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.     

不同构型和多点喷射对短环型燃烧室燃烧性能的影响

对短环形流燃烧室内有较强回流的湍流旋流流动进行了模拟,并从两个方面(燃烧室构型和多点喷射)对燃烧室性能的影响进行了分析。计算中采用Reynoldes应力湍流模型(RSM)、EBU-Arrheniue湍流燃烧模型和离散坐标辐射模型描述其燃烧流动,液相采用Lagrange法处理,气相采用SIMPLE法求解。研究表明:在燃料和空气总流量不变的情况下,燃烧室构型对燃烧室出口平均温度影响不大,对出口温度分布、燃烧室内空气流场有比较大的影响。喷嘴数目的改变对出口处的平均温度和平均速度影响不大,但是对出口截面处的温度分布影响比较大,在局部范围可能产生温度比较高的热斑。     

Large-Eddy Simulation of Lean Premixed Turbulent Flames of Three Different Combustion Configurations using a Novel Reaction Closure

This large eddy simulation (LES) study is applied to three different premixed turbulent flames under lean conditions at atmospheric pressure. The hierarchy of complexity of these flames in ascending order are a simple Bunsen-like burner, a sudden-expansion dump combustor, and a typical swirl-stabilized gas turbine burner–combustor. The purpose of this paper is to examine numerically whether the chosen combination of the Smagorinsky turbulence model for sgs fluxes and a novel turbulent premixed reaction closure is applicable over all the three combustion configurations with varied degree of flow and turbulence. A quality assessment method for the LES calculations is applied. The cold flow data obtained with the Smagorinsky closure on the dump combustor are in close proximity with the experiments. It moderately predicts the vortex breakdown and bubble shape, which control the flame position on the double-cone burner. Here, the jet break-up at the root of the burner is premature and differs with the experiments by as much as half the burner exit diameter, attributing the discrepancy to poor grid resolution. With the first two combustion configurations, the applied subgrid reaction model is in good correspondence with the experiments. For the third case, a complex swirl-stabilized burner–combustor configuration, although the flow field inside the burner is only modestly numerically explored, the level of flame stabilization at the junction of the burner–combustor has been rather well captured. Furthermore, the critical flame drift from the combustor into the burner was possible to capture in the LES context (which was not possible with the RANS plus k–ɛ model), however, requiring tuning of a prefactor in the reaction closure.     

A computational study of supersonic combustion behind a wedge-shaped flameholder

In this study, large eddy simulation (LES) has been used to examine supersonic flow, mixing, self-ignition and combustion in a model scramjet combustor and has been compared against the experimental data. The LES model is based on an unstructured finite-volume discretization, using monotonicity-preserving flux reconstruction of the filtered mass, momentum, species and energy equations. Both a two-step and a seven-step hydrogen–air mechanism are used to describe the chemical reactions. Additional comparisons are made with results from a previously presented flamelet model. The subgrid flow terms are modeled using a mixed model, whereas the subgrid turbulence–chemistry interaction terms are modeled using the partially stirred reactor model. Simulations are carried out on a scramjet model experimentally studied at Deutsches Zentrum für Luft- und Raumfahrt consisting of a one-sided divergent channel with a wedge-shaped flame holder at the base of which hydrogen is injected. The LES predictions are compared with experimental data for velocity, temperature, wall pressure at different cross sections as well as schlieren images, showing good agreement for both first- and second-order statistics. In addition, the LES results are used to illustrate and explain the intrinsic flow, and mixing and combustion features of this combustor.     

Effects of inlet guide-vane number on flowfields in a side-dump combustor

The flowfields of a side-dump combustor with various number of side-inlet guide-vane are measured using laser-Doppler velocimetry (LDV). The Reynolds number based on the bulk mean velocity and combustor diameter was 2.6×10⁴. Quantities such as mean velocity, turbulence intensity, turbulent kinetic energy, vorticity, friction factor, and wall static pressure oscillation are used to characterize the fluid flow. In the dome region of the inlet-jet plane, there is one pair of counter-rotating vortices for the no-vane, one-vane, and two-vane cases and two pairs of counter-rotating vortices for the three-vane case, respectively. This trend is reversed in the impinging plane. The combustor flowfield downstream of the X_c^*=2.5 station is found to be insensitive to the variation of inlet guide-vane number. In addition, the guide-vane number which provides the least pressure loss and the lowest pressure oscillation is identified for the first time. Based on the presented data, a better guide-vane number for practical reference is suggested.     

Hot flow analysis of swirling sudden-expansion dump combustor

Hot flow of a sudden-expansion dump combustor with swirling is analysed by employing an infinite chemical reaction rate. Turbulence properties are closed using one type of algebraic Reynolds stress model and two types of κ–? model. One of the κ–? models includes a swirling effect modification to the ε-equation. Computations have been performed by the SIMPLE-C algorithm with a power-law scheme. The calculated results of the momentum fields and turbulence quantities for swirling flow are compared with the available experimental data. It is shown that the standard κ–? model gives poor prediction of the mean velocity, particularly the tangential velocity. For the hot flow analysis of a sudden-expansion dump combustor with swirling flow it is suggested that it is necessary to use the modified κ–? model or algebraic Reynolds stress model.     

Diffusion flame analysis of an afterburner as a function of the air-fuel ratio

The diffusion flame of an afterburner as a function of the air-fuel ratio is analysed by employing the SIMPLE-C algorithm and the turbulence κ-κ model. In the present analysis, better combustion efficiency of an afterburner with a slightly fuel-lean mixture is shown. The velocity, fuel mass fraction, temperature and combustion efficiency distributions of reacting flow in an afterburner with two V-gutter flameholders as a function of the air–fuel ratio are also discussed and compared. The calculated results in the present analysis can be applied to the fundamental study of reacting flow in an afterburner.     

乙烯燃料超燃燃烧室流动特性与燃烧稳定性研究

在低飞行马赫数条件下,乙烯燃料超燃冲压发动机为实现成功点火及稳定燃烧,常使用先锋氢引燃乙烯,本文通过试验研究了多种喷注方案下的超燃燃烧室流动特性、火焰传播特性及燃烧稳定性,喷注方案包括单先锋氢、单乙烯和组合喷注方式.超燃燃烧室入口马赫数为2.0,总温为953 K,总压为0.82 MPa.多种非接触光学测量手段被应用于超...     

Experimental investigation of combustion mechanisms of kerosene-fueled scramjet engines with double-cavity flameholders

A scramjet combustor with double cavitybased flameholders was experimentally studied in a directconnected test bed with the inflow conditions of M = 2.64,Pt = 1.84 MPa,Tt = 1 300 K.Successful ignition and selfsustained combustion with room temperature kerosene was achieved using pilot hydrogen,and kerosene was vertically injected into the combustor through 4×φ 0.5 mm holes mounted on the wall.For different equivalence ratios and different injection schemes with both tandem cavities and parallel cavities,flow fields were obtained and compared using a high speed camera and a Schlieren system.Results revealed that the combustor inside the flow field was greatly influenced by the cavity installation scheme,cavities in tandem easily to form a single side flame distribution,and cavities in parallel are more likely to form a joint flame,forming a choked combustion mode.The supersonic combustion flame was a kind of diffusion flame and there were two kinds of combustion modes.In the unchoked combustion mode,both subsonic and supersonic combustion regions existed.While in the choked mode,the combustion region was fully subsonic with strong shock propagating upstream.Results also showed that there was a balance point between the boundary separation and shock enhanced combustion,depending on the intensity of heat release.     

飞行Ma12条件超燃发动机流场及燃烧特征分析

为提升针对高马赫数发动机的模拟能力,对计算方法进行了可压缩性修正,并针对飞行Ma12条件下超燃冲压发动机进行了多状态三维数值模拟,分析了发动机内波系、参数以及燃烧性能特征.研究结果表明:(1)修正后的方法计算所得激波位置及强度与试验值吻合,在激波串模拟、高马赫数发动机模拟上均展现了更优的能力.(2)发动机内形成激波与反...     

LES of a Multi-burner Annular Gas Turbine Combustor

In this study, Large Eddy Simulation (LES) has been used to predict the flow, mixing and combustion in both a single burner laboratory gas turbine combustor and in an 18 burner annular combustor, having identical cross sections. The LES results for the single burner laboratory combustor are compared with experimental data for a laboratory model of this combustor, and with other LES predictions, with good agreement. An explicit finite volume based LES model, using the mixed subgrid model together with a partially stirred reactor model for the turbulence chemistry interactions, is used. For the annular combustor, with the swirlers parameterized by jet inflow boundary conditions, we have investigated the influence of the a-priori unknown combustor exit impedance, the influence of the swirler characteristics and the fuel type. The combustion chemistry of methane–air and n-decane–air combustion is modeled by a two-step reaction mechanism, whereas NOx is separately modeled with a one-step mechanism. No experimental data exists for the annular combustor, but these results are compared with the single burner LES and experimental results available. The combustor exit impedance, the swirler- and fuel characteristics all seem to influence the combusting flow through the acoustics of the annular combustor. To examine this in greater detail time-series and eigenmodes of the combustor flow fields are analyzed and comparisons are made also with results from conventional thermoacoustic eigenmode analysis, with reasonable agreement. The flow and pressure distributions in the annular combustor are described in some detail and the mechanisms by which the burners interact are outlined.     

A 5-D Implementation of FGM for the Large Eddy Simulation of a Stratified Swirled Flame with Heat Loss in a Gas Turbine Combustor

Numerical simulations are foreseen to provide a tremendous increase in gas-turbine burners efficiency in the near future. Modern developments in numerical schemes, turbulence models and the consistent increase of computing power allow Large Eddy Simulation (LES) to be applied to real cold flow industrial applications. However, the detailed simulation of the gas-turbine combustion process remains still prohibited because of its enormous computational cost. Several numerical models have been developed in order to reduce the costs of flame simulations for engineering applications. In this paper, the Flamelet-Generated Manifold (FGM) chemistry reduction technique is implemented and progressively extended for the inclusion of all the combustion features that are typically observed in stationary gas-turbine combustion. These consist of stratification effects, heat loss and turbulence. Three control variables are included for the chemistry representation: the reaction evolution is described by the reaction progress variable, the heat loss is described by the enthalpy and the stratification effect is expressed by the mixture fraction. The interaction between chemistry and turbulence is considered through a presumed beta-shaped probability density function (PDF) approach, which is considered for progress variable and mixture fraction, finally attaining a 5-D manifold. The application of FGM in combination with heat loss, fuel stratification and turbulence has never been studied in literature. To this aim, a highly turbulent and swirling flame in a gas turbine combustor is computed by means of the present 5-D FGM implementation coupled to an LES turbulence model, and the results are compared with experimental data. In general, the model gives a rather good agreement with experimental data. It is shown that the inclusion of heat loss strongly enhances the temperature predictions in the whole burner and leads to greatly improved NO predictions. The use of FGM as a combustion model shows that combustion features at gas turbine conditions can be satisfactorily reproduced with a reasonable computational effort. The implemented combustion model retains most of the physical accuracy of a detailed simulation while drastically reducing its computational time, paving the way for new developments of alternative fuel usage in a cleaner and more efficient combustion.     

微小尺度下氢气预混燃烧的实验研究

对内径为1.66mm的不锈钢管燃烧室的氢气预混燃烧实验进行了描述，采用 红外测温仪测量了燃烧室壁面的温度场分布，获得了不同燃烧热功率下的运行界限．在突扩 段内高温回流区的作用下，在带有5mm长突扩段的燃烧室内可以实现完全预混燃 烧，最高运行界限可达1.415．由于较高的进气速度和较大的燃烧室壁面散热，在不带突扩 段的不锈钢管内无法实现完全预混燃烧．结果表明突扩段对微小尺度燃烧具有稳定火焰、拓 宽燃烧运行界限的作用．通过对火焰形状和结构的观察，结合突扩段燃烧流场的分析，合理 解释了燃烧室壁面温度场随过量空气系数的变化规律．     

双模态发动机的模态鉴别方法

双模态冲压发动机的不同燃烧模态具有不同的稳焰机制和流态特征,并且在模态转换时伴随着显著的推力变化. 因此,准确判断燃烧模态,对于捕捉发动机的燃烧区位置/范围、释热分布特征,以及为进一步优化燃烧室的设计(流道结构和供油布局) 具有重要意义. 目前尚无鉴别模态的有效试验方法,本文提出了一种模态鉴别的试验方法,并在超燃直连台上开展验证试验. 试验中使用的测量技术包括:壁面静压、高速阴影/纹影、多通道可调谐二极管吸收光谱和高能态碳氢自由基CH* 自发光成像. 利用多种测量方法的组合,可以同时获得燃烧室中气流静温、速度、马赫数分布,释热分布以及燃烧区位置/范围. 这些试验数据能够用于判别模态,并获得不同模态的流动和火焰特征.      

LES of Chemical and Acoustic Forcing of a Premixed Dump Combustor

This paper describes the first steps in the development of a large eddy simulation (LES) code able to compute combustion instabilities in gas turbines. This code was used to compute the forcing of an experimentally investigated premixed dump combustor. It is shown that the main effect of acoustic waves entering the combustion chamber is to create large vortices and unsteady heat release when these vortices burn. Another effect of waves entering the combustor is to modulate the fuel and air flow rates produced by the feeding lines. In this case the equivalence ratio of the mixture entering the combustor may also vary. This was investigated in a “chemical effect” simulation where the inlet equivalence ratio fluctuates but the total flow rate remains constant. For perturbations from stoichiometric burning, this mechanism was shown to induce less destabilizing effects than the purely aerodynamical mechanism due to vortex formation and combustion. It is shown that the LES methodology developed is able to reproduce the experimentally observed phase shift between acoustic excitation and total reaction rate in the chamber. This revised version was published online in July 2006 with corrections to the Cover Date.     

双燃式超燃发动机冷态内流场的数值研究

研究了双燃式一体化通道(包含进气道、双燃式燃烧室和尾喷管)的冷态内流场特性.首次在激波风洞中对内流场进行纹影照相,用TVD格式求解三维全N-S方程对喷管和一体化通道进行分区数值模拟,并考察了几何参数对内流场的影响.结果表明对典型工况(h     

Flow field characteristics of an axisymmetric sudden-expansion pipe flow with different initial swirl distribution

The results of an experimental investigation depicting the effects of swirl profile on confined flows in a sudden-expansion coaxial dump combustor are presented. Three swirlers (free vortex, forced vortex, and constant angle) with the same nominal swirl number were designed and fabricated to study the effects of swirl type on the isothermal dump combustor flow field. Imparting swirl to the inlet flow resulted in a considerable reduction of the corner recirculation length, a marked increase in turbulent mixing activity, and in one case creation of a central recirculation zone. This article highlights the importance of the combustor inlet swirl profile and shows that swirl type as well as swirl strength can affect the flow field significantly. The present database is well suited for numerical codes development and validation.     

当量比对汽油燃料两相旋转爆轰发动机工作特性影响实验研究

为了研究当量比对汽油燃料两相旋转爆轰发动机工作特性的影响,开展了以高总温空气为氧化剂的气液两相旋转爆轰实验研究。旋转爆轰发动机环形燃烧室外径、内径和长度分别为202、166和155 mm。汽油和高温空气采用高压雾化喷嘴与环缝对撞喷注的方式混合,以此提高推进剂的掺混效果与活性,采用预爆轰管作为点火装置。实验通过改变汽油质量流量改变推进剂当量比,并基于燃烧室内测得的高频动态压力和平均静压,对气液两相旋转爆轰波的传播模态和传播特性以及发动机的工作特性进行了详细分析。实验结果表明:在当量比为0.79～1.25时,燃烧室内均实现了旋转爆轰波的连续自持传播,且随着当量比的增加,爆轰波传播模态从双波对撞/单波的混合模态转变为单波模态;降低当量比至0.61～0.66,爆轰波传播稳定性变差,传播模态表现为间断爆轰以及零星爆轰;进一步降低当量比至0.53,爆轰波起爆失败。此外,燃烧室平均绝对压力与爆轰波平均传播频率均随着当量比的增加呈先增大后减小的趋势,极大值出现在当量比1.19附近。在此工况下获得了最佳实验结果,旋转爆轰波的平均传播频率为1 900.9 Hz,平均传播速度为1 110.8 m/s,与高频压力信号经快速傅里叶变换得到的主频基本一致,爆轰波传播速度存在严重亏损。     

Analytical series solution for the fully developed forced convection duct flow with frictional heating and variable viscosity

Laminar forced convection flow of a liquid in the fully developed region of a circular duct with isothermal wall is analyzed. The effects of viscous dissipation as well as of temperature dependent viscosity are taken into account. The coupled momentum and energy equations are solved analytically by means of a power series method. Then, reference is made to the Poiseuille model for the temperature change of viscosity. For a fixed value of the axial pressure gradient along the duct, dual solutions are found for the velocity and temperature fields. Although dual solutions correspond to the same value of the axial pressure gradient, they lead in general to different values of the average fluid velocity, of the average fluid temperature and of the wall heat flux. It is shown that, for a given fluid and for a fixed duct radius, the absolute value of the axial pressure gradient has an upper bound above which no steady laminar solution can exist.     

旋转爆震燃烧室轴向和周向长度对其出口流场压力和温度的影响

为了研究旋转爆震燃烧室与涡轮的匹配特性,利用二维欧拉方程数值研究了基于当量H₂/Air燃烧的旋转爆震燃烧室出口流场特性,对比分析了不同燃烧室轴向长度和周向长度出口总压脉动、总压畸变以及出口温度分布规律。结果表明:旋转爆震燃烧室在稳定工作状态下,其出口总压的脉动值会呈现周期性振荡;燃烧室尺度对发动机出口流场的不均匀性有很大影响,随着燃烧室轴向长度的增大或周向尺寸的减小,其出口总压脉动均值、畸变指数和出口温度分布系数均会减小,其出口流场均匀性提高。此外,爆震波高度随着周向尺寸的增大而增大;轴向尺寸对爆震波高度几乎不产生影响。