Evaluation of J-integral for surface cracked plates under biaxial loading using extended reference stress method

In this paper, reference stress solutions for plates with semi-elliptical surface cracks were firstly reviewed, and the applicability of the solutions was examined through the comparison with finite element analysis results under uniaxial loading. Next, an extended reference stress method was newly developed to evaluate J-integral for cracked plates under biaxial loading. The predictive accuracy of the method was validated through the comparison with finite element analysis results under biaxial loading. As a result, it was ascertained that the proposed method together with Lei's reference stress solution predicts J-integral with acceptable accuracy.     

An explicit J-V model of a solar cell for simple fill factor calculation

The J-V equation of a solar cell is implicit and requires iterative calculation to determine the fill factor and the maximum power point. Here an explicit model for J-V characteristic is proposed which is applicable to a large variety of solar cell. This model allows an easy estimation of fill factor from four simple measurements of the bias points corresponding to V_oc, J_sc, and any two voltage values lying between 0 and V_oc, where V_oc is the open circuit voltage and J_sc is the short circuit current density.     

An analysis of the numerical model influence on the ground temperature profile determination

The estimation of the ground temperature profile with respect to the depth and time is the key issue in many engineering applications which use the ground as a source of thermal energy. In the present work, the influence of the model components on the calculated ground temperature distribution has been analysed in order to develop an accurate and robust model for the prediction of the ground temperature profile. The presented mathematical model takes into account all the key phenomena occurring in the soil and on its top surface. The impact of individual model elements on the temperature of the soil has been analysed. It has been found that the simplest models and the most complex model result in a similar temperature variation over the simulation period, but only at a low depth. A detailed analysis shows that a larger depth requires more complex models and the calculation with the use of simple models results in an incorrect temperature and a theoretical COP estimation.     

The strictest energy requirements in the world: An analysis of the path dependencies of a self-proclaimed success

50 years of progressively strengthened energy requirements in the Danish building code appear to be a success, as the energy consumption has remained constant despite an increase in the total area in requirement of heating. This article however argues that the building code mechanism is heavily influenced by path dependent regime structuration processes, and that the mechanism constitutes a barrier to more radical developments within low energy housing. Few and poorly organized frontrunner activities within low energy housing have accordingly taken place in a Danish context during the past decades. Finally it is proposed that the current development within the energy system provides opportunities for cultivating an improved transitional awareness and for carrying out experimental activities that may challenge the path dependencies of prevailing regime structuration processes.     

An improved numerical scheme to evaluate the pressure gradient on unstructured meshes for two-phase flow analysis

In multi-dimensional two-phase flows, a local pressure is very important because it directly influences the phase change and it may lead to a great change in the flow field. This in turn puts emphasis on the accurate evaluation of local pressure gradient. This paper presents a new numerical scheme to evaluate the pressure gradient at cell centers on unstructured meshes for a three-dimensional thermal-hydraulic code, named CUPID. The results of the new scheme for a simple test function, a gravity-driven cavity, and a wall boiling two-phase flow are compared with those of the previous schemes in the CUPID code.     

Experimental and numerical analysis of natural ventilation with combined light/vent pipes

Both experimental and numerical procedures were developed for studying natural ventilation in combined light/vent pipes. An experimental tracer gas technique was used to measure ventilation rates in a test cell. Two different pipe terminals were simultaneously used, dividing the cell in two identical volumes, in order to allow a direct comparison of their performance. A 3D numerical model was developed using ansys5.5/flotran. Experimental and numerical results are shown and compared. It is concluded that pipe terminals significantly enhance ventilation, that one type of terminal has a better performance and that the numerical model underestimates measured flows.     

Electricity capacity expansion in Thailand: An analysis of gas dependence and fuel import reliance

Electricity generation in Thailand is highly dependant on natural gas. Recent research has revealed that the Thai economy would become more vulnerable from high gas dependence in the power sector. This paper aims to assess the economic impact of gas dependence in power generation in the coming decades. To fulfil this objective, two scenarios of electricity capacity planning were developed and the results were analysed to understand the changes in gas dependence and the effects on import reliance. It is found that from 2011 to 2025, the average cost of natural gas for power generation will account for 2.41% of gross domestic product (GDP) while high oil price in international energy markets would push this cost to 2.97% of GDP. In addition, reliance on fuel imports for power generation, particularly natural gas and coal, is going to be another crucial concern to the security of energy supply as the costs of these imports during the planning horizon will increase significantly at an average rate of 6.78% per year.     

蛇形太阳能空气集热器流道布置的优化分析

加装扰流板的蛇形太阳能空气集热器是平板式太阳能空气集热器的常见改进形式。文章对上风道、双风道、下风道蛇形太阳能空气集热器的集热效率和热量损失进行模拟分析。分析结果表明:3种太阳能空气集热器的集热效率随着测试时间的变化均呈现出先升高后降低的变化趋势;上风道、双风道、下风道蛇形太阳能空气集热器的平均集热效率分别为57.53%,64.69%,65.24%,热量损失分别为429.26,323.97,317.01 W。     

In-depth numerical analysis on the determination of amount of CO2 recirculation in LNG/O2/CO2 combustion

The determination of proper amount of CO₂ recirculation is one of the critical issues in oxy-fuel combustion technology for the reduction of CO₂ emissions by the capture and sequestration of CO₂ species in flue gas. The objective of this study is to determine the optimum value of O₂ fraction in O₂/CO₂ mixture to obtain similar flame characteristics with LNG–air combustion. To this end, a systematic numerical investigation has been made in order to resolve the physical feature of LNG/O₂/CO₂ combustion. For this, SIMPLEC algorithm is used for the resolution of pressure velocity coupling. And for the Reynolds stresses and turbulent reaction the popular two-equation (k–ε) model by Launder and Spalding and eddy breakup model by Magnussen and Hjertager were incorporated, respectively. The radiative heat transfer is calculated from the volumetric energy loss rate from flame, considering absorption coefficient of H₂O, CO₂ and CO gases. A series of parametric investigation has been made as function of oxidizer type, O₂ fraction and fuel type for the resolution of combustion characteristics such as flame temperature, turbulent mixing and species concentration. Further the increased effect of CO₂ species on the flame temperature is carefully examined by the consideration of change of specific heat and radiation effect. Based on this study, it was observed that the same mass flow rate of CO₂ with N₂ appears as the most adequate value for the amount of CO₂ recirculation for LNG fuel since the lower C_p value for the CO₂ relative to N₂ species at lower temperatures cancels the effect of the higher C_p value at higher temperatures over the range of flame temperatures present in this study. However, for the fuel with high C/H ratio, for example of coal, the reduced amount of CO₂ recirculation is recommended in order to compensate the increased radiation heat loss. In general, the calculation results were physically acceptable and consistent with reported data in literature. Further work is strongly recommended for a large-scale combustor such as coal-fired power plant to figure out important parameters caused by the effect of increased combustor size and the presence of particle phase, etc.     

An assessment of the accuracy of numerical solutions to the graetz problem

It has been well documented in prior studies that numerical solutions to the classical Graetz problem in the entrance region of pipes produce estimates of Nusselt number that are greatly in error. In this report, we examine this problem and attempt to provide plausible explanation for the discrepancy.     

Exploring the core factors and its dynamic effects on oil price: An application on path analysis and BVAR-TVP model

As the uncertainty of oil price increases, impacts of the influential factors on oil price vary over time. It is of great importance to explore the core factors and its time-varying influence on oil price. In view of this, based on the PATH-ANALYSIS model, this paper obtains the core factors, builds an oil price system VAR model, which uses demand, supply, price, and inventory as endogenous variables, and China's net imports as well as dollar index as exogenous variables. Then we set up a BVAR-TVP (Time varying parameter) model to analyze dynamic impacts of core factors on oil price. The results show that: (1) oil prices became more sensitive to oil supply changes, and the influence delays became shorter; (2) the impact of oil inventories on oil prices with a time lag of two quarters but has a downward trend; (3) the impact of oil consumption on oil prices with a time lag of two quarters, and this effect is increasingly greater; (4) the US dollar index is always the important factor of oil price and its control power increases gradually, and the financial crisis (occurred in 2008) further strengthens the influence of US dollar.     

Investigation of path dependence in commercial lithium-ion cells chosen for plug-in hybrid vehicle duty cycle protocols

There is a growing need to explore path dependence of aging processes in batteries developed for long-term usage, such as lithium-ion cells used in hybrid electric vehicle (HEV) or plug-in hybrid vehicle (PHEV) applications that may then be “retired” to be utilized in grid applications. To better understand the foremost influences on path dependence in the PHEV context, this work aims to bridge the gap between ideal laboratory test conditions and PHEV field conditions by isolating the predominant aging factors in PHEV service, which would include, for example, the nature and frequency of duty cycles, as well as the frequency and severity of thermal cycles. These factors are studied in controlled and repeatable laboratory conditions to facilitate mechanistic evaluation of aging processes. This work is a collaboration between Idaho National Laboratory (INL) and the Hawaii Natural Energy Institute (HNEI). Commercial lithium-ion cells of the Sanyo Y type (18650 configuration) are used in this work covering two initial independent studies of path dependence issues. The first study considers how the magnitude of power pulses and charging rates affect the aging rate, while the second seeks to answer whether thermal cycling has an accelerating effect on cell aging. While this work is in early stages of testing, initial data trends show that cell aging is indeed accelerated under conditions of high discharge pulse power, higher charge rates, and thermal cycling. Such information is useful in developing accurate predictive models for estimating end-of-life conditions.     

Steady and transient numerical analysis of the performance of annular fins

This article presents a simple, straightforward and accurate numerical technique that can be used to predict the steady and transient temperature distributions in an annular fin with uniform cross‐section. Both convection and/or thermal radiation ambient conditions are considered in this study. The heat transfer rate and fin efficiency are also calculated. Results are compared with the exact solution for the steady case where excellent agreement was observed for an insulated tip fin and with approximate solutions for the transient case for a non‐insulated tip condition. Copyright © 2001 John Wiley & Sons, Ltd.     

An experimental and numerical study on characteristics of laminar premixed H2/CO/CH4/air flames

The effects of variations in the fuel composition on the characteristics of H₂/CO/CH₄/air flames of gasified biomass are investigated experimentally and numerically. Experimental measurements and numerical simulations of the flame front position and temperature are performed in the premixed stoichiometric H₂/CO/CH₄/air opposed-jet flames with various H₂ and CO contents in the fuel. The adiabatic flame temperatures and laminar burning velocities are calculated using the EQUIL and PREMIX codes of Chemkin collection 3.5, respectively. Whereas the flame structures of the laminar premixed stoichiometric H₂/CO/CH₄/air opposed-jet flames are simulated using the OPPDIF package with the GRI-Mech 3.0 chemical kinetic mechanisms and detailed transport properties. The measured flame front position and temperature of the stoichiometric H₂/CO/CH₄/air opposed-jet flames are closely predicted by the numerical calculations. Detailed analysis of the calculated chemical kinetic structures reveals that the reaction rate of reactions (R38), (R46), and (R84) increase with increasing H₂ content in the fuel mixture. It is also found that the increase in the laminar flame speed with H₂ addition is most likely due to an increase in active radicals during combustion (chemical effect), rather than from changes in the adiabatic flame temperature (thermal effect). Chemical kinetic structure and sensitivity analyses indicate that for the stoichiometric H₂/CO/CH₄/air flames with fixed H₂ concentration in the fuel mixture, the reactions (R99) and (R46) play a dominant role in affecting the laminar burning velocity as the CO content in the fuel is increased.     

Two-dimensional numerical analysis of a rectangular closed-loop thermosiphon

The study presents a numerical investigation of the dynamical behavior of a rectangular natural circulation loop with horizontal heat exchanging sections. The study has been developed in a two-dimensional domain considering uniform wall temperatures, UWT, at the horizontal sections and thermally insulated vertical legs as thermal boundary conditions. The governing equations have been solved using a control volume method solving the velocity-pressure coupling with the SIMPLER algorithm. The analysis has been performed for a fixed geometry of the loop and for various Rayleigh numbers, separating the values of Rayleigh for which the system manifests stable and unstable dynamics. In the last case, it is shown that the vortices are responsible for the birth of the oscillations and for the growth of temperature gradients.     

A numerical analysis of buoyancy-driven melting and freezing

A numerical investigation of transient natural convective heat transfer with coupled phase change is presented. The numerical model attempts to capture the solid-fluid interface using a fixed-grid solution and is applied to two pure substance cases found in published literature, one considering the melting of 95% pure Lauric acid and the other involving the freezing of water. The governing equations are solved in a manner such that if the temperature falls below the freezing isotherm then the convection terms in the equations of motion are effectively disengaged. Variations in the specific heat of the material are incorporated in order to account for the phase change. A non-Boussinesq approach is considered which accounts for any density extrema in the flow, particularly for the density inversion found in water. In both of the cases considered the phase change occurs between fixed temperature boundaries and Rayleigh numbers rest well within the laminar flow regime. From the results obtained it is demonstrated that a relatively simple numerical technique can be applied to capture the physics of buoyancy-driven melting and freezing and that the results are in reasonable concurrence with experimental data.     

Pulse resistance effects due to charging or discharging of high-power lithium-ion cells: A path dependence study

Battery life estimations and state-of-health projections for commercial applications such as hybrid electric vehicles are highly dependent on accurate resistance monitoring. This study examined discharge/charge hysteresis (path dependency) effects on measuring resistance using two different lithium-ion cell chemistries. Cells were either discharged or charged to a target voltage, followed by a rest at open-circuit for electrochemical and thermal equilibration, immediately prior to a resistance measurement using a high-current pulse profile. Results show that a voltage hysteresis effect has an impact on cell resistance measurements, depending on the direction a target voltage is reached. Specifically, charging to a target condition yields different and less consistent resistance measurements compared to discharging to that same condition. Further, using slower rates to approach the target condition has a small impact on resistance on the discharge curve but does give a noticeable improvement on the charge curve. Unfortunately, slow charging and discharging are generally not practical for hybrid electric vehicle applications due to the rapidly changing power demands of the driver. Consequently, these results indicate that life estimates should be primarily based on resistances determined from pulses on the discharge curve.     

A numerical creep analysis on the interaction of twin semi-elliptical cracks

The interaction and coalescence of multiple flaws will significantly influence the service life of components. In this paper, the interaction of two identical semi-elliptical cracks in a finite thickness plate subjected to the remote tension is investigated. The results indicated that interaction of multiple cracks is different between the time-dependent fracture characterized by C^*-integral and linear elastic fracture noted by SIF. The magnifying factors of creep fracture are obviously larger than that of the linear elastic fracture cases. Therefore, the current interaction and coalescence rule developed from linear elastic fracture analysis may lead to a non-conservative result when it is used in the assessment of creep crack. At the end, an empirical equation is developed based on the numerical results.     

An example for the effect of round-off errors on numerical heat transfer

The effect of round-off errors on the solution of numerical heat transfer is illustrated by a simple example both analytically and numerically. It is found that the upper bound of the round-off error under both conditions with or without an inner heat source is proportional to the square of grid number—n². Increase in grid number might lead to larger round-off errors. The magnitude of relative round-off error is also determined by the specific problem. Proper treatment of the computation procedure can reduce the round-off error obviously. The precision can be improved with this method without occupation of additional computational resources.