test facility described herein, was designed and constructed to release hydrogen at a constant temperature and pressure, to study the dispersion and thermo-physical properties of cryogenic hydrogen releases and flames. In this study, a non-intrusive laser spark focused on the jet axis was used to measure the maximum ignition distance. The radiative power emitted by the corresponding jet flames was also measured for a range of release scenarios from 37 K to 295 K, 2–6 barabs through nozzles with diameters from 0.75 to 1.25 mm. The maximum ignition distance scales linearly with the effective jet diameter (which scales as the square root of the stagnant fluid density). A 1-dimensional (stream-wise) cryogenic hydrogen release model developed previously at Sandia National Laboratories (although this model is not yet validated for cryogenic hydrogen) was exercised to predict that the mean mole fraction at the maximum ignition distance is approximately 0.14, and is not dependent on the release conditions. The flame length and width were extracted from visible and infra-red flame images for several test cases. The flame length and width both scale as the square root of jet exit Reynolds number, as reported in the literature for flames from atmospheric temperature hydrogen. As shown in previous studies for ignited atmospheric temperature hydrogen, the radiative power from the jet flames of cold hydrogen scales as a logarithmic function of the global flame residence time. The radiative heat flux from jet flames of cold hydrogen is higher than the jet flames of atmospheric temperature hydrogen, for a given mass flow rate, due to the lower choked flow velocity of low-temperature hydrogen. This study provides critical information with regard to the development of models to inform the safety codes and standards of hydrogen infrastructure.

Effect of different inner secondary-air vane angles on combustion characteristics of primary combustion zone for a down-fired 300-MWe utility boiler with overfire air

To achieve significant reductions in NOx emissions without increasing the levels of unburnt carbon in the fly ash, a new combustion system was applied to a 300-MWe Babcock & Wilcox (B&W) down-fired boiler installed with swirl burners. The unit featured introduced overfire air (OFA) and decreased outlet area of the inner and outer secondary-air ducts of the swirl burners. Full-scale measurements (adjusting the inner secondary-air vane angle to 35°, 45°, and 55°) revealed that the influence of the high-temperature recirculating region under the arch upon the combustion and NOx emission characteristics of the boiler is greater than that of the high-temperature flue gas entrained by the swirl burner itself. The ignition distance of the coal/air flow is reduced by at least 1.8m compared with that of the original combustion system. For the inner secondary-air vane angle of 35°, the coal/air flow ignites earlier than for the vane angles of 45° and 55°. The measurements of the gas species concentrations in the zone near the sidewall indicates that at inspection port 1, the coal flame of the burners does not spread across the entire furnace cross-section for all three vane angles; however, for the vane angle of 35° the flame is spread across the entire furnace cross-section at inspection port 2. For this optimal (35°) inner secondary-air vane angle, the NOx emissions and carbon content in the fly ash reached levels of 674mg/m3 (6% O2), and 11.4%, respectively, achieving a significant NOx reduction of 51.9% without increasing the levels of unburnt carbon in the fly ash.

Hetero-/homogeneous combustion of fuel-lean methane/oxygen/nitrogen mixtures over rhodium at pressures up to 12 bar

The hetero-/homogeneous combustion of CH4/O2/N2 mixtures over rhodium was investigated experimentally and numerically at fuel-lean equivalence ratios φ=0.30–0.40, pressures 2–12bar, and catalyst temperatures 700–1250K. Experiments were performed in an optically accessible channel-flow reactor coated with rhodium and included in situ Raman measurements of major gas-phase species concentrations across the channel boundary layer for evaluating the catalytic processes, and planar laser induced fluorescence (LIF) of the OH radical for assessing homogeneous combustion. Computations were carried out with a 2-D elliptic code, using detailed heterogeneous and homogeneous chemical reaction mechanisms. Comparisons between Raman-measured and numerically-predicted transverse profiles of the limiting reactant (methane) mole fractions have led to the evaluation of the performance of detailed surface reaction mechanisms and to the construction of a one-step catalytic reaction suitable for pressures up to 12bar. For the investigated range 2–12bar, the catalytic reactivity of methane over rhodium exhibited an overall positive pressure dependence ∼p0.30, which was weaker compared to an earlier-reported pressure dependence of methane over platinum (∼p0.47). Comparisons of the planar OH-LIF measurements with numerical simulations employing different gas-phase reaction mechanisms demonstrated that the correct prediction of homogeneous ignition was particularly demanding at the present low equivalence ratios and moderate reactor temperatures. An elementary gaseous reaction mechanism was shown to reproduce the measured homogeneous ignition distances at all pressures, particularly when used in conjunction with the herein proposed one-step catalytic reaction.

Effect of steam on ignition of pulverized coal particles in oxy-fuel combustion in a drop tube furnace

Oxy-fuel combustion is recognized as a promising technology to reduce CO2 emissions from coal-fired power plants. Typical oxy-fuel combustion is O2/CO2 recycled combustion, in which the dilute gases are the mixture of O2 and CO2. Wet flue gas recycle combustion (O2/CO2/H2O atmospheres) should be promoted due to some merits over dry flue gas recycle combustion (O2/CO2 atmospheres). In this study the ignition of bituminous coal was studied using experimental and numerical methods under O2/CO2/H2O atmospheres. The experiments were conducted in a drop tube furnace using a high-speed camera with oxygen mole fractions of 21%, 30% and 40%. The substitution of CO2 by 5%, 10%, 20% and 30% steam were implemented to evaluate the effect of the mole fractions of steam on the ignition of coal in O2/CO2/H2O atmospheres. The experimental results reveal that the ignition distances are barely affected when low mole fraction of steam (5%, 10% and 20%) was added to O2/CO2 atmospheres. As the mole fraction of steam is increased to 30%, the ignition distance is significantly decreased. The effect of steam on the ignition of coal in O2/CO2/H2O atmospheres was particularly analyzed using a proposed CFD model. The numerical results indicate that the physical properties of steam lead to the ignition delay of coal when CO2 is replaced by steam in O2/CO2 atmospheres. The lower specific heat and higher diffusivity of steam compared to CO2 lead to the advanced ignition of coal, and the higher thermal radiation of steam leads to the ignition delay of coal. The chemical properties of steam are beneficial to the ignition. The steam shift reaction played a more important role in the advanced ignition of coal than the steam gasification reaction.

Industrial Application of an Improved Multiple Injection and Multiple Staging Combustion Technology in a 600 MWe Supercritical Down-Fired Boiler

To solve the water wall overheating in lower furnace, and further reduce NOx emissions and carbon in fly ash, continuous improvement of the previously proposed multiple injection and multiple staging combustion (MIMSC) technology lies on three aspects: (1) along the furnace arch breadth, changing the previously centralized 12 burner groups into a more uniform pattern with 24 burners; (2) increasing the mass ratio of pulverized coal in fuel-rich flow to that in fuel-lean flow from 6:4 to 9:1; (3) reducing the arch-air momentum by 23% and increasing the tertiary-air momentum by 24%. Industrial-size measurements (i.e., adjusting overfire air (OFA) damper opening of 20-70%) uncovered that, compared with the prior MIMSC technology, the ignition distance of fuel-rich coal/air flow shortened by around 1 m. The gas temperature in the lower furnace was symmetric and higher, the flame kernel moved upward and therefore made the temperature in near-wall region of furnace hopper decrease by about 400 °C, the water wall overheating disappeared completely. Under the optimal OFA damper opening (i.e, 55%), NOx emissions and carbon in fly ash attained levels of 589 mg/m(3) at 6% O2 and 6.18%, respectively, achieving NOx and carbon in fly ash significant reduction by 33% and 37%, respectively.

Experimental study on ignition behavior of pulverized coal particle clouds in a turbulent jet

Ignition behaviors of pulverized coal particle clouds in a jet with different turbulent intensity, O2 concentration and coal concentration were experimentally studied using an entrained flow reactor. Mie scattering technique and high-speed cameras were employed to record the particle motions and flame behavior. Results revealed that a cloud flame consisted of a number of parcel flames and stripe flames, which were located in the center and around boundary of the cloud flame, and resulted by the burning of the evolved volatile matter or the clusters of fine fuel particles, and the burning of the single particles respectively. As Reynolds number of the primary flow increased, the cloud flame changed from narrow and structured to wide and turbulent. At the same time, the particle dispersion became more intensive, leading to a lower flame incandescence. The increase of O2 concentration in the primary or secondary flow promoted ignition of cloud flames. For laminar cloud flame, ignition distance was more sensitive the O2 concentration in the primary flow, and a minimum value was found at certain coal concentration, but for turbulent cloud flame, ignition distance was shorter, and no obvious non-monotonic trend was found over the tested coal concentration range.

Effect of plasma on boron-based two-phase flow diffusion combustion

A parallel intake diffusion combustion physical model is designed to study the influence of plasma on the secondary combustion of boron-based gas in the after-burning chamber, with excluding mixing effects of the intake air. The flame images of the diffusion combustion of the boron-based gas in the after-burning chamber are obtained by a high-speed photographic apparatus. The diffusion combustion characteristics of the physical model and the secondary ignition distance of boron particles are analyzed. The King ignition model, finite-rate/eddy-dissipation model, particle-trajectory model, RNG k-ε model, and plasma model are adopted to simulate the influence of plasma on the diffusion combustion of boron-based two-phase flow in a certain condition. The results show that the secondary ignition distance of boron particles, which is based on the boron-based flame image, is consistent well with the numerical simulation result, which verifies the accuracy of the boron-based two-phase flow diffusion combustion numerical model and the calculation method. When the boron-based gas passes through the plasma area, the temperature of the boron particles increases while the diameter decreases significantly on their trajectory. The distribution area of the B2O3 mass fraction increases significantly, and more than 70% boron particles reach a 100% combustion efficiency before they arrive at the area of the two-thirds after-burning chamber. More heat is released by fully burning the boron particles under the influence of plasma, which results in a half increase of the central area. It can be indicated that plasma can obviously enhance the combustion process of the boron-based gas, which improves the combustion efficiency of boron particles and releases more energy.

Thermal and chemical effects of hydrogen addition on catalytic micro-combustion of methane–air

In this paper hydrogen assisted catalytic combustion of methane on rhodium is numerically modeled in steady condition. The aim of the work goes to better understand how the addition of hydrogen affects the combustion of methane–air. For this purpose, a micro flatbed channel is investigated by a three-dimensional simulation including an elementary-step surface reaction mechanism. It is clearly shown through a numerical study that appropriate hydrogen addition increases the conversion of methane and expands the lower limit of burnable equivalence ratio. In addition, the main effect of hydrogen is thermal when the mass fraction of hydrogen addition is less than 0.67%, while not only thermal effect but also chemical effect appears when the mass fraction is more than 0.67%. The sharp decreases of hydrogen fraction appear twice till hydrogen fraction increases from 0.67%. In addition, the first abrupt decline increases Rh(s) coverage to create favorable conditions for adsorption and oxidation of methane and it can suddenly reduce the ignition temperature 15 K and advance ignition distance 3%. Thanks to the second sharp decline, the adsorption–desorption equilibrium of oxygen slowly shifts towards desorption with increasing temperature to increase Rh(s) coverage.

Ignition delay time in swirling supersonic flow

Supersonic hydrogen–air cylindrical mixing layer is numerically analyzed to investigate the effect of inlet swirl on ignition delay time in scramjets. Combustion is treated using detailed chemical kinetics. One-equation turbulence model of Spalart and Allmaras is chosen to study the problem and advection upstream splitting method is used as computational scheme. The results show that swirling both fuel and oxidizer streams may drastically decrease the ignition distance in supersonic combustion, unlike using the swirl just in fuel stream which has no helpful effect.

Ignition time of hydrogen–air diffusion flames

The ignition time of hydrogen–air diffusion flames is a quantity of utmost interest in a large number of applications, with implications regarding the viability of supersonic combustion and the safe operation of gas turbines. The underlying chemistry and the associated ignition history are very different depending on the initial temperature and pressure. This article addresses conditions that place the system above the so-called second explosion limit, as is typically the case in SCRAMJET operation, so that a branched-chain explosion characterizes the ignition process. The roles of local radical accumulation, molecular transport, and chemical reaction in nonpremixed ignition are clarified by considering the temporal evolution of an unstrained mixing layer formed between two semi-infinite spaces of hydrogen and air. The problem is formulated in terms of a radical-pool mass fraction, whose evolution in time is studied with a WKB expansion that exploits the disparity of chemical time scales present in the problem, leading to an explicit expression for the ignition time. The applicability of the analytical results for obtaining predictions of ignition distances in supersonic-combustion applications is also considered.

Experimental study of oscillations in a scramjet combustor with cavity flameholders

Pressure and flame oscillations in a scramjet combustor are investigated experimentally. In cold flows, cavity oscillations appear to be dominated by Rossiter mode, and cavities with larger aft angle seem to exhibit pressure oscillations of higher frequency and intensity. When combustion occurs, both the frequency and intensity of the pressure oscillations shift to higher levels (15–20kHz, >170dB), indicating the existence of high-frequency, strong flow and combustion oscillations. The cavity flameholder with larger aft angle tends to exhibit stronger flame oscillations as well as shorter ignition distances, indicating moderate oscillations may be beneficial to the ignition and combustion.

Variation of hydrocarbon compositions and ignition locations on the radiative flame initiation characteristics through multi-dimensional DFCD incorporated image analysis

The experimental investigation of impinging diffusion methane (CH4) and propane (C3H8) ignition-to-combustion process under varying fuel flow rates and ignition positions have been characterised through multi-dimensional Digital Flame Colour Discrimination (DFCD) incorporated image processing methods. The DFCD post-processed digital flame images revealed that a 2D bluish-green flame kernel feature, similar to the common chemiluminescence-induced colour perception associated with premixed hydrocarbon flames, dominates the initial flame kernel propagation after the onset of ignition. The nominal yellowish soot colour feature typically associated with diffusion hydrocarbon flames was observed to trail the initial flame chemiluminescence emission entity by a time delay. The truncated 1D DFCD-processed colour signals illustrated discrepancies between the CH4 and C3H8 diffusion flame establishment process. It was found that under equivalent volumetric flow conditions, changes in the ignition position do not affect the typical C3H8 flame establishment procedure within the first 0.4s of the ignition-to-combustion event. Diffusion CH4 flame, on the other hand, can be significantly influenced by ignition position; where the increase in ignition distance downstream resulted in the increase of chemiluminescence bluish-green flame distribution. This can be attributed to the enhanced local air entrainment and mixing in the downstream ignition location which effectively promotes partial mixing of reactants, and consequently reduce the local equivalence ratio states. With near-plate ignition, the diffusion CH4 flame colouration becomes completely bluish-green, which provides further credence to the elevation of local fuel/air partial premixing along with increase in downstream ignition distance. The propagation velocity of the DFCD-derived chemiluminescence-induced flame kernel for both CH4 and C3H8 ignition matched well with the propagation velocity of the initial combustion-induced density disturbance observed from the schlieren images. A constant C3H8/CH4 velocity ratio of 1.1 was observed from both the DFCD-processed and schlieren-derived results under different fuel flow rates. In addition, the DFCD processing was incorporated with stereoscopic 3D flame reconstruction technique to provide a feasible multi-dimensional data processing scope for quantifying the temporal evolution of different flame colour entities into comparable diagnostic parameters. The mapping of spatial DFCD-processed colour signal ratio onto the reconstructed 3D flame structure provided another step towards the development of a practical analytical platform for abstract local fuel/air monitoring and diagnosis.

Turbulence and additive effects on ignition delay in supersonic combustion

Numerical study of two-dimensional supersonic hydrogen–air mixing layer is performed to investigate the effects of turbulence and chemical additive on ignition distance. Chemical reaction is treated using detail kinetics. Advection upstream splitting method is used to calculate the fluxes, and one-equation turbulence model is chosen here to simulate the considered problem. Hydrogen peroxide is used as an additive and the results show that inflow turbulence and chemical additive may drastically decrease the ignition delay in supersonic combustion.