A Computational Study of Auto-ignition and Flame Propagation in Stratified Mixtures Relevant to Modern Engines PDF Download
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Author: Ramanan Sankaran
Publisher:
ISBN:
Category :
Languages : en
Pages : 314
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Book Description
Author: Ramanan Sankaran
Publisher:
ISBN:
Category :
Languages : en
Pages : 314
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Author:
Publisher:
ISBN:
Category : Combustion
Languages : en
Pages : 210
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Book Description
Flame speed data reported in most literature are acquired in conventional apparatus such as the spherical combusion bomb and counter flow burner, and are limited to atmospheric pressure and ambient or slightly elevated unburnt temperatures. As such, these data bear little relevance to internal combustion engines and gas turbines, which operate under typical pressures of 10-50 bar and unburnt temperature up to 900K or higher. These elevated temperatures and pressures not only modify dominant flame chemistry, but more importantly, they inevitably facilitate pre-ignition reactions and hence can change the upstream thermodynamic and chemical conditions of a regular hot flame leading to modified flame properties. This study focuses on how auto-ignition chemistry affects flame propagation, especially in the negative-temperature coefficient (NTC) regime, where dimethyl ether (DME), n-heptane and iso-octane are chosen for study as typical fuels exhibiting low temperature chemistry (LTC). The structure of this thesis consists of the introduction of the combustion, the governing equations in thermodynamics and chemical reactions as well as the general structure of the flame. Then, the typicl experimental configuration exploited in the measurement of laminar flame speed is introduced, which is followed by the manifestation of the low temperature chemistry and the gap between the reality and the experimental understandings. Finally, the simulation results of laminar flame speed at constant pressure condition and HCCI engine condition are presented and discussed respectively. The computation of laminar flame speed of lean and stoichiometric mixtures of fuel/air was performed at different ignition reaction progress, by selecting the thermal chemical states corresponding to different residence times during auto-ignition as the flame upstream condition. Using scaling and budget analysis, it is shown that a well-defined flame speed for such a partially reactive mixture in the classical diffusion-reaction limit could still be feasible in the appropriate computational domain, especially with a sufficiently reduced induction length. The comparison of flame speed against different types of progress variables indicates a nearly linear relationship between the flame speed and progress variables based on the fuel mass fraction and temperature. The overall effect of the cool-flame reformation has been studied by comparing the flame speed of the initial mixture and that of the instantaneous mixture under the same thermodynamic conditions. It is found that the enhanced propagation is shown to be largely a thermodynamic effect, while chemistry nevertheless plays an overall retarding role. Sensitivity analysis has been performed to identify the key species which most influence flame propagation at different reaction progress. A general scheme of simplified mixture was constructed to describe flame propagation in a partially reactive mixture, for both lean and stoichiometric, as well as high pressures conditions. The findings and general simplified mixture scheme are validated in HCCI engine conditions.
Author: Andreas Manz
Publisher: Logos Verlag Berlin GmbH
ISBN: 3832542817
Category : Science
Languages : en
Pages : 263
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Book Description
Downsizing of modern gasoline engines with direct injection is a key concept for achieving future CO22 emission targets. However, high power densities and optimum efficiency are limited by an uncontrolled autoignition of the unburned air-fuel mixture, the so-called spark knock phenomena. By a combination of three-dimensional Computational Fluid Dynamics (3D-CFD) and experiments incorporating optical diagnostics, this work presents an integral approach for predicting combustion and autoignition in Spark Ignition (SI) engines. The turbulent premixed combustion and flame front propagation in 3D-CFD is modeled with the G-equation combustion model, i.e. a laminar flamelet approach, in combination with the level set method. Autoignition in the unburned gas zone is modeled with the Shell model based on reduced chemical reactions using optimized reaction rate coefficients for different octane numbers (ON) as well as engine relevant pressures, temperatures and EGR rates. The basic functionality and sensitivities of improved sub-models, e.g. laminar flame speed, are proven in simplified test cases followed by adequate engine test cases. It is shown that the G-equation combustion model performs well even on unstructured grids with polyhedral cells and coarse grid resolution. The validation of the knock model with respect to temporal and spatial knock onset is done with fiber optical spark plug measurements and statistical evaluation of individual knocking cycles with a frequency based pressure analysis. The results show a good correlation with the Shell autoignition relevant species in the simulation. The combined model approach with G-equation and Shell autoignition in an active formulation enables a realistic representation of thin flame fronts and hence the thermodynamic conditions prior to knocking by taking into account the ignition chemistry in unburned gas, temperature fluctuations and self-acceleration effects due to pre-reactions. By the modeling approach and simulation methodology presented in this work the overall predictive capability for the virtual development of future knockproof SI engines is improved.
Author: Mateen Mahmood Shaikh
Publisher:
ISBN:
Category :
Languages : en
Pages : 146
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Author: Linda Maria Beck
Publisher: Logos Verlag Berlin
ISBN: 9783832534264
Category :
Languages : en
Pages : 0
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The virtual development of future Spark Ignition (SI) engine combustion processes in three-dimensional Computational Fluid Dynamics (3D-CFD) demands for the integration of detailed chemistry, enabling - additionally to the 3D-CFD modelling of flow and mixture formation - the prediction of fuel-dependent SI engine combustion in all of its complexity. This work presents an approach, which constitutes a coupled solution for flame propagation, auto-ignition, and emission formation modelling incorporating detailed chemistry, while exhibiting low computational costs. For modelling the regular flame propagation, a laminar flamelet approach, the G-equation is used. Auto-ignition phenomena are addressed using an integrated flamelet approach, which bases on the tabulation of fuel-dependent reaction kinetics. By introducing a progress variable for the auto-ignition - the Ignition Progress Variable (IPV) - detailed chemistry is integrated in 3D-CFD. The modelling of emission formation bases on an interactively coupled flamelet approach, the Transient Interactive Flamelet (TIF) model. The functionality of the combined approach to model the variety of SI engine combustion phenomena is proved first in terms of fundamentals and standalone sub-model functionality studies by introducing a simplified test case, which represents an adiabatic pressure vessel without moving meshes. Following the basic functionality studies, the sub-model functionalities are investigated and validated in adequate engine test cases. It is shown, that the approach allows to detect locally occurring auto-ignition phenomena in the combustion chamber, and to model their interaction with regular flame propagation. Moreover, the approach enables the prediction of emission formation on cell level.
Author: Zhichao Tan
Publisher:
ISBN:
Category :
Languages : en
Pages : 240
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Author: Hanif Kasmani
Publisher:
ISBN:
Category : Flame
Languages : en
Pages : 45
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Book Description
This thesis deals with the flame propagation in spark ignition engine combustion process using Computational Fluid Dynamic (CFD). This study is based on flame propagation inside the combustion chamber which is important as flame propagation affects the engine efficiency, emission and some more. A 3-D model is created based on the Mitsubishi Magma 4G15 that act as a baseline engine. It is then simulated using CFD where its approaches make it a suitable platform to study the internal combustion engine phenomenon. The project simulates only 50o CA starting from the ignition until the completion of the combustion process. The flame radius obtain through simulation is then compared with the experimental data together with the literature review. However, there are discrepancies of the results due to improper boundary condition and inherit limitation of the model. For further simulation of combustion process must consider detail mixture properties, detail boundary condition and model extension for better accuracy data.
Author: John Claude Boutet
Publisher:
ISBN:
Category :
Languages : en
Pages : 216
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Author: Keshav S. Varde
Publisher:
ISBN:
Category :
Languages : en
Pages : 226
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Author: Xian Shi
Publisher:
ISBN:
Category :
Languages : en
Pages : 186
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Stratified combustion, combustion of fuel/air mixtures with temperature and/or mixture-composition stratification, is present in many combustion-related phenomena and applications such as forest wildfires, mining explosions, vessel ruptures, gas turbines, and reciprocating engines to name a few. A new generation of highly efficient internal combustion (IC) engines capable of satisfying stringent emission requirements, including modern direct-injection gasoline engines and gas turbines with lean premixed pre-vaporized (LPP) combustors, requires more comprehensive understanding and control of stratified combustion. Fundamentally, stratification of temperature or mixture composition affects a wide range of combustion characteristics such as flame speed, flammability, mode of combustion, instability, and others. This dissertation aims to identify, analyze and evaluate fundamental processes in the combustion of stratified mixtures, using theoretical analysis and advanced numerical simulation tools. ASURF-Parallel, a transient numerical solver of compressible reacting flow, is developed on the basis of the original A-SURF and exploited for stratified combustion simulations. A domain-decomposition parallelization scheme using Message Passing Interface (MPI) is developed and implemented in ASURF-Parallel to speed up the otherwise time-consuming numerical simulations. A significant speedup with the speed-up factor up to 10 is achieved on lab-scale servers. Effects of stratification on flame speeds, lean flammability limit, and modes of combustion are numerically investigated and studied. For flame speeds, laminar flame speeds of stratified flames propagating from rich mixtures to lean mixtures are generally faster than those of the corresponding homogeneous flames, primarily due to the preferential diffusion of lighter species and radicals such as H2, H and OH, i.e., the chemical effect. The degree of enhancement in flame speeds can be correlated to the degree of stratification, leading to the development of a transient local stratification level (LSL) model which is able to determine the stratified flame speeds incorporating both chemical effect and memory effect. For lean flammability limits, the extension introduced by stratification is very weak due to reduced overall reactivity and reduced degree of stratification. For modes of combustion, different modes can be realized by specific reactivity gradients, regardless of the sources of such gradients. Pressure waves introduced by ignition in a closed chamber can also lead to different modes of reaction front propagation and end-gas combustion. A transient reactivity gradient method is proposed to identify the onset of detonation.
