回热器板叠流动特性研究

回热器内交变流动的研究是回热式制冷机研究的重点,采用数值模拟的方法对回热器内交变流动特性进行了研究.结果表明回热器内压力、速度分布不再呈波动状分布,而是近似的呈直线分布.回热器内的交变流动阻力系数是时间与位置的函数,也具有明显的波动性.     

贝克利数对圆管流道交变流动传热的影响分析

从可压缩黏性流体的一般控制方程出发,通过数值模拟,研究管内可压缩层流交变流动传热现象,重点关注贝克利数对于传热的影响.采用离散傅立叶级数的形式表征管内交变流动传热特性,研究表明采用五阶级数拟合值与原始数据的相对误差可小于10%.给出了贝克利数为2和300时,一个周期内交变流动传热过程的计算结果,并进行了对比分析.根据模...     

交变流动传热的相似准则

为了利用相似原理研究管内工质交变流动传热特性,从可压缩粘性流体的一般控制方程出发,推导了描述理想气体管内交变流动传热的相似准则体系,包括Va、Remax、γ、Ma、Pr和Ar,并具体分析了各准则的物理意义,以及与其它常用相似准则的关系。     

竖直多孔表面通道内液氮沸腾换热实验研究

在液氮自然循环流动时，对竖直多孔表面管管内沸腾换热及外管单面加热时竖直多孔表面套管内沸腾换热，进行了实验研究，分析并讨论了通道的当量半径、热流密度及含气率对沸腾换热的影响。     

封闭三角形通道内热管的自然对流换热研究

论述和分析了封闭腔内自然对流换热的研究进展,运用Fluent软件对封闭三角形通道内的热管与壁面的二维散热问题进行了数值模拟,模拟了封闭腔内空气自然对流换热的温度场和速度场。     

二维多孔材料散热性能分析与设计

二维多孔材料存在一个易于流动的方向并具有较大的面密度 , 因此在具有良好的比刚度和比强度的同时也具有良好的散热性能 , 研究强迫对流下的散热性能对其多功能化设计具有重要意义。本文中利用数值方法求解考虑二维多孔材料内部流体流动规律、 热传导和对流换热影响的流固耦合热传输问题 , 分析了多孔率和微结构尺寸对散热性能的影响并进行了最优参数设计 ; 通过分析比较 5种具有典型微结构形式的二维多孔材料的散热性能 , 给出了微结构形式对散热性能的影响。提出了以需要满足的散热性能为约束条件 , 以满足需求的设计参数的可调范围(设计参数的允许变化范围)为设计目标的最优散热结构设计理念。以此理念得到的设计结果 , 更有利于根据其他性能的要求对材料进行多功能化设计。分析表明 , 具有正六边形微结构的二维多孔材料的散热性能最优 , 并有利于实现轻质多功能化设计。     

饱和多孔介质一维瞬态波动问题的解析分析

采用基于混合物理论的多孔介质模型,提出了饱和多孔介质一维动力响应的初边值问题。利用拉氏变换和卷积定理,分别得到了边界自由排水时在任意应力边界条件和任意位移边界条件下瞬态波动过程的解析表达。几种典型的数值算例同时给出了两类边界条件下瞬态波动过程中多孔固体的位移场、应力场和孔隙流体的速度场、压力场。结果表明,饱和多孔介质的波动过程是多孔固体和孔隙流体中以同一速度传播的两种波动的耦合过程,时效特性分析也揭示了饱和多孔介质固有的表观粘弹性性质。     

微极饱和土波动分析中的变分原理

主要给出饱和多孔微极介质波动方程变分所对应的泛函表达式和有限元离散化方程。首先对u-U形式的饱和多孔微极介质波动方程和边界条件进行Laplace 变换,形成力学中的非齐次边值问题,然后构造变分后满足波动方程和边界条件的泛函,最后将有限元插值形式代入泛函表达式得到单元体的有限元离散方程。此方程对微极饱和多孔介质的动力固结问题数值分析具有重要意义。     

液氦温区微小漏孔流动特性研究

低温环境下系统器件的密封性能至关重要,对于低温密封研究,了解低温下微小漏孔泄漏的流动特性很有必要.本文构建了具有随机粗糙度壁面的二维微通道,应用仿真软件研究液氦温区漏孔微通道内气流的流动特性,计算分析通道内各物理量的分布和变化.结果 表明随机粗糙度壁面对速度分布的扰动在近壁面影响最大,随着远离壁面的方向减小,对中心区域...     

微柱群多孔介质内部流场的实验研究

用PDMS加工了5个宽2mm、深100μm、长4cm的微通道,其内部充满方形排列的微柱群形成多孔介质模型,采用微流体粒子图像测速仪(Micro-PIV)获得该多孔介质内速度场的分布,在此基础上计算了剪切应力,得到剪切应力场分布及其均方根.实验结果表明,速度分布具有较好的对称性,符合低雷诺数条件下的流动规律.速度值沿展向呈现类谐波的周期性变化规律,不同流向位置的速度幅值不同.靠近圆柱壁面的对称位置存在两条流线,其上流速保持不变.由剪切应力分布可看到近壁区存在较大速度梯度,剪切强度与雷诺数成正比,与孔隙率的三次方呈近似反比关系.     

蜂窝板换热器内部流动传热特性研究

建立了蜂窝板换热器湍流流动的物理数学模型,并应用数值分析方法模拟了蜂窝板换热器的三维流动传热过程;分析了不同雷诺数下通道内流动阻力和换热性能及其随雷诺数的变化规律,并与相同当量直径的平行平板通道的流动换热性能进行了对比。结果表明,蜂窝板换热器在换热系数提高的同时流动阻力也增大了,在雷诺数Re=3000~15000的范围内,其传热努塞尔数比平行平板增大了0.93~2.12倍,阻力系数增大了2.24~2.35倍。最后从场协同理论的角度分析了蜂窝板强化传热的机理。     

一种压力波驱动的非共振型直流换热器的工作原理及实验

分析了传统交流型换热器的不足,提出了一种压力波(声波)驱动的非共振直流型换热器,对其基本工作原理进行了阐述,建立了原理性实验系统,初步研究了其换热性能,验证了该换热器用于交变流动热力系统的可行性.     

竖直矩形可视化流动沸腾换热实验台的设计及初步研究

为了了解矩形窄通道内流动沸腾及传热现象的机理,建立了单面加热竖直矩形窄通道可视化流动沸腾换热实验台进行了实验。实验结果表明:矩形窄通道流动沸腾过程的换热系数存在最大值;随着干度的增加(即热流密度的增加)其换热系数逐渐降低,转为以液膜蒸发为主的流动沸腾换热,此时需控制热流密度,避免干涸现象的发生。     

Heat transfer characteristics of oscillating flow regenerator filled with circular tubes or parallel plates

Under assumption of small perturbation, linear thermoacoustic theory was applied to analyze heat transfer characteristics of compressible oscillating flow in two kinds of simple regenerators filled with circular tubes or parallel plates. Based on the cross-sectional oscillating velocity and temperature distributions, the exact expressions of Nusselt number were derived in complex notation. The Nusselt number is the function of Prandtl number, kinetic Reynolds number Re_ω and the third dimensionless variable, D. Here, the D is defined as the ratio of heat transfer capability aroused by mean temperature gradient and gas compressibility. Both the gas compressibility and mean temperature gradient effects were discussed and two corresponding Nusselt numbers were given. In particular, simpler expressions for these two Nusselt numbers were deduced for extreme values of Re_ω and D. Finally, combined effect of gas compressibility and non-zero mean temperature gradient on heat transfer characteristics were analyzed via D. The analysis shows that the mean temperature gradient gives predominant contribution to the heat transfer performance of oscillating flow regenerator.     

Unsteady forced and free convection flow with mass‐transfer and chemical reaction past an infinite vertical porous plate with oscillating plate temperature

Abstract

An approximate analysis of a two‐dimensional unsteady flow of chemically reacting gases is presented under the following conditions: (1) Constant suction at the plate, (2) the plate temperature is oscillating about a non‐zero constant mean, (3) the uniform free‐stream velocity, (4) presence of foreign mass with negligible Soret‐Dufour effects, (5) first‐order chemical reaction, and (6) small amplitude. Solutions to the mean velocity, the mean temperature, the transient velocity, the transient temperature, the amplitude and phase of the skin‐friction and the rate of heat transfer are derived. It is observed that due to the presence of a chemical reaction, the mean skin‐friction decreases but the mean rate of heat transfer increases.     

Transferts thermiques en écoulements oscillants laminaires incompressibles

Phenomena concerning the temperature variations and the heat transfer are studied in the specific case of oscillating flow with null mean velocity circulating between two infinite walls. A 1D model is established and the interesting scale parameters are deduced from theoretical equations. The particular case of an oscillating laminar flow for incompressible fluid is detailed in order to illustrate and to discuss the effects of thermal interactions between the fluid and walls. Influence of wall length comparatively to the fluid displacement is studied. Conclusions for designing thermal heat exchangers of Stirling engines or PTR are proposed.     

弦月形狭缝通道内液氮受迫流动沸腾传热强化的研究

研究了液氮在弦月形狭缝通道中受迫流动沸腾时的传热特性。发现液氮在弦月形狭缝通道中的受迫流动沸腾具有很高的换热系数,有显著的强化换热效果。详细分析了弦月形狭缝通道内液氮沸腾传热及流动的偏心特性。研究对于进一步理解狭缝通道沸腾传热强化的机理和狭缝强化传热技术在工程中的应用有着重要的意义。     

Numerical and experimental studies on laminar forced convective heat transfer in a channel with surface‐mounted heated blocks

Abstract

The steady separated flow due to heated blocks mounted on one principal wall of a two‐dimensional channel has been numerically and experimentally studied. Numerical solutions of the Navier‐Stokes equations using the finite‐difference and the power‐law techniques have been obtained up to a Reynolds number of 2600. The effects of the Reynolds number and the block spacing on the fluid flow and heat transfer are investigated in detail. Results show that there exists two different types of flow between blocks, the D‐type and the K‐type flows. Furthermore, the Nusselt number monotonously increases or decreases along every face of the blocks. The calculated results of the reattachment length behind the second block and the local Nusselt number distribution compare well with the results obtained by the LDV and the naphthalene sublimation measurements, respectively. Heat transfer correlating equations are presented in terms of the Reynolds number and the block spacing.     

An Unsteady Oscillatory Flow of Generalized Casson Fluid with Heat and Mass Transfer: A Comparative Fractional Model

It is of high interest to study laminar flow with mass and heat transfer phenomena that occur in a viscoelastic fluid taken over a vertical plate due to its importance in many technological processes and its increased industrial applications. Because of its wide range of applications, this study aims at evaluating the solutions corresponding to Casson fluids’ oscillating flow using fractional-derivatives. As it has a combined mass-heat transfer effect, we considered the fluid flow upon an oscillatory infinite vertical-plate. Furthermore, we used two new fractional approaches of fractional derivatives, named AB (Atangana–Baleanu) and CF (Caputo–Fabrizio), on dimensionless governing equations and then we compared their results. The Laplace transformation technique is used to get the most accurate solutions of oscillating motion of any generalized Casson fluid because of the Cosine oscillation passed over the infinite vertical-plate. We obtained and analyzed the distribution of concentration, expressions for the velocity-field and the temperature graphically, using various parameters of interest. We also analyzed the Nusselt number and the skin friction due to their important engineering usage.     

Enhancing flow boiling heat transfer in microchannels for thermal management with monolithically-integrated silicon nanowires

Thermal management has become a critical issue for high heat flux electronics and energy systems. Integrated two-phase microchannel liquid-cooling technology has been envisioned as a promising solution, but with great challenges in flow instability. In this work, silicon nanowires were synthesized in situ in parallel silicon microchannel arrays for the first time to suppress the flow instability and to augment flow boiling heat transfer. Significant enhancement in flow boiling heat transfer performance was demonstrated for the nanowire-coated microchannel heat sink, such as an early onset of nucleate boiling, a delayed onset of flow oscillation, suppressed oscillating amplitudes of temperature and pressure drop, and an increased heat transfer coefficient.