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Статті в журналах з теми "Flamelet theory"
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Trouvé, Arnaud, and Thierry Poinsot. "The evolution equation for the flame surface density in turbulent premixed combustion." Journal of Fluid Mechanics 278 (November 10, 1994): 1–31. http://dx.doi.org/10.1017/s0022112094003599.
Повний текст джерелаАнотація:
One basic effect of turbulence in turbulent premixed combustion is for the fluctuating velocity field to wrinkle the flame and greatly increase its surface area. In the flamelet theory, this effect is described by the flame surface density. An exact evolution equation for the flame surface density, called the Σ-equation, may be written, where basic physical mechanisms like production by hydrodynamic straining and destruction by propagation effects are described explicitly. Direct numerical simulation (DNS) is used in this paper to estimate the different terms appearing in the Σ-equation. The numerical configuration corresponds to three-dimensional premixed flames in isotropic turbulent flow. The simulations are performed for various mixture Lewis numbers in order to modify the strength and nature of the flame-flow coupling. The DNS-based analysis provides much information relevant to flamelet models. In particular, the flame surface density, and the source and sink terms for the flame surface density, are resolved spatially across the turbulent flame brush. The geometry as well as the dynamics of the flame differ quite significantly from one end of the reaction zone to the other. For instance, contrary to the intuitive idea that flame propagation effects merely counteract the wrinkling due to the turbulence, the role of flame propagation is not constant across the turbulent brush and switches from flame surface production at the front to flame surface dissipation at the back. Direct comparisons with flamelet models are also performed. The Bray-Moss-Libby assumption that the flame surface density is proportional to the flamelet crossing frequency, a quantity that can be measured in experiments, is found to be valid. Major uncertainties remain, however, over an appropriate description of the flamelet crossing frequency. In comparison, the coherent flame model of Marble & Broadwell achieves closure at the level of the Σ-equation and provides a more promising physically based description of the flame surface dynamics. Some areas where the model needs improvement are identified.
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Peters, N. "A spectral closure for premixed turbulent combustion in the flamelet regime." Journal of Fluid Mechanics 242 (September 1992): 611–29. http://dx.doi.org/10.1017/s0022112092002519.
Повний текст джерелаАнотація:
Premixed turbulent combustion in the flamelet regime is analysed on the basis of a field equation. This equation describes the instantaneous flame contour as an isoscalar surface of the scalar field G(x,t). The field equation contains the laminar burning velocity sL as velocity scale and its extension includes the effect of flame stretch involving the Markstein length [Lscr ] as a characteristic lengthscale of the order of the flame thickness. The scalar G(x,t) plays a similar role for premixed flamelet combustion as the mixture fraction Z(x,t) in the theory of non-premixed flamelet combustion.Equations for the mean $\overline{G}$ and variance $\overline{G^{\prime 2}}$ are derived. Additional closure problems arise for the mean source terms in these equations. In order to understand the nature of these terms an ensemble of premixed flamelets with arbitrary initial conditions in constant-density homogeneous isotropic turbulence is considered. An equation for the two-point correlation $\overline{G^{\prime}({\boldmath x},t)G^{\prime}({\boldmath x}+{\boldmath r},t)}$ is derived. When this equation is transformed into spectral space, closure approximations based on the assumption of locality and on dimensional analysis are introduced. This leads to a linear equation for the scalar spectrum function Γ(k,t), which can be solved analytically. The solution Γ(k,t) is analysed by assuming a small-wavenumber cutoff at k0 = lT−1, where lT is the integral lengthscale of turbulence. There exists a $k^{-\frac{5}{3}}$ spectrum between lT and LG, where LG is the Gibson scale. At this scale turbulent fluctuations of the scalar field G(x,t) are kinematically restored by the smoothing effect of laminar flame propagation. A quantity called kinematic restoration ω is introduced, which plays a role similar to the scalar dissipation χ for diffusive scalars.By calculating the appropriate moments of Γ(k,t), an algebraic relation between ω, $\omega,\overline{G^{\prime}({\boldmath x},t)^2}$, the integral lengthscale lT and the viscous dissipation ε is derived. Furthermore, the scalar dissipation χ[Lscr ], based on the Markstein diffusivity [Dscr ][Lscr ] = sL [Lscr ], and the scalar-strain co-variance Σ[Lscr ] are related to ω. Dimensional analysis, again, leads to a closure of the main source term in the equation for the mean scalar $\overline{G}$. For the case of plane normal and oblique turbulent flames the turbulent burning velocity sT and the flame shape is calculated. In the absence of flame stretch the linear relation sT ∼ u′ is recovered. The flame brush thickness is of the order of the integral lengthscale. In the case of a V-shaped flame its increase with downstream position is calculated.
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Wirth, Martin, and Norbert Peters. "Turbulent premixed combustion: A flamelet formulation and spectral analysis in theory and IC-engine experiments." Symposium (International) on Combustion 24, no. 1 (January 1992): 493–501. http://dx.doi.org/10.1016/s0082-0784(06)80063-9.
Повний текст джерела
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Fiala, Thomas, and Thomas Sattelmayer. "Nonpremixed Counterflow Flames: Scaling Rules for Batch Simulations." Journal of Combustion 2014 (2014): 1–7. http://dx.doi.org/10.1155/2014/484372.
Повний текст джерелаАнотація:
A method is presented to significantly improve the convergence behavior of batch nonpremixed counterflow flame simulations with finite-rate chemistry. The method is applicable to simulations with varying pressure or strain rate, as it is, for example, necessary for the creation of flamelet tables or the computation of the extinction point. The improvement is achieved by estimating the solution beforehand. The underlying scaling rules are derived from theory, literature, and empirical observations. The estimate is used as an initialization for the actual solver. This enhancement leads to a significantly improved robustness and acceleration of batch simulations. The extinction point can be simulated without cumbersome code extensions. The method is demonstrated on two test cases and the impact is discussed.
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Füzesi, Dániel, Milan Malý, Jan Jedelský, and Viktor Józsa. "Numerical modeling of distributed combustion without air dilution in a novel ultra-low emission turbulent swirl burner." Physics of Fluids 34, no. 4 (April 2022): 043311. http://dx.doi.org/10.1063/5.0085058.
Повний текст джерелаАнотація:
Distributed combustion, often associated with the low-oxygen condition, offers ultra-low NOx emission. However, it was recently achieved without combustion air dilution or internal flue gas recirculation, using a distinct approach called mixture temperature-controlled combustion. Here, the fuel–air stream is cooled at the inlet to delay ignition and, hence, foster homogeneous mixture formation. This numerical study aims to understand its operation better and present a robust framework for distributed combustion modeling in a parameter range where such operation was not predicted before by any existing theory. Further, liquid fuel combustion was evaluated, which brings additional complexity. Four operating conditions were presented at which distributed combustion was observed. The reacting flow was modeled by flamelet-generated manifold, based on a detailed n-dodecane mechanism. The Zimont turbulent flame speed model was used with significantly reduced coefficients to achieve distributed combustion. The droplets of airblast atomization were tracked in a Lagrangian frame. The numerical results were validated by Schlieren images and acoustic spectra. It was concluded that the reactant dilution ratio remained below 0.25 through the combustion chamber, revealing that the homogeneous fuel–air mixture is the principal reason for excellent flame stability and ultra-low NOx emission without significant internal recirculation. The potential applications of these results are boilers, furnaces, and gas turbines.
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Zimont, Vladimir L. "A Two-Fluid Conditional Averaging Paradigm for the Theory and Modeling of Turbulent Premixed Combustion." Journal of Combustion 2019 (August 7, 2019): 1–27. http://dx.doi.org/10.1155/2019/5036878.
Повний текст джерелаАнотація:
This paper extends a recent theoretical study that was previously presented in the form of a brief communication (Zimont, C&F, 192, 2018, 221-223), in which we proposed a simple splitting method for the derivation of two-fluid conditionally averaged equations of turbulent premixed combustion in the flamelet regime, formulated more conveniently for applications involving unclosed equations without surface-averaged unknowns. This two-fluid conditional averaging paradigm avoids the challenge in the Favre averaging paradigm of modeling the countergradient scalar transport phenomenon and the unusually large velocity fluctuations in a turbulent premixed flame. It is a more suitable conceptual framework that is likely to be more convenient in the long run than the traditional Favre averaging method. In this article, we further develop this paradigm and pay particular attention to the problem of modeling turbulent premixed combustion in the context of a two-fluid approach. We formulate and analyze the unclosed differential equations in terms of the conditions of the Reynolds stresses τij,u, τij,b and the mean chemical source ρW¯, which are the only modeling unknowns required in our alternative conditionally averaged equations. These equations are necessary for the development of model differential equations for the Reynolds stresses and the chemical source in the advanced modeling and simulation of turbulent premixed combustion. We propose a simpler approach to modeling the conditional Reynolds stresses based on the use of the two-fluid conditional equations of the standard “k-ε” turbulence model, which we formulate using the splitting method. The main problem arising here is the appearance in these equations of unknown terms describing the exchange of the turbulent energy k and dissipation rate ε in the unburned and burned gases. We propose an approximate way to avoid this problem. We formulate a simple algebraic expression for the mean chemical source that follows from our previous theoretical analysis of the transient turbulent premixed flame in the intermediate asymptotic stage, in which small-scale wrinkles in the instantaneous flame surface reach statistical equilibrium, while the large-scale wrinkles remain in statistical nonequilibrium.
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CRETA, F., and M. MATALON. "Propagation of wrinkled turbulent flames in the context of hydrodynamic theory." Journal of Fluid Mechanics 680 (June 1, 2011): 225–64. http://dx.doi.org/10.1017/jfm.2011.157.
Повний текст джерелаАнотація:
We study the propagation of premixed flames in two-dimensional homogeneous isotropic turbulence using a Navier–Stokes/front-capturing methodology within the context of hydrodynamic theory. The flame is treated as a thin layer separating burnt and unburnt gases, of vanishingly small thickness, smaller than the smallest fluid scales. The method is thus suitable to investigate the flame propagation in the wrinkled flamelet regime of turbulent combustion. A flow-control system regulates the mean position of the flame and the incident turbulence intensity. In this context we study the individual effects of turbulence intensity, turbulence scale, thermal expansion, hydrodynamic strain and hydrodynamic instability on the propagation characteristics of the flame. Results are obtained assuming positive Markstein length, corresponding to lean hydrocarbon–air or rich hydrogen–air mixtures. For stable planar flames we find a quadratic dependence of turbulent speed on turbulence intensity. Upon onset of hydrodynamic instability, corrugated structures replace the planar conformation and we observe a greater resilience to turbulence, the quadratic scaling being replaced by scaling exponents less than one. Such resilience is also confirmed by the observation of a threshold turbulence intensity below which the propagation speed of corrugated flames is indistinguishable from the laminar speed. Turbulent speed is found to increase and later plateau with increasing thermal expansion, this affecting the average flame displacement but not the mean flame curvature. In addition, turbulence integral scale is also observed to affect the propagation of the flame with the existence of an intermediate scale maximizing the turbulent speed. This maximizing scale is smaller for corrugated flames than it is for planar flames, implying that small eddies that will be unable to significantly perturb a planar front could be rather effective in perturbing a corrugated flame. Turbulent planar flames, and more so corrugated flames, were observed to experience a positive mean hydrodynamic strain, which was explained in terms of the overwhelming mean contribution of the normal component of strain. The positive straining causes a decrease in the mean laminar propagation speed which in turn can decrease the turbulent speed. The effect of the flame on the incident turbulent field was examined in terms of loss of isotropy and vorticity destruction by thermal expansion. The latter can be mitigated by a baroclinic vorticity generation which is enhanced for corrugated flames.
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Tvrdojevic, Mijo, Milan Vujanovic, Peter Priesching, Ferry A. Tap, Anton Starikov, Dmitry Goryntsev, and Manolis Gavaises. "Implementation of the Semi Empirical Kinetic Soot Model Within Chemistry Tabulation Framework for Efficient Emissions Predictions in Diesel Engines." Open Physics 17, no. 1 (December 31, 2019): 905–15. http://dx.doi.org/10.1515/phys-2019-0096.
Повний текст джерелаАнотація:
Abstract Soot prediction for diesel engines is a very important aspect of internal combustion engine emissions research, especially nowadays with very strict emission norms. Computational Fluid Dynamics (CFD) is often used in this research and optimisation of CFD models in terms of a trade-off between accuracy and computational efficiency is essential. This is especially true in the industrial environment where good predictivity is necessary for engine optimisation, but computational power is limited. To investigate soot emissions for Diesel engines, in this work CFD is coupled with chemistry tabulation framework and semi-empirical soot model. The Flamelet Generated Manifold (FGM) combustion model precomputes chemistry using detailed calculations of the 0D homogeneous reactor and then stores the species mass fractions in the table, based on six look-up variables: pressure, temperature, mixture fraction, mixture fraction variance, progress variable and progress variable variance. Data is then retrieved during online CFD simulation, enabling fast execution times while keeping the accuracy of the direct chemistry calculation. In this work, the theory behind the model is discussed as well as implementation in commercial CFD code. Also, soot modelling in the framework of tabulated chemistry is investigated: mathematical model and implementation of the kinetic soot model on the tabulation side is described, and 0D simulation results are used for verification. Then, the model is validated using real-life engine geometry under different operating conditions, where better agreement with experimental measurements is achieved, compared to the standard implementation of the kinetic soot model on the CFD side.
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Veynante, D., A. Trouvé, K. N. C. Bray, and T. Mantel. "Gradient and counter-gradient scalar transport in turbulent premixed flames." Journal of Fluid Mechanics 332 (February 1997): 263–93. http://dx.doi.org/10.1017/s0022112096004065.
Повний текст джерелаАнотація:
In premixed turbulent combustion, the modelling of the turbulent flux of the mean reaction progress variable remains somewhat controversial. Classical gradient transport assumptions based on the eddy viscosity concept are often used while both experimental data and theoretical analysis have pointed out the existence of countergradient turbulent diffusion. Direct numerical simulation (DNS) is used in this paper to provide basic information on the turbulent flux of and study the occurrence of counter-gradient transport. The numerical configuration corresponds to twoor three-dimensional premixed flames in isotropic turbulent flow. The simulations correspond to various flame and flow conditions that are representative of flamelet combustion. They reveal that different flames will feature different turbulent transport properties and that these differences can be related to basic dynamical differences in the flame-flow interactions: counter-gradient diffusion occurs when the flow field near the flame is dominated by thermal dilatation due to chemical reaction, whereas gradient diffusion occurs when the flow field near the flame is dominated by the turbulent motions. The DNS-based analysis leads to a simple expression to describe the turbulent flux of , which in turn leads to a simple criterion to delineate between the gradient and counter-gradient turbulent diffusion regimes. This criterion suggests that the occurrence of one regime or the other is determined primarily by the ratio of turbulence intensity divided by the laminar flame speed, and by the flame heat release factor, τ ≡ (Tb — Tu)/Tu, where Tu and Tb are respectively the temperature within unburnt and burnt gas. Consistent with the Bray-Moss-Libby theory, counter-gradient (gradient) diffusion is promoted by low (high) values and high (low) values of τ. DNS also shows that these results are not restricted to the turbulent transport of . Similar results are found for the turbulent transport of flame surface density, Σ. The turbulent fluxes of and Σ are strongly correlated in the simulated flames and counter-gradient (gradient) diffusion of always coincides with counter-gradient (gradient) diffusion of Σ.
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See, Yee Chee, and Matthias Ihme. "Effects of finite-rate chemistry and detailed transport on the instability of jet diffusion flames." Journal of Fluid Mechanics 745 (March 25, 2014): 647–81. http://dx.doi.org/10.1017/jfm.2014.95.
Повний текст джерелаАнотація:
AbstractLocal linear stability analysis has been shown to provide valuable information about the response of jet diffusion flames to flow-field perturbations. However, this analysis commonly relies on several modelling assumptions about the mean flow prescription, the thermo-viscous-diffusive transport properties, and the complexity and representation of the chemical reaction mechanisms. In this work, the effects of these modelling assumptions on the stability behaviour of a jet diffusion flame are systematically investigated. A flamelet formulation is combined with linear stability theory to fully account for the effects of complex transport properties and the detailed reaction chemistry on the perturbation dynamics. The model is applied to a methane–air jet diffusion flame that was experimentally investigated by Füriet al.(Proc. Combust. Inst., vol. 29, 2002, pp. 1653–1661). Detailed simulations are performed to obtain mean flow quantities, about which the stability analysis is performed. Simulation results show that the growth rate of the inviscid instability mode is insensitive to the representation of the transport properties at low frequencies, and exhibits a stronger dependence on the mean flow representation. The effects of the complexity of the reaction chemistry on the stability behaviour are investigated in the context of an adiabatic jet flame configuration. Comparisons with a detailed chemical-kinetics model show that the use of a one-step chemistry representation in combination with a simplified viscous-diffusive transport model can affect the mean flow representation and heat release location, thereby modifying the instability behaviour. This is attributed to the shift in the flame structure predicted by the one-step chemistry model, and is further exacerbated by the representation of the transport properties. A pinch-point analysis is performed to investigate the stability behaviour; it is shown that the shear-layer instability is convectively unstable, while the outer buoyancy-driven instability mode transitions from absolutely to convectively unstable in the nozzle near field, and this transition point is dependent on the Froude number.
Дисертації з теми "Flamelet theory"
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Evans, Michael J. "Flame Stabilisation in the Transition to MILD Combustion." Thesis, 2017. http://hdl.handle.net/2440/119081.
Повний текст джерелаАнотація:
Emissions reduction and energy management are current and future concerns for governments and industries alike. The primary source of energy worldwide for electricity, air transport and industrial processes is combustion. Moderate or intense low oxygen dilution (MILD) combustion offers improved thermal efficiency and a significant reduction of CO and NOx pollutants, soot and thermo-acoustic instabilities compared to conventional combustion. Whilst combustion in the MILD regime offers considerable advantages over conventional combustion, neither the structure of reacting jets under MILD conditions, nor the boundaries of the MILD regime are currently well understood. This work, therefore, serves to fill this gap in the understanding of flame structure near the boundaries of the MILD regime. The MILD combustion regime has been previously investigated experimentally and numerically in premixed reactors and non-premixed flames. In this study, definitions of MILD combustion are compared and contrasted, with the phenomenological premixed description of MILD combustion extended to describe non-premixed flames. A simple criterion is derived analytically which offers excellent agreement with observations of previously studied cases and new, non-premixed MILD and autoignitive flames presented in this work. This criterion facilitates a simple, predictive approach to distinguish MILD combustion, autoignitive flames, and the transition between the two regimes. The adequacy of simplified reactors as a tool for predicting non-premixed ignition behaviour in the transition between MILD combustion and autoignition has not previously been resolved, and is addressed in this work. The visual lift-off behaviour seen in the transition between MILD combustion and conventional autoignitive flames seen experimentally is successfully replicated using simplified reactors. The location of the visible flame base in a jet-in-hot-coflow burner is shown to be highly sensitive to the relative location of the most reactive mixture fraction and the high strain-rate shear layer due to the strong coupling of between ignition chemistry and the underlying flow-field. Previous studies have demonstrated a strong dependence of ignition delay times to significant concentrations of minor species. Simulations presented in this work demonstrate that small concentrations of the hydroxyl radical (OH), similar to those expected in practical environments, significantly affect ignition delay and intensity of non-premixed MILD combustion, however have little effect on autoignitive flames. Importantly, such concentrations of OH do not result in a change in flame structure for the cases investigated. Whilst these results stress the importance of minor species in modelling the transient ignition of non-premixed MILD combustion, steady-state simulations do not demonstrate the same sensitivity to concentrations of minor species expected in hot combustion products. These results suggest that the temperature and oxygen concentration in the oxidant stream are the most important factors governing the boundaries of, the MILD combustion regime. Investigations of reaction zone structure and ignition in, and near the boundaries of, the MILD combustion regime have demonstrated the relative importance of different aspects of ambient conditions and differences in structure between non-premixed MILD and autoignitive flames. These findings build upon the understanding of this regime and provide critical insight for future studies towards both fundamental research, and the practical implementation, of MILD combustion.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2017
Частини книг з теми "Flamelet theory"
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Dold, J. W., L. J. Hartley, and D. Green. "Dynamics of Laminar Triple-Flamelet Structures in Non-Premixed Turbulent Combustion." In Dynamical Issues in Combustion Theory, 83–105. New York, NY: Springer New York, 1991. http://dx.doi.org/10.1007/978-1-4612-0947-8_4.
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Olmeda, R., P. Breda, C. Stemmer, and M. Pfitzner. "Large-Eddy Simulations for the Wall Heat Flux Prediction of a Film-Cooled Single-Element Combustion Chamber." In Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 223–34. Cham: Springer International Publishing, 2020. http://dx.doi.org/10.1007/978-3-030-53847-7_14.
Повний текст джерелаАнотація:
Abstract In order for modern launcher engines to work at their optimum, film cooling can be used to preserve the structural integrity of the combustion chamber. The analysis of this cooling system by means of CFD is complex due to the extreme physical conditions and effects like turbulent fluctuations damping and recombination processes in the boundary layer which locally change the transport properties of the fluid. The combustion phenomena are modeled by means of Flamelet tables taking into account the enthalpy loss in the proximity of the chamber walls. In this work, Large-Eddy Simulations of a single-element combustion chamber experimentally investigated at the Technical University of Munich are carried out at cooled and non-cooled conditions. Compared with the experiment, the LES shows improved results with respect to RANS simulations published. The influence of wall roughness on the wall heat flux is also studied, as it plays an important role for the lifespan of a rocket engine combustors.
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Barfusz, Oliver, Felix Hötte, Stefanie Reese, and Matthias Haupt. "Pseudo-transient 3D Conjugate Heat Transfer Simulation and Lifetime Prediction of a Rocket Combustion Chamber." In Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 265–78. Cham: Springer International Publishing, 2020. http://dx.doi.org/10.1007/978-3-030-53847-7_17.
Повний текст джерелаАнотація:
Abstract Rocket engine nozzle structures typically fail after a few engine cycles due to the extreme thermomechanical loading near the nozzle throat. In order to obtain an accurate lifetime prediction and to increase the lifetime, a detailed understanding of the thermomechanical behavior and the acting loads is indispensable. The first part is devoted to a thermally coupled simulation (conjugate heat transfer) of a fatigue experiment. The simulation contains a thermal FEM model of the fatigue specimen structure, RANS simulations of nine cooling channel flows and a Flamelet-based RANS simulation of the hot gas flow. A pseudo-transient, implicit Dirichlet–Neumann scheme is utilized for the partitioned coupling. A comparison with the experiment shows a good agreement between the nodal temperatures and their corresponding thermocouple measurements. The second part consists of the lifetime prediction of the fatigue experiment utilizing a sequentially coupled thermomechanical analysis scheme. First, a transient thermal analysis is carried out to obtain the temperature field within the fatigue specimen. Afterwards, the computed temperature serves as input for a series of quasi-static mechanical analyses, in which a viscoplastic damage model is utilized. The evolution and progression of the damage variable within the regions of interest are thoroughly discussed. A comparison between simulation and experiment shows that the results are in good agreement. The crucial failure mode (doghouse effect) is captured very well.
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Newman, William R. "Early Modern Alchemical Theory." In Newton the Alchemist, 64–87. Princeton University Press, 2018. http://dx.doi.org/10.23943/princeton/9780691174877.003.0004.
Повний текст джерелаАнотація:
This chapter argues that Newton's belief that metals are not only produced within the earth but also undergo a process of decay, leading to a cycle of subterranean generation and corruption, finds its origin in the close connection between alchemy and mining that developed in central Europe during the early modern period. Alchemy itself acquired a distinct, hylozoic cast that the aurific art had largely lacked in the European Middle Ages. Despite a common scholarly view that holds alchemy to have been uniformly vitalistic, the early modern emphasis on the cyclical life and death of metals was not a monolithic feature of the discipline across the whole of its history, but rather a gift of the miners and metallurgists who worked in shafts and galleries that exhibited to them the marvels of the underground world. The chapter concludes by describing sources used by Newton, such as his favorite chymical writer, Eirenaeus Philalethes, and the pseudonymous early modern author masked beneath the visage of the fourteenth-century scrivener Nicolas Flamel.
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Santoso, Muhammad A., Eirik Christensen, Hafiz M. F. Amin, Pither Palamba, Yuqi Hu, Dwi M. J. Purnomo, Wuquan Cui, et al. "GAMBUT field experiment of peatland wildfires in Sumatra: infrared measurements of smouldering spread rate." In Advances in Forest Fire Research 2022, 880–85. Imprensa da Universidade de Coimbra, 2022. http://dx.doi.org/10.14195/978-989-26-2298-9_133.
Повний текст джерелаАнотація:
Peatland wildfires present challenge to the mitigation of climate change due to the large amount of ancient carbon emission. Once ignited, the organic soils in peatland can burn for a long periods (weeks to months) and are difficult to extinguish. Peat fire is governed by smouldering combustion which is the slow, low temperature, flameless burning of charring porous fuel, and the most persistent type of combustion phenomena. The detection and monitoring of peatland wildfires are often conducted by remote sensing like satellite. However, there is currently a missing gap between spread of peat fires in the small laboratory scale and the large field scale. This work covers this gap by conducting field-scale controlled experiment of peatland wildfires. The experimental campaign, GAMBUT, was conducted in the peatland of Sumatra, Indonesia, covering an area of 374 m2. Smouldering spread rate was measured by infrared cameras and subsurface thermocouples. The smouldering sustained up to 10 days and nights, and survived against three rainfalls. Observation from infrared images show that horizontal smouldering spread rate fluctuates during propagation. However, no significant difference was found between average horizontal spread rates from the measurements of infrared camera and thermocouples, i.e. 0.3±0.13 cm/h to 0.8±0.2 cm/h. The spread rates here agree with the trend in the literature of laboratory experiments, fit within in the ranges of high moisture (MC) and inorganic (IC) contents of the soil (MC between 23 to 141% and IC between 49 to 72%). Even though slower, the fires thrived up to 10 days and against three rainfalls, demonstrating the persistency of smouldering peat fires and calling for a consideration of degraded peatland with high inorganic content to be consistently included in the mapping and monitoring of peatland area. GAMBUT presents a unique understanding of peatland wildfires at field conditions and aims to contribute to the better monitoring and mitigation acts.
Тези доповідей конференцій з теми "Flamelet theory"
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Briones, Alejandro M., Robert Olding, Joshua P. Sykes, Brent A. Rankin, Kyle McDevitt, and Joshua S. Heyne. "Combustion Modeling Software Development, Verification and Validation." In ASME 2018 Power Conference collocated with the ASME 2018 12th International Conference on Energy Sustainability and the ASME 2018 Nuclear Forum. American Society of Mechanical Engineers, 2018. http://dx.doi.org/10.1115/power2018-7433.
Повний текст джерелаАнотація:
Historically, combustion modeling is important for many transportation- and ground-based applications. More recently, modeling has been offered as an early screening tool in the evaluation of a potential alternative aviation jet fuel. This combustion evaluation path would in theory be conducted by gas turbine Original Equipment Manufacturers (OEMs) on proprietary geometries and conditions. Ideally, OEMs would have access to the latest combustion theory models and would thus have the highest predictive confidence in their model predictions. Unfortunately, the latest combustion theory codes are not written for commercial purposes. This work identifies and develops a conduit for OEM usage of latest flamelet theory for use in the evaluation of alternative jet fuel combustion properties. A so-called “common format routine” (CFR) software with two low-dimensional manifold combustion models that can be used for laminar and turbulent applications is developed, which can be implemented by OEMs on proprietary hardware. The two models are the flamelet prolongation of the intrinsic low-dimensional manifold (FPI), used for premixed combustion, and the flamelet progress variable (FPV), utilized for nonpremixed combustion. The three branches of combustion are computed using a hybrid tool that combines homotopic flamelet calculations with scaling laws and the two- and one-point flamelet continuation methods in order to resolve bifurcations. The mixture fraction and progress variable definitions can be chosen to be any summation of atomic and species composition, respectively. Diffusivity coefficients can be computed using unity Lewis number, mixture-averaged and multicomponent species composition. The turbulence-chemistry interaction is tabulated a priori using Beta probability density function (PDF) for the mixture fraction and Beta or Dirac-delta PDF for the progress variable. Parallel computing is necessary for industrial quality tabulation. The tabulated table can be used for k-ε and k-ω RANS, SAS, DES, and LES simulations. The software can also interact with liquid spray and exchange mass between the liquid and gaseous phase. The software is verified against previous numerical simulations of canonical triple flames, piloted flames and single-cup combustor. The numerical results are validated against experimental measurements of temperature and species mass fractions. The CFR software advances Cantera 2.3. Hence, the software contains an inner layer of C++ code, an intermediate layer of Python wrappers, and an upper layer (GUI) of C# code. The pre-tabulated chemistry is used for CFD simulations. The tables are bi-linearly interpolated for laminar simulations and tri-linearly interpolated for turbulent simulations. The tabulated chemistry can be hooked to commercial software such as Fluent through C and Scheme codes. The simulated flames presented here were computed with this software. The developed software is reliable for modeling and simulation of complex combustion phenomena.
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Verma, Ishan, Rakesh Yadav, Sourabh Shrivastava, and Pravin Nakod. "Turbulent Combustion Modeling of Swirl Stabilized Blended CH4/H2 Flames by Using Flamelet Generated Manifold." In ASME Turbo Expo 2022: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2022. http://dx.doi.org/10.1115/gt2022-82583.
Повний текст джерелаАнотація:
Abstract Hydrogen has been identified as one of the key elements of the decarbonization initiatives. The level of maturity with different original equipment manufacturers (OEMs) varies significantly for a 100% H2 gas turbine combustor. The typical standard short-term goal is to blend hydrogen with existing fuel as a promising alternative to meet regulatory standards for emission. A typical Dry Low NOx (DLN) combustion system can handle a certain level of hydrogen blending. However, due to fundamental differences between the properties of hydrogen and methane, existing designs of combustion systems are not capable of handling moderate to high levels of hydrogen blending. Therefore, prior knowledge of blend ratios that a given combustion system can handle is essential for the system’s stable operation. Computational Fluid Dynamic (CFD) simulation can help study the effect of different blend ratios on flame stability, peak temperature, pollutants, etc., without affecting the hardware. Thus, helping in reducing the overall cost and time spent deciding the allowable blend ratios. In this work, the accuracy and consistency of Flamelet Generated Manifold (FGM) with Large Eddy Simulation (LES) have been assessed to model swirling turbulent combustion of CH4/H2 blends for gas turbine engine combustors. FGM characterizes the extent of reaction using a reaction progress variable typically defined as a weighted sum of some representative product species of hydrocarbon combustion like CO and CO2. With H2 blending, the mixture now has multiple heat release time scales, and the prevailing choices of reaction progress definition are not optimal. Therefore, the first and foremost task is to correctly describe the reaction rate by choosing a reaction progress variable with validity over a range of H2 blending ratios and equivalence ratios. Additionally, the variation in the laminar properties of the blended mixture, e.g., thermal conductivity and viscosity, is enhanced when H2 is added to the fuel. In this work, we have used kinetic theory to compute these properties accurately as a function of temperature and composition. The flame configurations used to validate FGM in this work are CH4/H2 swirl flame (SMH1) and HM3e. The burner designs belong to a detailed and widely simulated database from Sydney Swirl Burner, with a CH4/H2 blend ratio of 1:1 (by volume). The FGM generates flamelets from opposed flow diffusion flames and freely propagates premix flame configuration. The solution of both the FGM approaches is compared with Finite Rate detailed chemistry solution, and definitive advantages/disadvantages of each approach are identified based on computational speed and accuracy. The results are then compared with experimental data for velocity, temperature, major and minor species distribution to establish the computational accuracy of each approach. Together with the inclusion of modifications in the modeling framework and usage of detailed chemistry with FGM-LES, these results provide important insights into the simulation of hydrogen-blended methane flames.
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Riesmeier, Elmar, Sylvie Honnet, and Norbert Peters. "Flamelet Modeling of Pollutant Formation in a Gas Turbine Combustion Chamber Using Detailed Chemistry for a Kerosene Model Fuel." In ASME 2002 Internal Combustion Engine Division Fall Technical Conference. ASMEDC, 2002. http://dx.doi.org/10.1115/icef2002-492.
Повний текст джерелаАнотація:
Combustion and pollutant formation in a gas turbine combustion chamber is investigated numerically using the Eulerian Particle Flamelet Model (EPFM). The code solving the unsteady flamelet equations is coupled to an unstructured CFD code providing solutions for the flow and mixture field from which the flamelet parameters can be extracted. Flamelets are initialized in the fuel rich region close to the fuel injectors of the combustor. They are represented by marker particles which are convected through the flow field. Each flamelet takes a different pathway through the combustor leading to different histories for the flamelet parameters. Equations for the probability of finding a flamelet at a certain position and time are additionally solved in the CFD code. To model the chemical properties of kerosene, a detailed reaction mechanism for a mixture of n-decane and 1,2,4-trimethylbenzene is used. It includes a detailed NOx submechanism and the build-up of polycyclic aromatic hydrocarbons (PAHs) up to four aromatic rings. The kinetically based soot model describes the formation of soot particles by inception, further growth by coagulation and condensation as well as surface growth and oxidation. Simulation results are compared to experimental data obtained on a high pressure rig. The influence of the model on pollutant formation is shown, and the effect of the number of flamelets on the model is investigated.
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Both, Ambrus, Daniel Mira, and Oriol Lehmkuhl. "ASSESSMENT OF TABULATED CHEMISTRY MODELS FOR THE LES OF A MODEL AERO-ENGINE COMBUSTOR." In GPPS Chania22. GPPS, 2022. http://dx.doi.org/10.33737/gpps22-tc-70.
Повний текст джерелаАнотація:
Tabulated chemistry methods present a compromise between computational cost and the ability to capture complex combustion physics in high-fidelity numerical simulations. The application of such models entails a number of modeling decisions, that may affect the simulation results significantly, especially in partially premixed combustion, where the assumption of the existence of underlying premixed or non-premixed flamelet structures is arguable. In this work, different classical tabulation strategies are assessed in terms of their ability to predict the lift-off induced by localized extinction in a model aero-engine combustion chamber: the Cambridge swirl spray flame. The lift-off dynamics of the stable n-heptane spray flame are compared using: i) premixed flamelets, ii) stable and unstable counterflow diffusion flamelets, iii) stable and unsteady extinguishing counterflow diffusion flamelets, iv) unsteady extinguishing and reigniting counterflow diffusion flamelets at a given strain rate. The extinction and reignition events associated to the lift-off are validated against OH-PLIF measurements, and the temporal evolution of the lift-off and reattachment is analyzed.
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Hergart, Carl, and Norbert Peters. "Applying the Representative Interactive Flamelet Model to Evaluate the Potential Effect of Wall Heat Transfer on Soot Emissions in a Small-Bore DI Diesel Engine." In ASME 2001 Internal Combustion Engine Division Spring Technical Conference. American Society of Mechanical Engineers, 2001. http://dx.doi.org/10.1115/ices2001-118.
Повний текст джерелаАнотація:
Abstract Due to the wide spectrum of turbulent and chemical length- and time scales occurring in a HSDI diesel engine, capturing the correct physics and chemistry underlying combustion poses a tremendous modeling challenge. The processes related to the two-phase flow in a DI diesel engine add even more complexity to the total modeling effort. The Representative Interactive Flamelet (RIF) model has gained widespread attention owing to its ability of correctly describing ignition, combustion and pollutant formation phenomena. This is achieved by incorporating very detailed chemistry for the gas phase as well as the soot particle growth and oxidation, without imposing any significant computational penalty. The model, which is based on the laminar flamelet concept, treats a turbulent flame as an ensemble of thin, locally one-dimensional flame structures, whose chemistry is fast. A potential explanation for the significant underprediction of part load soot observed in previous studies applying the model is the neglect of wall heat losses in the flamelet chemistry model. By introducing an additional source term in the flamelet temperature equation, directly coupled to the wall heat transfer predicted by the CFD-code, flamelets exposed to walls are assigned heat losses of various magnitudes. Results using the model in three-dimensional simulations of the combustion process in a small-bore direct injection diesel engine indicate that the experimentally observed emissions of soot may have their origin in flame quenching at the relatively cold combustion chamber walls.
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Vance, Robert, and Indrek S. Wichman. "Heat Loss Analysis of Flamelets in Near-Limit Spread Over Solid Fuel Surfaces." In ASME 2001 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2001. http://dx.doi.org/10.1115/imece2001/htd-24252.
Повний текст джерелаАнотація:
Abstract The profile of a spreading flamelet is analyzed by examining the heat losses to surrounding surfaces. The study addresses the reasons why flamelets have shapes ranging from round hemispherical “caps” to flat “coin-like” discs. A parabolic shape profile is used for the thin flame sheet, which provides both flame length and flame curvature. A third parameter specifies the height of the flame from the surface beneath it. Radiation and conduction heat losses from the flame sheet are calculated for various flame shapes. Overall heat losses as well as heat losses to the surface beneath the flamelet are examined. Some of the heat “losses” are misnamed because they produce the necessary surface decomposition for subsequent gaseous flame fuel vapors. Strictly, then, “losses” do not contribute appreciably to the maintenance of the flame. Physical arguments are made to explain observed flame spread behavior and flame shapes in response to prevailing flow and environmental conditions.
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Long, Yang, Saeed Jahangirian, and Indrek S. Wichman. "Flame-Surface Interaction and Flamelet Microstructure Over Single and Multiple Solid Fuel Segments in a Channel Cross Flow." In ASME 2006 International Mechanical Engineering Congress and Exposition. ASMEDC, 2006. http://dx.doi.org/10.1115/imece2006-15068.
Повний текст джерелаАнотація:
The goal of this research is to study flamelets near solid fuel segments embedded in a non-combustible binder material. Flame attachment to surfaces in problems such as spreading flames, fires, and surface-burning propellants is a complicated process. The flame heats the surface, which decomposes into volatiles, which leave the surface as the gaseous fuel that mixes with incoming oxidizer, feeding further combustion. An interesting flame shape called the "flamelet" appears in narrow-channel combustion, microgravity combustion, and combustion over heterogeneous fuels. The model examined here develops a simple description of flamelet attachment and examines the details of the localized near-surface heat transfer and gas phase combustion. In essence, the "flame microstructure" near the surface is described as it interacts with the surface, including the heat flux from the flame, the surface heat flux, their dependence on flame microstructural parameters, and the sensitivity of the flame structure in response to changes in these parameters. Where possible, numerical solutions are compared with analytical formulas and calculations, and extensions are made to describe more general cases.
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Croce, Giulio, Giulio Mori, Viatcheslav V. Anisimov, and Joa˜o Parente. "Assessment of Traditional and Flamelets Models for Micro Turbine Combustion Chamber Optimisation." In ASME Turbo Expo 2003, collocated with the 2003 International Joint Power Generation Conference. ASMEDC, 2003. http://dx.doi.org/10.1115/gt2003-38385.
Повний текст джерелаАнотація:
Different approaches for numerical simulation of premixed combustion are considered, in order to assess their usefulness as design tools for micro gas turbine systems. In particular, a flamelet concept routine by N. Peters has been developed taking into account both mixture fraction Z and G function as scalar flame locators, thus allowing computation of complex fully or partial premixed flame structure. The model can be used also in the thin reaction regime. Scalar transport equations for G, Z and their variance are added to the standard Navier Stokes and turbulence set of equation, in order to track the flame position. However, no chemical term appears explicitly in such equations, since the chemical effects are taken into account via pre-computed locally one-dimensional flamelet solutions. Here, the deep interaction between chemical and turbulence has been introduced through flamelets library built in non equilibrium conditions using CHEMKIN modules. The results of this model are compared the data obtained with a standard EBU model and different reaction mechanisms. Models validation has been carried out through experimental data coming from Aachen University for an axisymmetric Bunsen flame; finally, the code was applied to the analysis of a newly designed micro gas turbine combustor.
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Makino, I., T. Kawanami, and Y. Yahagi. "Local Quenching Recovery Processes of Premixed and Diffusion Interacting Flames in a Turbulent Opposite Flow." In ASME/JSME 2011 8th Thermal Engineering Joint Conference. ASMEDC, 2011. http://dx.doi.org/10.1115/ajtec2011-44441.
Повний текст джерелаАнотація:
A lean premixed CH4 air flame (LPF) impinges with a CH4 diluted with N2 diffusion flame (DF) having different turbulence conditions to create a lean heterogeneous combustion model such as a stratified combustion. The local quenching recovery processes of LPF and DF interacting with the turbulence in an opposed flow have been investigated experimentally using a Particle Image Velocimetry movie. The local quenching phenomena can be observed frequently with approaching the global extinction condition. The local quenching may trigger to global extinction. However, in many cases, the flame can recover from the local quenching phenomena and create the stable flame. There are three distinct local quenching recovery mechanisms namely a passive mode, an active mode, and an eddy transportation mode. These three modes depend on the local flame propagation mechanism, the bulk flow motion, and the eddy motion by turbulence. In the passive mode, the bulk flow plays an important role on the recovery process. The local quenching area is drifting outward from the stabilization point by the bulk flow and then, it is displaced by the stable flamelets. In the active mode, the local quenching area is recovered by the self-propagating wrinkled LPF from somewhere in the active zone. The active mode is observed only when the turbulence is added to the premixed flame side. In the eddy motion mode, the local quenching area is recovered by the eddy transportation. That is, the flamelet is transport by the eddy motion and the local quenching area is replaced. The wrinkled flamelet having self-propagation plays a very important role for the local quenching recovery mechanism. The turbulence on the premixed flame not only induces high possibility for the local quenching but also helps to recover from the local quenching.
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Manikantachari, K. R. V., Scott Martin, Ramees K. Rahman, Carlos Velez, and Subith Vasu. "A General Study of Counterflow Diffusion Flames for Supercritical CO2 Mixtures." In ASME Turbo Expo 2019: Turbomachinery Technical Conference and Exposition. American Society of Mechanical Engineers, 2019. http://dx.doi.org/10.1115/gt2019-90332.
Повний текст джерелаАнотація:
Abstract A counterflow diffusion flame for supercritical CO2 combustion is investigated at various CO2 dilution levels and pressures by accounting for realgas effects into both thermal and transport properties. The UCF 1.1 24-species mechanism is used to account the chemistry. The nature of important non-premixed combustion characteristics such as Prandtl number, thermal diffusivity, Lewis number, stoichiometric scalar dissipation rate, flame thickness, and Damköhler number are investigated with respect to CO2 dilution and pressure. The result show that, the aforementioned parameters are influenced by both dilution and pressure; the dilution effect is more dominant. Further, result shows that Prandtl number increases with CO2 dilution and at ninety percent CO2 dilution, the difference between the Prandtl number of the inlet jets and the flame is minimal. Also, the common assumption of unity Lewis number in the theory and modeling of non-premixed combustion does not hold reasonable for sCO2 applications due to large difference of Lewis number across the flame and the Lewis number on the flame drop significantly with increase in the CO2 dilution. An interesting relation between Lewis number and CO2 dilution is observed. The Lewis number of species drops by 15% when increasing the CO2 dilution by 30%. Increasing the CO2 dilution increases both the flow and chemical timescales; however chemical timescale increases faster than the flow time scales. The magnitudes of the Damköhler number signifies the need to consider finite rate chemistry for sCO2 applications. Further, the Damköhler numbers at 90% sCO2 dilution are very small, hence laminar flamelet assumptions in turbulent combustion simulations are not physically correct for this application. Also, it is observed that the Damköhler number drops non-linearly with increasing CO2 dilution in the oxidizer stream. This is a very important observation for the operation of sCO2 combustors. Further, the flame thickness is found to increase with CO2 dilution and reduce with pressure.