Matrix Burner Research Articles

A numerical study of highly-diluted, burner-stabilised dimethyl ether flames

Recently, a new burner was designed by Zhang et al. (Proc. Combust. Inst. 34 [2013], 763–770) to enable the investigation of 1D, premixed flames at atmospheric pressure with a temperature in the burnt gases near 1500 K. It consists of a matrix burner plate with alternating fuel and oxidiser feeds that, because of small-scale nozzles, mix quite rapidly. Flames at high dilution and reduced temperatures such as realised here are of relevance for the understanding of low-temperature combustion strategies. In this work, we examine the burner with regard to the validity of the 1D assumption for the investigated flames. Experimental measurements are conducted and 1D and 3D simulations are performed in which the chemistry is described by a detailed chemistry approach based on a reduced reaction scheme derived from the mechanism of Fischer et al. (Int. J. Chem. Kinetics 32 [2000], 713–740). The experimental results are compared to 1D simulations with two different temperature treatments. First, the unburnt temperature is set to the measured temperature closest to the burner surface; second, the experimental temperature profile is prescribed in the whole simulation domain without solving the energy equation. The comparison shows that the 1D simulation predicts the experimental results reasonably well, if the experimentally obtained temperature profile is prescribed in the simulation domain. Differences are found in the mole fractions of methyl and formaldehyde. Further comparisons of the experimental data with 3D simulation results and comparisons of 3D and 1D simulation results indicate that the differences between measured and computed mole fractions of these species are not a result of the 3D nature of the experimental flame and might be attributed to the chemical mechanism. The conclusion is that the measurement data can be used for validation purposes with the 1D simulation setup shown here if the measured temperature profiles are prescribed in the 1D simulation domain.

A new pattern of instability observed in an annular combustor: The slanted mode

In annular combustion chambers of aero-engines and gas turbines, acoustic coupling may arise from azimuthal modes which are less well damped than axial modes. Also, since the circumference is the largest length in the combustor, the azimuthal modes have the lowest resonance frequencies and are most prone to instability. Such a coupling raises many scientific issues which are considered in a small number of fundamental experiments. The present investigation focuses on this problem and provides experimental data on a special type of combustion instability in which the thermo-acoustic resonant coupling involves a combination of modes. This produces an unusual pattern of flame responses in which the distribution of heat release rate is slanted. Data are provided in the form of free radical light intensity patterns (interpreted as heat release rate distributions) and microphone signals detected in the plenum and chamber. It is shown that the slanted pattern is the signature of a combination of two modes with coinciding frequencies, the first being a standing azimuthal mode while the second is an axial mode. Measurements of the flame describing function on a single matrix burner at the fundamental frequency are used to explain the observed phase shift and amplitude in the flame responses of the different injectors in the annular combustor.

Partial Oxidation of Light Alkanes as a Base of New Generation of Gas Chemical Processes

Recent developments in unconventional natural gas production increase the need for principally new small-scale technologies for gas processing and transportation. The promising way for small-scale gas processing is its autothermal partial oxidation to syngas or direct partial oxidation to chemicals. The paper considers some prospective gas chemical processes based on the partial oxidation of light alkanes. Among them are the conversion of natural gas to syngas in volumetric (3D) matrix burners made of a gas permeable material and direct conversion of methane to methanol without its preliminary conversion to syngas (DMTM). As a more simple technology that lets to use fat associated oil gas often flaring in remote sites, it can be suggested the selective oxidative cracking of heavier components of natural gas. This process converts heavy methane homologues from propane to pentane and heavier into ethylene, methane, ethane, hydrogen, and carbon monoxide, thus increasing methane index (octane number) of gas and making it suitable for feeding modern gas piston and gas turbine power engines. One more interesting prospect is the creation of technologies making use of the subsequent processing of valuable oxycracking products, such as olefins, CO, and hydrogen, for example, by their catalytic co-polymerization without preliminary separation from gas phase. The co-polymerization of CO and ethylene, followed by the separation of resulting liquid products, can considerably improve the economic attractiveness of the oxycracing process. Thus, despite the absence of economically proved and industrial-scale tested smallcapacity direct and indirect gas chemical technologies, intensive efforts to develop such alternative technologies let to expect near bright future for them.

Analytical model of nonlinear thermo-acoustic effects in a matrix burner

This paper considers a fundamental thermo-acoustic test rig developed by Noiray (Linear and Nonlinear Analysis of Combustion Instabilities, Application to Multipoint Injection Systems and Control Strategies”, Ph.D. Thesis, École Centrale Paris, 2007) and models it with an entirely analytical approach. The measured flame describing function is represented by a surprisingly simple time-lag law, which is then used to derive the governing equation for a single acoustic mode in the test rig. This equation turns out to be that of a harmonic oscillator with a damping/amplification coefficient that depends on the velocity amplitude. On this basis we find analytically the pattern of the oscillation regimes in parameter space, in particular the frequency and amplitude of limit cycles at various tube lengths. There is good qualitative agreement with some, but not all, features of Noiray’s observations.

Effects of Non-Linearity on the Flame Response and Control of Combustion Instabilities in a Matrix Burner

This paper discusses the effect of non-linearities inside a matrix burner on the flame response and control of instabilities inside the matrix burner. In particular, the response of the flame to high inlet velocity modulation level and frequency of the forcing signal are discussed. The importance of the choice of the combustion model used in the numerical simulation of combustion instabilities is highlighted. The effect of non-linearities on the choice of the primary fuel actuator used in active control is discussed. Finally, it is shown that the occurrence of limit cycle behavior inside the combustor of the matrix burner led to hysteresis. The analysis begins with the discussion of the stability map of the matrix burner obtained from measurements.

Computed Tomography of Chemiluminescence (CTC): High resolution and instantaneous 3-D measurements of a Matrix burner

Computed Tomography of Chemiluminescence (CTC) is a diagnostic technique that provides instantaneous 3-D information on flame geometry and excited species concentrations. The technique reconstructs the 3-D chemiluminescence intensity field using Computed Tomography (CT) from integral measurements of chemiluminescence in the form of camera images. The CTC sensor has been demonstrated for a methane–oxygen Matrix burner consisting of 21 laminar diffusion jet flames using affordable commodity cameras. High resolution 3-D reconstructions obtained from 48 views show good agreement with the observed flame shape and resolve wavelengths of approximately 220μm. This is sufficient to capture the multiple flame fronts, showing the suitability of CTC for wrinkled turbulent flames. Instantaneous 3-D reconstructions using 10 simultaneous camera measurements also show good agreement with the observed flame shape and are seen to be tolerant of error in the camera location. Measurements with exposure times as short as 62μs were found to achieve more than sufficient signal-to-noise ratios for successful tomographic reconstructions. Obvious applications for CTC are the measurement of (instantaneous) flame-surface density, wrinkling factor, flame normal direction, and possibly heat release, as well as the observation of transient developments.

Modeling of laminar pulverized coal flames with speciated devolatilization and comparisons with experiments

In an earlier mathematical model of laminar pulverized coal–air combustion, supported by added CH 4, it was assumed that the volatiles from the coal consisted solely of CH 4 and HCN. A revised model is introduced with speciated devolatilization rate constants for tar, CH 4, CO, CO 2, H 2O, H 2, and HCN. It is assumed that these rate constants can also be applied to the devolatilization of the tar. In addition, it is assumed that the soot is predominantly carbon and is oxidized by the attack of O, H, OH, and O 2, in the same way as the coal char. Because the devolatilization rate is strongly dependent on particle temperature, the latter has to be determined accurately from the momentum and energy equations of the particle. The model is one-dimensional, with axial radiative transfer. The introduction of soot formation and speciation of the volatiles results in much improved accuracy in the prediction of species and temperature profiles in subatmospheric combustion on a flat flame matrix burner. It is possible to derive an overall global devolatilization rate constant that agrees reasonably with the measurements. These computations suggest that the effective area of the assumed spherical coal char particles is four times greater than that of the assumed sphere. Modeling of atmospheric pressure flames suggests that in this case, the value of 4 should be reduced, probably because, as pressure increases, the diffusion flux of reactant is reduced. Subatmospheric pressure laminar burning velocities are predicted with satisfactory accuracy over the full range of overall equivalence ratios. Previous measurements of laminar burning velocity at atmospheric pressure are reviewed. However, the various means of supporting a stable coal flame and the associated uncertain geometries make it impossible to apply the present model to the different conditions. It is suggested that burning velocities measured on a flat flame burner, with a controlled amount of methane to support the combustion of a pulverized coal/air mixture, would provide a good test of the reactivities of different coals.

Optimisation of a low-NO<SUB align=right>x combustion chamber using CFD simulations

Optimisation studies based on computer simulations of a 450kW combustion chamber are presented and discussed. A matrix burner which develops an array of premixed flames is used in this type of combustion chamber to achieve power modulation. The optimisation parameters were pollutant emissions, efficiency and simplicity of construction. In addition to the currently used combustion chamber, four other geometrical configurations or modifications were considered and simulated. Two of them were found to be better than the current combustion chamber geometry: (1) The insertion of a heat exchanger in the combustion chamber would reduce NOx emissions; (2) using a flat bottom configuration of the combustion chamber instead of a round bottom one would be a simpler configuration and would not change the other optimisation parameters.

Premixed turbulent flame instability and NO formation in a lean-burn swirl burner

Computed results are presented from a Reynolds stress, stretched laminar flamelet model, for premixed swirling combustion in a rotating matrix burner. These are in good agreement with flame photography and coherent anti-Stokes Raman spectroscopy (CARS) temperature measurements. The principal variable is the equivalence ratio, φ, while the swirl number and mean axial entry velocity remain constant. At higher equivalence ratios, the flame is stabilized by hot gas in both the inner and outer recirculation zones. As φ is reduced below 0.6 another steady-state solution appears, in which the flame is stabilized only by the hot gas in the inner zone. In this regime, the experiments show unstable combustion with low-frequency oscillations between the two states. This instability is shown to originate in flame quench in the regime between the two recirculations. At the values of φ below 0.6, most of the NO produced originates as prompt NO in the reaction zone. When φ is reduced to 0.56, there is only one steady-state solution. While the investigations were of a laboratory scale burner, some of the general findings are relevant to practical combustors.

Effects of natural gas composition on the nitrogen oxide, flame structure and burning velocity under laminar premixed flame conditions

Nitrogen oxide (NO x ), flame structure and burning velocity for Egyptian natural gas–air flames have been investigated experimentally and computationally for a laminar premixed flame. Experimentally, CO, O 2 and NO x concentrations as well as the gas temperature were measured throughout a flat flame on a matrix burner at an equivalence ratio, φ, of 0.5, atmospheric pressure and initial temperature of 300 K. Peak concentrations of CO and NO x were also measured at different φ. Computationally, a kinetic program has been developed to predict the above parameters for the premixed laminar natural gas–air mixture. The kinetic mechanism consists of 59 reactions, 25 chemical species as products and eight species as reactants. The results show that there is good agreement between the measured and computed NO x , CO, O 2 and gas temperature profiles. The results predicted from the model were used to interpret the kinetic factors responsible for the observations. Moreover, the effect of natural gas composition on NO x , flame structure and burning velocity was studied computationally. The results show that an increase of ethane concentration in the natural gas leads to an increase in the lean flammability limit, burning velocity, CO and NO x at very lean flames. The burning velocities of Egyptian natural gas–air flames were correlated at 300 K, 1 atm and different φ by: U l=0.37φ −0.35 exp[−5(φ−1.1) 2] (m s −1).

Low NO/sub x/ thermophotovoltaic generator

Fiber matrix burners incorporating "superemissive" ceramic materials were tested to obtain the relationship between power generation and the pollutant emissions. The tested variables were the following: equivalence ratio, fuel input, and inlet air temperature. One important objective was to achieve maximum efficiency of electricity generation at the lowest level of NO/sub x/ emissions. It was found that NO/sub x/ emission under these specific conditions does not depend on inlet air temperature in the ranges from 20 to 200/spl deg/C. This phenomena suggests that "thermal NO/sub x/" is not a significant contributor of NO/sub x/ emission in these types of burners. The test data indicate that the thermophotovoltaic (TPV) power output increased by a factor of 2 by increasing the combustion temperature while the NO/sub x/ emissions were stable. The result of these findings is that high efficiency, low polluting TPV-power generation is feasible. The basic technology responsible for the low NO/sub x/ emission and the electric power generation is being further developed for use in high-efficiency appliances, generators, and total energy systems.

Pollutant Emission Characteristics of Twin-Flame Matrix Burner.

Experimental studies were conducted on the pollutant emission characteristics of the twin-flame matrix burner which is composed of a number of counterflow burners arranged in a regular matrix. The burner consists of two opposing burner matrices which form the combustion chamber with two side plates and a bottom plate. The profiles of temperature and concentrations of some stable species were measured at the exit of the combustion chamber for methane flames. The NOx concentration in the combustion gas is far lower than the equilibrium concentration, because NOx formation was suppressed in the vicinity of the combustion chamber exit. A burner of this type can be operated for the fuel-lean mixture and the fuel is burned completely because the counterflow twin flames are stabilized even in the mixtures near the lean limit of flammability. This means that the burner of this type with a large turn-down ratio is also useful for the suppression of pollutant emission.

Combustion Characteristics of the Burner for High-Power Kerosene Stoves.

It is important for high-power kerosene stoves to have a wide TDR (turn-down ratio), because this kind of stove can be used in cold areas where the heating load is greatly changed. In order to develop high-power kerosene stoves, we prepared the matrix burner of counterflow premixed flames (twin-flame matrix burner), and investigated the performance of kerosene combustion. In this study, we clarified that the characteristics of blowoff and quenching in this burner are arranged between the calorific capacity of the combustion chamber and the premixed air ratio. Furthermore, in introducing the secondary air from slits and ports separately, we can improve the exhaust emission in this burner. Consequently, we have developed the twin-flame matrix burner for high-power kerosene stoves which has a wide TDR (5 : 1).

The formation of NO x in surface burners

Surface combustion of premixed methane/air mixtures within and near the downstream surface of a porous matrix burner were experimentally investigated. The experiments included measurements of radiant flux, surface temperature, gas temperature, and stable species concentrations. Particular attention was given to the burned gas temperature profiles in order to define the flame zone, and compare its position with that predicted by a theoretical model that utilizes large-activation-energy asymptotic methods. Although most of the methane is combusted (∼ 90%) within the porous medium, the maximum rate of heat release was found to occur at, or just outside, the surface. Prompt and thermal NO formation was modeled, and the vast majority of the NO was found to be formed by the prompt-NO mechanism.

Thermophotovoltaics: a new cogeneration technology for gas appliances

This technology is made possible by the development of a mixed oxide fiber matrix burner that provides narrowband selective radiation for the generation of photovoltaic power in addition to heat. The selective emission results in an extremely high conversion efficiency and provides significantly more electric power than a comparable photovoltaic system that utilizes typical blackbody radiation. Selective emission is tuned to the selective absorbability to provide the high conversion efficiency that results in high power output. This power is available to operate blowers, controls, and other electrical components within the appliance. This burner requires a minimum of excess air (increased efficiency), reduces emission of oxides of nitrogen, and is well suited for condensing combustion (increased efficiency) without the need for outside electric power. A design for a residual water heater, with a possible 94% efficiency, without the need for outside electric power is presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Attachment of flame chemielectrons to inert, fine particles

Micron size alumina particles were introduced into low pressure methane-air flat flames on a matrix burner. By means of electrostatic probes the concentrations of chemiions were measured and also the number of electrons attaching to the particles at different positions in the flame, for different particle concentrations. The maximum number of electrons so attached seems larger than that to be expected from a steady state analysis in which electrons and positive ion currents to a particle are equal. Quite small concentrations of particles can be effective in removing electrons from the gas phase. Because the particles are charged, they are efficiently removed by collector anodes at quite moderate voltages.

Effects of tetrafluorodibromoethane on the chemical ionization and chemiluminescence in propane-butane-air flames

In this paper, the effects of the inhibitor C/sub 2/F/sub 4/Br/sub 2/ on chemical ionization and chemical excitation occuring in propane-butane-air flames are considered. Flames burning at atmospheric pressure in a cylindrical burner were used as well as flames at 40-500 mm Hg with a planar matrix burner. The inhibitor vapor was supplied to the flow of fuel-air mixture from a heated cell. The inhibitor flow rate was determined by a gravimetric method. The emission intensities from CH* and C/sub 2/* and the concentration of positive ions as a function of tetrafluorodibromoethane flow rate indicate that the reaction for the formation of the primary CHO/sup +/ ion involves the HO/sub 2/ radical formed in the reaction H + O/sub 2/ + M ..-->.. HO/sub 2/ + M, whose rate is dependent on (H) and consequently on the inhibitor flow rate.

Studies of a Diffusion Flame Matrix Burner in a Combustion Chamber with Heat Exchanger

Experimental studies were carried out to make clear the combustion characteristics of a diffusion flame matrix burner in a combustion chamber with heat exchanger. The shape and the stability limits of the matrix burner flame were examined, and the concentrations of stable species and NOX in the exhaust gases were measured for methane flame, the burner surface/heat exchanger separation being the parameter. The minimum separation to maintain a stable flame with low emissions of pollutents was investigated. It has been found that this burner can be operated with good combustion and emission characteristics at a separation of only 10 mm, similarly to the matrix burner operated in the open atmosphere.

Studies of a Diffusion Flame Matrix Burner in a Combustion Chamber with Heat Exchanger

Studies of a Diffusion Flame Matrix Burner in a Combustion Chamber with Heat Exchanger

No&lt;SUB&gt;x&lt;/SUB&gt; Emission Characteristics of a Diffusion Flame Matrix Burner

Experimental studies were made of NOx formation in and its emission from a diffusion flame matrix burner, which is known as a typical shortflame burner. Profiles of temperature, stable-species and NOx concentrations were measured in detail for methane flame at atmospheric pressure, and the process of NOx formation and the NOx emission characteristics were examined. Measurements were made under various conditions of the air and fuel ejection-velocities for several types of burner matrices, and the effects of these parameters on the NOx formation in the flame zone and on the NOx emission were investigated. Results show that the formation of NOx is confined to a very narrow region about the flame zone and the NOx formation and emission are controlled mainly by the air ejection velocity. The NOx emission levels are found to be lower than those of usual combustion appliances.