Large-eddy structures of turbulent swirling flows and methane-air swirling diffusion combustion

Turbulent swirling flows and methane-air swirling diffusion combustion are studied by large-eddy simulation (LES) using a Smagorinsky-Lilly subgrid scale turbulence model and a second-order moment (SOM) SGS combustion model, and also by RANS modeling using the Reynolds Stress equation model with the IPCM+wall and IPCM pressure-strain models and SOM combustion model. The LES statistical results for swirling flows give good agreement with the experimental results, indicating that the adopted subgrid-scale turbulence model is suitable for swirling flows. The LES instantaneous results show the complex vortex shedding pattern in swirling flows. The initially formed large vortex structures soon break up in swirling flows. The LES statistical results of combustion modeling are near the experimental results and are as good as the RANS-SOM modeling results. The LES results show that the size and range of large vortex structures in swirling combustion are different from those of isothermal swirling flows, and the chemical reaction is intensified by the large-eddy vortex structures. The project supported by the Special Funds for Major State Basic Research (G-1999-0222-07). The English text was polished by Keren Wang.     

AN ANALYTICAL DESIGN METHODOLOGY FOR AN ANNULAR COMBUSTOR

SUMMARY

This paper describes a computational procedure for the optimization of the performance parameters of a simulated annular combustor. This method has been applied to analyze the influence of the performance parameters and geometries on the annular combustor characteristics and provide a good understanding of combustor internal flow fields, and therefore it can be used for guiding the combustor design process. The approach is based on the solution of governing nonlinear, elliptic partial differential equations for 3-D axisymmetric recirculating turbulent reacting swirling flows and the modelling of turbulence, combustion, thermal radiation and pollutant formation. The turbulence effects are introduced via the modified two-equation κ-ε model. Turbulent combustion is modelled using the κ-ε-g model and a two-step turbulent combustion model is employed for the excess emission of carbon monoxide CO. For the evaluation of the NO pollutant formation rate, the NO pollutant formation model, which takes into account the influence of turbulence, presented here. The radiative heat transfer is handled by the heat flux model. The predictions of the combustor character-istics and performance parameters are made using the present approach.

Predictions of velocity, length of the recirculation zone, combustion efficiency and wall temperature are compared with measurements. Agreement between the predictions and experimental data is very satisfactory.     

旋转式一孔测压管及其应用1）

由圆柱三孔型二元复合测压管的测速原理出发，提出用一孔测压管通过旋转实现平面气流速度大小与方向的自动测量，建立了相应的测试系统．通过大量实验，研究了该系统的性能．在多种回流和旋流流场测量中进行了应用．结果表明该系统测量原理正确，重复性好，精度高     

旋转式一孔测压管及其应用1）

由圆柱三孔型二元复合测压管的测速原理出发，提出用一孔测压管通过旋转实现平面气流速度大小与方向的自动测量，建立了相应的测试系统．通过大量实验，研究了该系统的性能．在多种回流和旋流流场测量中进行了应用．结果表明该系统测量原理正确，重复性好，精度高     

Development of a hybrid k-ε turbulence model for swirling recirculating flows under moderate to strong swirl intensities

A hybrid k-ε turbulence model, based on the concept that the modification of anisotropic effects should not be made in the flow regions inherent to small streamline curvatures, has been developed and examined with the swirling recirculating flows, with the swirl levels ranging from 0·6 to 1·23 in abrupt pipe expansion. A fairly satisfactory agreement of model predictions with the experimental data shows that this hybrid k-ε model can perform better simulation of swirling recirculating flows as compared to the standard k-ε model and the modified k-ε model proposed by Abujelala and Lilley.     

MODIFICATION OF THE κ-ε TURBULENCE MODEL FOR SWIRLING RECIRCULATING FLOW IN A PIPE EXPANSION

SUMMARY

A modified turbulence model, incorporating anisotropic effects into the standard κ-ε model, is proposed for the numerical simulation of swirling recirculating flow in a pipe expansion. The new modification follows the concept that the correction to the standard κ-ε model is made only in the flow regions where the anisotropic effects are appreciable. The performance of the present hybrid κ-ε model for two swirling flows with moderate and high swirl levels is better than that of the standard κ-ε model and the modified κ-ε model proposed by Abujelala and Lilley and is also competitive with our previously developed hybrid κ-ε model. Moreover, the present hybrid modification provides a relatively simpler form compared to our previously developed model; this also helps alleviate the numerical instability in the calculation procedure. Encouraging improvement in the prediction of the size of the central toroidal recirculation zone using the present hybrid modification is also exhibited     

Buoyancy effects on turbulent swirling flows

A three-parameter model of turbulence applicable to free boundary layers has been developed and applied for the prediction of axisymmetric turbulent swirling flows in uniform and stagnant surroundings under the action of buoyancy forces. The turbulent momentum and heat fluxes appearing in the time-averaged equations for the mean motion have been determined from algebraic expressions, derived by neglecting the convection and diffusion terms in the differential transport equations for these quantities, which relate the turbulent fluxes to the kinetic energy of turbulence, k, the dissipation length scale of turbulence, L, and the temperature covariance, T ′². Differential transport equations have been used to determine these latter quantities. The governing equations have been solved using fully implicit finite difference schemes. The turbulence model is capable of reproducing the gross features of pure jet flows, buoyant flows and swirling flows for weak and moderate swirl. The behaviour of a turbulent buoyant swirling jet has been found to depend solely on exit swirl and Froude numbers. The predicted results indicate that the incorporation of buoyancy can cause significant changes in the behaviour of a swirling jet, particularly when the buoyancy strength is high. The jet exhibits similarity behaviour in the initial region for weak swirl and weak buoyancy strengths only, and the asymptotic case of a swirling jet under the action of buoyancy forces is a pure plume in the far field. The predicted results have been found to be in satisfactory agreement with the available experimental data and in good qualitative agreement with other predicted results.     

强旋湍流气粒两相流动的PDPA研究

采用相多普勒颗粒分析仪（ＰＤＰＡ）对切向进气，轴向缩口出口的旋风筒内强旋单相和气粒两相流动进行了实验研究，给出了强旋流场中，两相湍流的运动及相互作用规律     

NUMERICAL INVESTIGATION OF TURBULENT SHALLOW RECIRCULATING FLOWS BY A QUASI-THREE-DIMENSIONAL k–ϵMODEL

A quasi-three-dimensional multilayer k– ϵ model has been developed to simulate turbulent recirculating flows behind a sudden expansion in shallow waters. The model accounts for the vertical variation in the flow quantities and eliminates the problem of closure for the effective stresses resulting from the depth integration of the non-linear convective accelerations found in the widely used depth- integrated models. The governing equations are split into three parts in the finite difference solution: advection, dispersion and propagation. The advection part is solved using the four-node minimax–characteristics method. The dispersion and propagation parts are treated by the central difference method, the former being solved explicitly and the latter implicitly using the Gauss–Seidel iteration method. The relative effect of bed-generated turbulence and transverse shear-generated turbulence on the recirculating flow has been studied in detail. In comparison with the results computed by the depth-integrated k–ϵ model, the results computed by the present model are found to be closer to the reported data.     

Determination of principal characteristics of turbulent swirling flow along annuli: Part 2: Measurement of turbulence components

Turbulence data for swirling flows along the annulus formed between two co-axial tubes are presented. The swirl was generated by a set of inlet guide vanes which produced, after settling and removal of wall boundary layers, a nominally free vortex flow over the complete entry plane of the test section. The work complements that described in Part 1 of the paper which considered the behaviour of time mean values under the same entry conditions¹. A hot wire anemometer was used to detect turbulence quantities and techniques have been developed for the measurement of these using the minimum number of simple probe geometries. The theory leading to the derivation of the separate turbulence parameters from hot wire measurements is described. A series of radial profiles are given as representative examples of the extensive data collected in addition to longitudinal variations of friction factor and shear stress. A brief discussion is given on the determination of eddy diffusivity as a function of radial and axial location.     

Introduction of turbulent model in a mixed finite volume/finite element method

The aim of this work is to present a new numerical method to compute turbulent flows in complex configurations. With this in view, a k-? model with wall functions has been introduced in a mixed finite volume/finite element method. The numerical method has been developed to deal with compressible flows but is also able to compute nearly incompressible flows. The physical model and the numerical method are first described, then validation results for an incompressible flow over a backward-facing step and for a supersonic flow over a compression ramp are presented. Comparisons are performed with experimental data and with other numerical results. These simulations show the ability of the present method to predict turbulent flows, and this method will be applied to simulate complex industrial flows (flow inside the combustion chamber of gas turbine engines). The main goal of this paper is not to test turbulence models, but to show that this numerical method is a solid base to introduce more sophisticated turbulence model.     

Direct numerical simulation of confined swirling jets

This study investigates the Lagrangian acceleration and velocity of fluid particles in swirling flows via direct numerical simulation. The intermittency characteristics of acceleration and velocity of fluid particles are investigated at different swirl numbers and Reynolds numbers. The flatness factor and trajectory curvature are used to analyse the effect of Lagrangian intermittency. The joint probability density function of Lagrangian acceleration and turbulence intensity is shown to explain the augmentation effect of Lagrangian intermittency by the strongly swirling levels under the relatively low intensity of turbulence. In addition, the correlation between the Lagrangian acceleration and the turbulence intensity is enhanced as the swirl level increases. It shows the important effect of swirl on the motion behaviour of fluid particles in the strongly swirling flows.     

大速差射流预燃室内三维回流两相湍流的数值模拟

本文由多流体两相流模型、气相湍流κ-ε模型和颗粒湍流代数模型出发,成功地模拟了真实形状大速差射流预燃室中三维湍流回流两相流动,得到了这类复杂的气固两相流中不同纵横截面上气相速度场、颗粒速度场及浓度场和两相湍流度场的分布,并且获得了与实验定性一致的合理结果,揭示了预燃室中气固两相流动与混合的主要物理特征,探讨了大速差射流技术稳焰和强化燃烧的两相流动机理。     

大速差射流预燃室内三维回流两相湍流的数值模拟

本文由多流体两相流模型、气相湍流κ-ε模型和颗粒湍流代数模型出发,成功地模拟了真实形状大速差射流预燃室中三维湍流回流两相流动,得到了这类复杂的气固两相流中不同纵横截面上气相速度场、颗粒速度场及浓度场和两相湍流度场的分布,并且获得了与实验定性一致的合理结果,揭示了预燃室中气固两相流动与混合的主要物理特征,探讨了大速差射流技术稳焰和强化燃烧的两相流动机理。     

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.     

统一二阶矩模型用于模拟旋流湍流两相流动

用统一二阶矩模型（ＵＳＭ）模拟了旋流数为０４７和１５的气粒两相流动，并和实验结果以及ｋ ε ｋｐ模型的模拟结果进行了对比．研究结果表明，提高旋流数减小了轴向速度反流区，增大了切向速度似固核区．ＵＳＭ和ｋ ε ｋｐ模型预报旋流数为０４７时的两相速度场差别不大，并都和实验结果接近，但前者预报的旋流数为１５的两相速度场比后者有改进，在两种情况下，前者都能揭示出后者无法预报的两相湍流各向异性规律．     

On basic equations of turbulent swirling gas-solid flows and their application in cyclones

The basic equations of turbulent gas-solid flows are derived by using the pseudo-fluid model of particle phase with a refined two-phase turbulence model. These equations are then applied to swirling gas-particle flows for analyzing the collection efficiency in cyclone separators.     

Prediction of Pressure Oscillations in a Premixed Swirl Combustor Flow and Comparison to Measurements

The most common and reliable technique used for flame stabilization of industrial combustors with high thermal loads is the application of strongly swirling flows. In addition to stabilization, swirl flames offer the possibility to influence emission characteristics by simply changing the swirl intensity or the type of swirl generation. Despite of these major advantages, swirling flows tend to evolve flow instabilities, that considerably constitute a significant source of noise. In general, noise generation is substantially enhanced, when such a swirling flow is employed for flames. Thus, the minimization of the resulting noise emissions under conservation of the benefit of high ignition stability is one major design challenge for the development of modern swirl stabilized combustion devices. The present investigation makes an attempt to determine mechanisms and processes to influence the noise generation of flames with underlying swirling flows. Therefore, a new burner has been designed, that offers the possibility to vary geometrical parameters as well as the type of swirl generation, typically applied in industrial devices. Experimental data has been acquired for the isothermal flow as well as swirl flames by means of 3-D-LDV-diagnostics comprising the components of long-time averaged mean and rms-velocities as well as spectrally resolved velocity fluctuations for all components. The noise emission data was acquired with microphone probes resulting in sound pressure levels outside the zone of the perceptible fluid flow. Along to the experiments, numerical simulations using RANS and LES have been carried out for isothermal cases with different burner outlet geometries. The results of the measurements show a distinct rise of the sound pressure level, obtained by changing both the test setup from the isothermal into the flame configuration as well as the geometrical parameters. This is also resembled by the LES simulation results. Furthermore, a physical model has been developed from experiments and verified by the LES simulation, that explains the formation of coherent flow structures and allows to separate their contribution to the overall noise emission from ordinary turbulent noise sources.     

Application of photobleaching molecular tagging velocimetry to turbulent bubbly flow in a square duct

To evaluate turbulence energy budget in bubbly flows, an image processing method in a photobleaching molecular tagging velocimetry is improved for accurate evaluation of velocity gradients. Turbulence properties in single-phase and two-phase dilute-bubbly flows in a square duct are measured using the improved method. As a result, the following conclusions are obtained: (1) The axial velocity and axial turbulent intensity measured by the present method agree well with those measured by laser Doppler velocimetry not only for the single-phase flow but also for the dilute-bubbly flow. (2) The present method can measure velocity components and velocity gradients in the vicinity of the wall, and therefore the present method is of great use in understanding the mechanism of turbulence generation and dissipation near the wall. (3) The present method can provide detailed information on turbulence structure such as turbulence kinetic energy budget. (4) Bubbles tend to increase not only the turbulence production but also the turbulence dissipation.     

环形通道内湍流旋流流动的数值模拟

本文应用一种考虑湍流－旋流相互作用及湍流脉动各向异性的新的代数Ｒｅｙｎｏｌｄｓ应力模型,对环形通道内的湍流旋流流动进行了数值模拟,研究了改主为旋流流数,进口轴向速度及半径比等参数对环形通道内湍流流动的影响,以及对强化环形通道内传热的作用。