600MW超临界循环流化床锅炉屏式受热面水动力及吸热量特性研究

针对高参数循环流化床(CFB)锅炉高温受热面热偏差特性直接影响锅炉安全运行的问题,根据超临界CFB锅炉炉膛内屏式过热器建立的复杂流动网络系统的数学模型以及吸热量模型,对某600 MW超临界CFB锅炉满负荷以及100 MW负荷2种不同运行工况下压降、质量流速分布、出口汽温分布以及沿工质流动方向壁温分布特性进行了计算分析,并进一步计算得到受热面吸热量分布。结果表明：屏式受热面在600 MW以及100 MW负荷下质量流速偏差分别为12.71%和13.96%,全屏出口汽温偏差分别为33 K和58.4 K,偏差均在安全范围内。600 MW负荷下,最高外壁温度为616.5℃,在材料允许范围内,吸热量分布呈靠近侧墙水冷壁及炉膛中心线处低、受热面中间处高的分布趋势。     

超临界锅炉螺旋水冷壁流量分配和壁温特性的研究

基于流体管网计算理论,建立了由流量测量回路(以下简称流量回路)、压力测量节点和连接管构成的600MW超临界锅炉螺旋水冷壁流动网络系统。根据质量守恒、动量守恒和能量守恒定律,建立了600 MW超临界锅炉螺旋水冷壁流量和壁温计算的数学模型。采用拟牛顿法对流量回路和节点方程组求解,并开发计算程序,获得了不同负荷下螺旋水冷壁内流动压降、流量分配及壁温特性。结果表明:在100%BMCR(锅炉最大连续蒸发量)、75%BMCR及30%BMCR负荷下,热偏差和流量偏差较小,热偏差最大为5℃,流量偏差最大为7.47%;流动压降与设计值符合较好;水冷壁壁温随炉膛高度增加而升高,最高壁温为469.4℃,各负荷下管壁温度均处在管材强度的允许范围内。     

H级机组余热锅炉高压直流蒸发器水动力特性计算与分析

采用直流设计的配H级联合循环机组的余热锅炉高压蒸发器(简称高压直流蒸发器),在水动力特性上与传统余热锅炉存在较大的区别.基于流动网络系统法,将高压直流蒸发器系统等效为由流量回路、压力节点和连接管组成的流动网络系统.根据质量守恒、动量守恒和能量守恒定律建立了流量分配计算模型.根据开口螺旋鳍片管的结构特点,建立了壁温计算模...     

大型电站锅炉过热器和再热器壁温计算方法的改进

本文作者在苏联热力计算方法的基础上,吸收了我国科研工作者在沿集箱轴向流量偏差和同屏辐射热量偏差研究的成果,在导出一次交叉流动汽温变化规律、U型管束交叉流动烟温和汽温的解法的基础上,最后得到综合考虑沿烟道宽度流动偏差、吸热偏差以及同屏的流量偏差、辐射热量分配、沿烟道高度烟温偏差及同屏流量偏差等因素的壁温计算方法。本文还介绍了作者基于上述方法所编制出的壁温计算程序概况和应用该程序于30万千瓦直流锅炉过热器、再热器的计算结果。作者认为,建立井采用合理的、考虑各方面因素的壁温计算程序用于大型电站锅炉的设计,对于提高我国大型机组设计水平、保证机组安全进行有重要的意义。     

超临界直流锅炉螺旋管圈水冷壁流量分配及壁温计算

根据回路和节点所遵守的质量守恒、动量守恒和能量守恒方程,建立了计算螺旋管圈水冷壁流量和壁温的数学模型.在此基础上,对1台350 MW超临界燃煤锅炉在不同负荷下的水冷壁流量分配及壁温分布进行了计算.结果表明:在锅炉最大连续蒸发量(BMCR)、75%BMCR及30%BMCR负荷下,螺旋管圈水冷壁的流量偏差不超过±6%,工质出口温差不超过10 K,流量偏差和热偏差均较小;各负荷下管壁温度均处于管子的许用温度范围之内,锅炉运行安全可靠.     

槽式太阳能聚光集热器传热性能研究

建立了槽式太阳能聚光集热器单侧吸收高热流密度的传热数学模型.该传热数学模型将金属吸热管壁面所能接受到的非均匀热流简化为矩形分布,考虑了金属吸热管管壁的周向导热,选取Dudley的试验数据验证了传热数学模型的可靠性并分析了影响集热器传热特性的主要因素.通过Fluent软件研究了槽式太阳能聚光集热器金属吸热管的周向温度分布规律.结果表明:该传热数学模型具有一定可靠性;流体温度、太阳辐射强度和流体体积流量是影响集热器管内换热特性和管壁周向温度分布的主要因素.     

槽式太阳能聚光集热器传热数学模型研究

为了计算分析槽式太阳能集热器的传热特性,将金属吸热管表面的热流密度不均匀特性考虑在内,建立了一种传热数学模型;该模型将金属吸热管管壁所能接受到的热流密度简化为矩形分布,考虑了管壁的周向和径向导热。本文以Dudley的试验数据为依据对其进行计算验证,同时也进行了不均匀热流边界条件下槽式太阳能吸热管的管内流动模拟研究。计算结果和模拟结果都与文献提供的数据有较好的吻合度,集热器出口流体温度和效率与试验数据的最大相对误差分别为0.91%和4.79%。该传热数学模型可用于槽式太阳能集热器传热热性的计算分析。     

平板太阳集热器列阵的二维模型与热性能

本文分析了管板式太阳集热器列阵的流动与传热。考虑了支管流量分布不均匀、吸热板二维导热及浮升力等因素,建立了离散型的二维数学模型,求得了数值解。计算结果表明,集热器列阵的布置方式和支管流量分布的均匀性对列阵的热性能有很大的影响。逆流布置(U型)的列阵由于流动的短路现象而使热效率明显降低。计算所得的数值结果与实验结果符合良好。     

电站锅炉对流过热器和再热器壁温数值计算方法的研究

对电站锅炉对流过热器和再热器的壁温计算方法进行了改进。将各管组、管段离散化为小单元,按工质流动顺序逐次计算小单元的辐射及对流传热关系,进而求出壁温分布。采用炉膛出口二维烟温分布,从基本的热平衡关系式入手,详尽考虑了影响管子传热的结构、位置、流动等偏差因素。该计算方法符合数值计算的特点,因此便于在计算机上实现快速准确的壁温计算。图７表１参８     

基于燃烧与水动力耦合模型的锅炉蒸汽管超温特性研究

提出一种基于燃烧与水动力耦合模型的锅炉蒸汽管壁温度数值模拟方法,对某660 MW超临界切圆燃烧锅炉壁温进行了计算分析。以均匀外壁温为边界条件,利用Fluent软件模拟了煤粉气固流动、燃烧和辐射等过程,获得了炉内不同位置受热管的传热热流。再以热流分布为边界,采用MATLAB软件建立了工质流动及气-壁-汽换热方程组,Fluent软件重新计算的壁温边界。通过编写模型间的网格映射函数,实现壁温的耦合计算。研究表明:壁温计算值与实测值的最大相对误差在2%以内;炉膛出口残余旋转使水平烟道左侧和右上方热流较大,高温再热器和末级过热器的外壁温沿炉宽方向呈双峰分布;高温再热器整级受热管出口壁温的峰谷差值远高于末级过热器,实际运行中应特别注意高温再热器靠烟道左侧管屏外圈管子向火侧弯头处的超温。     

矩形冷却通道内超临界RP-3航空煤油对流传热数值研究

对超临界压力下RP-3航空煤油在内截面宽为4mm、高为4mm、固体壁面厚为1mm、加热段长度为500mm的水平矩形冷却通道内的对流传热特性进行了数值模拟研究。分析了通道内速度场的分布规律,讨论了热流密度、压力、进口温度对传热的影响。计算结果表明:当主流温度处于拟临界温度附近时,流体物性参数变化剧烈,导致传热系数降低,传热出现恶化。在超临界压力下,较低的热流密度、增大压力、降低进口流体温度或提高质量流速均有利于改善冷却通道内的传热性能。     

Mechanism and influence factor analysis of heat transfer deterioration of transcritical methane

A three-dimensional simulation, of transcritical flow, and heat transfer of methane, under asymmetric heating conditions, were performed. The simulation results demonstrated that the drastic changes in density at the pseudo critical temperature lead to an M-type velocity distribution, which plays a dominant role in the deterioration of heat transfer. The specific heat affects the location of the deterioration, while the thermal conductivity and viscosity affect only the wall temperature magnitude, whereas they do not affect the occurrence and location of heat transfer deterioration. The M-type velocity gradually disappears with the inlet mass flow rate increasing, indicating that heat transfer deterioration was eliminated. In addition, there is a critical inlet pressure of 10 MPa. When the inlet pressure is less than critical inlet pressure, heat transfer is improved with the inlet pressure's increase. However, when the inlet pressure is higher than critical inlet pressure, with inlet pressure increasing further, the decrease in the peak specific heat value will weaken the heat absorption capacity of methane, making the deterioration more severe. The deterioration of heat transfer will be improved by increasing the wall roughness, while the pressure drop will also be increased. The optimal wall roughness of 7 μm can be selected by using the thermal performance factor.     

三叶膨胀管换热器壳程强化传热的数值研究

三叶膨胀管是一种新型强化传热管,针对纵向流换热器特点,设计了三种不同管束结构参数的三叶膨胀管自支撑纵向流换热器。应用FLUENT软件及Realizable k-ε湍流模型,对三种不同结构参数的三叶膨胀管换热器壳程强化传热特性展开了数值模拟,并通过与实验数据的对比,验证了计算模型的可靠性。计算了不同壳程介质流速下,三叶膨胀管换热器壳程的换热系数与压降值,并获得了壳程流体流线以及相应的温度场、速度场和二次流分布图。结果发现,在壳程水流速一致的情况下,管束横向间距越大的三叶膨胀管换热器,壳程拥有更高的综合换热性能和更低的压降值,但相应地,换热系数也更低。流场分析显示,壳程流体流线呈现出三维纵向旋流形态,二次流的出现改变了速度场和温度场分布,二次流的强度随着管束横向间距的减小而增大。     

Study on wall temperature distribution of oscillating tube

The wall temperature distribution and heat transfer process of the oscillating tube have been investigated in this paper using both numerical simulation and experimental method. The wall temperature of oscillating tube increases rapidly in the inlet and then decreases slowly, moreover, the rally phenomenon of wall temperature near the closed end is observed. With the increase of jet flow frequency, the highest wall temperature increases and the location of that moves towards the inlet. The velocity of pressure wave in the oscillating tube almost remains constant even its intensity changes. The quantity of heat transfer between the gas and inner wall of the oscillating tube determines the wall temperature of every location, and the pressure wave disturbance can cause the heat transfer quantity change. Each pressure wave has its own disturbance range. The wall temperature distribution can be explained by the change of pressure wave intensity and its disturbance time. Besides, the step and rally of wall temperature are discussed, which shows that the conditions of heat transfer can be improved due to intersection or reflection of pressure waves.     

Unsteady-state fluctuations analysis during bubble growth in a “rectangular” microchannel

Boiling in microchannels shows great potential for cooling systems and compact heat removal applications. However for confidence in this cooling technique, it is essential that any excursions from typical flow boiling are understood and predicted. Confined bubble growth can cause pressure fluctuations which interfere with bubble nucleation and growth and can also lead to flow reversal and instances of temperature excursions. Boiling experiments are performed in a single rectangular microchannel of hydraulic diameter 771 μm, using n-Pentane as the working fluid. A heating technique was incorporated on the exterior walls of the microchannel; a transparent, metallic, conductive deposit, which allows simultaneous uniform heating and visualisation to be achieved. In conjunction with obtaining high-speed imaging, an infrared camera is used to record the temperature profile at the microchannel wall, and sensitive pressure sensors are used to record the pressure drop across the microchannel over time. During flow boiling in the microchannel periodic and non-periodic fluctuations in both the channel pressure drop and channel temperature profile over time are apparent. In this paper we provide a full analysis of the temperature measurements and pressure data obtained during the growth of a vapour bubble in the microchannel. An augmentation of the heat transfer coefficient of over 216% has been achieved during periodic two-phase flow boiling in the microchannel. However overpressure (over 410% increase) in the microchannel occurs at corresponding instances to the heat transfer enhancement. The two time steps during the periodic bubble dynamics, namely the bubble expansion time period and the waiting time period in-between the bubble expansion fluctuations, are also investigated and modelled. It was determined that both the bubble dynamics and the channel wall heating time period are responsible for the pressure and temperature fluctuation time periods observed.     

Influence of natural convection on forced horizontal flow in saturated porous media for aquifer thermal energy storage

Estimation of the heat recovery rate in high-temperature underground storage (>50 °C) is required before such a system can be built. However, if high-temperature water is injected into and stored in the aquifer, large-scale natural convection could occur that might reduce the heat recovery rate. This study aims to clarify the universal quantitative condition under which natural convection appears and exerts an observable influence for a system with forced horizontal flow in the saturated porous medium. The authors investigated this using both experiments and computer simulations.A test section simulating an aquifer was made. Warm water was injected into the test section, which was filled with glass beads. The temperature distribution and the flow rate profile at the outlet were measured. The authors found that the limit condition at which natural convection influences the forced horizontal flow can be determined from the velocity profile and modified Rayleigh number Ra^*. In addition, the heat transfer coefficients of the upper and the lower side of walls were estimated. A computer simulation was made for calculation of the temperature field and the velocity vector in the porous medium under natural convection and under forced convection. As calculated temperature fields and flow rate distributions at the outlet were similar to the experimental results, it is thought that this program can be applied to evaluation of the temperature and the velocity of aquifer thermal energy storage. In addition, an index expressing the degree of influence of natural convection on forced convection was proposed.     

Equilibrium low pressure generator temperatures for double-effect series flow absorption refrigeration systems

The equilibrium temperatures at the low pressure (LP) generator for double-effect series flow lithium bromide–water vapour absorption chiller are evaluated and the system performance is estimated at these temperatures. Influence of temperatures at high pressure (HP) generator, evaporator, condenser and absorber, and the effectiveness of heat exchangers on equilibrium temperature and internal heat transfer at LP generator, circulation ratio, and coefficient of performance are studied. Dual-heat mode of operation of the system is also investigated utilising low grade waste heat at the LP generator. Correlations are presented for equilibrium LP generator temperature, internal heat transfer at the low pressure generator, circulation ratio, coefficient of performance, optimum HP generator temperature and optimum circulation ratio for maximum coefficient of performance in terms of operating temperatures, which are useful in the design and control of absorption system even at the off-design conditions.     

Optimization of high-pressure shell-and-tube heat exchanger for syngas cooling in an IGCC

This work investigates the flow field and the heat transfer characteristics of a shell-and-tube heat exchanger for the cooling of syngas. Finite volume method based on FLUENT software was used and the RNG k–ε turbulence model was adopted for modeling turbulent flow. The porosity rate, the distribution of the resistance and the distribution of the heat source are introduced to FLUENT by coupling the user defined function. The pressure drop, the temperature distribution and the variation of local heat transfer are studied under the effects of the syngas components and the operating pressure, and the effect of the arrangement of the baffles on the heat transfer is studied. The results show that higher operation pressure can improve the heat transfer, however brings bigger pressure drop. The components of the syngas significantly affect the pressure drop and the heat transfer. The arrangement of the baffles influences the fluid flow.     

粗糙圆管内超临界航空煤油湍流换热特性分析

给出了国产航空煤油RP-3的三组分热物性替代燃料模型。采用k-ε湍流模型结合增强壁面处理的方法对超临界压力下航空煤油RP-3在圆形粗糙冷却通道中的流动与换热过程进行数值研究。分析了粗糙元形状、高度以及间高比等因素对其超临界流动和传热特性的影响规律,探究了人为粗糙度强化超临界航空煤油换热的机理。结果表明,人为设置粗糙元能使壁面附近产生局部回流区和旋涡结构,强化煤油与受热壁面间的对流换热。通过合理布置粗糙元结构,能大幅降低圆形冷却通道的壁面温度,有效抑制航空煤油的超临界传热恶化现象的发生。     

Effect of developing flow and thermal regime on momentum and heat transfer in micro-scale heat sink

A developing micro-channel heat transfer and fluid flow has been investigated experimentally in rectangular micro-channels of D_h = 440 μm, having water as a working fluid. Infrared technique was used to design and built a micro-channel test section that incorporate internal fluid temperature measurements. The new method that provides information about the fluid temperature distribution inside the channel and provides validation for the methods used to determine the local and average Nusselt numbers. The experimental results have been compared with theoretical predictions from the literature and results obtained by numerical modeling of the present experiment. The experimental results of pressure drop and heat transfer confirm that including the entrance effects, the conventional theory is applicable for water flow through micro-channels.These results differ from the conclusions of several researches. It was shown that data presented by some researches can be due to entrance effects. The present results highlight the importance of accounting for common phenomena that are often negligible for standard flows such as accounting for profile of inlet velocity, axial heat conduction, effect of the design inlet and outlet manifolds.This paper, to the best of knowledge, is the first presentation on the method of the bulk fluid temperature measurements along micro-channel using IR technique, and calculation of the local heat transfer coefficient based on the local heat flux and the local temperature difference between the heated wall and the bulk fluid temperature.