The Analytical Modeling of Finite-Length Homogonous Micro-Combustor for a Hydrogen-Oxygen Mixture with Wall Temperature Effects (original) (raw)
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International Journal of Hydrogen Energy, 2014
The present paper aims to analytically investigate combustion phenomenon in microcombustors by using a two-dimensional model. The main objective is to analyze the effects of main parameters such as reaction zone thickness, maximum temperature and quenching distance throughout the combustor under catalytic and non-catalytic conditions. In solution of energy and mass equations, the temperature and mass dependant reaction rates are considered with an iterative procedure. The reaction zone thickness is considered as a variable and is predicted by the solution results of the present study. In order to validate the present model, the normalized magnitude of flame temperature is compared with published data, which shows an acceptable agreement and confirms the accuracy of the predicted data. The results show that the effect of catalytic surface on expanding flammability limits in a lean mixture is larger than the rich one.
Analytical two-dimensional modeling of hydrogen–air mixture in catalytic micro-combustor
Meccanica, 2015
This paper investigates the combustion phenomenon in catalytic micro-combustor by using a twodimensional model. In solution of energy and mass equations, the temperature and mass dependant reaction rates are considered with an iterative procedure where the series matching conditions are exerted between neighbor zones. The reaction zone thickness is considered as a variable parameter and predicted by the solution results of the present study. The study of main parameters effects such as normalized wall temperature, Peclet number and equivalence ratio is also attained. The limitations of low and high Peclet number are determined where at high Peclet numbers the maximum temperature is decreased and the trend is inversed at low Peclet numbers. In order to validate this model the results are compared with published experimental data, showing an acceptable agreement and confirming the accuracy of the predicted data.
International Journal of Hydrogen Energy, 2014
Lengthedepth ratio Flame-splitting limit Recirculation zone a b s t r a c t Combustion characteristics of H 2 /air mixture in a micro-combustor with wall cavities were investigated numerically. The effects of inlet velocity, equivalence ratio, and the length edepth ratio of the cavity were studied. The results show that at a high enough velocity the flame splits in the middle which leads to a large amount of fuel leakage and a sharp decrease in the conversion rate of hydrogen. Meanwhile, the flame splits at the inner wall which gives rise to two high temperature regions and double temperature peaks at outer wall. Moreover, the flame-splitting limit is extended at a higher equivalence ratio due to a more intensive reaction. Furthermore, the flame-splitting limit increases for a larger length edepth ratio of the cavity, whereas the wall temperature level decreases. Therefore, excessive large lengthedepth ratios are not beneficial for this type of micro-combustors if the combustor walls are used as heat sources of thermoelectric or thermal photovoltaic devices.
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.
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
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Preliminary model of a CH4 micro-combustor
2010
During ages combustion has been the most straightforward technology to gather energy from fuels for heating and power generation. The simplest way to make the combustion happen in gaseous streams is to join and ignite two flows of combustible and oxidant respectively; once stationary state is reached the flame is a diffusive type with a thin layer inside which fuel mixes with air and burns. The main advantage of this simple way of burning gas is the very high peak temperature reached in the combustion layer, which allows to operate without any catalyst, but a high temperature is reached also in the adjacent zones resulting in particulate matter formation in the fuel rich zone and in Nox production in the lean layer. Considering also that particulate matter quenches dramatically flame temperature by radiance, [1] its production lets more CO form and so we have to improve combustion techniques in order to decrease pollutant emission directly at their source. Premixed combustion eases ...
International Journal of Engineering, 2014
A micro combustor is one of important devices in heat generation to power miniaturized products such as microrobots, notebook computers, micro-aerial vehicles and other small scale devices. An integrated micro combustor with thermophotovoltaic (TPV) in a micro-size electric generator supplies electricity to these micro devices. There is a growing interest in developing micro combustors as a power source due to their inherent advantages of higher energy density, higher heat and mass transfer coefficients and shorter recharge times compared to electrochemical batteries. A new micro combustion concept is described in this work by introducing a new terminology in the micro combustion. The effects of Area to Volume Ratio (AVR) of the micro-combustors were studied to find the best performance of designed micro-combustors. In order to test the feasibility of the designed micro combustors before the actual experiment is conducted, simulation work was performed. There are three key parameters involved in the current study: Area to Volume Ratio (AVR), Flow Velocity of the mixture (u), and Fuel-Air Equivalent Ratio (Ø). Main results of this experiment are images of temperature contour, graphs of temperature distribution profile, and graphs of mean temperature profile. This study found there is a specific range of mixture flow velocity (0.50-0.56 m/s) which result a high and uniform temperature distribution as well as its best mean temperature of micro combustion process. The simulation work could also localized the specific range of AVR-value (1.40-2.01) which require further investigation in the future.
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.