截面形状对旋转扰流柱通道流动换热特性的影响

为合理设计航空发动机涡轮动叶尾缘冷却结构,采用ANSYS FLUENT软件模拟了四种扰流柱截面形状在旋转状态下的流动换热特性。对比分析了不同旋转数(Ro)和不同扰流柱截面形状时通道内部的三维流场分布、湍流动能分布、努塞尔数(Nu)分布以及阻力系数。其中扰流柱的截面形状包含圆形、菱形、矩形和椭圆形,Ro数包含0、0.2、0.4和0.6四种数值。模拟过程中通道入口雷诺数为7 000,壁面恒定热流为1 000 W/m²。结果表明,截面形状和旋转数对流动和换热状态有着显著影响,矩形扰流柱通道的换热系数和阻力系数最高;随着旋转数的增加,迎风面和背风面的换热系数差异逐渐增大;在旋转作用下,扰流柱尾缘区域出现了纵向二次流,该二次流显著破坏了壁面附近的边界层,有利于背风面换热增强。     

旋转对非圆形扰流柱排换热影响的数值模拟

对有非圆形叉排扰流柱排的旋转矩形通道的换热旋转效应进行了数值模拟。扰流柱排的几何参数为/=4,/=1.33,/=2.7。来流雷诺数=20000下,旋转数o=0～0.3时对三种不同形状(方形、钻石形和液滴形)的扰流柱旋转矩形通道的换热研究。结果表明:迎风面与背风面的平均数都随着o的增加而增大;钻石形扰流柱排通道的换热最强,其次为方形、液滴形;此外,研究发现随着旋转的增强,矩形通道端壁各段的换热变化呈现出不同的规律,进口区端壁的换热会增强,而扰流柱排区以及尾缘区的换热则先减后增。     

涡轮叶片尾缘典型结构的流动与换热特性研究

针对涡轮叶片尾缘"冲击+扰流柱"复合典型冷却结构,通过分析内部流场和换热特性,揭示"冲击+扰流柱"冷却结构中流动发展的过程以及冲击对压力分布和流场分布的影响,揭示涡轮叶片尾缘区内射流冲击扰流柱排通道内换热机理,详细分析了冲击下各个表面的换热情况。结果表明,压比的增大能够有效改善冷却通道端壁的换热性能,但同时增大了压力损失;在两种冲击距离下,n=3d换热效果优于n=6d,但是n=6d的下游换热覆盖效果优于n=3d。顺排时,冲击孔的平均换热系数大约是扰流柱的1.5倍;叉排时,冲击孔的平均换热系数大约是扰流柱的3倍,而其它部位的平均换热系数受排列方式的影响很小。因此,"冲击+扰流柱"冷却结构的匹配,对于优化涡轮叶片尾缘区域的换热及其重要。     

涡轮叶片尾缘端部冲击流动与换热特性研究

针对涡轮叶片尾缘吸力面热应力集中,容易造成叶片尾部烧毁的现象,提出端部冲击扰流柱结构,采用Realizable k-ε湍流模型和增强壁面函数分析涡轮叶片尾缘内部流场和吸力面换热特性,研究不同冲击孔与扰流柱排列方式的影响,揭示端部冲击扰流柱结构的流场与换热机理。研究表明,端部冲击扰流柱结构对于改善吸力面的换热效果要优于中间冲击扰流柱结构,对端壁的换热有显著提高;各表面平均换热系数均随着压比的增大而增大,顺排结构时,冲击孔换热最强,扰流柱换热次之;叉排结构时,冲击孔换热最强,隔板迎风换热次之;近距离冲击,顺排的综合效果优于其它几种结构,而远距离冲击,叉排的综合效果最好,其吸力面温度分布较均匀。     

扰流柱对间断交叉肋流动换热特性的影响研究

为了探究扰流柱对间断交叉肋通道流动与换热特性的影响,针对不同扰流柱数量和排布位置建立了不同的交叉肋模型,并通过数值模拟的方法,计算了各模型的阻力系数比、强化换热系数以及综合热效率3个性能指标的变化情况。研究结果表明：随着扰流柱数量的增大,阻力系数比和强化换热系数逐渐增大,而综合热效率不断下降;在进口雷诺数为20 000时,14柱模型与32柱模型相比,阻力系数比升高了15.4%,强化换热系数升高了32%,综合热效率提高了2.6%;将相同数量的扰流柱排布在通道内的不同位置对综合热效率的影响并不明显。     

分离结构扰流柱的叶片通道流动与换热数值研究

应用-两方程模型对分离结构扰流柱的叶片内冷通道的换热与流动进行了三维数值模拟研究,扰流柱的布置按简单叉排方式。计算结果表明:在本文的参数范围内,与完整扰流柱相比,分离结构扰流柱的换热效果略有增强,而阻力系数随之增大,分离位置居中的扰流柱通道的换热效果最好。随着开缝厚度的增加,通道表面换热效果和流动阻力系数均先略有增大而后逐渐减小。     

具有浇注缺陷的扰流柱流动与换热特性的对比分析

采用数值模拟方法对两种具有浇注缺陷的扰流柱通道的换热和流动阻力特性进行了模拟,重点研究了断裂结构和束腰结构扰流柱的影响,并与完整扰流柱通道进行了比较。结果表明所研究的扰流柱通道与完整扰流柱通道相比,其换热效果和压力损失系数相差不大。具有断裂结构和束腰结构的扰流柱减轻了叶片的重量,降低加工精度,对通道的流动和换热特性的影响不大,甚至具有一定的强化传热作用。     

带扰流片的矩形直通道内的流动与换热

建立了矩形直通道内三维可压缩流动与换热模型 ,对来流雷诺数Re在 1× 10 5～ 3× 10 5范围内 ,带顺排和错排扰流片的通道内部对流换热过程进行了模拟计算。计算结果表明 ,矩形扰流片具有明显的强化冷却效果 ,扰流片表面的对流换热系数明显高于光滑表面的值 ,有扰流片区域的壁面温度明显降低 ;错排扰流片与顺排扰流片相比 ,对流换热系数增大 4 %左右。计算结果归纳了传热和流动压降关系式。     

抽吸式自然对流换热及扰流柱对其影响

用稳态法进行了竖直平行平板间加扰流柱的抽吸式自然对流换热实验研究,全面考虑了包括辐射损失在内的各项修正,得到了精度不低于10％的经验公式。实验发现,对于不加扰流柱的平行平板通道,当温差大于50℃时,换热系数随间距的变化存在极值;小间距加扰流柱时,扰流柱对流动的阻滞起主要作用,换热系数低于无扰流柱时的值。     

抽吸式自动对流换热及扰流柱对其影响

用稳态法进行了竖直平行平板间加扰流柱的抽吸式自然对流换热实验研究，全面考虑了包括辐射损失在内的各项修正，得到了精度不低于１０％的经验公式。实验发现，对于不加扰流柱的平行平板通道，当温差大于５０℃时，换热系数随间距的变化存在极值，小间距加扰流柱时，扰流柱对流动的阻滞起主要作用，换热系数低于无扰流柱时的值。     

CALCULATION OF HEAT TRANSFER IN A RADIALLY ROTATING COOLANT PASSAGE

The three-dimensional flow field and heat transfer in a radially rotating coolant passage are studied numerically. The passage chosen has a square cross section with smooth isothermal walls of finite length. The axis of rotation is normal to the flow direction with the flow radially outward. The effects of Coriolis forces, centrifugal buoyancy, and fluid Reynolds number on the flow and heat transfer have all been considered. The analysis has been performed by using a fully elliptic, three-dimensional, body-fitted computational fluid dynamics code based on pressure correction techniques. The numerical technique employs a multigrid iterative solution procedure and the standard κ ? ε turbulence model for both the hydrodynamics and heat transfer. The effect of rotation is included by considering the governing equations of motion in a relative frame of reference that moves with the passage. The consequence of rotation is to bring higher velocity fluid from the core to the trailing surface, thereby increasing both the friction and heat transfer at this face. At the same time, the heat transfer is predicted to decrease along the leading surface. The effect of buoyancy is to increase the radial velocity of the fluid, thus generally increasing the heat transfer along both the leading and trailing surfaces. These effects and trends that have been predicted are in agreement with experimental heat transfer data available in the literature [1,2]. The quantitative agreement with the data was also found to be quite satisfactory.     

Enhanced Internal Heat Transfer on the Tip-Wall in a Rectangular Two-Pass Channel (AR = 1:2) by Pin-Fin Arrays

To improve gas turbine performance, the operating temperature has been increased continuously. However, the heat transferred to the turbine blade is substantially increased as the turbine inlet temperature is increased. Cooling methods are therefore much needed for the turbine blades to ensure a long durability and safe operation. The blade tip region is exposed to the hot gas flows and is difficult to cool. A common way to cool the tip is to use serpentine passages with a 180° turn under the blade tip cap taking advantage of the three-dimensional turning effect and impingement. Increasing internal convective cooling is however required to increase the blade tip life. In this article, enhanced heat transfer of a blade tip has been investigated numerically. The computational models consist of a two-pass channel with a 180° turn and arrays of pin-fins mounted on the tip-cap, and a smooth two-pass channel. Inlet Reynolds numbers range from 100,000 to 600,000. The computations are 3-D, steady, and incompressible. The detailed 3-D fluid flow and heat transfer over the tip surfaces are presented. The overall performance of the two models is evaluated. It is found that due to the combination of turning, impingement, and pin-fin crossflow the heat transfer coefficient of the pin-finned tip might be a factor of 1.84 higher than that of a smooth tip. This augmentation is achieved at the expense of a penalty of pressure drop around 35%. It is suggested that the pin-fins could be used to enhance blade tip heat transfer and cooling.     

Influence mechanism of rotation on the convective heat transfer and fluid-flow characteristics from a large diameter rotating isothermal cylinder

A micro-thermocouple is specially designed and employed to measure the temperature distribution in the boundary layer around a cylinder surface, and the influence mechanism of rotation on the convective heat transfer characteristics from a large diameter rotating isothermal cylinder has been experimentally investigated. The effect of rotation on the trailing vortex is observed and analyzed by using a schlieren apparatus. The results show that rotation has an obvious effect on the air flow and temperature distribution characteristics around the cylinder surface. There exists a worst convective heat transfer region which does not coincide with the trailing vortex region as previously reported.     

STUDY OF MIXED-CONVECTION HEAT TRANSFER FROM AN IMPINGING JET TO A SOLID WALL USING A FINITE-ELEMENT METHOD—APPLICATION TO COOKTOP MODELING

The mixed-convection flow from a hot vertical impinging jet on a colder horizontal disc has been studied. The geometry is analogous to a conventional burning gas cooktop. A numerical simulation of the system has been carried out using the finite-element method to study the dependence of fluid flow and heat transfer on the geometric, thermal, and fluid flow parameters. Results show that heat transfer efficiency versus several parameters such as inlet velocity magnitude and flue gas temperature has an optimum value, in which heat transfer efficiency is maximum. With thermal conductivity of the solid wall, velocity angle, and solid wall diameter heat transfer efficiency has increasing behavior. Finally, with solid wall height and solid wall thickness, heat transfer efficiency has diminishing behavior.     

Numerical Investigation into the Thermal Performance of Single Microchannels with Varying Axial Length and Different Shapes of Micro Pin-Fin Inserts

This study investigates numerically the thermal performance of combined microchannel heat sink with micro pin-fins with different cross-sectional shapes. The objective of this study is to investigate the best geometric configuration that maximizes the heat transfer from the heated base when the combined heat sink is subjected to a steady, laminar, incompressible convective fluid flow and heat transfer. The axial length of the solid substrate and microchannel is varied from 1 to 10 mm with fixed total volume of 0.9 mm³ while the number of rows of the different shapes of micro pin-fins was varied between three and seven. It was observed that best performance is obtained with a sixth row of circular-shaped micro pin-fins for the optimized combination of the microchannel and micro pin-fin heat sink. Results of the optimal axial length for fixed pressure drop range are also presented.     

Analysis of the heat transfer in the entrance region of optimised corrugated wall channel

In this work the analysis of the heat transfer in the entrance region of a channel composed by a corrugated profile and a flat wall is presented. The laminar and incompressible flow of a Newtonian fluid is assumed inside the channel, and an uniform heat flux is imposed on the external surface of the corrugated wall. The governing equations are solved with the help of a finite-element method, and the results are compared with the heat transfer coefficient in the entrance region of a flat channel. In order to investigate the sensitivity of the convective heat transfer coefficient to the Reynolds number under laminar conditions, the analysis have been performed for different values of the flow rate. The effect on the flowfield of the of the corrugated profile amplitude is also discussed.     

Numerical investigation of developing convective heat transfer in a rotating helical pipe

Numerical simulation is carried out for heat transfer characteristics of flow in rotating helical pipes. A good agreement has been achieved compared with experimental data from literature. The impacts of both co-rotation and counter rotation on local heat transfer enhancement are discussed in detail. Different developing modes of heat transfer enhancement in laminar and transitional regions are observed. Streamwise variation of circumferential distribution of heat transfer enhancement by rotation exhibits sensitivity to rotation speed, rotation direction and curvature ratio. Within the range of De and Ro under discussion, the impact of streamwise inertial force is the major factor of heat transfer enhancement for co-rotational cases while the effect of reversed flow and accompanied Dean vortex for counter rotational cases cannot be neglected.     

Convective heat transfer combined with surface radiation in a rotating square cavity with a local heater

The effect of surface radiation on laminar natural convection in a rotating cavity with a discrete heater has been analyzed numerically. The enclosure is insulated at the bottom and top, heated by a constant temperature from the discrete heater located on the bottom wall, and cooled by a constant temperature from the side walls. Governing equations with corresponding initial and boundary conditions formulated in dimensionless stream function, vorticity, and temperature have been solved by finite difference method of the second-order accuracy. The effects of surface emissivity, Rayleigh number, and Taylor number on the fluid flow and heat transfer have been studied. Obtained results have revealed that rotation can be a very good control parameter for heat transfer and fluid flow.     

Local heat transfer distribution and effect of instabilities during flow boiling in a silicon microchannel heat sink

Flow boiling of the perfluorinated dielectric fluid FC-77 in a silicon microchannel heat sink is investigated. The heat sink contains 60 parallel microchannels each of 100 μm width and 389 μm depth. Twenty-five evenly distributed temperature sensors in the substrate yield local heat transfer coefficients. The pressure drop across the channels is also measured. Experiments are conducted at five flow rates through the heat sink in the range of 20–80 ml/min with the inlet subcooling held at 26 K in all the tests. At each flow rate, the uniform heat input to the substrate is increased in steps so that the fluid experiences flow regimes from single-phase liquid flow to the occurrence of critical heat flux (CHF). In the upstream region of the channels, the flow develops from single-phase liquid flow at low heat fluxes to pulsating two-phase flow at high heat fluxes during flow instability that commences at a threshold heat flux in the range of 30.5–62.3 W/cm² depending on the flow rate. In the downstream region, progressive flow patterns from bubbly flow, slug flow, elongated bubbles or annular flow, alternating wispy-annular and churn flow, and wall dryout at highest heat fluxes are observed. As a result, the heat transfer coefficients in the downstream region experience substantial variations over the entire heat flux range, based on which five distinct boiling regimes are identified. In contrast, the heat transfer coefficient midway along the channels remains relatively constant over the heat flux range tested. Due to changes in flow patterns during flow instability, the heat transfer is enhanced both in the downstream region (prior to extended wall dryout) and in the upstream region. A previous study by the authors found no effect of instabilities during flow boiling in a heat sink with larger microchannels (each 300 μm wide and 389 μm deep); it appears therefore that the effect of instabilities on heat transfer is amplified in smaller-sized channels. While CHF increases with increasing flow rate, the pressure drop across the channels has only a minimal dependence on flow rate once boiling is initiated in the microchannels, and varies almost linearly with increasing heat flux.