Coupled finite-element/state-space modeling of turbogenerators inthe abc frame of reference-the short-circuit and load cases includingsaturated parameters

In this paper, a coupled finite-element/state space modeling technique is applied in the determination of the steady-state parameters of a 733 MVA turbogenerator in the abc frame of reference. In this modeling environment, the forward rotor stepping-finite element procedure described in a companion paper is used to obtain the various machine self and mutual inductances under short-circuit and load conditions. A fourth-order state-space model of the armature and field winding flux linkages in the abc frame of reference is then used to obtain the next set of flux linkages and forcing function currents for the finite-element model. In this process, one iterates between the finite-element and state-space techniques until the terminal conditions converge to specified values. This method is applied to the determination of the short-circuit, and reduced- and rated-voltage load characteristics, and the corresponding machine inductances. The spatial harmonics of these inductances are analyzed via Fourier analysis to reveal the impact of machine geometry and stator-to-rotor relative motion, winding layout, magnetic saturation, and other effects. In the full-load infinite-bus case, it is found that, while the three-phase terminal voltages are pure sinusoidal waveforms, the steady-state armature phase currents are nonsinusoidal and contain a substantial amount of odd harmonics which cannot be obtained using the traditional two-axis analysis     

A combined finite element-state space modeling environment forinduction motors in the ABC frame of reference: the no-load condition

A combined finite element state-space modeling environment capable of predicting no-load induction motor performance is introduced. The authors focus on the model derivation and the no-load simulation. The model is based on the natural ABC frame of reference and includes full effects of discrete winding layouts, magnetic saturation, and space harmonics. The model does not require the existence of actual motor hardware. The method is totally flexible and allows the design engineer to answer what-if questions before any resources are committed to prototyping. The method was shown to produce motor parameter results which correlate very well with corresponding test data of an example 1.2 hp 200 V three-phase induction motor     

Theoretical and finite-element modeling of autofrettage process in strain-hardening thick-walled cylinders

The optimum autofrettage pressure and the optimum radius of the elastic–plastic boundary of strain-hardening cylinders in plane strain and plane stress have been studied theoretically and by finite-element modeling. Equivalent von-Mises stress is used as yield criterion. Comparison of the results of the two methods shows good agreement. Although there is no explicit expression for the optimum autofrettage pressure in plane stress, the equation for plane strain can be used with good accuracy. It has also been observed that the optimum autofrettage pressure is not a constant value but depends on working pressure.     

Coupled electromagnetic acoustic and thermal-flow modeling of an induction motor of railway traction

In order to optimize the design of an enclosed induction machine of railway traction, a multi-physical model is developed taking into account electromagnetic, mechanical and thermal-flow phenomena. The electromagnetic model is based on analytical formulations and allows calculating the losses. The thermal-flow modeling is based on an equivalent thermal circuit which has the feature to consider the flow structure inside the machine. In this way, a numerical study has been carried out to evaluate this internal flow structure depending on the rotational speed. The results of the multi-physical model are confronted with experimental results.     

Gas-phase modeling of homogeneous boron/oxygen/hydrogen/carbon combustion

Boron has practical applications as an advanced fuel in propulsion systems due to its high energy content. The combustion of boron in the presence of hydrocarbon fuels is a complex problem involving heterogeneous particle oxidation followed by gas-phase kinetics of the volatilized boron species. In this study, we have modeled the high-temperature gas-phase combustion chemistry of the B/O/H/C system. We have examined the effects of recent experimental gas-phase kinetic measurements of several of the critical reaction rates and theoretical thermodynamic and transition state calculations on the previous model of boron combustion. Additional reactions that critically affect the combustion efficiency are identified for future experimental and theoretical study. The role of boron oxyhydrides, which are metastable species, is discussed.     

3D hybrid Cartesian/immersed-boundary finite-element analysis of heat and flow patterns in a two-roll mill

A fully three-dimensional time-dependent Navier–Stokes model with forced convection is developed to numerically investigate the heat and flow patterns of the two-roll mill system with two inner rotating cylinders. Such direct numerical simulations are usually limited by the difficulties from huge computational cost and complex boundary treatment. For a fast numerical process, we can use the operator-splitting scheme with the BTD term to advance the solution in temporal evolution. To implement the calculation over a Cartesian grid, the hybrid Cartesian/immersed-boundary finite-element method is employed for spatial discretization. In the authors’ previous study [D.L. Young, C.L. Chiu, C.M. Fan, A hybrid Cartesian/immersed-boundary finite-element method for simulating heat and flow patterns in a two-roll mill, Numer. Heat Transfer B 51 (3) (2007) 251–274], we have developed a simplified 2D numerical model to analyze the heat and flow patterns on the cross section of two-roll-mill flow under the assumption of infinite length in the third (vertical) direction. However, the 2D solutions could not completely represent the realistic physical phenomena unless a 3D algorithm is developed. In this study we then paid the particular attention to develop a 3D model to investigate the vertical heat and flow behaviors, including 3D features of the vortex structure, periodic oscillation and chaotic instabilities. It is found that the proposed 3D model is able to cover the 2D features if the assumptions of 2D conditions are fulfilled.     

Dynamic modeling of a hybrid wind/solar/hydro microgrid in EMTP/ATP

Microgrids are LV or MV electric networks which utilize various distributed generators (DG) to serve local loads. In this paper, dynamic models of the main distributed generators including photovoltaic (PV) cell, wind turbine, hydro turbine as well as the equivalent power electronic interfaces, battery unit of PV and excitation system of hydro turbine have been made in ElectroMagnetic Transient Program/Alternative Transient Program (EMTP/ATP) software package. Control strategies based on active power/frequency and reactive power/voltage droops for the power control of the inverters have been also developed. Case studies have been carried out in a distribution network to investigate the dynamic behavior of the micro-sources in both steady state and fault scenarios. Simulation results verify the feasibility of the proposed models.     

Multiscale modeling of thermal conductivity of polymer/carbon nanocomposites

Molecular dynamics simulation was used to estimate the interfacial thermal (Kapitza) resistance between nanoparticles and amorphous and crystalline polymer matrices. Bulk thermal conductivities of the nanocomposites were then estimated using an established effective medium approach. To study functionalization, oligomeric ethylene–vinyl alcohol copolymers were chemically bonded to a single wall carbon nanotube. The results, in a poly(ethylene–vinyl acetate) matrix, are similar to those obtained previously for grafted linear hydrocarbon chains. To study the effect of non-covalent functionalization, two types of polyethylene matrices. -- aligned (extended-chain crystalline) vs. amorphous (random coils) were modeled. Both matrices produced the same interfacial thermal resistance values. Finally, functionalization of edges and faces of plate-like graphite nanoparticles was found to be only modestly effective in reducing the interfacial thermal resistance and improving the composite thermal conductivity.     

State-of-the-art review of erosion modeling in fluid/solids systems

Erosion in fluidized-bed combustors, commercial process units used to burn coal cleanly, has surfaced as a serious issue that may have adverse economic effects. The evidence suggests that the key to understanding this erosion is detailed knowledge of the coupled and complex phenomena of solids circulation and bubble motion. The FLUFIX computer code has been developed for this purpose. Computed hydrodynamic results compare well with experimental data (including the bubble frequency and size and the time-averaged porosity and pressure distributions) taken in a thin ‘two-dimensional’ rectangular fluidized beds containing a rectangular obstacle and a few-tube approximation of the International Energy Agency Grimethorpe tube bank ‘C1’ configuration.

Six representative erosion models selected from the literature, comprising both single-particle and fluidized-bed models are critiqued. A methodology is described whereby the computed hydrodynamic results can be used with such erosion models. Previous attempts (none involving fluidized beds) to couple fluid mechanics and erosion models are reviewed. The energy dissipation models are developed, and are shown to generalize the so-called power dissipation model used to analyze slurry jet pump erosion. It is demonstrated, by explicitly introducing the force of the particle on the eroding material surface, that impaction and abrasive erosion mechanisms are basically the same. In doing so, it has been possible to unify the entire erosion literature developed for over a century.

Linkage is made to two previously developed single-particle erosion models: Finnie's and Neilson and Gilchrist's. The implementation methodology, which can be applied to any erosion model, be it single-particle or fluidized bed, is summarized. The monolayer energy dissipation (MED) erosion model is developed. The erosion rates computed from the EROSION code are compared with each other and for the cold few-tube approximation of the IEA Grimethorpe tube bank ‘C1’ fluidized-bed experiment, and with other available erosion data literature to validate the calculations. The simplified closed form MED (SCFMED) erosion models and erosion guidelines are developed using semi-empirical correlations in order to allow quick engineering estimates of erosion. Alternative methodologies to couple hydrodynamics and erosion using the kinetic theory of granular flow and discrete element method (DEM) models are briefly reviewed.

Finally, a critical review of the integrated experimental and computational fluid dynamics (CFD) pressurized fluidized-bed hydrodynamics and erosion research ongoing at Chalmers University is presented. This body of work has been influenced by the research at Argonne National Laboratory (ANL) and Illinois Institute of Technology (IIT) and reinforces the trends and conclusions reported in this review.     

A combined finite element-state space modeling environment forinduction motors in the ABC frame of reference: the blocked-rotor andsinusoidally energized load conditions

The combined finite element-state space (CFE-SS) modeling environment was used to predict the performance of a 1.2 hp, three-phase case-study squirrel cage induction motor under blocked rotor and typical load operating conditions. The nature of this CFE-SS environment allows one to rigorously account for the impact of space harmonics generated by the magnetic circuit, winding, and cage geometric, as well as layout peculiarities and magnetic saturation, on the current and torque profiles, and ohmic losses in the stator armature and cage. This includes the ability to predict the profiles of connector and bar currents. The results of the CFE-SS simulations compare favorably with blocked rotor and load experimental test data. Potential capabilities of this CFE-SS modeling environment, and its use in impacting motor design decisions, are discussed in the light of reported findings     

Thermal modeling of a cylindrical LiFePO4/graphite lithium-ion battery

A lumped-parameter thermal model of a cylindrical LiFePO₄/graphite lithium-ion battery is developed. Heat transfer coefficients and heat capacity are determined from simultaneous measurements of the surface temperature and the internal temperature of the battery while applying 2 Hz current pulses of different magnitudes. For internal temperature measurements, a thermocouple is introduced into the battery under inert atmosphere. Heat transfer coefficients (thermal resistances in the model) inside and outside the battery are obtained from thermal steady state temperature measurements, whereas the heat capacity (thermal capacitance in the model) is determined from the transient part. The accuracy of the estimation of internal temperature from surface temperature measurements using the model is validated on current-pulse experiments and a complete charge/discharge of the battery and is within 1.5 °C. Furthermore, the model allows for simulating the internal temperature directly from the measured current and voltage of the battery. The model is simple enough to be implemented in battery management systems for electric vehicles.     

Laminar flame speeds and kinetic modeling of hydrogen/chlorine combustion

In this study, laminar flame speeds at atmospheric pressure are accurately measured for H₂/Cl₂/N₂ mixtures at different equivalence ratios and N₂ mole fractions by the counterflow flame technique. A kinetic mechanism based on recently published and evaluated rate constants is developed to model these measured laminar flame speeds as well as the literature data on the concentrations of H₂, Cl₂, and HCl species in flat-burner flames and the ignition delay times from shock tube experiments. The kinetic model yields satisfactory comparison with these experimental data, and suggests that the reactions involving excited HCl(v) species and energy branching are not of substantial significance in combustion situations, and that the use of accurate elementary rate constants is instead crucial to the accuracy of the reaction mechanism.     

Microchannel reactor modeling for combustion driven reforming of iso-octane

Coupling of exothermic and endothermic reactions in parallel microchannels is investigated through parametric variation of geometric and material properties in the context of hydrogen production by steam reforming of iso-octane, the surrogate for gasoline. Heat required for the endothermic reforming reaction is provided by the catalytic combustion of methane, the model compound for natural gas. The combination of steam reforming and combustion is modeled for a microchannel reactor configuration in which reactions and heat transfer take place in parallel, micro-sized, square-shaped flow paths with wall-coated catalysts. Thickness of the wall between microchannels, side-length of the microchannels and channel texture (straight-through vs. micro-baffled) are the geometric parameters being studied, while the use of different materials of construction - α-alumina, AISI-steel and iron - is also investigated. Instead of a fully-fledged 3-D mathematical model, the parametric runs are performed on a 2-D unit cell model which is justified to give results close to that of the former. When the wall thickness is increased from 1 × 10⁻⁴ to 4 × 10⁻⁴ m, the hydrogen yield, defined as moles of hydrogen produced per mole of iso-octane fed, increases by 42% because axial heat conduction in the wall becomes more pronounced and spreads the energy released by combustion into the reforming channel more effectively. Increase in the yield is more remarkable (110%) when the channel side-length is doubled from 2.8 × 10⁻⁴ to 5.6 × 10⁻⁴ m. Use of micro-baffles also enhances the hydrogen yield: in the 1-4 × 10⁻⁴ m wall thickness range, the average increase is 16.5%, which is attributed both to the enhancement in heat transfer coefficients and axial conduction in the wall. As for the material property parameter, due to having the highest thermal conductivity, hence the ability of conducting heat axially, iron serves as the best wall material on the basis of highest hydrogen yield, compared with alumina and steel.     

An experimental and kinetic modeling study of n-butanol combustion

n-Butanol is a fuel that has been proposed as an alternative to conventional gasoline and diesel fuels. In order to better understand the combustion characteristics of n-butanol, this study presents new experimental data for n-butanol in three experimental configurations. Species concentration profiles are presented in jet stirred reactor (JSR) at atmospheric conditions and a range of equivalence ratios. The laminar flame speed obtained in an n-butanol premixed laminar flame is also provided. In addition, species concentration profiles for n-butanol and n-butane in an opposed-flow diffusion flame are presented. The oxidation of n-butanol in the aforementioned experimental configurations has been modeled using an improved detailed chemical kinetic mechanism (878 reactions involving 118 species) derived from a previously proposed scheme in the literature. The proposed mechanism shows good qualitative agreement with the various experimental data. Sensitivity analyses and reaction path analyses have been conducted to interpret the results from the JSR and opposed-flow diffusion flame. It is shown that the main reaction pathway in both configurations is via H-atom abstraction from the fuel followed by β-scission of the resulting fuel radicals. Several unimolecular decomposition reactions are important as well. This study gives a better understanding of n-butanol combustion and the product species distribution.     

Degradation modeling of InGaP/GaAs/Ge triple-junction solar cells irradiated with various-energy protons

Degradation modeling of InGaP/GaAs/Ge triple-junction (3J) solar cells subjected to proton irradiation is performed with the use of a one-dimensional optical device simulator, PC1D. By fitting the external quantum efficiencies of 3J solar cells degraded by 30 keV, 150 keV, 3 MeV, or 10 MeV protons, the short-circuit currents (I_SC) and open-circuit voltages (V_OC) are simulated. The damage coefficients of minority carrier diffusion length (K_L) and the carrier removal rate of base carrier concentration (R_C) of each sub-cell are also estimated. The values of I_SC and V_OC obtained from the calculations show good agreement with experimental values at an accuracy of 5%. These results confirm that the degradation modeling method developed in this study is effective for the lifetime prediction of 3J solar cells.     

基于BEM/FEM耦合技术的柴油机外声场模拟技术研究

采用BEM／FEM耦合技术对某高强化V6柴油机结构的声辐射情况进行了模拟,得到了整机辐射声功率、辐射效率、主要部件对声学结果的贡献以及近、远场声压(级)、整机外场声强矢量分布等声学结果,为该柴油机针对减振降噪的结构声学优化提供了详细信息与重要参考。在此基础上,重点对主要声辐射部件一下曲轴箱的结构进行了修改,并对修改方案的声辐射情况进行了重计算。主要声学量的对比结果表明,在所研究的频率域内,修改措施对改善低频域内整机外场声辐射情况具有较为明显的作用。     

Detailed modeling of hybrid reburn/SNCR processes for NOX reduction in coal-fired furnaces

Mechanism reduction has made the detailed kinetic modeling of combustion problems much easier; it also offers potential improvement of modeling accuracy and flexibility in comparison to global mechanisms. The present work applies mechanism reduction in conjunction with the CHEMKIN library and develops an automatic reduction program code. Regarding the hybrid re-burn/selective non-catalytic reduction (SNCR) (“advanced re-burning”) conditions in coal-fired furnaces and based on a full mechanism “GADM98,” a skeletal mechanism with 39 species, 105 reactions, and further a 10-step/14-species reduced mechanism were established. The reduced mechanism was implemented into a 3D-combustion computational fluid dynamics (CFD) code. The eddy-dissipation-concept model was used to describe the influence of turbulence on the combustion chemistry. A large number of simulations for reburning and hybrid reburn/SNCR processes in a coal-fired reactor were executed; the predicted results were compared with experimental measurements. The reduced mechanism and the comprehensive modeling give quite satisfactory results over a wide range of mole ratios for β = [NH₃]/[NO] and air/fuel equivalence ratios λ₂ in the reburn zone. From the modeling results, it was found that adding ammonia premixed with reburn fuel (CH₄) effects no further reduction of NO_x or even impairs the reduction efficiency compared to pure reburning, and in contrast, staged addition of ammonia downstream of the CH₄ injection in the reburn zone provokes a significant further reduction of NO_x over a wide range of parameters. According to the predictions, NO_x-reduction rates of 50-60% and of 70-80% can be achieved through pure reburning and hybrid reburn/SNCR approaches, respectively, at λ₂ = 0.95 and β = 1.5. Concerning the computational procedure, essential measures were taken to optimize convergence and computing time. The computing time with the present reduced mechanism is ∼2.5 times that with the traditional global mechanism for the same iteration number. Tabulation of the rate constants reduced the computing time of the reaction kinetics by ∼50%.     

Eulerian particle flamelet modeling of a bluff-body CH4/H2 flame

In this paper an axisymmetric RANS simulation of a bluff-body stabilized flame has been attempted using steady and unsteady flamelet models. The unsteady effects are considered in a postprocessing manner through the Eulerian particle flamelet model (EPFM). In this model the transient history of scalar dissipation rate, conditioned by stoichiometric mixture fraction, is required to generate unsteady flamelets and is obtained by tracing Eulerian particles. In this approach unsteady convective-diffusive transport equations are solved to consider the transport of Eulerian particles in the domain. Comparisons of the results of steady and unsteady calculations show that transient effects do not have much influence on major species, including OH, and the structure of the flame therefore can be successfully predicted by steady or unsteady approaches. However, it appears that slow processes such as NO formation can only be captured accurately if unsteady effects are taken into account, while steady simulations tend to overpredict NO. In this work turbulence has been modeled using the Reynolds stress model. Predictions of velocity, velocity rms, mean mixture fraction, and its rms show very good agreement with experiments. Performance of three detailed chemical mechanisms, the GRI Mech 2.11, the San Diego mechanism, and the GRI Mech 3.0, has also been evaluated in this study. All three mechanisms performed well with both steady and unsteady approaches and produced almost identical results for major species and OH. However, the difference between mechanisms and flamelet models becomes clearly apparent in the NO predictions. The unsteady model incorporating the GRI Mech 2.11 provided better predictions of NO than steady calculations and showed close agreement with experiments. The other two mechanisms showed overpredictions of NO with both unsteady and steady models. The level of overprediction is severe with the steady approach. GRI Mech 3.0 appears to overpredict NO by a factor of 2 compared to GRI Mech 2.11. The NO predictions by the San Diego mechanism fall between those of the two GRI mechanisms. The present study demonstrates the success of the EPFM model and when used with the GRI 2.11 mechanism predicts all flame properties and major and minor species very well, and most importantly the correct NO levels.     

Experimental investigation and modeling of a direct-coupled PV/T air collector

Photovoltaic/thermal (PV/T) systems refer to the integration of photovoltaic and solar thermal technologies into one single system, in that both useful heat energy and electricity are produced. The impetus of this paper is to model a direct-coupled PV/T air collector which is designed, built, and tested at a geographic location of Kerman, Iran. In this system, a thin aluminum sheet suspended at the middle of air channel is used to increase the heat exchange surface and consequently improve heat extraction from PV panels. This PV/T system is tested in natural convection and forced convection (with two, four and eight fans operating) and its unsteady results are presented in with and without glass cover cases. A theoretical model is developed and validated against experimental data, where good agreement between the measured values and those calculated by the simulation model were achieved. Comparisons are made between electrical performance of the different mode of operations and it is concluded that there is an optimum number of fans for achieving maximum electrical efficiency. Also, results show that setting glass cover on photovoltaic panels leads to an increase in thermal efficiency and decrease in electrical efficiency of the system.