| 1. |
- Wadekar, Sandip, 1989, et al.
(författare)
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Large Eddy Simulation of Stratified Combustion in Spray-guided Direct Injection Spark-ignition Engine
- 2018
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Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627. ; 2018-April
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Tidskriftsartikel (refereegranskat)abstract
- Stratified combustion in gasoline engines constitutes a promising means of achieving higher thermal efficiency for low to medium engine loads than that achieved with combustion under standard homogeneous conditions. However, creating a charge that leads to a stable efficient low-emission stratified combustion process remains challenging. Combustion through a stratified charge depends strongly on the dynamics of the turbulent fuel-air mixing process and the flame propagation. Predictive simulation tools are required to elucidate this complex mixing and combustion process under stratified conditions. For the simulation of mixing processes, combustion models based on large-eddy turbulence modeling have typically outperformed the standard Reynolds averaged Navier-Stokes methods. Therefore, we investigated spray-guided stratified combustion in a single cylinder engine using large-eddy turbulence modeling with a variant of the flame speed closure (FSC) model for premixed turbulent combustion. This model reveals the influence of the mixture composition on the flame speed. The effect of fluctuations in the composition were accounted for by using a presumed probability density function (PDF) approach for the mixture fraction. The fuel injection process was modeled with a standard Lagrangian spray model. More importantly, the measured in-cylinder pressure traces for three different loading cases with varying injection and ignition timings (leading to different levels of stratification) were accurately reproduced by the simulation. High-speed video images were used to evaluate the ability of the model to accurately simulate flame propagation under stratified conditions. The influence of mixture fluctuations on flame propagation was also investigated.
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| 2. |
- Lipatnikov, Andrei, 1961
(författare)
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Simulations of scalar transport in developing turbulent flames solving a conditioned balance equation
- 2010
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Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 182:7, s. 405-421
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Tidskriftsartikel (refereegranskat)abstract
- A balance equation for the difference in the conditioned velocities (u) over bar (b) and (u) over bar (u), derived and validated recently (Lipatnikov, 2008a, 2008b), is numerically solved in a statistically planar, one-dimensional case in order to (a) highlight the influence of premixed turbulent flame development on the direction of the mean scalar flux and (b) assess the equation by comparing computed trends with available experimental and DNS data. Numerical results show that (a) the flux (rho u '' c '') over bar gradient during an early stage of flame development followed by a transition to countergradient scalar transport (i. e., (rho u '' c '') over bar center dot del(c) over bar >0) at certain instant t(tr); (b) the transition time t(tr) is increased by the rms turbulent velocity and decreases when the density ratio or the laminar flame speed increases; and (c) even after the transition from gradient to countergradient scalar transport, the mean flame brush thickness grows because the mean rate of product creation overwhelms the transport term in the combustion progress variable balance equation and serves to not only control the turbulent burning rate, but also cause the growth of the thickness.
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| 3. |
- Lee, Hsu Chew, et al.
(författare)
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A DNS study of extreme and leading points in lean hydrogen-air turbulent flames - part II: Local velocity field and flame topology
- 2022
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Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 235
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Tidskriftsartikel (refereegranskat)abstract
- Data obtained in recent direct numerical simulations (Lee et al.) of statistically one-dimensional and planar, lean complex-chemistry hydrogen-air flames characterized by three different Karlovitz numbers Ka ranging from 3 to 33 are further analyzed in order to explore local characteristics and structure of (i) extreme points characterized by the peak (over the computational domain) Fuel Consumption Rate (FCR) or Heat Release Rate (HRR) and (ii) leading points that are also characterized by a high FCR or HRR, but advance furthest into unburned reactants. Results show that, on the one hand, common characteristics of flame perturbations (curvature, strain and stretch rates, displacement speed) fluctuate significantly in the extreme or leading, FCR or HRR points and are different in different flames. Moreover, other two-point local quantities such as the local gradients of combustion progress variables or species (e.g., the radical H) mass fractions are different in different flames. Therefore, a common simple configuration of a perturbed laminar flame cannot be used as a catchall model of the entire local structure of zones surrounding the discussed points at various Ka. On the other hand, single-point local characteristics (temperature, species mass fractions, rates of their production) of the FCR extreme points are comparable in all three turbulent flames and in the critically strained planar laminar flame. In particular, the FCRs in the extreme points fluctuate weakly and are approximately equal to each other and to the peak FCR in the critically strained laminar flame. The latter finding implies that (i) the maximum FCR evaluated in the critically strained laminar flame could be used to characterize, in a first approximation, the local FCR in the extreme or leading points in turbulent flames, thus, supporting the leading point concept, and (ii) almost the same extreme FCR can be reached in substantially different local burning structures.
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| 4. |
- Yu, Rixin, et al.
(författare)
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A DNS Study of Sensitivity of Scaling Exponents for Premixed Turbulent Consumption Velocity to Transient Effects
- 2019
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Ingår i: Flow, Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1386-6184 .- 1573-1987. ; 102:3, s. 679-698
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Tidskriftsartikel (refereegranskat)abstract
- 3D Direct Numerical Simulations of propagation of a single-reaction wave in forced, statistically stationary, homogeneous, isotropic, and constant-density turbulence, which is not affected by the wave, are performed in order to investigate the influence of the wave development on scaling (power) exponents for the turbulent consumption velocity UT as a function of the rms turbulent velocity u′, laminar wave speed SL, and a ratio L11/δF of the longitudinal turbulence length scale L11 to the laminar wave thickness δF. Fifteen cases characterized by u′/SL = 0.5,1.0,2.0,5.0, or 10.0 and L11/δF = 2.1, 3.7, or 6.7 are studied. Obtained results show that, while UT is well and unambiguously defined in the considered simplest case of a statistically 1D planar turbulent reaction wave, the wave development can significantly change the scaling exponents. Moreover, the scaling exponents depend on a method used to compare values of UT, i.e., the scaling exponents found by processing the DNS data obtained at the same normalized wave-development time may be substantially different from the scaling exponents found by processing the DNS data obtained at the same normalized wave size. These results imply that the scaling exponents obtained from premixed turbulent flames of different configurations may be different not only due to the well-known effects of the mean-flame-brush curvature and the mean flow non-uniformities, but also due to the flame development, even if the different flames are at the same stage of their development. The emphasized transient effects can, at least in part, explain significant scatter of the scaling exponents obtained by various research groups in different experiments, thus, implying that the scatter in itself is not sufficient to reject the notion of turbulent burning velocity.
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| 5. |
- Sathiah, Pratap, 1978, et al.
(författare)
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Effects of Flame Development and Structure on Thermo-Acoustic Oscillations of Premixed Turbulent Flames
- 2004
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Ingår i: 2004 ASME Heat Transfer/Fluids Engineering Summer Conference, Charlotte, North Carolina, USA, July 11-15. - : ASMEDC. ; 4, s. 107-114
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Konferensbidrag (refereegranskat)abstract
- Abstract:Non-stationary confined premixed turbulent flames stabilized behind a bluff body are studied. A simple kinematic model of such flames was developed by Dowling who reduced the combustion process to the propagation of an infinitely thin flame with a constant speed. The goal of this work is to extend the model by taking into account the real structure of premixed turbulent flames and the development of turbulent flame speed and thickness. For these purposes, so-called Flame Speed Closure model for multi-dimensional simulations of premixed turbulent flames is adapted and combined with the aforementioned Dowling model. Simulations of the heat release rate dynamics for ducted flames due to oncoming flow oscillations have been performed. Typical results show that the oscillations in the integrated heat release rate follow the oncoming flow velocity oscillations with certain time delay, which controls the sign of the well-known Rayleigh integral and, hence, the global instability or stability of the combustor. The delays computed using the Dowling and the above approach are different, thus indicating the importance of resolving the flame structure when modeling ducted flame oscillations.
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| 9. |
- Lipatnikov, Andrei, 1961
(författare)
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Fundamentals of Premixed Turbulent Combustion
- 2012
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Bok (övrigt vetenskapligt/konstnärligt)abstract
- Lean burning of premixed gases is considered to be a promising combustion technology for future clean and highly efficient gas turbine combustors. Yet researchers face several challenges in dealing with premixed turbulent combustion, from its nonlinear multiscale nature and the impact of local phenomena to the multitude of competing models. Filling a gap in the literature, Fundamentals of Premixed Turbulent Combustion introduces the state of the art of premixed turbulent combustion in an accessible manner for newcomers and experienced researchers alike.To more deeply consider current research issues, the book focuses on the physical mechanisms and phenomenology of premixed flames, with a brief discussion of recent advances in partially premixed turbulent combustion. It begins with a summary of the relevant knowledge needed from disciplines such as thermodynamics, chemical kinetics, molecular transport processes, and fluid dynamics. The book then presents experimental data on the general appearance of premixed turbulent flames and details the physical mechanisms that could affect the flame behavior. It also examines the physical and numerical models for predicting the key features of premixed turbulent combustion.Emphasizing critical analysis, the book compares competing concepts and viewpoints with one another and with the available experimental data, outlining the advantages and disadvantages of each approach. In addition, it discusses recent advances and highlights unresolved issues. Written by a leading expert in the field, this book provides a valuable overview of the physics of premixed turbulent combustion. Combining simplicity and topicality, it helps researchers orient themselves in the contemporary literature and guides them in selecting the best research tools for their work.
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