Effect of an External Magnetic Field on the 3-D Oscillatory Natural Convection of Molten Gallium During Phase Change

In this article, a numerical study of the effect of an external magnetic field on three-dimensional (3-D) oscillatory natural convection during phase change is carried out. A parallelepiped enclosure filled with a molten gallium and subjected to an external magnetic field applied in X-, Y-, and Z-directions, separately, is considered. The finite-volume method with enthalpy formulation is used to solve the mathematical model in the solid and liquid phases. The Hartmann number is fixed to Ha = 20.The computer program developed in this study was validated with the experimental data founded in the literature. The critical Grashof numbers Gr _Cr and critical corresponding frequencies F r _Cr are determined with and without magnetic field. The results show that the oscillatory natural convection during phase change are characterized by low-frequency oscillations in the presence and absence of the magnetic field. The pattern flow shows a spiral development of the flow in Z- direction. A strong dependence between the direction of the magnetic field and the critical Grashof number and their corresponding frequency is determined. A strong stabilization of the flow field is shown when the magnetic field is oriented horizontally.     

磁场对燃料分子微观状态的影响

本文对磁化后燃料分子的红外吸收光谱、紫外吸收光谱进行了分析，发现红外吸收峰向高波数明显位移，红外吸收强度、紫外吸收强度增强，表明磁化后燃料分子微观状态发生了改变．测试由分子微观状态决定的燃料性质也发生明显变化，证明燃料分子微观状态的改变是发动机磁化消烟节油技术的理论依据．     

矩形空腔噪声主动控制的研究

以压电陶瓷为执行元件,采用以控制噪声为目标,直接控制板件振动来控制空腔噪声的主动控制技术路线,将神经网络控制技术与自适应技术相结合,用于矩形空腔模型噪声主动控制系统中。采用同位配置和结构模态声辐射效率,进行传感器和作动器的位置布置。试验取得了较好的降噪效果,为多通道噪声主动控制理论在汽车等矩形空腔模型上应用提供了有益的探索和参考。     

Natural Convection Heat Transfer of Liquid Metal Gallium Nanofluids in a Rectangular Enclosure

A new mixed nanofluid (Cu/diamond–gallium [Cu/diamond–Ga] nanofluid) is proposed, and the mass ratio of Cu nanoparticles and diamond nanoparticles in the new mixed nanofluid is 10:1. The natural convection heat transfer of Cu/diamond–Ga nanofluid, Cu–gallium (Cu–Ga) nanofluid, and liquid metal gallium with different volume fractions in a rectangular enclosure is investigated by a single‐phase model in this paper. The effects of temperature difference, nanoparticle volume fraction and the kinds of nanofluid on the natural convection heat transfer are discussed. The natural convection heat transfer of the three kinds of fluids is compared. It is found that Nusselt numbers of the Cu/diamond–Ga nanofluid along with X direction increases with the nanoparticle volume fraction and temperature difference. Cu/diamond–Ga nanofluid can enhance the heat transfer by 73.0% and 9.7% at low‐temperature difference (ΔT = 1 K) compared with liquid metal gallium and Cu–Ga nanofluid, respectively. It also can enhance the heat transfer by 85.9% and 5.2% at high‐temperature difference (ΔT = 11 K) compared with liquid metal gallium and Cu–Ga nanofluid, respectively.     

Experimental Investigation of Heat Storage and Heat Transfer Rates during Melting of Nano-Enhanced Phase Change Materials (NePCM) in a Differentially-Heated Rectangular Cavity

In this work, an experimental study of melting heat transfer of nano-enhanced phase change materials(NePCM) in a differentially-heated rectangular cavity was performed. Two height-to-width aspect ratios of the cavity, i.e., 0.9 and 1.5, were investigated. The model Ne PCM samples were prepared by dispersing graphene nanoplatelets(GNP) into 1-tetradecanol, having a nominal melting point of 37℃, at loadings up to 3 wt.%. The viscosity was found to have a more than 10-fold increase at the highest l...     

Computation of Turbulent Natural Convection in a Rectangular Cavity with the Lattice Boltzmann Method

A numerical study of a turbulent natural convection in a rectangular cavity with the lattice Boltzmann method (LBM) is presented. The primary emphasis of the present study is placed on investigation of accuracy and numerical stability of the LBM for the turbulent natural-convection flow. A HYBRID method in which the thermal equation is solved by the conventional Reynolds-averaged Navier-Stokes equation (RANS) method while the conservation of mass and momentum equations are resolved by the LBM is employed in the present study. The elliptic-relaxation model is employed for the turbulence model and the turbulent heat fluxes are treated by the algebraic flux model. All the governing equations are discretized on a cell-centered, nonuniform grid using the finite-volume method. The convection terms are treated by a second-order central-difference scheme with the deferred correction method to ensure accuracy and stability of solutions. The present LBM is applied to the prediction of a turbulent natural convection in a rectangular cavity and the computed results are compared with the experimental data commonly used for the validation of turbulence models and those by the conventional finite-volume method. It is shown that the LBM with the present HYBRID thermal model predicts mean velocity components and turbulent quantities which are as good as those by the conventional finite-volume method. It is also found that the accuracy and stability of the solution is significantly affected by the treatment of the convection term, especially near the wall.     

Numerical Investigation of the Magnetic Field Effects on the Entropy Generation and Heat Transfer in a Nanofluid Filled Cavity with Natural Convection

This paper presents a numerical investigation of the entropy generation and heat transfer in a ferrofluid (water and 4% Fe₃O₄ nanoparticles) filled cavity with natural convection using a two phase mixture model and control volume technique. The effect of applying a nonuniform magnetic field on the entropy generation and heat transfer in the cavity and also the interaction of magnetic force and the buoyancy force are investigated. Based on the obtained results, applying a magnetic field will enhance the heat transfer mechanism. Furthermore, by applying the nonuniform magnetic field on the ferrofluid filled cavity with natural convection, the total entropy generation is decreased considerably at higher Rayleigh numbers. Therefore, applying a magnetic field can be considered as a suitable method for entropy generation minimization in order to have high efficiency in the system.     

重力场对液池内热-溶质毛细对流影响的数值研究

数值研究了不同重力场下液池内耦合热-溶质毛细对流流动特性,模型中考虑了热毛细效应和溶质毛细效应相当这一特殊情况。计算结果显示,当重力加速度较大时液池内存在周期性迁移的对流涡,而当重力加速度较小时,液池内的对流涡迁移消失,因而重力加速度能够促使热-溶质毛细对流失稳。随着重力加速的减小,监测点的温度和浓度振荡幅度减小。常重力条件下自由表面速度分布受浮力对流控制;微重力条件下,自由表面的速度分布基本一致,随着重力加速度减小自由表面速度略微减小。     

Effects of Magnetic Field and Inclination on Natural Convection in a Cavity Filled with Nanofluids by a Double Multiple-Relaxation-Time Thermal Lattice Boltzmann Method

Abstract

The present study numerically investigates the magneto-hydrodynamic flow and heat transfer of copper (Cu)-water nanofluids in an inclined cavity with one heat and one cold source. Simulations have been done via double multiple-relaxation-time thermal lattice Boltzmann method. Impacts of Hartmann number, Rayleigh number, inclination angle and the volume fraction of nanoparticles on the fluid flow and heat transfer performance are illustrated in terms of streamlines, isotherms, local, and average Nusselt numbers. Outputs demonstrate that the average Nusselt number decreases remarkably first as the inclination angle increases and then the average Nusselt number increases continuously and approaches a maximum value at a certain inclination angle for high Rayleigh numbers. In addition, the position where the average Nusselt number is maximized moves toward the lower inclination angle with increasing the Hartmann number for high Rayleigh numbers.     

Effect of Magnetic Field on Radial Dopant Segregation in Crystal Growth

A three-dimensional time-dependent computational model is developed to simulate the melt flow and the dopant transport during the growth of Si_1?x Ge _x semiconductor crystals. The effect of static magnetic fields of different orientations on radial segregation of dopant is analyzed. It is observed that the flow oscillations are damped and flow becomes steady for both horizontal and vertical applied magnetic fields as the Hartman number increases. It is found that there is an optimum value of Hartman number for each Rayleigh number beyond which little or no improvement in uniformity of concentration can be obtained at the growth interface.     

Heat and Moisture Transfer in a Rectangular Cavity Partially Filled with Hygroscopic Porous Media

Abstract

This work deals with turbulent natural convection heat and moisture transfer with thermal radiation in a rectangular cavity partially filled with hygroscopic porous medium. The governing equations for the momentum and heat transfer in both free fluid and hygroscopic porous media and moisture content transfer in hygroscopic porous medium were solved by the finite element method. Comparisons with experimental and numerical results in the literature have been carried out. Effects of thermal radiation, Rayleigh number on natural convection and heat transfer in both free fluid and porous medium and moisture content transfer in porous medium were analyzed. It was found that surface thermal radiation can significantly change the temperature and moisture content fields in the regions of free flow and hygroscopic porous medium. With the increase in Rayleigh number, the temperature of porous medium at the interface increased slightly, and the magnitude of moisture change becomes smaller.     

逆向流动边界下窄方腔内相变材料融化传热数值研究

为分析复杂流动边界条件对相变材料融化进程的影响,采用计算流体动力学方法进行数值模拟,研究了窄方腔内月桂酸在逆向流动边界中的融化传热特性。建立了填充有相变材料的窄方腔两侧流过参数相同但逆向的传热流体的物理模型。通过数值模拟得到了相变材料融化形态变化以及相对应的液体分数、温度、速度矢量分布等瞬态传热特性,从中总结出了腔体中相变融化机制和传热机理。选取雷诺数1×10⁴,3×10⁴,5×10⁴,7×10⁴和瑞利数3×10⁶,4×10⁶,5×10⁶,6×10⁶的7种工况,分析了不同边界条件对传热过程的影响。结果表明：腔内自然对流的产生大大增加了传热强度,导致顶部相变材料率先完全融化;融化阶段的传热方式依次为导热、导热与对流混合、对流主导传热;在两侧流体的共同传热作用下腔内外换热效果明显增强;雷诺数和瑞利数都对融化过程有积极的影响,但融化前期雷诺数影响较大,后期瑞利数逐渐起主导作用。     

The Effect of Simultaneously Developing Flow on Heat Transfer in Rectangular Tubes

A study of heat transfer in simultaneously developing flow through rectangular tubes is presented in this article. Heat transfer coefficients were measured for three different tube sizes and shapes (D h = 2.21 mm, f = 0.050; D h = 3.02 mm, f = 0.108; and D h = 1.74 mm, f = 0.029), which correspond to typical dimensions used in automotive heat exchangers. For each of these tubes, several different tube lengths were tested to measure the effect of developing flow on the Nusselt number. The results demonstrate that developing flow enhances Nusselt numbers, especially for short tubes. For the geometry range studied, the effect of aspect ratio was not very significant. Heat transfer correlations that accounted for the effects of Reynolds number (118 < Re < 10,671) Prandtl number (6.48 < Pr < 16.20), and bulk-to-wall property variations (0.243 < w b / w w < 0.630), and geometric features such as tube length, hydraulic diameter, and aspect ratio, were developed from the data.     

Numerical Study of Entropy Generation for Natural Convection in Rectangular Cavity with Circular Corners

This article numerically studies entropy generation due to natural convection in a rectangular cavity with circular corners. In this work, in order to solve the governing equations, an explicit finite-volume procedure and a time-marching method are utilized. Also, instead of the conventional algorithms of SIMPLE, SIMPLEM, and SIMPLEC, an artificial compressibility technique is applied for coupling the continuity to the momentum equations. Entropy generation, as a representation of irreversibility and efficiency loss in engineering heat transfer processes, is analyzed in detail. In this work, effects of the radius of walls corner, Rayleigh number, and distribution ratio on total entropy generation, Nusselt number, and Bejan number are also evaluated. The results show that entropy generation decreases with the increase of the radius of the walls’ corner and increases with the increase of Rayleigh number, aspect ratio, and irreversibility ratio.     

Characterization of the Rotor Magnetic Field in a Brushless Doubly-Fed Induction Machine

The large increase in wind generation could improve the final development of wind systems with brushless doubly-fed induction machines (BDFIM) as an alternative to the doubly-fed asynchronous machines. For this reason, a detailed study of several aspects of the BDFIM design, as well as of its rotor configuration, is absolutely essential. In this paper, the authors present an alternative formulation of the BDFIM operating principle in synchronous mode. Besides the basic equation of the machine operation, it presents as main advantage the precise characterization of all the magnetic field components in a BDFIM with idealized stator windings and an idealized rotor cage. Based on this formulation, the paper provides a standard that may be used to compare the fields created by different real BDFIMs. This standard has been validated by laboratory tests.      

Hydrothermal Behavior of a Ferrofluid in a Corrugated Channel in the Presence of a Magnetic Field

In this paper, results of applying a non‐uniform magnetic field on a dilute ferrofluid (water and 3% vol. Fe₃O₄) flow in a corrugated channel under a constant heat flux boundary condition have been reported. The thermal behavior of the flow is investigated numerically using a two‐phase mixture model and control volume technique. It is concluded that using a magnetic field with a negative gradient on a nanofluid flow in corrugated channels can be proposed as a suitable method to achieve higher heat transfer performance and augment the heat transfer coefficient and also reduces the wall temperature. This method can lead to the design of more compact heat exchangers. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(1): 80–92, 2014; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21060     

Natural Convection in a Vertical Rectangular Cavity Filled with a Non-Newtonian Power Law Fluid and Subjected to a Horizontal Temperature Gradient

A study of natural convection, in a vertical rectangular cavity filled with a non-Newtonian fluid and subjected to uniform heat flux along the vertical side walls, is carried out numerically by solving the full governing equations. In the limit of a tall enclosure, these equations are considerably reduced by using the parallel flow approximation. Solutions for the flow and temperature fields, and the heat transfer rate, are obtained as functions of the governing parameters. Good agreement is found between the results of the two approaches for a wide range of governing parameters.     

Effect of Heat-Generating Solid Body on Mixed Convection Flow in a Ventilated Cavity

The mixed convection flow and heat transfer characteristics inside a square ventilated cavity with a heat-generating solid circular body located at the center have been investigated numerically. The inlet opening is at the bottom of the left wall, while the outlet one is at the top of the right wall, and all the walls of the cavity are considered to be adiabatic. A Galerkin weighted residual finite element method is used to solve the governing equations of mass, momentum, and energy. The behavior of the fluid in the ranges of dimensionless cylinder diameter from 0.1 to 0.6 of the heat generating body, thermal conductivity ratio range from 0.2 to 50 between solid and fluid, and heat generating parameter range from 1 to 5 is described in detail. The medium considered is air with a Prandtl number of 0.71. It is found that the flow and temperature field is strongly dependent on the already-mentioned parameters for the ranges considered. The variation of the mean Nusselt number, the dimensionless average drag force, and the average temperature of the fluid versus Richardson number are presented for these parameters.     

磁场旋转方向对引发火焰旋转临界值的影响

通过对旋转火焰涡运动微分方程式的分析，研究旋转磁场引发火焰旋转时的磁场旋转临界速度与磁场参数之间变化的关系，以及旋转磁场方向的变化对旋转磁场引发旋转火焰时磁场旋转速度临界值的影响规律．实验研究结果表明，随着磁场强度的增加，引发火焰旋转的磁场上临界转速增加，下临界转速则下降；磁场顺时针方向旋转时，引发火焰旋转的磁场上临界转速较逆时针旋转时要小，而下临界转速较逆时针旋转时要大．     

磁场对旋转火焰特性影响的研究

通过磁场对旋转火焰特性影响的研究，发现磁场能够改变旋转火焰的燃烧特性和燃烧规律，使火焰的温度和高度都会发生一定程度的变化。因此，可以更准确地了解实际火灾过程中火旋风的燃烧特性，掌握实际火灾中火焰的燃烧特点，当实际火灾发生并产生火旋风时，了解地球磁场的存在对实际的火旋风的影响程度，从而根据其燃烧的规律性，采取有效的措施，控制火旋风的发展。