Search

This study proposes a straight-bladed vertical-axis turbine for ocean-wave energy conversion. The aim is to develop a novel air turbine suitable for an oscillating water column in a wave energy plant. To this end, the performances of straight-bladed vertical-axis turbines were investigated for different guide-vane geometries in a wind tunnel experiment. The rotor has four straight blades with an NACA0018 profile, a chord length of 80.5 mm, a pitch diameter of 460 mm, and a blade width of 490 mm. In this study, the guide-vane geometry was varied as arc, semi-arc, and semi-arc with a straight part. Under steady flow conditions, the turbine performance was evaluated by determining the torque, input and flow coefficients, and the turbine efficiency. The semi-arc blade geometry decreased the input coefficient and increased the torque coefficient. Moreover, the semi-arc blade with a straight part improved the starting characteristics of the turbine and its efficiency over a wide range of flow coefficients.

Wave energy can be converted to the electrical energy by using a wave energy converter. The wave energy converter with oscillating water column (OWC) is one of the most promising devices because of its simple structure and easy maintenance. In this device, an oscillating water column due to the wave motion is used to drive an air column. An air turbine is used to convert the pneumatic energy of this bi-directional airflow into the mechanical energy. The counter-rotating impulse turbine for wave energy conversion has been proposed and tested so far, and the average efficiency has been shown to about 0.3. On the contrary, in another offshore experiment, it has been reported that the power generation efficiency of this turbine is larger than Wells turbine in case of small waves. However, there is a scarcity of the detailed characteristics data of counter-rotating impulse turbine. In a previous study, the authors investigated the effect of rotor blade solidity and setting angle of guide vane on the performance of this turbine, and they clarified that the efficiency of this turbine is higher than impulse turbine with single rotor in the range of high flow coefficients. The present study aimed to investigate the effect of rotor blade profile on the turbine performance by using the computation fluid dynamic (CFD) analysis. The inner and outer angles of turbine rotor blade are changed in the range of 50° to 70°. The commercial CFD software of SCRYU/Tetra of Cradle Co. Ltd. was used in the present work. The Reynolds averaged Navier-Stokes (RANS) equations were used as the governing equations and the low Reynold’s number SST k-ω model was used to predict the turbulent stresses. As a result, it was found that the inner angle of γ = 70° and the outer angle of γ = 60° of the turbine rotor blades can give the best turbine efficiency and it shows the efficiency close to the impulse turbine with single rotor, even in the range of low flow coefficients.

Effects of the inlet guide vanes on the static characteristics, aerodynamic noise and internal flow characteristics of a small axial flow fan are studied in this work. The inlet guide vanes with different outlet angle are designed, which are mounted on the casing and located at the upstream of the impeller of the prototype fan. Both steady and unsteady flow simulations are performed. 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 the theoretical analysis, the inlet guide vanes with outlet angle of 60° are regarded as the optimal inlet guide vanes. The static characteristic experiment is carried out in a standard test rig and the aerodynamic noise is tested in a semi-anechoic room. Then, performances of the fan with optimal inlet guide vanes 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 static characteristics of small axial flow fan is improved obviously after installing the optimal inlet guide vanes. Meanwhile, the optimal inlet guide vanes have effect on reducing noise at the near field, but have little effect on the noise at the far field.

Recently a needle-free drug delivery device as an innovative injection device has been widely used in medical fields. Drug powders induced with the high momentum by a moving shock wave can be directly delivered into skin layers. The main component of this device is a contoured shock tube, which consists of a micro shock tube and an expanded nozzle. Drug powders and gas flows are induced by a moving shock wave generated in the micro shock tube and accelerated in the expanded nozzle. The most difficult operation is that the momentum of drug powders should be strictly controlled. In present experimental studies, a micro shock tube was designed to investigate the shock wave propagation. A sonic or a supersonic nozzle installed at the exit of the micro shock tube was studied to observe the particle acceleration. Different diaphragms for initializing incident shock waves were investigated as well. Particle tracking velocimetry (PTV) measurement was carried out to investigate the motion of solid particles. In order to visualize the shock wave propagation, Schlieren visualization method was also carried out. The primary incident shock wave and the reflected shock wave induced by the closed end of the driven section were clearly observed.

In this work, tip end-plate is used to improve the noise performance of small axial fans. Both numerical simulations and experimental methods were adopted to study the fluid flow and noise level of axial fans. Four modified models and the prototype are simulated. Influences of tip end-plate on static characteristics, internal flow field and noise of small axial fans are analyzed. The results show that on basis of the prototype, the model with the tip end-plate of 2 mm width and changed length achieved best noise performance. The overall sound pressure level of the model with the tip end-plate of 2 mm width and changed length is 2.4 dB less than that of the prototype at the monitoring point in specified far field. It is found that the mechanism of noise reduction is due to the decrease of vorticity variation on the surface of blades caused by the tip end-plate. Compared with the prototype, the static pressure of the model with the tip end-plate of 2 mm width and changed length at design flow rate decreases by 2 Pa and the efficiency decreases by 0.8%. It is concluded that the method of adding tip end-plate to impeller blades has a positive influence on reducing noise, but it may diminish the static characteristics of small axial fan to some extent.

A study on the half-ducted axial flow fan designed by a diagonal flow fan design method was conducted. The rotor which has NACA65 blades was designed, calculated numerically, manufactured and tested experimentally. As a result of the design and CFD, the meridional streamline and three distributions of the meridional, tangential and radial velocity at inlet and outlet go well as designed values of the half ducted fan. On the other hand, the values of the meridional velocity and the tangential velocity are little smaller than the design values at the hub side of the radial distribution. The improvement of the design is prospected for this point, that is, the approach between the design value and the actual flow is prospected if the tangential velocity is assigned small at hub and is assigned large at the tip so as to accord the actual flow in the vortex design of the rotor blade. Then the designed half-ducted rotor with four NACA65 blades was fabricated by a three-dimensional printer and tested in the wind tunnel in order to validate the half-ducted design method. For the comparison between the design values and the experimental values at the design flow rate coefficient of φ = 0.264, the experimental values of the pressure rise coefficient ψ and the efficiency η are rather small than the design values, while the experimental value of the torque coefficient τ is almost the same as the design value. However, the experimental value of approximately 0.45 of the maximum efficiency is comparably large value considering for the limitation of the situation of half-ducted. For the comparison between the experimental values and the CFD values at φ = 0.264, the CFD values are almost the same values as the experimental values for all the values of ψ, τ and η. In addition, the tendencies of the CFD values when the flow rate coefficient changes are almost similar as the experimental tendencies, though the flow rate coefficient for the CFD values when ψ or η takes the peak value shifts toward larger flow rate. For the case at rotor outlet at φ = 0.264, two values of the meridional velocity and the tangential velocity are larger than the design values at the tip side of the radial distribution.

A fixed oscillating water column (OWC)-type wave energy converter is composed of an air chamber for primary conversion and an air turbine for secondary conversion. In the optimal design method of a fixed OWC-type wave energy converter, it is necessary to develop a design method which can consider the characteristics of incident wave motion, the motion of the internal free surface affected in the structure such as a partly submerged wall, the fluctuation of air pressure in an air chamber, the rotation of the air turbine. In this paper, the 2-dimensional wave tank tests in regular waves for the performance evaluation of the air turbines in a fixed OWC-type wave energy converter were conducted to obtain the data needed to make this design method. As the results, the effects of the impulse turbine specification such as the rotor inlet/outlet angle, the guide vane's number and the vane's setting angle on the primary and secondary conversion efficiencies are clarified experimentally. Furthermore, the performances of the Wells turbines with different number of blade are presented for comparison of the operating condition.

Flow around two rotating side-by-side circular cylinders of equal diameter D is numerically studied at the Reynolds number Re=40-200 and various rotation rate theta_i. The incoming flow is assumed to be two-dimensional laminar flow. The governing equations are the incompressible Navier-Stokes equations and solved by the finite volume method (FVM). The ratio of the center-to-center spacing to the cylinder diameter is T/D=2. The objective of the present work is to investigate the effect of rotational speed and Reynolds number on the stability of the flow. The simulation results are compared with the experimental data and a good agreement is achieved. The stability of the flow is analyzed by using the energy gradient theory, which produces the energy gradient function K to identify the region where the flow is the most prone to be destabilized and the degree of the destabilization. Numerical results reveal that K is the most significant at the separated shear layers of the cylinder pair. With Re increases, the length of the wake is shorter and the vortex shedding generally exhibits a symmetrical distribution for theta_i, Comment: 10 pages; 10 figures

A detail description of the unsteady phenomena of three-dimensional shock vector control (SVC), including recirculation zones and shear layer regions, has been presented in this study. Shock vector control is a really efficient way to achieve flight direction control of high speed vehicle. Large eddy simulation (LES) has been applied to capture the unsteady characteristics of SVC method using bypass flow passage. Comparison of RANS and LES has been conducted in this study. LES model shows better results than others and it is able to capture the unsteady process very well. In this study, the separation bubble upstream of the injection port is the main source of flow unsteadiness. Large scale eddies in the whole flow field have been resolved by the LES model. Unsteady characteristics of SVC method at different nozzle pressure ratios (NPR) have been investigated. The time histories of thrust vector angle at different NPRs have been recorded by the LES model. The results indicate that it is possible to achieve SVC with the range of bypass mass flow ratio less than 7%. It is also revealed that nozzle pressure ratio has a strong effect on the unsteady phenomenon of SVC system.

This paper deals with the output improvement of heating and cooling cycle by using the work-fluid including phase change material. The experimental study is carried out by heat exchange between work-fluid and heat transfer surface. The work-fluid is flown to a high temperature or a low temperature heat transfer surface from the narrow path. In order to increase the amount of the heat transmission, a trace of Diethylether (boiling point 34.8 °C), as a phase change material (PCM), is added to the work-fluid. The parameters of the experiment are additive amount of PCM, the rotational speed of the displacer piston and the temperature of heat transfer surface. It is clarified that the increasing of engine cycle output is brought by the PCM addition. The effect of PCM addition is evaluated by output ratio which is defined from the experimental cycle output data. The requirements for acquiring the increasing effect of output by adding PCM are clarified.

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.

In recent years, separating and extracting technologies of condensate gas have been developed by combining a swirl flow with non-equilibrium condensation phenomena of condensate gas generated in a supersonic flow. The technology can reduce the size of the device and does not use chemicals. However, there are many unresolved problems for performance of the separation, extraction and operating principle. Therefore it is necessary to research further in order to improve the performance of the equipment. In the present study, the numerical study was carried out to clarify the effect of the heterogeneous condensation on the characteristics of the swirling flow field in a supersonic annular nozzle, and the differences between homogeneous condensation and heterogeneous condensation in the flow field. As the results, it is found that the condensation flow with a swirl affects the position of sonic line, the generating position of condensate and the radial distribution ratio of liquid phase.

Interaction between rotor and struts has great effect on the performance of small axial fan systems. The small axial fan systems are selected as the studied objects in this paper, and four square struts are downstream of the rotor. The cross section of the struts is changed to the cylindrical shapes for the investigation: one is in the same hydraulic diameter as the square struts and another one is in the same cross section as the square struts. Influence of the shape of the struts on the static pressure characteristics, the internal flow and the sound emission of the small axial fans are studied. Standard K-e turbulence model and SIMPLE algorithm are applied in the calculation of the steady fluid field, and the curves of the pressure rising against the flow rate are obtained, which demonstrates that the simulation results are in nice consistence with the experimental data. The steady calculation results are set as the initial field in the unsteady calculation. Large eddy simulation and PISO algorithm are used in the transient calculation, and the Ffowcs Williams-Hawkings model is introduced to predict the sound level at the eight monitoring points. The research results show that: the static pressure coefficients of the fan with cylindrical struts increase by about 25% compared to the fan with square struts, and the efficiencies increase by about 28.6%. The research provides a theoretical guide for shape optimization and noise reduction of small axial fan with struts.

Recently, micro shock tubes have been widely used in various fields such as aerospace, combustion and medical science. Needle-free drug delivery device is a typical example of the application of the micro shock tube used in the field of medical science. Compared to the macro shock tube, the flow viscosity and micro scale effects considerably influence the shock wave propagation in the micro shock tube, which makes shock wave behaviors significant difference from the theoretical prediction. Boundary layers which develop behind the moving shock wave attenuate the shock wave propagation as well. Additionally, the gas-particle flows are also shown the different characteristics compared to the single gas flow in the micro shock tube. Particles are induced by the shock wave inside the micro shock tube and accelerated in the supersonic nozzle. Supersonic flows and particle velocity make experimental studies difficult to be performed in the micro shock tube. Even though micro shock tubes have been studied for decades, shock flow characteristics and particle dynamics inside the micro shock tube are not well known to date. For the present study, pressure measurements and optical visualization have been carried out in a micro shock tube. Static pressure measurements were used for investigating the shock wave propagation in the driven section, and Pitot pressure measurements were used for obtaining flow characteristics at the exit of nozzles. Two high sensitive pressure transducers were used for recording pressure changes as the shock wave moved through pressure transducers and the shock Mach number was calculated. Particle tracking velocimetry was conducted to analyze particle-gas two-phase flows. Particle distribution and velocity were obtained in the present experiment study.

Supersonic cavity flows are characterized by compression and expansion waves, shear layer, and oscillations inside the cavity. For decades, investigations into cavity flows have been conducted, mostly with flows at zero pressure gradient entering the cavity in straight walls. Since cavity flows on curved walls exert centrifugal force, the features of these flows are likely to differ from those of straight wall flows. The aim of the present work is to study the flow physics of a cavity that is cut out on a curved wall. Steady and unsteady numerical simulations were carried out for supersonic flow through curved channels over the cavity with L/H = 1. A straight channel flow was also analyzed which serves as the base model. The velocity gradient along the width of the channel was observed to increase with increasing the channel curvature for both concave and convex channels. The pressure on the cavity floor increases with the increase in channel curvature for concave channels and decreases for convex channels. Moreover, unsteady flow characteristics are more dependent on channel curvature under supersonic free stream conditions.

Numerical experiments were carried out on the high speed driven cavity flows in 2D curved channels to investigate mainly the pressure field. A density-based algorithm in ANSYS Fluent 13.0 was used in the present URANS simulations. The SST k- ω model was used for modeling the turbulence within an unstructured mesh solver. Validation of the numerical code was accomplished, and the results showed a good agreement between the numerical simulation and experimental data. Three channels (straight, concave and convex) with a nominal height of H = 4 × 10 −3 m under the transonic flow conditions were considered in the study. The cavity studied is L = 12 × 10 −3 m long with the depth ranging from D = 12 × 10 −3 m to 48 × 10 −3 m to obtain the length-to-depth ratios of L /D=1 to 1 /4. The study comprised the analysis of the cavity surface pressures and the associated flow structures. The channel configuration influenced the cavity flowfield, and that influence finally resulted in a change in the surface pressure fluctuations in the cavity. The deep cavity attenuated the flowfield oscillation inside the cavity.

Experimental and numerical studies were carried out to evaluate the effect of vortex generator on a small cylindrical protrusion at Mach number 2.0. The experiments were performed using the supersonic blow down wind tunnel on different heights of cylindrical protrusion with vortex generator placed ahead of them. The upstream and downstream flow around the cylindrical protrusion is influenced by vortex generator as is observed using both visualization and pressure measurement techniques. Numerical studies using three dimensional steady implicit formulations with standard k-ω turbulence model was performed. Results obtained through the present computation are compared with the experimental results at Mach 2.0. Good agreements between computation and experimental results have been achieved. The results indicate that the aerodynamic drag acting on cylindrical protrusion can be reduced by adopting vortex generator.

On the basis of small axial fan with five blades, 6 types of small axial flow fans with different chord lengths splitter blades were designed. Numerical simulation of 6 fan models with splitter blades and prototype fan were done by using Fluent. Based on the obtained simulation results, internal flow characteristics and aerodynamic noise were analyzed and compared. It indicates that: splitter blades with suitable chord length have improved significantly on internal flow characteristics, which inhibits backflow from pressure surface to the suction surface at blade tip and leading edge and restrains flow separation. The 6 model fans are better than prototype fan on aerodynamic noise improvement, but too long or too short chord lengths are both disadvantage to improve aerodynamic noise. The results reveal that 2/6, 3/6 and 4/6 chord length model have relatively better acoustic characteristics and internal flow characteristics. The research program will offer a reference for structural improvements and noise reduction on small axial flow fan.

Recently, by combining a swirl flow with non-equilibrium condensation phenomena of condensate gas generated in a supersonic flow, a separating and extracting techniques of condensate gas have been developed. This technique can reduce the size of the device and don’t use chemicals. In the present study, by using a non-equilibrium condensation phenomenon of moist air occurred in the supersonic flow in the annular nozzle composed of an inner body and an outer nozzle with a swirl, the possibility of separation of the condensable gas and the effect of shape of nozzle inlet on the flow field were examined numerically.

At transonic flow conditions, unsteady self-excited shock waves are frequently observed in several modern internal aeronautical applications such as in turbine cascades, compressor blades, butterfly valves, fans, nozzles, diffusers and so on. The appearance of shock oscillation often causes serious problems such as aero-acoustic noise, non-synchronous vibration (NSV), intense drag rise, high cycle fatigue failure (HCF) and buffeting. In recent years, the effect of various passive means on the airfoil are investigated both experimentally and numerically to find the effectiveness as a shock control device. In the present study, the transonic internal flow around an airfoil using a cavity as a passive means of shock control was investigated numerically. Computational results are validated with available experimental data. Results showed that the airfoil with cavity significantly reduced the flow field unsteadiness such as the amplitude of pressure oscillation and root mean square of pressure oscillation.

Recently micro shock tubes have been widely used in many engineering and industrial fields, but the characteristics of unsteady flow are not well known to date in micro shock tubes. Compared to conventional shock tubes with macro scales, flows related to shock waves in micro shock tubes are highly complicated. Stronger viscous and dissipative interactions make shock wave dynamic behaviors significantly different from theoretical predictions. In the present study, a CFD work was applied to the unsteady compressible Navier-Stokes equations which were solved using a fully implicit finite volume scheme. The diaphragm pressure ratio and shock tube diameter were varied to investigate their effects on micro shock tube flows. Different wall boundary conditions were also performed to observe shock wave and contact surface propagation with no slip and slip walls. Detailed flow characteristics at the foot of shock wave and contact surface propagation were known from the present numerical simulations.

Compared with single rotor small axial flow fans, dual-rotor small axial flow fans is better regarding the static characteristics. But the aerodynamic noise of dual-rotor small axial flow fans is worse than that of single rotor small axial flow fans. In order to improve aerodynamic noise of dual-rotor small axial flow fans, the pre-stage blades with different perforation numbers are designed in this research. The RANS equations and the standard k-ɛ turbulence model as well as the FW-H noise model are used to simulate the flow field within the fan. Then, the aerodynamic performance of the fans with different perforation number is compared and analyzed. The results show that: (1) Compared to the prototype fan, the noise of fans with perforation blades is reduced. Additionally, the noise of the fans decreases with the increase of the number of perforations. (2) The vorticity value in the trailing edge of the pre-stage blades of perforated fans is reduced. It is found that the vorticity value in the trailing edge of the pre-stage blades decreases with the increase of the number of perforations. (3) Compared to the prototype fan, the total pressure rising and efficiency of the fans with perforation blades drop slightly.

In the previous researches, it is known that the swirl flow in circular pipe causes the temperature separation. Recently, it is shown that the temperature separation occurs in a vortex chamber when compressed air are pumped into this device from the periphery. Especially, in a cavity installed in the periphery of the chamber, the highest temperature was observed. Therefore, it is expected that this device can be used as a heat source in the engineering field. In recent researches, the mechanism of temperature separation in vortex chamber has been investigated by some researchers. However, there are few researches for the effect of diameter and volume of vortex chamber, height of central rod and position of cavity on the temperature separation. Further, no detailed physical explanation has been made for the temperature separation phenomena in the vortex chamber. In the present study, the effects of chamber configuration and position of the cavity on temperature separation in the vortex chamber were investigated experimentally.

The vacuum ejector-diffuser system has been widely used in many applications such as refrigeration systems, high altitude test facilities and fluid transportation devices. We simulated the starting transient flows of a supersonic vacuum ejector-diffuser system and its performance characteristics and analyzed by the numerical methods. Newly designed chevron lobes were installed at the inlet of the primary stream of the vacuum ejector-diffuser system for the purpose of its performance improvement. A CFD method based on transient scheme was applied to simulate the equilibrium flow behavior inside the secondary chamber. Primary numerical analysis results show that the performance of the vacuum ejector-diffuser system is considerably improved under chevron effects. Compared to the conventional ejector-diffuser system with a convergent nozzle, the updated system with a chevron nozzle leads to a lower equilibrium pressure inside the secondary chamber with less starting time. The flow characteristics obtained from the numerical results highlight the importance of the vortex generation effects of the chevron nozzle. The generation process of the longitudinal vortexes and its effects on the ejector-diffuser performance has been discussed in detail.

In the present study, a numerical investigation is carried out on the aerodynamic performance of a supercritical airfoil RAE 2822. Transonic flow fields are considered where self-excited shock wave oscillation prevails. To control the shock oscillation, a passive technique in the form of an open rectangular cavity is introduced on the upper surface of the airfoil where the shock wave oscillates. Reynolds Averaged Navier-Stokes (RANS) equations have been used to predict the aerodynamic behavior of the baseline airfoil and airfoil with cavity at Mach number of 0.729 and at angle of attack of 5°. The aerodynamic characteristics of the baseline airfoil are well validated with the available experimental data. It is observed that the introduction of a cavity around the airfoil upper surface can completely stop the self-excited shock wave oscillation and successively improve the aerodynamic characteristics.