镐型截齿温度场数值模拟与试验研究

以镐型截齿及其深冷处理环境为研究对象,基于Solidworks软件建立截齿的三维几何模型,基于ANSYS Fluent软件建立了镐型截齿深冷处理温度场数值模型。完成了某型截齿的温度场数值模拟,为验证该模型的合理性,完成了镐型截齿的深冷处理试验,通过截齿温度场模拟数据与试验数据对比,得出数值模拟温度与试验温度下降趋势一致,且时间相差8 min左右,表明镐型截齿温度场数值模拟具有一定的可行性。     

以最佳温度均匀度为目标的常压热循环系统结构参数优化

用数值模拟的方法对常压热循环系统试验箱内试件模型的降温过程进行了研究.研究了不同结构参数下试验箱内试件模型的降温速率及温度均匀度变化规律.结果表明,当模型位于试验箱中心位置时,由于试件模型的阻碍,在其上底面出现滞止区,而在底部产生高温回流区域.试件模型降温速率随离地高度的增加而逐渐加快,最终可在7 h内完成降温过程,最...     

送风速度对地板送风房间空气品质影响的数值模拟

针对全尺寸地板送风办公房间,采用数值模拟方法研究了当送风量和送风温度一定时,送风速度对室内流场、温度场和污染物浓度分布的影响。研究结果表明,当送风速度小于0.9m/s时,室内气流呈置换通风状态,主动式污染物CO2呈分层分布,室内空气质量较好;当送风速度大于0.9m/s时,室内气流呈混合通风状态,不利于主动式污染物CO2的排除,但有利于排除被动式污染物苯。     

Influence of the Air Gap between Adjacent Permanent Magnets on the Performance of NdFeB Guideway for HTS Maglev System

For the permanent magnet guideway (PMG) of the high temperature superconducting (HTS) magnetic levitation (Maglev) vehicle system, there should be many air gaps between two adjacent permanent magnets by connecting, which may fluctuate magnetic field in the propulsion direction of the vehicle. A three-dimensional (3D) model of a PMG is built up using FLUX 3D software. The magnetic field density of an NdFeB PMG is measured and simulated by 3D and 2D models. Comparison among their results indicate that the simulation results of the 3D model agree better with the measuring results than that of the 2D model, and thus the 3D model is more suitable to describe the PMG. By the model, the influence of the air gap on the uneven of the magnetic field density in the propulsion direction is studied. It is found that the magnetic field 15 mm above the PMG is roughly even in the propulsion direction, although the magnetic field at 2 mm is not even. Since the working height 15 mm is a sensitivity parameter for the onboard high temperature superconductor (HTSC), the levitation force at working height 15 mm above PMG is measured, which indicates that the influence of the air gap on the levitation force is very small so as to be ignored in the quasi-static state.     

24-hour cooling of a building by a PCM-integrated adsorption system

This study presents a theoretical investigation into integration of phase change materials (PCMs) with an adsorption cooling system in order to provide 24-hour air conditioning. A latent heat storage unit containing PCM is used to store solar energy during the daytime, and at nighttime the conserved thermal energy and an auxiliary heater drive the adsorption chiller. The system adopts a cooling channel to reduce the air temperature. The air flow to the channel is provided by use of fans and at different fresh air ratios (FR). Room temperature and the room's maximum cooling demand for which thermal comfort can be achieved are estimated. In addition, the effects of different parameters on room temperature and solar fraction are studied. It is indicated that an optimum ACH value exists for which the room temperature is the lowest. Also, rise of ACH and FR decrease solar fraction and increase auxiliary energy consumption. It is found that when ACH = 4 and FR = 20%, daily solar fraction is 0.76 and 217 MJ of auxiliary energy is required during the 24 hours. Under this condition, thermal comfort is achieved for a maximum cooling demand of 4000 W during the 24 hours.     

Numerical modelling of oxidized microstructure and degraded properties of 2D C/SiC composites in air oxidizing environments below 800 °C

In this paper, a numerical model which incorporates the modelling of oxidized microstructure and computing of degraded elastic moduli is presented for simulation of the oxidation behaviors of 2D C/SiC composites exposed to air oxidizing environments below 800 °C. Regarding the multi-scale characteristics of 2D C/SiC composite, the microstructure modelling is carried out on microscopic and macroscopic scale, respectively to compute the degraded elastic properties in terms of time duration, temperature and pressure, whose influences upon the oxidation microstructure morphology and degraded properties of 2D C/SiC composites are also investigated. It is shown that the simulation models well the microstructure morphology after oxidation and numerical results are found to be in good agreement with experimental data.     

The elastic properties of nanostructured Ag measured by laser ultrasonic technique

The elastic properties of nanostructuredAg (ns-Ag) and its changing behavior during densification and annealing were investigated by a laser ultrasonic technique. The ns-Ag with grain size d about 30 nm was synthesized by inert gas condensation and in-situ compaction. The experimental results indicate that the Poisson's ratio, and the shear as well as the Young's modulus for all ns-Ag specimens investigated are smaller than the corresponding values for polycrystalline Ag (p-Ag). It was found that the change of the Poisson's ratio with the density is non-monotonic. It decreases with decreasing density as the relative density is higher than ~93%, below which it increases with decreasing density. We found that the increase of elastic modulus with density is nonlinear. It exhibits three-stage behavior: (i) a rapid descent stage, when the relative density D < ~93%; (ii) a relatively stable stage (93% <D < 95%), and (iii) a rapid ascent stage (D > 95%). In addition, for annealed specimens, the modulus decreases with increasing annealing temperature. It is similar to the density change during annealing, while the variation of the Poisson's ratio resembles more the changing behavior of the grain size of ns-Ag. All these results were discussed based on the microstructural characteristics of ns-Ag.     

Analysis of perforated magnetic shields for electric power applications

The shielding performance of perforated magnetic shields for electric power applications is described. The shielding of an axisymmetric induction heating device is studied as a function of frequency, number of perforations and dimensions of the perforations. From the numerical point of view, the perforations cause the numerical model to be 3D. A numerical optimisation is carried out to find the optimal geometry with respect to the shielding factor and the volume of the shield. For the optimisation, two approaches are presented. The first approach is fast and easy-to-implement, but has limited accuracy. It uses a classical 2D axisymmetric model where the perforations are approximated by 'axisymmetric air gaps' resulting in a segmented shield. It is shown how to modify the 2D model to obtain results that are similar to the ones of a 3D model. The second approach is more accurate although quite fast, but more difficult to implement. It combines a 3D thinshell finite element model with the unmodified 2D model in a space mapping optimisation algorithm. The validation of both models is based on experimental work for an unperforated shield and for the optimised perforated shield.     

多级矩阵结构冷却除湿器的除湿性能研究

为了降低空分系统压缩机功耗,提高压缩机运行可靠性,本文提出一种用于压缩机进气除湿的多级矩阵结构的冷却除湿器,并搭建了多级冷却除湿实验台,测试了除湿器的除湿性能。实验结果表明,在进口空气含湿量和温度固定为11.7g/(kg干空气)和24.4℃时,当空气质量流量由0.48 kg/s增至0.78 kg/s,空气出口含湿量由7.1 g/(kg干空气)增至7.7g/(kg干空气);在进口空气质量流量和温度固定为0.53 kg/s和25.2℃时,当冷却水温度由6.9℃升至11.9℃,空气出口含湿量由7.1 g/(kg干空气)增至9.4 g/(kg干空气)。本研究针对除湿器内部传热传质过程建立了稳态数值模型,将模拟结果与实验结果进行对比,结果表明:该模型对于除湿器出口空气含湿量和温度的平均误差分别为8.6%和2.1%,显示出较好的可靠性;并进一步模拟研究了多级矩阵结构与单级叉流结构冷却除湿器的除湿性能,发现采用多级结构可以有效提高除湿效率,在进口空气质量流量和冷却水质量流量分别为0.53 kg/s和0.3 kg/s时,多级结构的除湿量可以提高4.3%,除湿效率可以提高2.5%;通过增加填料模块的长度...     

Shadowgraph visualization of condensing R134a flow through ejectors

Accurate simulation of ejector flows has been challenging due to the difficulties in modeling two-phase supersonic flows of fluids other than air or steam at high temperature and pressure conditions. Such modeling is necessary for ejector optimization, but the models currently available have yet to be validated. A transparent ejector test section is designed, fabricated, and operated in an ejector-based chiller loop using R134a as the working fluid. Undistorted visual access allows for detailed shadowgraph visualization of the motive jet in the mixing section at various degrees of condensation. High-speed imaging is used with measured temperatures and pressures at the ejector inlets to validate available analytical and CFD models. It is found that the best modeling techniques assume that the motive flow exits the motive nozzle at full thermodynamic equilibrium, and that the k-ε RNG turbulence model in ANSYS FLUENT captures these equilibrium conditions sufficiently.     

Effect of a wall on flow with dense particles

The behavior of dense gas–solid flows in engineering applications such as fluidized beds and pneumatic conveyers is highly complex and a reliable numerical model is required. Such flows are usually within solid walls that considerably affect the flow fields, and it is important to correctly include this effect in numerical models to improve their prediction capability. The observation of microscopic flows near walls can enhance our understanding of the flow behavior and assist in improving models. In this study, direct simulations are performed to investigate the effect of a wall on flow fields at a microscopic level. The effects of the bulk void fraction, particle Reynolds number, and particle diameter are investigated. The prediction performances of existing correlation equations usually used in mesoscopic model calculations are also investigated. It is found that the Ergun and Beetstra equations produce large discrepancies in the region within a distance equal to the particle diameter from the wall.     

R1234yf整车汽车空调泄漏扩散研究

新能源电动汽车热管理系统与传统乘用车不同,对采用热泵空调系统并利用液冷冷却电池的新能源电动汽车,制冷剂充注量比传统汽车空调增加了400～800 g.若使用可燃制冷剂,泄漏扩散至乘员舱,燃烧风险将增大.本文通过数值模拟对R1234yf制冷剂在蒸发器破损泄漏随送风进入乘员舱后的浓度变化过程和最高浓度进行了动态监测.结果表明...     

低温非热平衡圆柱温度分布特性的数值分析与实验

为研究复杂传热条件下,低温非热平衡圆柱表面温度分布特性及相关控制手段,本文以单纯顶部送风为基础,建立了附加局部送风、附加辐射屏两种传热条件下的非热平衡圆柱体的传热模型,通过数值模拟方法分析了该柱体的径向温度梯度、体温度不均匀度、面温度不均匀度等温度特性,并通过实验验证了数值模拟结果的准确性。在此基础上定量分析了不同传热方式和运行参数对温度特性的影响规律,从传热方式、结构设计、运行参数三个角度,提供了控制柱体温度特性的方案。结果发现:相比于只有单纯顶部送风,设置局部喷口送风可使柱体平均温度的降幅提升5.1%,柱体温度不均匀度降幅提升38.3%,同时降低了柱体面温度不均匀度,并将柱体温度梯度控制在近导热面侧;附加低温辐射屏后,对流和辐射的耦合传热可使柱体平均温度降幅减少2.3%,柱体温度不均匀度降幅提升57.5%,同时降低了柱体面温度不均匀度。     

An entropy-based approach for measuring complexity in supply chains

Today's supply chains are more complex than ever due to globalisation and its effects, especially on logistics activities. Therefore, understanding and managing complexity in supply chains are very popular topics nowadays. Measures for complexity in supply chains contribute to their manageability and controllability. This paper describes an approach to the measurement of complexity in supply chains based on Shannon's information entropy. The new proposed approach gives a formal approach that is able to measure and analyse the supply chain complexity. The main contribution of this study is to extend two formulas (structural and operational complexity) building on Shannon's entropy measure to evaluate the complexity of a supply chain. The aim is to measure complexity associated with information and material flows in the chain. A numerical example is presented to demonstrate the approach.     

Efficiency of free cooling using latent heat storage integrated into the ventilation system of a low energy building

This article presents the results of an investigation into the free cooling efficiency in a heavyweight and lightweight low energy building using a mechanical ventilation system with two latent heat thermal energy storages (LHTESs), one for cooling the fresh supply air and the other for cooling the re-circulated indoor air. Both LHTESs contain sphere encapsulated PCM (paraffin RT20). Using a developed and experimentally verified numerical model of the LHTES, the temperature response functions, based on the heat storage size, the air flow rates and the PCM's thermal properties, are established in the form of a Fourier series and empirical equations and used in the TRNSYS building thermal response model. Several mechanical ventilation, night cooling and free cooling operation modes were analysed and compared. It was found that the free cooling technique enables a reduction in the size of the mechanical ventilation system, provides more favourable temperatures and therefore enables better thermal comfort conditions, and in our studied case also fresh air for the occupants.     

A simplified experimentally tested theoretical model to reduce water consumption of a direct evaporative cooler for dry climates

The aim of this study is to introduce a simple modified experimentally tested theoretical model to mainly reduce the water consumption rate of a direct evaporative cooler type (DEC). The convective heat transfer coefficient correlations are introduced to the fundamental governing equations of this model and the saturation effectiveness of the DEC is calculated. The saline water at different concentrations is used as a feed water to the presented model. The impacts of inlet ambient air conditions and the salinity ratios on the amount of consumed water and the supply air temperature are investigated. The experimental runs are performed on a commercial type DEC and an average operating sprinkled water temperature is found to be 2.5 °C higher than inlet WBT of air. At 200,000 PPM salinity ratio, a reduction in water consumption by about 1.5 L h⁻¹ is reached but with 8.6% increase in the supply air temperature.     

Thermovibrational Convection in Microgravity: Preparation of a Parabolic Flight Experiment

This work describes the preparation of the future experiments on thermovibrational convection in microgravity during parabolic flights. The experimental setup for observing thermovibrational flows is designed. It consists of a cubic cell with liquid, which is subjected to controlled vibration, and equipment for registering velocity and temperature fields with a help of optical digital interferometry. The question of choosing working liquid and control parameters of the experiment is addressed. A 3D numerical simulation of thermovibrational convection in a cubic cavity is performed for real parabolic flight conditions. The study is aimed at estimating the values of physical quantities that manifest the presence of thermovibrational flows and can be experimentally measured during short microgravity time (20 s).     

Investigation the influences of geotextile on reducing the thermal reflective cracking using XFEM

Abstract

A 3D extended finite element model (XFEM) was constructed to investigate the thermal reflective cracking propagation and anti-cracking characteristic of geotextile. Influence factors of tensile modulus and placement location of geotextile were analysed. A temperature cycling test in the material test system was utilised in this work to validate the XFEM. Thermal simulations were performed considering the influences of solar radiation, air convection and air radiation. It has been found that crack initiated and propagated as temperature declining in earlier cycle, and crack length being fixed because of no more severe temperature variation based on maximum principal stress criterion. Geotextile with high tensile modulus had a significant influence on reducing the crack propagation compared with low tensile modulus geotextile or unreinforced situation. Geotextile placing at the bottom of asphalt surface had the best crack resistance influence, and geotextile placed at one-fourth depth from asphalt course bottom was more efficient than placed at other locations in terms of anti-crack propagation.     

地铁车厢条缝送风方式数值模拟

目前我国地铁列车内的人体热舒适性及空气质量难以达到要求。本文采用κ-ε湍流模型,对不同尺寸的条缝风口送风方式下的地铁车厢的三维空气流场和温度场进行数值模拟,综合分析车厢内温度、空气流速、粒子示踪、空气龄、CO2含量、PPD以及PMV等指标,说明各工况的空调效果及人体热舒适性,为改善地铁车厢空调制冷情况提供理论依据。     

Study of the specific features of pulsed-plasma generation in air at atmospheric pressure

The structural scheme and operational characteristics of a pulse-plasma device generating a pulsed plasma in air at atmospheric pressure are presented. The functional scheme and the operation algorithm for a high-voltage power supply are given. It is shown that the pulsed-plasma device can generate air plasma flows with a frequency of 1–10 Hz and maximum parameters of pressure, temperature, and velocity of 6.5 MPa, 12,000 K, 4 km ⁄ s, and 110 dB_A, respectively. Comparative analysis is presented of these quantities with characteristics of the plasma flow generated by the detonation pulsed-plasma device, which uses an air-propane mixture as the plasma-forming substance. It is shown that replacing the air-propane gas mixture with air provides a 70 dB_A decrease in the noise level of the pulsed-plasma treatment. Recommendations are given as to using the pulsed-plasma device for surface strengthening of the instrument and parts of the machine.