Measurement of Synchronous Machine Parameters by a Modified Frequency Response Method - Part I: Theory

De Mello and Hannett [1] have theoretically proposed a method for determining the operational inductances Ld(j2w), Lq(j2w) of a synchronous machine at low values of generator speeds. Eitelberg and Harley [2] extended these ideas to obtain the inductances and time constants from the Ld(j2w), Lq(j2w) curves by a numerical procedure which allows one to attach different levels of importance to either the magnitude or phase fitting of the operational inductance expressions to the computed curves. The first part of this two-part paper extends these ideas [1-2] by proposing an improved MODIFIED data-processing method which uses a Fourier analysis of the open-phase voltage and short-circuit current to compute the operational inductances Ld(j2w), Lq(j2w) and avoids integration of the d,q axes flux linkages; providing that the fundamental and the third harmonic present in the recorded waveforms are considered. The second part [3] of this paper applies the improved MODIFIED data-processing method to a 3 kVA, 220 V microalternator.     

Estimating Synchronous Machine Electrical Parameters from Frequency Response Tests

A method of obtaining synchronous machine d and q axis impedances by test as a function of frequency of d,q components has recently been proposed by de Mello and Hannett [1]. Their proposal ends by determination of the numerical values for the operational inductances Ld(j2w), Lq(j2w) or impedances Zd(j2w), Zq(j2w) at different values of W, the speed of the machine at which the tests are conducted; however, they do not show how the values of actual parameters such as inductances and time constants are to be obtained from the curves of operational inductance versus frequency.     

Asymmetrical non-imaging cylindrical solar concentrators

Asymmetrical Non-Imaging Cylindrical Concentrators have been proposed recently by Smith[5] and Rabl[6] which are related to the Symmetrical Non-Imaging Cylindrical Concentrators described by Winston [1, 2] and Baranov [3]. In this paper, both Parabolic and Non-Parabolic Asymmetrical Concentrators are examined and compared with symmetrical designs. Among the advantages of asymmetrical systems are: (1) a concentration vs time-of-day relationship which can be designed to compensate for projected solar area fall-off in early morning and late afternoon, allowing more uniform output where this is desirable; (2) greater operational flexibility for accommodating unexpected fluctuations in demand; (3) easier adaptation to vacuum-insulated receivers in one configuration; (4) the possibility of substantially increased concentration and energy collection per unit of mirror area for systems with receivers which can make use of large daily variations in energy input. A higher tracking frequency may be required, but a tilting adjustment every few days should be the maximum required for practical systems.     

Active control of fan noise

In the wake-rotor interaction fan noise, a number of the interacting modes at the blade passing frequency （BPF） and its harmonics are generated which are prescribed by the number of stator and rotor blades etc. In the present study, the dominant mode is tried to be suppressed by the secondary sound from the loudspeaker actuators. One of the novel features of the present system is the adoption of the control board with the Field Programmable Gate Array （FP（3A） hardware and the LabVIEW software to synchronize the circumferentially installed loudspeaker actuators with the relative location of rotational blades under arbitrary fan rotational speeds. The experiments were conducted under the conditions of three rotational speeds of 2004, 3150, and 4002 [rpm]. The reduction in the sound pressure level （SPL） was observed for all three rotational speeds. The sound pressure level at the BPF was reduced approximately 13 [dB] for 2004 [rpm] case, but not so large reduction was attained for other cases probably due to the inefficiency of the loudspeaker actuators at high frequencies     

Advanced fault diagnosis of a DC motor

This paper presents a model of the DCc motor with an eccentric rotor. The winding function theory shows the effect of eccentricity fault on the motor inductances and the simulation is done using a nonsymmetric air-gap function. A modified equation is presented to show the existence of rotor slot harmonics in the DC motor current. To detect the eccentricity fault, a pattern recognition technique is utilized. The proposed algorithm works at steady state and uses armature current as input. The rotor speed is needed in order to provide the appropriate feature for the classifier. Therefore, rotor speed is estimated from the armature current using the commutation harmonics. The experimental results obtained from a 1/3-hp shunt DC motor verifies the proposed method. In order to cover different motor conditions, data are collected at different shaft speeds for both a healthy dc motor and a dc motor with an unbalanced load which exhibits static eccentricity.     

SIF evaluation and stress analysis of drillstring threaded joints

In a study by Tafreshi and Dover [1], [2], [3] stress analysis of drillstring threaded joints under axial, bending and torsion loadings was carried out using the finite element method. Stress concentrations were determined on a variety of threaded joints in order to predict the fatigue life of these components. Here some of those results are presented in more detail. In this study it is shown that in the case of bending, axisymmetric solid elements with non-linear, asymmetric deformation with fourier interpolation can be employed which reduces the computational time and modelling in comparison with the full three-dimensional analysis. Quasi semi-elliptic circumferential internal and external cracks in tubes with the same dimensions of the pins or boxes of a series of joints are modelled and analysed using the J-integral method. The result of this fully three-dimensional crack analysis shows the same trend as experimental [3] and published results [4].     

直喷式柴油机燃烧室中喷雾的分布预测

本文介绍了三维准稳态喷雾模型。该模型是根据Dent~[1]二维平面准稳态喷雾模型,同时考虑缸内涡流和挤流的综合作用而建立的。其目的是为了预测在涡流和挤流作用下喷雾的分布。涡流和挤流强度可根据进气道稳流试验结果预测得到~[2]。本研究以高速直喷式6135柴油机为例,预测了喷雾的分布规律,将预测的喷束冲心线轨迹和可见廓线轨迹与实机的喷雾摄影作了比较,两者较为吻合,从而为研究直喷式柴油机的燃烧模型和性能提供了工具。     

Kinetic and fluid dynamics modeling of methane/hydrogen jet flames in diluted coflow

MILD combustion is a recent development in the combustion of hydrocarbon fuels which promises high efficiencies and low NO_x emissions. In this paper we analyze the mathematical and numerical modeling of a Jet in Hot Coflow (JHC) burner, which is designed to emulate a moderate and intense low oxygen dilution (MILD) combustion regime [1]. This paper initially discusses the effects of several modeling strategies on the prediction of the JHC flame structure using the CFD code FLUENT 6.3.26. Effects of various turbulence models and their boundary conditions have been studied. Moreover, the detailed kinetic mechanism adopted in the CFD simulations is successfully validated in the conditions of interest using recent literature data [2] on the effect of nitrogen dilution on the flame speeds of several CH₄/H₂/air lean mixtures. One of the aims of this paper is also to describe a methodology for computing pollutant formation in steady turbulent flows to verify its applicability to the MILD combustion regime. CFD results are post-processed for calculating the NO_x using a numerical tool called Kinetic Post Processor (KPP). The modeling results agree with the experimental results [1] and support the proposed approach as a useful tool for optimizing the design of new burners also in the MILD combustion regime.     

Brushless exciter modeling for small salient pole alternators using finite elements

In this paper, the self and mutual inductances of an exciter for a genset alternator are estimated for different rotor angular positions by the use of finite-element (FE) techniques. These inductances, which are described by complex functions, are measured using a stationary test and compared with calculated values. A real-time computer model for the exciter, the load and the diode bridge mounted on the rotor is then proposed, and a sample of performance prediction and measurement for a specific transient condition of the exciter is presented here as confirmation of the method. The model thus validated is ultimately to be included in the evolving total system model used for simulation of genset transient and steady-state performance.     

Thermal performance testing of flat-plate collectors

Existing standards for testing the performance of flat-plate solar collectors are documented in ASHRAE 93 [ANSI/ASHRAE Standard 93-2003, 2003. Methods of Testing to Determine Thermal Performance of Solar Collectors, ISSN: 1041-2336, ASHRAE, Inc., 1791 Tullie Circle, Ne, Atlanta, GA30329], ISO 9806-1 [ISO Standard 9806-1:1994(E), 1994. Test Methods for Solar Collectors – Part 1: Thermal Performance of Glazed Liquid Heating Collectors Including Pressure Drop, ISO, Case Postale 56, CH-1211 Geneve 20, Switzerland] and EN12975-2 [European Standard EN12975-2:2001, 2001. Thermal Solar Systems and Components – Solar Collectors – Part 2: Test Methods, CEN, Rue de Stasart, 36, B-1050, Brussels]. The ASHRAE 93 standard requires an experimental determination of the steady-state collector efficiency under prescribed environmental conditions for a range of collector fluid temperatures. Each test requires a minimum of 20 min and 22 tests are required to fully characterize a collector’s thermal performance. The ASHRAE 93 testing procedure is further complicated by the fact that the prescribed weather conditions do not often occur in some locations, which prolongs the time required to conduct the performance tests for a given collector. The EN12975-2 collector test procedure provides an alternative transient test method that can be conducted over a larger range of environmental conditions. This paper compares the results obtained by applying the EN12975-2 standard with results obtained from the ASHRAE 93 steady-state tests for a well-designed single-glazed selective surface flat-plate collector. The collector thermal parameters, F_R(τα)_e and F_RU_L obtained by the two test methods show good agreement. The incident angle modifier coefficient determined in the ASHRAE method, which uses a separate test for this purpose, was found to be more accurate than that determined in the transient method according to the EN12975-2 standard, which obtains this value and all other collector parameters in the same step. This transient method, however, uses a refined collector model that includes specific terms for the wind speed dependence and the collector thermal capacitance, which are absent in the ASHRAE model. The long-term collector thermal performance as a part of a water heating system was simulated using the efficiency curves derived from each of the test methods. The solar fractions obtained by simulation are within 7% for both cases.     

Heat and mass transfer in fruits and vegetables and measurement of thermal diffusivity

An experimental apparatus was designed to verify the method of measurement of thermal diffusivity of food produce proposed by the authors in an earlier paper [1]. Also, measurements were done for the time-temperature variation during transient cooling of food produce, and the temperature variation has been calculated by many methods available in the literature. It has been found that the calculation procedure proposed by the authors earlier [2] yielded consistently improved results.     

Lead-acid batteries in micro-hybrid applications. Part II. Test proposal

In the first part of this work [1] selected key parameters for applying lead-acid (LA) batteries in micro-hybrid power systems (MHPS) were investigated. Main results are integrated in an accelerated, comprehensive test proposal presented here. The test proposal aims at a realistic representation of the pSoC operation regime, which is described in Refs. [1] and [6]. The test is designed to be sensitive with respect to dynamic charge acceptance (DCA) at partially discharged state (critical for regenerative braking) and the internal resistance at high-rate discharge (critical for idling stop applications). First results are presented for up-to-date valve-regulated LA batteries with absorbent glass mat (AGM) separators. The batteries are close to the limits of the first proposal of pass/fail-criteria. Also flooded batteries were tested; the first out of ten units failed already.     

Uncertainty-quantification analysis of the effects of residual impurities on hydrogen–oxygen ignition in shock tubes

This study addresses the influences of residual radical impurities on the computation and experimental determination of ignition times in H₂/O₂ mixtures. Particular emphasis is made on the often-times encountered problem of the presence of H-atoms in the initial composition of H₂/O₂ mixtures in shock tubes. Two methods are proposed for quantifying experimentally H-residual impurities in shock tubes, namely, an a priori method that consists of detecting OH traces upon shocking unfueled mixtures, and a posteriori method in which the amount of impurities is inferred by comparing fueled experimental autoignition data with calculations. A stochastic Arrhenius model that describes the amount of H-radical impurities in shock tubes is proposed on the basis of experimental measurements as a function of the test temperature. It is suggested that this statistical model yields a probability density function for the residual concentration of hydrogen radicals in standard shock tubes. Theoretical quantifications of the uncertainties induced by the impurities on autoignition times are provided by using the 5-step short chemistry of Del Álamo et al. [1]. The analysis shows that the relative effects of H-impurities on delay times above crossover become more important as the dilution increases and as the temperature and pressure decrease. Below crossover, the effects of H-impurities on the ignition delay vanish rapidly, and are negligible compared to the departures produced by the non-ideal pressure rise that is seen in some shock-tube experiments at such low temperatures. The influences of kinetic uncertainties on the ignition time are typically negligible compared to the effects of the uncertainties induced by H-impurities when the short mechanism is used, except for air at high temperatures for which kinetic uncertainties dominate. Furthermore, calculations performed with the short mechanism show that correlations between the uncertainties in the rates of branching and termination steps have only some small influences on the ignition-time variabilities near crossover, where a global sensitivity analysis shows an increasing importance of the recombining kinetics. Computational quantifications of uncertainties are carried out by using numerical simulations of homogeneous ignition subject to Monte-Carlo sampling of the concentration of impurities. For the conditions analyzed, these computations show that the variabilities produced in ignition delays by the uncertainties in H-impurities are comparable to the experimental data scatter and to the effects of typical uncertainties of the test temperature when the Stanford chemical mechanism [2] is used. The calculations also unveil that the utilization of two other different chemical mechanisms, namely San Diego [3] and GRI v3.0 [4], yields variations in the ignition delays which are within the range of the uncertainties induced by the H-impurities. Finally, the effects of residual impurities in kinetic-isolation experiments and in supersonic-combustion ramjets are briefly discussed.     

Fast cycling of Li/LiCoO2 cell with low-viscosity ionic liquids based on bis(fluorosulfonyl)imide [FSI]

A charge–discharge cycling test of a Li/LiCoO₂ cell containing ionic liquids based on bis(fluorosulfonyl)imide ([FSI]⁻) as the electrolyte media, revealed significantly better rate properties compared to those of cells using conventional ionic liquids. The use of an 1-ethyl-3-methylimidazolium (EMI⁺) salt permitted the retention of 70% of the discharge capacity at a 4 C current rate. In contrast, similar performance of cells containing N-methyl-N-propylpyrrolidinium (Py₁₃⁺) and N-methyl-N-propylpiperidinium (PP₁₃⁺) salts of [FSI]⁻ was limited to operation at 2 and 1 C current rates, respectively. However, the charge/discharge cycling stability of the cell with Py₁₃[FSI] was much better than that of the cell using EMI[FSI].     

Handling zero values in the logarithmic mean Divisia index decomposition approach

The Logarithmic Mean Divisia Index (LMDI) decomposition approach has many desirable properties. It has been recommended by Ang [2004a. Decomposition analysis for policymaking in energy: which is the preferred methods? Energy Policy 32, 1131–1139] for adoption in energy and energy-related environmental index decomposition analysis (IDA). A complication associated with this approach is the treatment of zero values in the data set. Several studies show that replacing the zero values with a small number gives converging results. In a recent paper, Wood and Lenzen [Zero-value problems of the logarithmic mean Divisia index decomposition method. Energy Policy, in press] argue that this strategy is not necessarily robust and recommend using the analytical limits proposed in Ang et al. [1998. Factorizing changes in energy and environmental indicators through decomposition. Energy 23, 489–495]. We compare these two strategies and extend earlier works by generalizing the analytical limits of LMDI.     

Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications

New anion exchange membranes (AEMs) with high conductivity, good dimensional and alkaline stability are currently required in order to develop alkaline fuel cells into efficient and clean energy conversion devices. In this study, a series of AEMs based on 1, 2-dimethyl-3-(4-vinylbenzyl) imidazolium chloride ([DMVIm][Cl]) are prepared and investigated. [DMVIm][Cl] is synthesized and used as ion carriers and hydrophilic phase in the membranes. The water uptake, swelling ratio, IEC and conductivity of the AEMs increase with increasing the [DMVIm][Cl]. The imidazolium-based AEMs show excellent thermal stability, sufficient mechanical strength, the membrane which containing 30% mass fraction of [DMVIm][Cl] shows conductivity up to 1.0 × 10^?2 S cm^?1 at room temperature and good long-term alkaline stability in 1 M KOH solution at 80 °C. The results of this study suggest that this type of AEMs have good perspectives for alkaline anion exchange membrane fuel cell applications.     

柔性直流输电换流阀功率单元现场测试方法研究

  [目的]  在柔性直流输电工程中,换流阀功率单元及其备件需要定期进行测试和维护。功率单元工作正常是保证柔性直流输电系统安全稳定运行的重要前提。  [方法]  针对功率单元现场测试缺乏相关规范标准的现状,从功率单元的结构、工作机制和厂内测试项目入手,结合现场实际情况,提出了一种适用于现场的流程化的功率单元测试方法,并在该方法的基础上研制了一台可对功率单元进行自动测试的装置。  [结果]  所提出的测试方法和测试装置可有效提高功率单元现场测试的完备性、效率及准确性,该装置目前已在南澳多端柔性直流输电工程塑城换流站试点应用。  [结论]  工程实践表明:提出的测试方法贴近工程实际情况,为功率单元的运检、验收提供了有效的测试手段,也可为其他柔性直流输电工程的功率单元测试工作提供借鉴。     

The atmospheric radiation in Athens during the summer

In this paper, the downard flux of the longwave atmospheric radiation at the earth's surface and its variation with height is calculated for the clear days and nights of the summer period in Athens, by using a model developed by Atwater and Ball[1]. For this purpose upper air data for 12:00 G.M.T. and 00:00 G.M.T. were selected for a seven year period. From the obtained values of downward fluxes, sky temperatures were calculated by applying Stefan-Boltzmann law. Values of the longwave atmospheric radiation at the earth's surface were also estimated by applying the empirical formulae established by Idso and Jacson[2] and Brunt[3] and using the standard surface meteorological observations of temperature and absolute humidity at the National Observatory of Athens. It was found that the values obtained by the Idso and Jackson[2] formula were in better agreement to those computed by the model.     

Synthesis and photovoltaic properties of a star-shaped molecule with triphenylamine as core and benzo[1,2,5]thiadiazol vinylene as arms

A solution-processable and star-shaped molecule 4-((E)-2-(benzo[1,2,5]thiadiazol-4-yl)vinyl)-N,N-bis(4-((E)-2-(benzo[1,2,5]thiadiazol-7-yl)vinyl)phenyl)benzenamine (TPA-BT) has been designed and synthesized by palladium-catalyzed Heck reaction for the application in organic solar cells (OSCs). The molecule possesses a D-A structure with a triphenylamine core (donor unit) linked with three benzo[1,2,5]thiadiazole (acceptor unit) arms through double bonds. TPA-BT film shows a strong absorption peak in the visible wavelength range from 400 to 560 nm, which could be ascribed to the charge transfer band of the D-A structure of the molecule. The bulk-heterojunction OSCs with the device structure of ITO/PEDOT:PSS/TPA-BT:PCBM/Ca/Al (or Ba/Al) were fabricated by spin-coating the blend solution of TPA-BT and PCBM (1:3, w/w), in which TPA-BT was used as donor and PCBM as acceptor materials. The devices show a high open circuit voltage of ca. 0.9 V and a power conversion efficiency of 0.61%, under the illumination of AM 1.5, 100 mW/cm². The results indicate that TPA-BT is a promising solution-processable organic photovoltaic material.     

Numerical study of experimental feasible heat release rate markers for NH3–H2-air premixed flames

Ammonia (NH₃) and hydrogen (H₂) as carbon-free fuels have attracted much attention for combustion applications in recent years. Co-firing ammonia with hydrogen provides a solution to overcome the extremes in the reactivities of both pure ammonia and hydrogen fuels. Heat release rate (HRR) is one of the most important quantities in the study of turbulent combustion, but direct measurement of local HRR is not experimentally feasible. In this study, we explored several quantities, [NH], [O], and the gradient of [OH] (Grad [OH]) as potential experimentally feasible HRR markers for NH₃–H₂-air premixed flames using numerical simulations. The performance of these quantities over a wide range of equivalence ratios and H₂ blending ratios have been examined, and some key reactions have been identified to explain the corresponding variations of the correlation for [NH] and [O]. It is concluded that the [NH] and Grad [OH] can be used in general as a suitable HRR marker for NH₃–H₂-air premixed flames, and the use of [NH] is especially recommended for lean flame conditions. A strategy that slightly shifts the [NH] and Grad [OH] profiles to overlap the corresponding HRR shows a further improvement on the performance of [NH] and Grad [OH]. The use of [O] can be considered for rich flame conditions while cautions are needed for conditions with high H₂ blending ratios.