高海拔隧道施工机械动力性能研究

为探究高海拔低氧环境对施工机械动力性能的影响,文章以运输类施工机械为研究对象,通过测算牵引力比功率研究海拔变化对施工机械的动力性能影响。同时,以牵引力比功率作为运输类施工机械的工作效率量化指标,建立海拔-施工机械工作效率降低系数回归模型以及海拔-施工机械台班消耗调整系数模型。研究结果表明:海拔越高,机械动力性能越差,海拔4200 m处较2800 m处牵引力比功率下降23%,机械工作效率下降23%;海拔越高,机械台班消耗越大,海拔4200 m处较2800 m处台班消耗量增大24%。

Optimization Study of a Coanda Ejector

The Coanda effect has long been employed in the aerospace applications to improve the performances of various devices. This effect is the ability of a flow to follow a curved contour without separation and has well been utilized in ejectors where a high speed jet of fluid emerges from a nozzle in the ejector body, follows a curved surface and drags the secondary flow into the ejector. In Coanda ejectors, the secondary flow is dragged in the ejector due to the primary flow momentum. The transfer of momentum from the primary flow to the secondary flow takes place through turbulent mixing and viscous effects. The secondary flow is then dragged by turbulent shear force of the ejector while being mixed with the primary flow by the persistence of a large turbulent intensity throughout the ejector. The performance of a Coanda ejector is studied mainly based on how well it drags the secondary flow and the amount of mixing between the two flows at the ejector exit. The aim of the present study is to investigate the influence of various geometric parameters and pressure ratios on the Coanda ejector performance. The effect of various factors, such as, the pressure ratio, primary nozzle and ejector configurations on the system performance has been evaluated based on a performance parameter defined elsewhere. The performance of the Coanda ejector strongly depends on the primary nozzle configuration and the pressure ratio. The mixing layer growth plays a major role in optimizing the performance of the Coanda ejector as it decides the ratio of secondary mass flow rate to primary mass flow rate and the mixing length.

A Study of the Gas Flow through a LNG Safety Valve

A safety valve functions to control an upper limit of pressure inside the LNG line of transportation. If the pressure inside the safety valve nozzle exceeds a predetermined value on the valve sheet which plugs the nozzle, an excess of LNG discharges through the gap between the nozzle exit and valve sheet. In this situation, the forces acting on the valve sheet are gasdynamic forces generated by the discharge of LNG and mechanical forces supported by the spring behind the valve sheet. The flow through the gap is very complicated, involving vortices, flow separation, and shock waves. These affect adversely on the system accompanying with noise and vibration. The present study aims at understanding the flow physics of safety valve. A computational work using the twodimensional, axisymmetric, compressible Navier-Stokes equations is carried out to simulate the gas flow between the nozzle exit and valve sheet, and compared with the theoretical results. It has been found that there exists a distance between nozzle exit and valve sheet in which the thrust coefficient at the valve sheet increases abruptly.

Dynamic PIV Measurement of a Compressible Flow Issuing from an Airbag Inflator Nozzle

Among many equipment for passenger safety, the air bag system is the most fundamental and effective device for an automobile. The inflator housing is a main part of the curtain-type air bag system, which supplies high-pressure gases in pumping up the air bag-curtain which is increasingly being adapted in deluxe cars for protecting passengers from the danger of side clash. However, flow information on the inflator housing is very limited. In this study, we measure the instantaneous velocity fields of a high-speed compressible flow issuing from the exit nozzle of an inflator housing using a dynamic PIV system. From the velocity field data measured at a high frame-rate, we evaluate the variation of the mass flow rate with time. The dynamic PIV system consists of a high-repetition Nd：YLF laser, a high-speed CMOS camera, and a delay generator. The flow images are taken at 4000 fps with synchronization of the trigger signal for inflator ignition. From the instantaneous velocity field data of flow ejecting from the airbag inflator housing at the initial stage, we can see a flow pattern of broken shock wave front and its downward propagation. The flow ejecting from the inflator housing is found to have very high velocity fluctuations, with the maximum velocity at about 700 m/s. The time duration of the high-speed flow is very short, and there is no perceptible flow after 100 ms.

An Experimental Study on Aerodynamic Sound Generated from Wake Interference of Circular Cylinder and Airfoil Vane in Tandem

The purpose of this study is to investigate the characteristics of aerodynamic sound generated from wake interference of circular cylinder and airfoil vane located in tandem and to clarify the generation mechanism of the sound source with discrete frequency. The effects of the interval between the cylinder and the airfoil on the characteristics of aerodynamic sound are investigated by acoustic measurement, flow visualization and exploration test of sound source. The relation between the flow field and the sound field with discrete frequency noise（DFN） is shown, and then it is found that the downstream airfoil works as the sound source of DFN, which has the frequency of vortex shedding from the upstream cylinder, when the interval of two bodies is longer than a critical distance.

Curing kinetics and mechanical properties of bio-based composite using rosin-sourced anhydrides as curing agent for hot-melt prepreg

In this study, a novel bio-based thermosetting system has been developed from epoxy resin （EP）, with rosin-sourced anhydrides （maleopimaric acid, RAM） as curing agent and imidazole type latent catalyst （two amino imidazole salt complex, IMA）, to be used as matrix for hot-melt prepreg curing at mid-temperature. For comparison, the epoxy resin system with petroleum sourced hardener methylhexahydrophthalicanhydride （MHHPA） was also examined. The curing behaviour and mechanism were investigated by non-isothermal differential scanning calorimeter （DSC） analysis and Fourier transform infrared （FTIR） spectra. The results showed that the curing course of bio-based epoxy resin system containing RAM included two stages, which were the reaction between the free carboxyl group of RAM and oxirane ring under the acceleration of IMA, and the main reaction attributed to the reaction between anhydride and oxirane. According to Kissinger method, the reaction activation energy （E,） of two stages were 68.9 and 86.5kJmo1-1, respectively. The Eo of EP/MHHPA and EP/IMA resin system were 81.04 and 77.9kJmol-I. The processing property of EP/RAM/IMA system, i.e. the relationship between viscosity-temperature-time, was characterized by cone-plate viscometer aim to decide the processing parameter ofprepreg preparation. The effect of RAM content on mechanical performance and dynamic mechanical property was investigated. Noteworthily, compared with the laminates with EP/MHHPA as matrix, the laminates with RAM as hardeners achieved a 44%, 73% and 70℃ increase in bending strength, bending modulus and the glass transition temperature, respectively, due to the bulky hydrogenated phenanthrene ring structure incorporated into the cross-linking networks. When the fiber volume fraction reached 47%, the mechanical property of the laminates prepared with hot melt prepreg was superior or comparable to that of composites with pure petroleum sourced matrix. RAM as cross-linking agent of epoxy resin holds a great potential to satisfy the requirement of composites such as structure and secondary structure parts preparation.

Coupling effects of stress and ion irradiation on the mechanical behaviors of copper nanowires

By using molecular dynamics simulations,we studied the ion irradiation induced damage in mechanically strained Cu nanowires and evaluated the effects of damage on the mechanical properties of nanowires.The stresses in the pre-strained nanowires can be released significantly by the dislocation emission from the cascade core when the strain is greater than 1%.In addition,comparison of the stress-strain relationships between the defect-free nanowire and the irradiated ones indicates that ion irradiation reduces the yield strength of the Cu nanowires,and both the yield stress and strain decrease with the increase of irradiation energy.The results are consistent with the microscopic mechanism of damage production by ion irradiation and provide quantitative information required for accessing the stability of nanomaterials subjected to mechanical loading and irradiation coupling effects.

Study on air flow dynamic characteristic of mechanical ventilation of a lung simulator

As an important life support treatment, mechanical ventilation is usually adopted in clinics. With the development of the res-piratory diagnostic and treatment technologies, air flow dynamics of mechanical ventilation is usually referenced in the evaluation of pulmonary status and assessment of respiratory therapy. In order to improve the ventilation efficiency and provide a reference for pulmonary diagnostics, in this paper, a new mathematical model of mechanical ventilation system was set up. Furthermore, a prototype mechanical ventilation system for an artificial simulating lung was designed and experimentally studied. Lastly, in order to improve the ventilation efficiency and provide a reference for pulmonary diagnostics, the air flow dynamics of the mechanical ventilation system was illustrated through simulation and experimental studies. The study can be helpful to the optimization of the mechanical ventilation system.

Influence of Intertube Additional Atoms on Sliding Behaviors of Double-Walled Carbon Nanotube

The effects of intertube additional atoms on （DWCNTs） are investigated using molecular dynamics （MD） the sliding behaviors of double-walled carbon nanotubes simulation method. The interaction between carbon atoms is modeled using the second-generation reactive empirical bond-order potential coupled with the Lennard-Jones potential. The simulations indicate that intertube additional atoms of DWCNT can significantly enhance the load transfer between neighboring tubes of DWCNT. The improvement in load transfer is guaranteed by the addition of intertube atoms which are covalently bonded to the inner and outer tubes of DWCNT. The results also show that the sliding behaviors of DWCNT are strongly dependent of additional atom numbers. The results presented here demonstrate that the superior mechanical properties of DWCNT can be realized by controlling intertube coupling. The general conclusions derived from this work may be of importance in devising high-performance CNT composites.