Meso-scale Combustor Research Articles

Trap, ignition, and diffusion combustion characteristics of active carbon micro-particles at a meso-scale studied by optical tweezers

To study combustion characteristics of solid fuels at the meso-scale, this paper presents a study on trap, ignition, and diffusion combustion characteristics of active carbon micro-particles at a meso-scale by optical tweezers. In the meso-scale combustor, minimum trap power for active carbon micro-particles with a diameter of 7.0 m is 3.2 mW, and the trap velocity is in the range of 103.770.0 m/s. The active carbon micro-particles in static air flow can be ignited when the laser power is 3.2 mW. The effective diameter, perimeter, area and roundness of the particles have little effect on the minimum power for ignition. The ignition delay time is ～ 48 ms for active carbon micro-particles with a diameter of 3.0 m, and it will decrease till below 6 ms with increasing laser power. After ignited, the active carbon micro-particle shows flameless combustion first. The diffusion combustion velocity agrees with the diameter square linear-relationship, and the velocity is of 15.08.0 m/s. Then the active carbon micro-particle continues to carry out combustion reactions with bright flames repetitiously, and the flash frequency is 29.1 Hz. For the active carbon micro-particle with a diameter of 3.0 m, it can burn out thoroughly in an overall time ～ 0.648 s (including the heating and combustion processes). Results demonstrate that ignition of the active carbon micro-particle heated by high power density laser belongs to the combined ignition mode. Before volatile matter precipitates, the active carbon micro-particle is ignited heterogeneously and carries out a flameless combustion. However, after the volatile components are precipitated, it is ignited homogeneously, and the ombustion flame always shows a spheried shape.

Numerical and Experimental Studies of Mixing Enhancement and Flame Stabilization in a Meso-Scale TPV Combustor With a Porous-Medium Injector and a Heat-Regeneration Reverse Tube

Understanding the flow dynamics, chemical kinetics, and heat transfer mechanism within a miniature thermophotovoltaic (TPV) combustor is essential for the development of devices for combustion-based power microelectromechanical systems, which may have a much higher energy density than that of conventional batteries. In this study, methods for enhancing the intensity and uniformity of the combustion chamber wall (emitter) illumination through the design of combustion and thermal management of the combustor in a miniature TPV system are proposed, discussed, and demonstrated. The proposed miniature TPV system consists of a swirling combustor with the combustion chamber wall acting as the emitter, a heat-regeneration reverse tube, and mixing-enhancing porous-medium fuel injection, which improves the low nonuniform illumination or incomplete combustion problems associated with conventional miniature TPV systems. Experiments and numerical simulations are performed to analyze the details of the flame structure and flame stabilization mechanism inside the meso-scale combustor with and without a reverse tube. Results indicate that the proposed swirling combustor with a heat-regeneration reverse tube and porous medium can improve the intensity and uniformity of the combustion chamber (emitter) illumination and can increase the surface temperature of the chamber wall. From the systematic numerical and experimental analysis, suitable operational parameters for the meso-scale TPV combustor are suggested, which may be used as a guideline for meso-scale TPV combustor design.

환형 5중 동축관 연소기 내부에서의 예혼합 화염의 전파 특성 직접 관찰

Flame stabilization characteristics of premixed flames in an annular coaxial 5-tubes burner (AC5TB) were investigated experimentally. The AC5TB was made of five quartz tubes, and the flame stabilization conditions in that burner were investigated with the variation of equivalence ratio and the flow velocities. Flame behaviors inside of narrow annular tubes could be observed directly. Overall flame stabilization conditions were similar to that of the previous study, while the flame behaviors and structures were different mainly due to the controlled uniform distribution of the velocities in channels. Flame flashback conditions were thought to be governed by the competition between heat release rate, heat loss and heat recirculation in each channel. Stationary flames at a fixed location were compared in its velocity distribution and burned gas temperature across the channel. This AC5TB can be a basic configuration for the development of flame stabilization model of porous media combustors, and it will help understand about the real behavior of flames in meso-scale combustion spaces.

Investigation of Vortex Reacting Flows in Asymmetric Meso Scale Combustor

In the current study computational and experimental investigations of a turbulent asymmetric vortex flame is presented. The three dimensional flow fields have been described using a computational methodology that impalements the kε turbulence model. The computational model is validated for isothermal flow. Moreover, the visible flame structure was captured by direct photography at a wide range of equivalence ratios in order to emphasize the exceptional stability of such flame. The mechanism of flame stability and interaction with the forced vortex field is preliminarily discussed. Finally, the basic characteristics of the asymmetric vortex flames are concluded.

Flame propagation in a tube with wall quenching of radicals

In micro- and meso-scale combustion, there exists strong flame-wall interaction and flame can be quenched by thermal and kinetic mechanisms. The thermal quenching mechanism has been well studied while the kinetic quenching mechanism has received little attention. To provide an incremental advance to former analytical models, we conduct theoretical analysis on flame propagation in a tube with emphasis on both thermal and kinetic quenching mechanisms. A two-step chemistry model for gaseous combustion is employed and it consists of a chain-branching reaction and a completion reaction. To mimic the wall quenching of radicals, a one-step surface quenching reaction of radicals is considered. A general theoretical description of quasi-one-dimensional flame propagation in a tube with both heat loss and radical quenching is presented. An analytical correlation, which describes the change of the flame propagation speed with heat loss and radical quenching coefficients, is derived. Based on this correlation, the effects of radical Lewis number, cross-over temperature, wall temperature, and tube diameter on flame speed and quenching limit are examined. The results show that the impacts of both heat loss and radical quenching become stronger at smaller radical Lewis number. With the increase of cross-over temperature, flame extinction due to heat loss and that due to radical quenching are shown to be promoted and inhibited, respectively. With the increase of wall temperature, both the thermal and kinetic quenching limits are significantly extended. With the decrease of the tube diameter, flame extinction occurs due to thermal and/or kinetic quenching. It is found that the radical quenching effect becomes stronger at higher wall temperature and/or lower cross-over temperature.

좁은 다중 동축 석영관 내부에서의 예혼합 화염의 전파 특성에 대한 기초 실험

Flame stabilization characteristics of premixed flames in narrow annular coaxial tubes (NACT) were investigated experimentally. The NACT burner was proposed as a model of a cylindrical refractory burner, and it was made of quartz tubes. Flame stabilization conditions affected by the characteristic length of the burner was investigated with the variation of the equivalence ratio and the flow rates. Flame behaviors in narrow spaces could be directly observed. Conclusively, more wide flame stabilization conditions could be obtained at the case of the decreased channel scale. A flame instability, such as combustion noise was detected concerned with the flame oscillation observed at the surface of multi channel stage. Some flame propagation characteristics had complicated tendencies that may exist in practical porous-media combustors. Therefore, this NACT burner can be a basic configuration for the development of flame stabilization model in porous media combustor, and it will enhance our understanding about the behavior of flames in meso-scale combustion spaces.

Coupling micro and meso-scale combustion models of AP/HTPB propellants

Combustion simulations at the micro and meso-scales are coupled to give a more theoretically based and accurate description of AP (ammonium perchlorate)/HTPB (hydroxyl-terminated polybutadiene) composite propellant combustion. One and two-dimensional micro-scale combustion models of AP and AP/HTPB, which include detailed kinetics and species transport, are utilized to define semi-global kinetics for a meso-scale propellant combustion model capable of representing the complex morphology of an actual propellant. The AP/HTPB flame structure predicted with the micro-scale models is separated into four flames. Each flame is represented with a single reaction, thus a new four-step kinetic mechanism is implemented into the meso-scale model. Physical and kinetic parameters are determined with the micro-scale models. The meso-scale model is calibrated and validated with the micro-scale models to ensure the correct flame structure, as a function of pressure and AP particle sizes, is recreated with the four-step mechanism. The primary focus of this work is to outline the methodology used to couple the two numerical scales. Results of the meso-scale model are compared with the previous empirically-parameterized meso-scale model results and experimental data. Predictions are within 10% of experimental values for a range of AP/HTPB propellants.

Effects of inert gases on the vortex bursting in small diameter tubes

A flame propagation in a small diameter tube has been experimentally investigated with use of two meso-scale combustors. Methane was used as fuel, and four inert gases, Ar, N2, He and CO2 were used as diluents. Results show that, in the large diameter tube (Dg=10.0mm), the flame propagation range is the widest for Ar-dilution and becomes narrower in the order of N2, CO2 and He. In the small diameter tube (Dg=3.6mm), the propagation ranges of Ar and N2 are narrowed in fuel-rich region, where the Lewis number is greater than unity. For CO2 and He-dilution, however, the propagation has not been observed. Because the quenching diameters for CO2- and He-dilution are significantly large, the flames are strongly affected by the tube wall. Further experiments have shown that, under high oxygen concentrations, the flame can propagate in the small diameter tube for all diluents, and the propagation range is the widest for Ar-dilution and becomes narrower in the order of N2, CO2 and He. The flame is slender for CO2, whereas the flame is wide for He, as compared with Ar and N2. These differences in flame appearance clearly indicate that the Lewis number of deficient reactant affects strongly on the flame propagation in small diameter tube, and for CO2-dilution, as the Lewis number is sufficiently less than unity, the flame diameter can be small, and thus, the wall effects can be reduced. Whereas for He-dilution, as the Lewis number is sufficiently greater than unity, the flame diameter becomes large, and thus, the wall effects can be enhanced. The flame speed of He-dilution becomes considerably lower than the other three mixtures. This retardation seems to be resulted from an increase in the burned gas density due to the cooling by the enhanced wall effect.

Computational Study of a Mesoscale Combustor with Annular Heat Recirculation and Porous Inert Media

Combustion at small-scales is challenging because of the high heat loss and short flow residence time. This study features a combustor with porous inert media surrounded by an annulus. Reactants enter the annulus and are preheated by combustion heat that would otherwise be lost to the surroundings. Detailed computational fluid dynamics analysis is performed to identify design features with minimum heat loss. Analysis revealed coupled heat transfer pathways, i.e., reducing heat loss by axial conduction increases it by radiation and vice versa. Results show that the development of efficient small-scale combustion systems requires an accurate analysis of the coupled thermal pathways.

Numerical Studies of the Combustion of Methane-Air Mixture in a Meso-Scale TPV Combustor with a Heat-Regeneration Reverse Tube and a Porous-Medium Injector

The major objective of this paper is to develop a miniature combustor applied in a portable thermophotovoltaic (TPV) power generation system. The proposed miniature TPV system consists of a swirling combustor with the infrared thermal tube (ZrO2) acting as the emitter, a heat-regeneration reverse tube, and mixing-enhancing porous-medium fuel injection, which improves the low non-uniform illumination or incomplete combustion problems associated with conventional miniature TPV systems. A two-step global reaction mechanism was used to compute the chemical reactions of the reacting flows. Numerical simulations are performed to analyze the details of the flame structure and flame stabilization mechanism inside the meso-scale combustor with and without a reverse tube. In addition, these thermal conditions have strong effects on the combustion especially when the chamber dimension goes small and the ratio of surface area to volume becomes larger. Such as larger heat loss through the chamber wall to the ambient, may lead to the thermal quenching of meso-scale combustion. Therefore, the effect of various heat transfer conditions at chamber wall, e.g. with different wall heat conductivity on the combustion is delineated and discussed by numerical simulation. Results indicate that the proposed swirling combustor with a heat-regeneration reverse tube and porous medium can improve the intensity and uniformity of the combustion chamber (emitter) illumination, and can increase the surface temperature of the chamber wall. Correspondingly, the simulation results also indicated that the stable combustion in a meso-scale TPV combustor may be sustained through to control mass flow rate, heat loss conditions and maintaining proper thermal condition. Consequently, through such systematic numerical analysis, proper operational parameters for the meso-scale TPV combustor are suggested, which may be used as the guideline for meso-scale TPV combustor design.

Flame Imaging in Meso-scale Porous Media Burner Using Electrical Capacitance Tomography

This study was conducted to investigate the characteristics of meso-scale combustion. The technique of electrical capacitance tomography (ECT) was used to locate flame position and monitor the effect corresponding to varied air/fuel ratio in a meso-scale combustor. Combustion phenomena including igniting, quenching and unsteady combustion have been visualized using ECT. The method of metallization protecting ECT sensor from high temperature damage and the novel calibration method adapted to ECT monitoring of unknown permittivity flame have been shown to be successful. At the same time, electrical nature of combustion and dielectric characteristics of hydrocarbon flame were studied. The relationship between flame permittivity and state parameters of combustion gas was demonstrated preliminarily.

Micro and mesoscale combustion

A review of research and development on micro and mesoscale combustion is presented, with an emphasis on fundamental understandings achieved in the field during the last decade. Due to its small scale nature, increasing effects of flame–wall interaction and molecular diffusion are the characteristic features of micro and mesoscale combustion. After brief review of device developments, overview of fundamentals in micro and mesoscale combustion as well as possible future directions is presented.

Computational Fluid Dynamics Simulation of a Mesoscale Combustor: An Exercise in Verification and Validation

With the simulation of engineering processes via numerical methods on the rise comes the need for a quantitative measure of the agreement between computational results and experimental measurements. The use of quantitative methods in the comparison of the results of numerical and experimental analyses supersedes the traditional qualitative approach. In the present paper, the importance of the role of modeling assumptions in a verification and validation effort is illustrated through a mesoscale combustor example. The various types of uncertainties encountered in the experimentation and numerical simulation are investigated. Through the investigation the initial modeling assumptions proved to be insufficient, producing a comparison error outside of the acceptable range. Thus, the modeling assumptions were sequentially revised, minimizing the comparison error and producing a successful verification and validation effort.

Studies on non-premixed flame streets in a mesoscale channel

The effects of channel width, wall temperature, and flow rate on the dynamics of non-premix flames in a mesoscale combustor were studied in methane and propane–air mixing layers. A Flame Street structure which consists of multiple triple flamelets in the mixing layer of the reactants was observed experimentally for the first time. Depending on the flow rate, it was found that there are two different flame regimes, an unsteady bimodal flame regime and a Flame Street regime where there are multiple stable triple flamelets. It was found that the separation distance of the flamelets increased due to the dilution effect of the products and that the size of the flamelets was proportional to the width of the mixing layer. A scale analytical model was developed to qualitatively explain the mechanism of Flame Streets. The effects of flow velocity, wall temperature, and Lewis number were also investigated and the results showed that the occurrence of Flame Street regimes was a combined result of heat loss, curvature, diffusion, and dilution effect.

A novel meso-scale combustion system for operation with liquid fuels

A novel meso-scale combustion concept utilizing a flow-blurring injector to produce fine fuel droplets, a counter-flow heat exchanger to preheat reactants using product gas enthalpy, and porous inert medium to homogenize reactants and stabilize the flame is presented. The overall system is 30 mm long and 17 mm in diameter, with combustor diameter of 10 mm. For atmospheric pressure operation on kerosene fuel, the combustion system achieved heat release rate of up to 460 W, pertaining to energy density of 90 MW/m 3 based on the total volume and 230 MW/m 3 based on the combustor volume. The combustor performance was determined from surface temperatures measured by an infra-red camera, product gas temperature measured by an R-type thermocouple probe, and carbon monoxide (CO) and nitric oxide (NO x ) concentrations measured by a miniature sampling probe attached to gas analyzers. A comprehensive Computational Fluid Dynamics (CFD) model incorporating conjugate heat transfer, radiation heat exchange, flow and heat transfer in PIM, and heat release by combustion was developed and validated in this study. The CFD model accurately predicted the thermal performance of the combustion system. A 1D, burner-stabilized flame model incorporating detailed chemical kinetics was used to predict the flame structure. Model predictions are compared with measured CO and NO x concentrations, and subsequently, analyzed to identify the key reactions affecting the NO x production.

Combustion in a Meso-Scale Liquid-Fuel-Film Combustor with Central-Porous Fuel Inlet

Utilizing a metal-porous medium for a liquid fuel film combustor is an effective method to increase the contact surface area and conduction heat transfer for liquid fuel vaporization and flame stabilization. Based on this concept, a meso-scale liquid fuel film combustor with a central porous inlet is developed and tested. The effects of porous material type and bead size on the flame structures and combustion characteristics are examined. Porous media made of stainless steel and bronze are tested in the meso-scale combustor with different fuel and air flow rates, equivalence ratios, and bead sizes. The flame structure and its corresponding stabilization mechanism are different between the stainless steel and the bronze porous media combustor. In the stainless steel case, the high specific heat capacity enhances fuel vaporization and fuel-air mixing, and the flame anchor locates on the surface of the porous cap. In the bronze case, due to its low heat capacity, the flame is swept downstream where the recirculation zone above the porous cap offers a low velocity field to help anchor the flame. The flame structure and stabilization mechanism in the chamber can be related to a tribrachial flame. Chemiluminescence measurement and Abel deconvolution are performed to verify the flame structure in the vicinity of the porous cap. In addition, temperature measurements and exhaust gas analysis highlight the differences in combustion characteristics between the two kinds of porous media. As regards bead size effects, results indicate that there is no obvious difference in flame structure and flame anchoring position, but the stable operating ranges of a porous combustor decrease with decreasing bead size due mainly to the concomitant increase in thermal conductivity.

EFFECTS OF CHANNEL WIDTH AND LEWIS NUMBER ON THE MULTIPLE FLAME REGIMES AND PROPAGATION LIMITS IN MESOSCALE

The effects of channel width and Lewis number on the propagation limits and flame transition between multiple flame regimes in mesoscale combustion were studied experimentally and numerically using methane and propane-air mixtures. The experimental results showed that both methane and propane-air flames have multiple flame regimes. It was also shown that there existed non-monotonic dependence of flame speeds on equivalence ratio. Moreover, the results also showed that there are two different flame transitions, a direct transition and an extinction transition, depending on the channel width, Lewis number, and flow velocity. For the first time, a two-dimensional numerical simulation was conducted to predict the transition between different flame regimes and to examine the effects of channel width, Lewis number, and flow rates. The results showed that flame stretch had a significant effect on the extinction limit and the flame bifurcation. The predicted results agreed qualitatively well with the experiment.

Experimental study of spinning combustion in a mesoscale divergent channel

Quasi-steady and unsteady propagations of methane and propane–air premixed flames in a mesoscale divergent channel were investigated experimentally and theoretically. The emphasis was the impact of variable cross-section area and the flame-wall coupling on the flame transition between different regimes and the onset of flame instability. Experimentally, for the first time, spinning flames were observed in mesoscale combustion for both lean and rich methane and propane–air mixtures in a broad range of equivalence ratios. The spinning flames rotated in either clockwise or counterclockwise direction with equal probability. The results showed that for a fixed equivalence ratio, there was a critical flow rate, above which flame starts to spin. The spin frequency was approximately proportional to the flame speed. It was also found that the spinning flame only occurred after the transition from fast flame regime to slow flame regime. The flame propagation speed and the effective Lewis number were obtained analytically. Experimental observation and theoretical analysis suggested that regardless of the magnitude of mixture Lewis numbers, the flame-wall coupling will significantly increase the effective Lewis number and lead to a new mechanism to promote the thermal diffusion instability.

05/00316 Mesoscale combustion: a first step towards liquid fueled batteries: Kyritsis, D. C. et al. Experimental Thermal and Fluid Science, 2004, 28, (7), 763–770

05/00316 Mesoscale combustion: a first step towards liquid fueled batteries: Kyritsis, D. C. et al. Experimental Thermal and Fluid Science, 2004, 28, (7), 763–770

Mesoscale combustion: a first step towards liquid fueled batteries

The development of a liquid fueled mesoscale catalytic combustor is presented, based on the coupling of multiplexed electrosprays for liquid fuel dispersion and a stack of catalytic grids. The grids act as a compact catalytic reactor for combustion initiation, stabilization and enhanced conversion, as well as ground electrodes for the electrospray. The combustor has a volume on the order of few cm 3 and operates on various fuels, including JP8 jet fuel, which is electrosprayed at a flow rate on the order of 10 g/h and equivalence ratios varying from 0.35 to 0.70. Temperatures in the range 900–1300 K are achieved with a ±5% uniformity over the top circular surface of the burner. Using gas chromatographic analysis of the exhaust gases, a combustion efficiency in excess of 97% is estimated. This estimate was confirmed by exhaust gas composition measurements by photoionization mass spectrometry for operation with dodecane, the most abundant component of JP8. These measurements showed that alkenes and aldehydes are the major hydrocarbon products and that there is no significant molecular growth chemistry. The capability of the system to burn cleanly and efficiently even notoriously polluting fuels was demonstrated.