ELEVEN years ago appeared the comprehensive monograph entitled “Flame and Combustion in Gases” by Prof. W. A. Bone and D. T. A. Townend, followed a year or two later by “Gaseous Combustion at High Pressures” by Bone, Newitt and Townend. Except for a small volume on "Flame"by 0. C. de C. Ellis and W. A. Kirkby (1936), the subject has not since been treated as a whole in English, until the recent appearance of the work by Bernard Lewis and G. von Elbe now under review. Combustion, Flames and Explosions of Gases By Dr. Bernard Lewis Dr. Guenther von Elbe. (The Cambridge Series of Physical Chemistry.) Pp. xiv + 415. (Cambridge: At the University Press, 1938.) 21s. net.

Combustion, Flames and Explosions of Gases

Chemiluminescence imaging of an optically accessible non-premixed rotating detonation engine

Transverse combustion instabilities: Acoustic, fluid mechanic, and flame processes

Combustion instabilities represent a long known problem in combustion technology. The complex interactions between acoustics and turbulent swirling flames are not fully understood yet, making it very difficult to reliably predict the stability of new combustion systems. For example, the effects of fluctuations of swirl number on the heat release of the flame have to be investigated in more detail. In this paper a perfectly premixed, swirl stabilized burner with variable axial position of the swirl generator is investigated. In experiments, the position of the swirl generator has a strong impact on the dynamic flame response, although it does not influence the time-averaged distribution of the heat release significantly. This phenomenon is further investigated, using computational fluid dynamics combined with system identification. The generation of fluctuations of swirl number, their propagation to the flame, and their effect on the dynamic flame response are examined. A simple model based on convective time lags is developed, showing good agreement with experiments.Copyright © 2009 by ASME

Impact of Swirl Fluctuations on the Flame Response of a Perfectly Premixed Swirl Burner

Der speziellen Relativitatstheorie liegt folgendes Postulat zugrunde, welchem auch durch die Galilei-Newtonsche Mechanik Genuge geleistet wird: Wird ein Koordinatensystem K so gewahlt, das in bezug auf dasselbe die physikalischen Gesetze in ihrer einfachsten Form gelten, so gelten dieselben Gesetze auch in Bezug auf jedes andere Koordinatensystem K′, das relativ zu K in gleichformiger Translationsbewegung begriffen ist. Dieses Postulat nennen wir „spezielles Relativitatsprinzip“. Durch das Wort „speziell“ soll angedeutet werden, das das Prinzip auf den Fall beschrankt ist, das K′ eine gleichformige Translationsbewegung gegen K ausfuhrt, das sich aber die Gleichwertigkeit von K′ und K nicht auf den Fall ungleichformiger Bewegung von K′ gegen K erstreckt.

Die Grundlage der allgemeinen Relativitätstheorie

The hazardous potential of hydrogen/air mixtures has intensively been studied assuming a perfect mixture of fuel and oxidant. However, comprehensive risk assessment studies have shown that an inhomogeneous mixture with a vertical concentration gradient is much more likely to be generated in a real accident scenario. From a safety point of view, an open question is whether established criteria such as the 7λ criterion for the determination of the deflagration-to-detonation transition (DDT) limits can be applied to inhomogeneous mixtures as well. For the experimental investigation of DDT in such mixtures an injection mechanism has been developed that produces vertical fuel concentration gradients inside a horizontal channel with large aspect ratio. The channel is equipped with obstacles to enhance flame acceleration. Photodiodes and pressure transducers measure flame and shock arrival times. Schlieren measurements are conducted to track gas mixing behind the obstacles and to track the formation of the deton...

Deflagration-to-Detonation Transition in Hydrogen/Air Mixtures with a Concentration Gradient

In this study the hazardous potential of flammable hydrogen-air mixtures with vertical concentration gradients is investigated numerically. The computational model is based on the formulation of a reaction progress variable and accounts for both deflagrative flame propagation and autoignition. The model is able to simulate the deflagration-to-detonation transition (DDT) without resolving all microscopic details of the flow. It works on relatively coarse grids and shows good agreement with experiments. It is found that a mixture with a vertical concentration gradient can have a much higher tendency to undergo DDT than a homogeneous mixture of the same hydrogen content. In addition, the pressure loads occurring can be much higher. However, the opposite effect can also be observed, with the decisive factor being the geometric boundary conditions. The model gives insight into different modes of DDT. Detonations occurring soon after ignition do not necessarily cause the highest pressure loads. In mixtures with concentration gradient, the highest loads can occur in regions of very low hydrogen content. These new findings should be considered in future safety studies.

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Numerical Simulation of the Deflagration-to-Detonation Transition in Inhomogeneous Mixtures

In accident scenarios where detonations can occur a concentration gradient constitutes a more realistic initial condition than a perfectly homogeneous mixture. In this paper, the influence of a concentration gradient on detonation front shape, detonation instabilities and pressure distribution is studied. First, a simple method to determine the front shape from a given fuel distribution is presented. It is based on Huygens’ principle and includes a correction to satisfy the boundary conditions on the enclosing walls. Next, the presented highly resolved Euler computations demonstrate the influence of a concentration gradient on detonation instabilities. In configurations with a strong concentration gradient, Mach reflection occurs and leads to an asymmetric pressure load on the enclosing geometry. In this case, the impulse on the wall is higher than in configurations with homogeneous fuel distribution, although the fuel content is much lower.

Mach reflection in detonations propagating through a gas with a concentration gradient

In this paper we apply a new method to obtain the dynamical characteristics of premixed flames in acoustically complex systems like an annular combustor. For this a novel model based reduction method was applied which describes the acoustic field as a superposition of a discrete number of eigenmodes. The measured dynamic signals are decomposed into modal components which are then used to fit the model parameters. With this approach the complete characterisation of the annular test rig requires significantly less experimental effort, i.e. simultaneous pressure measurements, than before. After describing the new procedure, we show the validation of the acoustical model. Using this model, we were able to obtain the characteristics of the flame in the annular combustor with a much smaller number of pressure sensors as the model provides physical constraints that would otherwise have to be measured. The comparison of the characteristic flame parameters between the single burner and the annular combustor configurations for the same operating conditions shows that in the annular combustor slightly longer convective time delays are found which are consistent with the static flame characterisation showing longer flames in the annular combustor than in the single burner test rig.Copyright © 2008 by ALSTOM

Determination and Comparison of the Dynamic Characteristics of a Perfectly Premixed Flame in Both Single and Annular Combustion Chambers

In this paper alternative ways to obtain the thermo-acoustical characteristics of perfectly premixed flames given by their flame transfer matrix (FTM) is investigated. In particular a model based data reduction procedure which greatly reduces the experimental effort and therefore enables to provide this flame data for many more operation points than previously possible is proposed and validated. It is shown how the acoustic pressure field measured from two forcing states using the multi microphone method leads to the determination of the direct experimental FTM. The next relatively simpler method shown is the hybrid method which is based on Rankine-Hugoniot relations and the experimental flame transfer function (FTF) from OH*-chemiluminescence measurements for heat release fluctuations. Later to obtain the FTM using a network model based on Rankine-Hugoniot relations and an n-τ-σ FTF model representing the flame by regression analysis of the acoustical measurements is presented. Experimental results for the direct experimental FTM and the hybrid FTM are compared with the model based result. The results indicate very good consistency between the direct, hybrid and model based techniques providing a global check of the methods/tools used for analysing the thermoacoustic mechanisms of flames.© 2008 ASME

F. Ettner

Papers

Deflagration-to-Detonation Transition in Hydrogen/Air Mixtures with a Concentration Gradient

Numerical Simulation of the Deflagration-to-Detonation Transition in Inhomogeneous Mixtures

Mach reflection in detonations propagating through a gas with a concentration gradient

Determination and Comparison of the Dynamic Characteristics of a Perfectly Premixed Flame in Both Single and Annular Combustion Chambers

Flame Transfer Matrices of a Premixed Flame and a Global Check With Modelling and Experiments