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1.
This study describes a simple analytical method to compute the azimuthal modes appearing in annular combustion chambers and help analyzing experimental, acoustic and large eddy simulation (LES) data obtained in these combustion chambers. It is based on a one-dimensional zero Mach number formulation where N burners are connected to a single annular chamber. A manipulation of the corresponding acoustic equations in this configuration leads to a simple dispersion relation which can be solved by hand when the interaction indices of the flame transfer function are small and numerically when they are not. This simple tool is applied to multiple cases: (1) a single burner connected to an annular chamber (N = 1), (2) two burners connected to the chamber (N = 2), and (3) four burners (N = 4). In this case, the tool also allows to study passive control methods where two different types of burners are mixed to control the azimuthal mode. Finally, a complete helicopter chamber (N = 15) is studied. For all cases, the analytical results are compared to the predictions of a full three-dimensional Helmholtz solver and a very good agreement is found. These results show that building very simple analytical tools to study azimuthal modes in annular chambers is an interesting path to control them.  相似文献   

2.
The prediction and the control of combustion instabilities require the identification of the combustion chamber response. This identification is usually performed by forcing the combustor (for example, modulating its inlet velocity) and measuring its response. Two methods may be found in the literature to analyze this response: identification of transfer matrices (ITM) and flame transfer functions (FTF). In ITM approaches, the burner is considered as a “black box” and a two-port formulation (based on acoustic pressure and velocity perturbations) is used to construct a transfer matrix linking acoustic fluctuations on both sides of the burner. A drawback of this method is that in experiments, the measurement of unsteady pressure and velocity in burnt gases can be difficult. In FTF approaches, pressure measurements are replaced by a global heat release measurement (usually based on optical methods). The heat release fluctuations are then related to the flow velocity modulations at a reference point (usually the combustor inlet) through a transfer function. This paper uses a compressible numerical simulation of a forced laminar Bunsen flame to analyze FTF and ITM methods. Results show that FTF approaches lead to an ill-defined problem as soon as the reference point is not close enough to the chamber. This “compactness” limit is quantified here in terms of distance between the reference point and the local chamber. The source of the problem is that FTF approaches correlate heat release fluctuations to velocity oscillations only: extended FTF models are then proposed using the local unsteady pressure as well as the velocity upstream of the flame to predict the heat release oscillations. These models are tested numerically and provide consistent values when the reference point location changes or when upstream and downstream conditions are varied. These results lead to simple recommendations for system identification.  相似文献   

3.
Ramjet flows are very sensitive to combustion instabilities that are difficult to predict using numerical simulations. This paper describes compressible large eddy simulation on unstructured grids used to investigate nonreacting and reacting flows in a simplified twin-inlet ramjet combustor. The reacting flow is compared to experimental results published by ONERA in terms of mean fields. Simulations show a specific flow topology controlled by the impingement of the two air jets issuing from the twin air inlets and by multiple complex recirculation zones. In a second part, all unsteady modes appearing in the reacting LES are analyzed using spectral maps and POD (proper orthogonal decomposition) tools. A Helmholtz solver also computes the frequencies and structures of all acoustic modes in the ramjet. Pure longitudinal, transverse and combined modes are identified by all three diagnostics. In addition, a mode-by-mode analysis of the Rayleigh criterion is presented thanks to POD. This method shows that the most intense structure (at 3750 Hz) is the first transverse acoustic mode of the combustor chamber and the Rayleigh criterion obtained with POD illustrates how this transverse mode couples with unsteady combustion.  相似文献   

4.
Large-eddy simulations (LESs) of an industrial gas turbine burner are carried out for both nonreacting and reacting flow using a compressible unstructured solver. Results are compared with experimental data in terms of axial and azimuthal velocities (mean and RMS), averaged temperature, and existence of natural instabilities such as precessing vortex core (PVC). The LES is performed with a reduced two-step mechanism for methane-air combustion and a thickened flame model. The regime of combustion is partially premixed and the computation includes part of the swirler vanes. For this very complex geometry, results demonstrate the capacity of the LES to predict the mean flow, with and without combustion, as well as its main unstable modes: it is shown, for example, that the PVC mode is very strong for the cold flow but disappears with combustion.  相似文献   

5.
In this paper we describe the time-varying amplitude and its relation to the global heat release rate of self-excited azimuthal instabilities in a simple annular combustor operating under atmospheric conditions. The combustor was modular in construction consisting of either 12, 15 or 18 equally spaced premixed bluff-body flames around a fixed circumference, enabling the effect of large-scale interactions between adjacent flames to be investigated. High-speed OH chemiluminescence imaged from above the annulus and pressure measurements obtained at multiple locations around the annulus revealed that the limit cycles of the modes are degenerate in so much as they undergo continuous transitions between standing and spinning modes in both clockwise (CW) and anti-clockwise (ACW) directions but with the same resonant frequency. Similar behaviour has been observed in LES simulations which suggests that degenerate modes may be a characteristic feature of self-excited azimuthal instabilities in annular combustion chambers. By modelling the instabilities as two acoustic waves of time-varying amplitude travelling in opposite directions we demonstrate that there is a statistical prevalence for either standing m = 1 or spinning m = ±1 modes depending on flame spacing, equivalence ratio, and swirl configuration. Phase-averaged OH chemiluminescence revealed a possible mechanism that drives the direction of the spinning modes under limit-cycle conditions for configurations with uniform swirl. By dividing the annulus into inner and outer annular regions it was found that the spin direction coincided with changes in the spatial distribution of the peak heat release rate relative to the direction of the bulk swirl induced along the annular walls. For standing wave modes it is shown that the globally integrated fluctuations in heat release rate vary in magnitude along the acoustic mode shape with negligible contributions at the pressure nodes and maximum contributions at the pressure anti-nodes.  相似文献   

6.
This paper presents an experimental study into the structure and dynamics of the phase-averaged heat release rate during self-excited spinning and standing azimuthal modes in an annular combustion chamber. The flame response was characterised using two methods: high-speed OH chemiluminescence imaged above the annulus to investigate the structure of the phase-averaged fluctuations in heat release rate, and high-speed OH-PLIF measured across the centreline of two adjacent flames to investigate phase-averaged flame dynamics. Two-microphone measurements were obtained at three circumferential locations to determine the modes and the amplitude of the velocity fluctuations. It was found that the flame responds differently to spinning and standing wave modes. During standing wave modes, the amplitude of the unsteady heat release rate of each flame (sector) varied according to its location in the mode shape with maximum fluctuations occurring at the pressure anti-nodes and minimum fluctuations occurring at the pressure nodes. At the pressure anti-nodes, peak fluctuations result from the production of flame surface area by axisymmetric flame motions caused by the modulation of flow at the burner inlet by the pressure fluctuations. However, at the pressure nodes, an anti-symmetric, transverse flapping motion of the flame occurred producing negligible unsteady heat release rate over the oscillations cycle via the mechanism of cancellation. During spinning modes, the structure of the heat release rate was found to be asymmetric and characterised by the preferential suppression of shear layer disturbances depending on the spin direction.  相似文献   

7.
An experimental study on combustion instability is presented with focus on oxy-fuel type combustion. Oxidants composed of CO2/O2 and methane are the reactants flowing through a premixer-combustor system. The reaction starts downstream a symmetric sudden expansion and is at the origin of different instability patterns depending on oxygen concentration and Reynolds number. The analysis has been conducted through measurement of pressure, CH chemiluminescence, and velocity. As far as stability is concerned, oxy-fuel combustion with oxygen concentration similar to that found in air combustion cannot be sustained, but requires at least 30% oxygen to perform in a comparable manner. Under these conditions and for the sudden expansion configuration used in this study, the instability is at low frequency and low amplitude, controlled by the flame length inside the combustion chamber. Above a threshold concentration in oxygen dependent on equivalence ratio, the flame becomes organized and concentrated in the near field. Strong thermoacoustic instability is then triggered at characteristic acoustic modes of the system. Different modes can be triggered depending on the ratio of flame speed to inlet velocity, but for all types of instability encountered, the heat release and pressure fluctuations are linked by a variation in mass-flow rate. An acoustic model of the system coupled with a time-lag-based flame model made it possible to elucidate the acoustic mode selection in the system as a function of laminar flame speed and Reynolds number. The overall work brings elements of reflection concerning the potential risk of strong pressure oscillations in future gas turbine combustors for oxy-fuel gas cycles.  相似文献   

8.
The numerical analysis of thermoacoustic oscillation phenomena by means of time-dependent CFD simulations usually requires a great computational effort, which may not be reasonable in industrial design. On the other hand, CFD tools provide the only approach that includes all the physical and chemical aspects involved in the thermoacoustic coupling between flame heat release and the acoustic modes of the burner/combustion chamber system. This paper presents some guidelines to reduce the computational effort required to perform a CFD analysis of the thermoacoustic oscillations with commercial codes. These guidelines are organized in a procedure that can be followed to analyze thermoacoustic coupling conditions that actually lead to unstable oscillations or are identified as potentially critical in the design phase. This procedure is also illustrated by an example of application, the partially-premixed flame type burner of a real 10 MW industrial boiler which shows noisy pressure fluctuations at a low frequency.  相似文献   

9.
《Combustion and Flame》2014,161(2):525-540
In the context of large-eddy simulation (LES) of Diesel engine combustion, two LES combustion models are proposed. Their ability to predict autoignition delays and heat release of an autoigniting liquid α-methylnaphthalene/n-decane jet injected into a constant-volume chamber under Diesel-like conditions is assessed. These models retain the tabulation of a complex chemistry scheme using autoigniting homogeneous reactors (HR) at constant pressure. This allows accounting for the chemical complexity of heavy hydrocarbon fuels over the wide range of conditions representative for Diesel engines, at comparatively low CPU time overhead. The tabulated homogeneous reactor (THR) approach assumes the local structure of the reaction zone to be that of an HR, while the approximated diffusion flame (ADF) approach is based on autoigniting strained diffusion flames. Two variants of each approach are considered, either neglecting sub-grid-scale mixture fraction variance (THR and ADF models), or accounting for it via a presumed β-PDF (THR-pdf and ADF–PCM models). LES results indicate that the ADF model assuming diffusion flame structures tends to predict faster propagation of the combustion toward less reactive mixture fractions then the THR model. Moreover, neglecting the mixture fraction fluctuations strongly overestimates initial experimental heat release rates after autoignition. Comparison between models shows that this assumption yields higher reaction rates and temperature levels close to the stoichiometric mixture fraction zones. Predictions in terms of autoignition are remarkably close with all models, and exhibit very few variations from one realization to the other. Variations in global heat release rate become more apparent for different realizations at later instants, in relation to the interaction of large flow scales with combustion.  相似文献   

10.
This study describes an analytical method for computing azimuthal modes due to flame/acoustics coupling in annular combustors. It is based on a quasi-one-dimensional zero-Mach-number formulation where N burners are connected to an upstream annular plenum and a downstream chamber. Flames are assumed to be compact and are modeled using identical flame transfer function for all burners, characterized by an amplitude and a phase shift. Manipulation of the corresponding acoustic equations leads to a simple methodology called ANR (annular network reduction). It makes it possible to retain only the useful information related to the azimuthal modes of the annular cavities. It yields a simple dispersion relation that can be solved numerically and makes it possible to construct coupling factors between the different cavities of the combustor. A fully analytical resolution can be performed in specific situations where coupling factors are small (weak coupling). A bifurcation appears at high coupling factors, leading to a frequency lock-in of the two annular cavities (strong coupling). This tool is applied to an academic case where four burners connect an annular plenum to a chamber. For this configuration, analytical results are compared with a full three-dimensional Helmholtz solver to validate the analytical model in both weak and strong coupling regimes. Results show that this simple analytical tool can predict modes in annular combustors and investigate strategies for controlling them.  相似文献   

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