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。     

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。     

旋转对梯形扰流柱通道流动及换热的影响

采用数值模拟的方法,对在旋转情况下的有弦向出流的梯形扰流柱通道内的三维流场进行了模拟研究。重点研究了在固定出流比的情况下,不同转速对扰流柱通道内的流动情况及端壁平均换热系数的影响。计算结果表明：R_o不为0时,通道内的流场与静止时相比有较大变化,在扰流柱区域内的扰动强度明显增大,流动更加紊乱,在扰流柱区域和无柱区域内均有涡旋现象发生；当Re数一定时,通道的压降和端壁的平均换热系数随着旋转数R_o的增大而增加；旋转对扰流柱通道的压降和端壁换热系数有明显影响,旋转对扰流柱通道流动及换热的影响随通道数Re数的增大而增强。     

燃气透平叶片3种出气边冷却结构中流动与传热性能的比较

本研究通过流热耦合计算和实验验证,比较了圆柱排孔与两种劈缝的出气边冷却结构,在不同雷诺数和冷气质量流率下的流动与传热性能。结果表明:圆柱排孔流阻偏高,受供气压力限制,冷气流量不足,冷却效果下降,造成出气边烧蚀。叶片出气边压力面开设劈缝结构降低流阻,提高了冷气流量。劈缝内采用弦向肋和扰流柱结构强化换热,提高冷却效果。在相同冷气流量下,劈缝结构所需的流动压差明显下降,叶栅叶片的尾迹宽度也减小,对降低下游叶栅的流动损失和表面传热是有利的。     

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。     

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。     

不同形状多扰流柱流场特性的数值分析

湍流凝聚技术可以有效提高常规电除尘器的PM_(2.5)脱除效率,是国内外学者研究和厂家推广的热点技术。扰流柱扰流作用是凝聚器内超细颗粒物凝聚的主要动力源之一,采用数值方法计算不同形状多扰流柱工况的速度分布、湍流强度、湍流粘度、压力损失等参数,结果表明角钢扰流的速度和湍流扰动特性均优于圆柱扰流,但是压力损失也随之增大了很多,在实际工程应用中应在保证扰流效果的前提下尽量减小不必要的压力损失。     

纵向涡发生器作用下矩形通道内脉动流流动换热性能研究

对安装渐缩式纵向涡发生器与椭圆支柱组合的矩形通道内脉动流动换热性能进行了非稳态三维数值模拟研究,计算考察了不同Re、脉动频率ω以及振幅A对通道内强化传热和压力损失的影响。研究结果表明:在脉动流动的影响下纵向涡发生器的传热能力得到了强化。随着脉动频率ω和脉动振幅A的增加,矩形通道内整体传热能力的强化效果增强;随着Re的增大,强化效果逐渐减小。随着脉动频率ω和脉动振幅A的增大,矩形通道内E_f的波动振幅增加;随着Re的增大,E_f的波动振幅减小。     

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。     

扰流肋柱在流动方向排列密度对矩形通道表面传热影响的数值研究

运用数值计算的方法将流动方向扰流圆柱排列密度对涡轮叶片尾缘冷却通道中流动传热的影响进行了三维数值研究。研究了流动雷诺数、流动方向圆柱排列密度对肋柱扰流矩形通道表面传热影响的规律。计算结果表明:在研究范围内,肋柱表面的平均Nu均随着Re的增大而增大。在Re相同的情况下,随X/D取值的增大,肋柱表面平均Nu有所减小。Nu在通道进口附近逐渐增加,然后达到充分发展值。传热在迎向流动方向的圆柱侧较强,在流动向背侧表面传热较弱。沿圆柱高度方向在中部传热较强。     

两种波纹通道内周期性充分发展对流换热的数值研究

应用数值方法研究了流体在正弦型和三角型两种波纹通道内周期性充分发展的层流流动与换热特性,分析了恒壁温条件下,雷诺数Re对流动与换热性能的影响,并对两种通道的摩擦阻力系数厂与努谢尔特数Nu分别进行了比较。结果表明：两种通道流动阻力的相对大小因雷诺数Re的大小而异;正弦型波纹通道的换热速率比三角型通道的换热速率大,因此换热器通道表面的选择应综合考虑。     

Forced Convection Analysis of Discrete Heated Porous Convergent Channel

This study investigates numerically forced convection heat transfer and flow analyses of a passive heat exchanger for nonporous and partially filled porous channels with varying exit height (1, 0.5, and 0.25). Four discrete heat sources with uniform heat flux are simulated on the channel bottom wall. The partially filled porous channels are tested at two different porous block heights (0.5 and 1). The flow field and thermal analyses inside the channels are investigated across a wide range of Reynolds and Darcy numbers for Prandtl number of 0.71. The results reveal that the porous block and the exit height affect substantially the flow and heat transfer characteristics inside the tested channels. The Nusselt number is enhanced by 20–40% for the partially filled porous convergent channel (exit height = 0.25 and porous block height = 1) compared to the nonporous channel. Consequently, the heat exchanger size can be reduced by 37.5%. Moreover, the overall heat transfer performance parameter is enhanced with further increase in Darcy number at low Reynolds number. As a result, compact heat exchangers that provide superior heat transfer coefficients lead to development of macro- and microelectronic devices.     

Numerical investigation of thermo-hydraulic transport characteristics in wavy channels: Comparison between raccoon and serpentine channels

Numerical experiments have been performed to investigate the thermo-fluidic transport characteristics for laminar flow through sinusoidal wavy walled channel. The heat transfer and pressure drop characteristics are assessed for two different channels, namely, raccoon and serpentine for different values of amplitude and wavelength of the wall waviness. Our results reveal that the dependence of heat transfer on the geometry of the wall is strongly influenced by the wavelength of the wall waviness. For lower values of wavelength, the rate of heat transfer is almost same for both the channel, while the heat transfer for raccoon channel is always more than that for serpentine channel for higher values of wavelength and the difference appears to be more prominent for larger values of amplitude of wall waviness and Reynolds number. Furthermore, with the deployment of a performance parameter combining the enhancement in heat transfer and corresponding increase in pressure drop, we assess the thermo-hydraulic performance of the two channels. In contrast to the heat transfer characteristics, our results indicate that the performance factor of a serpentine channel is always more than that of a raccoon channel. The results of the present investigation may be considered as a basis for selection of geometry of channel wall for design of compact heat exchangers.     

Thermal and Hydrodynamic Performances of Chaotic Mini-Channel: Application to the Fuel Cell Cooling

Currently, heat exchangers allowing the thermal management of low-temperature fuel cells (PEMFC) are integrated in the bipolar plates and are constituted of a network of straight channels. The flow regime is laminar and thus unfavorable to intense convective heat transfer. In order to increase the power density of the fuel cells, the use of chaotic geometries in the cooling system is envisaged to promote high convective heat transfer. In the present study, several chaotic three-dimensional mini-channels of rectangular cross-section (2 millimeters × 1 millimeter) are evaluated in terms of heat transfer efficiency, mixing properties, and pressure losses. Their performances are compared both to those of the straight channel geometry currently used in the cooling systems of the PEMFC and those of a square-wave mixer. Two Reynolds numbers are considered: 100 and 200. It is shown that a 3-D chaotic channel geometry significantly improves convective heat transfer over that of regular straight or square-wave mixer channels. Of all the geometries studied, one induces higher heat transfer intensification (mean Nusselt number equal to 20) with a strong pressure loss. With an alternative geometry, a better compromise between heat transfer and pressure loss is obtained. However, all of the chaotic geometries present similar mixing rate for the two Reynolds numbers studied.     

Experimental Study of Heat Transfer and Pressure Loss in Channels with Miniature V Rib-Dimple Hybrid Structure

Abstract

In order to increase the thermal efficiency, the gas turbines are designed to operate at higher temperature, which requires highly efficient cooling structures for turbine blades. The dimples and ribs are effective surface structures to enhance the convective heat transfer in the gas turbine blade internal cooling. In the present study, a novel hybrid cooling structure with miniature V-shaped ribs and dimples is presented, and the heat transfer and pressure loss characteristics are obtained experimentally. The heat transfer performance of the rib–dimple structures, which include three different rib height-to-hydraulic diameter ratios of 0.017, 0.029 and 0.044 and one dimple configuration with the dimple depth-to-diameter ratio of 0.2, are studied by using the transient liquid crystal thermography technique for turbulent flow in rectangular channels within the Reynolds number range from 10,000 to 60,000. It is found that the miniature V-shaped ribs arranged upstream the dimples can significantly improve the heat transfer performance of the dimples, resulting in a more uniform heat transfer distribution on the surface. The V rib-dimple hybrid structure in the channel shows much higher heat transfer enhancement than the counterparts with only the dimples in the channels.     

Numerical analysis of laminar heat transfer in a channel with diamond-shaped baffles

Laminar periodic flow and heat transfer in a two dimensional horizontal channel with isothermal walls and with staggered diamond-shaped baffles is investigated numerically. The computations are based on the finite volume method, and the SIMPLE algorithm has been implemented. The fluid flow and heat transfer characteristics are presented for Reynolds numbers based on the hydraulic diameter of the channel ranging from 100 to 600. Effects of different baffle tip angles on heat transfer and pressure loss in the channel are studied and the results of the diamond baffle are also compared with those of the flat baffle. It is observed that apart from the rise of Reynolds number, the reduction of the baffle angle leads to an increase in the Nusselt number and friction factor. The computational results reveal that optimum thermal performance is at the baffle angle of 5° for baffle height and spacing of 0.5 and 1 times of the channel height, respectively. The thermal performance of the 5°–10°diamond baffle is found to be higher than that of the flat baffle for all Reynolds numbers used.     

Direct numerical simulation of fluid flow and heat transfer in periodic wavy channels with rectangular cross-sections

Fully-developed flow and heat transfer in periodic wavy channels with rectangular cross sections are studied using direct numerical simulation, for increasing Reynolds numbers spanning from the steady laminar to transitional flow regimes. The results show that steady flow is characterized by the formation of symmetric secondary flow or Dean vortices when liquid flows past the bends. It is found that the patterns of Dean vortices may evolve along the flow direction, thus leading to chaotic advection, which can greatly enhance the convective fluid mixing and heat transfer. With increasing Reynolds numbers, the flow undergoes transition from a steady state to a periodic one with a single frequency, and subsequently to a quasiperiodic flow with two incommensurate fundamental frequencies. Within these unsteady regimes, the flow is characterized by very complex Dean vortices patterns which evolve temporally and spatially along the flow direction, and the flow symmetry may even be lost. Further increase in Reynolds number leads to chaotic flow, where the Fourier spectrum of the velocity evolution becomes broadband. The bifurcation scenario in wavy channels may thus share some common features with the well-known Ruelle–Takens–Newhouse scenario. Heat transfer simulation in all flow regimes is carried out with constant wall temperature condition and liquid water as the coolant. It is found that due to the efficient mixing in wavy channels, the heat transfer performance is always significantly more superior to that of straight channels with the same cross sections; at the same time the pressure drop penalty of wavy channels can be much smaller than the heat transfer enhancement. The present study shows that these wavy channels may have advantages over straight channels and thus serve as promising candidates for incorporation into efficient heat transfer devices.     

HEAT TRANSFER IN 3-D SERPENTINE CHANNELS WITH RIGHT-ANGLE TURNS

Laminar flow and heat transfer in square serpentine channels with right-angle turns, which have applications in heat exchangers, were numerically studied. A finite volume code in FORTRAN was developed to solve this problem. For solving the flow field, a colocated-grid formulation was used, as opposed to the staggered-grid formulation, and the SIMPLE algorithm was used to link the velocity and pressure. The line-by-line method was used to solve the algebraic equations. The temperature field was solved for the uniform-wall-heat-flux boundary condition. The developed numerical code was validated by solving for fully developed flow and heat transfer in a square straight channel. The grid-independent solution was established for a reference case of serpentine channel with the highest Reynolds number. Periodically fully developed flow and heat transfer fields in serpentine channels were solved for different geometry parameters, for different Reynolds numbers, and for two different Prandtl numbers (for air and water, respectively). The enhancement of the heat transfer mechanism was explained by studying the plotted flow-field velocity vectors in different planes. The heat transfer performance of serpentine channels is better than that for straight channels for Pr = 7.0 and is worse than that for straight channels for Pr = 0.7.     

Influence of channel geometry on the performance of a counter flow microchannel heat exchanger

Microchannel heat exchangers (MCHE) can be made with channels of various geometries. Their size and shape may have considerable effect on the thermal and hydraulic performance of a heat exchanger. In this paper numerical simulation is carried out to solve 3D developing flow and 3D conjugate heat transfer of a balanced counter flow microchannel heat exchanger (CFMCHE) to evaluate the effect of size and shape of channels on the performance of CFMCHE for the same volume of heat exchanger. The effect of shape of the channels on its performance is studied for different channel cross-sections such as circular, square, rectangular, iso-triangular and trapezoidal. Results show that for the same volume of a heat exchanger, increasing the number of channels lead to increase in both effectiveness and pressure drop. Moreover circular channels give the best overall performance (thermal and hydraulic) among various channel shapes. New correlations are developed to predict the value of heat exchanger effectiveness and performance index as a function of relative size of channels with overall heat exchanger volume, Reynolds number and thermal conductivity ratio.     

Numerical Analysis of Flow and Thermal Performance of Liquid-Cooling Microchannel Heat Sinks with Bifurcation

Single-phase liquid-cooling microchannels have received great attention to remove the gradually increased heat loads of heat sinks. Proper changes of the flow path and/or heat transfer surface can result in much better thermal performance of microchannel heat sinks. In this study, a kind of rectangular straight microchannel heat sink with bifurcation flow arrangement has been designed, and the corresponding laminar flow and heat transfer have been investigated numerically. Four different configurations are considered. The effects of the bifurcation ratio (the initial channel number over the bifurcating channel number) and length ratio (the channel length before bifurcation over the bifurcation channel length) on laminar heat transfer, pressure drop, and thermal resistance are considered and compared with those of the traditional straight microchannel heat sink without bifurcation flow. The overall thermal resistances subjected to inlet Reynolds number and pumping power are compared for the five microchannel heat sinks. Results show that the thermal performance of the microchannel heat sink with bifurcation flow is better than that of the corresponding straight microchannel heat sink. The heat sinks with larger bifurcation ratio and length ratio provide much better thermal performance. It is suggested to employ bifurcation flow path in the liquid-cooling microchannel heat sinks to improve the overall thermal performance by proper design of the bifurcation position and number of channels.