bluuebunny/crossref_metadata_2025 huggingface dataset:
Some Contributions to the Theory of Near-Critical Bisexual Branching Processes
Authors: Molina M. , Mota M. , Ramos A.
Published: October 2007

We investigate the probabilistic evolution of a near-critical bisexual branching process with mating depending on the number of couples in the population. We determine sufficient conditions which guarantee either the almost sure extinction of such a process or its survival with positive probability. We also establish some limiting results concerning the sequences of couples, females, and males, suitably normalized. In particular, gamma, normal, and degenerate distributions are proved to be limit laws. The results also hold for bisexual Bienaymé–Galton–Watson processes, and can be adapted to other classes of near-critical bisexual branching processes.
On Some Ageing Properties of General Repair Processes
Authors: Finkelstein Maxim
Published: October 2007

We consider ageing properties of a general repair process. Under certain assumptions we prove that the expectation of an age at the beginning of the next cycle in this process is smaller than the initial age of the previous cycle. Using this reasoning, we show that the sequence of random ages at the start (end) of consecutive cycles is stochastically increasing and is converging to a limiting distribution. We discuss possible applications and interpretations of our results.
Markov Additive Processes and Repeats in Sequences
Authors: Spouge John L.
Published: October 2007

Computer analysis of biological sequences often detects deviations from a random model. In the usual model, sequence letters are chosen independently, according to some fixed distribution over the relevant alphabet. Real biological sequences often contain simple repeats, however, which can be broadly characterized as multiple contiguous copies (usually inexact) of a specific word. This paper quantifies inexact simple repeats as local sums in a Markov additive process (MAP). The maximum of the local sums has an asymptotic distribution with two parameters (λ and k), which are given by general MAP formulas. The general MAP formulas are usually computationally intractable, but an essential simplification in the case of repeats permits λ and k to be computed from matrices whose dimension equals the size of the relevant alphabet. The simplification applies to some MAPs where the summand distributions do not depend on consecutive pairs of Markov states as usual, but on pairs with a fixed time-lag larger than one.
Probabilistic Approximation of a Nonlinear Parabolic Equation Occurring in Rheology
Authors: Ben Alaya Mohamed , Jourdain Benjamin
Published: October 2007

In this paper we are interested in a nonlinear parabolic evolution equation occurring in rheology. We give a probabilistic interpretation to this equation by associating a nonlinear martingale problem with it. We prove the existence of a unique solution, P, to this martingale problem. For any t, the time marginal of P at time t admits a density ρ(t,x) with respect to the Lebesgue measure, where the function ρ is the unique weak solution to the evolution equation in a well-chosen energy space. Next we introduce a simulable system of n interacting particles and prove that the empirical measure of this system converges to P as n tends to ∞. This propagation-of-chaos result ensures that the solution to the equation of interest can be approximated using a Monte Carlo method. Finally, we illustrate the convergence in some numerical experiments.
Nonconstant Sum Red-And-Black Games with Bet-Dependent Win Probability Function
Authors: Pontiggia Laura
Published: October 2007

In this paper we investigate a class of N-person nonconstant sum red-and-black games with bet-dependent win probability functions. We assume that N players and a gambling house are engaged in a game played in stages, where the player's probability of winning at each stage is a function f of the ratio of his bet to the sum of all the players' bets. However, at each stage of the game there is a positive probability that all the players lose and the gambling house wins their bets. We prove that if the win probability function is super-additive and it satisfies f(s)f(t)≤f(st), then a bold strategy is optimal for all players.
Winning Rate in the Full-Information Best-Choice Problem
Authors: Gnedin Alexander V. , Miretskiy Denis I.
Published: October 2007

Following a long-standing suggestion by Gilbert and Mosteller, we derive an explicit formula for the asymptotic winning rate in the full-information best-choice problem.
A Note on Extinction Times for the General Birth, Death and Catastrophe Process
Authors: Pollett Phil , Zhang Hanjun , Cairns Benjamin J.
Published: October 2007

We consider a birth, death and catastrophe process where the transition rates are allowed to depend on the population size. We obtain an explicit expression for the expected time to extinction, which is valid in all cases where extinction occurs with probability 1.
Characterizations of Conditional Comonotonicity
Authors: Cheung Ka Chun
Published: October 2007

The notion of conditional comonotonicity was first used implicitly by Kaas, Dhaene, and Goovaerts (2000) and was formally introduced by Jouini and Napp (2004) as a generalization of the classical concept of comonotonicity. The objective of the present paper is to further investigate this relatively new concept. The main result is that a random vector is comonotonic conditional to a certain σ-field if and only if it is almost surely comonotonic locally on each atom of the conditioning σ-field. We also provide a new proof of a distributional representation and an almost sure representation of a conditionally comonotonic random vector.
Asymptotic Behavior of a Generalized TCP Congestion Avoidance Algorithm
Authors: Ott Teunis J. , Swanson Jason
Published: October 2007

The transmission control protocol (TCP) is a transport protocol used in the Internet. In Ott (2005), a more general class of candidate transport protocols called ‘protocols in the TCP paradigm’ was introduced. The long-term objective of studying this class is to find protocols with promising performance characteristics. In this paper we study Markov chain models derived from protocols in the TCP paradigm. Protocols in the TCP paradigm, as TCP, protect the network from congestion by decreasing the ‘congestion window’ (i.e. the amount of data allowed to be sent but not yet acknowledged) when there is packet loss or packet marking, and increasing it when there is no loss. When loss of different packets are assumed to be independent events and the probability p of loss is assumed to be constant, the protocol gives rise to a Markov chain {Wn}, where Wn is the size of the congestion window after the transmission of the nth packet. For a wide class of such Markov chains, we prove weak convergence results, after appropriate rescaling of time and space, as p → 0. The limiting processes are defined by stochastic differential equations. Depending on certain parameter values, the stochastic differential equation can define an Ornstein-Uhlenbeck process or can be driven by a Poisson process.
Stability of Processor Sharing Networks with Simultaneous Resource Requirements
Authors: Hansen Jennie , Reynolds Cian , Zachary Stan
Published: October 2007

We study the phenomenon of entrainment in processor sharing networks, whereby, while individual network resources have sufficient capacity to meet demand, the requirement for simultaneous availability of resources means that a network may nevertheless be unstable. We show that instability occurs through poor control, and that, for a variety of network topologies, only small modifications to controls are required in order to ensure stability. For controls which possess a natural monotonicity property, we give some new results for the classification of the corresponding Markov processes, which lead to conditions both for stability and for instability.
Strong Convergence for URN Models with Reducible Replacement Policy
Authors: Abraham R. , Dhersin J. S. , Ycart B.
Published: October 2007

A multitype urn scheme with random replacements is considered. Each time a ball is picked, another ball is added, and its type is chosen according to the transition probabilities of a reducible Markov chain. The vector of frequencies is shown to converge almost surely to a random element of the set of stationary measures of the Markov chain. Its probability distribution is characterized as the solution to a fixed point problem. It is proved to be Dirichlet in the particular case of a single transient state to which no return is possible. This is no longer the case, however, as soon as returns to transient states are allowed.
The Joint Density of the Surplus Before and After Ruin in the Sparre Andersen Model
Authors: Pitts Susan M. , Politis Konstadinos
Published: October 2007

Gerber and Shiu (1997) have studied the joint density of the time of ruin, the surplus immediately before ruin, and the deficit at ruin in the classical model of collective risk theory. More recently, their results have been generalised for risk models where the interarrival density for claims is nonexponential, but belongs to the Erlang family. Here we obtain generalisations of the Gerber-Shiu (1997) results that are valid in a general Sparre Andersen model, i.e. for any interclaim density. In particular, we obtain a generalisation of the key formula in that paper. Our results are made more concrete for the case where the distribution between claim arrivals is phase-type or the integrated tail distribution associated with the claim size distribution belongs to the class of subexponential distributions. Furthermore, we obtain conditions for finiteness of the joint moments of the surplus before ruin and the deficit at ruin in the Sparre Andersen model.
Optimal Smooth Portfolio Selection for an Insider
Authors: Hu Yaozhong , Øksendal Bernt
Published: October 2007

We study the optimal portfolio problem for an insider, in the case where the performance is measured in terms of the logarithm of the terminal wealth minus a term measuring the roughness and the growth of the portfolio. We give explicit solutions in some cases. Our method uses stochastic calculus of forward integrals.
The Critical Galton-Watson Process Without Further Power Moments
Authors: Nagaev S. V. , Wachtel V.
Published: October 2007

In this paper we prove a conditional limit theorem for a critical Galton-Watson branching process {Zn; n ≥ 0} with offspring generating function s + (1 − s)L((1 − s)−1), where L(x) is slowly varying. In contrast to a well-known theorem of Slack (1968), (1972) we use a functional normalization, which gives an exponential limit. We also give an alternative proof of Sze's (1976) result on the asymptotic behavior of the nonextinction probability.
Voter Model and Biased Voter Model in Heterogeneous Environments
Authors: Lanchier N. , Neuhauser C.
Published: October 2007

With the rapid adoption of transgenic crops, gene flow from transgenic crops to wild relatives through pollen dispersal is of significant concern and warrants both empirical and theoretical studies to assess the risk of introduction of transgenes into wild populations. We propose to use the (biased) voter model in a heterogeneous environment to investigate the effects of recurrent gene flow from transgenic crop to wild relatives. The model is defined on the d-dimensional integer lattice that is divided into two parts, Δ and Zd \ Δ. Individuals carrying the transgene and individuals carrying the wild type gene compete according to the evolution rules of a (biased) voter model on Zd \ Δ, while the process is conditioned to have only individuals carrying the transgene on Δ. Our main findings suggest that unless transgenes confer increased fitness in wild relatives, introgression of transgenes into populations of wild plants is slow and may even be reversible without intervention. Our study also addresses the effects of different spatial planting patterns of transgenic crops on the rate of introgression.
Integration by Parts for Point Processes and Monte Carlo Estimation
Authors: Privault Nicolas , Wei Xiao
Published: October 2007

We develop an integration by parts technique for point processes, with application to the computation of sensitivities via Monte Carlo simulations in stochastic models with jumps. The method is applied to density estimation with respect to the Lebesgue measure via a modified kernel estimator which is less sensitive to variations of the bandwidth parameter than standard kernel estimators. This applies to random variables whose densities are not analytically known and requires the knowledge of the point process jump times.
Explicit Optimal Strategy for the Vardi Casino with Limited Playing Time
Authors: Yao Yi-Ching
Published: October 2007

The Vardi casino with parameter 0 < c < 1 consists of infinitely many tables indexed by their odds, each of which returns the same (negative) expected winnings -c per dollar. A gambler seeks to maximize the probability of reaching a fixed fortune by gambling repeatedly with suitably chosen stakes and tables (odds). The optimal strategy is derived explicitly subject to the constraint that the gambler is allowed to play only a given finite number of times. Some properties of the optimal strategy are also discussed.
Clathrin-mediated endocytosis of a lipid-raft-associated protein is mediated through a dual tyrosine motif
Authors: Rollason Ruth , Korolchuk Viktor , Hamilton Clare , Schu Peter , Banting George
Published: October 2007

We have previously shown that the integral membrane protein CD317 has both a conventional transmembrane domain near its N-terminus and a C-terminal glycosyl-phosphatidylinositol (GPI) anchor. With the possible exception of a minor topological variant of the prion protein, there remain no other convincing examples of a mammalian protein with such a topology. CD317 is localised to cholesterol-rich lipid microdomains (`lipid rafts') in the plasma membrane and is internalised from the cell surface for delivery to a juxta-nuclear compartment (most probably the TGN). We have now investigated the mechanism by which CD317 is internalised and find that this raft-associated integral membrane protein is internalised through a clathrin-dependent pathway, internalisation is dependent upon a novel dual-tyrosine-based motif in the cytosolic domain of CD317, the cytosolic domain of CD317 can interact with the μ subunits of the AP2 and AP1 adaptor complexes, interaction with AP1 is required for delivery of CD317 back to the TGN, and removal of the GPI anchor from CD317 reduces the efficiency of CD317 internalisation. Collectively, these data indicate that CD317 is internalised and delivered back to the TGN by the sequential action of AP2 and AP1 adaptor complexes and that, surprisingly, the clathrin-mediated internalisation of CD317 occurs more efficiently if CD317 is localised to lipid rafts.
Sox17 plays a substantial role in late-stage differentiation of the extraembryonic endoderm in vitro
Authors: Shimoda Masafumi , Kanai-Azuma Masami , Hara Kenshiro , Miyazaki Satsuki , Kanai Yoshiakira , Monden Morito , Miyazaki Jun-ichi
Published: October 2007

Sox17 is a Sry-related HMG-box transcription factor developmentally expressed in both the definitive endoderm and extraembryonic endoderm (ExE). Although Sox17–/– mouse embryos have a defective definitive gut endoderm, their developing ExE is morphologically intact. Here, we aimed to investigate the role of Sox17 in ExE development by using an in vitro differentiation system of embryonic stem cells (ESCs). Although forced Sox17 expression in ESCs did not affect ExE commitment, it facilitated the differentiation of ESC-derived primitive endoderm cells into visceral and parietal endoderm cells. This event was inhibited by the forced expression of Nanog, a negative regulator of differentiation of ESCs into the ExE. Although Sox17–/– ESCs could differentiate into primitive endoderm cells, further differentiation was severely impaired. These results indicate a substantial involvement of Sox17 in the late stage of ExE differentiation in vitro. Furthermore, the expression of Sox7 – another Sox factor, concomitantly expressed with Sox17 in the developing ExE – was suppressed during the in vitro differentiation of Sox17–/– ESCs, but it was maintained at a high level in the extraembryonic tissues of Sox17–/– embryos. These findings possibly explain the discrepancy between the ExE phenotype derived from Sox17–/– ESCs and that of Sox17–/– embryos.
Tousled-like kinase in a microbial eukaryote regulates spindle assembly and S-phase progression by interacting with Aurora kinase and chromatin assembly factors
Authors: Li Ziyin , Gourguechon Stéphane , Wang Ching C.
Published: October 2007

The Tousled-like kinases are an evolutionarily conserved family of proteins implicated in DNA repair, DNA replication and mitosis in metazoans and plants. Their absence from the yeasts and other eukaryotic `microbes' suggests a specific role for them in the development of multicellular organisms. In this study, two closely related Tousled-like kinase homologs, TLK1 and TLK2, were identified in Trypanosoma brucei, a unicellular protozoan parasite. Only TLK1 plays an essential role in cell growth, and a deficiency in TLK1 led to an enrichment of S-phase cells, defective spindle formation and aberrant chromosome segregation. Although both TLK proteins localize to the nucleus, only TLK1 also concentrates in the spindle poles during mitosis. Both TLK proteins are phosphorylated by the Aurora kinase (AUK1), and both can autophosphorylate and phosphorylate histone H3 and the chromatin assembly factors Asf1A and Asf1B in vitro, but only TLK1 is autophosphorylated and capable of oligomerizing and interacting with AUK1, Asf1A and Asf1B in vivo. These discrepancies between the two TLK proteins can be attributed to minor differences between their N- and C-terminal sequences. In summary, TLK1 cooperates with Aurora kinase to regulate spindle assembly and chromosome segregation, and it performs a role in DNA replication probably by regulating histone modification in trypanosomes.
RPTPα is required for rigidity-dependent inhibition of extension and differentiation of hippocampal neurons
Authors: Kostic Ana , Sap Jan , Sheetz Michael P.
Published: October 2007

Receptor-like protein tyrosine phosphatase α (RPTPα)-knockout mice have severe hippocampal abnormalities similar to knockouts of the Src family kinase Fyn. These enzymes are linked to the matrix-rigidity response in fibroblasts, but their function in neurons is unknown. The matrix-rigidity response of fibroblasts appears to differ from that of neuronal growth cones but it is unknown whether the rigidity detection mechanism or response pathway is altered. Here, we report that RPTPα is required for rigidity-dependent reinforcement of fibronectin (FN)-cytoskeleton bonds and the rigidity response in hippocampal neuron growth cones, like in fibroblasts. In control neurons, rigid FN surfaces inhibit neurite extension and neuron differentiation relative to soft surfaces. In RPTPα–/– neurons, no inhibition of extension and differentiation is found on both rigid and soft surfaces. The RPTPα-dependent rigidity response in neurons is FN-specific, and requires clustering of αvβ6 integrin at the leading edge of the growth cones. Further, RPTPα is necessary for the rigidity-dependent concentration of Fyn and p130Cas phosphorylation at the leading edge of the growth cone, like it is in fibroblasts. Although neurons respond to rigid FN surfaces in the opposite way to fibroblasts, we suggest that the mechanism of detecting FN rigidity is similar and involves rigidity-dependent RPTPα recruitment of Fyn.
Regulation of P2X4 receptors by lysosomal targeting, glycan protection and exocytosis
Authors: Qureshi Omar S. , Paramasivam Anbalakan , Yu Jowie C. H. , Murrell-Lagnado Ruth D.
Published: October 2007

The P2X4 receptor has a widespread distribution in the central nervous system and the periphery, and plays an important role in the function of immune cells and the vascular system. Its upregulation in microglia contributes to neuropathic pain following nerve injury. The mechanisms involved in its regulation are not well understood, although we have previously shown that it is constitutively retrieved from the plasma membrane and resides predominantly within intracellular compartments. Here, we show that the endogenous P2X4 receptors in cultured rat microglia, vascular endothelial cells and freshly isolated peritoneal macrophages are localized predominantly to lysosomes. Lysosomal targeting was mediated through a dileucine-type motif within the N-terminus, together with a previously characterized tyrosine-based endocytic motif within the C-terminus. P2X4 receptors remained stable within the proteolytic environment of the lysosome and resisted degradation by virtue of their N-linked glycans. Stimulation of phagocytosis triggered the accumulation of P2X4 receptors at the phagosome membrane. Stimulating lysosome exocytosis, either by incubating with the Ca2+ ionophore ionomycin, for normal rat kidney (NRK) cells and cultured rat microglia, or the weak base methylamine, for peritoneal macrophages, caused an upregulation of both P2X4 receptors and the lysosomal protein LAMP-1 at the cell surface. Lysosome exocytosis in macrophages potentiated ATP-evoked P2X4 receptor currents across the plasma membrane. Taken together, our data suggest that the P2X4 receptor retains its function within the degradative environment of the lysosome and can subsequently traffic out of lysosomes to upregulate its exposure at the cell surface and phagosome.
Distinct endocytic pathways identified in tobacco pollen tubes using charged nanogold
Authors: Moscatelli Alessandra , Ciampolini Fabrizio , Rodighiero Simona , Onelli Elisabetta , Cresti Mauro , Santo Nadia , Idilli Aurora
Published: October 2007

In an attempt to dissect endocytosis in Nicotiana tabacum L. pollen tubes, two different probes – positively or negatively charged nanogold – were employed. The destiny of internalized plasma membrane domains, carrying negatively or positively charged residues, was followed at the ultrastructural level and revealed distinct endocytic pathways. Time-course experiments and electron microscopy showed internalization of subapical plasma-membrane domains that were mainly recycled to the secretory pathway through the Golgi apparatus and a second mainly degradative pathway involving plasma membrane retrieval at the tip. In vivo time-lapse experiments using FM4-64 combined with quantitative analysis confirmed the existence of distinct internalization regions. Ikarugamycin, an inhibitor of clathrin-dependent endocytosis, allowed us to further dissect the endocytic process: electron microscopy and time-lapse studies suggested that clathrin-dependent endocytosis occurs in the tip and subapical regions, because recycling of positively charged nanogold to the Golgi bodies and the consignment of negatively charged nanogold to vacuoles were affected. However, intact positively charged-nanogold transport to vacuoles supports the idea that an endocytic pathway that does not require clathrin is also present in pollen tubes.
Finite element method-based three-dimensional frictional contact analysis of turbocharger compressor
Authors: Liao A. H. , Zhang H. W. , Wu C. H.
Published: October 2007

In the long-distance transport of material, turbocharger compressors are playing an important role. The impeller mounted onto the compressor shaft assembly via interference fit is one of the key components of a centrifugal compressor stage. The assembly creates a complicated frictional contact problem on the interface of the contacting parts when the rotating speed is high. A locomotive-type turbocharger compressor with 24 blades under combined centrifugal and interference-fit loading is used for the analysis. The finite element parametric quadratic programming method that was developed based on the parametric variational principle is used for the analysis of the stress distribution of three-dimensional frictional contact problem between the impeller and the shaft assembly. The advantages of the parametric programming method when compared with the conventional ones are that the penalty factors can be cancelled and the solutions can be obtained directly without tedious iterative procedures such as general incremental iterative method. The effects of fit tolerance, coefficient of friction, the wall thickness of the shaft sleeve, rotational speed, and the contact stress distribution on the contacting surfaces are discussed in section 2. It is found that non-uniform initial amount of interference in the structural design would avoid relative displacement generated and ensure uniformity of the contact stress. To assure the quality of press-fitting, the amount of interference between the shaft sleeve and the shaft should be controlled strictly to avoid the rapid increase of the contact stress. The results obtained play a key role in deciding the proper fit tolerance and improving design and manufacturing technology of the compressor impellers. The current study also provides an effective approach which achieves more reliable interference-fitted connections and more precise assembly accuracy with lower manufacturing cost in the structural design.
Optimal, robust, and benign horizontal and vertical axis wind turbines
Authors: Farthing S. P.
Published: October 2007

The remarkable success of fixed speed and pitch ‘propellers’ in tapping the gusting and shifting wind begs explanation. Glauert's two-dimensional analysis of ‘horizontal axis wind turbines’ was an incomplete aside to aviation propeller theory in 1935, but is now the basis of most ‘Hawt’ algorithms. A very simple windmill optimum is found for his ‘blade element momentum’ theory, even for the movement inclined to the wind in ‘vertical axis wind turbines’. Optimizing the two-dimensional ‘Vawt’ pitch cycles for high speed ratios matches the best Hawt power.

Making the optimum ‘robust’ to variation of the wind further discovers a Hawt blade element chord and fixed pitch with a very broad quartic optimum around the design wind and ‘benign’ avoidance of high aerodynamic load coefficients further from it; and finds a fixed cycle of pitching to give a benign quartic Vawt.
Numerical and experimental study of the influence of drag reduction agent (carboxy methyl cellulose) on the central jet pump performance
Authors: El Gazzar M , Meakhail T , Mikhail S
Published: October 2007

Several experimental and theoretical investigations have been carried out to improve the performance of the jet pump through study of the influence of various geometric variables on its overall efficiency. To the best knowledge of the authors, none of these investigations studied the effect of the use of polymer solutions, e.g. carboxy methyl cellulose (CMC), as an agent of drag reduction on the jet pump performance, which is the objective of the present work.

The present numerical study is carried out and compared with the previous experimental results explained in Gazzar et al. [1].

A computational fluid dynamics code is used to predict the performance of the jet pump and to shed more light on the mechanism of the mixing process especially if the primary stream is a Newtonian fluid, while the secondary stream is (the polymer solution) assumed to be a non-Newtonian fluid.

Discussion and comparison between the experimental and theoretical results are included. The results of this comparison prove the validity of the computational code used, and illustrate the prominent effect of adding drag reduction agents (CMC) on jet pump performance.
Fundamentals of four-quadrant axial flow compressor maps
Authors: Gill A , von Backström T. W. , Harms T. M.
Published: October 2007

The current paper describes the experimental determination of a complete four-quadrant map for a three stage low-speed axial flow compressor. Non-dimensional pressure rise and torque coefficients and efficiency characteristics were determined. Six possible modes of operation were identified in the four quadrants. The characteristic for a compressor with a rotational speed of zero rpm, in terms of pressure rise coefficient and flow coefficient, is S-shaped and forms the dividing line between two alternate modes of operation which exist in the second and fourth quadrants. First quadrant operation is only possible during positive rotation, and third quadrant operation during negative rotation, as the zero-speed S-curve passes through the origin and so does not pass through these quadrants. Consequently, only one mode of operation is possible in each of these two quadrants. A system of notation for describing the various modes of operation has also been developed. Second quadrant operation for positive compressor rotation, and fourth quadrant operation with negative rotation are highly dissipative conditions of operation. First and third quadrant operation are compressor-like modes. Second quadrant negative rotation and fourth quadrant positive rotation operation are turbine-like modes.
Research on oil-free hermetic refrigeration scroll compressor
Authors: Wang L , Zhao Y , Li L , Bu G , Shu P
Published: October 2007

A prototype of the oil-free hermetic scroll compressor for the application of the refrigeration system of the aviation devices was constructed. The performance of this prototype oil-free scroll compressor was simulated using a thermodynamic model, and was measured. And it was concluded that the prototype oil-free scroll compressor is useful for the refrigeration system of the aviation devices. The simulated results were consistent with the measured ones within less than 10 per cent of the mean deviation. When the evaporating and condensing temperatures were 5 and 55°C, respectively, the volumetric efficiency, discharge temperature, and coefficient of performance were 0.61, 135°C, and 1.45, respectively. The prototype oil-free scroll compressor had been operating reliably for more than 200 h under 85.7 per cent duty cycle and 20 h for 100 per cent duty cycle.
Real-time commercial availability index of a steam power plant: Graph theory and matrix method
Authors: Mohan M , Gandhi O. P. , Agrawal V. P.
Published: October 2007

Worldwide, the structure of the power industry is changing to that of a market economy to ensure commercial accountability. Resources, therefore, are allocated for operation and maintenance from commerical considerations rather than technical alone, necessitating the introduction of a commercial approach to analysis of reliabilty, efficiency, and maintenance criticality for power plants. These requirements suggest the need to introduce performance monitoring systems in the form of a single composite index encompassing the consequences of non-performance with respect to all these parameters and which can effectively respond to market demands. Such performance monitoring system is developed in the current paper.

Graph theory has been applied to the complex system of a steam power plant to consider its structure explicitly and to derive reliability/availability, efficiency, and maintenance indices. Its representation at system level is modeled by system structure graphs to study reliability/efficiency indices (viz. RTRI and RTEI). The approach has already been applied by the authors for the development of an efficiency index, i.e. RTEI, and a maintenance criticality index to optimize the maintenance cost. In the current paper, it is applied to develop a single composite index, i.e., a real-time commercial availability index (RTAI)com. The various attributes that affect commercial availability are identified as also the interactions among them. With these, a commercial availability attributes' digraph is developed and its corresponding variable permanent commercial availability attribute interaction matrix is written. The permanent function of this matrix is then analysed to study the various aspects of commercial availability as well as to evaluate (RTAI)com. A scheme for implementation of the procedure to develop a single composite index has also been proposed. This will monitor the health of the mechanical equipment of a steam power plant as Supervisory Control and Data Acquisition (SCADA)® monitors its electrical equipment.
Prediction of hygro—thermal stress distribution in proton exchange membranes using a three-dimensional multi-phase computational fluid dynamics model
Authors: Al-Baghdadi M A R Sadiq , Al-Janabi H A K Shahad
Published: October 2007

A three-dimensional, multi-phase, non-isothermal computational fluid dynamics model of a proton exchange membrane fuel cell has been developed to simulate the hygro and thermal stresses in polymer membrane, which developed during the cell operation. The behaviour of the membrane during the operation of a unit cell has been studied and investigated. The model accounts for both gas and liquid phase in the same computational domain, and thus allows for the implementation of phase change inside the gas diffusion layers. The model includes the transport of gaseous species, liquid water, protons, energy, and water dissolved in the ion-conducting polymer. The new feature of the present model is to incorporate the effect of hygro and thermal stresses into actual three-dimensional, multi-phase, non-isothermal fuel cell model. In addition to hygro—thermal stresses, the model features an algorithm that allows for a more realistic representation of the local activation overpotentials, which leads to improved prediction of the local current density distribution in high accuracy, and therefore, high accuracy prediction of temperature distribution in the cell and then thermal stresses. This model also takes into account convection and diffusion of different species in the channels as well as in the porous gas diffusion layer, heat transfer in the solids as well as in the gases, and electrochemical reactions.
Contributions of self-absorption and soot on radiation heat transfer in a laminar methane—air diffusion flame
Authors: Datta A , Saha A
Published: October 2007

A numerical study is performed to understand the contributions of self-absorption and soot radiation in a laminar methane—air diffusion flame. Simplified models have been adopted for the combustion and soot chemistry. A gray radiation model was developed using the first-order moment method to evaluate the emission and absorption of radiation. The absorption coefficient is determined considering (i) the influence of the gaseous (CH4, CO2, H2O, and CO) species and soot (taken as the benchmark results for the current study), and (ii) only the influence of the gaseous species. A comparison of the two results identifies the contribution of soot on radiation. An optically thin radiation model is also used to estimate the contribution of self-absorption in the flame. Finally, a radiatively adiabatic treatment in the model is employed to account the total effect of radiation. The effects of each contribution have been ascertained by looking at the variation in the radiative heat loss and the resulting temperature distribution of the flame. Differences in the soot volume fraction and thermal NO formation with the different radiative treatments have also been studied.

Radiation is found to have very important role on the overall flame structure as well as on the soot formation and NO emission in a methane—air diffusion flame. Neglecting soot in the calculation of radiation underpredicts the radiative heat loss from the flame, particularly in the soot-laden region. Though the effect on the macroscopic flame shape is marginal, the peak temperature difference at the flame tip is found to be substantial. On the contrary, the predictions using the optically thin approximation are very close to the benchmark results.
Parametric simulation of steam injected gas turbine combined cycle
Authors: Srinivas T , Gupta A V S S K S , Reddy B V
Published: October 2007

In the current work, thermodynamic analysis has been conducted for a steam injected gas turbine (STIG) in the combined cycle system with the dual pressure heat-recovery steam generator. Effect of operating variables such as low-pressure (LP) steam temperature ratio, steam reheat pressure ratio, steam turbine inlet pressure, gas cycle pressure ratio and combustion chamber temperature on the efficiency of the combined cycle has been investigated. Exergy efficiency of the cycle is compared with and without the steam injection with respect to the above-mentioned parameters.

Maximum mass ratio of steam injection to fuel has been examined as 6 kg/kg fuel with the complete combustion of the fuel due to excess air supply in the combustion chamber and gas reheater. Results are plotted from the case of without steam injection to the maximum possible steam ratio. LP temperature ratio is identified as a dominant parameter having impact on the efficiency of the combined cycle as the steam is injected at this pressure. Component exergetic losses of the combined cycle as a fraction of exergy of fuel are compared with and without the steam injection. The results showed that the major exergetic loss in the combustion chamber decreased with the steam injection.
Aerodynamic effects of forward blade skew in axial flow rotors of controlled vortex design
Authors: Vad J , Kwedikha A. R A. , Horváth Cs , Balczó M , Lohász M. M. , Régert T
Published: October 2007

Comparative studies have been carried out on two axial flow fan rotors of controlled vortex design (CVD), at their design flowrate, in order to investigate the effects of circumferential forward skew on blade aerodynamics. The studies were based on computational fluid dynamics (CFD), validated on the basis of global performance and hot wire flow field measurements. The computations indicated that the forward-skewed blade tip modifies the rotor inlet condition along the entire span, due to its protrusion to the relative inlet flow field. This leads to the rearrangement of spanwise blade load distribution, increase of losses along the dominant part of span, and converts the prescribed spanwise blade circulation distribution towards a free vortex flow pattern. Due to the above, reduction in both total pressure rise and efficiency was established. By moderation of the radial outward flow on the suction side, being especially significant for non-free vortex blading, forward sweep was found to be particularly useful for potential reduction of near-tip loss in CVD rotors. Application of reliable CFD-based design systems was recommended for systematic consideration and control of both load- and loss-modifying effects due to non-radial blade stacking.
Characteristics of linear compressors under current source excitation
Authors: Wang J , Lin Z , Howe D
Published: October 2007

The current paper analyses the steady-state and dynamic performance characteristics of a direct-drive linear vapour compressor, and it is shown that it operates most efficiently and stably at resonance under current source excitation. Further, the piston amplitude and top clearance vary almost linearly with the amplitude of the excitation current. These features facilitate efficient operation and control of the linear compressor without the need for a discrete piston position sensor, and provide a simple means of regulating the cooling capacity according to the refrigerator demand. The findings are validated by measurements on a prototype linear compressor.
Model of real absorption heat pump cycle with a generalized heat transfer law and its performance
Authors: Qin X , Chen L , Sun F , He L
Published: October 2007

An irreversible four-temperature-level absorption heat pump cycle model with a generalized heat-transfer law of Q θ Δ ( T n) ( n ≠ 0) is established, which considers effects of heat resistances, heat leakages, and internal irreversibility. The general relation between the coefficient of performance and the heating load is deduced. The fundamental optimal relation and the performance limit, the optimal temperatures of working fluid, as well as the optimal heat-transfer surface area distributions with linear phenomenological heat-transfer law are derived analytically. Moreover, effects of heat-transfer law, heat leakage, and internal irreversibility on the performance of absorption heat pumps are analysed; and the performance comparison is performed for the distribution of the total heat-transfer surface area is optimized or not by numerical example. The results obtained herein are useful for optimal design and performance improvement of absorption heat pump cycles.
Numerical and experimental study of unsteady flow in a centrifugal fan
Authors: Younsi M , Bakir F , Kouidri S , Rey R
Published: October 2007

In the current work, computational fluid dynamics (CFD) technique coupled with experimental investigations has been used in order to study the complex phenomena related to the unsteady flow in a centrifugal fan.

The studied phenomena are the interactions and unsteadiness induced by the rotating blades motion relatively to the volute and their impact on the aeroacoustic behaviour of the fan. Thus, three-dimensional and two-dimensional unsteady simulations have been carried out using unsteady Reynolds averaged Navier-Stokes approach. Turbulence has been modelled with the k-ω-shear stress model and a sliding mesh technique has been applied to the interfaces in order to allow unsteady interactions between the rotating (impeller) and the stationary zones (volute casing). The overall performances predicted by the computations have been validated at different flowrates.

In order to locate the flow unsteadiness and perturbations, the near field wall pressure fluctuations at different strategic points on the volute surface have been computed. In addition to this, the unsteady flow variables provided by the CFD calculations have been used as inputs in the Ffowcs Williams-Hawkings equation in order to estimate the tonal noise generated by the centrifugal fan. Finally, the numerical results have been compared with the experimental measurements and a correlation between the wall pressure fluctuations and the far field noise signals has been found.
Exergoeconomic analysis of small industrial pasta drying systems
Authors: Ozgener L
Published: October 2007

In the current study, author performs an exergoeconomic analysis of an industrial pasta final drying process. The relations between capital costs and thermodynamic losses for the devices in the system are investigated. Thermodynamic loss rate-to-capital cost ratios are used to show that, for the devices and the overall system, a systematic correlation appears to exist between capital cost and exergy loss (total or internal), but not between capital cost and energy loss or external exergy loss. This correlation may imply that devices in a successful industrial pasta drying system is configured so as to achieve an overall optimal design, by appropriately balancing the thermodynamic (energy- and exergy-based) and economic (cost) characteristics of the overall system. Thermodynamic loss rate to capital cost values Ren and Rex are obtained as 0.016-0.004, while total energy rate input value to system change between 304.85 and 316.25 kW. Energetic and exergetic efficiencies of the system processes are determined in an attempt to assess their individual performances. The energy and exergy efficiencies of the overall system are found to be as 72.1 and 65.4 per cent, respectively.
Letter: Specific Isotope Labeling for the Identification of Free N-Terminal Peptides of Proteins Separated by Polyacrylamide Gel Electrophoresis
Authors: Sanchez Aniel , Ramos Yassel , Solano Yanni , González Luis Javier , Betancourt Lazaro , Gil Jeovanis , Padron Gabriel , Besada Vladimir
Published: October 2007

We report here a method for the identification of free N-terminal peptides of in gel digested isolated proteins. It is based on the difference between the isotopic ion distribution of the N-terminal peptide and internal peptides. After guanidination of lysine residues, the primary amino groups of the gel-entrapped protein are blocked with an equimolar mixture of normal and deuterated acetic anhydride. Upon MS analysis, internal peptides display a normal isotopic ion distribution while the N-terminal peptide shows a complex isotopic ion distribution.
Space Charge Effect in Spectrometers of Ion Mobility Increment with a Cylindrical Drift Chamber
Authors: Elistratov A.A. , Sherbakov L.A.
Published: October 2007

We have amplified the model for the drift of ions under a non-uniform, high-frequency electric field by taking space charge effect into account. By this means, we have investigated the effect of space charge on the dynamics of a single type of ions in a spectrometer of ion mobility increment with a cylindrical drift chamber. The counteraction of the space charge effect and the focusing effect is investigated. The output ion current saturation caused by the effect of the space charge is observed. The shape of the ion peak, taking into consideration the space charge effect, has been obtained. We show that the effect of the space charge is sufficient for the relative ion density greater than 10 ppt by order of magnitude (for a cylindrical geometry spectrometer with typical parameters).
Monitoring Conformational Changes in Protein Complexes Using Chemical Cross-Linking and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry: The Effect of Calcium Binding on the Calmodulin—Melittin Complex
Authors: Novak Petr , Havlicek Vladimir , Derrick Peter J. , Beran Kyle A. , Bashir Sajid , Giannakopulos Anastassios E.
Published: October 2007

Calmodulin is an EF hand calcium binding protein. Its binding affinities to various protein/peptide targets often depend on the conformational changes induced by the binding of calcium. One such target is melittin, which binds tightly to calmodulin in the presence of calcium, and inhibits its function. Chemical cross-linking combined with Fourier transform ion cyclotron resonance mass spectrometry has been employed to investigate the coordination of calmodulin and melittin in the complex at different concentrations of calcium. This methodology can be used to monitor structural changes in proteins induced by ligand binding and to study the effects these changes have on non-covalent interactions between proteins. Cross-linking results indicate that the binding place of the first melittin in the calcium-free calmodulin form is the same as in the calcium-loaded calmodulin/melittin complex.
Combining a Capillary with a Radio-Frequency-Only Quadrupole as an Interface for a Home-Made Time-of-Flight Mass Spectrometer
Authors: Guo Changjuan , Huang Zhengxu , Gao Wei , Nian Huiqing , Chen Huayong , Fu Jiamo , Zhou Zhen
Published: October 2007

A heated capillary tube combined with a radio-frequency-only quadrupole has been coupled with a home-made, high-resolution orthogonal-injection, time-of-flight mass spectrometer to improve ion transmission from the atmospheric pressure to the low-pressure regions. With an electrospray ion source, the performance of the interface on the intensity of spectra was investigated. For electrospray ionization, the ion intensity detected on the time-of-flight mass spectrometer was seen to increase three-fold compared with an orifice interface. It has been shown that the enhanced ion inlet designs can not only increase the ion translation efficiency, but also improve the detection limits of the mass spectrometer. Coupling atmospheric pressure matrix-assisted laser desorption/ionization with the improved interface resulted in an instrument detection limit as low as 2.5 fmol.