Analysis of Spontaneous Ignition of Grass: Chemical Oxidation and Water Vapor Sorption

Self-heating of biomass by chemical oxidation, which can cause spontaneous ignition, is a safety and management concern. This process can be accelerated by aerobic fermentation and water vapor sorption. The chemical oxidation and water vapor sorption of grass were studied in a laboratory oven, measuring the variations in weight and the internal temperature of a sphere with grass within a flexible polymeric network. Both processes were simulated to prove that the proposed mathematical model could fit the experimental data. It was observed that the water vapor sorption capacity of the grass was high, so the experimental increase in the internal temperature of a spherical body was around 47 K, from 73°C to 120°C. This fact can be very important because the chemical oxidation of grass accelerates at high temperatures. For scaling, simulation programs were used to study the sorption and oxidation processes with an increase in internal temperature in spherical bodies and infinite plane slabs. These results can be used to obtain those of other geometric symmetries by interpolation. It was deduced that at 70°C and with vapor sorption, the ignition time can be around 3 days to 5 days, while without vapor sorption, the ignition times can be around 110 days to 140 days. For 35°C the ignition times with vapor sorption can be around 12 days to 18 days, while without vapor sorption the ignition times can be around 3700 days to 4500 days. These results can be of interest for warehouses of similar biomass and for forestry research and management groups of wildfires. Graphical Abstract

Interaction between Screw Dislocation and Interfacial Crack in Fine-Grained Piezoelectric Coatings under Steady-State Thermal Loading

The mechanical behavior of fine-grained piezoelectric/substrate structure with screw dislocation and interface edge crack under the coupling action of heat, force and electricity are studied. Using the mapping function method, firstly, the finite area plane is transformed into the right semi-infinite plane, then the expression of the temperature field is given with the help of the complex function, and then the temperature field of the problem is achieved. By constructing the general solution of the governing equation with temperature function, the analytical expression of the image force is derived. Finally, the effects of material parameters, temperature gradient, coating thickness and crack size on image force are analyzed by numerical examples. The results show that the temperature gradient has a very significant effect on the image force, and thicker coating is conducive to the stability of dislocation and interface crack.

Hysteresis in Supersonic Flow Past a Plane Cascade of Cylindrical Rods

Abstract— The results of numerical simulation of the interaction of supersonic flow with a permeable screen in form of an infinite plane cascade (lattice) of circular cylinders are given. The interaction regime in which the shocks ahead of the cylinders are localized on the scale of the cascade step is considered. The multi-block computational technique in which the viscous boundary layers are resolved by means of local grids using the Navier–Stokes equations, while the effects of inteferrence between the shock-wave structures in supersonic wake are described within the framework of Euler’s equations. The action of shock waves induced by the neighboring elements of lattice to the near-wake region behind the intermediate elements can ambiguously affect the aerodynamic lattice performance as well as generate time-dependent phenomena in the wake. The flow regimes are classified depending on continuous increase and decrease in the free-stream supersonic air flow in the Mach number range from 2.4 to 4.2 with reference to the lattice of the 80% permeability. The sources of the hysteresis behavior of the lattice aerodynamic drag with respect to the Mach number and the mechanisms of the onset of self-oscillating wake flow regimes are discussed.

Implementation of planar 3D hydraulic fracture model in rock with layered compressive stress

Plane 3D model of hydraulic fracture propagation is implemented. Fluid flow inside the fracture, leak-of, rock deformation and breaking are taken into account. Asymptotic solution for tip of semi-infinite plane fracture is used to set boundary conditions for fluid flow problem and to calculate fracture front propagation velocity. Elastic and fluid flow equations are united in one system of nonlinear equations and solved simultaneously by Newton method with analytically calculated Jacoby matrix. The implemented model may be used as a start point for testing various methods of solution of “hydrodynamic-elasticity” problem and improving their convergence speed. Also model can be used for developed hydraulic fracture simulation.

Analytical Model of an I-core Coil for Nondestructive Evaluation of a Conducting Cylinder below an Infinite Plane Conductor

An analytical model for eddy current testing of an I-core coil located above a two-layer conductive material is presented. The upper layer is an infinite plane conductor, and the bottom layer is a conductive cylinder. The method of truncated region eigenfunction expansion (TREE) is used to solve this axisymmetric problem. First the magnetic vector potential of a filamentary coil coaxial with the I-core over the two-layer conductor is considered. Then the closed form expression for the impedance of the multi-turn coil with rectangular cross section is derived by using the principle of superposition from the filamentary coil field. For frequencies ranging from 0.1 kHz to 10 kHz, both the impedance changes of the I-core coil located above the infinite plane conductor without the conducting cylinder, and in the absence of the two-layer conductor are calculated using Mathematica, respectively. The influence of the conducting cylinder below the infinite plane conductor on the impedance change is analyzed. The analytical calculation results are verified by the finite element method and experiment, the results agree very well, which verifies the correctness of the analytical model.

A temperature problem for a square: An exact solution

We construct examples of exact solutions of the temperature problem for a square: the sides of the square are (i) free and (ii) firmly clamped. Initially, we solve the inhomogeneous problem for an infinite plane. The known exact solutions for a square, with which the boundary conditions on the sides of the square are satisfied, are added to this solution. The solutions are represented as series in Papkovich–Fadle eigenfunctions whose coefficients are determined from simple formulas.