超临界压力流体的传热恶化

本文根据一些试验研究的结果,分析了超临界压力流体发生传热恶化的现象,介绍了若干文献中报道的判别传热恶化的方法及锅炉机组受热管防止传热恶化发生的对策.     

近临界压力区低质量流速光管水冷壁临界热流密度试验研究

在近临界压力区,对垂直上升光管的临界热流密度(CHF)进行了试验研究。试验参数范围:压力18~22MPa,质量流速310~550kg·m-2·s-1,进口过冷度5~10℃。试验段的管径和壁厚为30mm和5.5mm。试验得出了各工况条件下光管壁温分布规律;分析了各参数对传热特性的影响;分别拟合出正常传热和传热恶化区域传热系数的计算式;拟合出了CHF的计算式并与4个典型的CHF计算式进行了对比;分析了各试验参数对CHF的影响;探讨了传热恶化产生的原因和汽泡大小的关系。试验结果表明:压力越靠近临界压力,质量流速越小,传热恶化越早发生;截面位置越靠前,发生传热恶化的干度越小;该试验中传热恶化以核态沸腾(DNB)传热恶化为主。     

亚临界压力下二氧化碳传热特性及临界热流密度预测模型研究

二氧化碳(CO₂)布雷顿循环系统有着紧凑、高效、灵活的特点在第三代光热系统和第四代核电系统中具有良好的应用前景，而CO₂传热恶化现象影响着机组的安全运行。为研究竖直上升管内CO₂的传热恶化现象，在实验室建立了CO₂传热特性系统，对比了亚临界与超临界状态下CO₂传热特性，获得了热工参数对CO₂传热恶化的影响规律，并建立了CO₂临界热流密度预测关联，预测值与实验值吻合良好，误差在±30%以内。研究结果表明：亚临界压力下，CO₂发生传热恶化时壁温峰值更高；远离临界压力，增加质量流量均有利于抑制传热恶化的发生。     

超超临界循环流化床锅炉膜式水冷壁管温度与应力分析

超临界循环流化床（CFB）锅炉在运行中发生传热恶化使水冷壁出现裂纹,增加水冷壁爆管和泄漏的概率。以某在建超超临界CFB锅炉为研究对象,建立膜式水冷壁的二维有限元计算模型,分析不同传热恶化程度对水冷壁的温度和热应力的影响。结果表明:正常传热与传热恶化时水冷壁管内外壁面温度随方位角θ的变化规律一致,传热恶化时水冷壁的温度整体升高,最高温度由向火侧鳍片中点转移至水冷壁管向火侧顶点;水冷壁管向火侧变形的变化趋势和温度的变化趋势相同,传热恶化时水冷壁的温度和热应力会超过材料的允许温度和屈服强度,反复的传热恶化是导致水冷壁爆管的重要因素。     

超超临界锅炉水冷壁管传热恶化对横向裂纹影响的有限元分析

为研究不同传热条件下水冷壁管的温度及热应力分布特征,以某1 000 MW超超临界锅炉水冷壁管为对象,建立了水冷壁管的三维有限元模型,分正常传热和传热恶化2种情况进行了计算。计算结果显示,向火侧壁面热负荷、管内传热系数对传热恶化后的温度和热应力有显著影响,在尚未达到屈服前,水冷壁管的热应力与温度基本呈线性关系。传热正常时,水冷壁管鳍片处温度和热应力值大于水冷壁管向火侧顶点相应值,而向火侧顶点发生破坏的可能性较小;在传热恶化时,水冷壁管向火侧顶点的温度和热应力迅速增大,并明显高于水冷壁管鳍片处,传热恶化的反复出现引起的交变应力最终导致水冷壁管横向裂纹的产生。     

亚临界及近临界压力区低质量流速垂直内螺纹管传热特性试验研究

在低质量流速条件下,对垂直上升内螺纹管内汽水两相流动沸腾传热特性进行了系统的试验研究。试验段采用了材料为SA-213T12的φ32mm×6.3mm四头内螺纹管。试验参数范围为压力p=12～21MPa,质量流速G=232～773kg/(m2·s),内壁热流密度q=132～663kW/m2。试验得到了不同工况下垂直上升内螺纹管的壁温分布特性,分析了压力、内壁热负荷和质量流速变化对内螺纹管传热特性的影响,探讨了传热恶化的发生机制,并给出了能用于工程实际的传热试验关联式。试验结果表明:在亚临界及近临界压力区,垂直上升内螺纹管会发生第2类传热恶化——干涸(dryout),而在试验中未观测到第1类传热恶化——膜态沸腾(departure from nucleate boiling,DNB)。压力与内壁热负荷的增大,以及质量流速的减小,均会导致干涸提前发生和干涸后的壁温飞升值增大。与亚临界压力区相比,内螺纹管在近临界压力区的传热特性变差,管壁温度显著升高,发生传热恶化时的临界焓值减小。     

一起燃烧器故障引起的停炉事故分析

结合一起直流锅炉螺旋段水冷壁管超温的事故,对直流锅炉蒸发受热面水动力不稳定性和传热恶化进行分析,提出对于摆动喷燃器燃烧系统,保持同层燃烧能防止发生水动力不稳定和传热恶化。     

大型锅炉膜式水冷壁暂态温度场有限元分析

在６００ＭＷ锅炉机组水冷壁热力试验的基础上,利用有限元分析的方法对低倍率锅炉膜式水冷壁管壁温度分布随传热工况的动态变化进行了分析。导致水冷壁管壁温度波动的最根本的原因是管内传热恶化;单面受热水冷壁在管内发生传热恶化时其向火侧管内外壁温差随时间的波动较小,而水冷壁周向温差则随向火侧外壁的壁温波动而剧烈波动。     

超临界二氧化碳在垂直光管内的传热特性

通过实验和模拟方法,研究超临界CO_2在恒定热流的垂直光管内的传热特性。实验段内径为10mm,全长2m;从实验数据中选取4个工况进行模拟研究,经过模型对比后选用SST k-w湍流模型模拟工质的传热传质过程;分别定义了正常传热和传热恶化现象,并对超临界流体的物性变化进行分析。流动热加速对于传热的影响显著,流体大比热区在径向截面上的位置与流体的主流温度可以作为判定传热恶化是否发生的依据。正常传热工况,在主流温度到达拟临界温度前,大比热区集中在过渡区中;当传热恶化现象出现时,主流温度与拟临界温度之差较大,大比热区已从过渡区移向核心区,导致雷诺切应力降低和湍流强度削弱,直到雷诺切应力方向改变,传热效果开始恢复。     

大型锅炉膜片水冷壁暂态温度场有限元分析

在６００ＭＷ锅炉机组水冷壁热力试验的基础上,利用有限元分析的方法对低培率锅炉膜式水冷壁管壁温度分布随传热工况的动态变化进行了分析,导致水冷壁管壁温度波动的最根本的原因是管内传热恶化,单面受热水冷壁在管内发生传热恶化时其向火侧管内外壁温差随时间的波动较小,而水冷壁周向温差则随向火侧外壁的壁温波动而剧烈波动。     

Procedure of calculating liquid flowrates in wall film and heat transfer crisis in tubes

A brief review of investigations into liquid distribution between the flow core and wall film in tubes previously carried out is given. The relationship between mass transfer processes and heat transfer crisis is shown. The procedure of calculating heat transfer crisis in tubes with uniform heat input along the length based on experimental data from the databank of the Physics and Power Engineering Institute (FEI) is presented.     

Regularities pertinent to heat transfer between torch gas layers and steam boiler firebox waterwalls. Part I. Geometrical and physical torch model as a source of heat radiation

The progress seen in the 19th–21st centuries in the development of methods for calculating heat transfer in torch furnaces, fireboxes, and combustion chambers is analyzed. Throughout the 20th century, calculations of heat transfer were carried out based on the law for radiation from solid bodies deduced by Y. Stefan and L. Boltzmann. It is shown that the use of this law for calculating heat transfer of a torch (a gaseous source of radiation) in heating furnaces and power-generating installations leads to incorrect results. It is substantiated that there is crisis of methods for calculating heat transfer in torch furnaces and power-generating installations. Geometrical and physical torch models in the form of radiating cylindrical gas volumes as sources of heat radiation are proposed for overcoming this crisis.     

Some results of the verification of SERPENT code while calculating heat transfer crisis in uniformly heated tubes

The results of calculating the parameters of heat transfer crisis regimes in uniformly heated tubes, obtained with the aid of look-up tables (LTs) Grnv-95 and FEI-91 are presented.     

System of closing relations of a two-fluid model for the HYDRA-IBRAE/LM/V1 code for calculation of sodium boiling in channels of power equipment

The system of equations from a two-fluid model is widely used in modeling thermohydraulic processes during accidents in nuclear reactors. The model includes conservation equations governing the balance of mass, momentum, and energy in each phase of the coolant. The features of heat and mass transfer, as well as of mechanical interaction between phases or with the channel wall, are described by a system of closing relations. Properly verified foreign and Russian codes with a comprehensive system of closing relations are available to predict processes in water coolant. As to the sodium coolant, only a few open publications on this subject are known. A complete system of closing relations used in the HYDRA-IBRAE/LM/V1 thermohydraulic code for calculation of sodium boiling in channels of power equipment is presented. The selection of these relations is corroborated on the basis of results of analysis of available publications with an account taken of the processes occurring in liquid sodium. A comparison with approaches outlined in foreign publications is presented. Particular attention has been given to the calculation of the sodium two-phase flow boiling. The flow regime map and a procedure for the calculation of interfacial friction and heat transfer in a sodium flow with account taken of high conductivity of sodium are described in sufficient detail. Correlations are presented for calculation of heat transfer for a single-phase sodium flow, sodium flow boiling, and sodium flow boiling crisis. A method is proposed for prediction of flow boiling crisis initiation.     

计及换热器传热特性的热电联合系统优化调度

换热器传热特性是影响热电联合系统优化调度的关键因素之一。针对热电联合系统,基于传热原理,采用抽汽的三级传热模型来描述换热器的传热过程,利用迭代法计算一定热负荷下的抽汽量,提出计及换热器传热特性的热电、火电、风电的联合调度模型。最后以最小化煤耗量为优化目标,分析换热器传热特性对抽汽传热过程和热电联合系统调度结果的影响。算例表明：增大换热器传热面积、降低换热器出口水温可以不同程度地提高风电消纳能力,降低运行成本,研究结果为电热联合系统优化调度方案提供换热参数选择的依据。     

计及换热器传热特性的热电联合系统优化调度

换热器传热特性是影响热电联合系统优化调度的关键因素之一。针对热电联合系统,基于传热原理,采用抽汽的三级传热模型来描述换热器的传热过程,利用迭代法计算一定热负荷下的抽汽量,提出计及换热器传热特性的热电、火电、风电的联合调度模型。最后以最小化煤耗量为优化目标,分析换热器传热特性对抽汽传热过程和热电联合系统调度结果的影响。算例表明：增大换热器传热面积、降低换热器出口水温可以不同程度地提高风电消纳能力,降低运行成本,研究结果为电热联合系统优化调度方案提供换热参数选择的依据。     

换热器冗余面积计算新方法

污垢是影响换热器传热性能的一个重要因素。在分析现有换热器冗余面积计算式存在局限性的基础上，提出新的换热器冗余面积计算式；对洁净换热器总传热系数与污垢换热器在不考虑污垢热阻时的传热系数之比，以及洁净换热器总传热系数和污垢热阻等参数对换热器冗余面积的影响进行了探讨。     

富氧燃烧循环流化床锅炉炉内传热特性

针对富氧燃烧循环流化床锅炉(circulating fluidized bed boiler,CFBB)炉内传热特性进行了研究。考虑气体辐射对传热系数的影响,建立了CFBB富氧燃烧下的传热模型。以一台440t/h循环流化床锅炉为例,通过模型分析了炉内传热情况,并和空气燃烧模式下的传热特性进行比较。进行了氧气浓度在30％、50％、70％气氛下的CFBB炉膛概念性设计。在循环流化床锅炉炉内传热中,灰占主导作用,烟气成分变化对传热系数影响不大。氧气浓度越高,越有必要设置外置换热器来维持炉膛正常运行。     

污垢热阻对空冷凝汽器换热面积影响特性研究

根据传热学基本原理,通过迭代计算出换热系数K,进而得到换热面积的数学模型。并以某600 MW直接空冷机组为例,充分考虑污垢热阻对换热面积的影响,得到对应的变化曲线,对直接空冷凝汽器换热面积的设计计算具有一定的参考价值。