Variable kinematic shell elements for composite laminates accounting for hygrothermal effects

This article presents advanced shell models for the steady-state hygrothermal analysis of composite laminates. The Carrera Unified Formulation is used to derive refined models that include both layer-wise and equivalent single layer models. The governing equations are derived from the principle of virtual displacement taking into account thermal and hygroscopic effects. The geometrical relations for the exact cylindrical geometry are here considered. Through-the-thickness variations of temperature and moisture concentration are calculated by solving the Fourier equation and the Fick law, respectively. The mixed interpolation of tensorial component method is applied to a nine-node shell element to contrast the membrane and shear locking phenomena. Simply supported cross-ply cylindrical shells with antisymmetrical lamination subjected to bisinusoidal thermal/hygroscopic loads are analyzed considering various thickness/curvature ratios. Results obtained with assumed linear and calculated temperature/hygroscopic profiles are presented. Variable kinematics are compared regarding both accuracy and computational costs. The results show that all the kinematics can approximate the transverse shear stress distribution through the thickness with satisfactory accuracy when su?cient expansion terms are adopted. In some cases, miscellaneous expansions can lead to significant reductions in computational costs. The results presented here can be used as benchmark solutions for future works.     

Numerical simulation for fan broadband noise prediction

In order to elucidate the broadband noise of fan,the numerical simulation of fan operating at two different rotational speeds is carried out using the three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) equations.The computed results are compared to experiment to estimate its accuracy and are found to show good agreement with experiment.A method is proposed to evaluate the turbulent kinetic energy in the framework of the Spalart-Allmaras one equation turbulence model.From the calculation results,the turbulent kinetic energy is visualized as the turbulence of the flow which leads to generate the broadband noise,and its noise sources are identified.     

An explicit dynamic model for direct reforming carbonate fuel cellstack

A nonlinear, lumped-parameter mathematical model of direct reforming carbonate fuel cell stack is extended by deriving an explicit set of differential equations for computer simulation. The equilibrium assumption used for the water-gas shift reaction results in an implicit equation set, previously solved using numerical techniques. An explicit equation set is derived by eliminating a key variable associated with the water-gas shift reaction. In addition, results are improved by incorporating a fuel cell performance model to account for reversible cell potential and polarization losses. This requires determination of intermediate gas composition at the cell anode inlet, resulting in additional computations. All results and physical data used are specific to a lumped 16-stack 2-MW system design, a precursor to a demonstration plant that had been operated at the City of Santa Clara, CA, USA. Steady state results are validated for several load points over the upper region of operation and transient results are provided for sudden load change     

Analytical simulation models for solar heating system design

Presented here is a method of simulating the performance of solar heating systems which can be incorporated into a microcomputer-based system design and sizing procedure. A general analytical solution to the storage energy balance is presented, and a scheme for varying the parameters of the general solution to represent changes in the operating mode and the driving forces is detailed. Three different simulation methods which employ the general solution are described. Two of these methods use hourly data and the third uses daily data. Measured results from an actual system are compared with simulated results to validate the models. Also, simulated results and computing speeds are compared with the general simulation model TRNSYS. Finally, an example of a microcomputer application is discussed.     

Strategies for high-performance supercapacitors for HEV

Our paper highlights the role of supercapacitors in hybrid electric vehicles (HEVs), the current response to the worldwide demand for a clean and low fuel-consuming transport. The main strategies for increasing the specific energy of supercapacitors, which are electrochemical energy storage/conversion systems of high-specific power, are discussed, with the focus on electrode material, electrolyte and electrode/electrolyte interface properties. Particular emphasis is given to the use of ionic liquids (IL), which are attracting much attention as green and solvent-free electrolytes, and to the development of high-voltage, IL-based hybrid supercapacitor with high surface area carbon negative electrode and poly(3-methylthiophene) positive. Based on the results of laboratory cells featuring N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide and 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonylimide) ILs, the specific energies of hybrid supercapacitor modules are evaluated and compared to those expected for double-layer carbon supercapacitors displaying the same ILs.     

Heat transfer model for small-scale air-cooled spark-ignition four-stroke engines

The heat transfer models proposed in previous studies are not suitable for small-scale spark-ignition engines, because they were developed primarily for large-scale engines. In order to improve the accuracy of the predicted heat transfer rate for small-scale engines, a heat transfer model using the Stanton number is proposed in this paper. Prediction results of instantaneous heat flux, global engine heat transfer, and cylinder pressure based on the proposed model are compared with the experimental results and prediction results of previous models. It is found that the proposed model has prediction results closer to the measured data than the previous models at most engine operating conditions.     

Optimal contracts for renewable electricity

Companies are increasingly choosing to procure their power from renewable energy sources, with their own set of potential challenges. This paper characterizes the contracts that minimize the cost of procuring a given amount of renewable energy from two risk averse, energy generators who are inherently unreliable (such as wind and solar). We contrast outcomes arising when investments are set in centralized and decentralized settings, with the absence of reliability addressed by either issuing orders in excess of what is needed or by investing in improved reliability. Our results suggest that future contracts may be geared towards a greater reliance on order inflation and lower investments in reliability as the cost of renewable energy keeps falling. The implications of these results for grid congestion and electricity spot market prices should be of interest to regulators and transmission system operators.     

基于MEEMD-GAELM组合模型的短期风电功率预测

考虑到风电功率短期预测的准确性对电网调度具有重要作用,提出了一种由改进的集成经验稳态分解(MEEMD)与基于遗传算法优化的极限学习机(GAELM)相结合的短期风功率组合预测模型,首先对原始风功率时间序列进行总体平均经验模态分解(CEEMD),通过排列熵剔除异常分量,再对剩余分量进行经验模态分解(EMD),其结果即为MEEMD分解所得分量,对分量分别建立GAELM预测模型,将各分量预测结果相加,即得到最终预测结果。对东北某风电场实测数据进行试验表明,与传统预测方法相比,组合预测模型有效提高了短期风功率预测的精确性。     

Critical evaluation of methods for wind-power appraisal

In the literature, several methods are being used to evaluate the contribution of wind energy conversion systems in a broader power-generating system. However, not all of them guarantee accurate results.This paper critically reviews several methods used in the literature for such reliability evaluations. There are three types of methods characterised by the details considered and the simplifications made: (1) methods using complete chronological data; measured or calculated; (2) methods using a truncated set of chronological data and (3) methods disregarding the chronology all together.In order to test the different possible simplifications, we calculate a case study for a potential wind farm. The result of the full chronological calculation is used as a benchmark to which the results of the simplified methods can be verified. The truncated method is found to be reliable when more than the top 25% chronological values are used. The methods in which the chronology is disregarded were not found accurate because of the significant correlation between consecutive wind-speed measurements embedded in the chronological data.We therefore conclude that the most accurate results are obtained when the full chronology of both the wind-power output and the power-generation requirement are used. Most simplifications considered with the aim to limit the amount of data needed and thus to save computation time and data-gathering effort are not really justified.     

Operating strategy for a hydrogen engine for improved drive-cycle efficiency and emissions behavior

Due to their advanced state of development and almost immediate availability, hydrogen internal combustion engines could act as a bridging technology toward a wide-spread hydrogen infrastructure. Extensive research, development and steady-state testing of hydrogen internal combustion engines has been conducted to improve efficiency, emissions behavior and performance. This paper summarizes the steady-state test results of the supercharged hydrogen-powered four-cylinder engine operated on an engine dynamometer. Based on these results a shift strategy for optimized fuel economy is established and engine control strategies for various levels of hybridization are being discussed. The strategies are evaluated on the Urban drive cycle, differences in engine behavior are investigated and the estimated fuel economy and NO_x emissions are calculated. Future work will include dynamic testing of these strategies and powertrain configurations as well as individual powertrain components on a vehicle platform, called ‘Mobile Advanced Technology Testbed’ (MATT), that was developed and built at Argonne National Laboratory.     

坝基渗流分析的边界点法

采用边界点法分析坝基渗流问题,编制了计算程序并与有限单元法和裂隙网络法的计算结果比较.结果表明,采用边界点法处理坝基渗流问题计算程序简单、速度快、精度高.     

A new tip correction for actuator line computations

The actuator line method (ALM) is today widely used to represent wind turbine loadings in computational fluid dynamics (CFD). As opposed to resolving the whole blade geometry, the methodology does not require geometry‐fitted meshes, which makes it fast to apply. In ALM, tabulated airfoil data are used to determine the local blade loadings, which subsequently are projected to the CFD grid using a Gaussian smearing function. To achieve accurate blade loadings at the tip regions of the blades, the width of the projection function needs to be narrower than the local chord lengths, requiring CFD grids that are much finer than what is actually needed in order to resolve the energy containing turbulent structures of the atmospheric boundary layer (ABL). On the other hand, employing large widths of the projection function may result in too large tip loadings. Therefore, the number of grid points required to resolve the blade and the width of the projection function have to be restricted to certain minimum values if unphysical corrections are to be avoided. In this paper, we investigate the cause of the overestimated tip loadings when using coarse CFD grids and, based on this, introduce a simple and physical consistent correction technique to rectify the problem. To validate the new correction, it is first applied on a planar wing where results are compared with the lifting‐line technique. Next, the NREL 5‐MW and Phase VI turbines are employed to test the correction on rotors. Here, the resulting blade loadings are compared with results from the blade‐element momentum (BEM) method. In both cases, it is found that the new correction greatly improves the results for both normal and tangential loads and that it is possible to obtain accurate results even when using a very coarse blade resolution.     

Thermal-shock problem for a hollow cylinder via a multi-dual-phase-lag theory

Abstract

An exact solution of a thermal shock for a circular cylinder is presented. A refined multi-dual-phase-lag generalized thermoelasticity model is proposed. The application of initial conditions without using Laplace transform is effected. The exact solutions of main physical fields are obtained analytically in the radial direction using the normal mode technique. For the case where the mechanical and thermal loads are applied on the inner and outer surfaces of the cylinder. Numerical results for the distributions of radial displacement, temperature, radial, hoop, and axial stresses are illustrated graphically. Extensive results are tabulated to show the accuracy of the present model. The results will also be used as benchmarks for forthcoming comparisons with other investigations. The results indicate that the effects of internal and external pressures and time are very pronounced.     

Evaluation of heat transfer correlations for HCCI engine modeling

Combustion in HCCI engines is a controlled auto-ignition of well-mixed fuel, air and residual gas. The thermal conditions of the combustion chamber are governed by chemical kinetics strongly coupled with heat transfer from the hot gas to the walls. The heat losses have a critical effect on HCCI ignition timing and burning rate, so it is essential to understand heat transfer process in the combustion chamber in the modeling of HCCI engines. In the present paper, a comparative analysis is performed to investigate the performance of well-known heat transfer correlations in an HCCI engine. The results from the existing correlations are compared with the experimental results obtained in a single-cylinder engine. Significant differences are observed between the heat transfer results obtained by using Woschni, Assanis and Hohenberg correlations.     

Liquid Sprays for Heat Transfer Enhancements: A Review

Sprays used for enhanced heat transfer are reviewed, starting from the spray characteristics, measurement methods, and spray dynamics, to spray heat transfer. Some results for spray heat transfer at large Reynolds numbers and surface boiling are also presented, including some recent results summarizing the effects of various injection parameters. It is recommended that basic principles of heat transfer be used to integrate various effects, such as coolant and surface temperatures, water and air flow rates, and injection conditions, into a concise form so that the results can be generalized and be applied to a large range of conditions.     

Heat-load modelling for large systems

Control strategies for operating district heating, the utilization of biomass boilers under optimal conditions, estimation of solar production by central solar-heating systems and much more, demand realistic modelling of heat loads/demands for district-heating systems. A dynamic system model is presented: the results from this simulation were then compared with results from alternative methods found in the literature. If knowledge is available for a given case or the load is to be found for a new system, the simulation approach is recommended. The method, however, involves many parameters that can lead to errors and uncertainties. Therefore, if knowledge of case-specific parameters is rare, the degree-day method can lead to realistic results. For low-energy, solar-optimized building area, the energy–signature method leads to reasonable results and if system-wide load data are available, the energy–signature method can even do better than the degree-day method.     

Probabilistic design factors for pipes used for hydrogen transport

Probabilistic design factors for pipes used for hydrogen transport are proposed for 3 locations: urban, peri-urban and rural areas.The used method involves a risk equation defined as the product of probability of leakage after failure, probability to have a gas flow greater than a prescribed value, probability of ignition, probability of lethal effects greater than a threshold value and probability of the presence of a person corrected by an environment factor and a risk reduction coefficient. The results obtained are less conservative than the deterministic values provided by ASME code.     

Thermal box-barrier for a direct measurement in high temperature environment

Thermal box-barriers (TBB) are used for the thermal protection of measuring systems during exposure in a high temperature environment of various furnaces. This paper deals with problems of TBB design. TBB characteristics and limitations are discussed. The four-layer TBB structure is proposed for the high temperature application. The computer simulation results are presented to show the typical time evolution and spatial distribution of temperature in TBB. Problems of optimum proposal of individual layers thickness are mentioned. Temperatures in the manufactured TBB measured by thermocouples during tests and applications in industrial reheating furnaces are introduced in order to verify the computational results and to show its real thermal insulation capabilities.     

Total deformation theory for non-proportional loading

This study presents a technique to apply a total deformation theory to non-proportional loading cases. A total deformation theory capable of analyzing a sequence of linear non-proportional load steps is proposed. Each linear loading path is defined with reference to its previous loading path, analogous to proportional loading. The application of the proposed formulation to tension–torsion loading of thin tubes and pressure–torsion loading of thick-walled cylinders is carried out. It is shown that for situations where stresses are known a priori, the proposed method gives the same plastic strain field as does incremental plasticity. For situations where stresses are not known a priori, a method to estimate the plastic strain for linear hardening materials is proposed. This method estimates the necessary stress fields using conventional deformation plasticity. These stresses are then used in the proposed total deformation formulation to predict plastic strains. The plastic strain field resulting from this method is compared with finite element results obtained using incremental plasticity. The results are in very good agreement and the proposed method significantly reduces computation time.     

Stochastic approach for daily solar radiation modeling

Mathematical modeling of solar radiation continues to be an important issue in renewable energy applications. In general, existing models are mostly empirical and data dependent. In this paper, a novel approach for solar radiation modeling is proposed and illustrated. The proposed application consists of hidden Markov processes, which are widely used in various signal processing topics including speech modeling with successful results. In the experimental work, mean of hourly measured ambient temperature values are considered as observations of the model, whereas mean of hourly solar radiation values are considered as the hidden events, which constitute the outcomes of the proposed mathematical model. Both solar radiations and temperatures are converted to quantized number of states. Finally, after a training stage that forms the transition probability values of the described states, the hidden Markov model parameters are obtained and tested. The tests are repeated for various numbers of states and observations are presented. Plausible modeling results with distinct properties in terms of accuracy are achieved.