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Existing Models (existing + models)
Kinds of Existing Models Selected AbstractsEffect of micro mass transfer through phase interface on numerical simulation of solidification processHEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 6 2004Yanhui Feng Abstract Existing models for the solute redistribution during solidification have been reviewed. Some typical models are applied for the numerical simulation of heat and mass transfer with phase change under experimental condition of inverse casting. The results show that the effect of micro mass transfer models on the formation of the new phase cannot be omitted. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(6): 393,401, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20024 [source] Selection power and selection labor for information retrievalJOURNAL OF THE AMERICAN SOCIETY FOR INFORMATION SCIENCE AND TECHNOLOGY, Issue 7 2007Julian Warner This study examines the relation between selection power and selection labor for information retrieval (IR). It is the first part of the development of a labor theoretic approach to IR. Existing models for evaluation of IR systems are reviewed and the distinction of operational from experimental systems partly dissolved. The often covert, but powerful, influence from technology on practice and theory is rendered explicit. Selection power is understood as the human ability to make informed choices between objects or representations of objects and is adopted as the primary value for IR. Selection power is conceived as a property of human consciousness, which can be assisted or frustrated by system design. The concept of selection power is further elucidated, and its value supported, by an example of the discrimination enabled by index descriptions, the discovery of analogous concepts in partly independent scholarly and wider public discourses, and its embodiment in the design and use of systems. Selection power is regarded as produced by selection labor, with the nature of that labor changing with different historical conditions and concurrent information technologies. Selection labor can itself be decomposed into description and search labor. Selection labor and its decomposition into description and search labor will be treated in a subsequent article, in a further development of a labor theoretic approach to information retrieval. [source] Risk-sensitive sizing of responsive facilitiesNAVAL RESEARCH LOGISTICS: AN INTERNATIONAL JOURNAL, Issue 3 2008Sergio Chayet Abstract We develop a risk-sensitive strategic facility sizing model that makes use of readily obtainable data and addresses both capacity and responsiveness considerations. We focus on facilities whose original size cannot be adjusted over time and limits the total production equipment they can hold, which is added sequentially during a finite planning horizon. The model is parsimonious by design for compatibility with the nature of available data during early planning stages. We model demand via a univariate random variable with arbitrary forecast profiles for equipment expansion, and assume the supporting equipment additions are continuous and decided ex-post. Under constant absolute risk aversion, operating profits are the closed-form solution to a nontrivial linear program, thus characterizing the sizing decision via a single first-order condition. This solution has several desired features, including the optimal facility size being eventually decreasing in forecast uncertainty and decreasing in risk aversion, as well as being generally robust to demand forecast uncertainty and cost errors. We provide structural results and show that ignoring risk considerations can lead to poor facility sizing decisions that deteriorate with increased forecast uncertainty. Existing models ignore risk considerations and assume the facility size can be adjusted over time, effectively shortening the planning horizon. Our main contribution is in addressing the problem that arises when that assumption is relaxed and, as a result, risk sensitivity and the challenges introduced by longer planning horizons and higher uncertainty must be considered. Finally, we derive accurate spreadsheet-implementable approximations to the optimal solution, which make this model a practical capacity planning tool.© 2008 Wiley Periodicals, Inc. Naval Research Logistics, 2008 [source] Viscosity models for New Zealand black liquor at low solids concentrationsASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2010S.B. Alabi Abstract Availability of accurate models for prediction of the viscosity of black liquor (BL) from the chemical pulping of pine will facilitate its online monitoring and control and subsequently the optimisation of combustion in a recovery boiler. New Zealand (NZ) BL viscosity data are limited, and no predictive model is available. The viscosities of the NZ BL samples at solids concentrations (SCs) < 50% were obtained at temperature of 25,85 °C and shear rate up to ,2000 s,1. The samples showed Newtonian behaviour. Existing models from the literature and a binomial model developed in this work were used to fit the viscosity data as a function of SC and temperature. Accuracies of these models were examined for both the log-transformed and the untransformed viscosity data using coefficient of correlation (R) and maximum absolute relative error (MARE) (between the actual and predicted viscosities), respectively, as indices. Although the existing models fit NZ BL viscosity data well when they were log-transformed, they performed poorly when not transformed. Conversely, the new binomial model gave accurate predictions with both the log-transformed and untransformed viscosity data (R = 0.9997; MARE = 5.7%). It is concluded that at low SCs, the viscosity of Newtonian BL can be accurately predicted using the new binomial model. Copyright © 2010 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Inductive Inference by Using Information CompressionCOMPUTATIONAL INTELLIGENCE, Issue 2 2003Ben Choi Inductive inference is of central importance to all scientific inquiries. Automating the process of inductive inference is the major concern of machine learning researchers. This article proposes inductive inference techniques to address three inductive problems: (1) how to automatically construct a general description, a model, or a theory to describe a sequence of observations or experimental data, (2) how to modify an existing model to account for new observations, and (3) how to handle the situation where the new observations are not consistent with the existing models. The techniques proposed in this article implement the inductive principle called the minimum descriptive length principle and relate to Kolmogorov complexity and Occam's razor. They employ finite state machines as models to describe sequences of observations and measure the descriptive complexity by measuring the number of states. They can be used to draw inference from sequences of observations where one observation may depend on previous observations. Thus, they can be applied to time series prediction problems and to one-to-one mapping problems. They are implemented to form an automated inductive machine. [source] Time-Dependent Discrete Network Design Frameworks Considering Land UseCOMPUTER-AIDED CIVIL AND INFRASTRUCTURE ENGINEERING, Issue 6 2010W. Y. Szeto Unlike existing models, the optimization frameworks can determine the optimal designs automatically without trial-and-error once the objective(s) is/are clearly defined. Moreover, these frameworks allow the evaluation of the impacts of the optimal designs on the related parties including landowners, toll road operators, transit operators, and road users, and help network planners and profit-makers with decision making by eliminating many alternative designs. A numerical study is set up to examine road network design's effects on these related parties under three road construction schemes: exact cost recovery, build-operate-transfer, and cross-subsidization. The results show that the changes in landowner profits are not the same after implementing any scheme. These unequal changes raise the issue of the landowner equity. This implies that the government has to consider trade-offs between parties' objectives carefully. [source] A theory for elastic properties of single crystals with microstructure and its application to diffusion induced segregationCRYSTAL RESEARCH AND TECHNOLOGY, Issue 9 2008T. Blesgen Abstract In this article a general theory for elastically stressed single crystals in the presence of microstructure is presented and an explicit formula for the resulting non-linear stored mechanical energy is obtained. The optimal microstructure under applied stress is characterised and the optimal laminates are identified in 2D. The analysis is based on a sharp lower estimate of the energy that relies on relaxation. The new theory is then used to extend existing models for diffusion induced segregation (DIS) in the case of (Zn,Fe)S single crystals. Numerical simulations based on finite elements are carried out and the results are compared with former computations of the homogeneous case. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] A general framework for neutral models of community dynamicsECOLOGY LETTERS, Issue 12 2009Omri Allouche Abstract Neutral models of community dynamics are a powerful tool for ecological research, but their applications are currently limited to unrealistically simple types of dynamics and ignore much of the complexity that characterize natural ecosystems. Here, we present a new analytical framework for neutral models that unifies existing models of neutral communities and extends the applicability of existing models to a much wider spectrum of ecological phenomena. The new framework extends the concept of neutrality to fitness equivalence and in spite of its simplicity explains a wide spectrum of empirical patterns of species diversity including positive, negative and unimodal productivity,diversity relationships; gradual and highly delayed declines in species diversity with habitat loss; and positive and negative responses of species diversity to habitat heterogeneity. Surprisingly, the abundance distribution in all of these cases is given by the dispersal limited multinomial (DLM), the abundance distribution in Hubbell's zero-sum model, showing DLM's robustness and demonstrating that it cannot be used to infer the underlying community dynamics. These results support the hypothesis that ecological communities are regulated by a limited set of fundamental mechanisms much simpler than could be expected from their immense complexity. Ecology Letters (2009) 12: 1287,1297 [source] Size distributions of suspended particles in open channel flow over bed materialsENVIRONMETRICS, Issue 2 2005B. S. Mazumder Controlled experiments have shown that the grain-size distribution of suspended sediments is related to bed material, flow velocity and height of suspension above the sand bed in an open channel flow. A theoretical model has been developed for computation of suspended grain-size distribution on the basis of continuity equations of sediment and water, using the computed bed-layer concentration as a reference. The proposed model includes the effect of suspension concentration into the mean velocity, turbulent and viscous shear stresses owing to the dynamic coupling between the flow and sediments in suspension. The effect of hindered settling due to the increased concentration in suspension is also taken into account. The model is considered to be a more general one than the existing models, and the results of the present model compare well with the experimental data. Copyright © 2004 John Wiley & Sons, Ltd. [source] A - scab (Apple-scab), a simulation model for estimating risk of Venturia inaequalis primary infections,EPPO BULLETIN, Issue 2 2007V. Rossi A-scab (Apple-scab) is a dynamic simulation model for Venturia inaequalis primary infections on apple. It simulates development of pseudothecia, ascospore maturation, discharge, deposition and infection during the season based on hourly data of air temperature, rainfall, relative humidity and leaf wetness. A-scab produces a risk index for each infection period and forecasts the probable periods of symptoms appearance. The model was validated under different epidemiological conditions: its outputs were successfully compared with daily spore counts and actual onset and severity of the disease under orchard conditions, and neither corrections nor calibrations have been necessary to adapt the model to different apple-growing areas. Compared to other existing models, A-scab: (i) combines information from literature and data acquired from specific experiments; (ii) is completely ,open' because both model structure and algorithms have been published and are easily accessible; (iii) is not written with a specific computer language but it works on simple-to-use electronic sheets. For these reasons the model can be easily implemented in the computerized systems used by warning services. [source] Models of reproductive skew: A review and synthesis (Invited Article)ETHOLOGY, Issue 1 2000Rufus A. Johnstone Animal societies vary markedly in reproductive skew, the extent to which breeding is monopolised by dominant individuals. In the last few years, a large number of different models have been developed to explain this variation. Here, I review existing models of reproductive skew, distinguishing between two basic types. Transactional models focus on group stability and the constraints this places on the division of reproduction. Compromise models, by contrast, ignore issues of group stability and view the division of reproduction as the outcome of a conflict in which each group member has a limited or partial ability to enforce its own optimum. I go on to show, however, that the division between transactional and compromise models is somewhat artificial, and that both approaches may be combined in a single, synthetic treatment. Different models of reproductive skew are thus better seen as special cases of a general underlying theory, rather than alternative paradigms. I conclude with a brief discussion of the possibilities and problems of empirically testing this unified theory of skew, and the prospects for future theoretical advances. [source] Preferential flow and transport in soil: progress and prognosisEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 1 2008B. E. Clothier Summary Soil is the first filter of the world's water; its buffering and filtering determine the quality and quantity of our reserves of subterranean and surface water. Preferential flow can either enhance, or curtail, the capacity of the soil to buffer and filter, and it can compromise, or boost, other ecosystem services. We ask ,when do preferential flow and transport matter?' We identify 12 of 17 ecosystem services that benefit from preferential flow and three that are affected detrimentally. We estimate by simple arithmetic the value of preferential flow to ecosystem services to be globally some US$304 billion (109) per year. We review the 1989 Monte Verità meeting on preferential flow processes and summarize the 2006 presentations, some of which are published in this issue of the Journal. New technologies and innovative experiments have increased our understanding of the conditions that initiate and sustain preferential flows. We identify contemporary exigencies, and suggest avenues for their resolution. We are progressing through observation-led discovery. Our prognosis is that new data will enable us to develop better models, and more aptly to parameterize existing models, and thereby predict the impact, benefits and detriments of preferential flow in soil. [source] Neural network models to predict cation exchange capacity in arid regions of IranEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 4 2005M. Amini Summary Design and analysis of land-use management scenarios requires detailed soil data. When such data are needed on a large scale, pedotransfer functions (PTFs) could be used to estimate different soil properties. Because existing regression-based PTFs for estimating cation exchange capacity (CEC) do not, in general, apply well to arid areas, this study was conducted (i) to evaluate the existing models and (ii) to develop neural network-based PTFs for predicting CEC in Aridisols of Isfahan in central Iran. As most researches have found a significant correlation between CEC and soil organic matter content (OM) and clay content, we also used these two variables for modelling of CEC. We tested several published PTFs and developed two neural network algorithms using multilayer perceptron and general regression neural networks based on a set of 170 soil samples. The data set was divided into two subsets for calibration and testing of the models. In general, the neural network-based models provided more reliable predictions than the regression-based PTFs. [source] Practitioners' Perspectives on Cultural Sensitivity in Latina/o Teen Pregnancy Prevention,FAMILY RELATIONS, Issue 3 2006Ada M. Wilkinson-Lee Abstract: This study examined practitioners' understandings of cultural sensitivity in the context of pregnancy prevention programs for Latina teens. Fifty-eight practitioners from teen pregnancy prevention programs in California were interviewed in a guided conversation format. Three themes emerged in our analysis. First, practitioners' definitions of cultural sensitivity were consistent with existing models in the published research literature and focused largely on reinforcing the cultural strengths of Latino youth and families. Second, strategies for cultural sensitivity were complicated by tension between traditional Latino culture and the culture of pregnancy prevention programs. Third, practitioners identified a blend of personal and professional characteristics associated with cultural sensitivity among staff. We discussed policy and practice implications for Latino pregnancy prevention. [source] Streaming potential dependence on water-content in Fontainebleau sandGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2010V. Allègre SUMMARY The electrokinetic potential results from the coupling between the water flow and the electrical current because of the presence of ions within water. The electrokinetic coefficient is well described in fluid-saturated media, however its behaviour under unsaturated flow conditions is still discussed. We propose here an experimental approach to investigate streaming potential variations in sand at unsaturated conditions. We present for the first time continuous records of the electrokinetic coefficient as a function of water content. Two drainage experiments have been performed within a column filled with a clean sand. Streaming potential measurements are combined with water pressure and water content measurements every 10 cm along the column. In order to model hydrodymanics during the experiments, we solve Richards equation coupled with an inverse problem to estimate the hydraulic parameters of the constitutive relations between hydraulic conductivity, water pressure and water content. The electrokinetic coefficient C shows a more complex behaviour for unsaturated conditions than it was previously reported and cannot be fitted by the existing models. The normalized electrokinetic coefficient increases first when water saturation decreases from 100 to about 65,80 per cent, and then decreases as the water saturation decreases, whereas all previous works described a monotone decrease of the normalized electrokinetic coupling as water saturation decreases. We delimited two water saturation domains, and deduced two different empirical laws describing the evolution of the electrokinetic coefficient for unsaturated conditions. Moreover, we introduce the concept of the electrokinetic residual saturation, Sr,ekw, which allows us to propose a new model derived from the approach of the relative permeability used in hydrodynamics. [source] Convergence in Structure and Productivity in European Manufacturing?GERMAN ECONOMIC REVIEW, Issue 1 2004Klaus Gugler Structural convergence; productivity convergence; growth of industries; European integration Abstract. We find fast convergence in productivity for 99 three-digit European industries over the 1985,98 period. Half of any productivity gap is closed on average in about 10,15 years. We explicitly formulate the steady-state assumptions for structural convergence to hold. Convergence in industrial structure is much slower than productivity catch-up with a half-life of around 50 years, a stylized fact which cannot easily be explained by the existing models of trade and growth. [source] A Simple Correction for Slug Tests in Small-Diameter WellsGROUND WATER, Issue 3 2002James J. Butler Jr. A simple procedure is presented for correcting hydraulic conductivity (K) estimates obtained from slug tests performed in small-diameter installations screened in highly permeable aquifers. Previously reported discrepancies between results from slug tests in small-diameter installations and those from tests in nearby larger-diameter wells are primarily a product of frictional losses within the small-diameter pipe. These frictional losses are readily incorporated into existing models for slug tests in high-K aquifers, which then serve as the basis of a straightforward procedure for correcting previously obtained K estimates. A demonstration of the proposed procedure using data from a series of slug tests performed in a controlled field setting confirms the validity of the approach. The results of this demonstration also reveal the detailed view of spatial variations in K that can be obtained using slug tests in small-diameter installations. [source] Latent class models for utilisation of health careHEALTH ECONOMICS, Issue 4 2006*Article first published online: 3 MAR 200, Teresa Bago d'Uva Abstract This paper explores different approaches to econometric modelling of count measures of health care utilisation, with an emphasis on latent class models. A new model is proposed that combines the features of the two most common approaches: the hurdle model and the finite mixture negative binomial. Additionally, the panel structure of the data is taken into account. The proposed finite mixture hurdle model is shown to fit the data substantially better than the existing models for a particular application to data from the RAND Health Insurance Experiment. The estimation results indicate a higher price effect for low users of health care. It is furthermore found that this results mainly from the difference of the price effects on the probability to visit a doctor, while the price effect on the conditional number of visits does not differ significantly between high and low users. Copyright © 2006 John Wiley & Sons, Ltd. [source] Lay perceptions of the desired role and type of user involvement in clinical governanceHEALTH EXPECTATIONS, Issue 1 2009Andrea Litva BA (Comb Hons) MA PhD Abstract Objective, The aim of this paper is to explore variations in lay perceptions of user involvement in clinical governance. Context, The English National Health Service has sought to build a dependable health service through enhanced effectiveness, responsiveness and consistency. Clinical governance, a policy for improving service quality, is a key pillar of these reforms. It is a statutory duty of primary care organizations to ensure that users are involved in all service planning and decision making, including clinical governance. Yet surveys indicated that user involvement in clinical governance was underdeveloped and underutilized. Design, Focus groups were conducted with different types of lay people to explore their perceptions around public involvement in different aspects of clinical governance policy. Results, Content analysis of the transcripts reveals that different groups of lay people varied in their desired role perspective and preferred type of involvement in different aspects of clinical governance policy. Drawing upon existing models of user involvement, we identified three role perspectives that lay people could take in user involvement , consumer, advocate and citizen. We compared our findings regarding the desired type of involvement with existing models of user involvement, and identified a new type of involvement, overseeing, that is relevant to clinical governance policy. Conclusions, These findings suggest that to facilitate user involvement in clinical governance, it would be necessary to use different strategies to accommodate the differing role perspectives and types of involvement desired by different groups of lay people. [source] Enhanced amygdala and medial prefrontal activation during nonconscious processing of fear in posttraumatic stress disorder: An fMRI studyHUMAN BRAIN MAPPING, Issue 5 2008Richard A. Bryant Abstract Biological models of posttraumatic stress disorder (PTSD) suggest that patients will display heightened amygdala but decreased medial prefrontal activity during processing of fear stimuli. However, a rapid and automatic alerting mechanism for responding to nonconscious signals of fear suggests that PTSD may display heightened rather than decreased MPFC under nonconscious processing of fear stimuli. This study used functional magnetic resonance imaging to examine blood oxygenation level-dependent signal changes during nonconscious presentation (16.7 ms, masked) of fearful and neutral faces in 15 participants with PTSD and 15 age and sex-matched healthy control participants. Results indicate that PTSD participants display increased amygdala and MPFC activity during nonconscious processing of fearful faces. These data extend existing models by suggesting that the impaired MPFC activation in PTSD may be limited to conscious fear processing. Hum Brain Mapp, 2008. © 2007 Wiley-Liss, Inc. [source] Effects of topography on the spatial distribution of evapotranspiration over a complex terrain using two-source energy balance model with ASTER dataHYDROLOGICAL PROCESSES, Issue 16 2009H. K. Kafle Abstract Spatial distribution of evapotranspiration (ET) over a complex terrain is estimated using a new approach of the conventional two-source energy balance (TSEB) model by considering the effect of topography (difference in slope and aspect). We name this approach topography considered two-source energy balance (T2SEB) model. The novelty of this model is the estimation of incoming shortwave solar radiation considering slope, aspect, altitude, latitude, longitude, and the day of calculation in the TSEB model, so that the new model should have wider applicability than existing models over topographically complex areas. In this study, high spatial resolution Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data and meteorological data are used. ET over a complex terrain of Nagoya, Japan, on three different dates, 4 November 2005, 25 May 2004 and 30 October 2003, is estimated using both TSEB and T2SEB models. To validate both models, estimated results are compared with ground observation data at the flux tower site. Moreover, estimated results from TSEB and T2SEB models are compared in five different locations of different topography within the study area. Variation of net radiation absorbed by the surface (Rn) with topographical variables is also studied with the help of scatter plots. Estimated results for all three dates agreed within ±75 W m,2 with calculated values from both models at the flux tower site. TSEB underestimated/overestimated ET in sunlit/shaded areas in hilly areas. The T2SEB model estimated ET in hilly areas better than the TSEB model. Copyright © 2009 John Wiley & Sons, Ltd. [source] Implicit integration of a chemo-plastic constitutive model for partially saturated soilsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 14 2008H. W. Zhang Abstract A chemo-plastic constitutive model for partially saturated soils is proposed in this paper based on the existing models developed in Hueckel (Int. J. Numer. Anal. Meth. Geomech. 1997; 21:43,72) and Gallipoli et al. (Geotechnique 2003; 53:123,135). The chemical softening effects due to the increase in contaminant mass concentration are considered based on Hueckel's chemo-plastic model. Gallipoli's model is used to simulate the effects of suction and degree of saturation on mechanical behavior of partially saturated porous materials. In order to implement the proposed model in a finite element code, a fully implicit backward-Euler integration algorithm is put forward. Numerical solutions for the tests at local level and the application of the algorithm to the real boundary value problem demonstrate the accuracy and convergence properties of the proposed integration scheme. Copyright © 2008 John Wiley & Sons, Ltd. [source] Shear band evolution and accumulated microstructural development in Cosserat mediaINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 10 2004A. Tordesillas Abstract This paper prepares the ground for the continuum analysis of shear band evolution using a Cosserat/micropolar constitutive equation derived from micromechanical considerations. The nature of the constitutive response offers two key advantages over other existing models. Firstly, its non-local character obviates the mathematical difficulties of traditional analyses, and facilitates an investigation of the shear band evolution (i.e. the regime beyond the onset of localization). Secondly, the constitutive model parameters are physical properties of particles and their interactions (e.g. particle stiffness coefficients, coefficients of inter-particle rolling friction and sliding friction), as opposed to poorly understood fitting parameters. In this regard, the model is based on the same material properties used as model inputs to a discrete element (DEM) analysis, therefore, the micromechanics approach provides the vehicle for incorporating results not only from physical experiments but also from DEM simulations. Although the capabilities of such constitutive models are still limited, much can be discerned from their general rate form. In this paper, an attempt is made to distinguish between those aspects of the continuum theory of localization that are independent of the constitutive model, and those that require significant advances in the understanding of micromechanics. Copyright © 2004 John Wiley & Sons, Ltd. [source] From monosilane to crystalline silicon, part II: Kinetic considerations on thermal decomposition of pressurized monosilaneINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 5 2006J. O. Odden Kinetic aspects of the thermal decomposition of monosilane at 690,830 K and initial pressures of 0.1,3.7 MPa in a free-space reactor are considered. Neglecting the preparatory initiation period for the reaction (which is difficult to evaluate under the present dynamic conditions), the onset temperature for the decomposition is stipulated to some 700,710 K, independent of the initial monosilane pressure. The overall reaction appears to be of first order throughout the progressing decomposition process. We observe considerably lower reaction rates under the high-pressure conditions than existing models in the literature suggest. A modified model is proposed that simulates the observed reaction rates within ±1% and moreover predicts credible concentrations of the involved gaseous species. A key feature of the modified model is incorporation of two third-body assisted surface reactions, which generate monosilane from disilane and disilane from trisilane. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 309,321, 2006 [source] Analytical and numerical investigation of the solar chimney power plant systemsINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 11 2006Ming Tingzhen Abstract There is a surge in the use of the solar chimney power plant in the recent years which accomplishes the task of converting solar energy into kinetic energy. As the existing models are insufficient to accurately describe the mechanism, a more comprehensive model is advanced in this paper to evaluate the performance of a solar chimney power plant system, in which the effects of various parameters on the relative static pressure, driving force, power output and efficiency have been further investigated. Using the solar chimney prototype in Manzanares, Spain, as a practical example, the numerical studies are performed to explore the geometric modifications on the system performance, which show reasonable agreement with the analytical model. Copyright © 2005 John Wiley & Sons, Ltd. [source] Carbon monoxide poisoning of proton exchange membrane fuel cellsINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 8 2001J. J. Baschuk Abstract Proton exchange membrane fuel cell (PEMFC) performance degrades when carbon monoxide (CO) is present in the fuel gas; this is referred to as CO poisoning. This paper investigates CO poisoning of PEMFCs by reviewing work on the electrochemistry of CO and hydrogen, the experimental performance of PEMFCs exhibiting CO poisoning, methods to mitigate CO poisoning and theoretical models of CO poisoning. It is found that CO poisons the anode reaction through preferentially adsorbing to the platinum surface and blocking active sites, and that the CO poisoning effect is slow and reversible. There exist three methods to mitigate the effect of CO poisoning: (i) the use of a platinum alloy catalyst, (ii) higher cell operating temperature and (iii) introduction of oxygen into the fuel gas flow. Of these three methods, the third is the most practical. There are several models available in the literature for the effect of CO poisoning on a PEMFC and from the modeling efforts, it is clear that small CO oxidation rates can result in much increased performance of the anode. However, none of the existing models have considered the effect of transport phenomena in a cell, nor the effect of oxygen crossover from the cathode, which may be a significant contributor to CO tolerance in a PEMFC. In addition, there is a lack of data for CO oxidation and adsorption at low temperatures, which is needed for detailed modeling of CO poisoning in PEMFCs. Copyright © 2001 John Wiley & Sons, Ltd. [source] Demand side management for water heating installations in South African commercial buildingsINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 4 2001P. G. Rousseau Abstract The largest percentage of the sanitary hot water used in South African buildings is heated by means of direct electrical resistance heaters. This is one of the major contributing factors of the undesirable high morning and afternoon peaks imposed on the national electricity supply grid. Water heating therefore continues to be of concern to ESKOM, the country's only electrical utility company. The so-called in-line water heating system design methodology was developed to address this problem. This paper investigates the potential impact of in-line systems on the national peak electrical demand. A computer simulation model was developed that combines a deterministic mathematical model with a statistical approach in order to predict the diversity factors associated with both the existing and in-line design methodologies. A study was also conducted to estimate the total installed water heating capacity in the national commercial building sector. This figure can be combined with the simulated diversity factor to determine the peak electrical demand. The deterministic model includes the detailed simulation of the hot water storage vessel, the electrical heater and the system control algorithm. The mathematical model for the storage vessel is based on an electrical analogue approach that includes the effects of conduction as well as forced and natural convection. This model was verified extensively with the aid of laboratory measurements and compared with existing storage vessel models. It was found that the new storage vessel model could predict the supply temperature within 2 per cent for a system configuration with the heater in parallel outside the reservoir and within 12 per cent for a configuration with the heater situated inside the reservoir. This compares favourably with existing models found in the literature. The complete simulation based on the statistical approach showed that extensive application of the new design methodology could result in a reduction of approximately 75 MW in the total maximum peak demand imposed on the electricity supply grid in wintertime. This is 58 per cent of the current peak demand due to commercial water heating and 12.5 per cent of the peak load reduction target set by ESKOM until the year 2015. Copyright © 2001 John Wiley & Sons, Ltd. [source] Binary encounter model for the electron impact K -shell ionization of atomsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 8 2008M. A. R. Patoary Abstract The electron impact K -shell ionization (EIKSI) cross sections on 18 atomic targets, with the atomic numbers 2 , Z , 92, are calculated using a modified version of the binary encounter approximation (BEA) model. The modified BEA (MBEA), which incorporates both ionic and relativistic corrections and is simpler in application than other existing models, is found to be immensely successful in describing the EIKSI data up to the incident energy of 1 GeV. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] A Review and Extension of Current Models of Dynamic CriteriaINTERNATIONAL JOURNAL OF SELECTION AND ASSESSMENT, Issue 3 2000Debra Steele-Johnson An important issue in personnel selection and test validation has been the nature of performance criteria and more specifically the existence of dynamic criteria. There is a continuing debate regarding the extent to which performance and validity coefficients remain stable over time. We examine research within work, laboratory, and academic settings and evaluate existing models of dynamic criteria. Building on previous models, we propose an integrative model of dynamic criteria that identifies important issues for ability and performance constructs and discusses how variables related to the task, job, and organization can affect the temporal stability of criterion performance and the ability-performance relationship. [source] Gay men's holiday destination choice: a case of risk and avoidanceINTERNATIONAL JOURNAL OF TOURISM RESEARCH, Issue 4 2002Howard Hughes Abstract Gay men are subject to discrimination and social disapproval in many parts of the world. This paper examines the possible impact of this on choice of holiday destination, drawing on the concepts of risk and destination avoidance. A number of interviews were held with gay men and it was concluded that risk of adverse reactions and of the need to modify behaviour were significant issues in destination avoidance and choice. It is possible, as a consequence, to classify holiday-taking by gay men into a typology including gay-centric and gay-related. In addition, it is appropriate to modify existing models of risk-avoidance through the addition of the risks that are more applicable to gay men than to other tourists. Copyright © 2002 John Wiley & Sons, Ltd. [source] | |||||||||||||||||||||||||||||||||||||||||||