Mixture Equivalence Ratio Research Articles

Premixed Flames Under Microgravity and Normal Gravity Conditions

Premixed conical CH4-air flames were studied experimentally and numerically under normal straight, reversed gravity conditions and microgravity. Low-gravity experiments were performed in Drop tower. Classical Bunsen-type burner was used to find out features of gravity influence on the combustion processes. Mixture equivalence ratio was varied from 0.8 to 1.3. Wide range of flow velocity allows to study both laminar and weakly turbulized flames. High-speed flame chemoluminescence video-recording was used as diagnostic. The investigations were performed at atmospheric pressure. As results normalized flame height, laminar flame speed were measured, also features of flame instabilities were shown. Low- and high-frequency flame-instabilities (oscillations) have a various nature as velocity fluctuations, preferential diffusion instability, hydrodynamic and Rayleigh-Taylor ones etc., that was explored and demonstrated.

Heat transfer investigations on methane-air premixed flame jet exiting from a circular nozzle and impinging over semi-cylindrical surfaces

The flame jet impingement on curved surfaces finds applications in various direct flame heating situations. The design of heating equipment demands the estimation of the surface temperature and of the convective heat flux to the target surface at steady state for various impingement parameters. The characterisation of flame jets in terms of Nusselt number and effectiveness aids the designer to numerically estimate the steady state surface temperature and heat flux by specifying the convective boundary condition with spatial distribution of reference temperature. The present study reports the heat transfer characteristics of single flame jet impinging on convex and concave surfaces of a semi-cylindrical target plate. The effects of mixture Reynolds number, equivalence ratio, curvature ratio and burner to plate spacing on the heat transfer behavior are investigated. The uniformity of the heat flux distribution and the overall thermal efficiency are determined from the local heat flux distributions. The flow fields along the curvilinear axes influence the overall thermal performance. The concave surface produces higher thermal efficiency and better uniformity of heat flux over the impingement surface as compared with the convex surface.

Experimental and numerical study of premixed flame penetration and propagation in multichannel system

ABSTRACTExperimental and numerical results on premixed flame penetration and subsequent propagation in a multichannel burner are presented. The burner consists of the set of planar straight quartz channels which transverse sizes can be varied. It is found that, depending on mixture flow rate, equivalence ratio and channels transverse sizes a big variety of combustion regimes can be observed. These regimes include burner-stabilized flames, upstream propagating flames, and flames stabilized under the burner external surface. The placement of different combustion regimes in equivalence ratio/flow rate plane is plotted by means of experimental and numerical studies. In wide range of parameters, the flame pulsations consisting of repetitive stages of flame ignition, upstream propagation, and quenching take place. Results of numerical simulations obtained in the framework of simplified thermal-diffusion model are found to be in a good qualitative agreement with experimental data and allow to explain experimental findings.

Effects of inlet parameters on combustion characteristics of premixed CH4/Air in micro channels

In order to improve the stability of combustion and the working performance of the micro-combustor, this paper focuses on the effects of inlet parameters such as inlet temperature, mass flow rate and equivalence ratio. Three micro-combustors with different channel-heights are fabricated, and the three-dimensional calculation model is built to research on the combustion characteristics of premixed CH4/Air mixture. It was found that with inlet gas mixture temperature increasing, the flammability limits of the combustion under micro-scale conditions were expanded, and the channel height under which the flame can exist in the combustor reduced from 3.0 mm to 2.0 mm after preheating. On the preheating basis, increasing the equivalence ratio of the gas mixture Ф improved the intensity of gas-phase reaction due to the simulation of the important free radical like OH. Furthermore, when the inlet mass flow rate of methane mch4•was increased and between m1ch4•and m5ch4•, it shows that the external wall temperature was higher in the micro-combustor of H = 2.5 mm compared with that of H = 3.0 mm. When in the micro-combustor of H = 3.0 mm, more fuel could be burned and mmaxch4• = 2.85 × 10−6 kg/s.

Quantitative Parametrization of Mixture Distribution in GDI Engines: A CFD Analysis

This paper presents an objective classification of mixture distribution in the combustion chamber of a gasoline direct injection (GDI) engine into homogeneous and non-homogeneous types. The non-homogeneous mixture distribution is further classified as properly stratified, improperly stratified and mal-distributed types. Based on this classification, four types of properly stratified mixture distributions viz., random, linear, Gaussian and parabolic are virtually simulated in the combustion chamber of a GDI engine using computational fluid dynamics to identify the mixture that results in maximum indicated mean effective pressure (IMEP). It is found that the IMEP is highest for the parabolic mixture distribution which is followed by Gaussian, linear and random types. The performance and emission characteristics of the virtual mixture distributions are compared with a late fuel injection case at different over all equivalence ratios ranging from 0.3 to 0.7. Then the variation of mixture equivalence ratio with the distance from the spark plug is parametrized for different virtual mixture distribution cases and expressed using a parameter called the “stratification index”. It is found that the stratification index based on Gaussian variation gives maximum information about the mixture distribution in the combustion chamber. Finally the stratification index of different virtual mixture distributions is compared with the late fuel injection case at various overall equivalence ratios. It is found that the late fuel injection case tends to produce highest IMEP when the stratification index is close to unity.

Instability Control by Actuating the Swirler in a Lean Premixed Combustor

A detailed study concerning a novel, dynamic control strategy for mitigating thermoacoustic instability in a swirl-stabilized lean premixed, laboratory combustor configuration is presented in this paper. The mitigation strategy is realized by rotating the otherwise static swirler, which is primarily meant for stabilizing the lean premixed flame. The proposed strategy is tested over a range of bulk flow velocities, mixture equivalence ratios, and swirler rotation rates for validating the robustness of this concept. A prominent reduction in the fundamental acoustic mode amplitude by about 25 dB is observed with this control technique for the cases that are studied. The physical mechanism responsible for the instability mitigation due to the rotating swirler is investigated by observing the distinct changes associated with the reacting flowfield using particle image velocimetry. An attempt is made to probe into the self-excited flame dynamics using high-speed intensified, chemiluminescence imaging and identifying the instability driving source locations from a spatial Rayleigh indices map. The rotating swirler induces vortex breakdown and increased turbulence intensity to decimate strongly positive Rayleigh indices regions (and eventually the acoustic energy source) to render quiet instability mitigated swirling flames.

Experimental Investigation of Flame Speed of Gasoline Fuel-Air Mixture

Flame speed is an important aspect of fuel combustion characteristics. It affects combustion system design and performance. In this study, a laboratory experimental rig has been built to investigate the effect of equivalence ratio of gasoline-air mixture in a centrally ignited constant volume combustion chamber on flame speed at an initial pressure of 1bar and an initial temperature of 483 K using high speed flame photography technique. The results of the outwardly expanding spherical flame showed that the flame speed depends on both equivalence ratio and flame radius. It decreases slightly with the increase of flame radius and increases with the equivalence ratio until slightly larger than stiochiometric and then decreases. The flame speed reached 1.45 m/s at Φ= 0.8, (1.6) m/s at Φ= 1 and 1.27 m/s at Φ= 1.5

Effect of Gravity on Premixed Methane–Air Flames

Premixed flames under different levels of gravity were studied experimentally and numerically. The experiments were carried out in the Bremen Drop tower. The object of investigation were conical premixed rich, lean, and stoichiometric CH4–air flames with wide range of flow regimes, at Reynolds numbers of 600–2000, which were generated on specially designed cone nozzle and premixed cylinder chamber with grids and beads. Planar laser-induced fluorescence OH radicals and high-speed video recording was performed under microgravity, terrestrial conditions, and inverted gravity. All the experiments were performed at atmospheric pressure. Our experiments confirm that gravity has a complex influence on laminar and weakly turbulent premixed flames. Gravity causes flame flickering, while under reduced gravity flames are stable and almost not flicker. Based on the experimental data and numerical simulation, a correlation of flickering frequency with different mixture equivalence ratio and gravity is formed.

Numerical investigation of injector geometry effects on fuel stratification in a GCI engine

Injectors play an important role in direct injection (DI) gasoline compression ignition (GCI) engines by affecting the in-cylinder mixture formation and stratification, which in turn impacts combustion and emissions. In this work, the effects of two different injector geometries, a 7-hole solid-cone injector and an outwardly opening hollow-cone injector, on fuel mixture stratification in a GCI engine were investigated by computational simulations. Three fuels with similar autoignition kinetics, but with different physical properties, were studied to isolate the effect of the combustion chemistry on combustion phasing. In addition, start of injection (SOI) sweeps relevant to low-load engine operating conditions were performed. The results show that physical properties of the fuel do not have significant influence when using a hollow-cone injector. Richer mixtures were observed at all the studied SOI (−40 to −14 CAD aTDC) cases, which can be attributed to the nature of the hollow cone spray. At later SOIs (−18 and −14 CAD aTDC), the richer mixtures are accompanied by lower mean in-cylinder temperature due to the charge cooling effect, which surpasses the equivalence ratio effect. The effect of fuel physical properties on combustion phasing was evident in multi-hole injection cases, which can be attributed to the differences in mixture stratification and equivalence ratio distribution at the time of ignition.

Compact design of planar stepped micro combustor for portable thermoelectric power generation

An efficient prototype of a micro power generator with integrated micro combustor has been developed in the present study. The proposed design of the integrated micro-combustor provides high surface temperature with superior temperature uniformity and enhanced flame stability limits, a prerequisite for a thermoelectric power generation system. This novel micro combustor configuration consists of three backward facing steps with a recirculation hole fabricated in a rectangular heating medium of aluminium material. Parametric studies are carried out by varying the mixture inlet velocity, equivalence ratio and coolant flow rate to obtain the optimized operating conditions for maximum power generation. Two thermoelectric modules are mounted on the system operating with liquefied petroleum gas as fuel. A maximum conversion efficiency of 3.3% is obtained at ϕ = 0.95 with a mixture velocity of 7.5 m/s and a load resistance of 4 Ω across the thermoelectric generator. The effect of porous media is investigated to enhance the flame stability limits in the micro combustor. Porous media significantly enhances the upper flame stability limits and maximum conversion efficiencies (3.8%, 4.03%, and 3.73% at ϕ = 1, 0.9 and 0.8 at 10 m/s). A significantly higher power density (∼50% higher than existing systems) of 0.12 mW/mm3 of system volume is achieved. A compact design of the prototype system with high conversion efficiency shows the possibility of its application for various systems requiring portable power for remote, stand-alone, military and aerospace applications.

Numerical study of inflow equivalence ratio inhomogeneity on oblique detonation formation in hydrogen–air mixtures

In this study, numerical simulations using Euler equations with detailed chemistry are performed to investigate the effect of fuel–air composition inhomogeneity on the oblique detonation wave (ODW) initiation in hydrogen–air mixtures. This study aims for a better understanding of oblique detonation wave engine performance under practical operating conditions, among those is the inhomogeneous mixing of fuel and air giving rise to a variation of the equivalence ratio (ER) in the incoming combustible flow. This work focuses primarily on how a variable equivalence ratio in the inflow mixture affects both the formation and characteristic parameters of the oblique detonation wave. In this regard, the present simulation imposes initially a lateral linear distribution of the mixture equivalence ratio within the initiation region. The variation is either from fuel-lean or fuel-rich to the uniform stoichiometric mixture condition above the oblique shock wave. The obtained numerical results illustrate that the reaction surface is distorted in the cases of low mixture equivalence ratio. The so-called “V-shaped” flame is observed but differed from previous results that it is not coupled with any compression or shock wave. Analyzing the temperature and species density evolution also shows that the fuel-lean and fuel-rich inhomogeneity have different effects on the combustion features in the initiation region behind the oblique shock wave. Two characteristic quantities, namely the initiation length and the ODW surface position, are defined to describe quantitatively the effects of mixture equivalence ratio inhomogeneity. The results show that the initiation length is mainly determined by the mixture equivalence ratio in the initiation region. Additional computations are performed by reversing ER distribution, i.e., with the linear variation above the initiation region of uniform stoichiometric condition and results also demonstrate that the ODW position is effectively determined by the ER variation before the ODW, which has in turn only negligible effect on the initiation length.

Experimental Investigation on Pressure Rise Characteristics in an Ethylene Fuelled Wave Rotor Combustor

Pressure rise is one of the key operation characteristics of a wave rotor combustor (WRC), which would affect the power performance of the propulsion system based on WRC. To find out the pressure rise characteristics of WRC, a simplified test system of WRC was established, which is a single-channel multiple-cycle WRC with a rotating inlet/outlet port. The effects of the filling speed of air-fuel mixture, the rotating speed of inlet/outlet port, and the equivalence ratio of combustible air-fuel mixture on pressure rise characteristics of WRC are presented. The pressure rise is derived from the flow stagnation of filling combustible air-fuel mixture and the constant volume combustion in channel. With the rotating speed of inlet/outlet port increasing, the pressure rise significantly improves and the maximum average pressure is 6.3 atm. When the rotating speed of inlet/outlet port is constant, there is a suitable filling speed of air-fuel mixture and an optimum equivalence ratio for combustible air-fuel mixt...

Effect of natural gas components on its flame temperature, equilibrium combustion products and thermodynamic properties

It is a known fact that the composition of natural gas varies widely from source to source and from time to time even in the same source. Such variations in gas composition cause variations in flame temperature, its combustion products and thermodynamic properties which can affect gas-fueled engine performance and its emissions. The purpose of this study is to investigate effects of varying amounts of natural gas diluent components such as ethane (C2H6), isobutane (C4H10), propane (C3H8), carbon dioxide (CO2) and nitrogen (N2) on methane–air combustion under different pressures, unburned mixture temperatures and equivalence ratios. Results show that adiabatic flame temperature is mostly increased by isobutane followed by propane and ethane however it is mostly influenced by CO2 content in the gas which decreases flame temperature 80 K in %70 CH4 - %30 CO2 mixture at stoichiometric conditions. As for specific heat, the highest increase is again for isobutane followed by propane and ethane content but it is decreased mostly by N2. Effect of increasing secondary fuel content on product species is greatest at equivalence ratio near unity except for CO and H2. Analysis results including equilibrium compositions are validated and expected to be a reference guide for scientists, engineers and NG consumers to evaluate and compare natural gas samples of different compositions.

Flame stability analysis of the premixed methane-air combustion in a two-layer porous media burner by numerical simulation

The flame stabilization within a two-layer porous burner was investigated numerically in the present study. Two-dimensional model was implemented to simulate the premixed methane-air combustion process. The governing equations including the radiative transport equation and separate energy equations for the solid and gas phases were solved by using the finite volume method. In order to validate the utilized model, the gas and solid temperature profiles within the burner were compared to the experimental temperature distribution and good agreement was observed. The results proved that the stability limit and flame temperature within a porous burner can be controlled by the equivalence ratio of the incoming mixture. Heat recirculation study showed that high convective heat recirculation occurred at low equivalence ratios caused higher stable inlet velocities. The radiative recirculation efficiency was almost constant over the stability limit at constant equivalence ratio while this parameter decreased with increasing equivalence ratio. Also, the effect of outlet diameter of the burner on the flame stabilization suggested that the optimum outlet diameter eventuating the highest flame stability limit was double the amount of inlet diameter. The study of the effect of the length of first porous layer on the flame status showed that the optimum ratio of preheat zone to combustion zone length was 0.33 to reach the highest burner operating range.

Thermal investigations on methane-air premixed flame jets of multi-port burners

The multi-port burners are utilised in various domestic and industrial heating appliances. The thermal performance of these appliances is usually performed with loading water test. An attempt is made in this work to thermally characterise the methane-air premixed impinging flame jets of multi-port gas burner employing air jet cooling technique. The present method has an advantage that apart from the thermal efficiency, the uniformity in surface heat flux, Nusselt number and effectiveness distributions can be found. The experiments are performed with inline and staggered configurations of multi-port plate burner. The thermal performance for varying mixture Reynolds number, equivalence ratio and burner to plate spacing is presented. The inline configuration performs better in terms of thermal efficiency. However, the staggered configuration produced more uniform surface heat flux.

Effects of applying non-thermal plasma on combustion stability and emissions of NOx and CO in a model gas turbine combustor

The effect of applying non-thermal plasma (NTP) (by adopting a dielectric barrier discharge) to a lean premixed model gas turbine combustor on flame stability and emissions of NOx and CO has been investigated experimentally by varying mixture nozzle exit velocity (U0) and equivalence ratio (∅). The results showed that the existence of flame enhanced streamer intensity. The streamer intensity decreased as equivalence ratio deceased. Applying NTP to the combustor extended the lean flammable limit, and augmented the flame stability in a manner that the stable flame regime was extended to lower equivalence ratio and the regimes of lifted and oscillating flames are reduced. It was also found that it resulted in increase of NOx emission slightly (however, having a maximum of 10ppm) and reduction of CO emission significantly due to exalting complete burning. Nonetheless, with applying the NTP extending the lean flammable limit for the baseline case without it, an optimal range in ∅ existed, i.e. <30ppm for CO emission and ⩽10ppm for NOx at 0.600⩽∅⩽0.650. These results suggest that applying NTP to a gas turbine combustor should be a useful tool to simultaneously reduce both emissions of NOx and CO.

Experimental study on the effects of mixture flow rate, equivalence ratio, oxygen enhancement, and geometrical parameters on propane-air premixed flame dynamics in non-adiabatic meso-scale reactors

In the present study, the effects of reactive mixture flow rate, adding oxygen to propane-air mixture, geometrical parameters, and equivalence ratio on propane-air/oxygen premixed flame dynamics in non-adiabatic meso-scale reactors were experimentally investigated. During the experiments, seven flame regimes of blow-off, blow-out, asymmetric stationary, stationary-repetitive extinction and re-ignition (RERI), forced/self-RERI, RERI-flash-back, and flash-back were observed. The results showed that increasing the reactive mixture flow rate could generally promote variety of the flame regimes and also improve flame stability in the non-adiabatic meso-scale reactors, especially in 40% and 80% oxygen-enhanced cases. Also, the results demonstrated that increasing the reactor inner diameter and equivalence ratio generally extended propane-air- oxygen flame stability and its presence range in the non-adiabatic meso-scale reactors. Moreover, it was shown that increasing the reactor length and also increasing the added oxygen to propane-air mixture more than 40% promoted flame instability and consequently restricted propane-air-oxygen flame presence range in the non-adiabatic meso-scale reactors. Also, it was shown that variations in the mixture flow rate, the reactor length and inner diameter, equivalence ratio, and oxygen concentration in propane-air mixture could significantly influence the flame average propagation speed, acoustic, and chemiluminescence in the non-adiabatic meso-scale reactors.

Numerical investigation of the flame stabilization in a divergent porous media burner

In the present work, a numerical study on the flame stabilization in a divergent porous media burner was carried out. The purpose of this study was to peruse the influence of different conditions on the flame status in a porous medium. Two-dimensional axisymmetric model was used to simulate the process of premixed methane–air combustion. Nonequilibrium condition between the solid and gas temperature was considered and heat recirculation in the porous medium was quantified. The present numerical method was validated by comparison of solid and gas temperature profiles against the experimental data. The results showed that the stable flame within the porous medium can be controlled by velocity and equivalence ratio of the incoming mixture. Also, it was proved that the alteration of divergence angle can change the flame stability limit so that the optimum divergence angle that results in the highest limit of flame stability range was 60°. The heat transfer analysis indicated that the heat recirculation efficiency decreases with increase in the equivalence ratio and inlet velocity.

Real time calculation of residual gas fraction utilising the polytropic coefficient of compression

An algorithm is developed to serve as a stand-alone residual gas fraction diagnostic tool which utilises the polytropic coefficient of compression and an inverse thermodynamic algorithm to determine trapped residual gas fraction. The model requires: 1) cylinder pressure resolved with at least crank angle resolution; 2) high accuracy manifold pressure (preferably crank angle resolved and measured very near the intake port; 3) equivalence ratio of the fuel-air mixture; 4) mass estimations of cylinder contents other than residual, namely air mass, and fuel mass; 5) cylinder geometry (bore, stroke, clearance volume); 6) timing of the intake valve closing event. This work outlines the algorithm methodology and assesses its performance on both an ensemble averaged and individual cycle basis. However, the current RGF calculation methodology is shown to produce unacceptable levels of error on an individual cycle basis. Suggestions are provided for algorithm enhancement.

Anatomy of the cooled EGR effects on soot emission reduction in boosted spark-ignited direct-injection engines

It has been extensively examined that cooled exhaust gas recirculation (EGR) is effective not only to reduce NOx emissions but also to improve fuel conversion efficiency in spark ignition (SI) engines. Recently, the benefit of cooled EGR on soot particle emission reduction in SI direct injection (DI) gasoline engines attracts increased attention. However, the mechanism behind the phenomena needs to be further clarified. The objective of this study is to anatomize the cooled EGR effects on soot emission reduction in SIDI gasoline engines by the experiments along with the computational fluid dynamics (CFD) simulation. The soot emission reductions up to 48% by cooled EGR are firstly demonstrated at three typical high load conditions of a 2.0L boosted SIDI engine. Since the engine-out soot emissions are the combined results of soot particle formation and oxidation, the effects of cooled EGR on these two processes are respectively considered. It is found that cooled EGR plays adverse effects on soot oxidation, owing to the reduced in-cylinder oxygen availability and lowered reaction temperature. Although the increased intake boost pressure and reduced fuel injection quantity with cooled EGR can help reduce remarkably the wall wetting during the intake stroke, the quantity of liquid fuel on the wall is only slightly lower than the case without EGR during the combustion processes. The in-cylinder mixture distribution and equivalence ratio (ϕ)-temperature (T) plots obtained by the CFD simulations show that the lowered temperature of “pool fire” is the primary contributor to the reduced soot formation with cooled EGR, while improvement of the local rich pockets in the cylinder and the dilution effect of EGR may also play positive roles in suppressing the soot formation.