共查询到20条相似文献,搜索用时 93 毫秒
1.
预混火焰拉伸和曲率效率的物理分析 总被引:1,自引:0,他引:1
湍流燃烧的基本火焰结构是拉伸的曲面涡管;拉伸流场中的管形火焰模拟了湍流燃烧的细微结构。本文对平面预混火焰、拉伸预混火焰和管形拉伸预混火焰进行了质量、能量和组分的守恒分析。通过对比这几种火焰,揭示了火焰拉伸效果是通过优先扩散改变火焰温度和熄火极限;而火焰曲率通过增强或削弱这种优先扩散效果来影响火焰温度,影响的大小和火焰厚度与火焰半径的比值呈正比。 相似文献
2.
本文利用5 kHz同步PIV/OH-PLIF实验装置,结合大涡模拟(LES)耦合PDF燃烧模型,系统研究燃料中心喷射下非预混旋流火焰中部熄火及再次稳燃机理,对比了两种旋流器出口结构的影响.结果 表明,LES-PDF模型可以准确地捕捉到旋流流场分布及火焰形态,包括中部熄火及火焰重新稳定.相比于直燃道的结构,扩张燃道的存在引导了更宽的回流区,改善了中部熄火现象,两种结构对其下游的主火焰再次稳燃高度影响不大.中央燃气射流带来的高标量耗散率使得热损失变大,从而引起中部熄火.同时进一步促进了CH4与空气的预先混合及部分反应前置物(CH2O),为非预混旋流火焰在下游重新稳燃提供了有利条件.通过对火焰再次稳燃处OH反应项及扩散项的分析,发现反应项占据主导地位,部分预混火焰传播为再次稳燃的主要机理. 相似文献
3.
预混气体燃烧火焰闪烁现象分析 总被引:1,自引:0,他引:1
在低速射流的预混火焰和扩散火焰中都存在火焰闪烁现象。对扩散火焰,其机理已比较明确,是由于浮力诱导引起的一种水力学不稳定性。而对预混火焰闪烁现象则存在水力学不稳定性和热驱动不稳定性两种观点。本文根据水力学不不稳定性观点,把预混火焰的闪烁现象看成是包围火焰锋面的已燃混气层中内、外区间在垂直方向上的相对脉动,应用Kelvin-Helmholtz不稳定性机理进行了分析,获得了火焰闪烁频率与重力和压力的关系式,并与已有的结果作了对比。 相似文献
4.
本文基于详细化学反应机理和输运性质,对氢气/空气部分预混火焰瞬态响应及脉冲不稳定性进行了数值分析.研究发现预混火焰区存在脉冲不稳定现象,振荡过程可形成极限环,此时火焰强度更依赖于O_2浓度,而对当地温度不敏感;扩散火焰区由于受到预混火焰区传热、传质影响,扩散火焰出现受迫振荡现象。研究结果表明增大拉伸率及当量比会抑制脉冲不稳定现象的发生,其原因是扩散火焰和预混火焰之间距离减小,具有更高温度的扩散火焰向预混火焰区导热增强,后者火焰强度增大,从而减小了Zeldovich数,抑制了不稳定现象的发生。 相似文献
5.
6.
7.
9.
10.
11.
12.
Kapil K. Singh Cheng Zhang Jay P. Gore Luc Mongeau Steven H. Frankel 《Proceedings of the Combustion Institute》2005,30(2):1707-1715
The occurrence of oscillating combustion and combustion instability has led to resurgence of interest in the causes, mechanisms, suppression, and control of combustion noise. Noise generated by enclosed flames is of greater practical interest but is more complicated than that by open flames, which itself is not clearly understood. Studies have shown that different modes of combustion, premixed and non-premixed, differ in their sound generation characteristics. However, there is lack of understanding of the region bridging these two combustion modes. This study investigates sound generation by partially premixed flames. Starting from a non-premixed flame, air was gradually added to achieve partial premixing while maintaining the fuel flow rate constant. Methane, ethylene, and ethane partially premixed flames were studied with hydrogen added for flame stabilization. The sound pressure generated by methane partially premixed flames scales with M5 compared to M3 for turbulent non-premixed methane flames. Also, the sound pressure generated by partially premixed flames of ethane and ethylene scales as M4.5. With progressive partial premixing, spectra level increases at all frequencies with a greater increase in the high-frequency region compared to the low-frequency region; flames develop a peak and later a constant level plateau in the low frequency region. The partially premixed flames of methane, ethylene, and ethane generate a similar SPL as a function of equivalence ratio when the fuel volume flow rate is matched. However, when fuel mass flow rate is matched, the ethane and ethylene flames produce a similar SPL, which is lower than that produced by the methane flame. 相似文献
13.
The mixing, reaction progress, and flame front structures of partially premixed flames have been investigated in a gas turbine model combustor using different laser techniques comprising laser Doppler velocimetry for the characterization of the flow field, Raman scattering for simultaneous multi-species and temperature measurements, and planar laser-induced fluorescence of CH for the visualization of the reaction zones. Swirling CH4/air flames with Re numbers between 7500 and 60,000 have been studied to identify the influence of the turbulent flow field on the thermochemical state of the flames and the structures of the CH layers. Turbulence intensities and length scales, as well as the classification of these flames in regime diagrams of turbulent combustion, are addressed. The results indicate that the flames exhibit more characteristics of a diffusion flame (with connected flame zones) than of a uniformly premixed flame. 相似文献
14.
Flame dynamics 总被引:1,自引:0,他引:1
Moshe Matalon 《Proceedings of the Combustion Institute》2009,32(1):57-82
This lecture describes recent theoretical developments associated with the dynamics of flames, obtained primarily by exploiting the various temporal and length scales involved in the combustion process. In premixed flames the focus is on flame-flow interactions that occur during the nonlinear development of hydrodynamically unstable large-scale flames, or during the propagation of curved flames in two-dimensional channels. The second part of the paper deals with non-premixed and partially premixed flames, where the focus is on understanding the nature of diffusive-thermal instabilities including the effect of thermal expansion, and on stabilization mechanisms of edge flames, which possess characteristics of both premixed and diffusion flames. The results presented in this talk illustrate how simplified models, when analyzed to their extreme, yield predictions of qualitative nature with physical insight that have advanced our understanding of combustion. This insight can be used to guide the experimental efforts, explain observations and validate large-scale numerical simulations. 相似文献
15.
《Proceedings of the Combustion Institute》2023,39(2):2249-2258
A new mixing time scale for PDF calculations of premixed combustion in the laminar flame limit is introduced. It is based on the characteristics of the “random walk” diffusion process and accurately captures scalar micro-mixing such that physical features of the premixed flames are preserved. The speed of a freely-propagating laminar flame can be captured accurately. Extreme stochastic events may, however, lead to flame acceleration, but flame stability can be recovered with the introduction of a conditioning variable that relaxes towards an Eulerian reference field. With conditioning the predicted flame speed is quite insensitive towards the exact value of the only modelling parameter. This modelling parameter then allows to control the minor scalar fluctuations and deviations from a flamelet structure which is thought to be one of the key features of conditioning methods. 相似文献
16.
17.
《Combustion Theory and Modelling》2013,17(4):423-428
This paper shows that the change in the number of moles of species during combustion can make a strong contribution to the acoustic power radiated by turbulent flames and cannot be systematically neglected. Starting from standard conservation equations, we derive an expression for the acoustic pressure radiated in the far field of a compact region of fluid where low Mach number non-isomolar combustion takes place. In this formulation, the contributions from ‘molar’ and thermal expansion appear explicitly. We also give a formulation in which the sound emission arising from purely non-stationary and from purely convective effects appear independently. As an application of the theory, we derive the acoustic power emitted by a premixed flame in the flamelet regime. Numerical evaluations show that the contribution of molar expansion to the acoustic power is between 2 and 5.6 dB (260% increase) for some common hydrocarbon-oxygen flames. 相似文献
18.
Jonathan Sosa Daniel A. Rosato Gabriel B. Goodwin Christian L. Bachman Elaine S. Oran Kareem A. Ahmed 《Proceedings of the Combustion Institute》2021,38(3):3513-3520
Experimental evidence of controlled detonation initiation and propagation in a hypersonic flow of premixed hydrogen-air is presented. This controlled detonation initiation is created in a hypersonic facility capable of producing a Mach 5 flow of hydrogen-air. Flow diagnostics such as high-speed schlieren and OH* chemiluminescence results show that a flame deflagration-to-detonation transition occurs as a combined result of turbulent flame acceleration and shock-focusing. The experimental results define three new distinct regimes in a Mach 5 premixed flow: deflagration-to-detonation transition (DDT), unsteady compressible turbulent flames, and shock-induced combustion. A two-dimensional implicit-LES (ILES) simulation, which solves the compressible, reactive Navier-Stokes equations on an adapting grid is conducted to provide additional insight into the local physical mechanism of detonation transition and propagation. 相似文献
19.
Sinan Demir Vitaly Bychkov Sri Hari Ramakrishna Chalagalla 《Combustion Theory and Modelling》2017,21(6):997-1022
To reveal the inner mechanisms of a combustion accident in a coalmine, the key stages and characteristics of premixed flame front evolution such as the flame shapes, propagation speeds, acceleration rates, run-up distances and flame-generated velocity profiles are scrutinised. The theories of globally spherical, expanding flames and of finger-flame acceleration are combined into a general analytical formulation. Two-dimensional and cylindrical mining passages are studied, with noticeably stronger acceleration found in the cylindrical geometry. The entire acceleration scenario may promote the total burning rate by up to two orders of magnitude, to a near-sonic value. Starting with gaseous combustion, the analysis is subsequently extended to gaseous-dusty environments. Specifically, combustible dust (e.g. coal), inert dust (e.g. sand), and their combination are considered, and the influence of the size and concentration of the dust particles is quantified. In particular, small particles influence flame propagation more than large ones, and flame acceleration increases with the concentration of a combustible dust, until the concentration attains a certain limit. 相似文献
20.
A numerical modeling of the acceleration of a flame by an additional energy input ahead of its front
M. F. Ivanov A. D. Kiverin V. A. Gal’burt 《Russian Journal of Physical Chemistry B, Focus on Physics》2009,3(3):363-367
The acceleration of a flame after an additional energy input ahead of its front was simulated using numerical methods. The combustion of a hydrogen-air mixture in a semiopen channel was considered. The calculations were performed within the framework of a two-dimensional hydrodynamic model of premixed flames, with consideration given to heat transfer, multicomponent diffusion, and chemical kinetics. It was demonstrated that, when the interaction of the flame front with the near-wall boundary layer is taken into account, even a moderate energy input could substantially promote the development of the Landau-Darrieus instability and, possibly, deflagration to detonation transition. 相似文献