直喷式柴油机主燃期量化研究

在采集了大量数据的基础上,用本文提出的基于K-L变换的GMDH改进方法建立了柴油机主燃期平均燃烧速率的数学模型.研究结果表明,所建模型具有较好的模型精度和计算稳定性.此种建模方法对柴油机燃烧过程的建模有一定意义.     

准均质充气压缩点燃（QHCCI）燃烧系统的模拟研究

建立了一个准维数学模型。将扩散燃烧和火焰传播燃烧同时分别加以计算,并有机结合,对准均质充气压缩点燃（QHCCI）燃烧系统进行了数学模拟。利用模型和试验结果;对QHCCI燃烧系统的工作机理进行了研究。研究结果表明,少量汽油被引燃柴油卷吸进入喷雾区,增加了扩散燃烧的燃油质量;当预混合气开始燃烧后,扩散燃烧并没有结束,一般地,QHCCI燃烧系统的燃烧放热率曲线具有特殊的“双峰”现象。     

Application of PDF mixing models to premixed flames with differential diffusion

Differential diffusion alters the balance of reaction and diffusion in turbulent premixed combustion, affecting the performance and emissions of combustion devices. Modelling combustion devices with Probability or Filtered Density Function (PDF or FDF) methods provides an exact treatment for the change in composition due to chemical reaction, while molecular mixing has to be modelled. Previous PDF molecular mixing models do not account for differential diffusion in a manner which satisfies realizability requirements. A new approach for treating differential diffusion, which ensures realizability, is proposed for pairwise-exchange mixing models in general, and applied in the Interaction by Exchange with the Mean (IEM) model of Dopazo [26], and in the Euclidean Minimum Spanning Tree (EMST) model of Subramaniam and Pope [5]. The new differential diffusion models are referred to as IEM-DD and EMST-DD respectively.Results from two and three-dimensional DNS of turbulent premixed methane–air combustion show that mixing rates and conditional statistics of species mass fractions depend on species diffusivities and the combustion regime. Zero-dimensional PDF model results obtained for the two-dimensional DNS case show that the EMST-DD model best reproduces the features that characterize differential diffusion in the DNS. The essential feature of the EMST-DD model, which accounts for its success in turbulent premixed combustion, is that differential mixing rates are imposed within a model which mixes locally in composition space.     

固体燃料燃烧的分形模型研究

在固体燃料燃烧的分形模型中,把固体燃料（煤）的燃烧用一个整体的分形动力－扩散方程来描述。在燃烧过程中,内部孔洞体积与反应表面积存在分维指数关系,而且反应面积的增长是两种分形增长模式的叠加。同时考虑孔洞合并的因素,得到了描述煤燃烧反应速率的分形模型。燃烧反应速率先增加后减少的规律不仅在理论上得到解释,而且与实验结果十分相符。     

An improved flamelet/progress variable modeling for supersonic combustion

In order to dynamically describe the transition of the scalar dissipation rate of the mixture fraction variance from near-wall regions to mainstream in near-wall combustion, the traditional flamelet/progress variable (FPV) model, which can be used either within the Reynolds Averaged Navier Stokes (RANS) framework or the large-eddy simulation (LES) framework, is modified to form an improved FPV model within the framework of the improved delayed detached eddy simulation (IDDES). The model blends respective expressions of the scalar dissipation rate within the RANS and LES frameworks into a unified form with an employment of the IDDES concept. It also adopts an analogy between the mixture fraction variance and turbulent kinetic energy. A hydrogen-fueled near-wall combustion is simulated to validate this model. Large-scale turbulent structures, which can either locally enhance combustion or cause local extinction in the reacting zone, are reproduced. Due to the blockage to lateral extents of spanwise turbulent structures, a three-dimensional effect, that the low temperature fluid at lower corners of flow passage is forced to move away from sidewalls, is also reproduced. The results show that the improved FPV model can avoid the scalar over-mixing effect of the traditional FPV model by delaying and mitigating the development of the mixture fraction variance at the near-wall regions. Therefore, the starting point of the high temperature zone predicted by the improved FPV model is downstream of that predicted by the traditional FPV model. It is indicated that the improved FPV model tends to delay combustion. This tendency is also demonstrated by the less combustion efficiency in the upstream region predicted by the improved FPV model. As a result, it can generate a moderate growth of the mixing and reacting zone in the longitudinal direction, which therefore improves the predictions of the species concentrations. In contrast, the traditional FPV model thickens the mixing and reacting zone, which is negative to the combustion efficiency in the downstream region. It is implied that a reasonable development of the mixture fraction variance at near-wall regions is essentially required for near-wall combustion simulation using the FPV model within the IDDES framework.     

A regenerative multiple zone model for HCCI combustion

A new conserved scalar approach, the so-called regenerative multiple zone (RMZ) model, is introduced to simulate combustion in homogeneous charge compression ignition (HCCI) engines with significant products of combustion. In this approach, two conserved scalars are introduced, the mixture fraction Z and the initial exhaust gas fraction J, to determine uniquely the state of the reactive system as a function of the two conserved scalars and time. For the numerical solution of the HCCI combustion, the conserved scalar plane is divided into different zones, which represent homogeneous reactors with constant initial exhaust gas level. Particularly, the zones are created based on the distribution of the initial exhaust gases and are mixed and regenerated at every time step during combustion in order to account for the history effects which are due to the finite rate chemistry. A proper methodology to create and initialize the new zones during the combustion, the so-called zone creation strategy (ZCS), is also proposed. For validation, the RMZ model is implemented in the 2DRD code, which is a computational fluid dynamics code that solves the governing equations for a two-dimensional reaction-diffusion problem. Initially, the consistency of the new model is validated in a one-dimensional reaction-diffusion (RD) case. Subsequently, the necessity for a proper zone creation strategy is demonstrated by a two-dimensional RD case. Next, a parametric study is performed to investigate the sensitivity of the new model on the maximum number of zones that is used. Finally, the limitations as well as the advantages of the RMZ model are discussed.     

Modeling of lean premixed combustion in stationary gas turbines

Lean premixed combustion (LPC) of natural gas is of considerable interest in land-based gas turbines for power generation. However, modeling such combustors and adequately addressing the concerns of LPC, which include emissions of nitrogen oxides, carbon monoxide and unburned hydrocarbons, remains a significant challenge. In this paper, characteristics of published simulations of gas turbine combustion are summarized and methods of modeling turbulent combustion are reviewed. The velocity–composition PDF method is selected for implementation in a new comprehensive model that uses an unstructured-grid flow solver. Reduced mechanisms for methane combustion are evaluated in a partially stirred reactor model. Comprehensive model predictions of swirl-stabilized LPC of natural gas are compared with detailed measurements obtained in a laboratory-scale combustor. The model is also applied to industrial combustor geometries.     

甲烷预混多喷嘴阵列燃烧器热声振荡模态实验研究

为研究燃气轮机模型燃烧室的非预混燃烧流场,采用大涡模拟方法分别结合火焰面生成流形模型（FGM）和部分预混稳态火焰面模型（PSFM）对甲烷/空气同轴射流非预混燃烧室开展了数值模拟研究,并与试验结果进行对比。结果表明：FGM所预测的速度分布、混合分数分布、燃烧产物及CO分布与试验结果更符合;两种模型均能捕捉到燃烧室中的火焰抬举现象;燃烧过程中的火焰结构较为复杂,同时存在预混燃烧区域和扩散燃烧区域,扩散燃烧主要分布在化学恰当比等值线附近,预混燃烧区域主要分布在贫油区。     

柴油喷雾着火过程的化学动力学模拟

在直喷式柴油机准维燃烧模型的基础上,建立了能描述柴油机着火前燃油的氧化历程及着火滞燃期的多步着火化学动力学子模型,它不仅可以很好地模拟实际柴油机的燃油氧化过程,而且可以较好地反映着火滞燃期随各种因素的变化关系,比以往的描述着火滞燃期的经验模型更具有合理性和先进性。整个燃烧模型的计算结果及其与试验结果的对比还表明,本模型能够合理地预测柴油机的燃烧过程。     

基于振动信号及瞬时转速信号的HCCI燃烧模式辨识

HCCI汽油发动机一般采用组合燃烧控制策略,根据发动机工况不同,HCCI汽油机分别采用SI燃烧模式、SI-HCCI燃烧模式和HCCI燃烧模式.在这种控制方式下,燃烧模式的辨识具有非常重要的作用与意义.笔者在装有全可变气门系统的汽油HCCI发动机上,测取HCCI发动机各工况下爆震传感器信号和瞬时转速信号,用时频分析方法从爆震传感器信号和瞬时转速信号中提取了特征量,分析了它们和HCCI汽油机燃烧模式之间的关系.通过辨识函数分析,基于爆震传感器信号特征量和瞬时转速信号特征量,建立了的HCCI燃烧模式辨识模型.分析表明,HCCI燃烧模式辨识模型能够较好地辨识出HCCI的燃烧模式,总体辨识成功率在75%左右.     

当量比对涡轮叶间燃烧性能影响的数值模拟

为探究涡轮叶间燃烧性能,设计了4种不同当量比的工况,利用 FLUENT 软件的 Realizable k-ε湍流模型、PDF 燃烧模型、DO 辐射模型和离散相模型对燃烧室的流动及燃烧进行数值模拟.结果表明:燃烧室能在广泛的当量比(2.59~0.81)下保持性能稳定,燃烧效率保持在96%以上、总压损失低于2.4%,气体温度提高650,K 左右;降低当量比,能够提高燃烧效率,降低 CO、UHC、NOx 等污染物排放,改善温度分布,但会造成更大的总压损失;最优当量比等于1.00,此时燃烧效率在99.95%以上,总压损失相对低(1.5%),出口径向温度呈抛物线型分布,最适合燃烧室设计.与文献对比发现,选取的工况合理,其结果对涡轮叶间燃烧室设计具有参考价值     

Numerical analysis of combustion and transient heat transfer processes in a two-stroke SI engine

A zero-dimensional model is presented to simulate the transient processes occurring within a two-stroke SI engine. A two zone combustion model, with a spherically expanding flame front originating from the spark location, is applied. The model is numerically solved using the network simulation model which allows coupling the combustion model with a heat transfer model where both radiant and convective heat contributions have been taken into account for the in-cylinder gases. The boundary conditions for this model are the convective heat transferred to the cooling medium. A gas mixture model has been used to obtain the influence of working fluid properties on combustion development.     

High pressure direct injection of gaseous fuels using a discrete phase methodology for engine simulations

Direct gaseous fuel injection in internal combustion engines is a potential strategy for improving in-cylinder combustion processes, performance and emissions outputs and, in the case of hydrogen, could facilitate a transition away from fossil fuel usage. Computational fluid dynamic studies are required to fully understand and optimise the combustion process, however, the fine grids required to adequately model the underexpanded gas jets which tend to result from direct injection make this a difficult and cumbersome task. In this paper the gaseous sphere injection (GSI) model, which utilises the Lagrangian discrete phase model to represent the injected gas jet, is further improved to account for the variation in the jet core length with better estimation due to total pressure ratio change. The improved GSI model is then validated against experimental hydrogen and methane underexpanded freestream jet studies, mixing in a direct injection hydrogen spark ignition engine and combustion in a pilot ignited direct injection methane compression ignition engine. The improved GSI model performs reasonably well across all cases examined which cover various pressure ratios, injector diameters, injection conditions and disparate gases (hydrogen and methane) while also allowing for relatively coarse meshes (cheaper computational cost) to be used when compared to those needed for fully resolved modelling of the gaseous injection process. The improved GSI model should allow for efficient and accurate investigation of direct injection gaseous fuelled engines.     

新型循环双流化床锅炉燃烧系统

建立了新型循环双流化床锅炉燃烧系统的通用动态数学模型,考虑了双流化床的流体动力、燃烧和传热特性,反映了双流化床内部的气固两相流动、宽筛分颗粒燃烧、气体化学反应、床内传热、系统压力平衡等物理化学过程.在仿真计算时,对传统计算方法作了改进,能满足实时仿真的要求.图4参5     

Study of the cycle-to-cycle variations of an internal combustion engine fuelled with natural gas/hydrogen blends from the diagnosis of combustion pressure

A methodology is presented for studying the influence of using alternative fuels on the cycle-to-cycle variations of a spark ignition engine which has been fuelled with mixtures of natural gas and hydrogen in different proportions (0–100%). The experimental facility consists of a single-cylindrical spark ignition engine coupled to an asynchronous machine with a constant engine rotation speed of 1500 rpm. A thermodynamic combustion diagnostic model based on genetic algorithms is used to evaluate the combustion chamber pressure data experimentally obtained in the mentioned engine. The model is used to make the pressure diagnosis of series of 830 consecutive engine cycles automatically, with a high grade of objectivity of the combustion analysis, since the relevant adjustment parameters (i.e. pressure offset, effective compression ratio, top dead center angular position, heat transfer coefficients) are calculated by the genetic algorithm. Results indicate that the combustion process is dominated by the turbulence inside the combustion chamber (generated during intake and compression), showing little dependency of combustion variation on the mixture composition. This becomes more evident when relevant combustion variables are plotted versus the Mass Fraction Burned of each mixture. The only exception is the case of 100% hydrogen, due to the inherent higher laminar speed of hydrogen that causes combustion acceleration and thus turbulence generation.     

Evaluation of a combustion model for the simulation of hydrogen spark-ignition engines using a CFD code

The present work deals with the evaluation of a combustion model that has been developed, in order to simulate the power cycle of hydrogen spark-ignition engines. The motivation for the development of such a model is to obtain a simple combustion model with few calibration constants, applicable to a wide range of engine configurations, incorporated in an in-house CFD code using the RNG k–? turbulence model. The calculated cylinder pressure traces, gross heat release rate diagrams and exhaust nitric oxide (NO) emissions are compared with the corresponding measured ones at various engine loads. The engine used is a Cooperative Fuel Research (CFR) engine fueled with hydrogen, operating at a constant engine speed of 600 rpm. This model is composed of various sub-models used for the simulation of combustion of conventional fuels in SI engines; it has been adjusted in the current study specifically for hydrogen combustion. The basic sub-model incorporated for the calculation of the reaction rates is the characteristic conversion time-scale method, meaning that a time-scale is used depending on the laminar conversion time and the turbulent mixing time, which dictates to what extent the combustible gas has reached its chemical equilibrium during a predefined time step. Also, the laminar and turbulent combustion velocity is used to track the flame development within the combustion chamber, using two correlations for the laminar flame speed and the Zimont/Lipatnikov approach for the modeling of the turbulent flame speed, whereas the (NO) emissions are calculated according to the Zeldovich mechanism. From the evaluation conducted, it is revealed that by using the developed hydrogen combustion model and after adjustment of the unique model calibration constant, there is an adequate agreement with measured data (regarding performance and emissions) for the investigated conditions. However, there are a few more issues to be resolved dealing mainly with the ignition process and the applicability of a reliable set of constants for the emission calculations.     

利用RIF模型对内燃机燃烧过程的模拟

代表性互动小火焰（RIF）模型在计算准确性和经济性上的优势使它非常适用于内燃机燃烧过程的模拟．论述了RIF模型的基本原理,并在KIVA-3V2平台上补充了层流小火焰求解程序和β函数形式的概率密度函数积分程序,开发出KIVA-RIF软件,从而实现了RIF模型的求解．应用正庚烷简化机理对一单缸Caterpillar3400...     

超声速湍流燃烧G/Z方程模型验证

为了研究超声速条件下的部分预混燃烧,引入一种基于Level set重构方法和稳态火焰面数据库的 G／Z方程模型,并利用德国宇航中心的DLR支板算例对 G／Z方程模型进行了验证。结果显示,超声速湍流燃烧G／Z方程模型可以捕捉到部分预混燃烧现象,数值模拟结果与实验结果吻合较好,验证了 G／Z方程模型运用到超声速部分预混条件下湍流燃烧流场计算的可行性。同时,超声速湍流燃烧 G／Z方程模型依赖于运用到的火焰传播速度模型与火焰面模型,模型精确度的提高有待进一步探究。     

An extended multi-regime flamelet model for IC engines

Modeling internal combustion engines is challenging due to the various coupled multi-physics phenomena. With the advent of modern supercomputing and advanced modeling techniques, studying and designing these engines through detailed simulations is becoming tractable. Since the combustion process is the primary controlling feature in these engines, a high fidelity combustion model is essential. This model must be efficient and valid across different combustion regimes, since modern engines might operate in hybrid modes. The Representative Interactive Flamelet (RIF) combustion model is a possible choice. This model has been developed to describe ignition, combustion, and pollutant formation in direct-injected diesel engines. However, it has recently been shown that the model has the correct asymptotic behavior for both diesel and Homogeneous Charge Compression Ignition (HCCI) regimes, and the model has been applied successfully for HCCI type combustion. In this study, the model is validated against two-dimensional direct numerical simulation data with multi-step finite rate chemistry to evaluate model performance in the diesel, the HCCI, and hybrid regimes. A wide range of temperature and mixture fraction stratification cases are simulated to evaluate the model performance across different modes. The model performs well for all cases considered, even when high levels of concurrent thermal and charge stratification are present.     

A comprehensive biomass combustion model

A combustion model for wheat straw is discussed and compared to that of a bituminous coal, Pittsburgh No. 8. The input data into the combustion model for both cases were generated using the FG-DVC pyrolysis model, which had been validated in part experimentally. The combustion behaviour of the two fuels are investigated using a laminar flow CFD model of a drop-tube furnace. The results indicate that, because of the low calorific nature of the straw volatiles, the combustion takes place at a lower temperature, but with rapid ignition and rapid devolatilisation. The straw char is highly microporous with relatively high ash and oxygen contents; consequently, the burnout is quicker than the analogous coal char burnout.