螺旋曲面三维肋强化传热的数值模拟研究

利用计算流体力学软件Fluent6.3研究普通直型三维肋和螺旋曲面三维肋分别用于矩形流体通道的流动换热特性,分析两者在雷诺数Re为2000～10000、迎流攻角β均为45°时的传热与阻力特性。研究结果表明:矩形流体通道内加螺旋曲面三维肋时平均努赛尔数Nu和平均阻力系数f皆大于普通直型三维肋,并随着Re的增大而增大。同时,利用综合评价准则数G对其传热进行综合判定,G值大于1。螺旋曲面三维肋的综合性能优于普通直型三维肋。     

斜截半椭圆柱面涡流发生器强化换热和压降特性的试验研究

在矩形通道内布置一对斜截半椭圆柱面涡流发生器(高宽比h/b=1/2),通过改变斜边倾角α、来流攻角β、前沿间距S、布置方式以及顺列、错列两排涡流发生器研究不同工况下的换热和压降特性.试验雷诺数范围为Re=700～26 800.结果表明,在试验条件下α=12°时的换热效果要好于α=20°,Re=1 300时两者的最高局部对流换热系数分别比平直通道高19.7%和10.7%,Re=26 800时分别为23.2%和18.33%.斜截半椭圆柱面涡流发生器(α=12°)的最优攻角为β=60°,最优前沿间距为S= 20 mm.当Re=26 800,β=60°,S=20 mm时的最高局部对流换热系数约比平直通道高23.2%,压力损失约比平直通道高33.7%.雷诺数较小时,由于回流滞止区的影响,布置两排涡流发生器后,换热效果反而弱于单排.随着雷诺数增大到4 000,双排涡流发生器的换热效果要明显优于单排,Re=26 800时,最高局部对流换热系数约比单排涡流发生器高14.4%,比平直通道高33.4%.通过试验得出斜截半椭圆柱面涡流发生器在β=60°,α=12°时的涡旋作用距离大约在250～300 mm.     

矩形带肋通道中蒸汽与空气换热特性比较试验研究

模拟实际燃气轮机叶片内冷通道几何及传热结构,研究了蒸汽和空气在两面带有肋片的矩形通道中当雷诺数为10000⁸0000时的换热和摩擦特性。试验通道宽高比(肋片在宽面上)为0.5,肋间距p/e为10,通道阻塞比为0.047,试验通道长度L为1000mm。试验结果显示,蒸汽和空气在带肋通道中的平均换热系数,平均摩擦系数和换热性能随雷诺数的变化趋势几乎相同;在相同试验条件下,蒸汽在带肋通道中的平均换热系数比空气高30.2%,平均摩擦系数比空气高18.4%;蒸汽在带肋通道带肋面和光滑面上的换热性能比空气分别高8.4%和7.3%。     

螺旋外肋管换热器壳侧流体流动与换热特性数值研究

利用CFD技术,研究了螺距及肋高对螺旋外肋管换热器壳程流体流动和换热性能的影响。建立了壳程周期性单元流道模型并进行数值计算,详细分析了不同螺距、肋高对壳程流体流动及换热的影响。结果表明,适当地减小螺距可以有效地强化管壁对流换热效果,同时伴随着摩擦阻力系数的增大,但综合换热性能更强。在高雷诺数区域,随肋高的增大,螺旋外肋管的换热性能随肋高的增加比较明显,而摩擦阻力系数的变化与肋高变化的程度大致相同。速度场和温度场的平均协同角随螺距的减小而减小,协同程度增强。提出了螺旋外肋管换热器壳程换热及阻力性能公式供相关工程设计参考,并对外肋管换热器壳程综合强化性能做出了评价。     

梯形带肋通道内空气流动与换热特性数值研究

带肋扰流冷却是航空发动机涡轮叶片内部冷却的一种主要冷却方式。采用CFX等计算流体动力学软件,通过气热耦合数值模拟方法,对比分析了梯形冷却通道和矩形冷却通道内空气的流动和换热特性,研究了雷诺数(Re=10 000~30 000)和肋角度(30°、45°、60°和90°)对梯形带肋通道内空气流动及换热特性的影响。     

螺旋内肋扭曲管换热与流阻性能研究

将两种现行的热流强化换热技术结合起来,提出螺旋内肋扭曲管这一新型强化换热管,利用ANSYS ICEM CFD对该强化换热管进行网格划分和网格独立性考核,利用Fluent分析比较了螺旋内肋扭曲管、扭曲管和圆管3种换热管的雷诺数Re在2300~50000范围内的换热和流阻性能,得出其努赛尔数Nu、阻力系数f、等泵功准则下综合评价指标PEC及热势容差-火积耗散ΔE的变化关系,结果表明,螺旋内肋扭曲管具有优于扭曲管的综合换热性能,具有较好的研发潜力和较高的工程应用前景。     

滑动轴承油水冷却器的换热与流阻特性

滑动轴承运行过程中油膜剪切和搅油产生的功耗约80%会转换成热量,导致油箱内润滑油温度升高,为将润滑油温度控制在设计值内,通常在油箱内安装油水冷却器带走热量,冷却器换热性能直接影响滑动轴承的运行温度和主机的正常工作。油水冷却器的散热能力主要由换热系数和流阻特性决定,对不同油流量条件下的油水冷却器进行试验测试,研究其换热和流阻特性,结果表明：在控制进油、进水温度及进水流量的前提下,油流速在一定范围内增加时,换热系数先增加后趋于稳定,压力损失增大,换热因子和阻力系数减小,冷却器可在一定油流速范围内获得最佳的换热和流阻特性;试验获得了对数坐标系下努赛尔数、欧拉数与雷诺数的拟合曲线,理论计算与试验拟合结果具有良好一致性,并在此基础上获得了油流速与换热系数及流阻的经验公式。     

雷诺数和半径比对90°圆弧弯管流动特性的数值研究

运用FLUENT软件中的RNG k-ε模型对不同雷诺数Re和不同半径比Rc/D下90°圆弧弯管内的流体流动进行了数值模拟,分析了管内流体的速度场分布、压力场分布以及二次流现象,考察了不同雷诺数Re(1×104Re1×106)和不同半径比Rc/D(0.6Rc/D2)组合下72种工况的局部阻力系数以及局部阻力影响长度。结果表明:同一Re下,Rc/D越大,局部阻力系数ξ越小。Rc/D一定时,当Re6×105时,局部阻力系数ξ随Re的增大迅速减小;当Re≧6×105时,局部阻力系数ξ随Re的变化很小,几乎趋于不变值。同一Re下,Rc/D越大,局部阻力影响长度越短。局部阻力影响长度随Re的增大先减小后趋于定值,且Rc/D越小,局部阻力影响长度越延后趋于定值。     

基于场协同理论的高温热管散热模拟分析

为了提高碟式太阳能热发电系统的高温热管的散热特性,根据场协同理论,利用变分法构造拉格朗日函数,从而求出附加体积力的动量方程,采用数值模拟的方法研究了纵向涡流强度和进口流体雷诺数对水夹套管内对流换热和阻力特征的影响,提出了流场优化强化换热的方法。研究发现基于场协同理论纵向涡流可以明显强化水夹套管内的对流换热,但同时流体流动阻力也随之增大,并且流动阻力的增加幅度要小于对流换热增强的幅度;纵向涡流强度越大,进口流体雷诺数越大,流体具有更为优良的综合强化换热特征。     

ZigZag微通道内S-CO_2流动传热性能分析

以超临界二氧化碳(S-CO_2)为工质,对其在Zig Zag半圆形横截面微通道内湍流状态下流动传热性能进行数值计算,分析了Zig Zag角度θ、单位周期流道轴向长度P对传热与流动阻力的影响。结果表明,流体在Zig Zag微通道内流动,在流道拐弯处有旋涡产生,导致流通面积减少,流体主流速度急剧增大并冲刷换热壁面,使边界层减薄或破坏,并且该位置附近湍动能急剧增大,增强了流体的扰动与混合,促进了能量传递,强化了换热;随着Zig Zag角度θ增大,微通道内传热性能提高而流动阻力急剧增大;随着单位周期流道轴向长度P增大,传热性能与流动阻力均下降;在文中所述计算条件下,θ=15°,P=15 mm时Zig Zag微通道内S-CO_2耦合传热综合传热性能最优。     

Experimental and numerical investigation of heat transfer characteristics in a square channel with various truncated ribs

Oblique ribs are widely applied to the internal cooling of turbine blades to promote the heat transfer between blade wall and coolant. In this study, the effect of several new types of truncated ribs on the heat transfer characteristics in 45° oblique rib channels is investigated experimentally and numerically. The numerical results obtained by the SST k-ω turbulence model agree well with the experimental data for the Reynolds number ranging from 10000 to 60000. The results indicate a significant entrance effect on the heat transfer in truncated rib channels. The numerical results show that ribs continuously truncated at 3.8 mm gives the best heat transfer performance among the newly truncated ribs. Compared with the original structure, the Nusselt number and heat transfer enhancement factor of newly truncated ribs increased by 24.6 % and 17.8 %, respectively. Concurrently, the friction factor is reduced by 5.1 %.

     

水平槽道内几种周期性结构的传热与流动研究

采用数值模拟方法,对流体在不同周期性结构二维水平槽道内的传热性能与流动特性进行研究。分析不同雷诺数条件下,布置于槽道下壁面的障碍块表面时均努塞尔数Nu及表面摩擦因数μ的变化规律。计算结果表明,当障碍块之间量纲一距离分别为0.50和0.25、雷诺数Re=1 000时;槽道上、下壁面均布置障碍块的传热性能较仅在槽道下壁面布置障碍块的传热性能有了显著提高,前者比后者的传热性能分别提高393%和574%。当雷诺数Re=500、障碍块之间量纲一距离为0.50时,对在槽道中间设置一斜板与不设置斜板两种情况进行比较后发现,设置斜板的槽道比不设置斜板的槽道传热性能提高27.1%。当在下壁障碍块右上角处放置一小圆柱体后发现:传热性能并未得到有效提高,但流动摩擦因数有所减小,从而使得输送流体的泵功降低。     

A parametric study of the 2D model of solar air heater with elliptical rib roughness using CFD

The heat transfer can be improved by providing artificial roughness on absorber plate of the solar air heat. Many studies are available on circular, semi-circular, triangular and rectangular rib roughened solar air heater. But in present study heat transfer enhancement by providing elliptical ribs on absorber plate was analyzed by developing CFD code on non-commercial ANSYS (Fluent) 12.1 software. The simulations were performed on 2-D CFD model and analysis was carried out to study the effect of relative roughness width, relative roughness height and relative roughness pitch on heat transfer and friction factor. The Reynolds number range from 4000 to 15000 and turbulence phenomena is modeled by using Reynolds-average Navier-Stokes equations (RANS). The mathematical modeling is validated and compared with available results. The strong vortex formation takes place in the main stream flow because of elliptical roughness, which improved heat transfer augmentation in the solar air heater. The local turbulence kinetic energy strongly influenced by orientation of the elliptical ribs. The value of average Nusselt number increases by increasing relative roughness height but it decreases with the increase of relative roughness width and relative roughness pitch. The rib width has significant effects on heat transfer enhancement and maximum Nusselt number is observed for relatively small roughness width (i.e., 0.5) among the considered range of 0.5 mm to 2.0 mm. The maximum value of Nusselt number and friction factor is observed for relative roughness width of 0.5, relative roughness height of 0.045, and relative roughness pitch of 6.

     

Heat transfer and friction factor enhancement in a square channel having integral inclined discrete ribs on two opposite walls

The influence of a gap provided in integral inclined ribs on heat transfer and friction factor enhancement is investigated. Experiments are conducted to obtain heat transfer and friction factor characteristics in a square channel with two opposite in-line ribbed walls for Reynolds numbers from 5000 to 40000. The test section of square channel composed of integral inclined ribs with a gap and has a length-tohydraulic diameter ratio (L/D _h ) of 20. The rib pitch-to-height ratio (p/e) is 10, the rib height-to-hydraulic diameter ratio (e/D _h ) is 0.060 and rib attack angle (α) varies in the range of 30⁰ to 90⁰ (4 steps). The relative gap position (d/W) and relative gap width (g/e) is varied in the range of 1/5–2/3 (5 steps) and 0.5–2.0 (4 steps), respectively. The enhancement in heat transfer and friction factor of this roughened duct was compared with smooth duct and duct roughened with continuous inclined ribs (with no gap) under similar flow condition. Presence of inclined ribs with a gap yields about 4-fold enhancements in Nusselt number and about 8-fold increase in the friction factor compared with smooth duct and about 1.3 times and 1.4 times higher than the case of continuous ribs (without gaps) for the entire range of parameters investigated. Ribs with relative gap width of 1.0 at relative gap position of 1/3 and attack angle of 60° provides maximum heat transfer and friction factor enhancement.     

车用小通道平行流换热器传热流动试验与分析

小通道平行流换热器是燃料电池汽车的主要散热部件。吸收了电堆废热的冷却液(50%乙二醇溶液),流过小通道换热管,由换热器外侧空气冷却。在进液温度、进风温度、冷却液流量以及风速变化的试验工况下,测试了换热器的传热流动性能。引入量纲一参数k,评估了各工况参数对换热量、阻力影响的强弱。接着,分析液侧努谢尔数Nu和摩阻系数f随雷诺数Re的变化趋势,结果显示:在小通道内(当量直径D=2.685 mm),冷却液从层流到湍流的转浪点Re_c=1 750,介于微尺度与常规尺度的临界值之间。在此基础上,通过多元回归法,拟合得到层流和湍流的液侧换热系数,摩阻系数的关联式,以及空气侧阻力f_a公式。Nu和f的计算值与试验值误差分别在[-7.06%,5.93%]和[-3.95%,4.11%]内,f_a的误差在[-2.22%,3.62%]内。基于这些关联式,建立数学模型,可在广泛多变的运行条件下,对换热器的运行性能进行理论预测和评估。     

An experimental study on heat transfer characteristics with turbulent swirling flow using uniform heat flux in a cylindrical annuli

An experimental study was performed to investigate heat transfer characteristics of turbulent swirling flow in an axisymmetric annuli. The static pressure, the local flow temperature, and the wall temperature with decaying swirl were measured by using tangential inlet conditions and the friction factor and the local Nusselt number were calculated for Re=30000-70000. The local Nusselt number was compared with that obtained from the Dittus-Boelter equation with swirl and without swirl. The results showed that the swirl enhances the heat transfer at the inlet and the outlet of the test tube.     

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.     

Numerical investigation of fluid flow and heat transfer characteristics in a helically-finned tube

In order to investigate the characteristics of flow and heat transfer rate in a Helically-finned tub (HFT), we used continuity, momentum and energy equations under a steady, three-dimensional and incompressible fluid flow assumptions. For the performance metrics, we considered the Darcy friction factor, Colburn j-factor, volume goodness factor and area goodness factor of the HFT. We could also evaluate the effect of geometry parameters on the results of local pressure coefficient, fluid vorticity and Nusselt number of the HFT. We carried out the CFD calculation for a range of laminar flow (Re = 100) and turbulent flow (Re = 2000 and 10000). In a laminar and turbulent flow regime, the friction factor increases with increasing the each geometric parameter. While the Colburn j-factor decreases as increasing these geometric parameters. Consequently, the thermal performance of HFT is poorer than that of single straight circular tube type because of having a small volume and area goodness factor as increasing the Reynolds numbers.     

Numerical investigation on developing laminar forced convective heat transfer and entropy generation in an annular helicoidal tube

In this study, numerical investigations are conducted for forced convective heat transfer in an annular helicoidal tube under uniform wall temperature condition for laminar flow including developing region. The numerical computations reveal the developments and distributions of heat transfer and flow fields in the annular helicoidal tube when the outer tube wall is heated and the inner tube wall is insulated. The effects of Reynolds number, curvature ratio, and coil pitch on the circumferential average friction factor and Nusselt number at different axial locations, and the non-dimensional entropy generation number of laminar convection in an annular helicoidal tube are investigated. In addition, the differences of flow and heat transfer characteristics between the annular helicoidal tube and circular helicoidal tube are also described.     

An experimental approach to turbulent heat transfer using a symmetric expanded plane channel

The flow in a symmetric expansion plane channel is known to deflect to one side of a channel even at a low Reynolds number due to the Coanda effect. Details of flow structure have been investigated by various authors; however, there have been a few works conducted in the area of on heat transfer. This paper presents experimental results of turbulent heat transfer in separated and reattached flows in a symmetric expansion plane channel. Experiments were conducted using a low-speed open-circuit wind tunnel. The step H was 20 mm high and 200 mm wide, with an expansion ratio of 2.0. The Reynolds number based on the uniform flow velocity at step and H was varied from 5,000 to 35,000, respectively. The mean and turbulent fluctuating velocities were measured using mainly two types of split film probe. A cold single wire probe was used for measuring the mean and turbulent fluctuating temperatures. It was found that the local Nusselt number profile was considerably different on the upper and lower walls due to the Coanda effect, which is was caused by instability between the upper and lower separated shear layers. Empirical formulae for the maximum Nusselt number in the reattachment region are hereby proposed for the upper and lower walls, respectively. The two formulae are well correlated with the previous general formula proposed. The location of maximum Nusselt number is found to be very close to the flow reattachment point. Details of the velocity and temperature fields were clarified and their correlations with the heat transfer characteristics described above investigated. Furthermore, the wavelet transformation methodology was employed to study instantaneous flow and temperature behaviors, which exhibited its usefulness in the study of the present complicated flow and temperature fields.