湍流边界层内钝体扰流的流动与传热特性

应用大涡模拟方法对小尺度开缝圆柱涡流发生器强化传热和流动减阻的机理进行研究。水平开缝圆柱置于充分发展湍流边界层内,分析不同间隙比对开缝圆柱尾流、湍流边界层拟序结构以及槽道底面流动与换热特性的影响。为验证所采用数值方法的准确性与可靠性,将矩形空槽道的计算结果与前人直接数值模拟结果及与采用相关准则关系式所得结果进行对比。计算结果表明：湍流边界层内钝体扰流的尾迹流与壁面边界层的相互作用能够显著提高槽道的换热性能。与未开缝的基准圆柱相比,间隙比小于2.0时,开缝圆柱通道的整体热性能较好;间隙比为2.0时,其综合性能系数最大;间隙比大于2.0时,整体热性能较差。与矩形空槽道相比,最大努塞尔数可提高17.45%,最小摩擦因数可减小4.94%。     

Study on the unsteady wakes past a square cylinder near a wall

Experimental and numerical studies on the unsteady wake field behind a square cylinder near a wall were conducted to find out how the vortex shedding mechanism is correlated with gap flow. The computations were performed by solving unsteady 2-D Incompressible Reynolds Averaged Navier-Stokes equations with a newly developed ε-SST turbulence model for more accurate prediction of large separated flows. Through spectral analysis and the smoke wire flow visualization, it was discovered that velocity profiles in a gap region have strong influences on the formation of vortex shedding behind a square cylinder near a wall. From these results. Strouhal number distributions could be found, where the transition region of the Strouhal number was atG/D=0.5 - 0.7 above the critical gap height. The primary and minor shedding frequencies measured in this region were affected by the interaction between the upper and the lower separated shear layer, and minor shedding frequency was due to the separation bubble on the wall. It was also observed that the position(y/G) and the magnitude of maximum average velocity (u/u∞) in the gap region affect the regular vortex shedding as the gap height increases.     

Numerical investigation of fluid flow past a square cylinder using upstream,downstream and dual splitter plates

A two-dimensional numerical study is carried out to analyze the drag reduction and vortex shedding suppression behind a square cylinder in presence of splitter plate arranged in upstream, downstream and both upstream and downstream location at low Reynolds number (Re = 160). Computations are performed using a Single relaxation time lattice Boltzmann method (SRT-LBM). Firstly, the code is validated for flow past a single square cylinder. The obtained results are compared to those available in literature and found to be in good agreement. Numerical simulations are performed in the ranges of 1 ≤ L ≤ 4 and 0 ≤ g ≤ 7, where L and g are the length of splitter plate and gap spacing between the splitter plate and main square cylinder, respectively. The effect of these parameters on the vortex shedding frequency, time-trace analysis of drag and lift coefficients, power spectra analysis of lift coefficient, vorticity contours visualization and force exerted on the cylinder are quantified together with the observed flow patterns around the main cylinder and within the gap spacings. The observed results are also compared with a single square cylinder without splitter plate. We found that at some combinations of L and g, the mean drag coefficient and Strouhal number reach either its maximum or minimum value. It is found that the drag is reduced up to 62.2 %, 13.3 % and 70.2 % for upstream, downstream and dual splitter plates, respectively as compared to a single square cylinder (without splitter plate). In addition, in this paper we also discussed the applications of SRT-LBM for suppression of vortex shedding and reduction of the drag coefficients.     

Axial slit wall effect on the flow instability and heat transfer in rotating concentric cylinders

The slit wall effect on the flow instability and heat transfer characteristics in Taylor-Couette flow was numerically studied by changing the rotating Reynolds number and applying the negative temperature gradient. The concentric cylinders with slit wall are seen in many rotating machineries. Six different models with the slit number 0, 6, 9, 12, 15 and 18 were investigated in this study. The results show the axial slit wall enhances the Taylor vortex flow and suppresses the azimuthal variation of wavy Taylor vortex flow. When negative temperature gradient exists, the results show that the heat transfer augmentation appears from laminar Taylor vortex to turbulent Taylor flow regime. The heat transfer enhancement become stronger as increasing the Reynolds number and slit number. The larger slit number model also accelerates the flow transition regardless of the negative temperature gradient or isothermal condition.     

斜截椭圆柱式涡流发生器强化传热的大涡模拟

对流体在放置斜截椭圆柱式涡流发生器矩形槽道内的流动与传热特性进行大涡模拟,得出流场中速度、温度与压力参数的瞬态变化特性,再现温度场、压力场及诱导旋涡的变化过程,并对流动结构及涡流发生器强化传热的机理进行分析。为验证大涡模拟计算结果的准确性,在相同条件下对未布置涡流发生器的空槽道分别采用湍流模型和大涡模拟进行对比计算,两者的计算结果符合较好。计算结果表明：流场中布置的涡流发生器可以诱导漩涡,而由其所诱导的流向涡对强化传热起主要作用。与相同条件下未布置涡流发生器的情况相比,局部对流换热系数可提高64%～105%,平均对流换热系数则可提高17%～36%;涡流发生器附近位置的对流换热系数提高幅度最大,传热面附近流体的流动状况及流动结构与传热密切相关。     

Combined Streamline Upwind Petrov Galerkin method and segregated finite element algorithm for conjugate heat transfer problems

A combined Streamline Upwind Petrov-Galerkin method (SUPG) and segregated finite element algorithm for solving conjugate heat transfer problems where heat conduction in a solid is coupled with heat convection in viscous fluid flow is presented. The Streamline Upwind Petrov-Galerkin method is used for the analysis of viscous thermal flow in the fluid region, while the analysis of heat conduction in solid region is performed by the Galerkin method. The method uses the three-node triangular element with equal-order interpolation functions for all the variables of the velocity components, the pressure and the temperature. The main advantage of the presented method is to consistently couple heat transfer along the fluid-solid interface. Four test cases, which are the conjugate Couette flow problem in parallel plate channel, the counter-flow in heat exchanger, the conjugate natural convection in a square cavity with a conducting wall, and the conjugate natural convection and conduction from heated cylinder in square cavity, are selected to evaluate efficiency of the presented method.     

组合涡发生器在螺旋板式换热器上的强化传热研究

在螺旋板式换热器螺旋通道内设置三角翼和椭圆柱组合涡发生器,利用流体计算软件Fluent进行三维数值模拟。研究了Re为4000～7000内组合涡发生器对通道平均Nu和平均阻力系数f的影响,并应用场协同原理进行了分析。与只加装椭圆柱涡发生器的螺旋板式换热器进行对比,结果表明,纵向涡发生器产生的二次流能改善螺旋通道内的速度场与温度场的协同性,起到强化换热作用。在正三角形排列方式下,组合涡发生器通道的平均Nu比椭圆柱涡发生器的平均Nu增大8.7%,阻力因子f减小23.7%,强化换热的效果较好。     

Investigation of a novel Couette flow with cylindrical baffles

The high-precision measure instrument for flow velocity is essential for industrial applications because the high-precision velocity can well reflect the physical characteristic of the flow. A restricted laminar Couette flow with cylindrical baffles, using a synthetic heat conduction liquid, was designed to obtain a steady vortex flow and wider work scope, according to Couette flow and Suspension flow characteristics. The heat transfer mechanism was investigated with a laminar flow model by the Fourier law. The research indicates that the heat transfer enhancement is related to the Temperature Boundary Layer (TBL). The TBL is affected by the Velocity Boundary Layer (VBL). The TBL thickness and Nusselt number (Nu) have a dependent relationship. The Reynolds number (Re) and the gap between the baffle and plate wall (Δh/h) can further affect Nu. The vortex flow generated by Couette flow can significantly enhance the heat transfer performance by a double spiral structure, which can rapidly mix heat fluxes and make the temperature converge to uniform. There is a sensitive and stable relationship between flow velocity and heat transfer. Notably, it is linear when Δh/h or Re is small, which can be used to design a high-precision thermal flow velocity meter.     

Numerical solution of flow past two circular cylinders in different arrangement rotating in various directions

The combined effects of different rotation types and the Reynolds number on the flow past two rotating circular cylinders about their axes in different arrangement (Side-by-side and tandem) were considered at a range of 520 ≤ Re ≤ 1570 and 0 ≤ ω ≤ 4 (ω is the rotational speed) at one gap spacing of L/D = 2 for a side-by-side arrangement, ω = 0, 2000 ≤ Re ≤ 21000 and L/D = 2 and 4/3 for tandem arrangement (L and D are the distance between the centers of two cylinders and the cylinder diameter, respectively). The results show that the variation of both rotation speed and Reynolds number have an important role in changing the pattern of vortex shedding. As the rotational speed further increases, the separation phenomenon in the boundary layers disappears at the attachment rotational speed. Regardless of Reynolds number, as ω increases, the lift decreases for up and down cylinders while the drag decreases for up cylinder and increases for down cylinder. Quantitative information is highlighted about the flow variables such as the pressure coefficient the Stanton number, the skin friction factor and wall viscous coefficient of the cylinders.     

A numerical study on mixed convection in a lid-driven cavity with a circular cylinder

A two-dimensional numerical simulation is carried out in this study to investigate mixed convection in a lid-driven cavity with an isothermal circular cylinder. The simulation is conducted at three Reynolds numbers of Re = 100, 500, and 1000 under a fixed Grashof number of Gr = 10⁵. The top wall of the cavity moves to the right at a constant velocity and is kept at a low temperature of T _c , whereas the stationary bottom wall is kept at a constant high temperature of T _h . The immersed-boundary method, which is based on the finite volume method, is adopted for the boundary of the circular cylinder that is present in the square cavity. The present study aims to investigate the effects of circular cylinder on fluid flow and heat transfer in a cavity at different locations. The fluid flow and heat transfer characteristics in the cavity strongly depend on the position of the circular cylinder as well as on the relative magnitude of the forced convection and the natural convection caused by the movement in the top wall of the cavity and the heating at the hot bottom wall, respectively.     

Numerical simulation of vortex-induced vibration of a square cylinder

Vortex-induced vibration (VIV) of a square cylinder in a cross flow is examined numerically. Both the rigid and elastic cases are simulated at a low Reynolds number of 100. The approach solves the unsteady flow field using a finite element method with a deforming grid to accommodate the moving cylinders. As for the cylinder motions, a two-degree-of-freedom structural dynamics model is invoked. Fluid-structure interactions are resolved through iteration at the same time step. The calculated results for the case of rigid cylinder indicated that the non-dimensional vortex shedding frequency (or the Strouhal frequency) of a square cylinder at rest is 0.13, which is in good agreement with the published results. For the elastic case, with the change of the cylinder’s natural frequency, “lock-in” and “beat” phenomena were successfully captured. The phenomena of resonance and galloping can also be indicated.     

基于流固耦合传热的摩托车发动机冷却水套优化设计

建立了摩托车发动机缸体与水套流固耦合传热仿真模型,进行了流体与固体之间的共轭传热仿真。结果表明,水套对缸体的散热效果基本满足散热要求;但缸体缸套区域水套内的冷却水流速分布较差,存在比较大范围的漩涡和流动死区,导致缸套局部过热,周向温差过大,为此对分水孔进行了优化设计,基本消除了流动死区,显著提高了水套冷却效能,缸体主要区域的温度显著减低,缸套局部过热也得到很大缓解。     

基于CFD的风冷摩托车发动机气缸体的传热仿真研究

针对单缸风冷摩托车发动机气缸体的传热问题,基于热力学第一定律建立了进气过程中气缸壁传热量的数学模型,分析了气缸周壁的传热方式,通过CAD三维软件建立了发动机气缸体的三维模型,并进行计算网格划分,给定边界条件,应用计算流体力学( CFD)软件对发动机气缸体传热过程进行数值模拟计算,得出气缸体的温度分布图,从而获得气缸壁传给进气气体的热量值,为进一步研究摩托车发动机气缸体的传热问题提供参考.     

斜截半椭圆柱面涡发生器在管道中的传热数值模拟研究

管内设置一种斜截半椭圆柱面涡发生器,利用计算软件F luent进行数值模拟研究,研究以烟气为加热介质,冷空气为冷却介质的换热方式在不同Re数下不同攻角和倾角的传热及阻力特性,并与光管进行了对比。结果表明:斜截半椭圆柱面涡发生器能明显提高换热性能,在所研究的纵向涡发生器中,攻角为60°,倾角为15°时,涡发生器强化传热综合效果最佳。     

Flow topology,heat transfer characteristic and thermal performance in a circular tube heat exchanger inserted with punched delta winglet vortex generators

To improve the heat transfer rate and thermal performance, the punched delta winglet vortex generators, DWVGs, were inserted in the middle of the circular tube heat exchanger. The effects of the flow attack angles and the flow directions were investigated numerically for the Reynolds number Re = 100–2000. The finite volume method and the SIMPLE algorithm were used to study. The results are reported in terms of the flow structure, heat transfer behavior and thermal performance evaluation and also compared with the smooth tube with no vortex generators. As the numerical results, the use of the DWVGs in the tube can improve the heat transfer rate and thermal performance by creating the vortex flow through the tested section. The rise of the flow attack angle results in the increasing strength of the vortex flows. The flow attack angle of 25° performs the highest heat transfer rate and thermal performance, while the flow attack angle of 0o gives the reversed results. The computational results reveal that the optimum thermal enhancement factor is around 2.80 at Re = 2000, α = 25°, with the winglet tip pointing downstream. The correlations on both the Nusselt number ratio and friction factor ratio for the DWVG in the tube heat exchanger are presented.     

Effect of Reynolds numbers on flow past four square cylinders in an in-line square configuration for different gap spacings

In this paper two-dimensional (2-D) numerical investigation of flow past four square cylinders in an in-line square configuration are performed using the lattice Boltzmann method. The gap spacing g = s/d is set at 1, 3 and 6 and Reynolds number ranging from Re = 60 to 175. We observed four distinct wake patterns: (i) a steady wake pattern (Re = 60 and g = 1); (ii) a stable shielding wake pattern (80 ≤ Re ≤ 175 and g = 1); (iii) a wiggling shielding wake pattern (60 ≤ Re ≤ 175 and g = 3) and (iv) a vortex shedding wake pattern (60 ≤ Re ≤ 175 and g = 6). At g = 1, the Reynolds number is observed to have a strong effect on the wake patterns. It is also found that at g = 1, the secondary cylinder interaction frequency significantly contributes for drag and lift coefficients signal. It is found that the primary vortex shedding frequency dominates the flow and the role of secondary cylinder interaction frequency almost vanish at g = 6. It is observed that the jet between the gaps strongly influenced the wake interaction for different gap spacing and Reynolds number combination. To fully understand the wake transformations the details vorticity contour visualization, power spectra of lift coefficient signal and time signal analysis of drag and lift coefficients also presented in this paper.     

螺旋曲面通道内组合涡发生器的强化传热及优化研究

对螺旋曲面通道壁面上设置三角翼和椭圆柱两种组合涡发生器进行了研究,利用计算机流体软件Fluent进行数值模拟,在雷诺数Re为4000～7000范围内,研究组合间距s、三角翼攻角α、椭圆柱攻角β对换热的影响。由正交实验对涡发生器的结构进行整体优化,得出影响换热因素的主次顺序及优化组合结构,并通过实验比较优化结构与常规结构的综合强化换热效果。结果表明当s＝90 mm、α＝45°、β＝45°时,其综合换热效果最佳。     

沟槽数量对泰勒涡流稳定性的影响

用粒子图像测速技术(Particle image velocimetry,PIV)对光滑壁面和沟槽数量为18的模型内的流场进行测量,初步获得沟槽的存在对于环隙内流场的影响规律;同时利用数值计算获得沟槽数量为18的模型内的流场分布,通过对比试验结果和数值模拟结果,验证数值计算方法可靠性。通过数值计算研究沟槽数量0、6、12、18模型内的流场分布,在比较沟槽模型与光壁模型环隙内的速度场和涡量场的基础上,得出以下结论：较少的沟槽数量增加了泰勒涡的轴向尺寸,随着沟槽数的增加,沟槽结构对流场影响不断加剧,环隙内最大径向速度和轴向速度均有不同程度的增加;对比不同轴向位置的流场分布,发现沟槽区域内存在明显的旋涡流动,不同轴向位置沟槽内的流体存在着进出环隙内的运动。     

近距离淹没冲击射流动态流场特性

近距离下淹没冲击射流具有重要的应用前景,但有关其动态流场特性的研究尚不充分。利用粒子图像测速技术（Particle image velocimetry,PIV）对冲击距离为H/D=1时的动态流场特性进行测量,研究雷诺数Re和喷嘴端面轮廓（壁面约束）对间隙内旋涡生成的影响,并对试验数据进行涡量分析与本征正交分解方法（Proper orthogonal decomposition,POD）分析。结果表明,雷诺数Re对不同喷嘴端面轮廓（壁面约束）的旋涡生成和迁移影响规律不同。雷诺数对直角型喷嘴的双涡环模式流态影响较小,但对圆角型喷嘴卷吸流态影响较大,且随雷诺数的增大流场出现动态对称涡旋,该对称涡旋逐渐向间隙外迁移且呈现出明显的周期性变化特征。涡量分析得到时均场的涡量大小与分布情况,通过与POD分析中瞬态场的含能大尺度结构分布相结合,揭示间隙外涡旋能量的来源与变化规律。通过前4阶瞬态脉动速度场重构,可以从微观时间尺度进一步说明瞬态速度场的突变性与周期性。研究成果将有助于加深对冲击射流本质的理解。     

半椭圆涡流发生器强化换热机理

对单列及双列方式排列的一种斜截半椭圆柱体涡流发生器,以稳态的气、水逆流换热方式为模型,用数值方法模拟了迎流攻角为0°～90°,雷诺数6 000～48 000范围内的传热特性。结果表明:涡流发生器可以诱导纵向涡与流向涡,起到强化传热的作用,纵向涡在强化传热中起主要作用。