超超临界锅炉水冷壁传热特性和温度偏差

叙述了变压运行超超临界锅炉水冷壁的运行特点,传热恶化、温度偏差与水动力稳定性的国内外研究结果,提出了对垂直内螺纹管水冷壁质量流速的选取和水动力不稳定性校核的看法.     

超临界W型火焰锅炉垂直水冷壁水动力特性研究

对超临界W型火焰锅炉垂直水冷壁的水动力特性进行研究,首先对锅炉应用的材料为SA-213T12的ф31.8mm×5.5mm四头优化内螺纹管在试验台进行阻力特性研究,得到摩擦阻力系数λ。根据摩擦阻力系数λ及运行参数,计算某电厂600 MW超临界机组直流锅炉炉膛垂直水冷壁的阻力特性、流量偏差及热负荷偏差,并对水冷壁的水动力特性进行分析研究。     

热敏感性对直流锅炉水冷壁安全运行的影响

根据汽液两相流动阻力的计算式,推导了流量敏感系数、出口焓值敏感系数的表达式。对直流锅炉内螺纹管、螺旋管圈水冷壁的热敏感系数进行了定量计算,分析了各种因素对热敏感性的影响,并对两种水冷壁管屏在热负荷不均时的温度偏差,焓值偏差进行了讨论,对直流锅炉安全运行具有一定的指导意义。     

超超临界锅炉垂直水冷壁的结构特点和水动力特性

叙述了超超临界锅炉垂直水冷壁的结构特点和水动力特性,并介绍了内螺纹管垂直管圈水冷壁主要设计参数的选取原则和水动力稳定性的判别方法。     

超临界和超超临界锅炉水冷壁的优化设计

阐述了超临界和超超临界锅炉水冷壁的优化设计，主要包括：下辐射区水冷壁采用内螺纹管，降低质量流速，准确掌握超临界流体热物性，减小工质焓增，减小热偏差和壁温偏差，降低局部热负荷，提高安全裕度等综合性措施。最低直流负荷优化选择综合考虑启动系统成本和变负荷运行范围以及对负荷快速变化的跟踪性能，以适应机组调峰。对1000MW超超临界锅炉垂直管屏水冷壁的流动特性进行了理论分析，强调应高度重视运行煤质变化的影响并严格监控水质。图5表1参7     

超临界循环流化床锅炉垂直并联内螺纹管内两相流不稳定性的研究

针对600 MW超临界循环流化床(CFB)锅炉水冷壁管在低质量流速条件下的水动力特性,进行了垂直并联内螺纹管内两相流不稳定性的试验研究,得到了不同条件下的水动力特性曲线和压力降、密度波2种脉动的典型脉动曲线.在试验参数范围内,分析了系统压力、质量流速、进1:2过冷度和上游可压缩容积对两相流不稳定性的影响.根据试验结果,采用均相流模型得到了不稳定发生的界限关系式,为超临界CFB锅炉垂直并联内螺纹管水冷壁的设计与安全运行提供了依据.     

变压运行600MW超临界锅炉垂直水冷壁计算特性

阐述了600MW超临界锅炉垂直水冷壁特点和设计要求。根据国内对φ28mm内螺纹管的传热与阻力特性试验以及国外的试验研究，详述了水冷壁的质量流速、出口温度、过热度及许可温差的选取。     

600MW超临界循环流化床锅炉水冷壁的选型及水动力研究

在超临界及亚临界压力范围内对不同尺寸的内螺纹管和光管的流动传热特性进行了实验研究,得到了偏离核态沸腾(DNB)和蒸干(DRYOUT)的发生规律、管子流动和传热的特性,建立了计算复杂流动网络系统回路流量及节点压力的数学模型,对非线性方程组采用迭代方法进行了求解,并对1台600 MW超临界循环流化床锅炉的水冷壁流量分配及壁温分布进行了计算.结果表明:在BMCR和在75%BMCR负荷下锅炉运行是安全可靠的.另外,笔者还研究了吸热偏差对流量分配及壁温分布的影响,结果表明流量与吸热量呈正响应特性.     

超(超)临界压力直流锅炉垂直管圈水冷壁特性的研究

介绍了代表不同技术流派的 3 台1 000 MW超(超)临界压力锅炉的设计与技术特点,以及实际运行时锅炉水冷壁的阻力情况,重点分析了垂直管圈水冷壁的阻力特性和在各种运行工况下的水动力特性.结果表明:在高负荷下运行时,垂直管圈水冷壁的阻力远大于螺旋管圈水冷壁;低负荷运行时炉膛热负荷偏差对水冷壁安全性的影响远大于高负荷时的影响.同时,提出了今后深入研究水冷壁的若干重点.     

改进型 UP 锅炉水冷壁管热敏感性的研究

针对国产改进型ＵＰ锅炉锅炉水冷壁经常爆漏的实际情况,分析了在ＵＰ锅炉中广泛使用的φ２２＊５．５ｍｍ四头内螺纹管的热敏感性。     

超超临界锅炉垂直水冷壁热偏差监测系统的应用

针对大容量双切圆锅炉水冷壁热偏差问题,根据水冷壁管结构特征及下水冷壁管出口汽温等运行数据,研究开发B/S架构垂直管圈水冷壁热偏差监测系统。系统实现了水冷壁管工质流量偏差,工质热偏差,沿炉宽、炉深烟气热负荷偏差的在线监测。监测结果为判定水冷壁管内异常阻力特性、分析炉膛热负荷分布规律、优化锅炉燃烧、避免超温引起的降参数运行,提供了有效可靠的数据支持,并为下一步水冷壁监测技术研究工作打下基础。     

Experimental investigation on heat transfer characteristics of low mass flux rifled tube with upward flow

An experiment for heat transfer of water flowing in a vertical rifled tube was conducted at subcritical and supercritical pressure. The main purpose is to explore the heat transfer characteristics of the new-type rifled tube at low mass flux. Operating conditions included pressures of 12–30 MPa, mass flux of 232–1200 kg/(m² s), and wall heat fluxes of 133–719 kW/m². The heat transfer performance and wall temperature distribution at various operating conditions were captured in the experiment. In the present paper, the heat transfer mechanism of the rifled tube was analyzed, the effects of pressure, wall heat flux and mass flux on heat transfer were discussed, and corresponding empirical correlations were also presented. The experimental results exhibit that the rifled tube has an obvious enhancement in heat transfer, even at low mass flux. In comparison with a smooth tube, the rifled tube efficiently prevents Departure from Nucleate Boiling (DNB) and delays dryout at subcritical pressure, and also improves the heat transfer of supercritical water remarkably, especially near pseudo-critical point. An increase in pressure or wall heat flux impairs the heat transfer at both subcritical and supercritical pressure, whereas the increasing mass flux has a contrary effect.     

Thermal–hydraulic analysis of a steam boiler with rifled evaporating tubes

Rifled evaporating tubes are applied in the steam boilers with the aim to increase the steam–water mixture turbulization and to prevent the burnout of tubes walls. The rifled evaporating tubes and the working fluid forced circulation are applied in the steam boiler at the Thermal Power Plant “Kolubara B” that is being built by the Electric Power Utility of Serbia. In order to investigate operating characteristics of the steam boiler of such an advanced design, a simulation and analysis of complex coupled thermal processes on the furnace gas side and thermal–hydraulics inside the evaporating tubes were performed for the whole range of plant operating loads. In this paper a methodology for hydraulic calculations of both forced and natural circulation loops is presented. The increased thermal safety margin of rifled tubes is predicted and compared with the safety margin of smooth tubes for uniform and variable heat loads among walls of the boiler furnace. The rifled tube provides an acceptable temperature of its wall even under a high void fraction of the coolant and prevents the occurrence of critical heat transfer conditions. The influence of the rifled tubes on an increase of the hydraulic resistance in the circulation loop is analyzed. Also, a range of operating conditions under which there is no need for the circulation pumps operation is determined.     

Experimental Investigation on Critical Heat Flux for Water Flowing in a Vertical Uniformly Heated Rifled Tube under Near-critical Pressures

This study experimentally investigated the critical heat flux(CHF) of departure from nucleate boiling(DNB) of water flowing under near-critical pressures in a 2 m-long vertical upward rifled tube with the size of Φ35 × 5.67 mm. Operating conditions included pressures of 18–21 MPa, mass fluxes of 475–1000 kg·m~(-2)·s~(-1), inlet subcooling temperatures of 3–5°C, and wall heat fluxes of 40–960 kW·m~(-2). Tube wall temperature distribution and heat transfer performance in different test conditions were obtained. The effects of the operating parameters on CHF were analyzed. Four heat transfer coefficient correlations were evaluated against our experimental data for further investigation of the two-phase heat transfer characteristics. A heat transfer correlation based on Martinelli number utilized in two-phase region and two empirical correlations used to predict the CHF in rifled tube at near-critical pressures were proposed. Meanwhile, experimental CHF data in rifled tube were compared with six widely used correlations and a CHF look-up table. The CHF enhancement effect in rifled tube is obvious as compared with the CHF data in smooth tube. Results show that DNB occurs at low vapor quality and subcooled region in the rifled tube at near-critical pressures. The increase in pressure leads to the early occurrence of DNB and the decrease in CHF, whereas the increase in mass flux delays the occurrence of DNB and results in the increase in CHF. DNB presents a tendency to move toward the inlet of the rifled tube. At individual operating conditions, DNB and dryout coexist at different sections of the rifled tube.     

超临界压力锅炉水冷壁倾斜并联内螺纹管内两相流不稳定性的试验研究

针对国产首台600 MW超临界机组在亚临界压力条件下的水动力特性,进行了倾斜并联内螺纹管内的汽液两相流不稳定性试验研究,观察到压力降型和密度波型两种类型的脉动。探讨了影响不稳定性的主要参数。并给出了发生不稳定性的阈值。试验表明:在超临界锅炉滑压运行设计参数条件下,水冷壁管内不会发生两相流不稳定性。图11表1参7     

Experimental Investigation of Two-Phase Flow Instabilities in Low-Mass-Flux Water Wall Tubes of Supercritical Circular Fluidized Bed Boilers

For the purpose of disclosing the hydrodynamic flow characteristics, under the low mass velocity conditions of the 600-MW supercritical circular fluidized beds boilers, experimental studies on instability of two-phase flow in parallel vertical internally ribbed tubes were conducted. Two kinds of oscillations, pressure-drop oscillation and density-wave oscillation, have been observed. In the range of test parameters the effects of pressure, mass flux, inlet subcooling, compressible volume, exit throttle, and asymmetric heat flux to the two-phase instability were explored and analyzed. Indications from experiment data are: To increase system pressure, mass flux and inlet subcooling will intensify the stability of water wall tubes. To increase exit throttle will intensify the instability of water wall tubes. The bounding pressure and bounding mass flux of density-wave oscillations and the bounding pressure of pressure-drop oscillation have been obtained. Based on the results of testing and using a homogeneous model, the threshold relational expressions of instability were obtained. The results may be used for the design and safe operation of parallel vertical rifled water wall tubes of supercritical circular fluidized beds boilers.     

Mathematical modeling and thermal-hydraulic analysis of vertical water wall in an ultra supercritical boiler

Water wall design is a key issue for an ultra supercritical boiler. In order to increase the steam–water mixture turbulization and to prevent the burnout of tubes walls, vertical rifled tubes are applied in Yuhuan power plant boiler which is the first 1000 MW ultra supercritical boiler in China and began to operate in December 2006. Mathematical modeling and thermal-hydraulic analysis are key factors for the successful design and operation of water walls. The water wall system is treated in this paper as a network consisting of circuits, pressure grids and connecting tubes. The mathematical model for predicting the mass flux distribution and metal temperature in water wall is based on the mass, momentum and energy conservation equations. An experiment on the heat transfer characteristics of vertical rifled tube was conducted with the aim to obtain the heat transfer performance and corresponding empirical correlations. The fitting computational formulas are applied in the mathematical model. The presented modeling method is more accurate than the conventional graphic method and can be applied to complex circuit structures. The mass flux distribution and the metal temperature in the water wall are calculated at 35%, 50% and 100% of the boiler maximum continuous rating (BMCR). The results show a good agreement with the plant data. The maximum relative difference between the calculated mass flux and the plant data is 9.7% at 50% BMCR load. The metal temperature difference in the tip of fins in lower circuit 8 is about 3–7 °C at 35% BMCR load. The results show that the vertical water wall in the ultra supercritical boiler of Yuhuan power plant can operate safely.     

两类过热器壁温分布特性的仿真研究

以一种分布式传热模型为基础,对后屏过热器和高温过热器的壁温分布特性进行了仿真研究.结果表明:这两类过热器的壁温分布特性存在较明显的差异:对于后屏过热器,随着高度方向烟温偏差的增大,各个管圈出口段壁温下降,水平段壁温显著升高;对于高温过热器,当高度方向烟温偏差较小时,壁温最高点位于尾部上升段内,且各点壁温相近;随着这一偏差的增大,壁温最高点逆工质流程向管屏下部移动.     

Critical heat flux performance for flow boiling of R-134a in vertical uniformly heated smooth tube and rifled tubes

In the present paper, critical heat flux (CHF) experiments for flow boiling of R-134a were performed to investigate the CHF characteristics of four-head and six-head rifled tubes in comparison with a smooth tube. Both of rifled tubes having different head geometry have the maximum inner diameter of 17.04 mm while the smooth tube has the average inner diameter of 17.04 mm. The experiments were conducted for the vertical orientation under outlet pressures of 13, 16.5, and 23.9 bar, mass fluxes of 285-1300 kg/m²s and inlet subcooling temperatures of 5-40 °C in the R-134a CHF test loop. The parametric trends of CHF for the tubes show a good agreement with previous understanding. In particular, CHF data of the smooth tube for R-134a were compared with well-known CHF correlations such as Bowring and Katto correlations. The CHF in the rifled tube was enhanced to 40-60% for the CHF in the smooth tube with depending on the rifled geometry and flow parameters such as pressure and mass flux. In relation to the enhancement mechanism, the relative vapor velocity is used to explain the characteristics of the CHF performance in the rifled tube.