外磁场作用下磁流体的对流换热特性

实验研究了外加磁场作用下水基磁流体的对流换热特性,分别测量了均匀磁场和梯度磁场条件下磁流体横掠加热细丝的对流换热系数,分析了外加磁场强度和方向对磁流体传热性能的影响.实验结果表明,外加磁场是影响磁流体对流换热的一个重要因素,应用外加磁场可以控制磁流体对流换热过程.     

温度敏感型磁流体热磁对流特性研究

建立了描述外磁场作用下温度敏感型磁流体热磁对流特性的数学模型,数值模拟了回路中热磁对流的流动与传热特性.搭建了磁流体热磁对流回路装置,采用粒子示踪测速技术(PIV)测量了磁流体的流速,用热电偶测量了磁流体的温度分布.通过实验值和数值模拟结果,分析了不同磁场大小、不同热负荷,以及不同冷却温度下回路的运行特性.     

磁流体结构的Lattice-Boltzmann方法模拟

本文运用Lattice-Boltzmann(LB)方法建立了宏观静止磁流体模型,提出磁性聚集体概念,充分考虑了磁性颗粒受到的各种内力与外力包括重力、布朗力、vanderWaals相互作用及静磁相互作用,对无外加磁场及外加竖直均匀磁场时磁流体的结构进行了模拟,并分析了磁能与热能之比对磁流体系统中粒子分布形态的影响。模拟结果表明:无外加磁场时磁流体结构易失去稳定性,外加均匀磁场时磁流体粒子沿外磁场方向排列,随着磁热能比例的增大,外磁场对粒子分布的影响逐渐明显。     

带引流条导电矩形管磁流体湍流大涡数值模拟

为研究引流条对磁流体湍流的影响，采用自主开发的低磁雷诺数流固耦合磁流体相干结构模型大涡模拟求解器，对均匀磁场作用下平行层内带引流条导电矩形管和标准导电矩形管中液态金属湍流进行了数值模拟研究。结果表明，外加垂直流动方向的均匀磁场与流动的导电流体相互作用产生与流动方向相反的洛伦兹力，能够抑制磁流体的湍流脉动，这种抑制作用随着哈特曼数增大而增强。在弱导电率条件下，当Re=16350、Ha=212 时，两种管道中的流动均转换为层流流动状态。管道内壁面摩擦系数随着哈特曼数的增大而增大。引流条能在其近壁局部区域增强横向速度，有效激发湍流，但在弱壁面导电率条件下，带引流条导电矩形管壁面摩擦系数较标准矩形管大。     

水平磁场对液态金属自然对流的影响

在聚变堆中,由于大温差和强磁场的影响,液态金属自然对流呈现出了不同于常见流体的流动和换热特征。本文基于自主研发的磁流体计算程序,通过三维直接数值模拟,研究了平行于温度梯度的水平磁场对封闭方腔中液态金属流动与传热的影响。分析了不同磁场强度下,方腔内的流动与传热,包括主环流、二次流、温度分布及Nu数分布等,并从机理上解释了磁场的影响。结果表明平行温度梯度的水平磁场对自然对流有抑制作用,磁场越强,抑制越明显,传热效率越低。     

梯度磁场下磁流体三维结构的格子-Boltzmann方法模拟

考虑磁性颗粒受到的各种内力与外力包括重力、布朗力、van der、Waaks力、磁偶极-偶极作用力以及外磁场作用力,建立了描述磁流体结构的两相格子-Boltzmann三维模型,对外加梯度磁场条件下磁流体的介观结构进行了模拟.模拟结果表明:外加梯度磁场时磁流体粒子沿梯度方向聚集并出现分层现象,且随时间推移和外加磁场增大,分层现象越来越明显.     

绕圆柱体自由表面磁流体流动和传热的研究

本文对在不同雷诺数下,绕圆柱体的磁流体自由表面流动及传热进行了模拟,分析了磁场对绕流圆柱尾迹和涡分离的影响,获得了两种雷诺数下的电磁力密度、流场和温度场分布。结果表明,磁场不仅影响了流动的形态,而且对湍流有抑制作用,降低了自由表面的更新机制,从而减少了传热能力;在相同的Hartmann数下,相比低雷诺数下的流动换热情况,高雷诺数下的湍流不能被完全抑制,自由表面与尾迹的相互作用也较强,因而自由表面换热也较强。     

磁流体粘度的实验研究

采用毛细法粘度计测量了水基Fe磁流体的粘度,分析了磁性粒子份额、表面活性剂含量以及外加磁场强度和方向对粘度的影响。实验结果表明,磁流体粘度随着磁性粒子和表面活性剂浓度的增加而增加;随着外加磁场强度的增大而增大,对于相同的磁流体,在外加磁场方向垂直于流动方向时的粘度大于外加磁场方向平行于流动方向时的粘度;表面活性剂含量的增大将减弱外加磁场对磁流体粘度的影响。     

三维立方体内辐射磁流体流动与传热研究

采用配置点谱方法求解了在存在壁面温差的立体方腔内有外加磁场作用下热辐射对磁流体流动与传热的影响。采用谱投影算法处理速度和压力的耦合,辐射传递方程的角向离散采用球带等差数列微元等分离散坐标(SRAPN)法进行离散,最后在相同的节点下采用配置点谱方法进行求解。在验证数值方法之后,分析了Ha数、介质光学厚度、以及导热辐射参数对等温面、流线投影等的详细影响规律。     

温度敏感型磁流体冷却回路的实验研究

建立了温度敏感型磁流体自动冷却回路实验系统,实验研究了不同加热功率、冷却温度、加热位置及磁场分布对其冷却性能的影响.结果显示,温度敏感型磁流体在外加磁场及温差时可以持续稳定地流动,无须机械泵的驱动而实现热量的传输.流动速度及冷却性能受磁场与温度场协同作用的影响.     

Ferrofluidic Open Loop Pulsating Heat Pipes: Efficient Candidates for Thermal Management Of Electronics

Thermal management of electronic devices is presently a serious concern. This article investigates the thermal performance of a five-turn open-loop pulsating heat pipe in both start-up and steady thermal conditions. The effects of working fluid, namely water and ferrofluid, heat input, charging ratio, ferrofluid concentration, orientation, as well as application of magnetic field, are explored. Experimental results show that using ferrofluid enhances the thermal performance in comparison with the case of distilled water under certain conditions. In addition, applying a magnetic field on the open-loop pulsating heat pipe charged with ferrofluid improves its thermal performance. Charging ratios that lead to lower thermal are mentioned. Optimum concentration of ferrofluid in steady-state performance is 2.5 g/L. This study helps to design electronic cooling devices more efficiently.     

Experimental investigation for enhanced ferrofluid heat transfer under magnetic field effect

This paper reports an experimental work on the convective heat transfer of ferrofluid flowing through a heated copper tube in the laminar regime in the presence of magnetic field. Significant enhancement on the heat transfer of ferrofluid by applying various orders of magnetic field is observed in this experiment. Also in this experiment, the effect of magnetic nanoparticles concentrations and magnet position have been investigated. The main reason for the enhancement of heat transfer coefficient could be caused due to remarkable changes in thermophysical properties of ferrofluid under the influence of applied magnetic field.     

磁性液体的介电特性

在外加磁化场的作用下,磁性液体的介电特性表现出与一般物质所不同的新效应——磁电方向效应,即介电常数不但依赖于外加磁化场的大小,而且还依赖于外加磁化场与电场之间的相对方向。根据实验观察到的磁性液体中的磁性颗粒呈球形,在外加磁化场的作用下,磁性颗粒呈长链形分布的这一实验事实,我们用长链模型进行理论分析,合理地解释了磁电方向效应。理论与实验结果符合。 关键词：     

磁性液体对射流冲击传热的强化

本文对磁液应用于射流冲击传热的影响进行了实验研究。采用变压器油和煤油基磁液ＣＩ－２０Ｂ作为工质,把 １３×５ ｍｍ的康铜金属膜作为加热面和传热面,进行自由表面液体窄缝射流冲击,使用的喷嘴窄缝宽度为 ２５０微米,射流速度变化范围 ５．７ ｍ／ｓ～８．５ ｍ／ｓ。分别对附加磁液前后的射流传热实验结果进行比较,表明磁液的使用对射流冲击传热具有一定的强化。     

Heating of magnetic fluid systems driven by circularly polarized magnetic field

A theory is presented to calculate the heat dissipation of a magnetic suspension, a ferrofluid, driven by circularly polarized magnetic field. Theory is tested by in vitro experiments and it is shown that, regardless of the character of the relaxation process, linearly and circularly polarized magnetic field excitations, having the same root-mean-square magnitude, are equivalent in terms of heating efficiency.     

Preparation of composite with silica-coated nanoparticles of iron oxide spinels for applications based on magnetically induced hyperthermia

It is reported a novel method to prepare magnetic core (iron oxide spinels)–shell (silica) composites containing well-dispersed magnetic nanoparticles in aqueous solution. The synthetic process consists of two steps. In a first step, iron oxide nanoparticles obtained through co-precipitation are dispersed in an aqueous solution containing tetramethylammonium hydroxide; in a second step, particles of this sample are coated with silica, through hydrolyzation of tetraethyl orthosilicate. The intrinsic atomic structure and essential properties of the core–shell system were assessed with powder X-ray diffraction, Fourier transform infrared spectrometry, Mössbauer spectroscopy and transmission electron microscopy. The heat released by this ferrofluid under an AC-generated magnetic field was evaluated by following the temperature evolution under increasing magnetic field strengths. Results strongly indicate that this ferrofluid based on silica-coated iron oxide spinels is technologically a very promising material to be used in medical practices, in oncology.     

Reciprocity of Faraday effect in ferrofluid: Comparison with magneto-optical glass

The transmission light intensity method is carried out on a classical platform to study the reciprocity of Faraday effect in water-based Fe₃O₄ ferrofluid and its diluents. Setting the polarization direction of the analyzer at an angle of 45° to that of the polarizer, the switchable DC magnetic field and the alternating magnetic field are imposed to ferrofluid. The ferrofluid film is replaced by magneto-optical glass for contrastive experiments. The results indicate that ferrofluid is different with magneto-optical glass. Even though the direction of magnetic field is reversed, the rotation direction of the polarized light does not change for ferrofluid. The theoretical model of magneto-optical rotation was used to describe the origin of the reciprocity of Faraday effect in ferrofluid and the non-reciprocity in magneto-optical glass. These findings suggest that the magnetic moments of nanoparticles in ferrofluid tend to the same orientation with the magnetic field because of the rotation of particles.     

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Based on the superparamagnetism of the ferrofluid which can receive the magnetic force after magnetization by the magnetic field, and there is no magnetic hysteresis after demagnetization, this paper presents a dynamic control method of the ferrofluid. The electromagnetic field is directly added to ferroliquid-column in the air domain to achieve the dynamic deflection. The surface tension, gravity force and magnetic force are added to the modified N-S control equation, and the magnetic induction equation is combined. The ferrohydrodynamics (FHD) model is established. The model of volume of fluid (VOF) is presented to describe ferrofluid jetting in the air domain with the secondary development of fluent. The liquid phase distribution and dispersion of ferrofluid are simulated under different magnetic field intensity. The effect of magnetism on its dynamical behavior is analyzed. The results show that with the increase of magnetic field intensity and jet distance, the ferrofluid velocity along the magnetic field direction and the offset increase, and its dispersion is gradually obvious.     

空气域中铁磁流体动态控制研究

根据铁磁流体磁化后会受到磁力,退磁后不存在任何磁滞的超顺磁性质,提出了铁磁流体的动态控制方法,对喷射在空气域中的铁磁流体液柱直接添加电磁场实现其动态偏转驱动。在修正后的纳维-斯托克斯(N-S) 控制方程中加入表面张力、重力及磁力,并结合磁感应方程,建立了铁磁流体动力学(FHD)模型。利用二次开发的Fluent流体仿真软件建立了铁磁流体在空气域中喷射的流体体积函数(VOF)多相流模型,仿真了在不同磁场强度下铁磁流体的液相分布及分散状况,分析了磁性对其动力学行为的影响。结果表明,随着磁场强度和喷射距离的增大,铁磁流体沿磁场方向速度及偏移量增大,其发散情况逐渐明显。     

The measurements of anisotropy of ultrasound propagation and magnetic susceptibility in viscous ferrofluid

Results of experimental study of anisotropy of ultrasound propagation and anisotropy of magnetic susceptibility in a ferrofluid of considerable viscosity are reported. The dependence of the propagation velocity and absorption coefficient of ultrasonic wave on the angle between the direction of measurement and that of the magnetic field provides important information on the ferrofluid structure in a magnetic field. The results show that the ultrasonic absorption coefficient of a wave propagating in a viscous ferrofluid in a DC magnetic field depends mainly on the rotational degree of freedom. Measurements of the anisotropy of the magnetic susceptibility of a ferrofluid bring the information on the magnetic moment of the magnetic particles and their mean radius.