高密实度H型风力机气动性能的数值分析

针对高密实度H型风力机在不同叶片数下的气动特性及风场布置等问题,将采用k-ω SST湍流模型进行数值模拟的方法应用到对高密实度H型风力机的研究中.开展了对风力机做功特性的分析,建立了流场与风力机功率之间的关系,提出了单个风力机设计选择叶片数时应综合考虑风力机效率和轴承安全这两个因素;在尾流场分析的基础上对不同叶片数的风力机在风场前后串列布置进行了评价.研究结果表明:叶片在上游(θ=90°附近)的气动性能决定整个风力机的性能;由正常工况点下的流场图显示,叶片数的增加导致流场复杂,以及叶片的内外压差逐渐减小,从而使得功率下降;尾流场流向速度恢复至来流速度的距离随叶片数的增加而减小了28.1％,这对风力机的前后串列布置提供了依据.     

Numerical predictions of roughness effects on the performance degradation of an axial-turbine stage

This paper describes a numerical investigation on the performance deteriorations of a low speed, single-stage axial turbine due to use of rough blades. Numerical calculations have been carried out with a commercial CFD code, CFX-Tascflow, by using a modified wall function to implement rough surfaces on the stator vane and rotor blade. To assess the stage performance variations corresponding to 5 equivalent sand-grain roughness heights from a transitionally rough regime to a fully rough regime, stage work coefficient and total to static efficiency were chosen. Numerical results showed that both work coefficient and stage efficiency reduced as roughness height increased. Higher surface roughness induced higher blade loading both on the stator and rotor which in turn resulted in higher deviation angles and corresponding work coefficient reductions. Although, deviation angle changes were small, a simple sensitivity analysis suggested that their contributions on work coefficient reductions were substantial. Higher profile loss coefficients were predicted by higher roughness heights, especially on the suction surface of the stator and rotor. Furthermore sensitivity analysis similar to the above, suggested that additional profile loss generations due to roughness were accountable for efficiency reductions.     

Method for measuring the resonant frequency of photoacoustic detector in the real-time mode

A method for measuring the resonant frequency of a photoacoustic detector (PAD) in the realtime mode in a wide temperature and gas-mixture-composition range is proposed. The method is based on measurements of natural frequencies of the resonance PADs, which are excited using an auxiliary acoustic emitter built into the PAD. The measurement procedure takes ≤0.1 s. When the PAD is filled with air or nitrogen, the high accuracy and reproducibility of the measurement results is experimentally shown. The relative measurement error of the PAD resonant frequency (~1700 Hz) is approximately 6 × 10^–5.     

Improving the performance of an RF resonant cavity water-cut meter using an impedance matching network

In multiphase flow measurement, one of the most challenging issues is to define an adequate technology for a specific scenario, taking into account the measurement accuracy, implementation feasibility and costs. The electromagnetic technology based on resonant cavities is often employed in water-cut meters to measure two-phase flows such as water/oil and water/gas mixtures. The main disadvantage of this technology is the electromagnetic signal attenuation that occurs as the water content decreases. This undesirable behavior is amplified due to the impedance mismatch between the sensor ports and the transmitter/receiver modules. This paper presents a study to implement an impedance matching network in order to improve the instrument performance. Impedance matching networks were built, taking into account the matching for a 100%, 50% and, also, for the worst case of 0% of water fraction where there is a significant signal attenuation. The implemented networks improved the signal amplitude ratio between the first resonant mode and the other modes, increasing the identification accuracy of the first resonance peak.     

Study of fluid damping effects on resonant frequency of an electromagnetically actuated valveless micropump

As fluid flow effects on the actuation and dynamic response of a vibrating membrane are crucial to micropump design in drug delivery, this paper presents both a mathematical and finite-element analysis (FEA) validation of a solution to fluid damping of a valveless micropump model. To further understand the behavior of the micropump, effects of geometrical dimensions and properties of fluid on the resonant frequency are analyzed to optimize the design of the proposed micropump. The analytical and numerical solutions show that the resonant frequency decreases with the slenderness ratio of the diffuser and increases with the opening angle, high aspect ratio, and thickness ratio between the membrane and the fluid chamber depth. A specific valveless micropump model with a 6-mm diameter and 65-μm thickness polydimethylsiloxane (PDMS) composite elastic membrane was studied and analyzed when subjected to different fluids conditions. The resonant frequency of a clamped circular membrane is found to be 138.11 Hz, neglecting the fluid. For a gas fluid load, the frequency is attenuated by slightly shifting to 104.76 Hz and it is significantly reduced to 5.53 Hz when the liquid fluid is loaded. Resonant frequency remarkably shifts the flow rate of the pump; hence, frequency-dependent characteristics of both single-chamber and dual-chamber configuration micropumps were investigated. It was observed that, although the fluid capacity is doubled for the latter, the maximum flow rate was found to be around 27.73 μl/min under 0.4-A input current with an excitation frequency of 3 Hz. This is less than twice the flow rate of a single chamber of 19.61 μl/min tested under the same current but with an excitation frequency of 4.36 Hz. The proposed double-chamber model analytical solution combined with the optimization of the nozzle/diffuser design and assuming the effects of damping proved to be an effective tool in predicting micropump performance and flow rate delivery.     

Numerical study on the improvement of resistance performance for fast-ferry with transom appendage

In recent, stern wedges and stern flaps are installed for the improvement of propulsion and resistance performance of fast-ferry. For example, U.S. Navy has achieved the development of stern wedges and stern flaps for destroyer to enhance powering performance. It is generally known that stern wave systems as well as bow wave systems play an important role in the wave making resistance performance for fast-ferry. The bow diverging wave system has been usually simulated by an interface tracking method (ITM). However, it is difficult to apply the ITM to the numerical simulation of the stern wave and spray phenomenon because of over-turning wave and wave-breaking. Therefore, to solve this problem an interface capturing method (ICM) is introduced. In the present study, a numerical method with the ICM is developed to evaluate the resistance performance of fast-ferry. Incompressible Navier-Stokes and continuity equations are employed in the present study and the equations are discretized by Finite Difference Method in the general curvilinear coordinate system. CIP (Constrained Interpolated Profile) method is used for the discretization of convection terms, respectively. The free surface location is determined by level set method. In order to validate the numerical method, numerical simulations for Wigley hull are performed and their results are compared with experimental results. Several numerical simulations of ship waves for fast-ferry are performed to find advantages of appendage installation. Through those simulations, the computed results, such as wave profile and resistance coefficient, are compared with the measured results which are achieved from Samsung Ship Model Basin (SSMB). The effects of transom appendage on the resistance performance are discussed with the computed results in this study.     

Numerical study on the operational performance of spray column direct contact heat exchangers

Spray columns have received considerable attention as direct contact heat exchangers due to their potential high throughput, as well as their near perfect counterflow operations if one-dimensional flow is obtainable. In practice, the degree of success in obtaining one-dimensional flow has depended on the design of the injectors for the disperse and continuous phases. In the past, the design of the injectors have been a trial and error procedure not sufficiently backed up by analyses. In this paper, theoretical analyses are carried out and several designs are evaluated. The influence of proper inlet conditions for the continuous phase to assure near one-dimensional flow is illustrated.     

Numerical investigation of the combined effects of slip and texture on tribological performance of bearing

Slider bearings are used in many applications. An increase in the load support may allow for saving of energy. In this work, in order to enhance the load support and decrease the friction force, a combined textured surface bearing using boundary slip is discussed. A modified Reynolds equation with slip is adopted. With the main goal of evaluating the effects of slip and texture, a parametric analysis is performed. For the given operating conditions, texturing features as well as slip pattern are analysed in detail. The numerical analysis is undertaken under the condition of different gap ratio values and the slip-textured area. The results show that combined techniques of slip and texture have a significant effect on the improvement of the tribological performance of bearing, that is, a high load support but low friction force. The gap ratio of the bearing is shown to have a significant effect on the lubrication behaviour. It is found that even with a smallest gap ratio (parallel gap), a high load support can be produced. However, it is also shown that the gap ratio appears to contribute to the generated friction force and the volume flow rate more than the boundary slip. Further analysis indicates that the optimum slip-text zones for certain gap ratio are highlighted. These findings may provide references for designing hydrodynamic-textured slider bearing considering boundary slip.     

Numerical study on performance of supersonic inlets with various three-dimensional bumps

Numerical investigations were performed with a supersonic inlet system installed with a three-dimensional bump which was substituted for a diverter or conventional ramp-type compression systems at Mach 2. The modified inlets were designed to have two oblique shocks and a terminal normal shock followed by a subsonic diffuser, with a circular cross-section throughout. A numerical analysis was conducted to understand the three-dimensional flow field including shock/boundary layer interactions that occur around a three-dimensional bump and to evaluate the performance of the supersonic inlets. The current numerical simulations showed a bump-type inlet based on a conventional ramp-type inlet can provide an improvement in total pressure recovery downstream of the shock/boundary-layer interaction over a ramp-type inlet.     

Numerical investigation into the effects of stroke trajectory on the aerodynamic performance of insect hovering flight

Various stroke trajectories may be observed in insect hovering flight in nature; however their influences on the flight performance of insect are not well estimated. In this study, a numerical investigation into the effects of stroke trajectories on the aerodynamic performance of insect hovering flight is carried out through the solution of the two-dimensional unsteady incompressible Navier-Stokes equations. An insect wing model with ellipse cross section in hovering flight is considered for the purpose and four types of idealized trajectories (Named linear, oval, figure-eight and double-eight) which possess different deviation characteristics from the stroke plane are examined. The influences of the deviation amplitude of trajectory, the attack angle of wing and the inclined angle of stroke plane on the aerodynamic characteristics of hovering wing are systematically analyzed. The results show that in the case of the wing in a normal hovering flight where the stroke plane is horizontal, the trajectory deviation from the stroke plane weakens the aerodynamic performance for each trajectory case considered, and this deteriorative effect becomes more serious as the amplitude of deviation increases. With regard to the influence of the angle of attack, the results show that the time-averaged drag force and power consumption increase monotonically with the angle, whereas the time-averaged lift force and the lifting efficiency increase first and then decrease as the angle increases further. In the case of a hovering flight with an inclined stroke plane, distinctly different trends from a normal hovering flight are obtained.     

喉管直径对旋风分离器性能影响的仿真研究

采用数值模拟方法研究了在普通Lapple型(d/D=0.5)旋风分离器气流出口处外加不同直径的喉管结构对于其流场及性能的影响.采用雷诺应力模型(RSM)模拟旋风分离器内的各向异性湍流,同时采用拉格朗日颗粒追踪(LPT)模型计算颗粒运动.结果表明,喉管直径能有效控制内旋涡的大小,从而影响旋风分离器的性能.随着喉管直径减小...     

零件的刚度对其热态几何精度影响的研究

通过了对具有相同截面积,相同材料,不同刚度零件的热态几何精度的有限元分析及实验分析,阐述了刚度大小对热态几何精度的影响规律,为减少大型零件（如大型精密机床床身）的热变形,优化零件结构提供了一种方法。     

Numerical prediction of switching gurney flap effects on straight bladed VAWT power performance

Journal of Mechanical Science and Technology - Based on the lift mechanism of the Gurney flap (GF) applied on isolated airfoil trailing edge, an active GF was designed to vary its height with the...     

几种参数对复合材料圆柱壳体振动特性的影响

以纤维增强复合材料层合壳体结构为研究对象,采用有限元分析软件,对纤维增强复合材料壳体进行了研究,分析边界条件、纤维铺层角度、铺层数目和壳体几何参数对壳体结构振动特性的影响.通过分析发现,不同的边界条件、纤维铺层角度和铺层数目对壳体的振动特性影响很大;在自由-自由边界条件下,长径比和径厚比的变化对壳体频率影响较小;随着边界条件的变化,长径比和径厚比的变化对壳体频率的影响逐渐增大.     

Numerical study on cooling performance of hybrid micro-channel/micro-jet-impingement heat sink

Journal of Mechanical Science and Technology - This study explores the cooling performance associated with single-phase hybrid micro-channel/micro-jet-impingement cooling method. A parametric study...     

Numerical study on mixing performance of separate-type HVAC system for industrial vehicles

Journal of Mechanical Science and Technology - Industrial vehicles have no vehicle-induced wind in operation mode, so they require more than one fan to secure enough air. Thus, a separate-type HVAC...     

Effects of geometrical parameters of an oil-water separator on the oil-recovery rate

As the economical and environmental damages due to the accidental oil-spills in marine environment increase gradually, more active countermeasure needs to be developed. In this respect, in the present study, we propose a new design of oil-water separation system and investigate the effects of several geometrical features on the oil-recovery rate, based on a two-dimensional numerical simulation. The working mechanism of current separator is to utilize the density difference between the oil and water, which is strengthened by adding momentum to the oil-water mixture flow through a “U-shaped” passage. Along the flow passage, we locate additional parts such as baffle plate, weir plate, and water outlet. While optimizing the conditions of these to maximize the oil recovery, it is found that the formation and stable retention of water layer between the water outlet and weir plate is critical to separate and store the above-layered oil. Finally, these findings are further confirmed with a basic experimental test with a three-dimensional oil-water separator model.

     

柱形含能破片对带壳炸药引爆能力的数值模拟

针对来袭目标导弹战斗部通常靠高速破片冲击引爆战斗部从而达到毁伤效果,但是靠一味地增加破片速度来冲击引爆战斗部已经受到限制.对一种新型复合结构破片(即含能破片)引爆模拟战斗部进行了研究,基于平头柱形含能破片撞击带铝壳炸药的模型,采用数值模拟的方法,研究了含能破片引爆带壳炸药的基本规律,给出了柱形含能破片引爆带壳炸药的速度...     

Effect of sinusoidal cylindrical surface of PCM on melting performance

Journal of Mechanical Science and Technology - A numerical analysis was conducted to examine the effect of capsule shape on phase change material (PCM) melting speed. The surface parameters of the...     

材料特性对行星式离心混合器混合性能影响的数值研究

针对行星式离心混合器(PCM)的使用中存在材料特性与混合性能之间关系难以确定的问题,采用计算流体动力学(CFD)模拟研究了流体黏度与密度对PCM混合性能的影响。首先,采用ICEM CFD对计算域进行了建模并划分了网格,对网格进行了无关性分析;然后,采用二阶有限体积法与离散相模型对PCM混合不同黏度与密度流体时的质点轨迹、低速区域以及离散粒子分布进行了求解,通过定义的混合指数对混合效果进行了定量表征;最后,分析了流体黏度与密度对PCM混合性能的影响机制。研究结果表明：流体黏度与密度主要通过影响流场结构使PCM的混合效果发生变化;流体黏度越大对应的最优转速比越大,能达到的最大混合指数值越小;流体密度越大对应的最优转速比越小,能达到的最大混合指数值越大;该研究结果可为PCM的使用与优化提供参考。