DNS on Autoignition and Flame Propagation of Inhomogeneous Methane–Air Mixtures in a Closed Vessel

2011 ◽  
Author(s):  
Makito Katayama ◽  
Naoya Fukushima ◽  
Masayasu Shimura ◽  
Mamoru Tanahashi ◽  
Toshio Miyauchi
Keyword(s):  
Heat Release ◽  
Flame Propagation ◽  
Heat Release Rate ◽  
Release Rate ◽  
Equivalence Ratio ◽  
Kinetic Mechanism ◽  
Spatial Inhomogeneity ◽  
Closed Vessel ◽  
Isothermal Wall ◽  
Detailed Kinetic Mechanism

Direct numerical simulations (DNSs) on autoignition and flame propagation of inhomogeneous methane–air mixtures in a closed vessel are conducted with considering detailed kinetic mechanism and temperature dependence of transport and thermal properties. The mixtures with spatial inhomogeneity of temperature or equivalence ratio are investigated. Periodic condition for non-heatloss cases or isothermal wall condition for heatloss cases is imposed on the boundaries. From the DNS results without heatloss, effects of spatial inhomogeneity of temperature and equivalence ratio on mean heat release rate are clarified. Increase of spatial variations of temperature or equivalence ratio suppresses drastic rise of mean heat release rate and reduces its maximum value. Autoignition process is affected by temperature more strongly than equivalence ratio. In the cases with heatloss, ignition delay increases and the maximum mean heat release rate decreases. After autoignition process, propagating flame is formed along walls. Heat transfer characteristics in a closed vessel are also discussed with combustion mechanisms.

Optical Measurement of Local and Global Transfer Functions for Equivalence Ratio Fluctuations in a Turbulent Swirl Flame

2013 ◽  
Author(s):  
Bernhard C. Bobusch ◽  
Bernhard Ćosić ◽  
Jonas P. Moeck ◽  
Christian Oliver Paschereit
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Equivalence Ratio ◽  
Transfer Functions ◽  
Optical Measurement ◽  
Low Frequencies ◽  
Fuel Flow ◽  
Calibration Measurement

Equivalence ratio fluctuations are known to be one of the key factors controlling thermoacoustic stability in lean premixed gas turbine combustors. The mixing and thus the spatio-temporal evolution of these perturbations in the combustor flow is, however, difficult to account for in present low-order modeling approaches. To investigate this mechanism, experiments in an atmospheric combustion test rig are conducted. To assess the importance of equivalence ratio fluctuations in the present case, flame transfer functions for different injection positions are measured. By adding known perturbations in the fuel flow using a solenoid valve, the influence of equivalence ratio oscillations on the heat release rate is investigated. The spatially and temporally resolved equivalence ratio fluctuations in the reaction zone are measured using two optical chemiluminescence signals, captured with an intensified camera. A steady calibration measurement allows for the quantitative assessment of the equivalence ratio fluctuations in the flame. This information is used to obtain a mixing transfer function, which relates fluctuations in the fuel flow to corresponding fluctuations in the equivalence ratio of the flame. The current study focuses on the measurement of the global, spatially integrated, transfer function for equivalence ratio fluctuations and the corresponding modeling. In addition, the spatially resolved mixing transfer function is shown and discussed. The global mixing transfer function reveals that despite the good spatial mixing quality of the investigated generic burner, the ability to damp temporal fluctuations at low frequencies is rather poor. It is shown that the equivalence ratio fluctuations are the governing heat release rate oscillation response mechanism for this burner in the low-frequency regime. The global transfer function for equivalence ratio fluctuations derived from the measurements is characterized by a pronounced low-pass characteristic, which is in good agreement with the presented convection–diffusion mixing model.

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.

DNS of Flame-Wall Interaction and Heat Transfer in a Constant Volume Vessel

2010 ◽  
Author(s):  
Akihiko Tsunemi ◽  
Yoshihiro Horiko ◽  
Masayasu Shimura ◽  
Naoya Fukushima ◽  
Seiji Yamamoto ◽  
...  
Keyword(s):  
Heat Flux ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Burning Velocity ◽  
Fluid Velocity ◽  
Pressure Rise ◽  
Kinetic Mechanism ◽  
Constant Volume ◽  
Wall Interaction

Direct numerical simulations of turbulent hydrogen/air and methane/air premixed flames in a rectangular constant volume vessel have been conducted with considering detailed kinetic mechanism to investigate flame behaviors and heat losses. For the hydrogen cases, since heat release rate increases with pressure rise due to dilatation during combustion in the constant vessel, heat flux on a wall also increases. For the methane cases, the pressure increase does not raise wall heat flux significantly because of the decrescence of heat release rate caused by thermo-chemical reaction near a wall. Pressure waves caused by wall reflection fluctuate flame propagation for the hydrogen flames. Flame displacement speed decreases remarkably at the moment when the pressure wave passes through flame fronts from unburnt side to burnt side. However, the turbulent burning velocity at that time does not decrease because of increases of fluid velocity normal to the flame fronts.

Comparison of Equivalence Ratio Transients on Combustion Instability in Single-Nozzle and Multi-Nozzle Combustors

2018 ◽  
Author(s):  
Xiaoling Chen ◽  
Wyatt Culler ◽  
Stephen Peluso ◽  
Domenic Santavicca ◽  
Jacqueline O’Connor
Keyword(s):  
Heat Release ◽  
Gas Turbine ◽  
Heat Release Rate ◽  
Release Rate ◽  
Equivalence Ratio ◽  
Combustion Instability ◽  
Driving Mechanism ◽  
Gas Turbine Combustor ◽  
Rate Distribution ◽  
Ratio Change

Low-emissions gas turbine combustion, achieved through the use of lean, premixed fueling strategies, is susceptible to combustion instability. The driving mechanism for this instability arises from fluctuations of pressure, fuel/air flow rate, and heat release rate. If these fluctuations are relatively in-phase, the combustion system will evolve to a self-excited state. The self-excited instability frequency and amplitude depend mainly on the operating condition and the geometry of the combustor. In this study, we consider the onset and decay of self-excited instabilities, resulting from transients in fuel/air ratio, in both single-nozzle and multi-nozzle combustors. In particular, we examine the differences in the instability onset and decay processes between these two flame configurations, as most gas turbine combustors have multiple nozzles, but most gas turbine combustor experiments utilize a single-nozzle. A nonlinear logistic regression analysis is applied to study the timescales of the decay and onset transients. Variations in the equivalence ratio change the heat release rate distribution inside the combustor, which is captured using chemiluminescence imaging. The normalized Rayleigh index, which shows the spatial distribution of the instability driving, is calculated to analyze the driving strength in different regions of the flame. Comparisons between the single- and multi-nozzle flame transients, including both center and outer flames for the multi-nozzle combustor, suggest that both confinement from the wall and flame-flame interaction are crucial to determining flame dynamics as the equivalence ratio transient changes the heat release rate distribution near corner recirculation zone and flame shear layers.

The Effect of the Degree of Premixedness on Self-Excited Combustion Instability

2020 ◽  
Author(s):  
Adam Howie ◽  
Daniel Doleiden ◽  
Stephen Peluso ◽  
Jacqueline O’Connor
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Equivalence Ratio ◽  
Feedback Loop ◽  
Relative Phase ◽  
Common Strategy ◽  
Rate Oscillations ◽  
Recirculation Zones ◽  
Increased Susceptibility

Abstract The use of lean, premixed fuel and air mixtures is a common strategy to reduce NOx emissions in gas turbine combustors. However, this strategy causes an increased susceptibility to self-excited instability, which manifests as fluctuations in heat release rate, flow velocity, and combustor acoustics that oscillate in-phase in a feedback loop. This study considers the effect of the level of premixedness on the self-excited instability in a single-nozzle combustor. In this system, the fuel can be injected inside the nozzle to create a partially-premixed mixture or far upstream to create a fully-premixed mixture, varying the level of premixedness of the fuel and air entering the combustor. When global equivalence ratio is held constant, the cases with higher levels of premixing have higher instability amplitudes. Highspeed CH* chemiluminescence imaging shows that the flame for these cases is more compact and the distribution of the heat release rate oscillations is more concentrated near the corner of the combustor in the outer recirculation zones. Rayleigh index images, which are a metric for quantifying the relative phase of pressure and heat release rate oscillations, suggest that vortex rollup in the corner region is primarily responsible for determining instability characteristics when premixedness is varied. This finding is further supported through analysis of local flame edge dynamics.

An Experimental Validation of Heat Release Rate Fluctuation Measurements in Technically Premixed Flames

2013 ◽  
Author(s):  
Poravee Orawannukul ◽  
Bryan D. Quay ◽  
Domenic A. Santavicca
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Equivalence Ratio ◽  
Premixed Flames ◽  
Reconstruction Technique ◽  
Chemiluminescence Intensity ◽  
Lean Premixed ◽  
Rate Of Heat Release

Knowledge of the effects of inlet velocity and inlet equivalence ratio fluctuations on the rate of heat release in lean premixed gas turbine combustors is essential for predicting combustor instability characteristics. This information is typically obtained from independent velocity-forced and fuel-forced flame transfer function measurements, where the global chemiluminescence intensity is used as a measure of the flame’s overall rate of heat release. The flame in an actual lean premixed combustor is referred to as a technically premixed flame and is exposed to both velocity and equivalence ratio fluctuations. Under these conditions the chemiluminescence intensity does not provide a reliable measure of the flame’s rate of heat release. The objective of this work is to experimentally assess the validity of a technique for making heat release rate measurements in technically premixed flames based on the linear superposition of fuel-forced and velocity-forced flame transfer function measurements. In the absence of a technique for directly measuring the heat release rate fluctuations in an air-forced technically premixed, the heat release reconstruction is validated indirectly by comparing measured to reconstructed chemiluminescence intensity fluctuations. Results are reported for a range of operating conditions and forcing frequencies which demonstrate the capabilities and limitations of this technique. A variation of this technique, referred to as a reverse reconstruction, is proposed which does not require a measurement of the fuel-forced flame transfer function. The air-forced flame transfer function gain and phase obtained using the reverse reconstruction technique are presented and compared to the results from the direct reconstruction technique.

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