Ethylene-air Mixtures Research Articles

Turbulent flame acceleration and deflagration-to-detonation transitions in ethane–air mixture

The deflagration-to-detonation transition (DDT) poses significant risks in the oil, gas, and nuclear industries, capable of causing catastrophic explosions and extensive damage. This study addresses a critical knowledge gap in understanding the DDT of ethane–air mixtures on a large scale, amid increasing industrial utilization and production of ethane. A novel computational framework is introduced, utilizing the finite-volume code named Morris Garages, which incorporates reactive compressible Navier–Stokes equations, adaptive mesh refinement, and correlations of turbulent burning velocities. This model integrates the most recent data on laminar and turbulent burning velocities for premixed ethane–air mixtures, simulating flame acceleration and DDT within a two-dimensional large-scale setting, measuring 21 m in length and 3 m in height, with obstacles mimicking pipe congestion. Two mixture scenarios, lean and near-stoichiometric, are analyzed to evaluate the effects of equivalence ratios on flame propagation and DDT. The simulations, validated against large-scale experimental data from Shell, show reasonable agreement and provide critical insights into the onset conditions of DDT, such as temperature, pressure, flame speed, and turbulent kinetic energy. Furthermore, the ξ–ε detonation peninsula diagram is utilized to explore autoignition and detonation behaviors in ethane–air mixtures.

Laminar flame speed measurement and combustion mechanism optimization for ethylene–air mixtures

AbstractEthylene plays a crucial role as an intermediate component in the cracking and combustion processes of large molecular alkane and olefins. In this article, the laminar flame speed of ethylene–air mixtures was measured using the heat flux method. The mechanism of ethylene was simplified by utilizing the error propagation directed relationship graph (DRGEP) and sensitivity analysis (SA), and the Arrhenius pre‐exponential factors for 20 selected reactions in the skeletal mechanism were optimized using the particle swarm optimization (PSO) algorithm. Finally, an ethylene optimization mechanism including 39 species and 85 reactions was obtained. The prediction results for flame speed, ignition delay time, and species concentration were compared with experimental data and other mechanisms, covering a wide range of temperatures (298–1725 K), pressures (1–22.8 atm), and equivalence ratios (0.5–2.0). The findings demonstrate that the optimization mechanism not only improves the prediction results of laminar flame speed in the rich combustion zone and low oxygen environment but also enhances the prediction accuracy of the ignition delay time at high pressure and in the lean combustion zone, as well as the prediction accuracy of C2H4 and H2O radicals. In conclusion, the optimized mechanism exhibits higher accuracy and broader applicability.

On the NTC behaviors in explosion limits of C1 to C3 n-alkane/air mixtures

In this study, the explosion limits of methane, ethane and propane with air under various equivalence ratios were investigated. For the methane-air mixture, a Negative Temperature Coefficient (NTC) response progressively appears with an increase in equivalence ratio. The NTC response for ethane-air mixture remains relatively constant, and the explosion boundary shifts to high pressure in the high-temperature region. In propane-air mixtures, both the explosion limit curve in the NTC response section and the high-temperature section shift to high pressure, causing a double NTC response under high equivalence ratios. Detailed reaction path analysis confirmed that the primary reaction path of methyl radicals shifted from oxygen addition reactions to recombination reactions, leading to the NTC response and reduced reactivity. The decomposition of C2H5O2H promotes the reactivity of the ethane-air mixture, thereby generating the NTC response. Under high equivalence ratios, both the reaction path of the C3H7 radical via secondary oxygenation at low temperatures and the oxygen addition reactions of small alkane at high temperatures are hindered, leading to the generation of a double NTC response. Temperature and species profiles are compared under the NTC region for these fuels. These findings provide valuable insights and guidance for applications involving fuel-rich hydrocarbon combustion processes.

Effects of Damköhler Number on Lean Premixed Jet Flames in Crossflow Using Direct Numerical Simulation

ABSTRACT In the present work, lean premixed jet flames in crossflow were studied using direct numerical simulation to explore the effects of Damköhler numbers () on turbulence and flame characteristics. The configuration is based on an experiment, where a jet of lean premixed ethylene-air mixtures ( = 0.6) was injected into a hot vitiated crossflow. The case with a higher and lower Da is denoted as case Da+ and case Da-, respectively. It was found that the turbulence is weakened around the reaction zone of case . In contrast, the turbulence of case is barely influenced by heat release. The vorticity dynamics are examined with the enstrophy transport equation budget terms, and the viscous dissipation term is the main sink for both cases. The windward and leeward flame branches of case are dominated by auto-ignition and premixed flame propagation respectively, while the auto-ignition and flame propagation modes coexist on the leeward flame branch of case . The scalar gradient magnitude in the leeward branch is generally lower compared with the windward branch so that the flame is overall thicker. The normal strain rate is negative, which tends to enhance the scalar gradient, but it becomes positive on the leeward side of case , which is influenced by the alignment between the scalar gradient and the principal strain rates.

Investigation on laminar burning velocity measurements of premixed Ethane-Air mixture at higher pressure and temperature conditions

The externally heated diverging channel (EHDC) method was utilized to evaluate the laminar burning velocity (LBV) of premixed ethane-air flames at higher mixture temperature (350-620 K) and pressure (1-5 atm) conditions over different mixture strengths (ϕ = 0.7-1.3). The maximum LBV was noted at ϕ = 1.1 for all pressure and temperature conditions. The inverted parabolic behavior of the temperature exponent was obtained with the minima at ϕ = 1.1. The pressure exponent (β) exhibits a parabolic variation with its maxima at ϕ = 1.0. The current results are further analyzed and compared with the literature measurements, and the comprehensive kinetic model predictions of USC mech II, San Diego mech, and Aramco mech 1.3. The present LBV measurements match well with the kinetic model predictions of Aramco mech 1.3 at most of the mixture and pressure conditions. The present results propose the variation of pressure exponent (β) as a function of temperature ratio, and temperature exponent (α) as a function of pressure ratio for different mixture conditions (ϕ). A revised power-law correlation for α and β variations is also recommended as, Su=Su0Tu/Tu0∝0+∝11-Pu/Pu0)Pu/Pu0β0+β11-Tu/Tu0. The sensitivity analysis indicates the maximum decrement in the positive sensitivity for the chain branching reaction HCO + M ↔ H + CO + M (R30) across all the stated mixture conditions, owing to an increment in the third body effects along with pressure and temperature. The reaction pathway analysis reveals a maximum net reduction in the elemental flux (∼56 %) for the reaction between the species carbon monoxide (CO) and carbon dioxide (CO2) through different reactions, due to a rise in pressure and temperature.

On the critical initiation of planar detonation in Noble-Abel and van der Waals gas

The critical decay rate theory (C.A. Eckett, J.J. Quirk, J.E. Shepherd, Journal of Fluid Mechanics 421 (2000) 147–183) on detonation initiation in unsteady flow was extended from ideal to Noble-Abel and van der Waals gas, in which the finite molecular volume effect (or repulsion force) and the inter-molecular attraction effect (or attraction force) were included. Considering the reactive Euler equations, the reaction zone temperature-gradient equation was established independently of the equation of state and wave geometry, and then solved asymptotically by assuming large activation energy, planar wave front and a one-step irreversible reaction model. A critical decay time was derived as the critical condition for detonation initiation. It was found that the inter-molecular attraction effect makes initiation more difficult by increasing the critical decay time, whereas the finite molecular volume effect promotes initiation. The real gas effects are enhanced when increasing the reduced activation energy and/or decreasing the heat capacity ratio of perfect gas. The applicability and generality of the asymptotic solutions were evaluated by comparison with quasi-unsteady simulation employing detailed reaction models for hydrogen-air and ethylene-air mixtures, and with the further simplified, fully analytical solutions based on strong-shock assumption. The critical conditions of the asymptotic solutions are qualitatively consistent with those obtained with the detailed mechanisms. The asymptotic solutions can also be well reproduced by the simple analytical solutions in most conditions. This study thus establishes a simple, yet reliable, method to evaluate the real gas effect on detonation initiation in high-pressure conditions.

Investigation of the ignition temperature of ethane-air mixtures at elevated pressure

Determining the ignition temperature of combustible substances is crucial in enhancing energy utilization efficiency and preventing fire and explosion accidents. This study investigates the ignition temperature of ethane-air mixtures with concentrations of 29.6%, 31.8%, and 34.4% at pressures up to 1 MPa in a 20 L spherical closed vessel, using a “heating” method. The high-pressure gas mixture was heated until spontaneous rapid reaction occurred in the reaction vessel. The study found that the ignition temperature of ethane in air decreases with an increase in initial pressure and ethane concentration. For example, at an ethane concentration of 31.8%, the ignition temperature decreased from 374℃ to 296℃ when the pressure increased from 0.3 MPa to 1 MPa. Additionally, larger container sizes resulted in lower ignition temperatures for ethane. Furthermore, the study simulated the self-oxidation reaction of ethane using the San Diego Mechanism kinetic reaction mechanism. The results revealed that the initial pressure's effect on important free radical reaction rates was the fundamental cause of autoignition. Under high-pressure conditions, the reaction between C2H5 and oxygen, C2H5 + O2<=>C2H4 + HO2, was the most critical reaction pathway. Overall, this study provides valuable insights into the ignition temperature of ethane-air mixtures and identifies key factors that influence autoignition. The findings can be utilized in improving safety measures and optimizing energy utilization in various industrial processes.

A direct numerical simulation study on the structures and turbulence–flame interactions of a laboratory-scale lean premixed jet flame in cross-flow

In the present work, direct numerical simulation of a laboratory-scale lean premixed reacting jet in cross-flow was performed to explore the flow–flame structures and turbulence–flame interactions. A jet of lean premixed ethylene–air mixtures (equivalence ratio $\phi = 0.6$ ) was injected into a hot vitiated cross-flow. Both non-reacting and reacting cases were simulated. It was found that the reacting jet penetrates deeper in the cross-flow with a weaker shear layer compared with the non-reacting one. The wake of the non-reacting and reacting jet is characterized by vertical vortices and recirculation zones, respectively. As for the flame structure of the reacting case, the reaction intensity varies considerably in different flame zones. The heat release rate on the leeward side is higher than that on the windward side, but lower than that of the corresponding laminar flame. The analysis of the turbulence–flame interactions of the reacting case showed that the large local Damköhler number ( $Da$ ) related to reaction-induced dilatations results in an increased tendency of the scalar gradient to align with the most extensive strain rate, which is more evident in the regions with high heat release rate on the leeward side. Negative dilatation regions with positive tangential strain rate and negative normal strain rate are observed on the windward side. High positive dilatations appear on the flame front of the leeward side. The tangential strain rate is negatively correlated with the normal strain rate and curvature. Regions with a high local $Da$ on the windward side correspond with high positive curvature regions.

Experimental Investigations on the Propagation Characteristics of Rotating Detonations Utilizing the Ethylene-Air Mixture

ABSTRACT In order to realize stable propagation of the ethylene-air rotating detonation wave with less velocity deficit in the annular combustion chamber, a series of experiments have been carried out while changing the combustor width (i.e. 10 mm, 15 mm, 25 mm, and a hollow combustor), mass flow rate of air, and equivalence ratio. The air and ethylene are supplied into the combustor through a slot-orifice impinging injection scheme, where air is injected through a slot with a height of 0.5 mm and ethylene is supplied by 160 orifices with a diameter of 0.35 mm. Experimental results indicate that four propagation modes have been obtained, i.e. the single wave mode, the sawtooth wave mode, the periodic oscillation mode, and the deflagration mode. The sawtooth wave mode and the periodic oscillation mode are considered unstable modes. In the single wave mode, the average propagation velocity of rotating detonations in the annular combustion chamber with a combustor width of 15 mm is between 1121 and 1215 m/s. The minimum velocity deficit is 21.9%, and the relative standard deviation of the propagation velocity is less than 5%, which is much lower than the experimental results of other studies utilizing ethylene and air in the annular combustors. In addition, the reasons may lead to the failure of producing rotating detonations have been analyzed. The effects of the chemical reactivity on the generation of rotating detonation waves have been demonstrated by installing a convergent nozzle at the exit of the combustor.

Dynamics of Pressure Variation in Closed Vessel Explosions of Diluted Fuel/Oxidant Mixtures

Nitrous oxide is widely used as oxidizer or nitriding agent in numerous industrial activities such as production of adipic acid and caprolactam and even for production of some semiconductors. Further, it is used as an additive in order to increase the power output of engines, and as an oxidizer in propulsion systems of rockets, because it has a large heat of formation (+81.6 kJ mol−1). N2O is highly exothermic, and during its decomposition a supplementary heat amount is released, so it needs special handling conditions. The combustion of fuels in nitrous oxide atmosphere can lead to high unstable and turbulent deflagrations that speedily self-accelerate and therefore a deflagration can change to a detonation. The peak explosion pressure and the maximum rate of pressure rise of explosions in confined spaces are key safety parameters to evaluate the hazard of processes running in closed vessels and for design of enclosures able to withstand explosions or of their vents used as relief devices. The present study reports some major explosion parameters such as the maximum (peak) explosion pressures pmax, explosion times θmax, maximum rates of pressure rise (dp/dt)max and severity factors KG for ethylene-nitrous oxide mixtures (lean and stoichiometric) diluted with various amounts of N2, at various initial pressures (p0 = 0.50–1.50 bar), in experiments performed in a spherical vessel centrally ignited by inductive-capacitive electric sparks. The influence of the initial pressure and composition on pmax, θmax and (dp/dt)max is discussed. The data are compared with similar values referring to ethylene-air mixtures measured in the same initial conditions. It was found that at identical C/O ratios with ethylene-air, ethylene-N2O-N2 mixtures develop higher explosion pressures and higher rates of pressure rise, due to the exothermic dissociation of N2O under flame conditions.

Influence of vent size and vent burst pressure on vented ethylene-air explosion: Experimental and numerical study

This paper describes the numerical simulation of the vented explosion of ethylene-air mixtures. The influence of concentration, vent size, and vent burst pressure on the vented ethylene-air explosion is investigated by comparing simulation results and experiments. The different phases of external flame propagation are concluded by comparing the experimental photos and the simulation results. The external explosion process is captured by the high-speed camera and simulation. It is found that the unburned mixture expanded to the external atmosphere, which was ignited by the flame. The flow velocity, velocity vector, and turbulence kinetic energy are applied to analyze the mechanism of external flame propagation. The velocity vector is deflected to both sides of the flame front and the reverse flow velocity is captured in the external flow field. The change of ethylene concentration and the venting flow velocity is used to explain the relationship between the pressure rise, chemical reaction rate, and venting flow. Results show that the internal pressure for AV= 0.18 m2 is significantly greater than the internal pressure for AV= 0.55 m2 in this case. The smaller vent size results in higher flow velocity and turbulence kinetic energy inside of the vessel. The vent burst pressure growth promotes the acceleration of the flame front inside of the vessel. The variation curve of ethylene concentration and venting flow velocity has a similar law with different vent burst pressures. The experimental and simulated results can provide technical and theoretical support for the design of gas explosion prevention in the field of process safety and risk engineering.

Computational Fluid Dynamics Modeling of Low Temperature Ignition Processes From a Nanosecond Pulsed Discharge at Quiescent Conditions

Abstract Recent interest in nonequilibrium plasma discharges as sources of ignition for the automotive industry has not yet been accompanied by the availability of dedicated models to perform this task in computational fluid dynamics (CFD) engine simulations. The need for a low-temperature plasma (LTP) ignition model has motivated much work in simulating these discharges from first principles. Most ignition models assume that an equilibrium plasma comprises the bulk of discharge kernels. LTP discharges, however, exhibit highly nonequilibrium behavior. In this work, a method to determine a consistent initialization of LTP discharge kernels for use in engine CFD codes like converge is proposed. The method utilizes first principles discharge simulations. Such an LTP kernel is introduced in a flammable mixture of air and fuel, and the subsequent plasma expansion and ignition simulation is carried out using a reacting flow solver with detailed chemistry. The proposed numerical approach is shown to produce results that agree with experimental observations regarding the ignitability of methane-air and ethylene-air mixtures by LTP discharges.

On the critical pressure pulse width for ignition of hydrogen–air and dimethyl ether–air mixtures

Under certain situations, such as a water hammer event, a volume of gas can undergo a rapid compression immediately followed by a rapid expansion, which could lead to auto-ignition under certain conditions. The goal of the present study was to investigate under which critical conditions such a pressure pulse could lead to an ignition event. To simulate the pressure pulse, a simplified Gaussian pressure pulse model was implemented in Cantera. Hydrogen–air and dimethyl ether–air mixtures were employed to study the ignition dynamics of chemical systems with a single and a double step of heat release. The numerical simulations were performed for three peak pressures of 101.3, 1013.2, and 5066 kPa, for a range of compression ratios between 5 and 45, and various pulse widths, which determine the characteristics of the pressure pulse. Depending on the width of the Gaussian pulse, either ignition or chemical quenching were observed. Critical ignition, just at the limit between ignition and quenching, is the result of the competition between chemical reaction and volumetric expansion due to the imposed specific volume changes. This competition can be summarized by the concept of critical pressure pulse width, which is an extension of the critical decay rate concept. To further investigate the mechanisms responsible for critical ignition and quenching, quantitative thermo-chemical analyses were performed for sub-critical, critical and super-critical conditions. A Damköhler number was defined as the ratio of characteristic time of expansion to ignition delay time to characterize the competition between ignition and quenching. A simplified non-dimensional pressure pulse model for which the derivative of the naperian logarithm of density was assumed to follow a linear law was established. It can be employed to derive the critical conditions for which ignition fails. The evolution of non-dimensional temperature was obtained numerically and showed the existence of a critical pulse width for successful ignition and that the location of ignition delay time moves from the expansion phase to the compression phase as the non-dimensional pulse width is increased.

Rotating Detonations in Hollow and Flow-Through Combustors

This paper focuses on rotating detonation combustor (RDC) experiments involving two configurations, a hollow and flow-through RDC, using hydrogen–air and ethylene–air mixtures. Two air injection areas with gaps of 1.778 and 3.81 mm were used at different air mass flow rates of 0.2, 0.3, 0.4, and 0.5 kg/s and equivalence ratios from 0.6 to 2 in 0.2 increments. Results show repeatable detonations for hydrogen–air mixtures in both configurations; however, the detonative range is wider, stronger, and faster in the hollow configuration. Speeds exceeding the Chapman–Jouguet velocity and pressures matching the ideal model were observed. Sustained detonations for ethylene–air mixtures were not attained with the flow-through configuration. Installing a back plate to convert the configuration into a hollow combustor promoted rotating detonations with the ethylene–air mixture at select operating points. The results of this study showcase the strong promise of producing and sustaining rotating detonation in nonannular RDC configurations.

Dynamics of Pressure Evolution during Gaseous Ethane–Air Mixture Explosions in Enclosures: A Review

The study here presents data from the literature regarding the characteristic parameters of explosion propagation in gaseous ethane–air mixtures. The maximum explosion pressures, maximum explosion times, maximum rates of pressure increase, and deflagration indices from experimental measurements are discussed and analyzed against the initial pressure, initial temperature, and equivalence ratio, as well as the explosion vessel characteristics. Ethane is used for ethylene production, as a refrigerant in cryogenic systems, as an alternative clean fuel in the power generation industry and automotive propulsion, and for many other applications. Therefore, the explosion characteristics of its mixtures with air are of great interest for explosions occurring after accidentally forming flammable mixtures, as well as for the prediction of combustors’ performances and/or engines that work in different conditions.

Updated conceptual design of hydrogen/ethylene fueled detonation ramjet: Test fires at Mach 1.5, 2.0, and 2.5

The conceptual design of a hydrogen/ethylene fueled detonation ramjet (DR) studied earlier is updated based on numerical simulations and test fires at airflow Mach numbers 1.5, 2.0, and 2.5. The goal of the design update is to increase the operation stability, reduce aerodynamic drag, and improve thrust performance at continuous detonation of hydrogen/ethylene in the DR combustor. The updated DR is fabricated and tested. The thrust performance of the updated DR is increased significantly. At on-design M=2.0, the range of stable operation of hydrogen fueled DR combustor is expanded in terms of the air-to-fuel equivalence ratio from 1.6 to 3.3. At the air-to-fuel equivalence ratio of 3.1, the fuel-based specific impulse reached 4760 s. The maximum value of total thrust is 1000 N. At off-design M=1.5 and 2.5, the fuel-based specific impulse / total thrust attained 2780 s / 740 N, and 3780 s / 1160 N, respectively. Replacement of hydrogen by ethylene leads to the significant narrowing of the DR stable operation domain: continuous-detonation of ethylene–air mixture is registered only for near-stoichiometric mixtures.

Reduction and optimization for combustion mechanism of dimethyl ether–air mixtures

AbstractAs an alternative fuel, dimethyl ether (DME) has attracted much attention due to its high efficiency and low emission. In this study, first, a skeletal mechanism of DME is obtained using a directed relation graph with error propagation (DRGEP) and sensitivity analysis (SA). Then the Arrhenius preexponential factors of the selected reactions in the skeletal mechanism are adjusted by the particle swarm optimization (PSO) algorithm to improve the prediction accuracy. Finally, an optimized mechanism with 30 species and 65 elementary reactions is proposed to describe the combustion of DME–air mixtures. This reaction mechanism is validated against the experimental data of ignition delay time, species concentration, and laminar flame speed, covering a wide range of initial temperatures (298–1600 K), pressures (1.0–60 bar), and equivalence ratios (0.5–2.0). Comparing the previous skeletal mechanisms, the optimized mechanism can excellently reproduce the experiment results, especially in the prediction of the ignition delay time of low‐temperature zone and laminar flame speed of high‐pressure conditions. It means that this mechanism corresponds to higher precision and more extensive applicability.

Experimental study on detonation propagation in industrial scale pipelines used in petrochemical plants

Based on the lack of systematic research on flame propagation in large-diameter and long-distance pipelines in the tank area of petrochemical plants, an experimental device for flame propagation in DN50-DN500 industrial-scale pipelines was designed and built. In this paper, effects of concentration of gas mixture for propagation characteristics of steady gaseous detonation waves in ethylene-air mixtures with DN50 pipeline were studied experimentally. The volume concentration of gas mixture was 5.6%, 5.93%, 6.6%, 7.15%, 8.0% ethylene in air. Homogeneous C2H4/air (6.6%) and C3H8/air (4.2%) mixtures were used with 9 kinds of pipelines which were from DN50 to DN500 to study the effects of pipeline diameter for propagation characteristics of steady gaseous detonation. The experimental results show that the concentration of combustible gas has an effect on flame propagation and detonation. The detonation runup distance is short and steady detonation is more likely to be formed when it is close to chemical equivalent concentration, when the combustible mixture is poorer or richer, the steady detonation will need more runup distance. The detonation flame speed and pressure are more affected by the type of combustible gas instead of pipe diameter. The detonation pressure of the mixture of 6.6% C2H4/air and the mixture of 4.2% C3H8/air is 15.17 and 14.47 times of the initial pressure, respectively, which is different from the reference value given by the ISO16852 standard where the ratio pm/p0 (the average value of the detonation pressure to initial pressure) increases with pipe diameter. The detonation pressure of pipeline below DN150 is far higher than the reference value which is 10 and 12. It’s suggested that in the design of pipelines and selection and installation of flame arresters for connecting pipelines in the tank areas, detonation pressure requires careful consideration and appropriate arresters should be selected in combination with the installation position.

Numerical study on oscillatory propagation dynamics and physics near the limits of planar freely propagating premixed flames

An oscillatory phenomenon of the freely propagating premixed flames for dimethyl ether–air mixtures at near-limit conditions was systematically analyzed to understand the underlying physicochemical processes that control the oscillation and extinction of hot-lean flames. The flame oscillation dynamics and extinction mechanism were first investigated using the analyses of flame oscillating structure and phase function. It was found that the leading cool-flame front and tailing hot-flame front present a significant separation during the oscillatory propagation, and the formation of flame oscillation was the consequence of the interaction between the two flame fronts which can be explained as the competition of reactions between low-, intermediate-, and high-temperature pathways. Meanwhile, due to the excessive heat losses of the lower half period, the flame will eventually extinguish in the divergent oscillating process. Furthermore, the key processes in the flame oscillation were further revealed by chemical explosive mode analysis-based diagnosis. The results show that the oscillations are primarily originated from the species and reactions of the intermediate-temperature pathway, including H2O2 and CH2O, and chain-branching/-termination reactions. These key processes eventually lead to the flame oscillatory extinction presenting an O(2) frequency.

Laminar flame speed measurements of ethylene at high preheat temperatures and for diluted oxidizers

Laminar flame speeds of ethylene are measured and the results are compared to predictions based on detailed chemical kinetic mechanisms. As important aspects of jet fuel chemistry involve ethylene, the flame speeds are determined at conditions relevant to jet engine combustors that have not been previously investigated, specifically high preheat temperatures (up to 650 K) and reduced O2 levels (down to 15% mole fraction in the oxidizer). The latter is relevant to staged-combustion and vitiation, where there is partial pre-burning of the oxidizing flow; so flame speeds are also measured with both CO2 and N2 dilution. Flame speed measurements of ethylene-air mixtures at room temperature conditions agree well (within 10%) of measurements with previous results from the literature and the chemical kinetic mechanism predictions. As the preheat temperature is increased to 650 K and with N2 dilution, the chemical mechanism predictions agree reasonably well with the measurements (generally within 10%). With CO2 dilution of the oxidizer, however, larger prediction discrepancies are found. The mechanisms over predict the measured flame speeds, and the prediction error increases with the amount of CO2 in the oxidizer. Analysis of different possible sources of discrepancy point to the third-body efficiency of CO2 as a likely reason; specifically, it may be underestimated for three-body association reactions such as H + O2 (+M) ⇔ HO2 (+M).