Large Eddy Simulation of Flame Response to Transverse Acoustic Excitation in a Model Reheat Combustor

2012 ◽  
Author(s):  
M. Zellhuber ◽  
C. Meraner ◽  
R. Kulkarni ◽  
W. Polifke ◽  
B. Schuermans
Keyword(s):  
Heat Release ◽  
Flame Propagation ◽  
Shear Layers ◽  
Harmonic Excitation ◽  
Single Frequency ◽  
Jet Flame ◽  
Eddy Simulation ◽  
Auto Ignition ◽  
Transverse Acoustic ◽  
Flame Response

The response of a perfectly premixed, turbulent jet flame at elevated inflow temperature to high frequency flow perturbations is investigated. A generic reheat burner geometry is considered, where the spatial distribution of heat release is controlled by auto-ignition in the jet core on the one hand, and kinematic balance between flow and flame propagation in the shear layers between the jet and the external recirculation zones on the other. To model auto-ignition and heat release in compressible turbulent flow, a progress variable / stochastic fields formulation adapted for the LES context is used. Flow field perturbations corresponding to transverse acoustic modes are imposed by harmonic excitation of velocity at the combustor boundaries. Simulations with single-frequency excitation are carried out in order to study the flame response to transverse fluctuations of velocity. Heat release fluctuations are observed predominantly in the shear layers, where flame propagation is important. The flow-flame coupling in these regions is analysed in detail with a filter-based post-processing approach, invoking a local Rayleigh index and providing insight into the interactions of flame wrinkling by vorticity and convection due to mean and fluctuating velocity.

Large Eddy Simulation of Flame Response to Transverse Acoustic Excitation in a Model Reheat Combustor

2013 ◽  
Vol 135 (9) ◽  
Author(s):  
Mathieu Zellhuber ◽  
Christoph Meraner ◽  
Rohit Kulkarni ◽  
Wolfgang Polifke ◽  
Bruno Schuermans
Keyword(s):  
Heat Release ◽  
Flame Propagation ◽  
Shear Layers ◽  
Harmonic Excitation ◽  
Single Frequency ◽  
Jet Flame ◽  
Eddy Simulation ◽  
Transverse Acoustic ◽  
Flame Response ◽  
Compressible Turbulent Flow

The response of a perfectly premixed, turbulent jet flame at elevated inflow temperature to high frequency flow perturbations is investigated. A generic reheat burner geometry is considered, where the spatial distribution of heat release is controlled by autoignition in the jet core on the one hand, and kinematic balance between flow and flame propagation in the shear layers between the jet and the external recirculation zones on the other. To model autoignition and heat release in compressible turbulent flow, a progress variable/stochastic fields formulation adapted for the LES context is used. Flow field perturbations corresponding to transverse acoustic modes are imposed by harmonic excitation of velocity at the combustor boundaries. Simulations with single-frequency excitation are carried out in order to study the flame response to transverse fluctuations of velocity. Heat release fluctuations are observed predominantly in the shear layers, where flame propagation is important. The flow-flame coupling in these regions is analyzed in detail with a filter-based postprocessing approach, invoking a local Rayleigh index and providing insight into the interactions of flame wrinkling by vorticity and convection due to mean and fluctuating velocity.

Numerical Investigation of Fuel Property Effects on Mixed-Mode Combustion in a Spark-Ignition Engine

2019 ◽  
Author(s):  
Chao Xu ◽  
Pinaki Pal ◽  
Xiao Ren ◽  
Sibendu Som ◽  
Magnus Sjöberg ◽  
...  
Keyword(s):  
Heat Release ◽  
Flame Propagation ◽  
Heat Release Rate ◽  
Release Rate ◽  
Octane Number ◽  
Mixed Mode ◽  
Spark Ignition ◽  
Spark Ignition Engine ◽  
Chemical Effect ◽  
Auto Ignition

Abstract In the present study, mixed-mode combustion of an E30 fuel in a direct-injection spark-ignition engine is numerically investigated at a fuel-lean operating condition using multidimensional computational fluid dynamics (CFD). A fuel surrogate matching Research Octane Number (RON) and Motor Octane Number (MON) of E30 is first developed using neural network based non-linear regression model. To enable efficient 3D engine simulations, a 164-species skeletal reaction mechanism incorporating NOx chemistry is reduced from a detailed chemical kinetic model. A hybrid approach that incorporates the G-equation model for tracking turbulent flame front, and the multi-zone well-stirred reactor model for predicting auto-ignition in the end gas, is employed to account for turbulent combustion interactions in the engine cylinder. Predicted in-cylinder pressure and heat release rate traces agree well with experimental measurements. The proposed modelling approach also captures moderated cyclic variability. Two different types of combustion cycles, corresponding to purely deflagrative and mixed-mode combustion, are observed. In contrast to the purely deflagrative cycles, mixed-mode combustion cycles feature early flame propagation followed by end-gas auto-ignition, leading to two distinctive peaks in heat release rate traces. The positive correlation between mixed-mode combustion cycles and early flame propagation is well captured by simulations. With the validated numerical setup, effects of NOx chemistry on mixed-mode combustion predictions are investigated. NOx chemistry is found to promote auto-ignition through residual gas recirculation, while the deflagrative flame propagation phase remains largely unaffected. Local sensitivity analysis is then performed to understand effects of physical and chemical properties of the fuel, i.e., heat of evaporation (HoV) and laminar flame speed (SL). An increased HoV tends to suppress end-gas auto-ignition due to increased vaporization cooling, while the impact of HoV on flame propagation is insignificant. In contrast, an increased SL is found to significantly promote both flame propagation and auto-ignition. The promoting effect of SL on auto-ignition is not a direct chemical effect; it is rather caused by an advancement of the combustion phasing, which increases compression heating of the end gas.

scholarly journals Effects of Flame Propagation Velocity and Turbulence Intensity on End-Gas Auto-Ignition in a Spark Ignition Gasoline Engine

Energies ◽  
2020 ◽  
Vol 13 (19) ◽  
pp. 5039
Author(s):  
Lei Zhou ◽  
Xiaojun Zhang ◽  
Lijia Zhong ◽  
Jie Yu
Keyword(s):  
Flame Propagation ◽  
Turbulence Intensity ◽  
Propagation Velocity ◽  
Gasoline Engine ◽  
Pressure Oscillation ◽  
Spark Ignition ◽  
Flame Propagation Velocity ◽  
Eddy Simulation ◽  
Auto Ignition ◽  
Large Eddy

Knocking is a destructive and abnormal combustion phenomenon that hinders modern spark ignition (SI) engine technologies. However, the in-depth mechanism of a single-factor influence on knocking has not been well studied. Thus, the major aim of the present study is to study the effects of flame propagation velocity and turbulence intensity on end-gas auto-ignition through a large eddy simulation (LES) and a decoupling methodology in a downsized gasoline engine. The mechanisms of end-gas auto-ignition as well as strong pressure oscillation are qualitatively analyzed. It is observed that both flame propagation velocity and turbulence have a non-monotonic effect on knocking intensity. The competitive relationship between flame propagation velocity and ignition delay of the end gas is the primary reason responding to this phenomenon. A higher flame speed leads to an increase in the heat release rate in the cylinder, and consequently, quicker increases in the temperature and pressure of the unburned end-gas mixture are obtained, leading to end-gas auto-ignition. Further, the coupling of a pressure wave and an auto-ignition flame front results in super-knocking with a maximum peak of pressure of 31 MPa. Although the turbulence indirectly influences the end-gas auto-ignition by affecting the flame propagation velocity, it can accelerate the dissipation of radicals and heat in the end gas, which significantly influences knocking intensity. Moreover, it is found that the effect of turbulence is more pronounced than that of flame propagation velocity in inhibiting knocking. It can be concluded that the intensity of the pressure oscillation depends on the unburned mixture mass as well as the local thermodynamic state induced by flame propagation and turbulence, with mutual interactions. The present work is expected to provide valuable perspective for inhibiting super-knocking of an SI gasoline engine.

Flame Response to Transverse Acoustic Forcing With Minimal Axial Coupling

2017 ◽  
Author(s):  
Travis Smith ◽  
Benjamin Emerson ◽  
William Proscia ◽  
Tim Lieuwen
Keyword(s):  
Heat Release ◽  
Gas Turbines ◽  
Axial Flow ◽  
Release Mechanism ◽  
Direct Role ◽  
Flow Disturbances ◽  
Transverse Excitation ◽  
Transverse Acoustic ◽  
Flame Response

Instabilities associated with transverse acoustic modes are an important problem in gas turbines. A number of studies have reported results on the response of flames to transverse excitation, in order to understand the acoustic-velocity-heat release mechanism associated with combustion instabilities. However, all forced and self-excited transverse studies to date have strong coupling between the transverse and axial acoustic fields near the flame. This is significant, as studies suggest that the actual transverse disturbances play a negligible direct role in generating spatially integrated oscillatory heat release. Rather, they suggest that it is the induced axial disturbances that control the bulk of the heat release response. As such, there is a need to control the relative amplitudes of the axial and transverse disturbances exciting the flame, and determine their relative roles in the overall heat release response. This paper presents experimental results to address this issue. The flow field and flame edge were measured using 5kHz simultaneous sPIV and OH-PLIF, and the relative heat release fluctuations were measured through OH* chemiluminescence. The flame was forced with both strong transverse and axial oscillations, with various degrees of coupling between them, showing quite consistently that it is the axial flow disturbances that excite heat release oscillations. These observations demonstrate that the key role of the transverse motions is to set the “clock” for the frequency of the oscillations, but have negligible effect on the actual heat release disturbances exciting the instability. Rather, it is the axial disturbances, induced by inherent multi-dimensional effects that lead to the actual heat release oscillations.

scholarly journals Control and optimization of spark ignition–controlled auto-ignition hybrid combustion based on stratified flame ignition

2018 ◽  
Vol 233 (12) ◽  
pp. 3057-3073 ◽  
Author(s):  
Tao Chen ◽  
Xinyan Wang ◽  
Hua Zhao ◽  
Hui Xie ◽  
Bangquan He
Keyword(s):  
Heat Release ◽  
Flame Propagation ◽  
Thermal Efficiency ◽  
Direct Injection ◽  
Combustion Process ◽  
Spark Ignition ◽  
Release Process ◽  
Auto Ignition ◽  
Flame Ignition ◽  
Injection Ratio

Spark ignition–controlled auto-ignition hybrid combustion, also known as spark assisted compression ignition, is of considerable interest in gasoline engines because of its potential to enlarge the operating range of gasoline diluted combustion. However, it was found that the spark ignition–controlled auto-ignition hybrid combustion process was often characterized with large cycle-to-cycle variations. In this research, a new approach by combining the traditional second-order derivative method and Wiebe function fitting method was proposed to identify different heat release stages of spark ignition–controlled auto-ignition hybrid combustion. The heat release characteristics of the spark ignition–controlled auto-ignition hybrid combustion based on stratified flame ignition strategy and its control methods were investigated in detail. The effect of control parameters, including spark timing, direct injection ratio and dilution strategy, on improving the thermal efficiency and decreasing the variation of heat release trace in spark ignition–controlled auto-ignition hybrid combustion based on stratified flame ignition strategy was analysed. The advance of flame propagation ending point and the increase in the average heat release rate in flame propagation stage benefitted the fuel economy and reduced the variations of heat release in spark ignition–controlled auto-ignition hybrid combustion. Although the increase in direct injection ratio contributed to the stability of heat release in the spark ignition–controlled auto-ignition hybrid combustion based on the stratified flame ignition strategy, the thermal efficiency of spark ignition–controlled auto-ignition hybrid combustion cannot be effectively optimized due to the decrease in combustion efficiency. The application of exhaust gas recirculation and air dilution could decrease the variations of heat release process and increase the thermal efficiency of spark ignition–controlled auto-ignition hybrid combustion based on stratified flame ignition strategy.

The comparison between traditional spark ignition and micro flame ignition in gasoline high dilution combustion

2021 ◽  
pp. 095440702098316
Author(s):  
Yifang Feng ◽  
Tao Chen ◽  
Kang Xu ◽  
Xinyan Wang ◽  
Hui Xie ◽  
...  
Keyword(s):  
Heat Release ◽  
Flame Propagation ◽  
High Efficiency ◽  
Combustion Process ◽  
Spark Ignition ◽  
Low Temperature Combustion ◽  
Reaction Fronts ◽  
Auto Ignition ◽  
Flame Ignition ◽  
Operational Range

Gasoline spark ignition (SI) – Controlled auto-ignition (CAI) hybrid combustion had previously been shown to expanding the operational range of high-efficiency low-temperature combustion and reducing fuel consumption. However, the spark ignition became ineffective when the mixture became highly diluted and the large cyclic variation and even misfire would occur. To achieve high-efficiency combustion in extended engine operational range and overcome the limitation of SI-CAI hybrid combustion, Micro Flame Ignition (MFI) was proposed and researched as a mean to providing multiple auto-ignition sites to initiate the combustion process of the diluted mixture. In this research, both engine experiments and Computational Fluid Dynamics (CFD) simulations were carried out to study the MFI combustion and SI-CAI hybrid combustion in a single-cylinder optical engine. Compared to the SI-CAI hybrid combustion, the flame propagation in MFI hybrid combustion was initiated by a large number of reaction fronts produced by the DME auto-ignition at multiple sites. MFI was found to deliver substantially more heat and ignition energy to the premixed mixture than the single spark ignition, enabling much faster initial heat release. However, the flame front expansion speed of MFI hybrid combustion dropped significantly to a similar value to that of the SI-CAI case because of the slower flame speed of diluted gasoline mixture. The MFI combustion exhibited three phases of autoignition stage, flame propagation stage and fast heat release stage. It is characterized by a higher peak heat release rate and shorter duration of the main combustion than those of the SI-CAI combustion. Besides, the use of spark ignition in the MFI operation promoted the autoignition of DME, leading to a shorter combustion duration and faster combustion than the MFI combustion without spark ignition. As a result, the spark assisted MFI strategy could be used to control the combustion phasing and optimize the MFI combustion process.

scholarly journals Large-Eddy Simulation of Laser-Ignited Direct Injection Gasoline Spray for Emission Control

Energies ◽  
2021 ◽  
Vol 14 (21) ◽  
pp. 7276
Author(s):  
Fabien Tagliante ◽  
Tuan M. Nguyen ◽  
Lyle M. Pickett ◽  
Hyung-Sub Sim
Keyword(s):  
Heat Release ◽  
Flame Propagation ◽  
Soot Formation ◽  
Direct Injection ◽  
Flame Structure ◽  
Dynamic Structure ◽  
Navier Stokes ◽  
Penetration Length ◽  
Eddy Simulation ◽  
Large Eddy

Large-Eddy Simulations (LES) of a gasoline spray, where the mixture was ignited rapidly during or after injection, were performed in comparison to a previous experimental study with quantitative flame motion and soot formation data [SAE 2020-01-0291] and an accompanying Reynolds-Averaged Navier–Stokes (RANS) simulation at the same conditions. The present study reveals major shortcomings in common RANS combustion modeling practices that are significantly improved using LES at the conditions of the study, specifically for the phenomenon of rapid ignition in the highly turbulent, stratified mixture. At different ignition timings, benchmarks for the study include spray mixing and evaporation, flame propagation after ignition, and soot formation in rich mixtures. A comparison of the simulations and the experiments showed that the LES with Dynamic Structure turbulence were able to capture correctly the liquid penetration length, and to some extent, spray collapse demonstrated in the experiments. For early and intermediate ignition timings, the LES showed excellent agreement to the measurements in terms of flame structure, extent of flame penetration, and heat-release rate. However, RANS simulations (employing the common G-equation or well-stirred reactor) showed much too rapid flame spread and heat release, with connections to the predicted turbulent kinetic energy. With confidence in the LES for predicted mixture and flame motion, the predicted soot formation/oxidation was also compared to the experiments. The soot location was well captured in the LES, but the soot mass was largely underestimated using the empirical Hiroyasu model. An analysis of the predicted fuel–air mixture was used to explain different flame propagation speeds and soot production tendencies when varying ignition timing.

Three-dimensional direct numerical simulation of a turbulent lifted hydrogen jet flame in heated coflow: flame stabilization and structure

2009 ◽  
Vol 640 ◽  
pp. 453-481 ◽  
Author(s):  
C. S. YOO ◽  
R. SANKARAN ◽  
J. H. CHEN
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Heat Release ◽  
Three Dimensional ◽  
Flame Stabilization ◽  
Flux Analysis ◽  
Jet Flame ◽  
Lifted Flame ◽  
Auto Ignition ◽  
Lagrangian Tracking

Direct numerical simulation (DNS) of the near field of a three-dimensional spatially developing turbulent lifted hydrogen jet flame in heated coflow is performed with a detailed mechanism to determine the stabilization mechanism and the flame structure. The DNS was performed at a jet Reynolds number of 11,000 with over 940 million grid points. The results show that auto-ignition in a fuel-lean mixture at the flame base is the main source of stabilization of the lifted jet flame. A chemical flux analysis shows the occurrence of near-isothermal chemical chain branching preceding thermal runaway upstream of the stabilization point, indicative of hydrogen auto-ignition in the second limit. The Damköhler number and key intermediate-species behaviour near the leading edge of the lifted flame also verify that auto-ignition occurs at the flame base. At the lifted-flame base, it is found that heat release occurs predominantly through ignition in which the gradients of reactants are opposed. Downstream of the flame base, both rich-premixed and non-premixed flames develop and coexist with auto-ignition. In addition to auto-ignition, Lagrangian tracking of the flame base reveals the passage of large-scale flow structures and their correlation with the fluctuations of the flame base. In particular, the relative position of the flame base and the coherent flow structure induces a cyclic motion of the flame base in the transverse and axial directions about a mean lift-off height. This is confirmed by Lagrangian tracking of key scalars, heat release rate and velocity at the stabilization point.

scholarly journals Effect of Temperature Conditions on Flame Evolutions of Turbulent Jet Ignition

Energies ◽  
2021 ◽  
Vol 14 (8) ◽  
pp. 2226
Author(s):  
Jiaying Pan ◽  
Yu He ◽  
Tao Li ◽  
Haiqiao Wei ◽  
Lei Wang ◽  
...  
Keyword(s):  
Flame Propagation ◽  
Early Stage ◽  
Turbulent Jet ◽  
Combustion Stability ◽  
Effect Of Temperature ◽  
Main Chamber ◽  
Detailed Chemical Kinetics ◽  
Jet Flame ◽  
Spherical Flame ◽  
Temperature Conditions

Turbulent jet ignition technology can significantly improve lean combustion stability and suppress engine knocking. However, the narrow jet channel between the pre-chamber and the main chamber leads to some difficulties in heat exchange, which significantly affects combustion performance and mechanical component lifetime. To clarify the effect of temperature conditions on combustion evolutions of turbulent jet ignition, direct numerical simulations with detailed chemical kinetics were employed under engine-relevant conditions. The flame propagation in the pre-chamber and the early-stage turbulent jet ignition in the main chamber were investigated. The results show that depending on temperature conditions, two types of flame configuration can be identified in the main chamber, i.e., the normal turbulent jet flame propagation and the spherical flame propagation, and the latter is closely associated with pressure wave disturbance. Under low-temperature conditions, the cold jet stoichiometric mixtures and the vortexes induced by the jet flow determine the early-stage flame development in the main chamber. Under intermediate temperature conditions, pre-flame heat release and leading pressure waves are induced in the jet channel, which can be regarded as a transition of different combustion modes. Whereas under high-temperature conditions, irregular auto-ignition events start to occur, and spherical flame fronts are induced in the main chamber, behaving faster flame propagation.

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