非设计工况下涡轮叶栅前缘壁角的作用

透平叶栅端区二次流具有复杂的涡系结构。Langston实验描述了两支马蹄涡和通道涡的演变和发展过程。基于Langston叶型构造出有效的前缘壁角,建立涡轮叶栅带有前缘壁角的端壁流动计算模型,分析前缘壁角对端壁流动与传热特性的影响,并评估其在非设计条件下的适应性。结果表明:在一定的非设计工况范围内,前缘壁角削弱了前缘马蹄涡和通道涡的强度,降低了流道内部的气动损失,增加了近端壁的流动损失。有效的前缘壁角使前缘附近端壁换热水平减弱,流道端壁换热整体减弱,端壁高换热区沿流向下移,尾缘附近换热有所增加。在一定的非设计工况范围内,前缘壁角都是有效的。     

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.     

Conjugate heat transfer analysis of a rotor blade with coolant channels

The present study deals with the conjugate heat transfer analysis of a cooled high-pressure turbine rotor blade consisting of eight cooling channels. Computational investigations are performed in a five-bladed cascade to determine characteristics of flow and heat transfer at different values of mainstream and coolant flow rate. Surface temperature measurements using infrared thermography are performed for validating the computational fluid dynamics results. The results show nonuniform variation of surface effectiveness: (a) higher average surface temperature on the pressure side than on the suction side, (b) peak surface temperature at the pressure side trailing edge region, (c) lowest temperature at midspan region of both pressure and suction sides, (d) intermediate values of temperature on the leading edge, and (e) these temperature patterns vary with the changes in mainstream Reynolds number and coolant flow rates, signifying the importance of carrying out conjugate heat transfer analysis. This study also emphasizes the importance of considering realistic coolant channel geometry over the idealized, an illustration showed that the maximum Nusselt number may vary up to 200% due to idealization.     

A computational study of unsteady radiative magnetohydrodynamic Blasius and Sakiadis flow with leading‐edge accretion (ablation)

The present study investigates the influence of the magnetic field, thermal radiation, Prandtl number, and leading‐edge accretion/ablation on Blasius and Sakiadis flow. The convective boundary condition is employed to investigate the heat transfer. The nondimensional governing boundary layer equations have been solved by the homotopy analysis method for different values of the pertinent parameters. The effects of these parameters on the dimensionless velocity, temperature, skin friction, and Nusselt number are also investigated for various values of relevant parameters affecting the flow and heat transfer phenomena. The most relevant outcomes of the present study are that enhancement in magnetic field strength undermines the flow velocity establishing thinner velocity boundary layer for both Blasius and Sakiadis flows while an increase in accretion/ablation effect at leading‐edge manifests in a deceleration in velocity for Blasius case and the opposite trend is observed for Sakiadis flow. Another important outcome is that an increase in radiation and accretion/ablation at leading‐edge upsurges the fluid temperature leading to enhancement in the thermal boundary layer. For both Blasius and Sakiadis flow, the skin friction coefficient and the heat transfer rate decline with the enhancement of the leading‐edge accretion parameter. The results are compared with the existing data and are found in good agreement.     

燃气涡轮无气膜动叶设计流程及分析

为了进行涡轮动叶冷却结构设计,采用一套设计流程,完成了某型涡轮第一列动叶无气膜方案设计。结果表明:管网计算设计中,得出调整后冷却结构的第一腔流量为16.85 g/s,第二腔流量为40.78 g/s,前缘最大温度为1 169 K,低于材料许用温度,满足设计要求。三维导热中的最大温度相比管网计算得出的温度有所上升,通过分析,管网计算未能考虑极值温度,因此三维导热计算是有必要的。从给出的三个截面温度场可以看出,前缘位置存在一定高温区,但最大温度低于设计温度,温度场符合设计要求。     

The Investigation of Flow Boiling for Flows in Channels with Cross-Corrugated Walls

The problem of heat transfer for flows in channels with corrugated walls is very complicated. Previously most investigators have focused their attention on studies of forced-convection, single-phase flow, and condensation of pure vapors and vapor gas mixtures. The results of new experimental investigations into heat transfer and pressure drop, for flow boiling in channels with corrugated walls, are discussed. The experimental section was a part of a pack of an industrial plate exchanger, which contained several pressed plates with longitude corrugations located under an angle to flow direction and had many contact points on the heat transfer area. The condensation method was used for the investigations. The relationship between the heat transfer ratios of the Nusselt number for single-phase flow was obtained. This correlation compares very favorably with the similar relationship obtained for flow bubble boiling in tubes proposed by Sterman [11]. For correlation of the pressure drop data, the Martinelli-Lockart approach was used, which also correlated well with the results of this study and the results of other investigations. The relationship obtained may be used in calculations for various types of industrial units which have corrugated patterns on their heat transfer surfaces.     

Field-Synergy and Figure-of-Merit Analysis of Two Oxide–Water-Based Nanofluids' Flow in Heated Tubes

Field-synergy analysis is performed on the water–oxide nanofluid flow in circular heat sinks to examine the synergetic relation between the flow and temperature fields for heating processes. By varying the Reynolds number and the nanoparticle volume fraction, the convective heat transfer of nanofluid is investigated based on the field synergy number. For heating, the degree of synergy between the velocity and temperature fields of nanofluid flow deteriorates with the Reynolds number increase, leading to a decreased heat transfer performance of the nanofluid. By increasing the particle volume fraction, the degree of synergy between the velocity and temperature fields of the nanofluid flow can be intensified, thus going to convection heat transfer enhancement. After generating results, one can notice that the heat transfer enhancement is strongly dependent on nanoparticle type, Reynolds number, and volume fraction. The results are similar, even if the thermal conductivity of the two considered oxide nanoparticles are quite different. Additionally, a convenient figure of merit that is known as the Mouromtseff number was used as base of comparison, and the results indicated that the considered nanofluids can successfully replace water in specific applications for single-phase forced convection flow in a tube.     

具有新型双层壁结构的透平叶片流热耦合研究

多通道壁面射流冷却结构是一种新型的燃气透平动叶内部冷却结构,具有消耗冷气少、压力损失小等优点。本文构建了简化的壁面射流冷却叶片与GE-E3冷却结构叶片模型,采用流热耦合方法对比研究了其流动与换热特性。结果表明,壁面射流冷却通道内的狭小空间抑制了横流的产生,冷气在冷却通道中形成了流向涡;前缘冷气流道中的大量冷气流经吸力侧冷却区,并从出口压力更小、面积更大的尾缘排出,使得前缘气膜孔出流的冷气流量和动量较小,冷气在叶片外表面的气膜覆盖特性更好;离心力的影响导致前缘冷气流道中叶根处的压力较低,叶根附近的气膜孔出现燃气主流入侵现象。相比于GE-E3叶片,壁面射流冷却叶片的前缘温度和温度梯度都较小,因此多通道壁面射流冷却在前缘具有更优异的冷却特性。     

凹槽式叶顶流动换热的数值研究

针对叶尖间隙高度对凹槽式叶顶流动与换热的影响展开数值研究,评估4种湍流模型在叶顶换热方面的预测能力.结果表明：凹槽肩壁顶部、凹槽腔底部近前缘区域和叶顶尾缘为高换热区,凹槽腔底的中部和尾部区域为低换热区;不同湍流模型对叶尖间隙泄漏量预测差别很小,但泄漏流流动状态差异很大,这是造成不同湍流模型对叶顶换热预测存在重大差别的原因;在研究的间隙范围内,叶尖间隙泄漏量和叶顶换热强度随间隙高度的增大而增加;在所选的4种湍流模型中,k-ω模型是叶顶换热数值模拟较好的湍流模型选择.     

Flow of Microencapsulated Phase Change Material Slurry through Planar Spiral Coil

Forced convection cooling is an effective method in thermal management that relies mainly on dissipating heat by pumping heat transfer fluid (HTF) through the heat source. In this paper, we investigate the thermal properties enhancement of dielectric water as the HTF. To enhance the properties of the HTF, microencapsulated phase change materials (MPCM) will be added to the base fluid. The MPCMs are composed of phase change material (PCM) encapsulated with shell materials. The PCM inside the capsules may undergo a phase change. This leads to a significant heat gain and release. The numerical model is developed to solve for continuity, momentum, and heat transfer equations using the finite volume method. The behavior of the MPCM slurry in curved channels, generates unique patterns due to different viscosity values and the centrifugal forces. Our preliminary numerical data on MPCM slurry through planar spiral coil heat exchangers show the new patterns of velocity and heat transfer curves. The current paper studies the steady condition of laminar flow at different boundary conditions. The velocity and temperature profiles, heat transfer data with different mass fractions of MPCM additives to the base fluid, and their heat removal capabilities are quantified and discussed in detail.     

The effects of vent channels and metal conductive base on thermal characteristics of the active graphite-composite cylindrical heat sink

This study proposed a novel configuration of active LED graphite-composite heat sink and experimentally investigated the effects of vent channels and metal conductive base on the fluid flow and heat transfer characteristics of this active heat sink. The heat sink was made of the graphite powders, aluminum-alloy powders, and adhesive mixed in specific proportion by the vacuum-pressure injection technique. The cost and weight of this graphite-composite material are much lower than those of aluminum alloy. The configuration of heat sink is a hollow circular cylinder with multiple radial fins. Different motor fans can be put in the chamber of heat sink, with various vent-channel positions and orientations (vertical vent channels, horizontal upper-row vent channels, and horizontal bottom-row vent channels) and numbers of channels (24, 36, 48, and 72) in the heat sink to enhance overall cooling performance by driving through airflow. The results indicate that the overall Nusselt number (Nu) of the graphite-composite heat sink with motor fan was 2.23–2.50 times that without motor fan. The numbers of vent channels in heat sink were positively related to the total flow rate of through air. Thus, the heat sink with the most vent channels had the maximum Nu in the motor-fan mode. When an additional annular aluminum-alloy conductive base was mounted in the graphite-composite heat sink with the most vent-channel configuration, the Nu was 35% higher than that without conductive base in motor-fan mode, proving the metal conductive base was effective. The optimal vent-channel configuration in this study was also used for the full aluminum-alloy heat sink, the corresponding Nu of the models without/with motor fan were compared with the full aluminum-alloy heat sink without vent channel, the heat transfer enhancements were about 13% and 127%, respectively.     

Conjugate Heat Transfer Analysis in the Trailing Region of a Gas Turbine Vane

Conjugate heat transfer calculations are performed on cambered converged channels with and without pin fins, simulating the trailing region internal cooling passages of a gas turbine vane. Simulations are carried out for an engine representative Reynolds number of 20,000, based on the hydraulic diameter at the entry of coolant channel. The effect of conjugation is brought out by varying the solid to fluid thermal conductivity ratio from 7 to 16,016. The interaction between the complex flow pattern and conjugate heat transfer is highlighted. The local values of pressure, wall and fluid temperature, area-averaged values of friction factor, and Nusselt number of the smooth and pinned channels are compared.     

微尺度通道内流动沸腾研究综述

阐述了微尺度通道内传热问题出现的工程背景——高密度微电子器件的冷却。对当前国内外微尺度通道内流动沸腾换热特性的研究现状进行了归纳。突出分析了工质种类、微尺度通道的几何参数和工质的工况参数等对微尺度通道内流动沸腾换热特性的影响。同时分析了微尺度通道内流动沸腾换热的强化机理、流动阻力特性、压降关联式和沸腾换热关联式的理论和实验研究。最后根据分析对今后的工作提出了一些建议。     

Steady free convection boundary layer over a vertical flat plate embedded in a porous medium filled with water at 4 °C

This paper reports a theoretical investigation of the boundary layer flow over a vertical flat plate embedded in a porous medium filled with water near the vicinity of its density maximum associated with the temperature of 3.98 °C at atmospheric pressure. The study aims at determining similarity solutions of the governing boundary layer equations for a class of problems where the variable wall temperature (VWT), variable heat flux (VHF), or variable heat transfer coefficient (VHTC), vary as power functions of the distance from the leading edge of the plate. The existence and uniqueness of the solutions are considered and studied. The analytical and numerical solutions of the similarity form of the boundary layer equations yield velocity and temperature profiles as well as values of the stream function at the edge of the boundary layer, the heat transfer coefficient and the temperature on the plate.     

基于流热耦合的燃气轮机透平叶顶冷却设计

燃气轮机透平叶顶区域存在复杂的流动和换热问题,承受很高的热负荷。为了降低透平动叶叶顶温度,在透平叶顶现有结构的基础上提出气膜冷却和气膜+内冷通道冷却两种叶顶冷却方案,并通过流热耦合计算分析冷却升级前后叶顶区域的换热和流动特性。研究发现：叶顶气膜冷却方案可有效降低叶顶温度,特别是叶顶前缘至中弦区域;而气膜＋内冷通道冷却方案基于外部气膜冷却,结合内部冷却通道设计,可进一步降低叶顶尾缘的温度;与原型叶片相比,气膜+内部冷气通道的复合冷却设计可以使叶顶尾缘最高温度降低24 K。     

Computations of flow and heat transfer in a three-dimensional multilouvered fin geometry

This study presents computational results in a complex three-dimensional louver geometry. The three-dimensionality occurs along the height of the fin, where the angled louver transitions to the flat landing and joins with the tube surface. The transition region is characterized by a swept leading edge and decreasing flow area between louvers. The results show that for Re_b=1100, the flow on the angled louver is dominated by spanwise vortex shedding, which is weakly three-dimensional. On the other hand, the flow in the transition region exhibits strong three-dimensionality. A high-energy compact vortex jet forms in the vicinity of the louver junction with the flat landing and is drawn under the louver. The top surface experiences large velocities in the vicinity of the surface and exhibits high heat transfer coefficients. Although the flow slows down at the flat landing, the large induced velocities on the top surface increases the heat transfer coefficient on the tube surface.     

Experimental and analytical investigation on an automobile radiator with CuO/EG‐water based nanofluid as coolant

In the present study, experimental and analytical thermal performance of automobile radiator using nanofluids is investigated and compared with performance obtained with conventional coolants. Effect of operating parameters and nanoparticle concentration on heat transfer rate are studied for water as well as CuO/EG‐water based nanofluid analytically. The results are presented in the form of graphs showing variations of net heat transfer rate for various coolant flow rate, air velocity, and source temperature for various CuO/EG‐water based nanofluids. Experimental results indicate that with the increase in coolant flow rate and air velocity, heat transfer rate increases, reaches maximum and then decreases. Experimental investigation of a radiator is carried out using CuO/EG‐water based nanofluids. Results obtained by experimental work and analytical MATLAB code are almost the same. Maximum absolute error in water and air side is within 12% for all flow condition and coolant fluids. Nusselt number of nanofluid is calculated using equation number 33[9]. The results obtained from experimental work using 0.2% volume CuO/EG‐water based nanofluids are compared with the results obtained from MATLAB code. The results show that the maximum error in the outlet temperature of the coolant and air is 12% in each case. Thus MATLAB code can be used for different concentration of nanofluids to study the effect of operating parameters on heat transfer rate. Thus MATLAB code developed is valid for given heat exchanger applications. From the results obtained by already validated MATLAB code, it is concluded that increase in coolant flow rate, air velocity, and source temperature increases the heat transfer rate. Addition of nanoparticles in the base fluid increases the heat transfer rate for all kind of base fluids. Among all the nanofluid analyzed in this study, water‐based nanofluid gives highest value of heat transfer rate and is recommended for the heat exchanger applications under normal operating conditions. Maximum enhancement is observed for ethylene glycol‐water (4:6) mixture for 1% volume concentration of CuO is almost equal to 20%. As heat transfer rate increases with the use of nanofluids, the heat transfer area of the radiator can be minimized.     

Measurements of net change in heat flux as a result of leakage and steps on the contoured endwall of a gas turbine first stage nozzle

The first stage vane section of a modern gas turbine engine is assembled with a gap between the combustor and the vane platform and gaps on the platform. To prevent ingression, leakage flow is provided through each gap. In this paper, the measured net effects on endwall heat flux rates due to certain changes in the gap regions are presented. The geometry has axial contouring of the cooled endwall and several step configurations for each gap. The steps reflect assembly or differential thermal growth misalignment. Various leakage rates are applied through each gap to allow assessment of the change in heat flux rates with changes in leakage rate. Thus, the results presented herein show how the gaps, steps, and leakage rates alter the endwall cooling that can be achieved with leakage flow. The results are given as the net surface heat flux change. They are computed from heat transfer coefficient and adiabatic wall temperature distributions of comparison cases and those of a nominal case. Generally, changes in heat transfer coefficients due to gaps, steps, or increased leakage are observed to be partially offset with changes in film cooling effectiveness values such that the heat flux changes are mild. Nevertheless, elevated leakage at the transition section gap reduces heat flux at the leading edge of the platform and a fore step at the transition section gap increases heat transfer rates at the leading edge of the platform. Also, steps at the slashface gap change endwall heat flux immediately downstream of the slashface gap.     

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.     

舰载燃气轮机间冷器内部流动的三维数值模拟

板翅式换热器是舰载燃气轮机首选换热器形式。针对平直型翅片的矩形通道的结构特点,建立了流动换热分析的耦合计算模型,采用计算流体动力学（CFD）方法对间冷器的通道流场进行了数值模拟。给出并分析了计算区域内多个截面的温度、压力、速度、局部传热系数等参数的分布图形和变化趋势,并考察了不同工况下间冷器的工作能力。