Buoyancy induced flow in a nanofluid filled enclosure partially exposed to forced convection

A numerical study was performed on natural convection for water–CuO nanofluid filled enclosure where the top surface was partially exposed to convection. The cavity has a square cross-section and differentially heated. Except exposed convection part on the top, all sides are adiabatic on horizontal walls. Effects of Rayleigh number (10³ ? Ra ? 10⁵), Biot number (0 ? Bi ? ∞), length of partial convection (0.0 ? L ? 1.0) and volume fraction of nanoparticles (0.0 ? φ ? 0.1) on heat and fluid flow were investigated. The results showed that for the case of high Biot number that heat transfer along the heated was enhanced by increasing the Rayleigh number mainly at the upper portion of the heated wall. When the top wall was totally exposed to convection, the results prevail that the heat transfer was more effective at high Biot number especially at the upper portion of the heated wall. For the case of high Biot number, the results prevailed that the heat transfer at the upper portion of the heated wall increases considerably at high exposed length to convection (L); however, for L ? 0.75 the effect of L was less pronounced. Contour maps for percentage of heat transfer enhancement were presented and it was shown that the location of maximum enhancement in heat transfer was sensitive to Ra, φ and L.     

Axi symmetric 2D simulation and numerical heat transfer characteristics for the calibration furnace in a rectangular enclosure

This paper presents axi symmetric 2D numerical investigation of the spherical thermocouple calibration furnace in a rectangular enclosure. The focus is on the flow structure inside the Saturn (a hollow spherical cavity), external flow behavior due to annulus block heating and the surface temperature uniformity. Mesh sensitivity analysis is adopted to extract the mesh with minimum number of nodes but with fast convergent finite element solution. The basic strategy here is that temperature perturbation error at a single point instead of a single element contributed to the total perturbation error qualitatively remains the same. Agreement between numerical simulation results and the experiment results is good with a maximum temperature deviation 10 °C for the cavity temperature 400 °C. Finally, standard numerical temperature uncertainty due to variation in thermal conductivity is computed through the sensitivity coefficient using uncertainty analysis.     

Conduction-radiation effect on natural convection flow in fluid-saturated non-Darcy porous medium enclosed by non-isothermal walls

The combined effect of conduction-convection-radiation on natural convection flow of an optically thick Newtonian fluid with gray radiant properties, confined in a porous media square cavity with Darcy-Brinkman-Forchheimer drag is studied numerically. For a gray fluid, Rosseland diffusion approximation is considered. It is assumed that (i) the temperature of the left vertical wall varies linearly with height, (ii) the right vertical and top walls are at a lower temperature, and (iii) the bottom wall is uniformly-heated. The governing equations are solved using the alternate direct implicit method together with the successive over relaxation technique. The investigation of the effect of governing parameters, namely, the Forschheimer resistance (Γ), the temperature difference (Δ), and the Plank number (Rd), on the flow pattern and heat transfer characteristics is carried out. It can be seen that the reduction of flow and heat transfer occur as the Forschheimer resistance is increased. On the other hand, both the flow strength and heat transfer increase as the temperature ratio Δ is increased.     

数字扬声器技术与未来发展

主要介绍了目前具有代表性的2种数字扬声器技术。对数字换能器阵列和多音圈扬声器的结构、声压场机理,及其它潜在的实现技术进行了对比评述。讨论了数字扬声器对2种驱动技术的要求。评估了6位数字样机系统的声学特性,同时对于扩展至16位系统的可行性,包括半导体大规模制造技术的使用,过采样和噪声整形技术的应用也进行了探讨。     

高保真扬声器系统设计的探索

综合利用客观的测量结果、计算机模拟分析结果和主观的听感音质评价结果,进行了一个二分频高保真扬声器系统的设计。探索以主观听感的艺术性要求去进行高保真扬声器系统的技术性设计,追求艺术与技术的相互结合。     

模态控制技术与龙骨振膜扬声器——现代扬声器设计的发展趋势

提出了扬声器的声信噪比问题及其测量方法,指出扬声器的声信噪比是评价扬声器音质的重要指标。介绍了现代扬声器的设计方法——模态控制技术及采用模态控制技术设计的龙骨振膜扬声器。在提高振膜的信噪比、音质和有效传递驱动力方面．采用模态控制技术设计的扬声器龙骨振膜结构优于现有的其他结构。     

不同用途扬声器系统的频响特性及特征

扬声器系统在不同使用声压级、不同使用环境和播放不同音乐时都可能对音色表现有不同的要求。尝试通过一个基本达到“高保真扬声器系统最低性能的要求”的扬声器系统的设计,进行其不同使用声压级时依据主观听感的需求调整其频率响应特性的试验。该扬声器系统通过调整后能适应不同声压级的使用要求．并分析和总结出不同状态下该扬声器系统的不同频率响应特性及其特征。     

Experimental and numerical analysis of thermo-magnetic convection in a vertical annular enclosure

In the present paper an experimental and numerical analysis of a thermo-magnetic convective flow of paramagnetic fluid in an annular enclosure with a round rod core and a cylindrical outer wall is presented. It is complemented by an experimental analysis of natural convection depending on the inclination angle to show the stability of the present configuration. Convection in an annulus between two vertical co-axial cylinders resulting from gravitational and magnetic environments has been investigated. A strong magnetic field can be an alternative to heat transfer enhancement. The effect of the magnetic field on the convection of the paramagnetic fluid in the annular vessel in various positions was compared. The numerical analysis was done based on the continuity, momentum and energy equations. A term related to the magnetic buoyancy force was added to the momentum equation. The distributions of Nusselt number present minima in two positions of the enclosure, which depends on the reciprocal relationship between the gravitational and magnetising forces.     

扬声器非线性的几种数字补偿方法

对扬声器非线性的数字补偿方法作了综述,扼要介绍分析了该领域研究人员所采用过的5种主要方法.展望了扬声器非线性数字处理方法的应用前景.     

Numerical simulation of MHD natural convection flow in a wavy cavity filled by a hybrid Cu-Al_2O_3-water nanofluid with discrete heating

We consider the combined effect of the magnetic field and heat transfer inside a square cavity containing a hybrid nanofluid(Cu-Al_2O_3-water). The upper and bottom walls of the cavity have a wavy shape. The temperature of the vertical walls is lower,the third part in the middle of the bottom wall is kept at a constant higher temperature,and the remaining parts of the bottom wall and the upper wall are thermally insulated.The magnetic field is applied under the angle γ, an opposite clockwise direction. For the numerical simulation, the finite element technique is employed. The ranges of the characteristics are as follows: the Rayleigh number(10~3≤Ra≤10~5), the Hartmann number(0≤Ha≤100), the nanoparticle hybrid concentration(?_(Al_2O_3),?_(Cu) = 0, 0.025, 0.05),the magnetic field orientation(0≤γ≤2π), and the Prandtl number P_r, the amplitude of wavy cavity A, and the number of waviness n are fixed at P_r = 7, A = 0.1, and n = 3, respectively. The comparison with a reported finding in the open literature is done,and the data are observed to be in very good agreement. The effects of the governing parameters on the energy transport and fluid flow parameters are studied. The results prove that the increment of the magnetic influence determines the decrease of the energy transference because the conduction motion dominates the fluid movement. When the Rayleigh number is raised, the Nusselt number is increased, too. For moderate Rayleigh numbers, the maximum ratio of the heat transfer takes place for the hybrid nanofluid and then the Cu-nanofluid, followed by the Al_2O_3-nanofluid. The nature of motion and energy transport parameters has been scrutinized.