Increasing Equivalence Ratio Research Articles

OH visualization of ethylene combustion modes in the exhaust of a fundamental, supersonic combustor

Abstract This work examined combustion modes in a fundamental, axisymmetric, supersonic combustor at flight equivalent Mach numbers ranging from 7.5 to 9.0. Ethylene was injected at a variety of mass flow rates to examine both scram-mode, jet wake stabilized and dual-mode combustion. At higher flight Mach number conditions, the fuelling rate required to transition from scram-mode to dual-mode combustion increased. Distributions of the OH radical were observed using planar laser-induced fluorescence (PLIF) in a cross-sectional plane immediately downstream of the combustor exit, and analyzed according to their variation in the radial and circumferential dimensions. It was observed that radial centroids of the ring-like OH PLIF signal for the scram-mode cases approached the centerline linearly with increasing equivalence ratio, but the dual-mode cases appeared to randomly fluctuate in the observed exhaust cross-section. The distributions of scram- and dual-mode cases were clustered in specific regions on plots of circumferential variation as a function of radial centroid, with a jet wake stabilized case appearing between the two clusters. These clusters may be used to help identify scram- and dual-mode combustion in future experiments.

Evaluation of unrestricted hydrogen and hydrogen-methane explosion venting through duct

This work is aimed at evaluating unrestricted duct-vented explosion of hydrogen-air mixture and hydrogen-methane-air mixture. The flame behavior in the venting zone and explosion pressure inside explosion chamber are obtained in the experiments. On the assumption of only fresh mixture venting or only combustion product venting, a mathematical model of unrestricted duct-vented explosion is established by considering chocked venting. The results indicated that the chocked venting could be achieved for unrestricted hydrogen and hydrogen-methane explosion venting due to the fact that the Mach disc structure is appeared in venting zone. Maximum explosion pressure and maximum pressure rise rate could be reduced significantly by unrestricted duct-venting. For the unrestricted hydrogen explosion venting, maximum explosion pressure and maximum pressure rise rate increases until Φ = 1.5, then decreases monotonously with the increase of equivalence ratio. For the unrestricted hydrogen-methane explosion, maximum explosion pressure and maximum pressure rise rate continue to decrease with the increase of methane addition. Maximum explosion pressure in the experiment is between theoretical values of fresh mixture venting and combustion product venting. Maximum explosion pressure and pressure rise rate are increased with increasing burning velocity whether it is fresh mixture venting or combustion product venting. Maximum explosion pressure is increased linearly with increasing nondimensional vent coefficient.

Effect of equivalence ratio and ignition location on premixed syngas-air explosion in a half-open duct

A detailed experimental work is conducted to address the response of premixed syngas-air (H2/CO = 50%/50%) explosion with the effect of equivalence ratio and ignition location. The equivalence ratio, Φ, ranges from 0.8 to 3.0, and three different ignition locations, i.e., left closed end (Ig-1), central duct (Ig-2), and right open end (Ig-3), are considered. Results show that the flame oscillates more violently as the ignition location is set close to the open end. But for a given ignition location, the flame undergoes a similar shape change as the equivalence ratio increases. The pressure dynamics suggest that the pressure waves do a little on the flame propagation as the flame is ignited at Ig-1. But when the flame is ignited at Ig-2 and Ig-3, both the flame tip speed and overpressure oscillate and coincide well with each other, and the pressure waves play an important role in the flame oscillation. What’s more, the parameters, including flame propagation time (t), the maximum flame propagation speed (νMax), and the maximum overpressure (PMax), are obtained. The results indicate that the equivalence ratio and the ignition location are coupled to affect the premixed syngas-air explosion, and the extremum, i.e., minimum t, maximum νMax and PMax, are attained at Φ = 1.4 for Ig-1, at Φ = 1.6 for Ig-2, and at Φ = 1.8 for Ig-3. Finally, the results suggest that the flame ignited at Ig-3 is less destructive, because it has the longest flame propagation time, but the smallest νMax and PMax.

Numerical investigation on a multi-channel micro combustor fueled with hydrogen for a micro-thermophotovoltaic system

Targeting at improving the energy output of the micro-thermophotovoltaic (MTPV) system, the heat transfer performance and energy output of the conventional micro combustor (CMC) and the multi-channel micro combustor (MCMC) with various channel shapes in the MTPV system are compared. The mean outer wall temperature of the MCMC with diamond channel is improved by 86.5 K than that of the CMC at the inlet hydrogen mass flow rate (m˙in,hydrogen) of 1.64 × 10-6 kg/s. Furthermore, four arrangements of MCMC are compared in this work. The double-layer MCMC with layers counterflow exhibits the highest total energy conversion efficiency of 13.2% when the m˙in,hydrogen is 8.10 × 10−7 kg/s. The equivalence ratio (Φ) of premixed hydrogen/air plays a crucial role on the thermal performance of DMCMC-layer counter flow. The outer wall temperature and its uniformity becomes better as the equivalence ratio increases. The MPTV system with a DMCMC-layers counterflow can obtain an energy output of 19.3 W when the m˙in,hydrogen is 1.64 × 10-6 kg/s and Φ is equal to 1. Namely, the multi-channel micro combustor has the potential to be a novel energy resource for the MTPV system.

Thermodynamic Modeling of a Turbocharged Diesel–Hydrogen Dual-Fuel Marine Engine

In this work, a thermodynamic model is used to analyze the influence of some usual engine parameters such as compression ratio, ambient temperature, turbocharger compressor pressure ratio, equivalence ratio, and engine speed on the performances of a diesel–hydrogen dual-fuel marine engine. The model takes into consideration the change in the composition of the working fluid resulting from the combustion cycle as well as the reactive mixture. This model considers also the dependence of the working fluid specific heat on temperature. Results show that the increase in the equivalence ratio yields to an improvement of the brake specific fuel consumption, the brake power output, the brake thermal, and exergy efficiencies with an increase in the exhaust gas temperature. Nevertheless, it produces a drop in volumetric efficiency. The engine speed has a slight effect on the brake thermal efficiency, exergy efficiency, and the brake specific fuel consumption. The engine speed affects the engine power output mainly at higher values of the fuel–air equivalence ratio due to the reduction in heat losses and the fuel energy consumed by the engine. The increase in the turbo-compressor pressure ratio has a significant effect on the improvement of engine performance. However, it has an insignificant effect on the exhaust gas temperature. The exergy destroyed in the engine dominates. It accounts for 88.2% of the total exergy destroyed in the overall system. The remaining components (turbo-compressor, intercooler, mixer, catalytic converter, and turbine) are responsible for only 11.2%.

Research on the operating boundary of the dual mode scramjet with a constant area combustor through thermodynamic cycle analysis

The maximum pressure ratio of the isolator is important for studying the operating boundary of the dual mode scramjet. In this paper, the pressure rise mechanism of the dual mode scramjet engine and the characteristic of the isolator maximum pressure rise are analyzed. The operating range and performance of scramjet under the pressure rise limit are investigated using the constant cross-sectional area combustor. The maximum pressure ratio of the isolator confines the back pressure from the combustor so that the scramjet engine has a minimum freestream Mach number of each equivalence ratio. When the freestream Mach number is smaller than the minimum freestream Mach number, an expansion section may be arranged between the combustor and the isolator to balance the pressure of the combustor and that of the isolator. The expression of the minimum freestream Mach number is derived and it is found that the minimum freestream Mach number of the dual mode scramjet in both modes increases with the increase of equivalence ratio and compression ratio of the inlet. A variable divergent section is suggested to balance the pressure under different freestream Mach numbers and equivalence ratios.

Ignition delay times of iso-butane activated by DME under various equivalence ratios in the shock tube

The ignition delay time of the high cetane number dimethyl ether (DME), high octane number i-butane, and their binary blends were investigated in a shock tube over the temperature range of 1091–1797 K, pressures of 2–10 atm, and equivalence ratios of 0.5–2.0. The LLNL iso-Octane Model and the DME sub-set of the Aramco3.0 Model were assembled together to reproduce the ignition delay behavior of the binary blends. Chemical kinetic analyses were conducted to understand the interplay between DME and i-butane at high temperatures. The ignition delay of DME and i-butane show the opposite equivalence ratio dependence under the current conditions. The promoting effect of DME addition on the ignition delay of i-butane becomes more significant as the equivalence ratio increases. The research result shows that there is no significant fuel to fuel interactions between DME and i-butane in the present conditions. The high reactivity of DME and its rapid accumulation of the free radicals lead to the promoted auto-ignition of i-butane.

Accelerating Laminar Flame Speed of Hydrous Ethanol via Oxygen-Rich Combustion

To investigate the laminar flame characteristics of hydrous ethanol, experiments were conducted in a constant volume combustion chamber. The laminar burning velocity and Markstein length of ethanol with water content from 0 to 20% by volume were measured over a wide range of equivalence ratios from 0.7 to 1.4 under the initial condition of 388 K and 0.1 MPa. Calculations of the laminar burning velocity with the constant pressure method (CPM) and constant volume method (CVM) were compared and discussed. Apart from experiments, numerical studies were carried out in CHEMKIN based on the Marinov’s and Olm’s ethanol oxidation mechanism. Moreover, the combustion of hydrous ethanol with 20% water by volume in oxygen-enriched air (O2 = 23% by volume) was also studied. Results indicated that the overall trends of laminar burning velocity versus the equivalent ratio were basically consistent between the experiment and the simulation, but the results obtained by CVM were larger than those obtained by CPM and closer to the simulation results. The peak value of laminar burning velocity decreased from 0.638 to 0.591 m/s when the water content increased from 0 to 20% in CVM calculation, but the peak laminar burning velocity of hydrous ethanol with 20% water in oxygen-enriched air enhanced to 0.771 m/s, meaning that the oxygen-enriched air can effectively offset the negative impacts of water. Moreover, the Markstein length of hydrous ethanol decreases with increasing equivalence ratios.

Lean-burn characteristics of a turbocharged opposed rotary piston engine fuelled with hydrogen at low engine speed conditions

Opposed rotary piston (ORP) engines have high power density and compact designs which meet the requirements of power sources of hybrid vehicles. Hydrogen applications to ORP engines can effectively decrease greenhouse gas emissions; however, hydrogen combustion in ORP engines around stoichiometric ratio generated large quantities of nitrogen oxides (NOx), especially for low engine speed conditions. Lean-burn as an effective method to decrease NOx emissions was adopted in this research. A 3D numerical simulation method was used to explore the effect of equivalence ratio (≤1) on combustion and NOx emission characteristics of this ORP engine fuelled with hydrogen. The results indicated that peak in-cylinder pressure increased with equivalence ratio for 1000 revolutions per minute (RPM); however, the value over the equivalence ratio of 0.9 was the maximum among 2000 RPM scenarios. The effect of equivalence ratio on heat release rates was greatly dependent on the engine speeds. Start of combustion over 1000 RPM engine speed was advanced with the increase of equivalence ratio; and it was the earliest over the equivalence ratio of 0.9 at 2000 RPM conditions. During the exhaust stroke, in-cylinder pressure of free discharge was much higher than atmosphere pressure, which significantly increased the pumping losses of exhaust stroke. Accumulated NOx emissions over the equivalence ratio of 0.9 reached the maximum value for given engine speeds; and the NOx emissions were almost zero for severe lean-burn (<0.7) conditions. Engine power output decreased with the drop of equivalence ratio for 1000 and 2000 RPM engine speeds; indicated thermal efficiency over the equivalence ratio of 0.9 was the maximum at 2000 RPM engine speed.

Effect of CO2 on self-acceleration properties of syngas/air turbulent premixed flame at different equivalence ratios

For studying the effect of carbon dioxide (CO2) on the self-accelerating properties of syngas/air mixture's turbulent premixed flame, an experimental study was carried out on 90%H2/10%CO/air mixture at the turbulence intensity of 1.31 m/s. The effects of various equivalence ratios (0.6, 0.8 and 1.0) and carbon dioxide contents (10%, 20% and 30%) on the flame evolution and propagation speed were studied. The effects of the CO2 contents and equivalence ratios on the self-acceleration properties in the flame propagation transition stage were evaluated. The effects of effective Lewis number and Zel'dovich number on the fractal excess and acceleration exponent were analyzed. The results showed that, as the CO2 contents increased, the development process of the flame slowed down, and the rate of growth of the flame's radius and the turbulent premixed flame propagation speed decreased at the same radius. The fractal excess and acceleration exponent increased with the increasing CO2 contents and decreased with the increasing equivalence ratio. Under different equivalence ratios and CO2 contents, the syngas/air premixed flame acceleration exponent was less than 1.5 in the transition stage.

Exhaust emission characteristics of a gardener compression ignition engine fuelled with rapeseed methyl ester and fossil diesel

This paper presents an experimental investigation into the exhaust emissions characteristics of a gardener Compression Ignition (CI) Engine fuelled with rapeseed methyl Esther (RME) and fossil diesel under lean equivalence ratios (0.2≤ φ ≤0.8). The experiments were carried out at engine speeds of 750 and 1250 rpm under five different loads. The experimental results showed that NOx and CO2 emissions increased while emissions of HC, O2 and CO decreased with increasing equivalence ratio, exhaust temperature, brake mean effective pressure and specific fuel consumption. All exhaust emissions were found to decrease with increasing engine speed from 750 to 1250 rpm. There was reduction in exhaust emissions of RME over fossil diesel by 0.06% for O2, 84% for CO and 4.7% for CO2 at 750rpm. At higher speed of 1250rpm however, RME was observed having higher NOx and CO2 but relatively lower O2 and CO than the fossil diesel.&#x0D; Keywords— Exhaust Emission, Compression ignition engine, rapeseed methyl Esther, engine speed, fossil diesel

Effects of Additional Cavity Floor Injection on the Ignition and Combustion Processes in a Mach 2 Supersonic Flow

Effects of additional cavity floor injection on the ethylene ignition and combustion processes in a cavity-based scramjet combustor are investigated experimentally in a Mach 2.0 supersonic flow using flame luminosity and CH* (CH radical) spontaneous emission methods and static pressure measurements. Numerical calculation is performed to study the non-reacting flow-field structures prior to ignition. Two injection schemes, including the cavity upstream injection scheme and the combined injection scheme with an additional cavity floor injection, are compared to study the effects of the additional cavity floor injection on the ignition and combustion processes. It is found that there exists an equivalence ratio upper limit for maintaining stable combustion for the cavity upstream injection scheme. As the equivalence ratio further increases, the fuel jet penetration is improved accordingly, and thus, the interaction between the fuel jet and the cavity is weakened, which can lead to the ignition failure and flame blowout during combustion. On the contrary, although the combined injection scheme has a minor effect on combustion enhancement at the same global equivalence ratio, it can also provide a more favorable flow-field environment that enables more successful ignitions and better flame stabilizations. For the combined injection scheme, as the equivalence ratio increases, the initial flame propagations are observed to perform different routines during the ignition process, and the major combustion reaction zone tends to move further downstream the cavity shear layer. It is concluded that the advantages of the combined injection scheme with an additional cavity floor injection are more significant when the equivalence ratio is higher, as well as that the interaction between the fuel jet and the cavity becomes weaker.

Numerical studies and validation of combustor and annular isolator interactions of hydrocarbon based axisymmetric dual combustion ramjet

Combustor Intake interactions study is very important for design of Ramjet/Scramjet propulsion systems. High Combustion-induced back pressures make the shock train propagate upstream further and interfere the flow of inlet and the scramjet would unstart. The study has more relevance especially for the propulsion systems like Dual Combustion Ramjet (DCR) due to its wider range operation of Mach number. In this paper, numerical simulation of flow field in an axisymmetric scramjet/ramjet combustor with annular isolator is carried out. Configuration chosen for simulation is Dual Combustion Ramjet, which has a dump subsonic combustor, main supersonic Combustor and an annular isolator. Objective of the work is to evolve a CFD procedure for combustor-isolator interaction and validation of the model with experimental data. Geometry of model DCR is taken from literature for simulations and studies are carried out for Isolator inlet Mach No. 1.79 and 2.23. Favre Averaged equations are solved using Commercial code–ANSYS FLUENT. Premixed composition of air and Kerosene is introduced into dump combustor. Reactive flow is modelled using 7 species and 4 step chemistry. Numerical simulations have been carried out with various turbulence models namely k–ε (Standard), (RNG) k–ε, kω-Standard, kω-SST and combustor wall pressures were compared with experimental results. Error in estimation of starting location of Shock train, Maximum Pressure, Average pressure in combustor is compared. (RNG) k–ε predicted shock train location with an error difference of 5% compared to k–ε (Standard). Further, effect on shock train position in isolator with change in gas generator equivalence ratio is also studied for free stream Mach numbers 4 and 5. Shock train moves upstream in Isolator with increase in gas generator equivalence ratio. Various parameters such as Wall static Pressure, Shock train location, length and static pressure along shock train centre, Velocity profile and Combustion efficiency are compared for reactive flow turbulence models.

An investigation of ammonia primary flame combustor concepts for emissions reduction with OH*, NH2* and NH* chemiluminescence at elevated conditions

With developing interest in NH3 as a prospective energy carrier, combustor designs and fuelling concepts require optimisation to reduce NOx emissions. Through the introduction of staged combustor concepts, pathways have previously been identified that limit NOx production whilst improving combustor efficiency and reducing unburned NH3. However, the efficacy of secondary air staging is sensitive to the primary flame behaviour, and whilst low NOx emissions can be achieved at rich conditions, high unburned NH3 leads to greater global NOx concentrations from downstream production. Here, time-resolved OH*, NH2* and NH* chemiluminescence were employed together for the first time for NH3-air and NH3H2-air flames to investigate a primary flame configuration that produced the lowest combined emissions concentration. A generic, fuel-flexible burner was developed to enable partial and full premixing, together with operation of a swirl-stabilised non-premixed flame. Initially, NH3H2-air flames were employed in a range of configurations and produced markedly different chemiluminescence and emissions results as functions of global equivalence ratio. The performance of a pure NH3-air flame was subsequently investigated and compared to the blended fuel results. Optical trends complemented changes in sampled exhaust emissions, enabling analysis of intermediate chemistry. Burner inlet temperature and pressure were then increased proportionally to maintain equivalent bulk nozzle exit velocities. Contrasting trends were identified as functions of fuel composition and equivalence ratio, with a comprehensive database of optical and analytical results generated. Results obtained for NH3H2-air suggest the most favourable configuration resulted from a partially premixed flame employing H2 as a pilot, operating under rich conditions (Φ=1.2). However, at higher temperatures and pressures, the trends observed for non-premixed NH3-air flames will lead to superior performance, particularly with a small increase in equivalence ratio.

Performance analysis of a medium-scale downdraft gasifier using Lantana camera biomass as feeding material

ABSTRACT Lantana camera , an invasive weed species, is widely available in different agro-climatic zones of India and has a huge potential in the gasification process. In the present investigation, the fuel properties of Lantana camara stems, e.g. energy value, volatile matter content, ash content, fixed carbon content and elemental detection (Carbon, Hydrogen, Nitrogen, Sulfur and Oxygen analysis) were investigated. In addition, thermo-gravimetric analysis of Lantana camara biomass at different temperatures was also carried out under atmospheric conditions. To explore the gasification characteristics of Lantana camara biomass, a downdraft biomass gasifier is designed and fabricated. The performance of the downdraft gasifier system is examined in terms of equivalence ratio, zone temperatures, heating value, gas yield, gas composition, carbon conversion efficiency, and gasifier efficiency. The gasification experiments were performed using air as an oxidizing agent. The results showed that the gas yield increases with increasing equivalence ratio and the obtained producer gas was composed of about 20–22% carbon monoxide (CO), 18–20% hydrogen (H2), 3–5% carbon dioxide (CO2), 1–4% methane (CH4), and rest nitrogen (N2). The higher heating value of producer gas achieved from 5.9 to 7 MJ/Nm3 while lower heating value varied from 5.47 to 6.42 MJ/Nm3. The maximum carbon conversion and thermal efficiency of the present biomass gasifier were observed to be around 46% and 64%, respectively.

Flame stability analysis of premixed hydrogen/air mixtures in a swirl micro-combustor

A swirl micro-combustor for premixed hydrogen/air flames is proposed to improve flame stability. The swirl micro-combustor consists of a cylindrical combustion chamber and an inlet section where a swirler with 6 straight vanes at an angle of 45° is placed in an annular inlet duct. The flame stabilization mechanism and combustion characteristics at different velocities, equivalence ratios, and solid materials are analyzed using three–dimensional numerical simulations. The results indicate that the role of the inner recirculation zone on flame anchoring is dominating while the corner recirculation zone is favorable to the preheating effect. The coupling of the heat recirculation from the solid walls and the recirculating flow in the recirculation zones contributes to the improvement of flame stability. Increasing inlet velocity weakens the preheating effect. The flame temperature, combustion intensity, and preheating energy increase with an increasing equivalence ratio. The preheating effect is enhanced by using highly thermally conductive materials, thereby facilitating the initiation of the chain reactions.

Effects of nitrogen and carbon dioxide on hydrogen explosion behaviors near suppression limit

By varying inert gas content, equivalence ratio and initial pressure, this study is aimed at investigating flame propagation behaviors and explosion pressure characteristics near suppression limit. For carbon dioxide, the weakest flame floating phenomenon is observed at Φ = 1.5 and the buoyant instability is enhanced when the equivalent ratio deviates to the rich and lean sides. For nitrogen, the buoyant instability decreases with increasing equivalent ratio. Both maximum explosion pressure and maximum pressure rise rate increase firstly and then decrease with the increase of equivalence ratio, and they decrease significantly with increasing content of carbon dioxide and nitrogen. For carbon dioxide, the critical suppression ratio of Φ = 0.6, 0.8, 1.0, 1.5 and 2.0 is 7.50, 7.18, 5.74, 3.83, and 2.87. For nitrogen, the critical suppression ratio of Φ = 0.6, 0.8, 1.0, 1.5 and 2.0 is 15.83, 11.87, 9.50, 6.33 and 4.75. Compared to nitrogen, the carbon dioxide is more effective on suppressing hydrogen explosion pressure. The adiabatic flame temperature, thermal diffusivity and mole fraction of active radicals continue to decrease with increasing content of carbon dioxide and nitrogen, which contributes to the decrease of laminar burning velocity.

OH* and CH* chemiluminescence characteristics in low swirl methane-air flames

Chemiluminescence information is of great significance for characterization of flame structure and combustion characteristics. An atmospheric low swirl burner was developed to investigate the chemiluminescence characteristics of OH* and CH* in low swirl flames, with the equivalence ratio varying from 0.8 to 1.2 and the swirl number from 0.2 to 0.6. The chemiluminescence images were captured via ICCD cameras coupled with narrow-bandpass filters, and an Abel inversion method was introduced to transform the line-of-sight-integrated image into two-dimensional radial distributions. The results show that the distribution of CH* is smaller than that of OH* and concentrated more upstream of the flame near the burner. The equivalence ratio has a relatively more direct influence on chemical reactions, while the swirl number has a more evident effect on the flame structure. As the equivalence ratio increases, the peak value of OH* and CH* increases and the peak position moves downstream of the flame, suggesting that the chemical reactions become more intense. In contrast, the height and width of chemiluminescence distribution increase linearly with increasing swirl number. Moreover, it is found that the equivalence ratio and swirl number can be feasibly estimated based on chemiluminescence measurement results, using the correlation between them derived from this study.

Effect of DME addition on flame dynamics of LPG/DME blended fuel in tail space of closed pipeline

In order to recognize and reveal the explosion characteristics and hazards of LPG blended fuel with active fuel additions, flame dynamics of liquefied petroleum (LPG)/dimethyl ether (DME) blended fuel in the tail space of a closed pipeline was experimentally investigated. A high-speed camera and a pressure transducer were used to acquire the flame structure and explosion overpressure, and the displacement and speed were further calculated by MATLAB programming. The research results show that the flame under the conditions of stoichiometric and lean-fuel concentration presents a more remarkable local suppressive propagation compared with the rich-fuel condition. In the vicinity of stoichiometric concentration, the increase of equivalence ratio leads to the increase of flame front temperature, meanwhile, the process also improves the molecular diffusion coefficient and heat transfer efficiency. The reverse propagation of flame brings obvious temperature loss to fuel system. The existence of reflected reverse flow makes it difficult for the flame to propagate to the end of the closed space. The increase of equivalence ratio and DME blended ratio is helpful for the improvement of flame suppressive mechanism and flame acceleration mechanism, and the effect of the latter is more obvious. The knowledge obtained about flame dynamic characteristics can improve the assessment and recognition of explosion hazards of blended fuel.

Cellular instabilities of n-butane/air flat flames probing by PLIF-OH and PLIF-CH2O laser diagnosis

The structure and instability characteristics of non-adiabatic fuel-rich n-butane/air cellular flames on McKenna burner were experimentally investigated at atmospheric pressure. Planar Laser Induced Fluorescence (PLIF-OH and PLIF-CH2O) flame diagnosis technology was utilized to probe the flame structure under varied equivalence ratio and inflow mixture velocity respectively. Results show that, the equivalence ratio plays an important role in forming cellular flames. The flat, wrinkled and cellular flames appear in turn when increasing equivalence ratio and inflow velocity. Differed to the separated cells of cellular flames appeared in direct Digital camera images and PLIF-OH images, the PLIF-CH2O images clearly display that all cellular flames are connected. In the PLIF-OH images of cellular flames, the OH radical fluorescence signal intensity is higher in the convex region toward unburned mixture than in the concave region, but in PLIF-CH2O images, the fluorescence signal intensity is nearly unchanged in both convex and concave regions. Quantitative stand-off distance and amplitude data probed by PLIF-OH and PLIF-CH2O diagnosis together reveal that the onset of non-adiabatic n-butane/air cellular flames on flat flame burner is dominantly governed by diffusive-thermal mechanism.