756 research outputs found
Reaction regimes in supersonic combustion
No full textLarge-eddy simulations (LES) of supersonic combustion are considered to be an important tool to better understand the physics of supersonic reactive flows. To correctly predict the flame anchoring and the flame structure, the implementation of a appropriate SGS scale model is crucial, and its physics and derivation are analyzed in the present work. In fact, when modelling mixing and combustion at small scales, all effects of compressibility must be accounted before attempting to reproduce experimental results and predict performance. The theoretical analysis in [1], shows that at high Mach, high Re numbers, mixing and combustion are driven not only by transfer of kinetic energy by vortex stretching, as in subsonic reacting flows, but also by compressibility and baroclinic effects: this affects the classical -5/3 Kolmogorov scaling for the energy transfer from large to small scales. Further, compressibility favours combustion by increasing reaction rates. The increase of the reaction rate leads to an increase of flame speed: as a consequence, reaction regimes at Ma > 1(in particular those where some form of premixing occurs) must be re-examined. In fact, depending on the reaction rate, Mach and turbulence intensity, different regimes are possible that are analyzed. Based on these, an appropriate turbulence-chemistry coupling model must follow. Such model (dubbed ISCM) has been derived and implemented in a CFD LES code to simulate SCRJ combustion. Results show good agreement with experimental data. Copyright © 2009 by Copyright © 2008 by Antonella Ingenito and Claudio Bruno
A comparative study of combustion between biofuels and fossil fuels
No full textCurrent global energy supplies are dominated by fossil fuels, with much smaller contributions from nuclear power and hydropower. Bioenergy provides about 10% of the total energy supplies, making it by far the most important renewable energy source used; solar, wind and other renewable energy sources have nowadays a very small contribution. On average, in the industrialized countries biomass contributes less than 10% to the total energy supplies, but in developing countries the proportion is as high as 20-30%. Bioenergy could play a bigger role, especially in the industrial countries that consume a lot of fossil energy and are therefore the main contributors to atmospheric pollution and global warming. Therefore, biofuels can contribute to reducing the dependency on fossil fuels and to lowering greenhouse gas emissions due to transport and other activities and can be considered as both renewable and sustainable energy sources. Indeed, these are recognized as'net zero CO emission' fuels due to the very short-term carbon cycle. Thus the goal of this paper is to investigate the possibility to reduce NOx and CO emissions still maintaining the same conventional fuel performance. In particular, in the following, a comparative study of the bio-fuels ignition delay time, flame temperature ad emissions at different operative conditions with those of conventional fuels, has been performed. Results have actually shown that renewable fuels could play a critical role in pollutants abatement and for the next generation energy sources. © 2012 by Antonella Ingenito
Mixing and turbulent kinetic energy scaling in compressible reacting flows
No full textPrevious work by these authors analyzed in depth vorticity generation and transport in supersonic flows in order to understand the physics of supersonic combustion and to improve air-hydrogen mixing. In fact, the short combustor residence time (10-3- 10-4 s) minimizes the chance to completely mix and burn the fuel. Thus it becomes imperative to create there very energetic vortex structures, and a solution is to inject hydrogen in crossflow. In this paper, the 3D LES of the supersonic combustor flight tested in the HyShot project showed that the interactions between the airstream entering the combustor and the H2 sonic jet produce average vorticity of order 105 Hz, with much higher localized peaks. The interaction between the hydrogen jets and the supersonic airflow leads to a bow shock formation in front of each jet and boundary layer separation. This separation allows H2 to be convected upstream through spanwise vortices created by the baroclinic effect. Once created, vortices are tilted, stretched, compressed and expanded as predicted by the fully compressible vorticity transport equation. This paper is meant to expand and complement earlier works showing how vortices affect combustion and analyzing the main species distribution along the combustor. The spectral analysis of turbulent kinetic energy obtained by LES results demonstrates that where compressibility is not negligible, the turbulent kinetic scaling differs from Kolmogorov. © 2011 by Antonella Ingenito & Claudio Bruno
Theoretical and Numerical analysis of the Turbulence scaling in Supersonic Flows
No full textSupersonic mixing and combustion is critical to advanced airbreathing propulsion systems able to push vehicles well beyond M=4. Research in this field is of interest around the world. In fact, vehicles capable of such speed are being tested now in the US (HyTech, HyV), Russia and the UK-Australia (HyShot), in Japan, India, China and Korea. EU is funding the project LAPCAT to study the feasibility of a long range hypersonic commercial transport. In a SCRJ, the air stream flow captured by the inlet is decelerated but still maintaining supersonic conditions. Since the residence time is very short (~1ms), the study of a efficient mixing and combustion is a key issue in the ongoing research in compressible flows. Due to experimental difficulties in measuring complex high-speed unsteady flowfields, the most convenient way to understand unsteady features of supersonic mixing and combustion is the use of computational fluid dynamics. The complexity of physics involved makes the problem of considerable interest also from a numerical point of view. Therefore, resolution of a turbulent compressible reacting flow imply a threefold requirement: 1. a highly accurate non dissipative numerical scheme to properly simulate the strong gradients in the vicinity of the shock waves and the turbulent structures away from these discontinuities; 2. a proper modelling of the small subgrid scales for supersonic combustion, including the effect of compressibility on mixing and combustion; 3. a highly detailed kinetic scheme accounting for the radicals formation and recombination to properly predict the flame anchoring. A hybrid method capable of capturing shocks and, at the same time, of resolving with low dissipation turbulent structures away from discontinuities has been implemented in the present paper. A new subgrid scale model accounting for the nature of the turbulent in compressible regime is proposed. The introduction of detailed chemistry (the scheme of Warnatz, including 9 species and 38 reactions to account for radicals formation) results in more expensive computer run times and storage requirements. High velocity and density gradients, and high hydrogen diffusivity also poses some numerical critical issues. This work, based on this subgrid physical model and using LES shows that, in supersonic flows, the baroclinic and dilatational effects pump vorticity in the flow influencing the turbulent KE decay and the dissipative turbulence scale. © 2011 by Antonella Ingenito and Claudio Bruno. Published by the American Institute of Aeronautics and Astronautics, Inc
Theoretical Investigation of Air vitiation effects on hydrogen fuelled Scramjet performance
No full textThe renewed interest in the next generation hypersonic vehicles for commercial and military applications requires combined efforts between multidisciplinary teams involved in computational and experimental work. A common feature of experimental tests, at least for studies on air breathing propulsion systems, is the relatively short test time, limited by the test flow conditions high dynamic pressure and total enthalpy, i.e. 10 MPa at Mach 8 flight conditions. In general, it can be stated that test times decrease as the flow Mach number increases, with changes of several orders of magnitude: a few tens of microseconds for shock tube based tunnels (Mach numbers above 10–15) to tens of seconds or even minutes for the high supersonic range (Mach 4–7.5). In many cases the test flow chemical composition differs from the standard air composition because of pollutants production by the different techniques adopted to increase the flow stagnation enthalpy (combustion products, ionized species, dust, ...). The real flow conditions are thus not perfectly duplicated, but only partially simulated in terms of a few main parameters, such as velocity, pressure, and temperature. Few studies have been performed until now, therefore a deep investigation of the air vitiation effects on combustion is mandatory to extrapolate ground to flight correlations. Numerical simulations accounting for the effects of vitiation are also scarce and a characterization of the chemical and turbulence models is still lacking. In this context, CFD simulations of the LAPCAT II MR2 combustor configuration have been performed to provide useful information in terms of efficiency at various vitiation percentages. Theoretical laws and remedies have been proposed for the ground to flight data extrapolation
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