超超临界锅炉流动不稳定性的形成机理及研究进展

以煤炭为主要能源的国家火电机组,尤其是煤电机组持续低负荷运行或深度调峰在未来几年将成为常态。在深度调峰过程中,机组负荷多数偏离设计工况,很有可能产生流动不稳定问题。笔者主要研究了现代机组运行的流动不稳定性形成机理及影响因素,分析超超临界机组的流动不稳定性的研究方法。按发生特性归类,流动不稳定性可分为静态不稳定性和动态不稳定性。而在超超临界锅炉系统变负荷运行过程中,主要存在密度波型流动不稳定性、压力降型流动不稳定性和热力型流动不稳定性,几种不稳定现象都影响系统的正常运行。流动不稳定性的主要影响因素包括热负荷分布、管道结构及系统流动参数等。由于分析和计算工具的发展,流动不稳定性的发生条件及其变化规律能较准确预测,大量试验及数值研究表明,热流密度越小,系统压力越大,进口节流系数越大,出口节流系数越小,则系统越趋于稳定。从管道结构上来看,加热长度越短,管道内径越大,则系统越稳定,且具有交叉连接的系统比没有交叉连接的系统和单通道系统更稳定。针对超超临界水流动不稳定性的研究,主要有试验和数值模拟2种方法。试验方法的优势在于可以有针对性地以实际物理系统为研究对象,为相应的数值模拟研究提供有价值的参考。考虑到水在超超临界压力和温度下的流动不稳定试验系统极为复杂,所需费用庞大,数值模拟就成为一种重要的研究手段,其可以借鉴成熟的两相沸腾研究成果,能够方便分析各种参数对流动不稳定性的影响规律。针对超超临界流体系统的流动不稳定性的数值模拟研究,其分析方法通常可分为频域法和时域法。频域分析方法的缺点在于不能很好地解决非线性问题,为有效解决频域分析方法非线性效应消失的问题,可通过Hopf分岔技术来确定极限环的振幅。时域法作为用于分析诸如振荡周期和混沌等非线性效应的最常用方法,结合一系列无量纲数,能在保留动态变化的同时,有效地描述亚临界及超超临界流体的流动不稳定边界。

Formation mechanism and research progress of flow instability in ultra-supercritical boiler

For countries where coal is the main energy source,thermal power units,especially coal-fired power units,will continue to operate with low load or deep peak adjustment in the next few years. In the process of depth peak adjustment,the load of the unit deviates from the design condition,which may lead to flow instability. In this paper,the formation mechanism and influencing factors of the flow instability of ultra-supercritical units was analyzed and studied in order to provide guidance for the design of heat transfer equipment and safe variable load operation and the research methods of flow instability in ultra supercritical units were analyzed. According to occurrence characteristics,flow instability can be divided into static instability and dynamic instability. In load changing process of ultra-supercritical boiler flow system,there are flow instabilities such as density wave,pressure drop and thermal flow instability,which will have a serious impact on the normal operation of the system. The main influencing factors of flow instability include heat load distribution,pipeline structure and system flow parameters. Due to the development of analysis and calculation tools,the occurrence conditions and change rules of flow instability can be predicted accurately. A large number of experiments and numerical studies show that the lower the heat flux density is,the higher the system pressure is,the larger the inlet throttle coefficient is,and the smaller the outlet throttle coefficient is,and the more stable the system is. From the perspective of pipeline structure,the shorter the heating length is and the larger the inner diameter is,the more stable the system is,and the system with cross-connection is more stable than the system without cross-connection and the single-channel system. There are two methods to study the instability of ultra-supercritical water flow: experimental test and numerical simulation. The advantage of the experimental method is that it can focus on the actual physical system as the research object and provide valuable reference for the corresponding numerical simulation. Considering that the experimental system of water flow instability under ultra-supercritical pressure and temperature is extremely complex and requires a large amount of cost,numerical simulation has become an important research method. Numerical simulation can learn from mature two-phase boiling research results,which can easily analyze the influence law of various parameters on flow instability. For the numerical simulation of the flow instability of supercritical fluid system,the analysis methods can be divided into frequency domain method and time domain method. The disadvantage of the frequency-domain analysis method is that it can’t solve the nonlinear problem well. The solution to the disappearance of the nonlinear effect of frequency-domain analysis method is to determine the amplitude of the limit cycle through the Hopf bifurcation technology. To analyse the flow instability of ultra supercritical fluid,the time domain method is most commonly used in the analysis for period of oscillation and chaotic nonlinear effect. With a series of dimensionless number,both dynamic changes and the flow instability boundary of subcritical and supercritical fluid can be effectively described.