Effect of different turbulence models on combustion and emission characteristics of hydrogen/air flames (original) (raw)
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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.
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.
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.
Numerical Simulations of a Micro Combustion Chamber
47th AIAA Aerospace Sciences Meeting including The New Horizons Forum and Aerospace Exposition, 2009
The goal of this paper is to investigate the performance of microcombustors for microturbines and for propulsion. Such field is currently under rapid development because of new market requirements. In particular, main areas of interest for microcombustion are propulsion, e.g., for UAVs, and micro-electrical power generators. This study is focused on a cylindrical microcombustor fed by methane and air, with diameter and height 0.006m and 0.009m, respectively. Following a preliminary scaling analysis, two combustion models were tested, and 3D RANS numerical simulations were performed. The two combustion models simulating micro-combustor flames are the eddy dissipation model with fast chemistry and the flamelet model. Both use a novel 2-step reduced kinetics mechanism: this was properly tuned for the present device. Results indicate that the two models predict similar results for what concerns the chamber maximum temperature and outlet temperature; they differ in predicting combustion efficiency: in particular the eddy dissipation model underpredicts the measured combustion efficiency while the flamelet model overpredicts it. Compared to the eddy dissipation model, the advantage of the flamelet model is its enormous computational time saving. This work should be seen as an advance in the understanding of how to design, and what to expect from future microcombustors applications.
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.
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.
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.
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.
Presumed PDF modeling of microjet assisted CH 4 -H 2 /air turbulent flames
sds, 2016
The characteristics of microjet assisted CH 4-H 2 /air flames in a turbulent mode are numerically investigated. Simulations are performed using the Computational Fluid Dynamics code Fluent. The Presumed PDF and the Discrete Ordinates models are considered respectively for combustion and radiation model-ing. The k-e Realizable model is adopted as a turbulence closure model. The Tesner model is used to calculate soot particle quantities. In the first part of this paper, the Presumed PDF model is compared to the Eddy Dissipation model and to slow chemistry combustion models from literature. Results show that the Presumed PDF model predicts correctly thermal and species fields, as well as soot formation. The effect of hydrogen enrichment on CH 4 /air confined flames under the addition of an air microjet is investigated in the second part of this work. The found results show that an inner flame was identified due to the air microjet for the CH 4-H 2 /air flames. Moreover, the increase of hydrogen percentage in the fuel mixture leads to mixing enhancement and consequently to considerable soot emission reduction.
A COMPARATIVE STUDY OF TURBULENCE MODELLING IN HYDROGEN-AIR NONPREMIXED TURBULENT FLAMES
Combustion Science and Technology, 2006
The goal of this paper is to investigate the predictive capability of two turbulence models which are the k-ε model and the Reynolds Stress Model (RSM) within flamelet approach. A co-flow axisymetric turbulent non-premixed hydrogen flame investigated experimentally by and is used as a test case. The chemical mechanism of Yetter's and al. (1991) is adopted for the generation of the flamelet library. It consists of 10 chemical species and 21 reactions. The comparisons with experimental data demonstrate that predictions based on the Reynolds stress turbulence model are slightly superior to those obtained using the k-ε model. Overall, profile predictions of axial velocity, turbulent kinetic energy, mixture fraction, flame temperature and major species are in reasonable agreement with data and compare favourably with the results of earlier investigations.