活塞环普雷斯特理论分析

本文对普雷斯特理论进行了较深入分析,对普氏理论接触压力、数学分析及性线设计函数进行了探讨,给出了普氏理论中正确的接触压力表达式及活塞环性线设计函数表达式,指出了普氏理论中所采用的简化过程且对其误差进行了必要分析。众所周知,活塞环设计理论中以普雷斯特、阿诺尔德及金茨布尔格最具代表性,这三种理论具有各自的优点及适用性。尽管国内出版的有关活塞环的书刊中一再提及这三种理论,然而实际设计中根本无法采用,原因在于每种理论的前提条件、逻辑分析及数学表达式的可疑性,尤其是函数表达式的正确性更为可疑。这对于活塞环产品开发、研究有一定的影响。本文对普氏理论进行了分析,对其接触压力函数作了分析,并在计算机上计算了五种接触压力形式。指出了普氏理论中接触压力的正确表达式,根据该表达式推出了正确的径向位移,及角位移表达式,并将普氏公氏与该式进行了对比,最后对普氏理论中所采用的筒化过程及其误差作了分析说明。     

一种改进的燃煤理论燃烧温度计算方法

在燃煤理论燃烧温度计算中,内插值近似法需预先假设理论燃烧温度值并重复计算或需编制烟气焓温表,而比热近似法则精确度不够,适用条件有限。为了更简捷、准确地计算燃煤理论燃烧温度,给出了一种基于方程求解方法的理论燃烧温度计算模型,同时考虑了煤灰分对理论燃烧温度的影响,借助于Matlab强大的数学运算功能,可快速求解该计算模型。此外,验证了该计算模型对于120种普遍煤种的适用性,且其理论燃烧温度四次方程根的分布特征为一对共轭复根和两相异实根。     

喷射器一维设计理论的研究进展

主要概述气体喷射器一维设计理论研究的近期进展,分别介绍了定压混合理论、定常面积混合理论及其实验研究情况,最后提出了作者的一些展望。     

火力发电厂项目施工标段划分研究

本文对火力发电厂项目的工程特点做了简要介绍,并选择了当前研究工程标段划分的一种理论计算模型-基于交易费理论的标段划分模型,使用该理论模型对越南沿海电厂一期工程总承包项目的标段划分进行了计算分析,根据理论计算结果对基于交易费理论的标段划分模型进行优缺点分析,并提出该理论计算模型在实际应用中的改进。     

喷射式制冷系统关键问题研究进展

对喷射器现有设计理论,如定压混合理论、定常面积混合理论、等动量变化理论、等马赫数梯度法进行了简介。通过对喷射器特点及其在制冷方面应用的研究综述,从制冷剂发展在喷射器及制冷系统中的应用、喷射器内关键部件几何尺寸的研究等内容展开分类总结,以促进喷射器的研究。     

基于GIS系统图模转换的配电网线损理论计算

针对配电网线损理论计算方法,结合配电网GIS信息系统数据资源,提出了一种基于GIS系统的图模转换自动实现配电网线损理论计算的方法,通过对GIS系统CIM文件进行简析,生成配电网线损理论计算所需基础数据,进而进行线损理论计算.算例表明本文方法不但能够减少线损理论计算数据人工采集工作量,提高工作效率,进一步提高线损理论计算...     

相似理论在发动机快速磨损试验中的应用

本文应用相似理论提出发动机快速磨损试验的理论及方法,并建立了应用于CC195柴油机的快速磨损试验的模拟模型。通过试验与实际对比分析表明,作者提出的理论及方法是可行的。     

矿山生态修复可持续发展分析

生态修复是运用系统生态学的原理,应用生态工程、环境工程和社会工程等手段,保护和合理利用土地及其他自然资源,提高复合生态系统的自我调控能力。本文借助循环经济理论、可持续发展理论、生态学理论和工业生态学理论,对矿区生态修复规划可持续发展提出了相应的解决措施和建议,从而使矿区生态系统健康、稳定的持续发展。     

经济供热半径与经济比摩阻关系的研究

以供热管网寿命周期成本最小的最优理论为基础,建立合适的数学模型,对经济供热半径与经济比摩阻的关系进行探讨,并应用微分法得到微分方程,通过解微分方程得到合理的理论关系公式;然后将所推导的理论关系运用于实例,根据市场现行价格,得出不同经济比摩阻取值,对应的经济供热半径;同时通过COMSOl软件进行仿真计算,对所推导的理论关系进行验证;最后根据推导的理论关系以及举例计算,采用Matlab软件编制经济供热半径求解软件,证明了理论推导的实用性。     

煤垛温度分布的研究

本文针对横截面为梯形的成型煤堆进行了理论分析和计算,并通过现场实测验证了理论分析的正确性。为堆煤场煤垛的自然问题提供了理论基础及解决的途径。     

1951～2013年南京市降水变化特征研究

南京市气候变化较敏感,受亚热带季风气候影响,近年来短历时强降雨等极端天气增多,研究南京市降水变化特征对水资源规划管理有重要意义。基于南京站1951~2013年逐日降水资料,分别采用5年滑动平均法、线性倾向估计法、Mann-Kendall法、Morlet小波分析法和R/S分析法,分析了南京市降水量年际、年内四季变化特征。结果表明:南京市年降水量呈增加趋势,存在2、7、9、17年4个主周期,未来年降水量将呈减小趋势;春季降水量呈减小趋势,存在4、6、16年3个主周期,未来春季降水量将继续减少;夏季降水量呈显著增加趋势,存在4、9、18年3个主周期,未来夏季降水量将继续增加;秋季降水量呈减小趋势,存在3~5、8、12~13、21年4个主周期,未来秋季降水量将继续减少;冬季降水量呈增加趋势,存在5、12~13、18~19年3个主周期,未来冬季降水量将继续增加。     

The concept of the gas turbine‐based hybrid vehicle: system,design and configuration issues

The object of this study is a theoretical and experimental analysis of a new hybrid propulsion system for a passenger sedan in which the thermal engine is a small gas turbine set. Some preliminary results on the possibility of replacing the conventional ICE of a hybrid ‘series’ configuration by a turbogas were presented and discussed in previous papers by the same authors: several possible designs were examined under both a thermodynamic and an operative point of view. This paper presents a summary of the project and constitutes an attempt to put things in a proper engineering perspective: the technical feasibility of the project is assessed via a calculation of the required mission loads, a preliminary design of the most important elements of the propulsive system, the choice of the mission control strategy and the implementation of a numerical system simulator. The experiments that provided a verification for the assumed component efficiencies were carried out, in cooperation with the Research Centre of ENEA‐Casaccia, on an ELLIOTT TA‐45 group. Our results, though only preliminary, allow for a direct comparison between a GT‐hybrid vehicle and a modern diesel car, and indicate that the GT‐hybrid may be actually a competitor for the FC‐powered vehicle concept. Our ‘optimal’ configuration is a combination of a 100 kg battery pack and two turbogas set of 5 and 16 kW, respectively. Copyright © 2005 John Wiley & Sons, Ltd.     

Design and analysis of a new wave energy converter based on a point absorber and a hydraulic system harvesting energy from waves near the shore in calm seas

The aim of this paper is to illustrate the design of a new wave energy converter, composed of a point absorber and a hydraulic system (power take off) and sized for recovering energy in calm seas from waves near the shore. The point absorber is consisting of a rectangular shaped buoy integrating a piston pump. The set buoy‐pump oscillates under the waves action and moves natural water in a closed circuit hydraulic system (power take off) composed of a piping connecting the piston pump itself, a pressurized reservoir, a hydraulic turbine and a discharge tank. The methodology adopted for designing the main constituents involves a 1D mathematical model, settled for understanding the motion of the buoy under the hypothesis of regular waves and fully developed sea, and a sizing procedure applied for the design of all the components of the hydraulic system. The project related to the Calabrian site of Cetraro (Mediterranean Sea—south Italy) led to designing a system with a 4 m large buoy, associated with a small 13 cm diameter micro Pelton turbine, so that more than 22 000 kWh could be recovered in a year.     

Critical insulation thickness of a rectangular slab embedded with a periodic array of isothermal strips

We address the problem of two-dimensional heat conduction in a solid slab embedded with a periodic array of isothermal strips. The surfaces of the slab are subjected to a convective heat transfer boundary condition with a uniform heat transfer coefficient. Similar to the concept of critical insulation radius, associated with cylindrical and spherical configurations, we show that there exists a critical insulation thickness, associated with the slab, such that the total thermal resistance attains a minimum, i.e. a maximum heat transfer rate can be achieved. This result, which is not observed in one-dimensional heat conduction in a plane wall, is a consequence of the non-trivial coupling between conduction and convection that results in a 2D temperature distribution in the slab, and a non-uniform temperature on the surface of the slab. The findings of this work offer opportunities for improving the design of a broad range of engineering processes and products.     

Report of the first prototype of non-imaging focusing heliostat and its application in high temperature solar furnace

Following the publication on the principle and theory of a newly proposed non-imaging focusing heliostat, this paper presents a report on the design, optical alignment and application of the first prototype heliostat. In the architecture of the first prototype, 25 mirrors, each with a dimension of 40×40 cm, are arranged into five rows and five columns to form a total reflective area of 4 m². The design of the essential part of the first prototype heliostat will be discussed in this paper, which consists of two primary elements; a rotation–elevation system for tracking a mirror support frame which carries 25 mirror facets, and a separate two-axis tracking system for compensating (each second) off-axis aberrations of 24 slave facets relative to the central mirror, which is fixed in the mirror frame. The rotation–elevation system consists of a pedestal supporting a rotational tracking mechanism carrying a U-shaped arm and a second tracking system for tracking a moving frame in elevation. The moving frame carries a central stationary (relative to the frame) mirror, called a master mirror. Slave mirrors are arrayed in five rows and five columns, and eight stepper motors drive the outer four rows and columns relative to the master mirror via a computer programme implementing newly proposed formulas to eliminate the first-order aberration. With a second stage concentrator comprising a small aperture size parabolic mirror (diameter of 60 cm), a cost-effective high temperature solar furnace was constructed. In our experiment, the highest furnace temperature of 3400°C has been recorded through the melting of pure tungsten wires.     

Performance model for two-stage optical concentrators for solar thermal applications

A performance model has been developed for evaluating benefits associated with the addition of a nonimaging secondary concentrator to a conventional paraboloidal solar dish. The model uses a Monte Carlo ray-trace procedure to determine the focal plane distribution as a function of optical parameters and, by evaluating the trade-off between thermal losses and optical gain, calculates the corresponding optimized concentration and thermal efficiency as a function of temperature, both with and without the secondary. These comparative optimizations, carried out over a wide range of design parameters, show that the efficiency of a two-stage concentrator is always greater than that of a single stage if all other design parameters are the same. For example, for a reference design corresponding to a dish with a focal length to diameter ratio of 0.6 and a characteristic slope error of 5 milliradians operated at a receiver temperature of 1000°C, the optimized efficiency with a secondary is 0.70 compared to 0.59 for the primary alone. At fixed focal ratio, the relative performance advantage with a secondary increases, if either the temperature or the primary slope error or both, are increased, whereas it decreases if they are decreased. However, the advantage remains significant at temperatures above 400°C, even in the “high performance limit” of slope errors <2 milliradians.     

汽轮机最有利真空循环水泵变频驱动控制系统

在火电厂中循环水流量大多采用节流调节方式,控制效果差,不能保证凝汽器工作在最有利真空值,而且循环水泵电耗严重。研制了一种循环水泵变频调速控制系统,利用PM(可编程控制器)设计合理的控制逻辑,实现平稳启停和切换,保证最有利真空值的实现,使汽轮机经济运行。同时分析了汽轮机的经济运行方式,给出了最有利真空值的计算原则;详细介绍了系统的控制原理、硬件结构及PM程序的设计,并给出了应用效果。     

Analysis and synthesis of the variability of irradiance and PV power time series with the wavelet transform

The irradiance fluctuations and the subsequent variability of the power output of a PV system are analysed with some mathematical tools based on the wavelet transform. It can be shown that the irradiance and power time series are nonstationary process whose behaviour resembles that of a long memory process. Besides, the long memory spectral exponent α is a useful indicator of the fluctuation level of a irradiance time series. On the other side, a time series of global irradiance on the horizontal plane can be simulated by means of the wavestrapping technique on the clearness index and the fluctuation behaviour of this simulated time series correctly resembles the original series. Moreover, a time series of global irradiance on the inclined plane can be simulated with the wavestrapping procedure applied over a signal previously detrended by a partial reconstruction with a wavelet multiresolution analysis, and, once again, the fluctuation behaviour of this simulated time series is correct. This procedure is a suitable tool for the simulation of irradiance incident over a group of distant PV plants. Finally, a wavelet variance analysis and the long memory spectral exponent show that a PV plant behaves as a low-pass filter.     

Transcritical CO2 refrigerator and sub‐critical R134a refrigerator: A comparison of the experimental results

This paper describes experiments comparing a commercial available R134a refrigeration plant subjected to a cold store and a prototype R744 (carbon dioxide) system working as a classical ‘split‐systems’ to cool air in residential applications in a transcritical cycle. Both plants are able to develope a refrigeration power equal to 3000 W. The R744 system utilizes aluminium heat exchangers, a semi‐hermetic compressor, a back‐pressure valve and a thermostatic expansion valve. The R134a refrigeration plant operates using a semi‐hermetic reciprocating compressor, an air condenser followed by a liquid receiver, a manifold with two expansion valves, a thermostatic one and a manual one mounted in parallel, and an air cooling evaporator inside the cold store. System performances are compared for two evaporation temperatures varying the temperature of the external air running over the gas‐cooler and over the condenser. The refrigeration load in the cold store is simulated by means of some electrical resistances, whereas the air evaporator of the R744 plant is placed in a very large ambient. The results of the comparison are discussed in terms of temperature of the refrigerants at the compressor discharge line, of refrigerants mass flow rate and of coefficient of performance (COP). The performances measured in terms of COPs show a decrease with respect to the R134a plant working at the same external and internal conditions. Further improvements regarding the components of the cycle are necessary to use in a large‐scale ‘split‐systems’ working with the carbon dioxide. Copyright © 2009 John Wiley & Sons, Ltd.