A hybrid level set method for modelling detonation and combustion problems in complex geometries

We present an accurate and fast wave tracking method that uses parametric representations of tracked fronts, combined with modifications of level set methods that use narrow bands. Our strategy generates accurate computations of the front curvature and other geometric properties of the front. We introduce data structures that can store discrete representations of the location of the moving fronts and boundaries, as well as the corresponding level set fields, that are designed to reduce computational overhead and memory storage. We present an algorithm we call stack sweeping to efficiently sort and store data that is used to represent orientable fronts. Our implementation features two reciprocal procedures, a forward ‘front parameterization’ that constructs a parameterization of a front given a level set field and a backward ‘field construction’ that constructs an approximation of the signed normal distance to the front, given a parameterized representation of the front. These reciprocal procedures are used to achieve and maintain high spatial accuracy. Close to the front, precise computation of the normal distance is carried out by requiring that displacement vectors from grid points to the front be along a normal direction. For front curves in two dimensions, a cubic interpolation scheme is used, and G ¹ surface parameterization based on triangular patches is used for the three-dimensional implementation to compute the distances from grid points near the front. To demonstrate this new, high accuracy method we present validations and show examples of combustion-like applications that include detonation shock dynamics, material interface motions in a compressible multi-material simulation and the Stephan problem associated with dendrite solidification.     

Flow Structure of Lifted Swirling Jet Flames

Swirling combustion is widely applied in various applications such as gas turbines, utility boilersor waste incinerators. This article contributes to the ongoing research by providing experimentaldata that are gathered in the mixing zone of a lifted swirling premixed natural gas flame. Theobjective of this paper is fivefold: (1) to introduce the lifted swirling flame featuring lowNO _x emissions (2) to provide experimental data such as major species distributions, temperature and streamlines of the flow pattern, (3) to report on velocity bias in probability density function (PDF) distributions and to present PDF sequences of velocities in medium scale swirling flows, (4) to make an assessment on the local small-scale turbulence that is present in the swirling mixinglayer and (5) to provide new experimental data for model verification and development.The PDFs are corrected in order to compensate for the velocity bias phenomenon, which is typicalfor randomly sampled LDA data. Sequences of axial PDF data are presented and measurement locationsof interest are selected to look at the PDF characteristics of the internal and externalrecirculation zones, the mixing layer and the onset of the reacting flow into detail. The mixinglayer PDFs covered a wide velocity range and revealed bimodality; even the concept ofmulti-modality is suggested and explored. Analysis showed that a sum of two Gaussian distributionscan accurately envelop the experimental PDFs. The reason for this broadband turbulence behavior isto be found in combination of precessing and flapping motion of the flow structures, and also incombustion generated instabilities of the lifted flame. As a result, the flame brush is wide (largescale motion) and the mixing (small-scale turbulence) flattens any high temperatures in thecombustion process.The multi-scale turbulence concept is subsequently used to make anassessment of the local turbulence characteristics in the mixing layer.The idea is that the PDFs capture both contributions of the flow-inherent fine grain turbulence (u _l ) which is superposed on slowlarge scale fluctuating structures. It is this u _l that will be of interest in continued research on the classification of the lifted flame into acombustion regime diagram (e.g. Borghi diagram). Finally, the bimodalitycharacter in reacting flows and the prediction of large-scale structuresmay be a challenge for LES researchers.     

Noise Reduction in Non-Premixed Lifted-Jet Flames

The characteristic changes in non-premixed lifted flames when excited by hole tones from a cavity, placed in the flow path of the fuel gas, were studied. A significant reduction of the sound pressure level was observed in the low-frequency noise at the flame base of the lifted flame when the hole tones were induced in the jet. The liftoff height and the mean diameter of the flame base decreased for a given jet Reynolds number. The blow-off velocities also increased suggesting improved flame stability in the presence of the hole tones induced by the cavity. Incorporation of the cavity upstream of a burner nozzle is demonstrated to give a quieter lifted flame with improved stability characteristics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Experimental Study on Stabilization of Lifted Swirl Flames in a Model GT Combustor

This study reports on experimental investigations on isothermal and reacting swirled non-premixed flows under varying pressure conditions. In this configuration, a central high speed fuel jet was surrounded by a heated swirling air flow. For the reacting case natural gas served as fuel whereas for isothermal conditions fuel was replaced by a mixture of helium and air to achieve Reynolds-similarity. The optically accessible combustor allowed for application of laser diagnostics. Here we report on Laser Doppler Anemometry and planar laser-induced fluorescence (PLIF) experiments used to characterize the flow field and visualize selected scalars, respectively. Acetone served as a fluorescence marker for mixture fraction investigations. The hydroxyl radical was used to provide general features of the reaction zone such as flame shape and mean stabilization. To expose the influence of pressure on the flame structure three different operating points were investigated varying the combustor pressure between 2 and 6 bar while the inflow bulk velocities remained the same. Striking features of the present configuration are a detached flame, multiple recirculation zones, and complex coherent flow structures.     

适应于纵数据的随机效应模型中参数的局部影响诊断

本根据纵数据既包含个体又包含个体不同状态的特点，针对适应于纵数据的随机效应模型提出两种便于合理分析数据的扰动方案，并给出扰动对参数估计局部影响的各种计算公式和寻找影响点的方法。通过对Ｃａｍｂｒｉｄｇｅ过滤嘴中提取尼古丁含量的实验室间数据进行分析表明我们的分析结果不但包含了以前许多学用不同的方法对这组数据所进行的所有有关影响点方面的分析结果，而且还获得了一些新的结果。     

Large Eddy Simulation of flame stabilisation dynamics and vortex control in a lifted H2/N2 jet flame

Flame stabilisation in (highly) preheated mixture is common in several industrial applications. When the reactants are injected separately in the device (usually at high-speed), the flame is lifted so that the fuel and oxidant first mix to give an ignitable mixture. If the temperature of the mixture is adequate, it auto-ignites stabilizing the flame. Here we focus on an academic lifted jet flame and Large Eddy Simulation (LES) is used to capture the flame and auto-ignition dynamics. Comparisons with experimental data show that LES simulates accurately high OH fluctuation levels at the stabilisation location. The vortex dynamics linked to these fluctuations is analyzed and it is found that small scale coherent structures play a vital role in the auto-ignition process. These structures are axial vorticity tubes (braids) and are located relatively far (in the radial direction) from the shear-layer. As a consequence, the lift-off height varies dramatically in time leading to OH fluctuations of the order of the mean OH concentration. This scenario is monitored in the compositional space highlighting the simultaneous evolution of OH, HO ₂ and temperature. Further, different strategies for open-loop control of the flame lift-off height are tested. In order to anchor the flame at different positions downstream of the nozzle, the vortex dynamics in the shear-layer was modified. Promoting successively vortex ring and braids, the auto-ignition region was moved significantly. In particular, modified nozzle geometries impacted the formation of braids and ensured a good premixing very close to the nozzle. As a consequence, it was possible to reduce significantly the lift-off height and stabilise the flame few diameters downstream of the nozzle.     

Level‐set based flamelet approach for simulating turbulent lifted jet flame

The present study focuses on numerically investigating the flame structure, flame liftoff, and stabilization in a lifted turbulent H₂/N₂ jet flame with a vitiated coflow. To realistically represent the turbulent partially premixed nature in the flow region between nozzle exit and flame base, the level‐set approach coupled with the conserved scalar flamelet model has been applied. The unstructured‐grid level‐set approach has been developed to allow the geometric flexibility and computational efficiency for the solution of the physically and geometrically complex reacting flows. The pressure–velocity coupling is handled by the multiple pressure‐correction method. The predicted flame pattern is in good conformity with the measured one. In terms of the liftoff height, the agreement between prediction and experiment is quite good. Even if there are noticeable deviations in a certain region, the predicted profiles for the overall flame structure agree reasonably well with the experimental data. These numerical results indicate that the present level‐set‐based flamelet approach in conjunction with the unstructured‐grid finite‐volume method is capable of realistically predicting the essential features and precise structure of the turbulent‐lifted jet flame with computational efficiency. Copyright © 2008 John Wiley & Sons, Ltd.     

Heat release rate as represented by [OH] × [CH2O] and its role in autoignition

Data from a recent instantaneous, simultaneous, high-resolution imaging experiment of Rayleigh temperature and laser induced fluorescence (LIF) of OH and CH₂O at the base of a turbulent lifted methane flame issuing into a hot vitiated coflow are analysed and contrasted to reference flames to further investigate the stabilization mechanisms involved. The use of the product of the quantified OH and semi-quantified CH₂O images as a marker for heat release rate is validated for transient autoigniting laminar flames. This is combined with temperature gradient information to investigate the flame structure. Super-equilibrium OH, the nature of the profiles of heat release rate with respect to OH mole fraction, and comparatively high peak heat release rates at low temperature gradients is found in the kernel structures at the flame base, and found to be indicative of autoignition stabilization.     

New 5-designs constructed from the lifted Golay code over ℤ4

It is shown that the lifted Golay code over ℤ₄ contains several 5-designs. In particular, a 5-(24, 12, 1584) design and a 5-(24, 12, 1632) design are constructed for the first time. © 1998 John Wiley & Sons, Inc. J Combin Designs 6: 225–229, 1998     

On Recognizing Frame and Lifted‐Graphic Matroids

We prove that there is no polynomial with the property that a matroid M can be determined to be either a lifted‐graphic or frame matroid using at most rank evaluations. This resolves two conjectures of Geelen, Gerards, and Whittle (Quasi‐graphic matroids, to appear in J. Graph Theory).