新型无阀微泵扩张微流道中流体动力学特性的仿真分析

对新型收缩/扩张型无阀微泵泵腔结构中扩张微流道内流体的流动特性进行CFD仿真分析,得到扩张角度、雷诺数（Re）等参数对微流道中流体阻力损失系数的影响。仿真结果显示,对于任一确定的扩张角度和雷诺数,都对应一个最小阻力损失系数,但对于不同的扩张角度,阻力损失系数随雷诺数的不同呈明显的非线性依赖特征;扩张角度较小时,阻力损失系数随雷诺数的增大而减小,而当扩张角度较大时,阻力损失系数随雷诺数的增大而增大。     

Experimental study of the effect of tangential swirling injection on diffuser effectiveness

The experimental data on studying the tangential swirling injection effect on the effectiveness of a conical diffuser with an aperture angle of 18° and the expansion ratio equal to four are presented. It has been indicated that the effect becomes maximal for a given diffuser at an insignificant initial thickness of the boundary layer, when the injected jet swirling angle is 15° and the relative injection rate is 7.7%, as a result of which the pressure increases from 0.48 to 0.79 if the injected jet energy is taken into account.     

Numerical Investigation of a Periodic Heating Thermal-Bubble Actuated Diffuser–Nozzle Valveless Pump

A valveless micropump actuated by thermal bubbles which generated by an electrode heater mounted with a pair of diffuser nozzles has been numerically studied by commercial CFD software FLUENT. The relationship between the net flow rate and the superheating and heat supplying frequency has been investigated. The depth of the diffuser–nozzle micropump used in current numerical simulation model is 200 μm, and the diameter of micropump chamber is 1 mm. The pair of diffuser–nozzles are with gaps expanding from 30 to 274 μm and open angles of 7°. The working fluid is methanol in present study. The results show that the pump has different optimal driving frequency with different superheating. The cycle composed of bubble growth and shrinking costs more time at higher superheating. The maximum volume flow rate and the maximum pump pressure will increase with increasing superheating, simultaneously; and the optimal pulse duty, the maximum volume flow rate and pump pressure decrease with increasing superheating. The maximum volume flow rate and the maximum pump pressure are 29.6 μL/min and 680 Pa at ΔT = 15°C, respectively.     

基于格子-大涡理论的高频换向阀压力特性研究

为揭示高频换向阀的阀口压力特性,降低压力损失,采用格子Boltzmann方法,并结合大涡理论模拟高雷诺数下的阀口流体流动。建立基于格子-大涡理论的高频换向阀格子模型,模拟阀不同尺寸参数下的阀口特性,得出进油腔体积和最大过流面积均对阀口压力有较大的影响,且对于一定的最大过流面积,开口度对阀口压力影响最大。结果表明格子Boltzmann方法具有准确、计算效率高等优点,可以应用于液压激振系统的分析研究。     

基础振动下直动式减压阀动态特性分析

针对基础振动对TBM液压元件性能的影响,为TBM液压元件优化设计提供理论依据,选择直动式减压阀为研究对象,分析其工作原理,建立减压阀的动态响应数学模型,仿真研究基础振动幅值和频率对减压阀波动特性的影响规律,分析减压阀不同结构参数对压力波动的影响.结果表明：基础振动会引起减压阀出口压力波动,波动幅值随振动幅值增加而增大;当基础振动频率大于50 Hz时,压力波动幅值随频率的增加明显增大;减小背压腔初始容积和回流通道直径能提高减压阀的动态稳定性.     

Experimental validation of a prototype ejector designed to reduce throttling losses encountered in transcritical R744 system operation

This study presents experimental results obtained from a transcritical R744 system using a refrigerant ejector. The results were compared to that of a conventional system with an expansion valve. For the test conditions considered, the cooling capacity and COP simultaneously improved by up to 8% and 7%, respectively. Experiments were analyzed to quantitatively assess the effects on system performance as a result of changes in basic ejector dimensions such as motive nozzle and diffuser sizing. Small angles of 5° yielded best results for the static pressure recovery of the high-speed two-phase flow entering the diffuser. Experiments confirmed that like in a conventional transcritical R744 system with expansion valve, the high-side pressure control integrated into the ejector could be used to maximize the system performance. Numerical simulation results helped identifying this basic trend. Due to difficulties in the ejector throat pressure measurements, a more practical performance metric was introduced in order to quantify overall ejector efficiencies. According to this definition, the prototype ejector was able to recover up to 14.5% of the throttling losses.     

Design and characterization of poly(dimethylsiloxane)-based valves for interfacing continuous-flow sampling to microchip electrophoresis

This work describes the fabrication and evaluation of a poly(dimethyl)siloxane (PDMS)-based device that enables the discrete injection of a sample plug from a continuous-flow stream into a microchannel for subsequent analysis by electrophoresis. Devices were fabricated by aligning valving and flow channel layers followed by plasma sealing the combined layers onto a glass plate that contained fittings for the introduction of liquid sample and nitrogen gas. The design incorporates a reduced-volume pneumatic valve that actuates (on the order of hundreds of milliseconds) to allow analyte from a continuously flowing sampling channel to be injected into a separation channel for electrophoresis. The injector design was optimized to include a pushback channel to flush away stagnant sample associated with the injector dead volume. The effect of the valve actuation time, the pushback voltage, and the sampling stream flow rate on the performance of the device was characterized. Using the optimized design and an injection frequency of 0.64 Hz showed that the injection process is reproducible (RSD of 1.77%, n = 15). Concentration change experiments using fluorescein as the analyte showed that the device could achieve a lag time as small as 14 s. Finally, to demonstrate the potential uses of this device, the microchip was coupled to a microdialysis probe to monitor a concentration change and sample a fluorescein dye mixture.     

Optimal Twist and Polarization Angles for the Reflective Liquid Crystal Light Modulator

Abstract

We determine the twist angle and the input polarization angle that optimize the efficiency, modulation sensitivity, and contrast ratio for the reflective liquid crystal modulators (especially the liquid crystal light valve). If a monochromatic light source is used, and when the input polarization is parallel to the front molecular director of the liquid crystal, the conventionally used 45° twist has a theoretical maximum reflectance of only 81%. However, a 63·6° twist angle yields the highest efficiency (theoretical maximum reflectance of 100%) as well as a higher modulation sensitivity. When the input polarization is not parallel, different options that yield a high efficiency and an increased modulation sensitivity are available. If the light source is not monochromatic but has a narrow bandwidth, the dispersion effect tends to reduce the contrast ratio, so that a tradeoff between contrast and sensitivity must be made. We show that a configuration with 65–75° twist angle has an efficiency close to 100% with continuously increasing sensitivity but decreasing contrast as the input polarization changes from 0° to ?30°.     

Internal Flow Characteristics and Aft-cone Angle on Performance of Swirlmeter

The swirler is the main component that leads the fluid spirally into a swirlmeter. The structure of the swirler thus directly affects the internal flow field and meter performance. The present study conducted a numerical simulation and experiment to investigate the effects of the aft-cone angle β of the swirler on the vortex precession characteristics and pressure loss inside a swirlmeter. The RNG k–ε turbulent model was adopted in three-dimensional unsteady simulation and a sonic nozzle device was used for calibration in the experiment. The numerical and experimental results obtained for four values of β (0°, 20°, 30° and 40°) revealed the flow characteristics in detail. The results show that the pressure pulsation at the throat is stronger than that in the convergent region, the swirling flow through the swirler is affected by different outlet velocities with various values of aft-cone angle β, and the aft-cone angle directly affects both the pressure loss and vortex precession frequency of the swirlmeter; i.e., smaller β results in higher pulsation frequency and larger pressure loss.     

The droplet ejection of an inkjet mechanism controlled by electrorheological fluid

In a new design of printhead with ER fluid acting as a working medium, the actuating element and the ER fluid valve control the ink droplet ejection [Smart Mater. Struct. 9 (2000) 839–847]. In this paper, the transient behavior of the ER valve printhead incorporated with ink chamber is investigated. The pattern of ink droplet ejected from the printhead is examined by using the digital optical system for visualization. A quasi-static modeling of the system based on the bulk compressibility of the fluid and the stiffness of the elastic membrane is performed. The fluid pressure in the ink chamber is investigated, both theoretically and experimentally under the following controlled parameters: the actuating voltage and frequency of the piezoelectric transducer; and the controlled electric field strength of the ER value. It is found that the velocity of the ejected ink droplet increases with actuation voltage applied to the piezoelectric transducer. And a better ink droplet without satellite can be obtained with voltage impulse of magnitude 4 V, and pulse width of 6.5 ms. On the other hand, by applying 200 V/mm electric field on the ER valve, the ink ejection can be effectively stopped. Although the frequency of the ink ejection is limited to less than 10 Hz, due to the dynamic resonance of the system, it is believed that the operating bandwidth can be further raised by minimizing the dimensions of the system.     

Utilisation of the heat of condensation from medium capacity refrigerating units for heating waterL'utilisation de la chaleur de condensation de groupes frigorifiques de moyenne puissance pour le chauffage de l'eau

This paper is concerned with the utilisation of condenser heat rejected by a refrigerator cooling milk. The condenser pressure is raised so that hot water can be produced in a recuperator, any residual refrigerant vapour being passed to an air cooled condenser. The performance and control of such equipment with varying heat demand and ambient temperature are considered.To cool 1000 dm³ of milk from 35°C down to +5°C, the quantity of heat recuperation increases from 177 500 to 182 000 kJ when the ambient temperature drops from 20° to 10°C. However, this 2.5% gain in recoverable heat will result in a 33.3% increase in the running time of the compressor and an 8.6% increase in the power consumption.A higher capacity expansion valve can be fitted in order to improve the functioning of the refrigerating unit during cold periods, but this will entail adjustments to the valve during warm periods.Another method is to fit a constant pressure valve between the recuperator and the condenser to maintain the desired pressure in the recuperator and thus directly control the fluid flow volume. This will involve the fitting of a suitably sized accumulator and increasing the amount of refrigerant. It also brings control problems.A secondary study has analysed the difficulties arising when the heated water temperature is raised from 40°C to 50°C. For a flow of 1300 dm³ at 40°C an economy of 51 kWh was achieved. The latter temperature was found impossible to achieve, the maximum being 47.5°C and this required an additional power consumption of 31.9%.A properly designed installation will provide the daily cooling of 1000 l of milk with an annual saving of 16 000 kWh.     

Numerical modeling of startup and shutdown transients in reciprocating compressors

Several phenomena that affect energy consumption, noise level and reliability of compressors are associated with transient effects that occur during the compressor startup and shutdown. This paper presents a simulation methodology, experimentally validated, developed to analyze the compressor in such transients. Because the time scale associated with the compressor thermal behavior is much larger than that related to the mechanical physics, a thermal equilibrium condition has been considered herein for convenience. Results for valve displacement, piston motion, pressure in the compression chamber and resistive torques are provided to illustrate the application of the methodology and to assist an understanding of the physical aspects that affect the compressor performance throughout the startup and shutdown. It is observed that the dynamics of suction and discharge valves are greatly affected. Moreover, the model is employed to estimate the minimum voltage required for the compressor startup as a function of the equalized pressure and the auxiliary coil actuation time.     

Experimental study on control fins of a small flying vehicle using piezo-composite actuators

A camber morphing control fin design and an all-moving control fin design using piezo-composite unimorph actuators are presented in this paper. The control fin of a small flying object is usually actuated using a servo motor system with an electromagnetic motor. Much research has been conducted to solve the structural complexity of servo actuation systems to convert the rotation of a servo motor to a linear actuation motion. To simplify this structural complexity, several types of smart actuators have been developed, such as bimorph or unimorph actuators with piezoelectric material layers and shape memory alloy actuators. In this study, a camber morphing type control fin and an all-moving type control fin actuated using piezo-composite actuators are designed to evaluate their ability to simplify the structural complexity of the gear transmission and electromagnetic servo motor system or hydraulic actuator system. Within the skin of the control fin, a piezo-composite actuator is mounted and the other end inserted in a slot of the control fin. As the piezo-composite actuator is excited by an electric field, the pitch angle of the control fin is changed. Experimental testing for the pitch rotation angle of a control fin in a 450 V electric field showed the deflection angle of the camber morphing control fin was 1.4° and the rotational angle of the all-moving control fin was 5.4°, which is obtained from the rotation angle magnification linkage structural system.     

Flow of a supersonic jet into channels of various shapes

The author examines the process of flow from one or more supersonic nozzles into a chamber with a diffuser or a wide cylindrical tube. The characteristic regimes are established. An analysis of the experimental data shows that the chamber pressure is at a minimum when critical flow conditions exist at the diffuser outlet.     

Novel transcatheter aortic heart valves exhibiting excellent hemodynamic performance and low-fouling property

Transcatheter aortic heart valves (TAHVs) have been widely used for aortic valve replacements, with less trauma and lower clinical risk compared with traditional surgical heart valve replacements. In the present study, composites of poly(ethylene glycol) diacrylate (PEGDA) hydrogels and anisotropic high-shrinkage polyethylene terephthalate/polyamide6 (PET-PA6) fabric (PEGDA/PET-PA6) were fabricated as artificial heart valve leaflets. Dynamic mechanical analyses (DMA) indicated that PEGDA/PET-PA6 composites possessed anisotropic mechanical properties (i.e., storage moduli ～23.30 ± 1.36 MPa parallel to the aligned fabric fibers and ～9.68 ± 0.90 MPa perpendicular to the aligned fibers at 1 Hz) that were comparable to aortic valve leaflets. The PEGDA/PET-PA6 composites with smooth surfaces were highly hydrophilic (contact angle ～41.6° ± 3.8°) and had low-fouling properties without platelet adhesion, suggesting a low risk of thrombogenicity when they interacted with blood. Furthermore, transcatheter aortic heart valves were fabricated using nitinol self-expanding frames and PEGDA/PET-PA6 composites as artificial leaflets, which presented excellent hemodynamic performance with a large orifice area (1.75 cm²) and low regurgitation (3.41%), thus meeting the requirements of ISO 5840-3 standard. Therefore, PEGDA/PET-PA6 composites had suitable mechanical properties, good biocompatibility, and low-fouling properties, indicating that they might be used for TAHVs in the future.     

有机朗肯循环模拟及涡旋式膨胀机的性能研究

近些年来,太阳能作为一种可再生能源受到了广泛的关注。其中利用太阳能集热器实现100℃以下高效的热量回收,是一种普遍且有效的太阳能利用方式。采用有机朗肯循环与100℃的低温热源相结合进行发电,目前也逐渐受到了研究人员的关注。考虑到膨胀机是有机朗肯循环的核心部件,本文选择了R600制冷剂作为ORC系统的工质,对其进行了计算以及热力学性能分析。同时搭建了利用压缩空气来驱动的涡旋式膨胀机性能研究的实验台。从ORC的理论分析得,当热源温度为78~97℃,环境温度为30℃,可以获得0.7~1kW的电量,效率为0.84~0.89。利用压缩空气模拟R600,当温度从75℃变化到95℃,对应的压力从0.8MPa变化到1.2MPa,膨胀机出口压力控制在0.28MPa,等熵效率维持在0.7左右。膨胀机的功电转化效率随着膨胀机理想输出功的增加而降低。     

基于CFD的舌簧排气阀流动特性研究

气阀是活塞式压缩机最为关键的部件,其工作性能直接影响压缩机的能效。本文首先对气阀的流动特性进行分析,然后基于CFD的方法,在Fluent中模拟了舌簧排气阀稳态和瞬态流动时的压力场、速度场的分布,对造成气阀压力损失的流截面比、扩散角以及阀片升程这些因素进行分析。研究表明:较小的通流截面比和扩散角有利于减小气阀压力损失;较大的阀片升程虽然能降低阀隙速度,明显减小气阀压力损失,但是过大的升程不能有效降低压力损失,反而会增加阀片的磨损,减少阀片的使用寿命。最后,论文以某型号压缩机为例,实验验证了仿真分析结论的正确性。通过对舌簧排气阀流动特性的分析,为气阀优化设计、提高气阀效率提供了参考依据。     

坑道口部内爆炸情况下扩散室中冲击波的数值分析

扩散室是一类重要的防护工程坑道口部消波措施,其结构形式和几何尺寸对消波效果影响显著.为使其消波效率最大化同时具有较高性价比,对化爆荷载作用下坑道中不同特征尺寸扩散室的消波规律进行探讨,提出了数值分析的计算方案,并取得了理想的消波模拟效果.在此基础上与试验进行比较,对模拟结果展开分析,验证了计算结果的准确性和计算方法的可行性.计算给出了扩散室中冲击波瞬变位置取值的表达式,并指出费效比合理的扩散室长径比为3,为扩散室在防护工程坑道结构设计中的应用提供了技术参考.     

Inverse design optimisation of a novel range hood based on intelligent algorithms and computational fluid dynamics simulations

Our aim in the present study is to increase grease separation by optimising the discharge performance of cooking oil fumes. A novel range hood with an outer cylinder and guide vanes of exhaust (GVE) is proposed. The geometric configuration of the hood is optimized by a reverse design method developed by coupling back propagation neural networks, mind evolutionary algorithms, and computational fluid mechanics. The primary objective of the range hood is to maximise the exhaust airflow rate with good grease separation. At the optimised conditions, the inlet and outlet blade angles are 80° and 176°, respectively, while the heights of the blade and guide vane are 0.180 and 0.245 m, respectively. The optimal number of blades and guide vanes are 29 and 7, respectively, while the optimal diffuser diameter is 0.340 m. The proposed range hood exhibits excellent performance of grease particle separation compared to multi-blade centrifugal fans with volutes. In addition, the outer cylinder and GVE of the range hood exhibit a large separation efficiency for particles in the size range of 1–4 μm, while impeller blades mainly separate grease particles in the size range of 4–10 μm.     

A study on diffusion bonding of steel and copper alloy

In combustion chamber of liquid propellant launch vehicle, the inner shell of the chamber is copper alloy with cooling channels for regenerative cooling and outer shell is steel to maintain high pressure inside the chamber. The purpose of this study is to find the optimum condition for diffusion bonding of copper and steel and the experimental conditions were 3 different pressures at temperatures from 800°C to 950°C. In order to characterize the flow strength of materials at high temperatures, several tensile tests were performed at several temperatures from 800°C to 950°C. This information is used to estimate the test condition for diffusion bonding and superplastic forming. Mechanical properties of bonded specimen were evaluated with single lap joint tests and shear tests. Microstructure of bonded layer has been also observed with SEM with EDX. It is shown that the optimum condition of diffusion bonding is 7 MPa at 890°C, for one hour. Pressurization test of bonded specimen with cooling channels was performed with hydraulic pressure of 87 MPa without failure.