The effect of magnetic fields on low frequency oscillating natural convection with pressure gradient

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Conjugate Natural Convection in a Porous Enclosure with Non-Uniform Heat Generation

Effects of a conductive wall on natural convection in a square porous enclosure having internal heating at a rate proportional to a power of temperature difference is studied numerically in this article. The horizontal heating is considered, where the vertical walls heated isothermally at different temperatures while the horizontal walls are kept adiabatic. The Darcy model is used in the mathematical formulation for the porous layer and finite difference method is applied to solve the dimensionless governing equations. The governing parameters considered are the Rayleigh number (0 ???Ra ???1000), the internal heating and the local exponent parameters (0 ????? ???5), (1 ????? ???3), the wall to porous thermal conductivity ratio (0.44 ???Kr ???9.9) and the ratio of wall thickness to its width (0.02 ???D ???0.5). The results are presented to show the effect of these parameters on the fluid flow and heat transfer characteristics. It is found a strong internal heating can generate significant maximum fluid temperature more than the conductive solid wall. Increasing value thermal conductivity ratio and/or decreasing the thickness of solid wall can increase the maximum fluid temperature. It is also found that at very low Rayleigh number, the heat transfer across the porous enclosure remain stable for any values of the thermal conductivity ratio.     

Optimal discrete distribution of heat flux elements for in-tube laminar forced convection

A new technique is proposed to enhance the heat transfer from a discretely heated pipe to a developing laminar fluid flow. Unlike the common heating situation where the fluid is continuously heated along the pipe wall with uniform heat flux, the proposed technique consists in heating the fluid with stepwise distributed heat flux, namely by placing insulated segments between the heated segments. Applying this technique, the effective length of the thermal entrance region is enlarged and as a result, the average heat transfer is invigorated. In order to maximize the heating performance, an optimal placement of the insulated segments between the heated segments is calculated according to constructal design. This serves to describe the optimal stepwise distribution of the heat flux. Owing that the total heat load is considered fixed, the maximization of the heating performance translates into the minimization of the peak temperature (‘hot spot’) of the pipe wall. The analytical results demonstrate that the optimal location of the insulated segments along with the reduction of the peak temperature strongly depend on the Graetz number. It is also shown that for intermediate values of the Graetz number, the peak temperatures are remarkably reduced in response to the optimal placement of the insulated/heated segments.     

Magnetothermal force on heated or cooled pipe flow

The effects of the magnetothermal force on the flows of heat and fluid through a pipe are investigated numerically when the pipe wall is either heated or cooled at constant heat flux. The flow is laminar and a paramagnetic fluid is presumed as the working fluid. Because the magnetic susceptibility of a paramagnetic fluid depends on the inverse of its temperature, the magnetothermal force is induced by coupling of the temperature field and magnetic induction. First, the effects are discussed using the case of a magnetic field induced by a single-turn concentrically placed electric coil. It is found that the effects of the magnetothermal force differ according to whether the pipe is cooled or heated. When cooled, the heat and fluid flows are affected behind the coil; the flow is repelled from the wall to the center and the thermal boundary layer thickens. By decomposing the force into the radial and axial directions in the heated and cooled cases, it is clarified that the axial force changes from positive to negative depending on the coil location in the heated case. Therefore, it can be concluded that the effects are not simply oppositional in the heated and cooled cases. In relation to the heat transfer, only when the coil is placed at the threshold of the heating/cooling zone do the effects on the local heat transfer become the opposite of each other. At other coil locations, the suppression of heat transfer is dominant ahead of the coil in the heated case, as indicated in previous work by our group. However, in the cooled case, this effect occurs behind the coil. For a more practical case, a solenoid coil is employed in the simulation. It is then found that the effect on the heat transfer becomes remarkable at the solenoid edges, especially for the heat-transfer suppression in both the heated and cooled cases.     

低雷诺数翼型蒙皮主动振动气动特性及流场结构数值研究

针对低雷诺数(Re)翼型气动性能差的特点,文章通过对翼型柔性蒙皮施加主动振动的方法,提高翼型低Re下的气动特性,改善其流场结构.采用带预处理技术的Roe方法求解非定常可压缩Navier-Stokes方程,对NACA4415翼型低Re流动展开数值模拟.通过时均化和非定常方法对比柔性蒙皮固定和振动两种状态下的升阻力气动特性和层流分离流动结构.初步研究工作表明在低Re下柔性蒙皮采用合适的振幅和频率,时均化升阻力特性显著提高,分离泡结构由后缘层流分离泡转变为近似的经典长层流分离泡,分离点后移,分离区缩小.在此基础上,文章更加细致研究了柔性蒙皮两种状态下单周期内的层流分离结构及壁面压力系数分布非定常特性和演化规律.蒙皮固定状态下分离区前部流场结构和压力分布基本保持稳定,表现为近似定常分离,仅在后缘位置出现类似于卡门涡街的非定常流动现象.柔性蒙皮振动时从分离点附近开始便产生分离涡,并不断向下游移动、脱落,表现为非定常分离并出现大范围的压力脉动.蒙皮振动使流体更加靠近壁面运动,大尺度的层流分离现象得到有效抑制.      

极高超声速稀薄气体原子辐射效应的p-DSMC方法

极高超声速流动激波层内的高温导致内能模态的激发并伴随热辐射发生, 过高的温度使得空气分子完全解离, 原子组分对辐射热的贡献将达到80%以上. 本文基于优化的原子辐射模型, 提出追踪光子?直接模拟蒙特卡罗(p-DSMC)方法, 研究了稀薄流区不同马赫数下的高超声速二维圆柱绕流的壁面辐射加热, 获得了有无激发辐射效应的壁面压力和热流以及沿驻点线变化的平动、振动和转动温度. 在不考虑激发辐射效应的情况下, 得到的壁面压力和热流与已有的模拟结果符合的非常好, 误差均在5%以内, 尤其是在驻点位置, 误差在1%以内; 获得的平动、振动以及转动温度均与文献结果符合的很好. 在相同的来流条件下, 考虑辐射效应后发现, 来流速度低于10 km/s时, 辐射加热不明显, 在驻点区域, 辐射加热占对流加热比重在7%左右; 来流速度大于10 km/s时, 在驻点区域, 辐射加热占对流加热比重将超过30%. 考虑辐射效应后, 对非平衡区的平动、转动和振动温度的最大值影响不大. 此外, 另一个重要结论是, 流场中原子的浓度是影响壁面辐射热流大小的一个重要因素.      

Structure of the temperature field in a flow over heated waves

Liquid crystal thermometry (LCT) was used to quantify temperature fields in a flow over resistively heated waves and assess the effect of the large-scale longitudinal structures that were previously obtained in the velocity field for an isothermal flow (A. Günther and P. Rudolf von Rohr, submitted article, 2002). The wavelength 6 was 10 times larger than the amplitude, and the considered Reynolds numbers were 725 and 3300, defined with the bulk velocity and the half-channel height. A constant heat flux was imposed at the wavy bottom wall. For the first time, LCT was used to determine the fluid temperature in a wall-bounded flow with heat transfer. The dominant spanwise scale obtained from a proper orthogonal decomposition (POD) of the fluid temperature field above an uphill location of the wavy wall was 1.56. It agrees well with the one previously obtained for a decomposition of the streamwise velocity.     

A new algorithm of global tightly-coupled transient heat transfer based on quasi-steady flow to the conjugate heat transfer problem

Concerning the specific demand on solving the long-term conjugate heat transfer(CHT) problem,a new algorithm of the global tightly-coupled transient heat transfer based on the quasi-steady flow field is further put forward.Compared to the traditional loosely-coupled algorithm,the computational efficiency is further improved with the greatly reduced update frequency of the flow field,and moreover the update step of the flow field can be reasonably determined by using the engineering empirical formula of the Nusselt number based on the changes of the inlet and outlet boundary conditions.Taking a duct heated by inner forced air flow heating process as an example,the comparing results to the tightly-coupled transient calculation by Fluent software shows that the new algorithm can significantly improve the computational efficiency with a reasonable accuracy on the transient temperature distribution,such as the computing time is reduced to 22.8% and 40% while the duct wall temperature deviation are 7% and 5% respectively using two flow update time step of 100 s and 50 s on the variable inlet-flow rate conditions.     

A Numerical Study of Oscillating Peristaltic Flow of Generalized Maxwell Viscoelastic Fluids Through a Porous Medium

This article presents a numerical study on oscillating peristaltic flow of generalized Maxwell fluids through a porous medium. A sinusoidal model is employed for the oscillating flow regime. A modified Darcy-Brinkman model is utilized to simulate the flow of a generalized Maxwell fluid in a homogenous, isotropic porous medium. The governing equations are simplified by assuming long wavelength and low Reynolds number approximations. The numerical and approximate analytical solutions of the problem are obtained by a semi-numerical technique, namely the homotopy perturbation method. The influence of the dominating physical parameters such as fractional Maxwell parameter, relaxation time, amplitude ratio, and permeability parameter on the flow characteristics are depicted graphically. The size of the trapped bolus is slightly enhanced by increasing the magnitude of permeability parameter whereas it is decreased with increasing amplitude ratio. Furthermore, it is shown that in the entire pumping region and the free pumping region, both volumetric flow rate and pressure decrease with increasing relaxation time, whereas in the co-pumping region, the volumetric flow rate is elevated with rising magnitude of relaxation time.     

Flow and heat transfer of an electrically conducting fluid of second grade in a porous medium over a stretching sheet subject to a transverse magnetic field

An analysis is performed for flow and heat transfer of a steady laminar boundary layer flow of an electrically conducting fluid of second grade in a porous medium subject to a transverse uniform magnetic field past a semi-infinite stretching sheet with power-law surface temperature or power-law surface heat flux. The effects of viscous dissipation, internal heat generation of absorption and work done due to deformation are considered in the energy equation. The variations of surface temperature gradient for the prescribed surface temperature case (PST) and surface temperature for the prescribed heat flux case (PHF) with various parameters are tabulated. The asymptotic expansions of the solutions for large Prandtl number are also given for the two heating conditions. It is shown that, when the Eckert number is large enough, the heat flow may transfer from the fluid to the wall rather than from the wall to the fluid when Eckert number is small. A physical explanation is given for this phenomenon.     

Study of Heat Transport in a Porous Medium Under G-jitter and Internal Heating Effects

In this article we study the combined effect of internal heating and time-periodic gravity modulation on thermal instability in a closely packed anisotropic porous medium, heated from below and cooled from above. The time-periodic gravity modulation, considered in this problem can be realized by vertically oscillating the porous medium. A weak non-linear stability analysis has been performed by using power series expansion in terms of the amplitude of gravity modulation, which is assumed to be small. The Nusselt number has been obtained in terms of the amplitude of convection which is governed by the non-autonomous Ginzburg?CLandau equation derived for the stationary mode of convection. The effects of various parameters such as; internal Rayleigh number, amplitude and frequency of gravity modulation, thermo-mechanical anisotropies, and Vadász number on heat transport has been analyzed. It is found that the response of the convective system to the internal Rayleigh number is destabilizing. Further it is found that the heat transport can also be controlled by suitably adjusting the external parameters of the system.     

Magnetic Heat Transfer Enhancements on Fin-Tube Heat Exchangers

通过DNS方法解耦合的三维非稳态流动和固流体能量方程组,本文研究了两平行磁质平板和圆管所组成的肋片式圆管换热器单元与震荡流体间的传热过程.对不同的磁场频率和振幅的三维动态流热场的模拟结果表明增强磁场频率和振幅能很有效地增加周期平均传热强度达到强化传热的目的.     

Seed bubbles stabilize flow and heat transfer in parallel microchannels

Seed bubbles are generated on microheaters located at the microchannel upstream and driven by a pulse voltage signal, to improve flow and heat transfer performance in microchannels. The present study investigates how seed bubbles stabilize flow and heat transfer in micro-boiling systems. For the forced convection flow, when heat flux at the wall surface is continuously increased, flow instability is self-sustained in microchannels with large oscillation amplitudes and long periods. Introduction of seed bubbles in time sequence improves flow and heat transfer performance significantly. Low frequency (∼10 Hz) seed bubbles not only decrease oscillation amplitudes of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures, but also shorten oscillation cycle periods. High frequency (∼100 Hz or high) seed bubbles completely suppress the flow instability and the heat transfer system displays stable parameters of pressure drops, fluid inlet and outlet temperatures and heating surface temperatures. Flow visualizations show that a quasi-stable boundary interface from spheric bubble to elongated bubble is maintained in a very narrow distance range at any time. The seed bubble technique almost does not increase the pressure drop across microsystems, which is thoroughly different from those reported in the literature. The higher the seed bubble frequency, the more decreased heating surface temperatures are. A saturation seed bubble frequency of 1000–2000 Hz can be reached, at which heat transfer enhancement attains the maximum degree, inferring a complete thermal equilibrium of vapor and liquid phases in microchannels. Benefits of the seed bubble technique are the stabilization of flow and heat transfer, decreasing heating surface temperatures and improving temperature uniformity of the heating surface.     

研究了雷诺数Re=200, 1000, 线速度比$\alpha =0.5$,

研究了雷诺数Re=200, 1000, 线速度比$\alpha =0.5$, 2.0, 4.0, 强迫振荡频率$f_{s}=0.1\sim 2.0$情况下的旋转振荡圆柱绕流问题. 通 过基于非结构同位网格有限体积法对Navier-Stokes方程进行数值求解. 对流项、扩 散项和非恒定项的离散格式均具有二阶精度，利用SIMPLE算法处理压力-速度耦合. 计算得到了作用力系数随不同控制参数的变化规律. 通过对升力系数的频谱分析得到 自然脱落频率和强迫振荡频率下的作用力振幅. 通过对不同频率作用力幅值的分析， 得到频率之间的竞争关系，进而定量地给出了不同尾迹涡脱落模式的分区图.     

DAMPING OF VERTICALLY EXCITED SURFACE WAVE IN WEAKLY VISCOUS FLUID

In a vertically oscillating circular cylindrical container, singular perturbation theory of two-time scale expansions is developed in weakly viscous fluids to investigate the motion of single free surface standing wave by linearizing the Navier-Stokes equation. The fluid field is divided into an outer potential flow region and an inner boundary layer region. The solutions of both two regions are obtained and a linear amplitude equation incorporating damping term and external excitation is derived. The condition to appear stable surface wave is obtained and the critical curve is determined. In addition, an analytical expression of damping coefficient is determined. Finally, the dispersion relation, which has been derived from the inviscid fluid approximation, is modified by adding linear damping. It is found that the modified results are reasonably closer to experimental results than former theory. Result shows that when forcing frequency is low, the viscosity of the fluid is prominent for the mode selection. However, when forcing frequency is high, the surface tension of the fluid is prominent.     

随机激励下受热翼面的模态频率特性试验研究

现代高速飞行器结构热模态频率特性试验研究,对这类飞行器设计校核和飞行安全具有重要意义。根据飞行过程中遭受的气动加热特性设计了瞬态热环境模拟系统,同时,根据高温环境的特点对测试中的激励和测量方式进行了重新设计,成功地将普通激振器应用于高温结构模态试验,最终将热环境模拟系统与振动测试系统组合,形成一套考虑瞬态热影响的热模态试验系统,实现了瞬态热环境下结构模态的地面测试。对一个切尖三角翼测量了各个加热区的温度随加热时间的变化,验证了加热温度控制的精确性;在纯随机激励下对测得的激励和振动响应信号采用短时傅里叶变换(Short Time Fourier Transformation,STFT)进行时变模态参数辨识,获得了前四阶模态频率随加热时间的变化,并与结构有限元数值计算结果进行了比较,试验与计算结果吻合得很好,验证了该试验方法对热模态测试问题的有效性和准确性。通过分别对瞬态和稳态热环境下结构模态频率试验和计算结果的分析,探讨了结构瞬态温度场对模态频率影响的机理,揭示了结构内部存在的热应力和材料属性的变化,是决定模态频率随加热时间变化趋势的内在原因。     

Experimental and numerical study on flow characteristics and heat transfer of an oscillating jet in a channel

A fluidic oscillator can produce self-induced and self-sustaining oscillating jet by fluid supply without moving parts. This device has attracted research interest in heat and mass transfer enhancement in recent years. In the current study, a double-feedback fluidic oscillator was numerically investigated based on three-dimensional unsteady Reynolds-averaged Navier-Stokes equations (3D-URANS) while the operating fluid is an incompressible flow. Then, the results were validated with experimental data by two-dimensional time-resolved particle image velocimetry (2D-TR-PIV) and thermographic phosphor thermometry (TPT) for the velocity and temperature field, respectively. A grid sensitivity study was done by comparison of instantaneous and time-averaged flow fields. Additionally, the proper orthogonal decomposition (POD) method was used to find the phase information of the oscillating jet, and fast Fourier transform (FFT) analysis was used to find the frequency of the oscillating jet to validate the numerical results. The effect of the working fluid was also studied. Finally, in order to determine the effect of the Reynolds number on heat transfer enhancement, the Q-criterion was calculated to provide detailed insight into the oscillating mechanism. The results show that the non-dimensional frequency of oscillation is independent of either the working fluid or mass flow rate. Additionally, for a given fluid, increasing Re causes strong vortices and increases the frequency of oscillation. However, the convection heat transfer did not change significantly when varying the mass flow rate because the convection velocity of vortices increases as the mass flow rate is enhanced. A comparison with a free jet reveals that the oscillating jet in a channel is useful in terms of covering a larger area.     

Effects of aspect ratio on unsteady solutions through curved duct flow

The effects of the aspect ratio on unsteady solutions through the curved duct flow are studied numerically by a spectral based computational procedure with a temperature gradient between the vertical sidewalls for the Grashof number 100 ≤ Gr ≤ 2 000. The outer wall of the duct is heated while the inner wall is cooled and the top and bottom walls are adiabatic. In this paper, unsteady solutions are calculated by the time history analysis of the Nusselt number for the Dean numbers Dn = 100 and Dn = 500 and the aspect ratios 1≤γ≤ 3. Water is taken as a working fluid （Pr =7.0）. It is found that at Dn = 100, there appears a steady-state solution for small or large Gr. For moderate Gr, however, the steady-state solution turns into the periodic solution if γ is increased. For Dn = 500, on the other hand, it is analyzed that the steady-state solution turns into the chaotic solution for small and large Gr for any γ lying in the range. For moderate Gr at Dn = 500, however, the steady-state flow turns into the chaotic flow through the periodic oscillating flow if the aspect ratio is increased.