多层保温地下直埋热力管道传热的边界元分析

用边界元方法计算复合保温的地下直埋热力管道保温材料和土壤温度场。管道周围的土壤是半无限域,应用半无限域格林函数的基本解建立边界方程时,无需人为划定边界。在保温层内则采用无限域的基本解。计算方法可用于复合保温结构的优化设计。     

大口径供热管道直埋敷设的初步探讨

本文分析了七十年代以前热力管道直埋敷设技术没有得到发展的原因,并就直径1220×10,水温150/70℃的热力管道直埋敷设研究的几个主要技术问题进行了计算和探讨,提出热力管道直埋敷设,应用弹塑性理论进行管道强度计算,除了出土端和阀门外,全部直管段可不设补偿器,管材可采用普通A_3钢。并对管道直埋敷设进行了详细热力计算还绘出了管道周围土壤温度场图,提出热力管道直埋敷设热损失很小,优于相同条件下地沟敷设。还指出必须加强对直埋管道的防腐保温推荐采用“管中管系统,采用聚氨脂泡沫塑料保温,高密度聚乙烯作保护管。     

长距离热水输送管道保温结构热损失研究

为研究保温管道不同保温材料在埋地敷设和架空敷设时的保温情况,通过Ansys软件建模计算架空管道和直埋管道在不同保温材料下运行前期和运行稳定后热损失随时间变化的情况,模型考虑了保温材料和土壤的比热容。数据结果表明:在管道运行初期,直埋管道和架空管道的热损失几乎一致,保温效果主要取决于保温材料的导热系数。当管道运行稳定在管壁周围形成一个稳定的温度场后直埋管道的保温效果稍好于架空管道的保温效果,且随着保温材料导热系数的增加,埋地管道的保温效果越明显。在管道架空敷设时,选用导热系数小的保温材料收益更高。     

供热管道敷设方式的综合分析

本文对供热管道的三种敷设方式——直埋敷设、架空敷设和地沟敷设进行了比较,分析了各种敷设方式的技术特点、经济性及保温结构与性能,阐述了各自的特点与应用。     

大口径供热管道无补偿直埋敷设方法

由于传统供热管道敷设方法的保温效果较差,且在敷设安装过程中易出现管道质量问题,研究大口径供热管道无补偿直埋敷设方法。以某地区集中供热工程为例,利用轴向与环向应力的反作用力,确定弯头应力的变化与强度条件;基于“弯头弹性抗弯铰”解析法,设置管道最大弯臂长度;明确管道正常工作温度与最低温度,将预热温度控制在二者之间;采用焊接方式完成管道直埋敷设,并按照要求控制施工质量。结果显示：使用无补偿直埋敷设方法后,供热管道长度从0增至60 m时,管道外部最高温度不超过10℃,由此验证设计的敷设方法质量更好,保温效果更好。     

柔性石墨复合接地材料在线路-管道交叉点过电压防护中的应用研究

柔性石墨复合材料相比传统金属材料具有诸多优势。将柔性石墨复合接地材料应用于输电线路-管道交叉跨越点管道过电压防护,提出改进交叉线路杆塔接地网形状和敷设防护线两种限压方法,并对比分析柔性石墨复合接地材料与传统金属材料的防护效果。仿真计算结果表明：改进接地网形状可有效降低管道感应电压和涂层感应电压以及涂层内外电势差;在接地体外延线长度相同条件下,石墨复合接地材料和金属材料的防护性能相仿;敷设防护线可明显减少线路对管道的持续电磁干扰,且可限制线路遭受雷击时的管道过电压。相关研究结论可为油气管道绝缘层防护的材料选型与设计施工提供参考。     

基于变导热系数的蒸汽长距离输送模拟

为了提高长距离输送蒸汽参数的计算精度,建立了蒸汽热力和水力耦合计算方程,并在能量方程中考虑了蒸汽流动过程中的相态变化。对于具有多层保温结构管道的节点热阻,提出了一个导热系数随温度变化的热阻计算方法,在介质流动方向上将长距离管道保温热阻进行分段计算。通过实测数据对模型进行验证,结果表明,与将平均温度作为保温材料定性温度相比,新计算模型模拟出的蒸汽温降和压降值更小且计算精度更高,其误差分别不超过1%和4%。     

保温结构热工缺陷对管道散热的影响研究

由于施工质量、外力破损、保温材料性能等多方面原因,管道保温结构存在不同程度的热工缺陷。热工缺陷的存在导致保温结构局部外表面温度过高,散热损失过大,对整条管线热损失有一定影响。而目前所进行的管道保温效果测试与评价工作很少考虑保温结构热工缺陷的影响,导致管线散热损失计算存在一定偏差。本文介绍了管道保温结构热工缺陷位置散热损失测试与计算方法,并提出热工缺陷散热附加系数来衡量热工缺陷对管道散热损失的影响,为今后研究保温结构热工缺陷散热损失提供方法指导。     

两例高温蒸汽管道直埋工程的经验与教训

本文简要介绍了同时设计的两例高温蒸汽管道直埋敷设供热管网工程,所用的材料性能不同,保温结构形式不同,因而运行效果也不同。     

两例高温蒸汽管道直埋工程的经验与教训

本文简要介绍了同时设计的两例高温蒸汽管道直埋敷设供热管网工程，所用的材料性能不同，保温结构形式不同，因而运行效果也不同。     

Heat and mass transfer laws for fully turbulent wall flows

The general method of Izakson and Millikan for the derivation of the well-known Prandtl-Nikuradse skin friction law is applied to the analysis of turbulent heat and mass transfer in pipes, channels, and boundary layers. The formula for the heat (or mass) transfer coefficient (or the Nusselt number) is obtained which contains the dimensionless coefficients of the universal logarithmic equations for the velocity and temperature profiles as parameters of the formula. One of these parameters is a universal function of Prandtl (or Schmidt) number and all the others are constants. The existing velocity and temperature profile measurements in various turbulent wall flows permit the determination of all the necessary coefficients with fair accuracy. The resulting calculations are in satisfactory agreement with numerous experiments on heat and mass transfer in pipes and boundary layers on a flat plate over the Prandtl (or Schmidt) number range from 6 × 10⁻³ to 10⁶ and over two orders of magnitude of Reynolds (or Péclét) number variations.     

A solar water heater based on phase-changing material

A water heater utilising a material which changes phase for storage of solar energy is discussed. The transient analysis is carried out by replacing the solid-liquid block of the phase-changing material by a fictitious solid and assuming that the solid-liquid boundary remains stationary throughout the day. The efficiency of the system and the outlet water temperature during the evening hours increase with the increase in the thermal conductivity of the solid-liquid phases of the materials. Hot water can be obtained throughout the day if water pipes are placed near the surface of the storage material. The outlet water temperature curve becomes flat if the pipes are placed near the bottom of the storage material.     

Unsteady state temperature fields in a slab induced by line sources

Solutions for the unsteady state temperature fields and heat fluxes from the surfaces due to single or multiple line sources in a slab and in a semi-infinite solid are derived by using integral transform techniques. The convective boundary condition (boundary condition of the third kind) is applied at the surfaces. Responses to a step change in the coil power are given. Also a cyclic case is studied, in which power in coils is alternately switched on and off as described by a cyclic square wave function. As an application the use of a floor or a wall as storage of heat from electric cables is discussed and equations applicable for dimensioning of the heat storage system and simulation of operation are given. The temperature isotherms generated by a line source are approximately circular in the vicinity of the line source. Based on this an approximate method to calculate heat losses in steady state from single or several pipes in a slab or ground with convective heat transfer at surface is illustrated.     

Entry flow in heated curved pipes

The flow in the entrance region of heated curved pipes is analysed. Two cases of heating—a constant temperature at the wall, and a constant flux of heat at the wall—are considered. Using boundary layer approximations and the method of matched asymptotic expansions, the combined effects of curvature, entrance region and the buoyancy is studied. It is found that buoyancy disturbs the symmetric secondary motion induced by curvature, the deviation depending on the type of thermal input at the wall. It is also found that the oscillatory nature of the Nusselt number in the constant temperature case decreases as the Peclet number is increased.     

Studying the effect of the force convection boundary condition and radius magnetic field on fluid flow and heat transfer between coaxial pipes

An investigation of the heat transfer of Newtonian fluid flow through coaxial two pipes with variable radius ratio has been conducted with the boundary conditions of forced convection on the inner pipe walls and a radius magnetic field. This paper presents an exact analytical solution to the momentum equation and a novel semi-analytic collocation method for solving the full-term energy equation that takes Joule heating into account as well as viscous dissipation. Based on the results of the numerical fourth-order Runge–Kutta method, it was found that increasing the magnetic parameter decreased the amount of friction on the surface of the pipe walls and the rate of heat transfer. As the radius ratio of the two pipes increases, so does the skin friction and heat transfer rate on the internal pipe walls. As Eckert (Ec) and Prandtl (Pr) numbers increase, the mean temperature as well as the dimensionless temperature between the two pipes increases. The increase in Biot number (Bi) has the opposite impact on the mean temperature. As Ec, Pr, and Bi increase, so does the rate of heat transfer on the inner wall of the pipe.     

Development of a numerical model for the simulation of vertical U-tube ground heat exchangers

Vertical U-tube ground heat exchangers are a key component in geothermal energy utilization systems like ground source heat pumps (GSHPs). This paper presents a three-dimensional unstructured finite volume model for them. The model uses Delaunay triangulation method to mesh the cross-section domain of the borefield (borehole field), and consequently may intactly retain the geometric structure in the borehole. To further improve the computational accuracy, the soil is divided into many layers in the vertical direction in order to account for the effect of changing fluid temperature with depth on the thermal process in the borefield. The inlet temperature of the ground heat exchanger (GHE) is used as a boundary condition, and the inside and outside surfaces of the U-tube pipes are treated as the conjugated interfaces in the domain. Thus, the conjugate thermal processes between the fluid in the pipes and the soil around it and between the two pipe legs may be accounted fully. A comparison of the model predictions and experimental data shows that the model has good prediction accuracy.     

A numerical study of flow and heat transfer in internally finned rotating straight pipes and stationary curved pipes

In this article the effects of internal fins on laminar incompressible fluid flow and heat transfer inside rotating straight pipes and stationary curved pipes are numerically studied under hydrodynamically and thermally fully developed conditions. The fins are assumed to have negligible thickness with the same conditions as the pipe walls. Two cases, constant wall temperature and constant heat flux at the wall, are considered. First the accuracy of the numerical code written by a finite volume method based on SIMPLE algorithm is verified by the available data for the finless rotating straight pipes and stationary curved pipes, and then, the numerical results for those internally finned pipes are investigated in detail. The numerical results for different sizes and numbers of internal fins indicate that the flow and temperature field analogy between internally finned rotating straight pipes and stationary curved pipes still prevail. The effects of Dean number (K_L) versus friction factor, Nusselt number, and other non-dimensional parameters are studied in detail. From the numerical results obtained, an optimum fin height about 0.8 of pipe radius is determined for Dean numbers less than 100. At this optimum value, the heat transfer enhancement is maximum, and the heat transfer coefficient appears to be 6 times as that of corresponding finless pipes.     

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。     

On transient heat losses from buried district heating pipes

The theory of steady‐state heat loss determination from buried heating pipes has been reviewed as well as previous approaches to treat transient heat losses in the case of constant water temperatures. A new method has been developed to find an undisturbed ground temperature by which the transient heat loss can be calculated by the use of the steady‐state heat loss equations. By numerical simulations, as well as by experiment, the position of this undisturbed ground temperature has been found. The position of this ground temperature is closer to the ground surface in the case of uninsulated pipes—or pipes with the insulation in poor condition—than in the case of insulated pipes. For pre‐insulated pipes the position corresponds approximately to the top of the pipe casing. Copyright © 2000 John Wiley & Sons, Ltd.     

柴油机换气过程缸内压力模拟计算

本文建立了内燃机换气过程缸内压力模拟计算的数学模型。其特点是缸内过程由开口系控制方程控制,进排气管内的过程由特征线法进行计算,进排气管与气缸之间由相应的边界方程相联系。通过实例计算,证明计算结果与实测值能较好地吻合。应用本文提出的方法,可以预测柴油机的换气损失等参数以及结构参数变化对换气过程的影响