Experimental investigation of the influence of solidity on the performance and flow field aerodynamics of vertical axis wind turbines at low Reynolds numbers

This paper shows the results of an experimental investigation into the effect of changes in solidity on the performance of a Vertical Axis Wind Turbine. Two VAWT configurations are used, one of solidity σ = 0.26 (chord C = 0.03 m) and the other with σ = 0.34 (C = 0.04 m). The turbine performance coefficient (C_p) was measured over a range of tip speed ratios and Particle Image Velocimetry (PIV) was used to determine the flow field around both turbine configurations.Performance (C_p–λ) curves for the two VAWTs are compared at the same Reynolds numbers to investigate the effects of solidity alone on the performance and aerodynamics of each configuration. The higher solidity (σ = 0.34) VAWT attained a similar maximum C_p but with a narrower C_p–λ curve than the lower solidity VAWT. The performance differences between the two VAWT configurations at two tip speed ratios are explained in detail using PIV around both VAWT rotor blades. This allows the linking of detailed aerodynamics to the performance and it was shown that the generation and shedding of stall vortices started earlier on the lower solidity VAWT than the higher solidity VAWT, thus limiting the rotor efficiency.     

Numerical and experimental investigation on aerodynamic performance of small axial flow fan with hollow blade root

To reduce the influence of adverse flow conditions at the fan hub and improve fan aerodynamic performance,a modification of conventional axial fan blades with numerical and experimental investigation is presented.Hollow blade root is manufactured near the hub.The numerical and experimental results show that hollow blade root has some effect on the static performance.Static pressure of the modified fan is generally the same with that of the datum fan,while,the efficiency curve of the modified fan has a different trend with that of the datum fan.The highest efficiency of the modified fan is 10% greater than that of the datum fan.The orthogonal experimental results of fan noise show that hollow blade root is a feasible method of reducing fan noise,and the maximum value of noise reduction is about 2 dB.The factors affecting the noise reduction of hollow blade root are in the order of importance as follows: hollow blade margin,hollow blade height and hollow blade width.The much smoother pressure distribution of the modified fan than that of the datum fan is the main mechanism of noise reduction of hollow blade root.The research results will provide the proof of the parameter optimization and the structure design for high performance and low noise small axial fans.     

仿生叶片水动力特性实验研究

文章通过水槽实验,研究了雷诺数为1.2×105,1.6×105和2.2×105时,采用NACA0018翼型的标准直叶片和带前缘突起的仿生叶片的水力学性能。实验结果表明:在3个不同雷诺数下,迎角小于33°时,标准叶片在失速时出现升力系数的忽然减小和阻力系数的忽然增加;带前缘突起的仿生叶片能够推迟叶片失速的发生,且在失速区域,仿生叶片比标准叶片的升力系数提高约9.4%,阻力系数减小约22.75%,升阻比提高约33.6%;前缘突起不仅能够为仿生叶片在失速区域提供更大的升力,而且可以起到减阻的效果。实验证明了仿生叶片在失速区域比标准叶片有更好的水力学性能。     

Effect of the uneven circumferential blade space on the performance of small axial flow fan

Effects of the uneven circumferential blade space on static characteristics and aerodynamic noise of a small axial flow fan are studied in this work.The blade angle modulation is adopted to design a series of unequally spaced fans,which have different maximum of modulation angular displacement.The steady flow is simulated by the calculations of Navier-Stokes equations coupled with RNG k-epsilon turbulence model,while the unsteady flow is computed with large eddy simulation.According to theoretical analysis,a fan with a maximum of modulation angular displacement of 6° is regarded as the optimal unequally spaced fan.The experiment of static characteristic is carried out in a standard wind tunnel and the aerodynamic noise of both fans is tested in a semi-anechoic room.Then,performances of the optimal unequally spaced fan are compared with those of the prototype fan.The results show that there is reasonable agreement between the simulation results and the experimental data.It is found that the discrete noise of the optimal unequally spaced fan is lower than that of the prototype fan at the near field monitoring point.This can be explained that the total pressure fluctuation of the optimal unequally spaced fan is much more regular than that of the prototype fan.     

低Re下板式换热器性能的实验研究及热力学分析

实验测定了两种不同板式换热器在低Re条件下(200相似文献   

Wall shear rate in the Taylor-Couette-Poiseuille flow at low axial Reynolds number

An experimental investigation was done by the use of visual observations and the electrochemical technique in order to study the appearance of hydrodynamic instabilities at low Re_axg in the gap between two coaxial cylinders, with radius ratio R_r/R_s=0.615 and aspect ratio L/e=24. A motor drove the inner cylinder and the outer cylinder was fixed. A Newtonian fluid (Emkarox HV45) and two non-Newtonian fluids (aqueous solutions of guar and CMC) have been used. The analysis of the evolution of the size of the Taylor vortices is carried out for Re_axg<4.0. For Re_axg<2 we showed that the vortices underwent a contraction phase up to Ta_g≈80, and a stretching phase for Ta_g>80. For higher Re_axg, the contraction phase vanished and the vortices progressively stretched with increasing Ta_g. A dimensionless representation was also proposed for wall friction generalized to non-Newtonian fluids following the Ostwald law.     

Interaction between instream axial flow hydrokinetic turbines and uni-directional flow bedforms

A series of experiments were completed to investigate the interactions between relatively large-scale sediment dunes and axial flow hydrokinetic turbines. Baseline experiments were completed under clear water conditions to assess local scour impacts of single and two-turbine installations. Spatio-temporal measurements of bed elevation were obtained simultaneously with turbine voltage output, from which a measure of the instantaneous rotor angular velocity was used as a proxy for turbine response to unsteady loads. These experiments were completed in a mobile bed of 1.8 mm coarse sand with migrating bedforms. A bedform tracking routine was used to analyze streamwise bed elevation profiles to estimate bedform geometric characteristics and their corresponding impact on turbine performance. Cross-correlation analysis was also performed, investigating critical relationships between approaching bedform crest location, height and the corresponding turbine voltage output. In parallel with the analysis on bedform effects on turbine performance, an extended region downstream of the rotor location was analyzed to investigate how bedform geometric characteristics changed along the channel after turbine deployment.     

Characteristics of force acting on adjustable axial flow pump blade

The internal three-dimensional turbulent flow of adjustable axial-flow pump arrangement was simulated, and the force acting on the blade surface was calculated under different operating conditions. Based on the calculated results, finite element method (FEM) was adopted to analyze stress and strain distributions of the adjustment blade in different operations. Hydraulic moment, centrifugal moment and friction moment which must be conquered by adjusting the blades were also calculated. __________ Translated from Journal of Shanghai University (Natural Science), 2007, 13(3): 314–319 [译自: 上海大学学报]     

Characteristics of force acting on adjustable axial flow pump blade

The internal three-dimensional turbulent flow of adjustable axial-flow pump arrangement was simulated, and the force acting on the blade surface was calculated under different operating conditions. Based on the calculated results, finite element method (FEM) was adopted to analyze stress and strain distributions of the adjustment blade in different operations. Hydraulic moment, centrifugal moment and friction moment which must be conquered by adjusting the blades were also calculated.     

Experimental and numerical investigation of the unsteady tip leakage flow in axial compressor cascade

For convenience of both measurement and adjusting the clearance size and incidence, the current research is mainly conducted by experiments on an axial compressor linear cascade. The characteristics and the condition under which the unsteadiness of tip leakage flow would occur were investigated by dynamic measuring in different clearances, inlet velocities and incidences. From the experiment it is found that increasing tip clearance size or reducing rotor tip incidence can affect the strength of the tip clearance flow. Then the experimental results also indicate the tip leakage shows instability in certain conditions, and the frequency of unsteadiness is great influenced by inflow angle. The condition of occurrence of tip leakage flow unsteadiness is when the leakage flow is strong enough to reach the pressure side of the adjacent blade. The main cause of tip leakage flow unsteadiness is the tip blade loading.     

Radial- and mixed-flow turbines for low head microhydro systems

This paper describes the design of two different specific speed microhydro turbines operating at heads between 6 and 12 m, at small scale and up to heads of 50 m at larger scales. The features are specifically tailored for ease of manufacture and uniquely resistant to debris blockage. Test machines are described and test results given; hydraulic efficiencies of over 70% have been achieved in all test models despite the fact that the turbine blades are made from flat plate, specifically to simplify manufacture. Outline drawings are given with key dimensions for each reference model, along with the equations for scaling to arbitrary sizes. These turbines are the mixed- and radial-flow members of a family of turbines developed to cover the microhydro range from 2 to about 50 m of head, which is below the range where Pelton wheels are applicable.     

Experimental investigation of the influence of lubricating oil on heat pump performance

Experimental results are presented which demonstrate the effect of lubricating oil on the COP and evaporator performance of refrigeration and heat pump systems. These are compared with the predictions of a simple theoretical analysis and it is shown that the agreement is satisfactory. It is demonstrated that evaporator heat extraction can be degraded by as much as 50 per cent and the COP by up to 30 per cent when compared to the performance indicated by the properties of pure refrigerants for systems containing up to 15 per cent oil. An interesting phenomenon is also demonstrated, wherein there is a clearly identifiable optimum setting for the evaporator superheat control, and the way in which this varies with oil concentration is indicated.     

The influence of wedge diffuser blade number and divergence angle on the performance of a high pressure ratio centrifugal compressor

Wedge diffuser is widely used in centrifugal compressors due to its high performance and compact size. This paper is aimed to research the influence of wedge diffuser blade number and divergence angle on centrifugal compressor performance. The impact of wedge diffuser blade number on compressor stage performance is investigated, and then the wedge diffusers with different divergence angle are studied by varying diffuser wedge angle and blade number simultaneously. It is found that wedge diffuser with 27 blades could have about 0.8% higher adiabatic efficiency and 0.14 higher total pressure ratio than the wedge diffuser with 19 blades and the best compressor performance is achieved when diffuser divergence angle is 8.3°.These results could give some advices on centrifugal compressor design.     

Effects of a kind of non-smooth blade on the performance of an axial fan

This paper presents an experimental investigation of effects of a kind of streamwise-grooved blade on theperformance of an axial-flow fan.The flow field at 25% chord downstream from the trailing edge at hub wasmeasured using a 5-hole pressure probe at different mass-flow conditions.The fan performance of the grooveblades was compared with that of the smooth blades.The measurement results indicate that:(1)the non-smoothblades increase mass flow of the fan at the same throttle conditions except a near stall condition;(2)thenon-smooth blades reduce the relative total pressure loss in the rotor passage and increase the fan's total pressurerise at the test mass-flow conditions except the near stall condition;(3)Negative benefits are obtained at a nearstall condition when the smooth blades are replaced by the non-smooth ones.The fan mass flow decreases 0.9%while the total-pressure rise decreases 2.4% at the near stall condition.     

Numerical investigation into influence of geometry on flow in a rotating cavity with an axial throughflow

For cooling design, in modern gas turbine engines, it is important to understand and predict the fluid flow and heat transfer in the high pressure compressor drum cavities, which have complex and varying geometries. Unsteady, three dimensional (3-D), Computational Fluid Dynamics (CFD) techniques are used to illustrate the great influence the geometry of these cavities has on flow and heat transfer. Comparisons are made with heat transfer measurements and general agreement is found. Predictions suggest that for certain geometries less computationally expensive axisymmetric CFD or boundary layer integral computations may be suitable for design purposes.     

Experimental investigation on the influence of different grades of wood chips on screw feeding performance

Although basic investigations on wood chip material properties haven been carried out, only few studies deal with transport of wood chips, despite the fact that significant problems are commonly observed when feeding biomass in industrial applications. Within the work presented, basic bulk material properties were measured and experiments carried out with a system consisting of a hopper, agitator and a screw conveyor. The aim of this study was to investigate how three different wood chip grades and two blends of wood chips influence typical design parameters, such as mass flow and driving torque, of a biomass feeding system. The measured basic bulk properties are in good overall agreement with the literature. However, discrepancies were discovered between two standardized methods for determination of the bulk density. The results for the driving torque, mass flow and mass-related energy consumption showed that different grades of wood chips can alter these values considerably. Between two wood chip grades, a twice as high torque was recorded, while a third grade could not be fed due to jamming. One of the major findings of this study is that mixing a rather small amount of a high-fine content grade with the non-feedable grade of wood chips resulted in a blend which inhibited jamming during the screw feeding process.     

Pressure measurement on casing wall and blade rows interaction of contra-rotating axial flow pump

An application of contra-rotating rotors has been proposed against a demand for developing higher specific speed axial flow pump. The blade rows interaction between front and rear rotors should be clarified for its stable operation and reduction of unsteady losses. In this paper, the static pressure distributions on casing wall are provided by measuring with the phase locked sampling method. The measurements are carried out for two types of the rear rotors with different blade number and chord length, and it is found that, for both types of rotors, the unsteady pressure fluctuations are more remarkable in the front rotor than in the rear rotor and they are caused by the rear rotor pressure field. The effects of pressure fluctuations will be discussed in more details toward understanding the blade rows interaction in the contra-rotating axial flow pump.     

低雷诺数下凹坑结构对翼型层流分离影响的数值研究

在低雷诺数Re工况下,翼型表面容易发生流动分离,形成的层流分离泡会导致翼型气动性能恶化,且分离泡在尾缘周期性脱落,会诱发振动,影响叶片的结构安全.文章以NACA4415翼型为例,采用大涡模拟(LES)方法,在低Re下,对光滑翼型及布置凹坑结构翼型的层流分离进行了研究.研究结果表明:凹坑结构对翼型在低Re下出现的层流分离...     

Numerical investigation on the effect of blade sweep on the performance of Wells turbine

A Wells turbine is one of the simplest and promising self-rectifying air turbines which is basic to the needs of the near future and likely to be economically viable. With the recent development in computer hardware and software, it has now become practicable to conduct a reasonable computation of three-dimensional turbulent flows through complex geometry. To investigate the effect of blade sweep on the performance of the Wells turbine, the numerical investigation was carried out under steady flow condition with a fully 3-D Navier–Stokes code for two kinds of blades, NACA0020 and CA9. As a result, it was found that the performance of the Wells turbine is considerably influenced by the blade sweep. The optimum blade sweep ratio (f=0.35) for the NACA0020 was found. This value is just the same as one obtained experimentally by the authors in the past. It was also found that the overall turbine performance for the NACA0020 is better than that for the CA9. It was shown that the numerical method is able quite well to predict the effect of blade sweep of the Wells turbine. The detailed flow patterns for several blade sweeps were also shown and discussed in this paper.     

冷凝器翅片积灰对风冷机组性能影响的实验研究

近年来,风冷式机组在中央空调市场的比重逐渐增加。机组长期在室外运行后,会出现翅片积灰的问题。利用焓差实验室,在不同环境温度下,对1台风冷热泵机组在翅片积灰前、后进行电耗功、制冷量以及制冷性能系数(COP)测试。结果表明,翅片积灰会导致机组制冷量降低,机组电耗功增加,机组COP降低;相比于环境温度35℃时,环境温度在43℃时的翅片积灰对机组性能的影响略小。