冲击热负荷作用下垂直自然循环传热特性的试验研究

鉴于对炼钢转炉锅炉研究的重要意义,设计了一套简单、可靠的试验系统,对炼钢转炉余热锅炉周期性的不稳定传热过程进行试验模拟。对其在冲击热负荷下垂直自然循环的传热特性进行了深入的试验研究,得到其传热特性的一般变化规律,并分析了冲击热负荷和工质欠焓对其传热特性的影响,通过回归试验数据得到传热系数的准则关系式。对改善炼钢转炉余热锅炉的传热特性和水循环安全性提出了建议。     

自然循环在冲击热负荷作用下的流动特性实验研究

冲击热负荷条件下的自然循环是工程中常见的汽液两相流动与传热过程 ,同时也是一个复杂的非稳态过程。转炉余热锅炉是一种典型的运行于冲击热负荷下的大型换热设备 ,冲击热负荷严重地影响着其水循环流动特性 ,从而影响它的寿命。在实验模拟氧气顶吹炼钢转炉余热锅炉的实际工作过程的基础上 ,研究了冲击热负荷、压力和初始速度等因素对不稳定传热和流动的影响。实验结果表明 ,自然循环在受到冲击热负荷后 ,水循环流量迅速达到最大值 ,呈现较快的响应速度 ,随后在最大流量附近脉动。循环流量的主要影响因素是冲击热负荷的强度 ,而压力和初始速度对循环流量的影响则较小。     

联合循环参数和系统结构优化研究

研究了如何提高余热锅炉型三压再热联合循环系统的效率,应用分析的方法建立了系统效率数学模型,以联合循环系统效率最高作为系统性能的评判标准。在亚临界范围内,对余热锅炉的蒸汽参数进行了优化;针对余热锅炉进气温度对余热锅炉性能的影响进行分析,在此基础上提出燃气轮机排气部分回热利用,并研究了回热利用对联合循环效率的影响。计算结果表明:经余热锅炉优化和排气部分回热利用,在基本负荷下,PG9351FA机组的联合循环热效率可提高1.33%;在75%和50%的负荷下,效率分别提高2.11%和4.17%;而具有再热的GT26机组热效率高达60.73%。     

余热锅炉汽包壁温差控制策略

阐述燃气-蒸汽联合循环启停操作过程中汽包壁温差大的现象，从余热锅炉汽包结构特点和启停过程 传热特性入手，详 热锅炉汽包 的原因，找到热锅炉负荷 汽包 大的 影 响因素。 运 优化操作包括 热锅炉汽包 、改汽包升温升 率、加快热锅炉 ， 停 汽包上、下 大，对于优化余热锅炉启停操 制汽包上、下.差 的燃气 电厂有借鉴 。     

水泥余热锅炉低负荷下排烟温度过高的原因分析

针对水泥余热锅炉低负荷下排烟温度过高的问题,通过锅炉热量平衡计算和数值模拟的方法,对存在该问题的日产水泥5 000 t的水泥厂余热锅炉进行分析.利用测试机构的测试数据、余热锅炉烟风系统图、热力系统图、余热锅炉建模和数值模拟计算结果,发现锅炉炉膛内实际流动速度过低是造成低负荷下排烟温度过高的原因.     

转炉余热锅炉水套中的复合循环

炼钢转炉余热锅炉的洼口烟道(水套)最易受到具有侵蚀性的金属胶状喷渣和急剧变化的热负荷(局部达到600千瓦/米~2)的影响。因此人们制造了作为转炉余热锅炉(OKT)组体的水套并又将这种水套设计成多次强制循环式的水套。 为了使冷却介质能沿水套管壁均匀分布,对水套入口段进行了节流,并决定采用10HKY—7或HKY—630/80型高压循环泵(0.7～0.8兆帕)。例如,为了冷却180吨炼钢转炉水套(属于OKT—180锅炉)的受热面,要求安装两台工作循环泵和一台备用循环泵。每台泵     

余热锅炉鳍片管受热面传热特性

文中介绍了在一热态试验台上进行余热锅炉鳍片管受热面传热特性试验研究的结果,对于大型余热锅炉的设计和运用具有重要意义。     

600MW高水分褐煤锅炉变负荷特性试验

由于高水分褐煤的低发热量和易结渣特性,锅炉一般采用非常低的炉膛容积热负荷和非常低的炉膛断面热负荷.该类型的褐煤锅炉变负荷速率低于普通烟煤锅炉.进行了一次风压、燃烧器负荷分配、二次风配风调整等对锅炉变负荷影响的试验.试验结果表明:在机组需要快速升负荷时,先提高一次风压,然后增加给煤量可以提高锅炉的升负荷速率.如果锅炉的氧...     

镇海300MW联合循环电站余热锅炉调试问题的探讨

介绍了镇海300MW燃气－蒸汽联合循五发电工程两台余热锅炉在调试过程中发现的问题及其解决措拖,如高压循环泵、狡热锅炉水位定值问题以及燃气轮机负荷升隆速率对余热锅炉影响等。     

焚烧炉—余热锅炉系统不同负荷下运行能效与分析

针对现役垃圾焚烧发电机组效率低、能耗大的技术难题,现场测试不同负荷下机组主要运行参数,定量分析不同负荷下焚烧炉—余热锅炉能效及热损失分布,根据不同负荷下能效计算结果分析机组存在问题的根本原因。结果表明：100%最大连续出力(MCR)、85%MCR及75%MCR负荷下,焚烧炉—余热锅炉效率均较低,分别为77.44%、76.85%及76.59%,造成焚烧炉—余热锅炉效率偏低的主要原因为排烟温度过高,导致排烟热损失较大。100%MCR、85%MCR及75%MCR负荷下,系统总热量中排烟热损失所占比例分别为19.12%、19.68%、19.76%。针对排烟温度过高的问题,现场测试机组尾部换热设备性能,结果表明：由于机组运行年限较长,机组尾部换热设备换热性能变差。不同负荷下过热器的余热利用率仅为34.91%～37.84%,100%负荷下,实测过热器的余热利用率比设计值低4.58%,实测省煤器的余热利用率较设计值低12.64%。研究结果对焚烧炉—余热锅炉系统运行优化及尾部换热设备的改造具有参考意义。     

Unsteady conjugated heat transfer in laminar pipe flows with step change in ambient temperature

This study aims to numerically investigate the effects of wall heat capacity on the unsteady forced convection heat transfer in the flow in a long circular pipe resulting from a step change in the ambient temperature. Two non-dimensional parameters, Nu_o and a^*, are identified in the study, and their effects on the non-dimensional duct wall heat flux and the time evolution of temperature field are discussed in great detail. Comparisons are made with zero thermal capacity wall solution. It is found that the wall heat capacity has decisive impact on the unsteady heat transfer in the flow.     

On the role of the fluid effusivity in unsteady heat transfer between a boundary layer and a solid wall

The problem of obtaining the wall heat flux in the presence of unsteady heat transfer in two-dimensional, turbulent boundary layer flow is re-examined. A novel expression to produce estimates of the amplitude of the fluctuating wall heat flux has been proposed for the foregoing conditions. This expression is based on flow field measurements instead of measurements in the solid wall substrate, thus allowing us to take the flow dynamics directly into account in the analysis. The fluid effusivity, a measure of the ability of the fluid to exchange thermal energy with its surroundings, was shown to be the dominant parameter controlling the unsteady heat transfer process.     

Unsteady Natural Convective Boiling in a Narrow Vertical Channel

The purpose of this work is to describe the two-phase flow structure and heat transfer of unsteady natural convective boiling in a narrow vertical channel. The experiments are performed with saturated n-pentane at a pressure of 1 bar. An unheated plate is placed parallel to the heating surface and lateral sides are closed. The distance between the heated surface and the confinement plate is 800 μm. Void fraction measurements are performed using capacitive sensors. The void fraction increases with heat flux and reaches a maximum of 0.80 in the mid-height of the channel when the heat flux is equal to 90% of the critical heat flux. Flow observations using a high-speed video camera show an unsteady thermo-hydraulic behavior. The frequency of the cycles increases with the wall temperature during nucleate and transient boiling. Local velocities of the bubble meniscus developing within the confined space are determined during the boiling cycles. The time-averaged liquid flow rate increases significantly with heat flux and reaches a maximum for heat flux close to the critical heat flux.     

Heat transfer reduction at the separation point on a spinning sphere in mixed convection

The unsteady laminar thermal boundary‐layer flow over an impulsively started translating and spinning isothermal body of revolution in the case of mixed convection is investigated. Velocity components and temperature are obtained as series of functions in powers of time. The general results are applied to a spinning sphere and the development of the surface heat flux evaluated at the separation point as it advances upstream is determined. The surface heat flux evaluated at the separation point as it moves forward decreases due to the increasing magnitude and influence of the centrifugal force and it is augmented by the opposing flow and reduced by the aiding flow. Reduction of the surface heat flux at the separation point is as low as 50 per cent as compared to the heat flux at the front stagnation point. Copyright © 2002 John Wiley & Sons, Ltd.     

Flow due to a porous stretching/shrinking sheet with thermal radiation and mass transpiration

This study investigates the behavior of carbon nanotubes (CNT) approaching an unsteady flow of a Newtonian fluid over a stagnation point on a stretching surface employing porous media. It flows when the liquid begins to move with the progression of time. Heat exchange with the environment has an impact on the flow. The implicitly limited component technique is used to solve the nondimensional partial differential equation with an associated boundary layer, which is an unstable system. Analytically, the solutions, as well as the required boundary conditions, are obtained. The effects of mass transpiration, volume fraction, and heat radiation on Newtonian fluid flow through porous media are explored. Single- and multi-walled CNTs are used as well as water, as base fluids in the experiment. The impact of thermal radiation and heat source/sink is shown in the energy equation, which is solved under four different cases: uniform heat flux case, constant wall temperature case, general power-law wall heat flux case, and general power-law wall temperature case. By supplying distinct physical characteristics, a theoretical analysis of the existence and nonexistence of unique and dual solutions may be explored. These physical parameters determine the velocity distribution and temperature distribution. Prescribed surface temperature (PST) and prescribed wall heat flux (PHF) heat transfer solutions can be written using confluent hypergeometric equations, and generic power-law PST and PHF situations can also be expressed using confluent hypergeometric equations. The graphical representations assist in the discussion of the current study's findings.     

HEAT AND FLUID FLOW ACROSS A SQUARE CYLINDER IN THE TWO-DIMENSIONAL LAMINAR FLOW REGIME

The flow structure and heat transfer characteristics of an isolated square cylinder in cross flow are investigated numerically for both steady and unsteady periodic laminar flow in the two-dimensional regime, for Reynolds numbers of 1 to 160 and a Prandtl number of 0.7. The effect of vortex shedding on the isotherm patterns and heat transfer from the cylinder is discussed. Heat transfer correlations between Nusselt number and Reynolds number are presented for uniform heat flux and constant cylinder temperature boundary conditions.     

Unsteady mixed convection flow in the stagnation region of a heated vertical plate due to impulsive motion

The unsteady mixed convection in the stagnation flow on a heated vertical plate is studied where the unsteadiness is caused by the impulsive motion of the free stream velocity and by sudden increase in the surface temperature (heat flux). The short time as well as the long time solutions are included in the analysis. Both prescribed surface temperature and prescribed surface heat flux conditions are considered. The partial differential equations governing the flow and the heat transfer have been solved numerically using an implicit finite difference scheme. Also, the asymptotic behaviour of the solution for large value of the independent variable is examined when the flow becomes steady. There is a smooth transition from the small-time solution to the large-time solution. The surface shear stress and the heat transfer increase with time and buoyancy parameter. The heat transfer increases with the Prandtl number, but the surface shear stress decreases.     

Experimental study of unsteady convective boiling in heated minichannels

Convective boiling in narrow channels may under specific conditions display an unsteady behavior. An experimental set-up has been elaborated to investigate heat and mass transfer and to analyze two-phase flow instabilities in rectangular microchannels with a hydraulic diameter of 889 μm. Depending on the operating conditions two types of behavior are observed: a steady state characterized by pressure drop fluctuations with low amplitudes (from 0.5 to 5 kPa/m) and no characteristic frequency; a non-stationary state of two-phase flow. The pressure signals exhibit fluctuations with high amplitudes (from 20 to 100 kPa/m) and frequencies ranging from 3.6 to 6.6 Hz. Steady and unsteady thermo-hydraulic behaviors depending on the two control parameters (heat flux and mass velocity) are analyzed and given in this paper.     

Impact of viscous dissipation and Dufour on MHD natural convective flow past an accelerated vertical plate with Hall current

The present research work concentrates on viscous dissipation, Dufour, and heat source on an unsteady magnetohydrodynamics natural convective flow of a viscous, incompressible, and electrically conducting fluid past an exponentially accelerated infinite vertical plate in the existence of a strong magnetic field. The presence of the Hall current induces a secondary flow in the problem. The distinguishing features of viscous dissipation and heat flux produced due to gradient of concentration included in the model along with heat source as they are known to arise in thermal-magnetic polymeric processing. The flow equations are discretized implicitly using the finite difference method and solved using MATLAB fsolve routine. Numerical values of the primary and secondary velocities, temperature, concentration, skin friction, Nusselt number, and Sherwood number are illustrated and presented via graphs and tables for various pertinent parametric values. The Dufour effect was observed to strengthen the velocity and temperature profile in the flow domain. In contrast, due to the impact of viscous dissipation, the local Nusselt number reduces. The study also reveals that the inclusion of the chemical reaction term augments the mass transfer rate and diminishes the heat transfer rate at the plate.