Comparative Study on Combustion and Flame Characteristics of Laminar Methane/Air and N-Butane/Air Flames in a Micro-Slot Burner (original) (raw)
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Thermal and chemical structures formed in the micro burner of miniaturized hydrogen-air jet flames
Proceedings of the Combustion Institute, 2015
The thermal and chemical structure formed in the micro burner potentially leading to the unique stability mechanism of miniaturized jet diffusion flame via excess heat recirculation through the burner wall, is studied numerically. 2-D axis-symmetric heat and mass transport processes with chemical reactions are considered in gas phase, while the heat transport process is considered in solid phase. The burner materials and fuel ejecting velocities are considered as the main numerical parameters in order to examine the role of the burner on the thermal and chemical structure inside the burner. A skeletal reaction mechanism consists of twelve steps chemical reactions and nine species is applied to consider the radical generation/consumption, and their transport in the burner. It is found that the tiny flame is stabilized even with extraordinary small Reynolds number, at which an extinction is generally experienced, when the low conductivity burner is adopted. This is due to the effective usage of the transferred heat from the flame to the burner to improve the stability owing to (1) preheating the incoming fuel, (2) the least heat loss from the flame toward the burner tip, and (3) enhancing the reactivity inside the burner. In such low Reynolds number jet flow with the low conductive burner, ambient air can easily diffuse back into the burner and oxidative reactions at the vicinity of the burner tip is then promoted. Accordingly, the flame structure along the axis is dramatically modified beyond the typical 1-D flame. It is suggested that the further chemical process could be promoted in the substantially-heated micro burner by considering the catalytic reaction at the inner burner surface or using oxygenated fuel likely alcohol or ether.
Microscale combustion: Technology development + fundamental research
2011
Technical paper by Yiguang Ju + Kaoru Maruta, 2011 Princeton University + Tohoku University Deals with combustion in demanding environments + under more difficult conditions, including rotary engine, quenching ... 47 pages > Content ... > Abstract 1 > Contents 1 > 1. Introduction 2 » 1.1 Needs of micro-scale combustion 2 » 1.2 Scaling parameters of micro-scale combustion 3 » 1.3 Development + challenges of micro-power generators 4 » 1.4 Challenges in fundamental research of micro-scale combustion 5 » 1.5 Focus of previous + the present reviews 5 > 2. Meso + micro-scale combustors 6 » 2.1 Micro-thrusters 6 » 2.2 Micro internal combustion engines 8 » 2.3 Micro reactors 10 > 3. Flame dynamics of micro-scale combustion 14 » 3.1 Premixed combustion 14 » 3.1.1 Flammability limit + quenching diameter 14 » 3.1.2 Heat recirculation 16 » 3.1.3 Flame + structure coupling 17 » 3.1.4 Thermal + kinetic quenching 20 » 3.1.5 Weak flame regimes with temperature gradient 23 » 3.2 Catalytic micro-combustion 25 » 3.2.1 Stability + flammability limits of catalytic combustion 25 » 3.2.2 Interaction / transition between gas-phase + surface reactions 27 » 3.2.3 Ignition of catalytic reaction 29 » 3.3 Non-equilibrium combustion 31 » 3.4 Flame instability 32 » 3.4.1 Repetitive extinction + reignition instability 33 » 3.4.2 Spinning instability 35 » 3.4.3 Spiral flames + pattern formations 37 » 3.5 Non-premixed combustion 38 » 3.5.1 Mixing liquid fuel vaporization 39 » 3.5.2 Formation of diffusion flame cells + flame streets in mesa + micro-scale combustion 41 > 4. Future research of micro-combustion 42 » 4.1 Low temperature meso-scale combustion for advanced engines 42 » 4.2 Microreactors for fuel reforming 43 » 4.3 Microreactors for boundary layer flow control 43 » 4.4 Micro-combustion launching new concept fundamentals 43 > References 43 german keywords: Wankelmotor / Kreiskolbenmotor English Keywords: Wankel Engine / Wankel Rotary Engine / Rotary Piston Engine / Rotary Combustion Engine The high energy density of hydrocarbon fuels creates a great opportunity to develop combustion based micro-power generation systems to meet increasing demands for portable power devices, micro unmanned aerial vehicles, micro-satellite thrusters, and micro chemical reactors and sensors. In this paper, the recent technological development of micro-power systems and progress in fundamental understanding of micro-scale combustion are reviewed. At first, micro-scale combustion regimes are categorized by using different physical and chemical length and time scales and the resulting nondimensional parameters and their correlations to various combustion regimes for micro and mesoscale combustion are discussed. Secondly, the recent successful developments and technical challenges of micro-thrusters, micro internal combustion engines, and micro chemical reactors summarized. Thirdly, the underlying fundamental mechanisms and ignition and flame dynamics in micro-scale combustion are reviewed, respectively, in premixed, non-premixed, catalytic, and non-equilibrium, micro-scale combustion systems. The conventional concepts of combustion limits such as the flammability limit, quenching diameter, and flame extinction and heat recirculation are revisited. The unique thermal and chemical transport mechanisms such as flame structure interaction, radical quenching, non-equilibrium transport appearing in micro-scale combustion are discussed. New flame regimes and instabilities such as flame bifurcation, weak flames, flame cells/streets, thermal and kinetic quenching, flameless low temperature catalytic combustion, repetitive extinction and ignition, spinning flames, spiral and multibranched flames, symmetric and asymmetric oscillating flames are discussed. Finally, an overview of future research and conclusion are made. The goal of this review is to present an overview of the development of micro-power generators by focusing more on the advance in fundamental understanding of micro-scale combustion.
Fuel, 2018
Main challenges for a fuel efficient micro scale combustion are rooted from size restriction of micro combustors which results with inappropriate residence time of fuel/air mixture and intensified heat losses due to relatively high surface to volume ratio of such devices. One way of increasing energy output of micro combustors is to optimize its geometry by considering simplicity and easy manufacturability. In this study, effect of combustor geometric properties on combustion behavior of premixed hydrogen/air mixtures was numerically investigated. For this purpose, an experimentally tested micro combustor's geometric properties were modified by establishing a backward facing step which is varying distance from combustor inlet and has varying step height, and adding opposing cavities which are varying distance from combustor inlet and have constant length to depth ratio into the flow area. Modeling and simulation studies were performed using ANSYS Design Modeler and Fluent programs, respectively. Combustion behavior was analyzed by means of centerline and outer wall temperature distributions, amount of heat transferred through combustor wall, conversion ratio of input chemical energy to utilizable heat, and species distributions. Turbulence model used in this study is Renormalization Group (RNG) k-ε. Multistep combustion reaction scheme with 9 species and 19 steps was simulated using Eddy Dissipation Concept model (EDC). Results showed that backward facing step in the flame region alters reaction zone distribution, flame length and shape, and consequently temperature value and distribution throughout the combustor. Lastly cavity was found to slightly increase peak temperature value.
An engineering model of a propane-fueled miniature combustor was developed for ultra-micro gas turbines. The combustion chamber had a diameter of 20 mm, height of 4 mm, and volume of 1.26 cm 3 . The flat-flame burning method was applied for lean-premixed propane-air combustion. To create the stagnation flow field for a specific flat-flame formation, a flat plate was set over the porous plate in the combustion chamber. A burning experiment was performed to evaluate the combustion characteristics. The flame stability limit was sufficiently wide to include the design operation conditions of an equivalence ratio of 0.55 and air mass flow rate of 0.15 g/s, and the dominant factors affecting the limit were clarified as the heat loss and velocity balance between the burning velocity and the premixture flow velocity at the porous plate. CO, total hydrocarbons (THC), and NO x emission characteristics were established based on the burned gas temperatures in the combustion chamber and the temperature distribution in the combustor. At an air mass flow rate of less than 0.10 g/s, CO and THC emissions were more than 1000 ppm due to large heat loss. As the air mass flow rate increased, the heat loss decreased, but CO emissions remained large due to the short residence time in the combustion chamber. NO x emission depended mainly on the burned gas temperature in the combustion chamber as well as on the residence time. To reduce emissions despite the short residence time, a platinum mesh was placed after the combustion chamber, which drastically decreased the CO emissions. The combustor performance was compared with that of other miniature combustors, and the results verified that the present combustor has suitable combustion characteristics for a UMGT, although the overall combustor size and heat loss need to be reduced.
Energy, 2017
In this study, effect of micro combustor geometry on combustion and emission behavior of premixed hydrogen\air mixtures is numerically investigated. An experimentally tested micro combustor geometry is varied by establishing a cavity or a backward facing step or micro channels inside the combustor. Considering effect of combustor geometry on the amount of heat transferred through wall based on outer wall and combustor centerline temperature distributions, combustion behavior is analyzed. Emission behavior is examined by means of mixing conditions, combustion efficiency and maximum temperature value which are highly bound to geometric properties of a micro combustor. Turbulence model used in this study is Renormalization Group k-ε. For turbulence chemistry interaction, Eddy Dissipation Concept model is used. Multistep combustion reaction scheme includes 9 species and 19 steps. Numerical results obtained from this study are validated with published experimental data. Results of this study revealed that combustion in such combustors can be improved by means of quality of mixing process, residence time, combustor centerline and outer wall temperature distributions, conversion rate of input chemical energy to utilizable heat and emanated NO x levels from combustor outlet with proposed geometric variations.
Flame Stability and Combustion Characteristics in Catalytic Micro-combustors
The flame stability and combustion characteristics in catalytic micro-combustors were studied using an elliptic two-dimensional computational fluid dynamics model that includes detailed homogeneous and heterogeneous chemical reaction schemes, heat conduction in the solid wall, surface radiation heat transfer, and external heat losses. Simulations were carried out to investigate the effects of wall thermal conductivity, wall thickness, inlet velocity, and operating conditions on combustion characteristics and the steady-state, self-sustained flame stability of hydrogen-air mixtures. Simulation results reveal that the reaction is limited by heat transfer near the entrance and by mass transfer further downstream, despite the small scales of this system. Large transverse and axial gradients are observed even at these small scales under certain conditions. Wall thermal conductivity and thickness are very important as they determine the upstream heat transfer, which is necessary for micro-flame ignition and stability, and the material's integrity by controlling the existence of hot spots. Wall thermal conductivity is vital in determining the flame stability of the system, as the walls are responsible for the majority of the upstream heat transfer as well as the external heat losses. Thin walls exhibit large axial temperature gradients, resulting in hot spots. Thicker walls have a large cross-sectional area, which allows for greater heat transfer and more uniform, lower temperatures. Inlet velocity plays a competing role in flame stability. Low flow velocities result in reduced power generation, and high flow velocities decrease the convective timescale below that of the upstream heat transfer through the walls. There exists a range of flow velocities that allow stabilized combustion in catalytic micro-combustors.
2007
Abstract Experimental investigations on combustion stability limits for methane–air mixtures in a radial micro-scale combustor configuration are herein reported. To study the flame stability characteristics in this radial combustor configuration, two circular quartz plates were arranged parallel to each other and a fuel–air mixture was supplied at the center of the plates. The plates were externally heated to create a positive temperature gradient condition in the flow direction to simplify the heat recirculation process through the solid walls.
Numerical study on stabilization of flame inside micro-combustor
1ST INTERNATIONAL CONFERENCE ON MANUFACTURING, MATERIAL SCIENCE AND ENGINEERING (ICMMSE-2019), 2019
This work aims to numerically study the stabilization of flame in a MEMS based methane-air adiabatic microcombustor. Three different micro-combustor geometries were computationally simulated using premixed methane air-fuel in quartz micro-combustor with different configurations in order to determine the optimal micro-combustor design for the highest efficiency. The study was conducted to determine the design and characteristics of flame stabilization behavior in the micro-combustor. ANSYS Fluent application software was used to simulate the models. Premixed fuel of methane-air is used to stabilize the flame with a stoichiometric ratio of 0.6. The design of micro-combustor is modified by creating backward steps in the model, in order to create a recirculation zone to slow down the flow, which intends to stabilize the flame at center of the micro combustor. The model was analyzed for different inlet velocities, ranging from 0.5 to 2 m/s, of the premixed fuel. It is clear from the analysis that a micro-combustor with one backward step stabilizes the flame with highest efficiency for different inlet velocities.
Journal of Thermal Science and Technology
Microscale hydrogen (H2) combustion is one of the promising technologies for renewable miniaturized heat sources. This study analyzes the oxygen combustion of H2 in small-scale counterflow burners, with carbon dioxide (CO2) added for safe hydrogen treatment (flame visualization and reduction of flame propagation velocity). The effects of burner inner diameter, burner gap, and gas flow rate on the flame shape/size (thickness and diameter) are measured through flame image analysis. The experimental results show that the flame thickness and diameter monotonically decrease with a decrease in the burner inner diameter, burner gap, and H2 flow rate. The flame thickness decreases with an increase in the flame stretch rate, and the approximate curve representing this relationship varies depending on the burner inner diameter and H2 flow rate. Accordingly, the flame thickness normalized by H2 flow velocity and burner inner diameter is newly proposed, which strongly correlates with the flame stretch rate and converges on a single line, i.e., inverse of the square root of the flame stretch rate. These findings are also applicable to biogas (CH4-CO2 mixture)-O2 micro counterflow diffusion flames with the same CO2 concentration in the fuel gas and apparent equivalence ratio.