A new general model for phase-change heat transfer of waxy crude oil during the ambient-induced cooling process

The thermal process during shutdown (a stoppage state of the pipeline), of which the essence is an irregular phase-change process accompanied by natural convection, non-Newtonian behavior, and sometimes turbulence, is a critical problem in crude oil transportation engineering. An accurate calculation of the thermal process during shutdown is more than necessary for the safety of crude oil pipeline; however, it faces some challenges due to the complexity of the phase change. In this study, the phase change of waxy crude oil during the cooling process is divided into four stages, which includes a pure liquid natural convection, solid/liquid dispersion natural convection, coexistence of dispersion system natural convection and porous media natural convection, and pure porous media convection, according to different heat transfer mechanisms on different stages. Based on this division, a general phase-change heat transfer model is proposed for the thermal calculation of waxy crude oil during shutdown. Compared with the previous research, this model appropriately includes the influences of non-Newtonian behavior, phase evolution as well as turbulence. With the proposed model, the temperature drop characteristic of a sample pipeline is analyzed and the influencing factors are investigated.     

双水平井SAGD预热阶段温度场计算

采用满足质量守恒的流函数方法,编制有限容积法程序,模拟计算了双水平井蒸汽辅助重力泄油(steam assisted gravity draining,SAGD)蒸汽循环启动预热阶段温度场演化情况。对比了纯导热与导热及热对流共同作用下所需预热时间的差别,给出了单位长度井散热速度和两井中间位置温度随时间的变化,同时,也分析了井间距对预热时间的影响。结果表明,单位井长度的散热速度随时间的延长逐渐减少,在初始阶段变化较快,然后以较慢的速度降低。这表明,以井筒恒热流边界条件的温度叠加原理分析方法,建立预测阶段温度分布模型,预测热联通需要的加热时间是不合适的。不同井间距工况预热阶段数值计算表明,影响预热时间的主要因素为两井间距,间距扩大一倍,需要的预热时间延长约为原来的4倍;原油黏度很大且随温度变化不太敏感或井间距较小的情况,自然对流换热作用很弱,在预测预热时间时可以按照纯导热模型处理,而相对不太黏稠的原油或井间距较大时热对流作用强,不能忽略对流传热的影响,自然对流换热可大大缩短预热时间。     

埋地原油管道停输期间温降及原油凝固传热模型及数值模拟

假设原油凝固区域为一固相和液相组成的动态多孔介质区域,建立了土壤、管道能量方程与原油质量、动量和能量方程相互耦合的传热模型,并对埋地原油管道停输温降过程进行了数值模拟.数值模拟结果能够合理解释停输期间温度场、凝固界面和自然对流规律.     

原油管输不可逆过程火用的传递和转换规律研究

基于不可逆过程热力学的基本理论,探讨了原油管输过程各不可逆过程的强度量势场梯度驱动力作用下的火用流传递现象,结合原油管道输送过程的基本场平衡方程组和普遍化热力学体系的火用平衡动力学方程,推导了管输过程的热火用和压火用传递和转换方程,进一步建立管输过程的总火用传递方程。以某原油管道为例,对管道输送过程进行数值分析得到管输过程中热火用和压火用的传递和转换火用流分布规律,为将火用的传递转换规律应用到管道节能体系中提供理论基础和合理依据。     

A numerical investigation into the charge behaviour of a spherical phase change material particle for high temperature thermal energy storage in packed beds

基于高温相变材料,对填充床储热系统中储热单元球体的储热性能进行了模拟研究.研究了不同传热流体温度和球体直径对球体储热性能的影响规律,对导热为主的相变储热过程与导热和自然对流共同作用的相变储热过程进行了比较分析,同时还探讨了高温辐射换热的影响.结果表明,相变时间随球体直径的增大而增大,随传热流体温度的增大而减小.当考虑相变区域自然对流时,总的相变时间显著减少,和单纯导热相比,完全相变时间缩短了近16%.在导热和自然对流的基础上加上辐射传热后可以看出,辐射换热强化了球体内的传热过程,加快了相变材料的熔化速度,强化了自然对流的作用.     

稠油管输节能技术研究与应用

原油管道运输量大,费用低,是最主要的输送方式。稠油管输方式及设备研究表明,密闭输送方式实现了在不与大气接触的状态下输送,不仅大大减少了原油轻组分的挥发,还可节约输送动能,适宜长距离稠油输送。螺杆泵运送液体的种类和黏度范围宽广,具有结构简单、维修方便、体积小、流量稳定、压力高、耐高温等优点。变频调速装置可实现无极调速,调速范围大、平滑性好,动态响应快,便于自动化调节,能大量节省电能,适用于长输管道稠油输送。对于稠油的管道输送,推荐采用相变加热炉形式,其热效率通常达到85%以上,节能效果明显。聚氨酯硬质泡沫塑料导热系数低,具有安装简便、工程造价低、施工周期短等优点,每100m降温为0.1℃,每100m热损失率为2.07%。适宜长输管道保温。辽河油田锦-沥输油管道应用以上工艺进行稠油输送,防腐保温效果明显,每年可节省天然气消耗176×104m3,每年节省管道运行费用约200万元。     

Numerical analysis of unsteady heat conduction in regular solid bodies comprising natural convection to nearby fluids

The present paper addresses unsteady, unidirectional heat conduction in regular solid bodies (vertical plate, horizontal cylinder, and sphere) that exchange heat by natural convection with a neighboring fluid. From thermal physics, natural convection constitutes a worst-case scenario for forced convection cooling. Under the premises of natural convection heat transfer, the unsteady, 1-dimensional heat conduction equation consists in a linear parabolic partial differential equation with a dominant natural convection boundary condition represented by the mean convective coefficient that depends upon temperature. As expected, the nonlinear unsteady, unidirectional heat conduction problem is complex and does not admit an exact, analytical solution. Instead, the nonlinear unsteady, unidirectional heat conduction problem forcibly necessitates approximate numerical treatment with the finite difference method. The computed dimensionless center, surface, and mean temperatures varying with dimensionless time are obtained numerically and are graphed for 3 solids: iron, aluminum, copper exposed to 3 fluids: air, water, oil; the 6 media are used in numerous engineering applications.     

Further study on the thermal characteristic of a buried waxy crude oil pipeline during its cooling process after a shutdown

Since the temperature drop of waxy crude oil after a shutdown determines whether the pipeline is able to restart successfully or not, it is necessary to calculate the temperature drop and clarify the characteristic of the thermal process of waxy crude oil pipeline after the shutdown. However, the relevant techniques proposed in the previous researches for this calculation are not accurate enough, due to the complex phase change, non-Newtonian behavior of the fluid, and the transition of different heat transfer mechanisms involved within the physical problem. Therefore, in a companion piece to this paper, a general and accurate mathematical model was proposed for the phase-change heat transfer of waxy crude oil. In this paper, the mathematical model of the waxy crude oil pipeline system after its shutdown is established, based on the phase-change heat transfer model proposed in the companion piece, and the numerical procedure is established for the calculation of the model. With the proposed techniques, the thermal process of the shutdown of waxy crude oil is investigated in detail, and the temperature drop characteristic is clarified on the level of heat transfer mechanism. The research will provide theoretical support for the establishment of shutdown scheme and thermal preservation method for waxy crude pipeline.     

原油管道伴热工艺热力分析

根据原油管道伴热方式,建立原油管道伴热系统能量平衡模型,对管道伴热系统进行数值模拟,计算原油和伴热水的沿程温度变化,并对比分析不同伴热方式下的伴热效果,根据管线长度,优化选择伴热方式。     

A direct three-dimensional simulation of radiation-induced natural convection in a shallow wedge

A three-dimensional numerical simulation is carried out to investigate the natural convection in a shallow wedge subject to solar radiation. The study reveals three distinct stages of the flow development from an isothermal and stationary state: an initial stage, a transitional stage and a quasi-steady stage. The heat transfer at the initial stage is dominated by conduction from the sloping bottom. The transitional stage starts with the onset of instabilities, and the quasi-steady state is characterised by steady growth of a spatially averaged temperature. The present results have confirmed the earlier observations in a flow visualisation experiment and a two-dimensional simulation.     

Investigation of the influence of free convection on the heat transfer of cylinders with different aspect ratios

IntroductionThe heat trallsfer from Hot-Wires in low-velocityforced flows can be significantly influenced by buoyancy effects. In Order to quantify this buoyant contribuhon tO the total heat trallsfer, experiments underweighhessness were compared with laboratory measurementS. The ndcrogravity ig measurementS havebeen performed in the DrOP TOwer Bremen whichprovides a reduced gravity level of 10-5 g during aperiod Of 4.7 s. In this work tWo wires with differentaspect ratios F==l/d, where …     

壳管式相变储能换热器性能研究与场协同效应分析

建立壳管式相变储能换热器仿真模型,利用Fluent软件对蓄能、释能工况分别进行模拟。从温度场、速度场、固液交界面3个方面分析逐时换热强度的变化原因;探讨在蓄热过程中热源管壁温、热管口径规格对传热效率的影响。研究结果表明:蓄热工况下管间热扰及自然对流作用明显,10000 s时液相分数达到95%。放热工况以导热为主,60000 s时相变材料仅凝固了75%。从场协同效应分析,采用小尺寸热源管能缩短时间,但单位面积换热强度有明显下降。换热温差≤20℃时,其强化效果开始衰弱。     

Limitations of the lumped model for in‐tube laminar forced convection interacting with cross forced convection flows

The short communication addresses forced convection heat transfer of a viscous fluid flowing in a tube with fully developed laminar velocity and uniform entrance temperature that exchanges heat convection with a surrounding fluid. The idea is to implement the lumped model following the footsteps of the potent lumped model within unsteady heat conduction, instead of the differential model. The computed mean bulk temperatures of the viscous fluid expressed by a simple exponential function agrees well with the baseline numerical mean bulk temperatures when the internal fluid is air, water or crude oil and the external fluid is air for small values of the modified Biot numbers and maximum Reynolds numbers are satisfied.     

Transient heat transfer in a heat‐generating fin with radiation and convection with temperature‐dependent heat transfer coefficient

The transient heat transfer in a heat‐generating fin with simultaneous surface convection and radiation is studied numerically for a step change in base temperature. The convection heat transfer coefficient is assumed to be a power law function of the local temperature difference between the fin and its surrounding fluid. The values of the power exponent n are chosen to include simulation of natural convection (laminar and turbulent) and nucleate boiling among other convective heat transfer modes. The fin is assumed to have uniform internal heat generation. The transient response of the fin depends on the convection‐conduction parameter, radiation‐conduction parameter, heat generation parameter, power exponent, and the dimensionless sink temperature. The instantaneous heat transfer characteristics such as the base heat transfer, surface heat loss, and energy stored are reported for a range of values of these parameters. When the internal heat generation exceeds a threshold the fin acts as a heat sink instead of a heat source. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21012     

Freezing of Water in a Slab with Boundary Conditions of the Third Kind

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Analysis of the influence of operating conditions on fouling rates in fired heaters

Fouling due to chemical reaction in preheat trains for the processing of crude oil plays a key role in the operation and maintenance costs and on greenhouse emissions to atmosphere in crude processing plants. A preheat train consists of a set of heat transfer units that provide the crude oil stream the required amount of thermal energy to reach its target temperature either by heat recovery or by direct firing. Fired heaters supply external high temperature heating through the burning of fuel which result in complex heat transfer processes due to the large temperature and pressure changes and vaporization that takes place inside the unit. In this work, a thermo-hydraulic analysis of the performance of fired heaters is carried out through the application of commercial software to solve the mathematical models using finite difference methods; the analysis is applied to the crude side of a vertical fired heater in order to evaluate the impact of process conditions such as throughput and crude inlet temperature (CIT) on the fouling that take place at the early stages of operation. Using a fouling rate model based on thermo-hydraulic parameters, fouling rates are predicted assuming steady state operation and clean conditions. Although variations in process conditions are known to influence fouling rates, little work has been done on the subject. In this work excess air and steam injection are studied as a means to mitigate fouling. Results show that throughput reduction brings about a marked increase in the fouling rates. A decrease in CIT affects only the convection zone and it is found that this effect is negligible. In terms of excess air, it is found that although it affects negatively the heater efficiency it can be used to balance heat transfer between the convection and radiation zone in a way that fouling rates are reduced; however this strategy should be considered right from the design stage. Finally it is observed that steam injection is an effective method to reduce fouling rates since it results in lower film temperatures and larger shear stress.     

回填土质对埋地热油管道传热影响的研究

针对不同导热系数的土样回填管沟,忽略管道轴向温降建立二维非稳态传热模型,环境温度采用周期性边界条件。利用有限容积法,对方程进行离散,数值模拟了管道投产第一年内正常运行非稳态传热过程,同时应用"焓-多孔度"技术,并考虑原油凝固潜热对温降的影响,数值模拟了管道停输非稳态温降过程。结果表明:采用导热系数相对较小的土质回填管沟,管壁及保温层外壁热流密度明显减小,而且回填区温度较高散热较慢,由计算的安全停输时间可知,回填土质导热系数越小安全停输时间越长,模拟结果符合实际。     

Numerical investigation of the thermal mechanism of the solid-liquid phase changing process

由于相变换热储能技术可以协调能量在时间和空间尺度的分配,成为了目前研究的热点问题。本工作用焓值法分别对充填低温无机盐相变材料的二维和三维管壳式相变储能换热器模型的储/放热特性进行了模拟研究,采用Boussinesq近似研究了液相区密度变化引起的自然对流的影响。研究表明换热器的入口温度对相变换热效率影响显著;在储热过程中自然对流发挥了重要作用,换热效率与液相区的运动状态直接相关,而放热过程中的热交换主要依靠热传导完成;三维模拟的结果表明换热管出口温度与管壁的平均努赛尔数高度相关,且换热管水平放置的换热效率略低于竖直放置。     

Role of tree-shaped fins in charging performance of a latent heat storage unit

The performance enhancement of latent heat storage (LHS) units is of great consequence for the development of sustainable energy. In this article, the transient models of phase-change heat transfer processes in LHS units with consideration of natural convection are developed and numerically analyzed, in an effort to explore the role of the fractal tree-shaped fin in the energy charging performance. The solid-liquid interface evolution and dynamic temperature response in the tree-shaped finned LHS unit are presented and compared with the wheel-shaped one. Moreover, the influence of the fin number is investigated for maximizing the energy charging performance. The results indicate that the tree-shaped fin has merits of effective layout optimization for the point-to-area heat transfer and the time-coordination of energy charging and discharging process. Compared with the wheel-shaped fin, the melting duration time is decreased only a little for the presence of tree-shaped fin, however, the solidification process is decreased significantly. The presence of tree-shaped fin leads to a lower energy charging rate during the early and middle stages of the charging process due to the natural convection suppression, however, the energy charging rate is faster during the later stage due to the thermal conduction dominated interior heat transfer. For maximizing the melting performance, the appropriate fin number in practical applications is 16.     

环状掺水工艺降温集输试验技术研究

大庆油田外围低产、低渗透油藏原油集输物性差,油井产量低,单位产能建设投资高,集输能耗大。其中,油气集输环节分别由集油、脱水、原油稳定处理及储运能耗4部分组成。各部分中,集油部分的能耗更是占到集输系统总能耗的60%～80%,设法降低集油环节的能耗是集输流程节能的关键。对此,开展了确定安全混输温度界限的室内外试验,实现原油在凝固点附近安全运行。根据中高含水期油田开发的特点,对高产液、高含水的集油环开展不加热集输试验。试验区试验结果表明,原油含水率达到并超过转相点后,其流动阻力明显下降,有利于低温集输;对于部分产液量大于20t/d、含水率大于80%的集油环,可采取季节性停掺冷输工艺;对于长期低温运行的集油环出现的开井困难情况,可采取管线热洗、通球物理清洗,增加管道泵、混输泵或工艺改造等措施加以解决;对于污水处理反冲洗效果变差的情况,需要加强适合低温处理药剂的研究与应用。