共查询到20条相似文献,搜索用时 109 毫秒
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罗茨泵以抽速作为主性能考核指标不够确定,它不能真实反映罗茨泵的质量水平,建议以零流量压缩比作为考核罗茨泵抽气能力的性能指标。 相似文献
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罗茨泵转子型线是罗茨泵的关键。零流量压缩比高的罗茨真空泵、湿式罗茨泵真空泵和罗茨鼓风机均要求泵工作时转子与壳体之间及两转子之间气体泄漏较小,这样可改善泵的低压下性能,即提高泵的抽气效率。转子型线为“圆弧-摆线-渐开线”就可以满足这一要求。文中给出了泵的参数选择图,从图中可以看出泵的参数选择范围,文中说明了该图边界的性状.由该图可查出泵的容积利用系数λ同转子顶圆半径Rm与节圆半径R之比 Rm/R以及同渐开线压力角α的关系。 λ=f(Rm/R,a)。图中用计算机绘出了几个比较特征参数下转子型线及其啮合情况,借此可根据实际需要来选择系的转子型线参数。此种转子型线工艺性良好。可仿型法加工(只需两个工位)。还可以用范成法加工其渐开线部分,余下部分用成型铣刀加工。若以范成法加工型线渐开线部分时,设计转子型线时可选取渐开线基圆半径r为整数.以便于其工艺装备的设计。 相似文献
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国外关于声景的探索经历了数十年的发展,对其进行系统梳理与总结对于中国声景研究具有重要的借鉴价值。文章以"Web of Science"核心数据库中近20年的相关英文文献作为研究对象,依托科学计量分析软件CiteSpace的知识图谱绘制功能,展现国外声景研究的学科及期刊分布特征、理论基础与研究热点。研究发现,国外声景研究呈现出明显的多学科特征。在研究热点上,从接续Schafer的理念、关注人类对声景的感知,到兼顾生物圈中非人类生物对声景的响应,再到以"格局-过程"为指导关注区域层面的声景生态特征与演变机制,是国外声景研究的主要发展脉络。声景研究框架的拓展使国外声景研究得以越来越全面和科学地解释和理解声景现象,并为声景实践的开展奠定了良好的基础。不足之处在于研究内容有待深化、研究学科有待融合、声景标准化建设有待加强、声景普及有待提高,而这也是国内声景研究实现跨越发展需要积极补足的方面。 相似文献
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The aim of this paper is the detailed analysis of different well-known thermodynamic efficiencies usually used to characterize hermetic compressors. Attention is focussed on the volumetric efficiency, the isentropic efficiency, and the combined mechanical–electrical efficiency. A procedure is presented to detach these efficiencies into their main components (physical sub-processes) to get deeper insight on the overall behavior.The volumetric efficiency is split into partial efficiencies related to pressure drop and heat transfer effects, supercharging effects, superdischarging effects, leakages, etc. The isentropic efficiency is detached using two different points of view: the work associated to the individual sub-processes (compression, discharge, expansion, suction), and the work associated to the underpressures, overpressures, and between the inlet and outlet mean compressor pressures. Finally, the combined mechanical–electrical efficiency is related to the heat transfer losses/gains, and to the exergy transfers and exergy destroyed.Even though some of the concepts introduced in the paper can be applied to different kinds of compressors, the discussion is specially focussed on hermetic reciprocating compressors. An advanced simulation model developed by the authors has been used to generate data to illustrate the possibilities of the detailed thermodynamic characterization proposed. The criteria developed are useful tools for comparison purposes, to characterize compressors, and to assist designers during the optimization process. 相似文献
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Anna Devlin Jane McGillivray Judith Charlton Georgia Lowndes Virginie Etienne 《Accident; analysis and prevention》2012
While there is a large body of research indicating that individuals with moderate to severe dementia are unfit to drive, relatively little is known about the driving performance of older drivers with mild cognitive impairment (MCI). The aim of the current study was to examine the driving performance of older drivers with MCI on approach to intersections, and to investigate how their healthy counterparts perform on the same driving tasks using a portable driving simulator. Fourteen drivers with MCI and 14 age-matched healthy older drivers (aged 65–87 years) completed a 10-min simulator drive in an urban environment. The simulator drive consisted of stop-sign controlled and signal-controlled intersections. Drivers were required to stop at the stop-sign controlled intersections and to decide whether or not to proceed through a critical light change at the signal-controlled intersections. The specific performance measures included; approach speed, number of brake applications on approach to the intersection (either excessive or minimal), failure to comply with stop signs, and slower braking response times on approach to a critical light change. MCI patients in our sample performed more poorly than controls across a number of variables. However, because the trends failed to reach statistical significance it will be important to replicate the study using a larger sample to qualify whether the results can be generalised to the broader population. 相似文献
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Runway surface distortions such as rutting allow water to accumulate on the runway surface which may cause aircraft operational risks related to hydroplaning and loss of friction. This may increase the runway excursion risk during wet weather conditions. Several standards relating to airfield pavement maintenance have documented that rutting can cause safety hazards to aircrafts due to hydroplaning risk. This paper proposes an analytical approach to evaluate the hydroplaning potential for aircrafts due to the flooded ruts of a runway pavement and evaluate the operational risk. A finite element model is developed to simulate aircraft tyre hydroplaning under given set of operating conditions. By performing this evaluation for different rut depths, an assessment of the relative severity levels of different rut depths with respect to hydroplaning can be made. A numerical example is presented to illustrate the application of the procedure to evaluate operational risks to aircraft from runway rutting. 相似文献
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Gregorio Rebuldela Jeffrey A. Jargon 《Journal of research of the National Institute of Standards and Technology》1992,97(6):673-687
The National Institute of Standards and Technology has established a measurement capability to support high power systems and devices. The automated wattmeter calibration system operates at power levels of 1 to 1000 W for frequencies from 1 to 30 MHz and 1 to 500 W from 30 to 400 MHz. A cascaded coupler technique is used to extend power measurements to high levels which are traccable to a 10 mW standard thermistor mount. This technique uses an arrangement of nominal 10, 20, 30, 40, and 50 dB couplers with sidearm power meters. The initial step transfers the calibration of the 10 mW standard to the 10 dB coupler/power meter. The standard is then replaced with a wattmeter to be calibrated. RF power is increased 10 dB and the calibration is transferred to the adjacent 20 dB eoupler/power meter. This sequence is repeated with the remaining coupler/power meters until the wattmeter is calibrated at the desired power levels and frequencies. Power ratios calculated from simultaneous power measurements made at each transfer are used to calculate the incident power at the wattmeter. Due to nonideal components, corrections are made for nonlinearities, mismatch, and other errors. Two types of wattmeters have been evaluated at selected frequencies and power levels. Total uncertainties are based on the random and systematic components. 相似文献
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中国传统图案对现代礼品包装设计的启示 总被引:3,自引:3,他引:0
论述了中国传统图案如何运用到现代礼品包装设计中,如何结合现代工艺手段,使现代礼品包装充满文化底蕴和艺术性,满足人们的需求,同时,丰富现代包装设计的面貌,为平面设计带来新的形式. 相似文献
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James Clarke John F. Gamble John W. Jones Mike Tobyn Richard Greenwood Andy Ingram 《Advanced Powder Technology》2019,30(5):920-929
For image based particle characterisation approaches one of the most common discussion points is determining the number of particles required to have statistical confidence that the measurement is able to adequately describe the distribution of the sample. This topic becomes significantly more challenging when applied to the extraction of single component size distributions from multi-component samples.The aim of this work was to propose a means to accurately assess the particle number requirements using a method specific approach. The method applies a sub-sampling method to the original imaged dataset in order to provide an understanding of the impact of sub-sampling on the ability to accurately reproduce the original distribution.The method was applied to understand the particle number requirements for two batches of theophylline anhydrous with varied particle size distributions, using the input size distribution to guide the requirements for the subsequent multi-component samples of both materials.The results demonstrate the utility of the method to determine the appropriate number of particles required to recreate the size distributions. Whilst the minimum number of particles required to be sampled can be estimated, how those particles are sampled can also affect the validity of the measurement and must be taken into consideration. 相似文献
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《工程(英文)》2018,4(2):286-290
Tissue engineering, which involves the creation of new tissue by the deliberate and controlled stimulation of selected target cells through a systematic combination of molecular and mechanical signals, usually involves the assistance of biomaterials-based structures to deliver these signals and to give shape to the resulting tissue mass. The specifications for these structures, which used to be described as scaffolds but are now more correctly termed templates, have rarely been defined, mainly because this is difficult to do. Primarily, however, these specifications must relate to the need to develop the right microenvironment for the cells to create new tissue and to the need for the interactions between the cells and the template material to be consistent with the demands of the new viable tissues. These features are encompassed by the phenomena that are collectively called biocompatibility. However, the theories and putative mechanisms of conventional biocompatibility (mostly conceived through experiences with implantable medical devices) are inadequate to describe phenomena in tissue-engineering processes. The present author has recently redefined biocompatibility in terms of specific materials- and biology-based pathways; this opinion paper places tissue-engineering biocompatibility mechanisms in the context of these pathways. 相似文献
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感官代偿设计在产品中的应用 总被引:4,自引:2,他引:2
通过对人的感知系统进行分析,将感官代偿这一生理现象应用到设计中,不仅能够方便产品的使用,也能够锻炼感官障碍者的感官代偿功能,从而验证感官代偿设计是一种行之有效的设计方法,让其人本关怀的特征发挥的淋漓尽致。 相似文献
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The regulations being applied to liquefied natural gas (LNG) import terminal siting in the United States are reviewed. There are no requirements for exclusion zones to protect the public from LNG spills onto water. Serious problems with current practices used to determine exclusion zones on the land-based part of the facility are identified. Many of the questions that are considered relate to the use of computational fluid dynamic (CFD) models, which appear to offer the best potential for realistic modeling to determine vapor cloud exclusion zones that result from LNG spills into impounded areas with or without dispersion in the presence of other obstacles to the wind flow. Failure to use CFD models, which are already approved by the regulation, and continued use of practices which have been demonstrated to be in error, raises important questions of credibility as well as denies the applicant full use of scientific tools that are available to optimize the design of such facilities so as to best provide for safety of the public. 相似文献