The self-oscillation determination to a category of nonlinear closed-loop systems

A geapho-analytical method is presented for the determination of self-oscillations in nonlinear closed-loop systems with two nonlinearities without memory. The method relies on the graphical solution of the system equations, derived by harmonic linearization of the nonlinear functions that describe the two nonlinearities.     

Limit cycle oscillations in CW laser-driven NEMS

Limit cycle, or self-oscillations, can occur in a variety of NEMS devices illuminated within an interference field. As the device moves within the field, the quantity of light absorbed and hence the resulting thermal stresses changes, resulting in a feedback loop that can lead to limit cycle oscillations. Examples of devices that exhibit such behavior are discussed as are experimental results demonstrating the onset of limit cycle oscillations as continuous wave (CW) laser power is increased. A model describing the motion and heating of the devices is derived and analyzed. Conditions for the onset of limit cycle oscillations are computed as are conditions for these oscillations to be either hysteretic or nonhysteretic. An example simulation of a particular device is discussed and compared with experimental results.     

Modeling of self-oscillations and a search for new self-oscillatory flows

A possible mechanism of self-oscillations in flows with shock waves and contact discontinuities is studied. Flows are investigated in the heliosphere, near a blunt cone in an inhomogeneous stream, and in the vicinity of a blunt cylinder with an outflowing supersonic jet, which is supposed to be, according to this mechanism, of a self-oscillatory nature. Two-dimensional Reynolds equations with algebraic turbulent viscosity are solved by the implicit third order Runge-Kutta scheme. The results of numerical studies are presented.     

Increasing the reactant conversion through induced oscillations in a continuous stirred tank reactor by using PI control

We report a strategy to increase the reactant conversion in a continuous stirred tank reactor (CSTR) to produce propylene glycol through induced oscillations generated by two controllers PI₁ and PI₂ that manipulate the reactor outlet flow and the coolant flow rate respectively. It is shown that an adequate parameter choice for the PI controllers allows one to derive sustained oscillations in the concentrations and reactor temperature, which in turn allows increasing the propylene glycol production. For a suitable choice of the PI₁ and PI₂ controller parameters, we use a complete reactor model that provides with physically feasible parameters. The issues of external disturbance rejection, self-oscillations and stability have also been discussed. The analytical calculations are verified by means of full numerical simulations.     

强化悬浮式分解炉内煤粉燃烧过程模拟的研究

分解炉是新型干法水泥生产线上预分解技术的核心设备和关键技术装备之一,其性能优劣直接影响到系统能耗、环保排放以及整个系统的生产效率。将数值模拟方法应用于分解炉的复杂燃烧过程研究,以强化悬浮式分解炉为研究对象,结合气固二相流理论,对煤粉的燃烧模拟采用非预混燃烧模型,对挥发份的燃烧采用概率密度函数模型,焦碳的燃烧采用表面反应PDF模型,挥发份的析出模型选用两步竞争反应模型,燃烧的辐射传热采用P-1模型进行模拟。数值计算采用二阶迎风差分和SIMPLE算法,求解方程采用TDMA逐面迭代和低松弛因子。对分解炉内煤粉的燃烧过程进行的数值模拟揭示分解炉内煤粉的燃烧机理,为煤粉内优化燃烧提供理论依据和有利的信息。     

Pulse-Frequency Characteristics in Bifurcation Problems

Generation of self-oscillations under unstable stationary states of nonlinear control systems, which may contain delay of complicated nature (the Andronov–Hopf bifurcation), and approximate design of self-oscillating states and their asymptotes are studied with the help of pulse-frequency characteristics of the linear component of a system.     

Online adaptive least squares support vector machine and its application in utility boiler combustion optimization systems

Boiler combustion optimization is a key measure to improve the energy efficiency and reduce pollutants emissions of power units. However, time-variability of boiler combustion systems and lack of adaptive regression models pose great challenges for the application of the boiler combustion optimization technique. A recent approach to address these issues is to use the least squares support vector machine (LS-SVM), a computationally attractive machine learning technique with rather legible training processes and topologic structures, to model boiler combustion systems. In this paper, we propose an adaptive algorithm for the LS-SVM model, namely adaptive least squares support vector machine (ALS-SVM), with the aim of developing an adaptive boiler combustion model. The fundamental mechanism of the proposed algorithm is firstly introduced, followed by a detailed discussion on key functional components of the algorithm, including online updating of model parameters. A case study using a time-varying nonlinear function is then provided for model validation purposes, where model results illustrate that adaptive LS-SVM models can fit variable characteristics accurately after being updated with the ALS-SVM method. Based on the introduction to the proposed algorithm and the case study, a discussion is then delivered on the potential of applying the proposed ALS-SVM method in a boiler combustion optimization system, and a real-life fossil fuel power plant is taken as an instance to demonstrate its feasibility. Results show that the proposed adaptive model with the ALS-SVM method is able to track the time-varying characteristics of a boiler combustion system.     

Turbulent premixed flame model based on a recent dispersion model

A model of turbulent premixed combustion is formulated based on a recent dispersion model and on earlier flame models. The dispersion model accounts for the effect of velocity correlation on turbulent dispersion without using parameters that are explicitly dependent on time or space. The earlier flame models are used as a basis for formulating an appropriate chemical source term. The resulting model is more general than the earlier models.The proposed model and one of the earlier models are validated against various experimental flames. It is shown that the proposed model is more accurate and requires less calibration. Furthermore, the proposed model is tested successfully against general criteria for premixed combustion modeling formulated in earlier works. It is therefore concluded that the dispersion model is a useful tool for premixed combustion modeling.     

Coal-fired utility boiler modelling for advanced economical low-NOx combustion controller design

This paper focuses on developing a control-oriented coal-fired utility boiler model for advanced economical Low-NO_x combustion (ELNC) controller design. Two boiler combustion models are proposed in this paper: one is a mathematical model describing the key dynamics of the real-time boiler thermal efficiency and the furnace one-dimensional NO_x concentration distribution under conventional fuel and overfire air operations; the other recast from the first model is a control-oriented grey-box model with a data-driven furnace combustion submodel. Simulation studies on static and dynamic properties of the first mathematical model indicate that the model can function as a real-time simulator for both advanced boiler combustion control laws testing and generating training and validation data for the control-oriented grey-box model. At the end of this paper, the control-oriented grey-box modelling procedure as well as an optional discrete time linear state-space model are summarised to facilitate model-based advanced combustion controllers design.     

燃烧动力学平台设计

燃料的相关研究在发动机研制中扮演着重要角色,数据是燃烧模型耦合的基础,反应机理是燃烧数值模拟的关键.针对我国在燃烧数据库建设方面的短板问题和个性化燃烧反应机理构建的迫切需求,本文设计了燃烧动力学平台,包括燃烧数据库和燃烧反应机理在线计算系统.燃烧数据库系统通过数据的标准化和分级管理建立了高效的搜索流程,形成了国内独有的...     

二进制流量控制算法的性能分析

自适应比特(available bit rate,简称ABR)流量控制是ATM网络中拥塞控制和流量管理的一种有效手段.在大规模的高速网络中,简洁、有效的实现算法对优化交换机的性能是至关重要的.二进制流量控制策略以其实现上的简洁性吸引了广泛的注意,但标准EFCI算法作用下队列和速率的振荡却给性能造成了负面影响,使人们对二进制流量控制机制产生了疑虑,不得不放弃二进制算法的简洁性,转而研究相对复杂但有效的显式速率反馈算法.在已建立模型的基础上,用非线性控制理论中描述函数的分析方法系统地评价和分析了标准EFCI算法的性能.得到的结论是:依赖于直觉的启发式非线性控制算法诱发的自激振荡是造成队列和速率振荡的本质原因,而非二进制流量控制机制本身固有的属性,仿真试验的结果证实了理论分析的结论.最后,提出了一种通过修改交换机参数配置策略来优化现已有EFCI交换机性能的方案.     

The torque ratio concept for combustion monitoring of internal combustion engines

This work presents a method to analyze combustion events in an internal combustion engine, called the torque ratio concept. The method is based on crankshaft torque measurements, but an extension to angular speed measurements is possible. The torque ratio concept provides a parametrized model for the combustion progress from which, e.g. combustion phasing can be extracted. The torque ratio concept is derived mathematically and related theoretically to other combustion analysis methods, such as pressure ratio and net heat release. Finally, analysis on recorded data from a five cylinder spark ignited engine verifies the relationships between the three methods. For combustion phasing, the 50% torque ratio is an equivalent measure to 50% pressure ratio and can be transformed into the 50% net heat release position by using a derived volume ratio function.     

Comparison between optically excited vibrations of silicon cantilever and bridge microresonators

A comparison, of the optically excited vibrations, including self-oscillations, of cantilever and bridge silicon microresonators is presented. The theoretical analysis and experimental investigations show that not only has the cantilever microresonator a greater vibration amplitude than the bridge structure, but also its resonance frequency is independent of the strain induced by boron doping and the deformation of the silicon wafer caused by assembly. These advantages could be of great value in achieving practical all-fibre optically addressed silicon microresonator sensors, which are used to measure physical parameters except for force or pressure, such as small masses, chemical components, vibration, temperature, etc.     

Nonlinear identification of a gasoline HCCI engine using neural networks coupled with principal component analysis

Homogeneous charge compression ignition (HCCI) is a futuristic combustion technology that operates with high efficiency and reduced emissions. HCCI combustion is characterized by complex nonlinear dynamics which necessitates the use of a predictive model in controller design. Developing a physics based model for HCCI involves significant development times and associated costs arising from developing simulation models and calibration. In this paper, a neural networks (NN) based methodology is reported where black box type models are developed to predict HCCI combustion behavior during transient operation. The NN based approach can be considered a low cost and quick alternative to the traditional physics based modeling. A multi-input single-output model was developed each for indicated net mean effective pressure, combustion phasing, maximum in-cylinder pressure rise rate and equivalent air–fuel ratio. The two popular architectures namely multi-layer perceptron (MLP) and radial basis network (RBN) models were compared with respect to design, prediction performance and overall applicability to the transient HCCI modeling problem. A principal component analysis (PCA) is done as a pre-processing step to reduce input dimension thereby reducing memory requirements of the models. Also, PCA reduces the cross-validation time required to identify optimal model hyper-parameters. On comparing the model predictions with the experimental data, it was shown that neural networks can be a powerful approach for non-linear identification of a complex combustion system like the HCCI engine.     

固体火箭冲压发动机三维数值计算

利用FLUENT中的概率密度函数非预混模型对固体火箭冲压发动机补燃室内的气相湍流燃烧进行数值模拟.主要目的是解决在固体火箭冲压发动机含镁铝推进剂的补燃室中,存在上百种中间及最终产物.复杂的反应机理使采用有限反应速率模型难以模拟补燃室中复杂的湍流燃烧的问题.模拟结果有助于提高对固体火箭冲压发动机补燃室内部流场流动的了解.模拟结果表明:补燃室内发生着复杂的三维化学反应流动,存在对掺混燃烧有重要影响的头部回流和轴向涡流.补燃室内温度分布与空气与燃气的掺混、燃烧及流动状态有密切关系.提高空燃比,可增强补燃室中燃气的回流和轴向涡流强度,加大掺混力度.提高燃烧效率.     

改进的Bilger型湍流燃烧反应速率模型及其数值模拟的验证

基于旋涡耗散原理,结合平均生成速率正比于化学计量比下的混合分数的概率密度的理论分析,发展Bilger型燃烧反应速率模型,使之能够模拟富氧和贫氧燃烧工况。模型中利用快速反应假设,辐射由多射线模型模拟,结果与Steckler的燃烧实验数据比较,并与文献中多个模型比较。温度、速度分布以及总体量的比较表明,当前模型比传统的EDM、Brizulea-Bilger模型在模拟贫氧燃烧方面好。对不同模型的应用提出了建议。     

基于SVM与遗传算法的燃煤锅炉燃烧多目标优化系统*

电站燃煤锅炉燃烧优化要求在保证燃烧效率的基础上降低NOx的排放,针对锅炉燃烧系统多变量、强耦合、强干扰、大滞后的复杂特性,提出利用支持向量机(SVM)对锅炉燃烧特性建模,利用遗传算法实现运行工况寻优,从而获得锅炉燃烧优化调整方式.仿真实验和实践结果表明,该系统实现了锅炉高效低氮的燃烧优化,满足实时性的要求.     

强化悬浮式RSP型分解炉石油焦燃烧特性实验及数值模拟研究

水泥生产中分解炉内主要化学物理过程是煤粉的燃烧.与煤粉相比,石油焦是一种经济的替代燃料.本文首先通过实验测试出石油焦燃烧的物理化学特性,根据这些特性建立了石油焦在强化悬浮式RSP型分解炉中燃烧的数值模拟模型.该模型结合气固二相流理论,采用快速反应的PDF模型,对分解炉内石油焦的燃烧过程进行了数值模拟,揭示了分解炉内石油焦的燃烧机理.研究结果表明RSP型分解炉的炉型适宜于石油焦的燃烧,并且数值模拟的结果能为石油焦在分解炉内优化燃烧提供了理论依据和有利的信息.     

Self-tuning gross heat release computation for internal combustion engines

The paper describes a novel method for self-tuning gross heat release computation in internal combustion engines suitable for both online usage in combustion phasing control applications and post-processing of cylinder pressure measurements. The method estimates the polytropic exponents and cylinder pressure offsets immediately preceding and succeeding the combustion event, respectively, using a fast nonlinear least-squares method. The polytropic exponent and the pressure offset are subsequently interpolated during the combustion event and a net heat release computation is performed based on the interpolated exponent and pressure. The result is a self-tuning gross heat release algorithm with no need to model heat losses, crevice losses and blow-by explicitly.     

Operating regime based dynamic engine modelling

In this paper the nonlinear model identification of combustion engines using operating regime model structures is discussed. The split of the engine process into several local operating regimes, characterised by the dominant influence of load and speed, allows one to overcome the highly nonlinear dynamic complexity of the engine. Additionally, local limits in terms of engine protection and combustion stability can be considered much easier. Such a model, with physical a priori information embedded in the model structure, provides excellent generalisation capability and a real-time application of the model is easily possible.