Runaway Modes Research Articles

Energy loss mechanisms of transition from pump mode to turbine mode of an axial-flow pump under bidirectional conditions

Pump-as-turbine stations with ultra-low head often face the deterioration risk during load rejection operations when the demands in water delivery and power generation are satisfied. In the course of load rejection, significant energy transfer and hydraulic instability may ensue from the pump-to-turbine transition mode, and vice versa. In order to investigate the variation of energy loss during the transition process of a bidirectional axial-flow pump, the entropy production method is herein utilized for visualization and quantitative analysis of high energy loss regions within key components. In the process of transient simulation, the torque balance equation is applied for calculating the rotational speed of impeller. Two-way transitions are considered with the bidirectional pump for comparing two distinct forms of pumps equipped with front and rear guide vanes. Results show that the simulated external characteristic curves and transition parameters of the pump are in good accordance with the experimental data. The transient process of the transition successively goes through four modes, i.e. pump, braking, turbine and runaway modes. In the process of the direction switch of the main flow and rotation, the different positions of guide vanes also deeply influence the inflow angle and flow pattern in impeller, and then change the distribution of the pressure coefficient, vorticity and entropy production rate. In pump mode, the entropy production rate and pressure coefficient at the impeller inlet edge under backward transition condition (BTC) is higher than that under forward transition condition (FTC), due to the smaller inflow angle caused by the front guide vanes under BTC. In runaway operation, the total entropy production (TEP) under FTC is obviously higher than that under BTC, because the energy loss is reduced by the broken shedding vortices caused by the obstruction of the guide vanes under BTC. This study's results can be referred to while seeking for the operation safety and stability of the pump stations consecutively used to pump and generate power.

DYNAMICS OF SPRINGY SYSTEMS OF PACKAGING LINES FOR FOOD PRODUCTS

The purpose of the study was to develop mathematical models and proposals for improving the technology and equipment for the supply of roll packaging materials. The analysis of features of systems for transportation of packing materials, dynamics of transient processes, research methods concerning movements of rolled materials, features of loadings in systems of cyclic action with springy elements are resulted in the article. It is shown that the modern theoretical basis of synthesis of technological machines on the basis of use of film rolled materials combines possibilities of the account of technological requirements, indicators of high productivity, neighborhood conditions, restriction of power parameters. Mathematical formalizations concerning the ratios of force factors in systems of transportation of loads of cyclic action at the levels of external force actions, generated friction forces, geometrical, kinematic and dynamic parameters were obtained. Mathematical models concerning runaway modes and estimation of stiffness of springy bonds, on the basis of film materials, methods of estimation of system parameters were offered. Synchronized combination of operations, the use of optimal laws of movement of working bodies, limitation of dynamic loads was carried to the positive directions of creating efficient technological machines and lines. It is proposed to determine the ratio of the run-out time of rolls of variable mass by the ratio of the squares of the initial and final radii. The solution of the problem of kinetic energy recovery in systems of periodic action is simultaneously connected with the restriction of the non-uniformity of the drive of machines.

Main modes of runaway electron generation during a breakdown of high-pressure gases in an inhomogeneous electric field

Using our recently proposed method, we have identified three modes of runaway electron generation. This method is based on the recording of a dynamic displacement current using a current collector. It makes it possible to unambiguously interpret the dynamics of ionization processes occurring at the stage of formation of a high-voltage nanosecond discharge in a strongly inhomogeneous electric field and dense gaseous media. Simultaneous and separate recording by the current collector of a runaway electron beam current and the dynamic displacement current as well as the recording of discharge characteristics and high-speed framing of plasma glow at the breakdown stage made it possible to interpret the runaway electron generation modes in terms of the rate of ionization processes. It has been shown how the reduced electric field strength, the gas kind, the design and material of a high-voltage cathode with a small radius of curvature, the amplitude and rise time of a voltage pulse as well as the delay in the onset of the explosive emission process at the cathode affect the implementation of one or another generation mode.

Thermal runaway propagation behavior within 18,650 lithium-ion battery packs: A modeling study

Abstract Thermal runaway and subsequent propagation are the main factors to cause catastrophic consequences in lithium-ion battery packs. Exploring the thermal runaway propagation is thus of great fundamental and practical interest in understanding the mechanism of battery safety. A thermal runaway propagation mathematical model is established by combining the 0 D thermal runaway, and electrical and thermal conduction models that are verified by a series of experiments where thermal runways are triggered by mechanical abusive loading. Two thermal runaway propagation modes are observed and it is found that overheating of the local area or high overall temperature determines the propagation mode. The governing factors of thermal runaway propagation speed, including ambient temperature, packing spacing, and stacking form, are further analyzed. Our analysis reveals a complete link between engineering design variables and the thermal runaway behaviors of a specific battery pack. Our study paves a novel avenue to design the safer and higher energy density lithium-ion battery pack and elevates the limits of battery pack energy density without sacrificing safety risks.

Influence and assessment of AIBN on thermal hazard under process situations

Free radical polymerization is an effective method for large-scale production of various olefin polymers, in which the initiator is one of the emphases in the study of different chain reaction mechanisms in chemical industry. The azo compounds are widely used in organic synthesis to initiate chain reactions. Azos are divided into different types according to the applicable temperature range. 2,2′-Azobis(2-methylpropionitrile) is suitable for moderate temperatures and well known. In the past, the chemical and free radical reactions of 2,2′-Azobis(2-methylpropionitrile) have been studied. However, the kinetic model and reaction mechanism of initiators under process conditions are still lacking. The thermal hazard characteristics evaluated by process situation of 2,2′-Azobis(2-methylpropionitrile) were discussed in this study. The kinetics analysis on AIBN is based on detailed information including rate constants, reaction orders and activation energies of the decomposition reaction. Simulated heat exchange patterns of initiators between bulk packaging and the environment can be used to collect relevant heat hazard parameters. The results of the analysis are directly related to the storage and transportation safety of the 2,2′-Azobis(2-methylpropionitrile). Finally, a series of evaluation models about the thermal safety of initiators are constructed and analyzed by simulating the runaway modes of the 2,2′-Azobis(2-methylpropionitrile) at different temperatures.

Phase-Inversion Polymer Composite Separators Based on Hexagonal Boron Nitride Nanosheets for High-Temperature Lithium-Ion Batteries.

By preventing electrical contact between anode and cathode electrodes while promoting ionic transport, separators are critical components in the safe operation of rechargeable battery technologies. However, traditional polymer-based separators have limited thermal stability, which has contributed to catastrophic thermal runaway failure modes that have conspicuously plagued lithium-ion batteries. Here, we describe the development of phase-inversion composite separators based on carbon-coated hexagonal boron nitride (hBN) nanosheets and poly(vinylidene fluoride) (PVDF) polymers that possess high porosity, electrolyte wettability, and thermal stability. The carbon-coated hBN nanosheets are obtained through a scalable liquid-phase shear exfoliation method using ethyl cellulose as a polymer stabilizer and source of the carbon coating following thermal pyrolysis. When incorporated within the PVDF matrix, the carbon-coated hBN nanosheets promote favorable interfacial interactions during the phase-inversion process, resulting in porous, flexible, free-standing composite separators. The unique chemical composition of these carbon-coated hBN separators implies high wettability for a wide range of liquid electrolytes. This combination of high porosity and electrolyte wettability enables enhanced ionic conductivity and lithium-ion battery electrochemical performance that exceeds incumbent polyolefin separators over a wide range of operating conditions. The hBN nanosheets also impart high thermal stability, providing safe lithium-ion battery operation up to 120 °C.

Spacetime instability due to quantum gravity

We show that quantum gravity yields exponentially growing gravitational waves. Without a mechanism to stop these modes from growing, the universe would go through a gravitational collapse. For Minkowski background, we propose a solution by choosing an integration contour in Fourier space that does not enclose the problematic modes, thus preventing them from showing up in the effective theory. It turns out that this is only possible when the modes are removed altogether. For an expanding universe, we argue that the runaway modes can be managed accordingly to the dynamics of the Hubble constant, leading to important implications for astrophysics.

Ultra-light and strong: The massless harmonic oscillator and its singular path integral

In classical mechanics, a light particle bound by a strong elastic force just oscillates at high frequency in the region allowed by its initial position and velocity. In quantum mechanics, instead, the ground state of the particle becomes completely de-localized in the limit [Formula: see text]. The harmonic oscillator thus ceases to be a useful microscopic physical model in the limit [Formula: see text], but its Feynman path integral has interesting singularities which make it a prototype of other systems exhibiting a “quantum runaway” from the classical configurations near the minimum of the action. The probability density of the coherent runaway modes can be obtained as the solution of a Fokker–Planck equation associated to the condition [Formula: see text]. This technique can be applied also to other systems, notably to a dimensional reduction of the Einstein–Hilbert action.

Self-induced neutrino flavor conversion without flavor mixing

Neutrino-neutrino refraction in dense media can cause self-induced flavor conversion triggered by collective run-away modes of the interacting flavor oscillators. The growth rates were usually found to be of order a typical vacuum oscillation frequency Δ m2/2E. However, even in the simple case of a νe beam interacting with an opposite-moving ν̄e beam, and allowing for spatial inhomogeneities, the growth rate of the fastest-growing Fourier mode is of order μ=√2 GF nν, a typical ν–ν interaction energy. This growth rate is much larger than the vacuum oscillation frequency and gives rise to flavor conversion on a much shorter time scale. This phenomenon of “fast flavor conversion” occurs even for vanishing Δ m2/2E and thus does not depend on energy, but only on the angle distributions. Moreover, it does not require neutrinos to mix or to have masses, except perhaps for providing seed disturbances. We also construct a simple homogeneous example consisting of intersecting beams and study a schematic supernova model proposed by Ray Sawyer, where νe and ν̄e emerge with different zenith-angle distributions, the key ingredient for fast flavor conversion. What happens in realistic astrophysical scenarios remains to be understood.

Collective neutrino flavor conversion: Recent developments

Neutrino flavor evolution in core-collapse supernovae, neutron-star mergers, or the early universe is dominated by neutrino–neutrino refraction, often spawning “self-induced flavor conversion,” i.e., shuffling of flavor among momentum modes. This effect is driven by collective run-away modes of the coupled “flavor oscillators” and can spontaneously break the initial symmetries such as axial symmetry, homogeneity, isotropy, and even stationarity. Moreover, the growth rates of unstable modes can be of the order of the neutrino–neutrino interaction energy instead of the much smaller vacuum oscillation frequency: self-induced flavor conversion does not always require neutrino masses. We illustrate these newly found phenomena in terms of simple toy models. What happens in realistic astrophysical settings is up to speculation at present.

Self-induced flavor conversion of supernova neutrinos on small scales

Self-induced flavor conversion of supernova (SN) neutrinos is a generic feature of neutrino-neutrino dispersion. The corresponding run-away modes in flavor space can spontaneously break the original symmetries of the neutrino flux and in particular can spontaneously produce small-scale features as shown in recent schematic studies. However, the unavoidable ``multi-angle matter effect'' shifts these small-scale instabilities into regions of matter and neutrino density which are not encountered on the way out from a SN. The traditional modes which are uniform on the largest scales are most prone for instabilities and thus provide the most sensitive test for the appearance of self-induced flavor conversion. As a by-product we clarify the relation between the time evolution of an expanding neutrino gas and the radial evolution of a stationary SN neutrino flux. Our results depend on several simplifying assumptions, notably stationarity of the solution, the absence of a ``backward'' neutrino flux caused by residual scattering, and global spherical symmetry of emission.

Evidence for non-conservative current-induced forces in the breaking of Au and Pt atomic chains.

This experimental work aims at probing current-induced forces at the atomic scale. Specifically it addresses predictions in recent work regarding the appearance of run-away modes as a result of a combined effect of the non-conservative wind force and a ‘Berry force’. The systems we consider here are atomic chains of Au and Pt atoms, for which we investigate the distribution of break down voltage values. We observe two distinct modes of breaking for Au atomic chains. The breaking at high voltage appears to behave as expected for regular break down by thermal excitation due to Joule heating. However, there is a low-voltage breaking mode that has characteristics expected for the mechanism of current-induced forces. Although a full comparison would require more detailed information on the individual atomic configurations, the systems we consider are very similar to those considered in recent model calculations and the comparison between experiment and theory is very encouraging for the interpretation we propose.

-symmetric interpretation of the electromagnetic self-force

In 1980 Englert examined the classic problem of the electromagnetic self-force on an oscillating charged particle. His approach, which was based on an earlier idea of Bateman, was to introduce a time-reversed (charge-conjugate) particle and to show that the two-particle system is Hamiltonian. Unfortunately, Englert’s model did not solve the problem of runaway modes, and the corresponding quantum theory had ghost states. It is shown here that Englert’s Hamiltonian is symmetric, and that the problems with his model arise because the symmetry is broken at both the classical and the quantum level. However, by allowing the charged particles to interact and by adjusting the coupling parameters to put the model into an unbroken -symmetric region, one eliminates the classical nonrelativistic runaway modes and obtains a corresponding nonrelativistic quantum system that is in equilibrium and ghost free.

Absence of long-wavelength Cerenkov radiation with isotropic Lorentz andCPTviolation

Modified theories of electrodynamics that include violations of Lorentz symmetry often allow for the possibility of vacuum Cerenkov radiation. This phenomenon has previously been studied in a number of Lorentz-violating theories, but none of the methods that have previously been developed are sufficient to study a theory with a timelike Chern-Simons term ${k}_{AF}$, because such a term may generate exponentially growing solutions to the field equations. Searching for vacuum Cerenkov radiation in a theory with a purely timelike Chern-Simons term using only elementary methods, we find that, despite the presence of the runaway modes, a charge in uniform nonrelativistic motion does not radiate energy, up to second order in the velocity.

Evidence for a resonant cyclotron interaction between runaway electrons and MHD modes in the experimental advanced superconducting tokamak

In the past, the resonant cyclotron interaction between runaway electrons and lower hybrid waves via anomalous Doppler broadening was experimentally investigated, and it was shown to be able to create a barrier to the energy that could be reached by the runaway electrons [E. Li et al., Nucl. Instrum. Methods Phys. Res. A 621, 566 (2010)]. In this paper, to our knowledge for the first time, experimental evidence will be provided for a resonant cyclotron interaction between runaway electrons and magnetohydrodynamics modes in a stochastic magnetic field in the experimental advanced superconducting tokamak (EAST), which has been theoretically proposed as a mechanism able to limit the maximum attainable energy by runaway electrons in tokamak plasmas [J. R. Martín-Solís and R. Sánchez, Phys. Plasmas 15, 112505 (2008)].

Momentum constraint relaxation

Full relativistic simulations in three dimensions invariably develop runaway modes that grow exponentially and are accompanied by violations of the Hamiltonian and momentum constraints. Recently, we introduced a numerical method (Hamiltonian relaxation) that greatly reduces the Hamiltonian constraint violation and helps improve the quality of the numerical model. We present here a method that controls the violation of the momentum constraint. The method is based on the addition of a longitudinal component to the traceless extrinsic curvature , generated by a vector potential wi, as outlined by York. The components of wi are relaxed to solve approximately the momentum constraint equations, slowly pushing the evolution towards the space of solutions of the constraint equations. We test this method with simulations of binary neutron stars in circular orbits and show that it effectively controls the growth of the aforementioned violations. We also show that a full numerical enforcement of the constraints, as opposed to the gentle correction of the momentum relaxation scheme, results in the development of instabilities that stop the runs shortly.

Runaway sexual selection and modes of inheritance

Assume additive gene action and no selection on the preference. Rescale the display trait and preference such that T = 0 and P = 0 is the equilibrium. The dynamic equations for the evolutionary change in the display and preference means near the equilibrium are DTf = tTf ≠Tf Tf + H1 tTf ≠Tf L HTm + GmT HkT Tm + kPT PfLL Tf; DTm = tTm ≠Tm HTm + GmT HkT Tm + kPT PfLL + H1 tTm ≠Tm L Tf Tm; DPf = tPf ≠Pf Pf + H1 tPf ≠Pf L HPm + GmPT HkT Tm + kPT PfLL Pf; DPm = tPm ≠Pm HPm + GmPT HkT Tm + kPT PfLL + H1 tPm ≠Pm L Pf Pm; where Tf and Tm are respectively the mean values for the display trait in females and males, and Pf and Pm the mean values for the preference. The notation for the transmission coefficients is: Here KJB tP f ≠Pf tiff ≠iff + tiff ≠ifm tPm ≠Pm timm ≠imf + timm ≠imm tT f ≠Tf t jff ≠ jff + tjff ≠ jfm tTm ≠Tm t jmm ≠ jmf + t jmm ≠ jmm Stability

Polar interface vibrations in GaN/AlN quantum dots: Essential effects of crystal anisotropy

The inherent anisotropy of crystal lattices of the nitride semiconductor compounds is found to essentially determine the character of surface polar vibrations of a GaN quantum dot in AlN matrix. The interface phonons are analyzed within the framework of the anisotropic macroscopic dielectric continuum model. Analytical solutions are obtained for surface modes on a quantum dot of oblate spheroidal form. These modes can exist in continuous frequency regions, in contrast to quantized frequencies that are characteristic for isotropic case. The period of spatial oscillations in these modes varies substantially over the dot surface, so that the oscillations can have condensation points at the dot poles. Along with truly localized surface states, there are two other types of phonon modes. First, runaway modes, which freely leave the dot surface through escape roots in equatorial regions. Second, quasistationary (leaky) states, in which the areas of spatial oscillations are separated from the escape root regions by the areas of exponential behavior. The leaky states can provide effective energy relaxation of the confined electrons.

Delay equations and radiation damping

Starting from delay equations that model field retardation effects, we study the origin of runaway modes that appear in the solutions of the classical equations of motion involving the radiation reaction force. When retardation effects are small, we argue that the physically significant solutions belong to the so-called slow manifold of the system and we identify this invariant manifold with the attractor in the state space of the delay equation. We demonstrate via an example that when retardation effects are no longer small, the motion could exhibit bifurcation phenomena that are not contained in the local equations of motion.

Effective Dipole-Radiation-Field Theory: III. Three-Dimensional Oscillator

The novel analysis of the interaction between a harmonically bound, nonrelativistic "isotropic point" charge and the electromagnetic field as presented in paper I [Int. J. Mod. Phys.B8, 2307 (1994)] and II [Int. J. Mod. Phys.B10, 1211 (1996)], is finally extended to the case of a three-dimensional oscillator. The coupling between the electron and the field is treated through all orders beyond the standard dipole model. After a statistical linearization of the highly nonlinear dynamics, the problem is solved exactly in terms of the system's normal modes. The procedure intrinsically involves a generalized mass renormalization. The solution is free of runaway modes. The quantum mechanical ultraviolet divergence known from the standard dipole model is shown to be suppressed by the generalized coupling. Inter alia an effective equation of motion for the charge is derived. It is also shown that the zero-coupling and the infinite-system limits do not commute.