交叉三角形波纹板流道的流动与换热特性研究

本文实验研究了不同的热流密度和波纹高度对交叉三角形波纹板流道的流动与换热的影响。利用低Re数k-ε湍流模型进行了数值模拟,二者吻合情况较好。并且得到了三维复杂计算区的速度场。研究表明:交叉三角形波纹板流道具有良好的换热效果。热流密度和波纹高度越大,换热效果越好。底部区域产生强烈的顺时针旋涡,并且具有明显的周期性。     

人字形波纹板相变流动及换热特性研究

针对人字形波纹板流道内蒸发换热过程进行了三维建模和数值模拟,研究了不同波纹倾角下人字形波纹板蒸发换热时流动及换热特性,并与单相流换热特性进行了对比。结果表明,蒸发相变对人字形波纹板内流体的流动形态影响不大,波纹倾角对人字形波纹板表面气相体积分数分布影响较大;相同波纹参数下,蒸发换热换热系数比单相流换热换热系数提高20%~100%;随着波纹倾角的增大,蒸发换热增强,波纹倾角为75°时换热性能最优。     

非均匀壁面棒束间流场结构的数值分析

本文采用CFD方法对棒束内的流动状况进行非稳态模拟,利用已知的实验结果对计算模型进行验证,详细分析了非均匀壁面粗糙度条件下雷诺数对冷却剂交混流动特性的影响。结果表明:非均匀壁面粗糙度对棒柬间速度场分布及交混系数有明显的影响。并且,当流动雷诺数提高时,棒束间的交混强度先增强然后减弱,存在一个最佳的流动雷诺数条件,在此雷诺数下,非均匀壁面粗糙度棒束间的湍流交混和相干涡结构最强烈。     

高负荷低压涡轮边界层转捩预测及其机理分析

本文利用数值模拟方法在不同雷诺数、自由来流湍流度(FSTI)下对某高负荷低压涡轮叶型边界层转捩流动进行了数值模拟,并与实验测试结果进行了对比。结果表明:该转捩模型在不同的雷诺数和FSTI情况下均可以得到满意的数值计算结果,而仪仪利用湍流模型得不到合理的计算结果;当进口特征雷诺数较低时,叶型表面长分离泡的出现在很大范围内改变了壁面处的静压分布;随着自由来流湍流度的提高,叶型吸力面分离点的位置靠后且其对应的再附点位置靠前,即分离泡长度较短,高来流湍流度可以减小叶型吸力面的分离区域,减小叶型损失。而较低的自由流湍流度则在叶型尾缘对应着较长的分离泡,增大了流动分离再附的预测难度。     

一种可压缩湍流RANS模拟的浸入边界方法

提出一种可压缩湍流RANS(Reynolds-Averaged Navier-Stokes)模拟的浸入边界方法。该方法的解重构中,不再定义用于插值的参考点。利用其周围计算内点上已知的流动变量值,解域内紧靠壁面的网格节点上的流动变量通过距离倒数加权方法插值确定。为降低高雷诺数湍流模拟的近壁区域网格分辨率要求,采用一种显式壁面函数将无滑移边界条件转化为无穿透边界条件和当地剪切应力条件;该壁面函数无需迭代求解,提高了计算效率。RAE2822翼型亚、跨音速绕流的数值模拟验证了方法的可靠性。     

不同粒径粗糙元对壁面湍流拟序结构的影响

采用氢气泡流动显示技术,研究了不同粒径粗糙元对壁湍流拟序结构的影响。实验中粗糙元布置在氢泡丝前,均匀排成一排,基于平均速度和水力直径的雷诺数从14300到48000变化,离散粗糙元直径分别为0.9、1.6、2.3、4、5和6mm,粗糙元间距为2 cm。实验发现:光滑壁面和粗糙元壁面近壁湍流条带数量和条带高度都随雷诺数增大而增加。相同雷诺数下,粗糙元壁面流动产生的条带数量和条带高度随粗糙元粒径增大而增加。粗糙元壁面湍流条带数量和条带高度均比光滑壁面大。该研究对壁湍流的流动控制工程应用具有参考价值。     

竖直圆管中超临界压力CO2在低Re数下对流换热研究

本文对超临界压力CO2自下而上流过内径为2mm的加热圆管,在低进口Re (Rein≈1700)条件下的对流换热进行了数值模拟,并与实验结果进行了比较.结果表明,在进口雷诺数较低(Rein≈1700)而热流密度较高时,由于密度变化导致浮升力对流动产生扰动,流动从层流提前转变为湍流,换热大大增强并导致壁面温度的异常分布.使用LB湍流模型可以较好地模拟此时流动从层流向湍流的过渡现象,而采用层流与湍流相结合的分区计算方法的结果与实验测量值吻合得更好.由于浮升力的影响,径向速度呈M型分布,速度最大值在靠近壁面某处;当热流密度很大时,在管子中心区会出现回流.     

混合板式换热器板通道的数值模拟研究

本文采用二维非稳态数学模型、周期性充分发展边界条件对混合板式换热器中的板通道内的流动和换热进行数值模拟。在Re=4407～22035工况范围内,计算了半圆型、半椭圆Ⅰ型和半椭圆Ⅱ型三种不同纵截面形状波纹板通道内的流动和换热性能。数值模拟结果分析表明,波纹通道内的流动会因为流体从凸面流向凹面时产生回流而形成涡,即产生扰动,强化了后面的换热,但同时增大了阻力。并得出半圆型通道换热最强,Nu为椭圆Ⅱ型的1.4倍;但其阻力最大,压降为椭圆Ⅱ型的1.9倍。     

分流流道内流动结构的测试与分析

分叉管道常见于工程上的流量分配装置、飞行器内外涵道结构,其中分流流道的流动结构影响着主流道的通流量,且回流涡的产生与扩大会引起分流流道的阻塞,使之失去分流的作用。本文采用高速摄影对分流流道内部的流动结构进行识别,发现在其入口处存在明显的回流涡,进而对回流涡处壁面进行了压力动态测量。压力动态测试与高速摄影的结果显示出分流通道中回流涡流场变化的频率信息。在不同的管道入口雷诺数(Re_(in)=80249到179414)下,回流涡处的壁面压力变化具有混沌特性,随着入口雷诺数的增加,压力脉动的幅值增大,而随机性却减小,确定性和稳定性增强,且在回流涡产生位置尤为显著,影响整个分流流道的通流能力。     

不同排列粗糙元对湍流拟序结构的影响

采用氢气泡流动显示技术,研究了壁面不同排列粗糙元对近壁湍流拟序结构的影响。实验中基于平均速度和水力直径的雷诺数分别为14300、31200、48000。氢泡丝上游布置的离散粗糙元直径为2．3、4和6mm,得到了光滑壁面和5种不同排列方式的离散粗糙元壁面湍流条带和湍流斑块特征。结果发现：相同雷诺数和相同排列方式下条带间距...     

使用RSM模型对旋风除尘器内湍流各向异性的数值模拟

旋风除尘器内三维湍流流场具有典型的各项异性,本文采用雷诺应力模型求解该流场得到了三维湍流流场的详细数据,得出了雷诺应力6个分量的分布图,揭示了这类强旋流流场湍流各向异性的特征。典型的几个算例的实践表明:采用雷诺应力模型求解这类流场非常必要,它能较好地反映这类流场的真实物理流动;与相关实验数据的比较表明吻合性较好,显示了本文算法的可行性;计算中发现,发展非结构网格更有利于模拟壁面附近的流动,并节省计算机内存。     

Lattice Boltzmann simulation of viscous-fluid flow and conjugate heat transfer in a rectangular cavity with a heated moving wall

In the present work, conjugate heat transfer in a rectangular cavity with a heated moving lid is investigated using the lattice Boltzmann method (LBM). The simulations are performed for incompressible flow, with Reynolds numbers ranging from 100 to 500, thermal diffusivity ratios ranging from 1 to 100, and Prandtl numbers ranging from 0.7 to 7. A uniform heat flux through the top of the lid is assumed. Results show that LBM is suitable for the study of heat transfer in conjugate problems. Effects of the Reynolds number, the Prandtl number and the thermal diffusivity ratio on hydrodynamic and thermal characteristics are investigated and discussed. The streamlines and temperature distribution in flow field, dimensionless temperature and Nusselt number along the hot wall are illustrated. The results indicate that increase of thermal diffusivity yields the removal of a higher quantity of energy from lid and its temperature decreases when increasing the Reynolds and the Prandtl numbers.     

Experimental Investigation of Opposing Mixed Convection in a Channel with an open Cavity Below

Abstract

In this article, mixed convection in an open cavity with a heated wall bounded by a horizontal unheated plate is investigated experimentally. The heated wall is on the opposite side of the forced inflow. The results are reported in terms of wall temperature profiles of the heated wall and flow visualization. The range of pertinent parameters used in this experiment are Reynolds numbers (Re) from 100 to 2,000 and Richardson numbers (Ri) from 4.3 to 6,400. Also, the ratio between the length and the height of cavity (L/D) ranges from 0.5–2.0, and the ratio between the channel and cavity height (H/D) is equal to 1.0. The lack of experimental results on mixed convection in a channel with an open cavity below was an impetus for investigating this configuration when one cavity vertical wall is heated at uniform heat flux. The present results show that at the lowest investigated Reynolds number, the surface temperatures are lower than the corresponding surface temperatures for Re = 2,000 at the same ohmic heat flux. The flow visualization shows that for Re = 1,000, there are two nearly distinct fluid motions: a parallel forced flow in the channel and a recirculation flow inside the cavity. For Re = 100, the effect of a stronger buoyancy determines a penetration of thermal plumes from the heated plate wall into the upper channel. Moreover, the flow visualization shows that for lower Reynolds numbers, the forced motion penetrates inside the cavity, and a vortex structure is adjacent to the unheated vertical plate. At higher Reynolds numbers, the vortex structure has a larger extension while L/D is held constant.     

Wall Shear Stress and Heat Transfer of Downward Bubbly Flow at Low Flow Rates of Liquid and Gas

The effect of suppression of turbulence in a downward bubbly flow and its impact on the wall shear stress and heat transfer are discussed. Measurements were carried out for Reynolds numbers Re = 5000–10000, which were calculated from the velocity of the liquid phase and with the gas volumetric flow rate ratio β = 0–0.05. Data on the size of bubbles detaching from the edges of an array of capillaries in a liquid flow are given. The influence of the disperse phase dimensions on the wall shear stress and heat transfer is discussed. It is shown that change in the size of the dispersed phase can lead to both intensification and deterioration of heat transfer as compared with a single-phase flow at constant flow rates of liquid and gas at the channel inlet. The cause of the heat transfer deterioration is “laminarization” of the flow in the near-wall region. An analysis of the spectral power of signals is given.     

翅片管束式管壳式换热器三维数值模拟研究

本文提出了运用多孔介质模型、分布阻力模型和k-ε湍流模型对壳侧为翅片管束的壳管式换热器壳侧速度场与温度场进行三维数值模拟的方法,并对一相应类型换热器壳侧的流动与换热进行了数值模拟,得出了壳侧流场参数的图示以及壳侧进出口压降,温差,换热量随壳侧Re变化的特性曲线。     

超临界压力CO2在水平圆管内流动传热数值分析

采用SST k-w低雷诺数湍流模型对加热条件下超临界压力CO2在内径di=22.14 mm,加热长度Lh=2440 mm水平圆管内三维稳态流动与传热特性进行了数值计算.通过超临界CO2在水平圆管内的流动传热实验数据验证了数值模型的可靠性和准确性.首先,研究了超临界压力CO2在水平圆管内的流动传热特点,基于超临界CO2在类临界温度Tpc处发生类液-类气“相变”的假设,揭示了水平圆管顶母线和底母线区域不同的流动传热行为.然后,分析了热流密度qw和质量流速G对水平圆管内超临界压力CO2流动换热的影响,通过获取流体域内的物性分布、速度分布和湍流分布等详细信息,重点解释了不同热流密度qw和质量流速G下顶母线内壁温度Tw,i分布产生差异的传热机理,分析结果确定了类气膜厚度d、类气膜性质、轴向速度u和湍动能k是影响顶母线壁温分布差异的主要因素.研究结果可以为超临界压力CO2换热装置的优化设计和安全运行提供理论指导.     

A restricted nonlinear large eddy simulation model for high Reynolds number flows

Recent numerical studies of the restricted nonlinear (RNL) model have demonstrated its ability to reproduce important features of wall turbulence despite a severe reduction in the number of degrees of freedom. In these prior studies, the RNL model included full resolution of the viscous term. In this work, we extend the RNL model to arbitrarily high Reynolds numbers by developing a RNL large eddy simulation (LES) framework along with a method to systematically identify an appropriate streamwise wavenumber support based on spectral properties of wall turbulence. This method leads to a band-limited RNL–LES system which is successful in reproducing some of the most important statistical features captured in previous low to moderate Reynolds number simulations, e.g. the mean velocity and second-order moment profiles. The RNL–LES framework offers a new approach to understanding the connection between coherent structures and the momentum transfer mechanisms of wall turbulence at arbitrarily high Reynolds numbers, where resolution of the viscous terms can become computationally expensive even with the relatively low computational complexity afforded through the dynamical restriction of the RNL model.     

Turbulent passive scalar transport under localized blowing

The effect of blowing through a localized slot on the wall turbulence dynamics and heat transfer process is analyzed by direct numerical simulations in a fully developed turbulent channel flow. The severity parameter is mild and there is no flow separation induced by the blowing. The shear stress transport and temperature energy budget is discussed in detail. The wall shear and flux decreases immediately downstream the slot in a similar manner but the Reynolds analogy does not hold over the slot. The physical process is governed by the production and pressure redistribution over the slot in a complex manner. The turbulent transport and especially the advection play an essential role in the heat transfer mechanism.     

低阶格式在大涡模拟计算中的适用性

采用三维Taylor-Green涡作为研究对象,利用工程中常用的低阶数值格式,研究格式本身的数值误差对大涡模拟计算的影响.结果表明:三种数值格式的数值耗散行为都与亚格子模型行为类似,即在小雷诺数下,流场比较光滑时,耗散很小,当雷诺数增加,流动转捩为湍流,流场梯度增大,耗散显著增大.对于MUSCL格式和二阶有界中心格式,在高雷诺数下,亚格子尺度模型没有明显改善计算结果,但也没有使计算结果恶化.中心格式相比其它两种格式,数值耗散最小,但是在高雷诺数湍流情况下,中心格式的数值耗散仍然主导了能量的耗散,再添加亚格子模型,计算结果反而变得稍差.对于工程中的低阶格式而言,采用中心格式计算大涡模拟是比较好的选择,而且在计算不存在稳定性问题时,采用不添加亚格子模型的隐式大涡模拟效果更好.     

A Reynolds stress model for turbulent flows of viscoelastic fluids

A second-order closure is developed for predicting turbulent flows of viscoelastic fluids described by a modified generalised Newtonian fluid model incorporating a nonlinear viscosity that depends on a strain-hardening Trouton ratio as a means to handle some of the effects of viscoelasticity upon turbulent flows. Its performance is assessed by comparing its predictions for fully developed turbulent pipe flow with experimental data for four different dilute polymeric solutions and also with two sets of direct numerical simulation data for fluids theoretically described by the finitely extensible nonlinear elastic – Peterlin model. The model is based on a Newtonian Reynolds stress closure to predict Newtonian fluid flows, which incorporates low Reynolds number damping functions to properly deal with wall effects and to provide the capability to handle fluid viscoelasticity more effectively. This new turbulence model was able to capture well the drag reduction of various viscoelastic fluids over a wide range of Reynolds numbers and performed better than previously developed models for the same type of constitutive equation, even if the streamwise and wall-normal turbulence intensities were underpredicted.