Experimental study on radial temperature gradient effect of a Taylor-Couette flow with axial wall slits

The flow between two concentric cylinders with the inner cylinder rotating and an imposed radial temperature gradient was studied using a digital particle image velocimetry method. The flow transition process under both a positive and negative temperature gradient with four different models of a stationary outer cylinder without and with differing numbers of slits (6, 9 and 18) was studied. The results showed that the buoyant force due to the temperature gradient clearly generated a helical flow when the rotating Reynolds number was small. For the plain and 6-slit models, the transition to a turbulent Taylor vortex flow was not affected by the temperature gradient considered in this study. In addition, the transition process of a larger number of slits (9-, 18-slit models) was accelerated due to the slit wall. As the temperature gradient became larger, the critical Reynolds number of the transition process decreased.     

Experimental investigation of irreversibility of a proton exchange membrane fuel cell at different channel geometry

The geometry of the channels of a fuel cell is very important for performance and efficiency of it. For this reason, a thermodynamic analysis is performed for a PEM fuel cell at different channel geometry that three different fuel cells with rectangular, elliptical and triangular serpentine channels have constructed. The active area of each cell is 25?cm² that its weight is 1300?g. The material of the gas diffusion layer is carbon clothes, the membrane is Nafion 112 and the catalyst layer is a plane with 0.004?g/cm² Platinum. Also a test bench designed and constructed for testing the cell and a series of experiments are carried out to investigate the influence of the geometry of the cell on irreversibility under the normal conditions. The results show that the performance of the cell at $ T_{{{\text{O}}_{2} }} $ ?=?55?°C, $ T_{{{\text{H}}_{2} }} $ ?=?55?°C, $ T_{\text{cell}} $ ?= 60?°C, oxygen flow rate?=?0.5?L/min, hydrogen flow rate?=?0.3?L/min and P?=?2.905 bar is higher about 12?% and 18?% when the geometry of the channels is rectangular in comparison of elliptical and triangular channels and the irreversibility is lower about 17?% and 33?% when the geometry of the channels is rectangular in comparison of elliptical and triangular channels.     

旋转爆震燃烧室轴向和周向长度对其出口流场压力和温度的影响

为了研究旋转爆震燃烧室与涡轮的匹配特性,利用二维欧拉方程数值研究了基于当量H₂/Air燃烧的旋转爆震燃烧室出口流场特性,对比分析了不同燃烧室轴向长度和周向长度出口总压脉动、总压畸变以及出口温度分布规律。结果表明:旋转爆震燃烧室在稳定工作状态下,其出口总压的脉动值会呈现周期性振荡;燃烧室尺度对发动机出口流场的不均匀性有很大影响,随着燃烧室轴向长度的增大或周向尺寸的减小,其出口总压脉动均值、畸变指数和出口温度分布系数均会减小,其出口流场均匀性提高。此外,爆震波高度随着周向尺寸的增大而增大;轴向尺寸对爆震波高度几乎不产生影响。     

Experimental studies on thin-walled grooved tubes under axial compression

We study experimentally the axial crushing behavior and crashworthiness characteristics of thin-walled steel tubes containing annular grooves. The grooves determine the positions of the folds and control the buckling mode of deformation. In the present work we aim to improve the uniformity of the load-displacement behavior and to predict the energy absorption capacity of the tubes. Grooves are cut circumferentially and alternately inside and outside the tubes at predetermined intervals. Quasi-static axial crushing tests are performed with different groove distances. Photographs are taken during axial buckling and the specimens after crushing are sectioned axially to carry out the measurements. The deformation modes and load-displacement curves are described and energy absorption and mean post-buckling load are determined. The convolutions are achieved by folding in an axisymmetric concertina mode about the circumferential grooves. The results show that the load-displacement curve and energy absorbed by the axial crushing of tubes can be controlled by the introduction of grooves with different distances.     

Experimental studies on buckling of ring-stiffened conical shells under axial compression

The buckling of integrally external ringstiffened conical shells under axial compression was investigated experimentally. Experimental results were compared with theory to find the effect of the stiffener parameters (e ₂ /h), (A ₂ /a ₀ h) and (I ₂₂ /a ₀ h ³ ) as well as of shell geometry. Agreement between classical linear theory and experiments was found to be governed primarily by the area parameter (A ₂ /a ₀ h), and correlation with theory was significantly affected in the range 0.1<(A ₂ /a ₀ h)<0.5 of that parameter. Beyond this region there is practically no improvement with increase in ring area, whereas the weight of the shell continues to increase linearly. An approximate formula is proposed for calculation of critical loads and found to yield results very close to the more exact critical values calculated by linear theory. A modified “Southwell plot” method was applied and both the intercept method and slope method were used. Critical loads computed from the strain records were found to be below the classical linear-theory predictions and closer to experimental ones.     

轴向冲击下C型冷弯卷边槽钢构件的动力响应

针对轴向冲击载荷下C型冷弯薄壁钢构件的动力响应,采用商业有限元软件Abaqus建立了能够反映冲击过程的有限元模型。通过对比有限元模拟和落锤实验中利用数字散斑技术采集的数据以及试样残余变形,验证了数值模型的可靠性。采用该模型分析了在不同冲击速度下翼缘、腹板和卷边质点的轴向位移-时间曲线以及腹板横向挠度的变化,结果表明:在较低冲击能量的作用下,翼缘对卷边的约束作用明显;而在较高冲击能量的加载过程中,冲击端卷边的轴向位移和速度明显大于翼缘和腹板,卷边破坏严重;随着冲击速度的提升,C型冷弯薄壁钢构件的动态屈曲临界载荷相应提升。     

Analysis of piezoelectric semiconductor fibers under gradient temperature changes

Piezoelectric semiconductors(PSs) possess both semiconducting properties and piezoelectric coupling effects, making them optimal building blocks for semiconductor devices. PS fiber-like structures have wide applications in multi-functional semiconductor devices. In this paper, a one-dimensional(1D) theoretical model is established to describe the piezotronic responses of a PS fiber under gradient temperature changes. The theoretical model aims to explain the mechanism behind the resistance chang...     

Deformation and failure mechanisms of single crystal superalloy under temperature gradient

The crystallographic constitutive model under temperature gradient is developed and introduced to study the deformation and failure mechanisms of single crystal superalloy. Tensile tests of thin-walled pipe specimen at different temperatures without cooled air flow were carried out. Based on the experimental results, the temperature dependence of constitutive model was studied and the basic parameters of constitutive model were obtained. To investigate the deformation and failure mechanisms, the thin-walled pipe specimen with cooled air flow under temperature gradient were tested. Considered the fluid-solid interface (FSI), a finite element method (FEM) was proposed to simulate the process of tension. In FEM, the activation rate of slip system was defined as the failure law of specimen under temperature gradient. The simulation result was in good agreement with the experiment result. Furthermore, the fracture surface of the specimen was observed by the scanning electron microscopy (SEM). The microstructure revealed that the slip deformation belonged to {1 1 1} crystalplane is a principal failure mechanism of single crystal superalloy under temperature gradient. The results of the SEM also implied that the proposed FEM method can be used to systemically study the deformation and failure behavior of single crystal superalloy cooled blade.     

Experimental investigation on longitudinal vortex control over a dihedral bluff body

The present paper addresses an experimental investigation of the dynamic and the control of a longitudinal vortex emanating from the front pillar of a dihedral bluff body corresponding to a simplified geometry of an automotive vehicle. The control system is based on a thin rectangular slot located along the lateral edge of the windscreen and provides steady suction or blowing normal to the lateral face of the geometry. Qualitative results obtained with dye visualizations and Schlieren photos provide an overview of the impact of the control on the topology of the vortex flow. Quantitative Stereo-PIV measurements and unsteady forces measurements are used to characterize the interaction between the control and the longitudinal vortex. Controls that change significantly the topology of the vortex core and reduce aerodynamic drag are identified.     

Experimental study of thin cylindrical shells under local axial loadings

A thin cylindrical shell was subjected to local axial loads. These loads were transferred to the shell through uniformly spaced pads mounted inside the cylinder. The stress distribution in the vicinity of these pads was determined experimentally, and the results were then compared with theoretical findings. Parameters examined included the number and size of pads and the eccentricity of the loads applied to the pads with respect to the middle surface of the cylindrical skin.     

Experimental investigation of fluid convection in a closed toroidal channel

The results of investigating convective fluid motion in a closed toroidal channel are analyzed and compared with the results of calculations based on the Lorenz model. It is shown that this model adequately describes the fluid convection only in the case of a large aspect ratio of the mean radius of the torus to its cross-sectional dimension and small heating power. The measurements and visual observations record substantial deviations from the assumptions underlying the Lorenz model in the domain of problem parameters in which, in accordance with the calculations, bifurcations and instability of steady solutions of the Lorenz model should take place.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 4, pp. 20–28, July–August, 1995.     

Experimental investigation of turbulent natural convection flow in a channel

This paper presents the results of velocity measurements of natural convection in symmetrically heated vertical channel using the particle image velocimetry (PIV) system. Velocity measurements were conducted at three different sections on the horizontal plane to validate the flow two-dimensionality and at three different heights in the vertical plane to establish vertical mean velocity profiles. The results indicate a considerable influence of the Rayleigh number and aspect ratio on the mean velocity profile. The results also indicate significant diffusion rates of the vertical mean velocity component and normal Reynolds stresses towards the center of the channel.     

Parametric instability and nonlinear oscillations of an FRP channel section column under axial load

Nonlinear Dynamics - The range of applications of fiber-reinforced polymer structures has become increasingly diverse in recent years. However, little is known on their performance under dynamic...     

Convective instability of Marangoni-Poiseuille flow under a longitudinal temperature gradient

An exact solution is obtained for the problem of steady flow in a system of two horizontal layers of immiscible fluids with a common interface. The stability of the flow is studied by a linearization method. It is shown that the occurrence of instabilities is due to the different governing parameters of the fluids (thickness, heating conditions, viscous and thermal conductivity of the fluids). It is found that under constant gravity conditions, the perturbations are monotonic, and in zero gravity, oscillatory thermocapillary instability occurs.     

Experimental investigation about the effect of non-axisymmetric wake impact on a low speed axial compressor

Non-axisymmetric wake impact experiments were carried out after the best exciting frequency for a low speed axial compressor had been found by axisymmetric wake impact experiments. When the number and circumferential distribution of inlet guide vanes （IGV） are logical, the wakes of non-axisymmetric IGVs can exert beneficial unsteady exciting effect on their downstream rotor flow fields and improve the compressor＇s performance. In the present paper, four non-axisymmetric wake impact plans were found working better than the axisymmetric wake impact plan. Compared with the base plan, the best non-axisymmetric plan increased the compressor＇s peak efficiency, and the total pressure rise by 1.1 and 2%, and enhanced the stall margin by 4.4%. The main reason why non-axisymmetric plans worked better than the axisymmetric plan was explained as the change of the unsteady exciting signal arising from IGV wakes. Besides the high-frequency components, the nonaxisymmetric plan generated a beneficial low-frequency square-wave exciting signal and other secondary frequency components. Compared with the axisymmetric plan, multifrequency exciting wakes arising from the non-axisymmetric plans are easier to get coupling relation with complex vortices such as clearance vortices, passage vortices and shedding vortices.     

Experimental procedure for testing of piles under varying axial and lateral loads

Experimental Techniques -