A Study on a Modified Model for the Hold Time of a Clean Extinguishing Agent

When estimating the theoretical hold time of a Clean Extinguishing Agent (CEA), the hold time is predicted using a leakage exponent and leakage characteristics that are measured through an Enclosure Integrity Test (EIT). In particular, the leakage exponent n is conventionally applied as 0.5, but recently, a variable exponent has been applied through measurement (approximately 0.48 to 0.85). When variable n is applied, the hold time varies greatly depending on the size of n. Therefore, in this study, a modified theoretical model for the hold time of a CEA is proposed to improve the accuracy of the hold time depending on the leakage exponent n. The modified theoretical model applies a different analysis of the inflow and outflow volumetric flow, and the results enable a more accurate hold time prediction. The modified theoretical equation shows an improvement of up to 8.6% for outflow volumetric flow V˙o and a maximum improvement of approximately 10.7% for hold time th.

Design and Analysis of a Contact-Aided Variable Stiffness Flexure Hinge (CVSFH)

Flexure hinges are the basis of compliant mechanisms. The stiffness is one of the important indexes to evaluate the performance of a flexure hinge, and the rotation angle when the stiffness changes affects its motion characteristics. Thus, based on the constant rectangular cross-section flexure hinge and contact interaction, this paper proposed a contact-aided variable stiffness flexure hinge (CVSFH). With the deformation under an external load, the contact interaction with different parts of the CVSFH itself can achieve the purpose of variable stiffness. The equivalent mechanical model is built and the theoretical equation of the stiffness is given. CVSFHs with different dimensions are designed, and a finite element analysis (FEA) is done. The FEA results of the design examples are coincide with the theoretical results, which verifies the feasibility of the design and the correctness of the theoretical equation.

Theoretical and Numerical Study on Electrical Resistivity Measurement of Cylindrical Rock Core Samples Using Perimeter Electrodes

The estimation of hydraulic and mechanical properties of bedrock is important for the evaluation of energy-related structures, including high-level nuclear waste repositories, hydraulic fracturing wells, and gas-hydrate production wells. The hydraulic conductivity and stress–strain curves of rocks are conventionally measured through laboratory tests on cylindrical samples. Both ASTM standards for hydraulic conductivity and compressive strength involve the use of the planar bases of a cylindrical sample. Hence, an alternative test method is required for the simultaneous measurement of hydraulic conductivity and stress–strain curves. This study proposes a novel electrical resistivity estimation method using two perimeter electrodes for the estimation of hydraulic properties. The theoretical background for the perimeter electrode setup is derived and the COMSOL MultiPhysics® finite element numerical simulation tool is employed to verify the derived theoretical equation. The accuracy of the numerical simulation tool is first validated by simulating the ASTM standard testing method for electrical resistivity. The electrical resistance values derived from the theoretical equation and numerical simulation are compared for different electrical resistivity and electrode radius. The assumed equidistant, circular equipotential surface results in a theoretical lower bound for the measured electrical resistance in the cylindrical specimen. The introduction of a phenomenological distortion factor to correct for the theoretical equipotential surface results in a good fit with the numerical simulation results. The effects of electrode length and equivalent strap electrodes were investigated to assess the applicability of the suggested method for laboratory testing. Consequently, this study presents an effective alternative theoretical assessment method for the lower bound electrical resistivity of cylindrical rock core samples under confining conditions when the installation of base electrodes is infeasible.

Influence of Effective Pressure on Percolation Rate of Coal

To study the role of the effective pressure on the percolation rate of coal, the effect of gas pressure, gas adsorption, and temperature on coal is taken into consideration comprehensively. Starting from the research direction of effective pressure and porosity, the coal body percolation rate and effective pressure equations considering the influence of temperature and gas pressure are established. The experiments on the percolation characteristics of raw coal are implemented under different effective pressure by using the independently developed thermos-solid-gas linking triaxial servosystem. The theoretical equation of the effective pressure and percolation rate of coal is calculated with the finite element software COMSOL, and the results obtained from the theoretical equation agree well with the experimental results. The results show that the percolation rate of coal gradually decreases with the growth of the effective pressure when the gas pressure and temperature are kept unchanged. Therefore, the results of the research are of certain reference value for the effective prevention of gas disaster.

Electron paramagnetic resonance of manganese-doped strontium titanate

Electron paramagnetic resonance (EPR) spectra of manganese-doped strontium titanate were investigated for several Mn concentrations. The spectra of Mn2+ and Mn4+ ions were observed and attributed, respectively, to Mn ions occupying Sr2+ and Ti4+ sites. The relative intensity of the spectra suggested that the manganese ions occupy preferentially Ti4+ sites. The results showed that the EPR peak-to-peak linewidth of the Mn4+ spectrum increases with manganese concentration according to the theoretical equation DHpp=0.45+210.f.(1-f)80 (mT). This suggested that the exchange interaction between tetravalent manganese ions in strontium titanate has an approximate range of 0.96 nm, comparable to that of Gd3+ in the same compound.

Influence of liquid bridge formation process on its stability in nonparallel plates

The effect of liquid bridge formation process on its stability was discussed to obtain the theoretical equation for determining the stability of the liquid bridge.

Theoretical description of drawing body shape in an inclined seam with longwall top coal caving mining

Understanding the characteristics of drawing body shape is essential for optimization of drawing parameters in longwall top coal caving mining. In this study, both physical experiments and theoretical analysis are employed to investigate these characteristics and derive a theoretical equation for the drawing body shape along the working face in an inclined seam. By analyzing the initial positions of drawn marked particles, the characteristics of the drawing body shape for different seam dip angles are obtained. It is shown that the drawing body of the top coal exhibits a shape-difference and volume-symmetry characteristic, on taking a vertical line through the center of support opening as the axis of symmetry, the shapes of the drawing body on the two sides of this axis are clearly different, but their volumes are equal. By establishing theoretical models of the drawing body in the initial drawing stage and the normal drawing stage, a theoretical equation for the drawing body in an inclined seam is proposed, which can accurately describe the characteristics of the drawing body shape. The shape characteristics and volume symmetry of the drawing body are further analyzed by comparing the results of theoretical calculations and numerical simulations. It is shown that one side of the drawing body is divided into two parts by an inflection point, with the lower part being a variation development area. This variation development area increases gradually with increasing seam dip angle, resulting in an asymmetry of the drawing body shape. However, the volume symmetry coefficient fluctuates around 1 for all values of the seam dip angle variation, and the volumes of the drawing body on the two sides are more or less equal as the variation development volume is more or less equal to the cut volume. Both theoretical calculations and numerical simulations confirm that the drawing body of the top coal exhibits the shape-difference and volume-symmetry characteristic.