Numerical investigation on the fatigue failure characteristics of waterbearing sandstone under cyclic loading

The strength of sandstone decreases significantly with higher water content attributing to softening effects.This scenario can pose a severe threat to the stability of reservoirs of pumped storage power stations developed from abandoned mines,especially when subjected to the cyclic loading condition caused by the repeated drainage and storage of water(fatigue damage).Based on this,it is essential to focus on the fatigue failure characteristics.In this study,the mineral composition of the used sandstone of Ruineng coal mine in Shanxi Province,China,was first tested to elucidate the rock softening mechanism after absorbing water.Next,a numerical model for replicating the mechanical behavior of water-bearing sandstone was established using twodimensional particle flow code(PFC2D)with a novel contact model.Then,16 uniaxial cyclic loading simulations with distinct loading parameters related to reservoir conditions(loading frequency,amplitude level,and maximum stress level)and different water contents were conducted.The numerical results show that all these three loading parameters affect the failure characteristics of sandstone,including irreversible strain,damage evolution,strain behavior,and fatigue life.The influence degree of these three parameters on failure behavior increases in the order of maximum stress level,loading frequency,and amplitude level.However,for the samples with different water contents,their failure characteristics are similar under the same loading conditions.Furthermore,the failure mode is almost unaffected by the loading parameters,while the water content plays a significant role and causing the transformation from the tensile splitting with low water content to the shear failure with higher water content.

Convolution Neural Network-based Load Model Parameter Selection Considering Short-term Voltage Stability

The recently proposed ambient signal-based load modeling approach offers an important and effective idea to study the time-varying and distributed characteristics of power loads.Meanwhile,it also brings new problems.Since the load model parameters of power loads can be obtained in real-time for each load bus,the numerous identified parameters make parameter application difficult.In order to obtain the parameters suitable for off-line applications,load model parameter selection(LMPS)is first introduced in this paper.Meanwhile,the convolution neural network(CNN)is adopted to achieve the selection purpose from the perspective of short-term voltage stability.To begin with,the field phasor measurement unit(PMU)data from China Southern Power Grid are obtained for load model parameter identification,and the identification results of different substations during different times indicate the necessity of LMPS.Meanwhile,the simulation case of Guangdong Power Grid shows the process of LMPS,and the results from the CNNbased LMPS confirm its effectiveness.

Equivalent Impedance Parameter Calculation of Three-phase Symmetrical Loads for Harmonic Source Location

The equivalent impedance parameters of loads have been widely used to identify and locate the harmonic sources.However,the existing calculation methods suffer from outliers caused by the zero-crossing of the denominator.These outliers can result in inaccuracy and unreliability of harmonic source location.To address this issue,this paper proposes an innovative method of equivalent impedance parameter calculation of three-phase symmetrical loads that avoid outliers.The correctness and effectiveness of the proposed method are verified by simulations on Simulink using actual monitoring data.The results show that the proposed method is not only simple and easy to implement but also highly accurate.

Theoretical Study of Combustion Processes in Aero Gas Turbines for Air Craft Engines

Combustion process involves various physical and chemical processes which govern and control flames initiation in aero gas turbine engines. During certain flying conditions, at full load, unexpected critical situation may take place in such engines called blow off conditions, which leads to flames diminishing in the combustion chamber of such engines. Gas motion, flow velocity and turbulence kinetic energy are the most important parameters in ensuring flame stabilities. These parameters play a tremendous role and effects on this phenomenon. In gas turbines, the flame exists within a high velocity, non-uniform and intensely turbulent flow field, therefore careful temperature control is vital. Another important factor which must be considered to avoid blow off conditions, is mixture strength. Nearly, all modern gas turbines, due to emissions restrictions, operate on lean mixture conditions which are hard to ignite and lower flame temperatures and thus more risk to reach blow off conditions which leads to a complete flame extinction. These conditions may exist in an air craft engines due to sharp changes in loading parameters, (θL): pressure (Pu), temperature (Tu), mass flow rate (), and cross sectional area (Au). At present there is no detailed theory of gas turbine combustion. Therefore, we must resort to simple models and experimental correlations. This paper investigates the blow-off phenomena in aero gas turbine engines, its causes and estimation of required energy to ensure recovery (re-ignition) again inside the combustion chamber. Identifying the conditions at which blow-off takes place and associated loading parameters (θL) which are a function of (A, T, P, and ). The paper also, quantify the recovery conditions (required energy to re-ignition, change in loading parameter (Δq) Power, Required VHRR: (Volumetric Heat Release Rate) and changes in other loading variables (ρ: density, T: Temperature, P: Pressure, and : mass flow rate) tarts with discussing causes of blow off along with effecting operating conditions.

Discharge Characteristics of Large-Area High-Power RF Ion Source for Positive and Negative Neutral Beam Injectors

A large-area high-power radio-frequency（RF） driven ion source was developed for positive and negative neutral beam injectors at the Korea Atomic Energy Research Institute（KAERI）. The RF ion source consists of a driver region, including a helical antenna and a discharge chamber, and an expansion region. RF power can be transferred at up to 10 kW with a fixed frequency of 2 MHz through an optimized RF matching system. An actively water-cooled Faraday shield is located inside the driver region of the ion source for the stable and steady-state operations of high-power RF discharge. Plasma ignition of the ion source is initiated by the injection of argongas without a starter-filament heating, and the argon-gas is then slowly exchanged by the injection of hydrogen-gas to produce pure hydrogen plasmas. The uniformities of the plasma parameter,such as a plasma density and an electron temperature, are measured at the lowest area of the driver region using two RF-compensated electrostatic probes along the direction of the shortand long-dimensions of the driver region. The plasma parameters will be compared with those obtained at the lowest area of the expansion bucket to analyze the plasma expansion properties from the driver region to the expansion region.

Describing failure in geomaterials using second-order work approach

Geomaterials are known to be non-associated materials. Granular soils therefore exhibit a variety of failure modes, with diffuse or localized kinematical patterns. In fact, the notion of failure itself can be confusing with regard to granular soils, because it is not associated with an obvious phenomenology. In this study, we built a proper framework, using the second-order work theory, to describe some failure modes in geomaterials based on energy conservation. The occurrence of failure is defined by an abrupt increase in kinetic energy. The increase in kinetic energy from an equilibrium state, under incremental loading, is shown to be equal to the difference between the external second-order work,involving the external loading parameters, and the internal second-order work, involving the constitutive properties of the material. When a stress limit state is reached, a certain stress component passes through a maximum value and then may decrease. Under such a condition, if a certain additional external loading is applied, the system fails, sharply increasing the strain rate. The internal stress is no longer able to balance the external stress, leading to a dynamic response of the specimen. As an illustration, the theoretical framework was applied to the well-known undrained triaxial test for loose soils. The influence of the loading control mode was clearly highlighted. It is shown that the plastic limit theory appears to be a particular case of this more general second-order work theory. When the plastic limit condition is met, the internal second-order work is nil. A class of incremental external loadings causes the kinetic energy to increase dramatically, leading to the sudden collapse of the specimen, as observed in laboratory.