CFD simulation of transverse vibration effects on radial temperature profile and thermal entrance length in laminar flow

Radial heat transfer in laminar pipe flow is characterized by a wide temperature distribution over the pipe cross‐section. We use a validated Computational Fluid Dynamics (CFD) model to show that the superimposition of a transverse vibration on the steady laminar flow of a Newtonian fluid moving in a pipe with an isothermal wall, generates considerable chaotic flow and radial mixing which result in a large enhancement in wall heat transfer as well as a considerably more uniform radial temperature field. Transverse vibration also causes the temperature profile to develop very rapidly in the axial direction reducing the thermal entrance length by a large factor. These effects are dependent on vibration amplitude and frequency, and fluid viscosity. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

回路热管微通道换热器蒸发冷却实验

采用微通道换热器作为数据中心回路热管冷却换热器,在微通道冷凝器侧进行了蒸发冷却实验。实验利用焓差平台测量了不同因素对采用蒸发冷却技术的微通道冷凝器换热影响,并分析了室内外不同温差、室外不同湿度条件下采用蒸发冷却的微通道冷凝器及蒸发器的进出口空气温差。实验结果表明：采用蒸发冷却可使冷凝器入口空气温度实现大幅温降,并增加冷凝器及蒸发器的进出口空气温度差值,喷淋后可使流经热管空气降温0.3～1.9℃,增大2%～20%的系统换热量,冷凝器换热效率得以提升,并可扩大回路热管换热器使用地区,延长回路热管年利用时间。     

新型外分液结构调控水平管间歇流流型

相变传热广泛存在于换热领域,而两相流中的厚液膜是恶化相变传热的根本原因。提出在换热管壁面设计新型网状外分液结构,利用金属网进行间歇流气液非能动相分离,从而调控流型、强化传热。通过搭建空气-水水平管冷态实验台,证实当间歇流经过外分液结构时,气体在表面张力作用下被金属网孔拦截,液桥和厚液膜则被分离出换热管,使管内液相减少,气相汇聚,有效增大气相与管壁的接触概率;多级分液后气液两相长度比呈振荡上升,甚至将间歇流调控为分层流。减小液速或增大气速,分液效果越好,间歇流最终转变为分层流所需经历的外分液级数越少,预期此结构可显著提高相变传热效果。     

单环路液氢温区脉动热管高充液率工况计算流体动力学（CFD）模拟

针对液氢温区单环路脉动热管建立了二维数值模型,分析了80%充液率条件下脉动热管不同阶段的流动及传热特性。在本研究中,流体体积函数（VOF）方法被用于追踪气液相界面,恒热通量和恒温分别作为蒸发段和冷凝段的边界条件,蒸发段的加热量逐渐从0.27W增加到1W。模拟的传热热阻与实验值的误差不超过15%,验证了模型的有效性。同时得到了脉动热管初始阶段的气液分布与压力分布,通过气体体积分数云图分析了启动阶段气塞的运动,结果显示在本研究中工质经过两次循环完成启动,根据工质流动方向的变化每次循环又可以分为三种流动方式。进入稳定阶段后,脉动热管内的工质主要呈顺时针循环流动,蒸发段的壁面温度呈周期性的振荡。在加热量较低时,温度振荡频率较低,传热热阻较大;随着加热量的增加,振荡频率增加并趋于稳定,热阻先减小而后不变。     

On turbulent pipe flow with heat transfer and chemical reaction

A theoretical model has been developed to describe heat transfer to a fluid which is flowing turbulently in a pipe and within which a first-order endothermic irreversible chemical reaction is taking place. It is based on the penetration model for the fluid boundary layer close to the pipe wall and enables the calculation of the effect of chemical reaction on the heat transfer coefficient of turbulent pipe flow.The model has been checked against experimental results for flow distribution and heat transfer in turbulent pipe flow without chemical reaction. Good agreement between the calculated and experimental results was found. The measured heat transfer coefficients for turbulent pipe flow with a uniformly distributed heat source within the fluid were also well predicted. Unfortunately no accurate experimental data on the effect of a first-order chemical reaction on the heat transfer coefficient of turbulent pipe flow are available. However, calculations made with the model show that this effect can be considerable and thus may not always be neglected in practice.     

振荡热管内不同形态纳米颗粒流动及传热特性

主要针对不同形态的纳米颗粒在振荡热管内的流动及热传输特性进行了实验研究。在相同的压力、相同的热管倾角、不同的充液率条件下,对振荡热管内工质分别为Cu-水纳米流体以及Cu-水纳米流体中Cu纳米颗粒沉积后溶液的流动以及热传输特性进行了实验研究,并与工质为蒸馏水时进行了对比实验分析,以此来研究振荡热管内气、液以及纳米颗粒多相流动存在时,对热管传输特性的影响。实验表明：当振荡热管内存在气、液以及不同形态的纳米颗粒多相流动时,对其传热特性会产生很大的影响,在一定条件下会起到强化传热的作用。     

Coating the inner surfaces of pipes with high-viscosity epoxy in annular flow

Applying a thin, protective coating of a nontoxic, chemically resistant epoxy to the interior of existing pipes is an alternative method to pipe replacement. In order to find the controlling parameters in this method, in this study, viscous epoxy was propelled by compressed air through clear polyvinyl chloride (PVC) pipes. Epoxy flow was annular, and it hardened to form a thin, uniform coating on the inner pipe surface. A video camera was employed to record fluid motion, and the thickness of the coating was measured using an image analysis program named ImagJ. Tests were done with varying air temperature, airflow rate, piping configuration, and epoxy temperature. A one-dimensional numerical algorithm was developed to model fluid flow, heat transfer, and epoxy curing. Heating the epoxy makes it move faster because liquid viscosity decreases with increasing temperature. The coating was significantly thicker at the bottom of a horizontal pipe than at the top due to sagging of the epoxy coating after it had been applied, resulting in flow from the top to the bottom of the pipe. Sagging could be reduced by maintaining airflow until curing was almost complete and the epoxy had hardened enough to prevent it from moving easily. The combination of the experimental results and numerical modeling showed that the most important parameters controlling the speed of the epoxy and coating thickness were the air flow rate and temperature, since they determine the shear forces on the epoxy layer and the rate at which the epoxy cures. Raising air temperature increases the reaction rate and therefore decreases the time required for the epoxy to cure inside the pipe. The results of the simulation showed a very good agreement with the experimental results in pipes with 1-in diameter or less.     

重力径向偏心热管的温度特性

研制了不锈钢-水重力径向偏心热管,在70～100℃范围内实验研究了该热管的温度特性及其影响因素。当冷凝段处于自然对流时,较大充液率、上部控温方式和较低运行温度有利于提高冷凝段及蒸发段壁面温度的稳定性和均匀性;冷凝段内壁中心处的温度稳定性2 h内为±0.024℃,其中心段20 cm内温度均匀性为0.142℃。当采用强制对流冷却时,增大充液率、降低冷却能力可提高蒸发段平均温度,并改善蒸发段外壁面温度均匀性。此外,分析了热管通过相变换热及其热容来提高温度稳定性和均匀性的传热机理。此类热管在温度校准领域具有较好的应用前景。     

复合中空热管两相流的观测及充液比对热管等温性的影响

为了解决工业中大量低品质烟气余热的回收利用和烟气酸露点腐蚀导致设备容易失效的工程问题,提出了一种复合中空热管传热元件,介绍了其结构及工作原理。采用可视化实验对管内气液两相流的传热传质机理进行了分析,同时,对不同充液比情况下管壁的等温性进行了实验研究。研究结果表明:在内管采用酒精炉加热、外管采用空气自然对流冷却的条件下,复合中空热管管内液池为核态沸腾换热,外管壁内侧是饱和蒸气层流膜状凝结;复合中空热管在外管壁温度30℃时,加热时间2 min之内就能正常快速启动工作;充液比33%时,复合中空热管的等温性好。实验研究结果为复合中空热管换热器的工程应用提供了基础。     

用于空调能量回收的板式脉动热管换热器

为提高脉动热管换热器在空调系统排风能量回收中的换热效率,提出了一种新型并联槽道板式脉动热管及由其组成的换热器。首先对单片热管在空调排风夏季工况下的能量回收情况进行了传热性能实验研究,影响因素包括槽道当量直径、充液率、工质种类、风速、风温、微倾角;然后对一组由7片热管顺排形成的板式脉动热管换热器的换热效率进行了计算。研究表明：新型板式脉动热管的适用工质为R141b,最佳充液率为25%;传热性能随新风温度及风速的升高而增强,新风、排风温差小于6℃时热管不启动;随风速增加,换热量增加,但换热效率有所降低;给定工况下板式脉动热管散热器的换热效率为44.1%;微倾角可使空调能量回收系统在保证良好换热效率的同时实现换季不换向,热管安装宜采用+2°左右的微倾角。     

A Numerical Study of Heat Transfer Mechanisms in Gas-Solids Flows Through Pipes Using a Coupled CFD and DEM Model

A two dimensional numerical model has been developed to simulate heat transfer in gas-solids flows through pipes, in which the gas phase is modelled as a continuum using the Computational Fluid Dynamics (CFD) approach and the solids phase is modelled by the Discrete Element Method (DEM). This allows interactions between gas, particles, and pipe wall to be accounted at the scale of individual particles and convective and conductive heat transfers to be calculated using local gas and solids parameters. The predicted changes to the flow structures and the various heat transfer mechanisms due to the presence of particles were analyzed and compared with other workers' findings. This study has quantitatively demonstrated the crucial effect of particle transverse motion on heat transfers due firstly to the thermal energy transport by rebounding particles and secondly to the modification of the fluid thermal boundary layer characteristics.     

Gravity pipe flow of polymeric bulk solids in pneumatic conveying system

Various physical parameters of gravity pipes such as gravity pipe diameter, the inclination of the gravity pipe, the cone angle, temperature of the discharged material and the rate of air counter flow into the gravity pipe were studied. The results obtained from the pipe diameter and cone angle experiments showed that the mass flow rate was proportional to (D-1.4d)^2.5 and also to θ^-0.5. The results from the pipe inclination experiment showed the existence of an angle of inclination for maximum flow, which was also reported by Wieghardt [Uber einige versuche an stromungen in sand. Ingenieur Archived 20, 109-115]. The air flow experiment also showed that the mass flow rate was inversely proportional to the air counter-flow rate, and strongly influenced by the material properties. Results from the temperature experiment showed that the temperature of the material had slight effect on the mass flow rate for the temperature range that was used in the experiment. Flow visualization images showed formation of solid plugs in the pipe that played a part in influencing the behaviour and mass flow rate of solids in the system.     

Enhancing radial temperature uniformity and boundary layer development in viscous Newtonian and non-Newtonian flow by transverse oscillations: A CFD study

Radial heat transfer in laminar pipe flow is limited to slow thermal conduction which results in a wide temperature distribution over the pipe cross-section. This is undesirable in many industrial processes as it leads to an uneven distribution of fluid heat treatment. Often the fluids involved are relatively viscous and processing them under turbulent conditions is impractical and/or uneconomical. On the other hand, the use of static in-line mixers to promote radial mixing may be prohibited in hygienic processes because they are difficult to keep clean. In this paper, we use a validated Computational Fluid Dynamics (CFD) model to show that the imposition of a transverse vibration motion on a steady laminar flow generates sufficient chaotic fluid motion which leads to considerable radial mixing. This results in a large enhancement in wall heat transfer as well as a near-uniform radial temperature field accompanied by a substantial heating of the inner region of the flow. Vibration also causes the temperature profile to develop very rapidly in the axial direction reducing the thermal entrance length by a large factor, so that much shorter pipes could in principle be used to achieve a desired temperature at the outlet. The effects are quantitatively demonstrated for Newtonian and non-Newtonian pseudoplastic fluids of different viscosities, for a wide range of vibration amplitudes and frequencies. For processes where vibrational motion can be implemented the benefits can be very significant.     

水热管等温特性的数值模拟分析

为了研究水热管内腔里的温度均匀性,进行了水热管内腔的温度场和速度场的数值模拟。建立了包括管壁、吸液芯及内腔里的液体水和水蒸气3个耦合传热区域的水热管数理模型,采用VOF模型对水热管内部的流动与传热传质过程进行了数值模拟分析,得到了水热管稳定运行时的温度场和速度场分布,并探讨了加热温度、加热段长度和充液量等参数对热管内腔温度场分布的影响规律,计算结果与实验结果的对比结果表明两者能够较好吻合。本文研究结果可为水热管的设计、运行和改进提供理论依据和参考。     

双套管双管板换热器流动及传热性能数值模拟

采用CFD软件方法,研究双套管双管板换热器传热及流动特性.对隔绝腔连同内外套管的环形间隙内分别充入空气、水、甲苯气体和氩气4种介质进行模拟.结果表明:不同介质对传热有不同影响,充入水时的换热效果最好,甲苯气体的最差;壳程介质在流体接管进、出口附近存在回流和绕流且速度较小,管程介质流动较为均匀,受壁面边界层影响,速度在管...     

无吸液芯径向热管的传热机理

研究了无吸液芯径向热管的传热机理,由可视化实验推测其换热机理为液体沸腾换热和蒸汽对流换热,在碳钢-水径向热管实验中,所测管壁温度分布证实了以上推测的机理,由实验数据计算沸腾换热量,蒸汽对流换热系数随其流速变化而不断改变,推导出对流换热系数表达式。将换热系数表达式导入计算中,做为传热边界条件,模拟管壁温度以及蒸汽温度和流速。管壁温的计算结果与实验所测值能很好的吻合,这进一步验证了无吸液芯径向热管的传热机理。     

卧式螺旋管内流动换热壁温分布特性

螺旋管是一种高效换热管型,在卧式螺旋管内流动时受到重力和离心力的作用,工质流动方向时刻改变,产生二次回流,从而表现出了不同于其他管型的流动与传热特性。以卧式螺旋管为研究对象,选取底部截面、上升段截面、顶部截面三个具有代表性的位置截面,采用逐渐增大热通量法研究了其在单相对流和过冷沸腾条件下壁温的分布特性。为螺旋管换热器的设计与应用提供了参考依据。     

重力热管冷凝段运行特征的可视化实验研究

应用电容层析成像技术（ECT）对重力热管冷凝段的流动换热进行了可视化实验研究。重力热管以乙醇为工作介质,通过加热器控制重力热管蒸发段加热温度,冷凝段采用冷却水与乙醇蒸气进行逆向对流换热。通过ECT测量系统对冷凝段乙醇蒸气的冷凝过程进行监测,观察不同工况条件下重力热管冷凝段的气、液分布特性和液膜的形成及发展过程。摒弃了传统电容传感器的屏蔽罩结构,通过将测量电极用绝水层密封实现了传感器在液下环境工作,有效地拓展了ECT技术的应用领域。实验结果显示：当蒸发段加热温度较低时,乙醇蒸气在冷凝段壁面凝结形成条索状流动;随加热温度升高,冷凝液流动过渡至环状流;加热温度超过一定限值后,冷凝段出现液膜增厚甚至闭合脱落的周期性现象,并且频率随温度升高而升高。重力热管与垂直方向夹角为30°倾斜放置时,在高加热温度条件下同样存在液膜增厚甚至闭合脱落的周期性现象。     

人工糙管中湍流传热与温度分布

以横肋管为典型人工糙管,空气为试验介质,恒热流操作,进行湍流传热时径向温度分布和传热特性的测定,试验Re为20000～70000.所得湍流核心区温度分布可以对数分布律和温度亏损律表示,式中常数与粗糙几何参数有关.由测得的温度分布和速度分布计算了湍流粘度和湍流导温系数.此外,还进行了温度脉动特性的测定.传热系数测定结果表明,人工粗糙管比光滑管强化150%～300%,传热膜系数可用简单关联式估算Nu/Nu_s=1+26(k/d)~(0.49)(S/K)~(-0.46)式中Nu_s=0.022Re~(0.8)Pr~(0.5)