One of the most important issues in combustion researches is the prediction and suppression of combustion instability, which induces flashback, generates combustion noise and damages combustor. Although a number of studies on combustion instability have been performed, the underlying physics have not been well clarified yet, especially for spray combustion. In this article, authors' recent numerical works on Large-eddy Simulations (LES) of combustion instabilities are introduced. The combustion instabilities of gas and spray combustion in back-step flows are demonstrated in terms of LES, and effects of initial droplet diameter on the combustion instability are investigated for spray combustion. Methane and kerosene are used as fuels for gas and spray combustion, respectively, and two-step global reaction models are used for calculations of the reactions. A dynamic thickened flame model is employed as a turbulent combustion model.
This article introduces a research on thermoacoustic instability based on direct numerical simulation (DNS) of a hydrogen-air turbulent swirling premixed flame in a cuboid combustor. To investigate thermoacoustic instability and dynamic modes of a turbulent swirling premixed flame, dynamic mode decomposition (DMD) and spectral analysis are applied to results of the DNS. DMD of pressure field shows that the quarter-wave mode in longitudinal acoustics has the largest energy, while DMD of heat release rate field reveals that the quarter-wave mode in longitudinal acoustics is not a dominant mode in heat release rate field. It has found that oscillation modes caused by large scale vortical motions have large energy not only in pressure field but also in heat release rate field. Dynamic modes of the source term of classical acoustic energy equation are also discussed with the vortical motion and the resonance mode of the combustor.
Large-eddy simulation (LES) is performed to simulate high-frequency combustion instability in a single-element atmospheric combustor. Simulations are conducted for the corresponding combustion experiments, and the self-excited combustion instability observed in the experiments is successfully reproduced. The first tangential (1T) mode of the combustion chamber is excited in the LES, and the amplitude and frequency of the pressure fluctuations are consistent with the experimental observations. The 1T mode in the LES was observed at 1 kHz and the peak-to-peak amplitude was approximately 4% of time-averaged pressure. The coupling mechanism between the flame and acoustic velocity fluctuations is explored based on the LES results. The periodic ignition of the unburnt H2/O2 mixture produces a lifted combustion in a pulsating motion at the 1T mode frequency. This unsteady pulsating flame behavior is caused by the coupling between the fuel injection and the 1T mode acoustic oscillations. The Rayleigh index indicates that a primary driving factor of the instability is the acoustically coupled pulsating flame motion. The present results demonstrate that the LES can accurately capture the unsteady heat release and its coupling with pressure oscillations. The LES results clarify details of flame structures that are not completely understood solely from the experimental data, and therefore are valuable for understanding the coupling mechanism of the flame and acoustic mode in a combustion chamber.
Because of increasingly stringent regulations on jet engines pollutant emissions, new ways of reducing them are needed. A promising way is the use of Lean Premixed Prevaporized (LPP) combustion where the liquid fuel is vaporized and mixed with air in excess before burning. In the present work, a model gas turbine combustor fed with liquid dodecane is studied experimentally. It is equipped with two fuel injection stages: a pressurized nozzle called the pilot and a multipoint device. When the split of the fuel injection between the stages is changed, several phenomena can be observed. The flame shapes can change drastically and present different behaviors, some of them showing a strong acoustic activity. In particular, two different flame shapes can be obtained for the exact same operating conditions depending on the burner history. The first flame, named sV, is stabilized thanks to an internal reaction zone that modifies the air flow compared to non-reacting conditions. The second state, associated with a Lifted flame, exhibits a strong thermo-acoustic instability linked to the quarter-wave mode of the chamber. A switch from the sV to the Lifted state can be triggered by air flow rate modulations while the opposite change can occur naturally as the fuel split is changed. These complex phenomena originate from multiple interactions between the fuel spray, the gaseous flow and the flame itself.
The radial / tangential modes of acoustically resonant oscillatory combustion have been observed in the large scale tubular flame burners. The triggering and amplification mechanisms of those modes of oscillations can be understood in detail with tubular flame burners because (1) those modes of oscillations have been found to be occurred in tubular flame burners at relatively low thermal input conditions with high reproducibility, and thus, the experiments can be repeatedly conducted under the well-controlled conditions, and (2) the modes of oscillations are easily identified because the natural frequencies of the tubular flame burners are easily obtained due to its simple geometry, and furthermore (3) the heat release position can be easily identified because the tubular flame is the thin laminar flame front. At present, it was found that (n, m) = (0, 1) mode of oscillation which has non-axisymmetric sound pressure profile is the dominant mode of oscillation, where n and m is the radial and tangential mode number, respectively. As results of flow field measurements, the Presessing Vortex Core motion was observed in the burned gas region of the tubular flame burner, which can cause the non-axisymmetric sound pressure structure in the tubular flame burner.
Gasoline vehicles, particularly, Gasoline Direct Injection (GDI) vehicles were subjected to regulations of PM (Particulate Matter) emissions because of the sales increase of GDI vehicles in the market. In this article, the emissions standards of PM for gasoline vehicles in Japan, EU and USA (EPA) are reviewed, and the latest regulatory trend are compared. The outline of PM generation in a GDI engine are explained as the fundamental information. In cold-fast-idle condition, for example, PM emissions in GDI engine originate from the wall fuel films produced by the fuel jet. The reported chemical compositions of exhaust PM are summarized. The major component is found to be Elemental Carbon (EC). Finally, the need for the modeling of PM generation is discussed for the numerical simulation in order to investigate the GDI engine with lower PM emission in the future.
The structures and thermodynamic aspects of the PAHs and soot emitted from combustion as well as their formation processes in high-temperature environment are briefly reviewed and introduced. Firstly, the morphology of the soot and the mechanism of formation inferred from the macro- and microstructures and chemical analyses are described. Then the chemical structures and the stability of the PAHs based on the π-resonance electronic structures in termed of the Kekulé count and the Pauling bond order are discussed. The thermodynamic aspect of the soot formation is explained based on the overall combustion equilibrium state, and the thermodynamics governing the microscopic equilibrium between key intermediate species is introduced for the understanding of the temperature-limiting phenomena. Finally the coagulation step of the graphene platelets to graphite crystallite is discussed based on the thermodynamics of the elementary processes.
Nanometer size particulate matter (PM) emission exhausted from internal combustion engine is one of the major air pollutants, and its reduction technologies are required. In this study, to make clear the influence of flame quenching on PM exhaust, characteristics such as composition, size distribution and density of PM exhausted by metal mesh quenching of laminar diffusion flame with benzene were measured by a combustion type PM analyzer (MEXA-1370PM), a Scanning Mobility Particle Sizer (SMPS) and an electric hydrometer. From these results, it was confirmed that PM emitted from quenched flame included SOF with high mass concentration. Its size was smaller than PM emitted from the normal benzene flame (free flame). Density of PM deposition on the quenched mesh at > 10min was higher than that of PM from free flame, though density of PM emission from quenched flame was lower than that.