Picard–Fuchs Equations and Gauss–Manin Systems with a View Towards the Riemann–Hilbert Problem

We study Abelian integrals associated with a tame polynomial function and their Picard–Fuchs equations using the theory of algebraic Gauss–Manin systems. Especially, we look for a basis of the Petrov module, in which the Picard–Fuchs equations become as simple as possible. As an application, we discuss the related Riemann–Hilbert problem and prove that it has a positive answer under some conditions. In this case, we compute the Jordan normal form of the residue matrices of the corresponding Fuchsian system in terms of local data. 2000 Mathematics Subject Classification. 32S40, 34C20.     

Hydrogen embrittlement associated with strain localization in a precipitation-hardened Fe–Mn–Al–C light weight austenitic steel

Hydrogen embrittlement of a precipitation-hardened Fe–26Mn–11Al-1.2C (wt.%) austenitic steel was examined by tensile testing under hydrogen charging and thermal desorption analysis. While the high strength of the alloy (>1 GPa) was not affected, hydrogen charging reduced the engineering tensile elongation from 44 to only 5%. Hydrogen-assisted cracking mechanisms were studied via the joint use of electron backscatter diffraction analysis and orientation-optimized electron channeling contrast imaging. The observed embrittlement was mainly due to two mechanisms, namely, grain boundary triple junction cracking and slip-localization-induced intergranular cracking along micro-voids formed on grain boundaries. Grain boundary triple junction cracking occurs preferentially, while the microscopically ductile slip-localization-induced intergranular cracking assists crack growth during plastic deformation resulting in macroscopic brittle fracture appearance.     

A new type of a gas–steam turbine cycle with increased efficiency

A new system with a gas–steam turbine particularly effective for district heating systems is proposed. Compared to the existing combined installations, including a gas turbine, a waste heat utilization and a steam turbine, it shows significantly lower capital cost and an increase of the thermodynamic efficiency. The simultaneous expansion of gas and steam in one turbine, as well as the utilization of the heat of condensation of the waste gases at the installation outlet allows the overall thermodynamic efficiency, calculated on the basis of the lower calorific value, to reach 108.7%.     

High glass forming ability correlated with microstructure and hydrogen storage properties of a Mg–Cu–Ag–Y glass

Thermal characterization of an as-cast Mg₅₄Cu₂₈Ag₇Y₁₁ bulk metallic glass revealed that this alloy exhibits excellent glass forming ability. High-resolution X-ray diffraction study and transmission electron microscopy show that heating and isothermal annealing treatment results in the nucleation of nanocrystals of several phases. The average size of these nanocrystals (∼15–20 nm) only slightly varies with prolonged annealing, only their volume fraction increases. High-pressure calorimetry experiments indicate that the as-cast fully amorphous alloy exhibits the largest enthalpy of hydrogen desorption, compared to partially and fully crystallized states. Since the fully crystallized alloy does not desorb hydrogen, it is assumed that hydrogen storage capacity correlates only with the crystalline volume fraction of the partially crystallized Mg₅₄Cu₂₈Ag₇Y₁₁ BMG and additional parameters (crystalline phase selection, crystallite size, average matrix concentration) do not play a significant role.     

Design of a compact absorber with a hydrophobic membrane contactor at the liquid–vapor interface for lithium bromide–water absorption chillers

In this study, design of a compact plates-and-frames absorber possessing a hydrophobic microporous membrane contactor at the aqueous solution–water vapor interface is performed analytically. The absorber is a component of a 5 kW cooling capacity single-effect lithium bromide–water absorption chiller that incorporates a hot water thermally driven generator and a water-cooled absorber and condenser. Good agreement prevailed for the analytically evaluated water vapor mass transfer flux and aqueous solution outlet temperature when compared with measured values at similar operating conditions. At design point conditions, the main design parameters obtained are a membrane contactor area of 6.06 m², a ratio of the mass transfer area to absorber net volume (_Am/_Vnet$A_m/V_{\mathit{net}}$) of 130.1(m²/m³), and ratio of the membrane area (mass transfer area) in this design configuration to the area required for heat transfer is 1.162, respectively. The results clearly indicate that the aqueous solution channel thickness is the most significant design parameter that affects the absorber size compactness; the thinner the thickness of the solution channel, the higher the ratio (_Am/_Vnet$A_m/V_{\mathit{net}}$). The results also show the countercurrent refrigerant flow with the aqueous solution has positive effects on the absorber size compactness.     

Bifurcation Behaviors Analysis on a Predator–Prey Model with Nonlinear Diffusion and Delay

In this paper, a class of predator–prey model with nonlinear diffusion and time delay is considered. The stability is investigated and Hopf bifurcation is demonstrated. Applying the normal form theory and the center manifold argument, we derive the explicit formulas for determining the properties of the bifurcating periodic solutions. Some numerical simulations for justifying the theoretical analysis are also provided. Finally, main conclusions are included.     

MHD mixed convection of Cu–water nanofluid in a two-sided lid-driven porous cavity with a partial slip

ABSTRACT

A numerical simulation of magneto-hydrodynamic mixed convection flow and heat transfer of Cu–water nanofluid in a square cavity filled with a Darcian porous medium with a partial slip is numerically investigated. The left and right walls of the cavity are moving up with a constant speed in vertical direction, and the partial slip effect is considered along these walls. The top and bottom walls of the cavity are assumed to be adiabatic. The right vertical wall of the cavity is assumed to be kept at a lower temperature, while the left vertical wall is kept at a higher temperature. The developed equations of the mathematical model are nondimensionalized and then solved numerically subject to appropriate boundary conditions by the finite-volume method. A parametric study is performed and a set of graphical results is presented and discussed to demonstrate interesting features of the solution.     

Analysis of flows driven by a torsionally–oscillatory lid in a fluid-saturated porous enclosure with thermal stable stratification

The convection flow caused by a torsionally–oscillatory lid with thermal stable stratification in an enclosure filled with porous medium is studied in this paper. The governing continuity, momentum and energy transport equations are solved by a semi-implicit projection finite element method. The Grashof number and Reynolds number for ranges 10⁴⩽Gr⩽10⁶, 10²⩽Re⩽10³ are involved in this research for Darcy number 10⁻², 10⁻⁴, 10⁻⁶ and porosity 0.4, 0.6. The results present that the permeability of porous medium deeply dominates the flow field and diminishes the flow strength as the permeability is decreased. The increase in the value of Gr/(Re²·Da) retards the stable stratification from the top surface to extend into the interior domain. The influence of oscillatory frequency is so serious in heat flux variation at some particular frequency corresponding to the resonant frequency. There is an evident resonant frequency at Da=10⁻² condition; however, this phenomenon is not clear for Da⩽10⁻⁴ at different oscillatory frequencies.     

Hydrogen generation from hydrolysis of aluminum/graphite composites with a core–shell structure

Hydrogen generated by hydrolysis of metal aluminum with water is promising for portable fuel cell applications. However aluminum would not react with water to yield hydrogen at ordinary conditions due to the passive oxide film formed on its surface. In the present investigation, the aluminum/graphite composite were prepared by a ball milling process in an attempt to improve the reactivity of aluminum, using sphere-shape aluminum particles and laminate graphite as the initial materials and 2 wt% NaCl as the milling-assisted agent. The TEM observation showed that the Al particles are covered by graphite to form a core–shell structure. Such a Al/graphite composite material exhibited a pronounced hydrolysis reactivity with tap water to generate hydrogen while Al alone did not react with water. The presence of graphite could lower the hydrogen generation reaction temperature below 45 °C. Increasing the reaction temperature could obtain an increased hydrogen generation rate and the maximum hydrogen generation rate of 40 cm³ min⁻¹ g⁻¹ Al was obtained when the reaction temperature was increased to 75 °C. Prolonging milling time could also improve the Al hydrolysis reactivity in the composite particularly at a relatively low temperature. The XRD results identified that the hydrolysis byproducts are bayerite (Al(OH)₃) and boehmite (AlOOH). The microstructure-related hydrolysis reaction mechanism was finally proposed.     

Entropy generation in a hollow cylinder with temperature dependent thermal conductivity and internal heat generation with convective–radiative surface cooling

This article investigates entropy generation in an asymmetrically cooled hollow cylinder with temperature dependent thermal conductivity and internal heat generation. The inside surface of the cylinder is cooled by convection on its inside surface while the outside surface experiences simultaneous convective–radiative cooling. The thermal conductivity of the cylinder as well as the internal heat generation within the cylinder are linear functions of temperature, introducing two nonlinearities in the one-dimensional steady state heat conduction equation. A third nonlinearity arises due to radiative heat loss from the outside surface of the cylinder. The nonlinear system is solved analytically using the differential transformation method (DTM) to obtain the temperature distribution which is then used to compute local and total entropy generation rates in the cylinder. The accuracy of DTM is verified by comparing its predictions with the analytical solution for the case of constant thermal conductivity and constant internal heat generation. The local and total entropy generations depend on six dimensionless parameters: heat generation parameter Q, thermal conductivity parameter β, conduction–convection parameters Nc₁ and Nc₂, conduction–radiation parameter Nr, convection sink temperature δ and radiation sink temperature η.     

Performance and emission characteristics of a diesel engine fueled with coconut oil–diesel fuel blend

The objective of the present study is to reveal the effects of pure coconut oil and coconut oil–diesel fuel blends on the performance and emissions of a direct injection diesel engine. Operation of the test engine with pure coconut oil and coconut oil–diesel fuel blends for a wide range of engine load conditions was shown to be successful even without engine modifications. It was also shown that increasing the amount of coconut oil in the coconut oil–diesel fuel blend resulted in lower smoke and NO_x emissions. However, this resulted in an increase in the BSFC. This was attributed to the lower heating value of neat coconut oil fuel compared to diesel fuel.     

Partially-Averaged Navier–Stokes method with modified k–ε model for cavitating flow around a marine propeller in a non-uniform wake

Unsteady cavitating turbulent flow simulations need to be responsible for both cavitation and turbulence modeling issues. The Partially-Averaged Navier–Stokes (PANS) computational model developed from the RANS method and the k–ε turbulence model are used to model turbulent cavitating flow with a mass transfer cavitation model in the present paper. An objective of this study is to pursue more accurate estimates of unsteady cavitating flows with large-scale fluctuations at a reasonable cost. Firstly, the unsteady cavitating flow simulations over a NACA66-mod hydrofoil are performed using the PANS method with various values of the resolution control parameters (f_k = 1 ～ 0.2, f_ε = 1) to evaluate the numerical methods based on experimental data. The comparison with the experiments show that the numerical analysis with a f_k = 0.2 can predict the cavity evolution and shedding frequency fairly well. Then, cavitating flow around a marine propeller in non-uniform wake was simulated by PANS method. The calculations show that large cavity volume pulsation as the blade passes through the wake region is resolved better by the PANS method with f_k = 0.2 than by the RANS method with the k–ε or k–ω SST turbulence models. This can be contributed to the fact that a smaller f_k give larger cavity volume pulsations leading to increased cavity volume accelerations and larger pressure fluctuations above the propeller, while a larger f_k overestimates the turbulent viscosity along the rear part of the cavity. Finally, it is confirmed from the simulation by the PANS method with f_k = 0.2 that the whole process of cavitating flow evolution around the propeller in non-uniform wake can be very well reproduced including cavitation inception, sheet cavitation and tip vortex cavitation observed experimentally.     

Economic efficiency of a renewable energy independent microgrid with energy storage by a sodium–sulfur battery or organic chemical hydride

The present study uses numerical analysis to investigate the operating methods and costs of an independent microgrid incorporating a sodium–sulfur (NAS) battery or an energy storage system using organic hydrides. Details relating to the operation of the system and its installed capacity and cost were clarified, assuming the introduction of an independent microgrid in Kitami City, a cold region in Japan. Analysis results indicate that energy storage technology using the organic hydride system is economically inferior to the NAS battery owing to large losses associated with the water electrolyzer and dehydration reactor. Therefore, for the widespread use of the organic hydride system, it is necessary to improve the efficiency of these components.     

Sam＇s Choice： building a cabin with iron or a castle with sand？

The road of economic recovery in America has been a long and bumpy one. Yet all the economic indicators have been pointing at the improving and healthy conditions the state of the economy has, for already quite some time. And many are predicting economic growth to gather more steam in the second semester, with more job creation and sounder finances.     

Experimental investigation of heat transfer and friction factor with water–propylene glycol based CuO nanofluid in a tube with twisted tape inserts

Convective heat transfer and friction factor characteristics of water/propylene glycol (70:30% by volume) based CuO nanofluids flowing in a plain tube are investigated experimentally under constant heat flux boundary condition. Glycols are normally used as an anti-freezing heat transfer fluids in cold climatic regions. Nanofluids are prepared by dispersing 50 nm diameter of CuO nanoparticles in the base fluid. Experiments are conducted using CuO nanofluids with 0.025%, 0.1% and 0.5% volume concentration in the Reynolds numbers ranging from 1000 < Re < 10000 and considerable heat transfer enhancement in CuO nanofluids is observed. The effect of twisted tape inserts with twist ratios in the range of 0 < H/D < 15 on nanofluids is studied and further heat transfer augmentation is noticed. The increment in the pressure drop in the CuO nanofluids over the base fluid is negligible but the experimental results have shown a significant increment in the convective heat transfer coefficient of CuO nanofluids. The convective heat transfer coefficient increased up to 27.95% in the 0.5% CuO nanofluid in plain tube and with a twisted tape insert of H/D = 5 it is further increased to 76.06% over the base fluid at a particular Reynolds number. The friction factor enhancement of 10.08% is noticed and increased to 26.57% with the same twisted tape, when compared with the base fluid friction factor at the same Reynolds number. Based on the experimental data obtained, generalized regression equations are developed to predict Nusselt number and friction factor.     

Investigation of buoyancy-driven flow and heat transfer in a trapezoidal cavity filled with water–Cu nanofluid

A numerical study is conducted to investigate the transport mechanism of free convection in a trapezoidal enclosure filled with water–Cu nanofluid. The horizontal walls of the enclosure are insulated while the inclined walls are kept at constant but different temperatures. The numerical approach is based on the finite element technique with Galerkin's weighted residual simulation. Solutions are obtained for a wide range of the aspect ratio (AR) and Prandtl number (Pr) with Rayleigh number (Ra = 10⁵) and solid volume fraction (? = 0.05). The streamlines, isotherm plots and the variation of the average Nusselt number at the left hot wall are presented and discussed. It is found that both AR and Pr affect the fluid flow and heat transfer in the enclosure. A correlation is also developed graphically for the average Nusselt number as a function of the Prandtl number as well as the cavity aspect ratio.     

FC/UC hybridization for dynamic loads with a novel double input DC–DC converter topology

Due to the fact that the environmental issues have become more serious recently, interest in renewable energy systems, such as, fuel-cells (FCs) has increased steadfastly. Among many types of FCs, proton exchange membrane FC (PEMFC) is one of the most promising power sources due to its advantages, such as, low operation temperature, high power density and low emission. However, using sole PEMFC for dynamic loads may not be feasible to satisfy the peak demand changes. Therefore, hybridizing PEMFC and an energy storage system (ESS) decreases the FC cost and improves its performance and life. Ultra-capacitor (UC) is the most powerful candidate to hybridize with PEMFC for dynamic loads. The DC–DC converter is the key enabling technology for hybridization of PEMFC and UC. Generally, the efficiency and performance of hybridization is largely limited by the converter topology employed for the mentioned hybridization. Integrating each source (PEMFC and UC) with a DC–DC converter is not feasible in terms of cost, performance, and control. Due to the above mentioned reasons, an attractive converter topology which can combine PEMFC and UC is strongly required. In this regard, the objective of this study is to design and simulate a novel double input DC–DC converter based on current additivity concept, in order to combine two different types of energy systems (PEMFCs and UCs).