Hot Spot Radius Research Articles

Numerical study on the critical characteristics of internal short-circuit hot spot in lithium-ion batteries

High current density resulting from internal short circuit (ISC) in lithium-ion batteries leads to rapid local temperature rise, forming a hot spot. This study quantitatively characterized the critical properties of ISC hot spot in batteries. ISC hot spot in batteries exhibit two different evolution patterns. When the short circuit resistance approaches the internal resistance of the battery, the hot spot temperature reaches its peak at 1165 K. The thermal runaway characteristics of batteries vary depending on the short circuit resistance, with the hot spot growing and spreading rapidly when the resistance value is 30 mΩ. In addition, a modeling study was conducted to examine the critical characteristics of short circuit resistance. At a hot spot radius of 2 mm, the critical resistance was determined to be 281 mΩ, accompanied by a critical hot spot temperature of 562 K. Under subcritical conditions, the SEI film decomposes completely, while the electrolyte undergoes minimal decomposition. Interestingly, it was observed that the critical resistance and critical hot spot temperature decrease as the hot spot radius increases. Finally, the study quantified the effects of hot spot thermal conductivity, convective heat transfer coefficient, and hot spot position on the critical threshold of hot spots. These findings provide valuable references for the design of batteries with high critical thresholds.

Multi-regime reaction front and detonation initiation by temperature inhomogeneity

Auto-ignition, reaction front propagation, and detonation development are foundational events in combustion, and are relevant to the occurrence of engine knock. It is generally understood that different auto-ignition modes can be initiated by non-uniform initial temperatures, manifesting the transition from supersonic to subsonic combustion modes with increasing temperature gradients. In this work, we have investigated the auto-ignition and reaction front propagation of syngas/air mixtures initiated by wide-ranging temperature gradients, in both spherical and planar coordinates, and have identified a universal detonation response diagram with multiple, non-monotonic boundaries of auto-ignition modes under engine-relevant conditions. Specifically, it is shown that with increasing gradient steepness, in addition to the conventional three regimes of supersonic auto-ignition deflagration, detonation development, and subsonic auto-ignition deflagration, the reaction front propagation speed would first decrease dramatically and then increase, hence inducing additional detonation regimes. Consequently, two detonation peninsulas are identified, with the first corresponding to the well-established Bradley detonation peninsula and the second manifesting a broader detonation regime. Both detonation peninsulas depend on the hotspot size and they can connect together when the hotspot radius becomes sufficiently large. The transient auto-ignition processes and chemical-gas dynamic interactions agree with the typical characteristics of various auto-ignition modes. Finally, auto-ignition modes are summarized in the detonation diagram, in which the Bradley detonation peninsula is well reproduced and the new detonation peninsula is quantitatively determined. The present study demonstrates that auto-ignition modes are significantly affected by the non-monotonic behavior of reaction front propagation, and the use of actual propagation speed is necessary for steeper temperature gradients in order to determine more accurate dimensionless parameters.

An Investigation of Effective Parameters in Deuterium-Tritium Fuel Gains in Shock Ignition Method Driven by Laser Beams

Shock ignition (SI) is one of the methods considered in the concept of inertial confinement fusion (ICF). This two-step ICF process separates fuel assembly and ignition, relaxing the driver requirements and promising high gains. In SI scheme, a strong spherical shock wave converging at the end of the initial laser pulse ignites the pre-compressed fuel. In shock ignition, when the hot spot pressure is much higher than the surrounding cold fuel pressure, the fuel structure is considered non-isobaric. In this research, ignition conditions and fuel efficiency in shock ignition method were investigated. Then, the fuel efficiency correlations of total fuel energy, fuel gain and hot-spot radius in a non-isobaric model of fuel assembly were improved and compared with the numerical results of deuterium-tritium (DT) homogeneous fuel in SI scenario. Calculations showed that the hot spot formation conditions depend on the hot spot density and the surrounding cold fuel. Furthermore, using the improved energy efficiency equation, the role of physical parameters such as fuel mass and different ratios of hot spot pressure to the surrounding cold fuel pressure in energy efficiency were investigated.

Effects of strain rate and Lewis number on forced ignition of laminar counterflow diffusion flames

Forced ignition is one of the most fundamental and important combustion problems. It is used in advanced engines and closely related to fire safety and accidental explosion. In this study, forced ignition of laminar counterflow diffusion flames with one-step chemistry is studied through two-dimensional simulations. The initial ignition kernel is simply modelled as a hot spot of unburned gas with a specified radius and uniform high temperature inside it. We focus on the subsequent ignition kernel development, during which an expanding axisymmetric triple flame appears under certain conditions. Non-monotonic change of the flame kernel radius with time is observed near the critical ignition conditions. The effects of strain rate and Lewis number on the ignition kernel development and critical ignition conditions are assessed. For the same initial hot spot, successful ignition can be achieved at relatively low strain rate and fuel Lewis number; while the ignition kernel quenches at relatively high strain rate or fuel Lewis number. The strain rate and Lewis number are shown to have great impact on the critical hot spot radius and on the minimum ignition energy. At relatively low strain rates, the minimum ignition energy increases linearly with the strain rate. This is consistent with previous theory on flame ball in a mixing layer. Besides, the optimum ignition position and the influence of the hot spot shape are investigated. It is demonstrated that the optimum ignition position is close to the stoichiometric surface and it depends on the Lewis number and global equivalence ratio.

Thermal explosion of aluminum particles suspended in airflow

Thermal explosion of aluminum powder ASD-4 suspended in an air flow was studied experimentally using the model of hotspot ignition. A critical hotspot radius and ignition condition have been determined, and the eff ect of turbulence intensity on the development of the hotspot has been established.

Numerical and experimental study of heat-transfer characteristics of needle-to-ring-type ionic wind generator for heated-plate cooling

Ionic wind generators have shown significant application potential in devices for cooling, air actuation, and flow control. In this study, a needle-to-ring electrode ionic wind generator with optimized parameters was employed to cool a heated copper plate. A three-dimensional numerical simulation was conducted to obtain the electric, flow, and temperature fields of the ionic wind generator. To verify the ionic wind generator's capacity to cool the heated plate, an ionic wind generator prototype was fabricated and experimentally tested. The temperature distribution of the plate heated by a uniform distributed heat flux and non-uniform heat flux with a local hot spot was analyzed. Results show that the applied voltage should be less than the threshold voltage of spark discharge to ensure effective and stable operation of the ionic wind generator. When the plate is heated by a uniform heat flux, the temperature distribution in most areas around the center of the plate is relatively uniform. If a hot spot exists, the temperature of the central plate is higher than that of the surrounding areas, and the radial temperature difference gradually increases with the increase of the hot-spot heat flux. Moreover, the high-temperature area in the center of the plate gradually expands with increasing hot-spot radius. Besides, the increase in the copper plate thickness has little influence on the final temperature distribution. The heated plate with a uniform heat flux below 2 kW/m2 could be cooled to below 80 °C by the ionic wind generator at an applied voltage of 11 kV. When the plate is heated by a non-uniform heat flux with a hot spot, compared with the experimental results of natural convection, ionic wind can effectively reduce the temperature of the plate (a temperature drop of at least 45 °C was observed). This study provides a potential solution for commercial chip cooling with needle-to-ring-type ionic wind generators.

A theoretical model for low-mode asymmetries in ICF implosions

Low-mode asymmetry is known to be a main source of yield degradation in implosion experiments performed at the National Ignition Facility at the Lawrence Livermore National Laboratory. In this paper, we present a theoretical model of the deceleration phase to investigate low-mode asymmetries, which is derived by considering the main fuel layer to be composed of thin shell pieces and neglecting the interaction between these pieces in the longitudinal direction. The model is able to characterize the evolution of low-mode asymmetries and assess the corresponding performance degradation, as validated numerically using the radiation hydrodynamics code LARED-S. The deceleration phases of implosions modulated separately by P2 (in Legendre polynomials) asymmetries in the shell mass, shell velocity, and hot-spot radius are studied using this model. It is found that asymmetries in the shell velocity and hot-spot radius have more pronounced effects than shell mass asymmetry on capsule distortion, resulting in greater yield degradation. The results obtained using this model indicate that yield degradation is mainly caused by the increase in residual kinetic energy at stagnation time, which is identical for all three types of asymmetries.

Visceral leishmaniasis: Spatiotemporal heterogeneity and drivers underlying the hotspots in Muzaffarpur, Bihar, India.

BackgroundDespite the overall decrease in visceral leishmaniasis (VL) incidence on the Indian subcontinent, there remain spatiotemporal clusters or ‘hotspots’ of new cases. The characteristics of these hotspots, underlying transmission dynamics, and their importance for shaping control strategies are not yet fully understood and are investigated in this study for a VL endemic area of ~100,000 inhabitants in Bihar, India between 2007–2015.Methodology/Principal findingsVL incidence (cases/10,000/year) dropped from 12.3 in 2007 to 0.9 in 2015, which is just below the World Health Organizations’ threshold for elimination as a public health problem. Clustering of VL was assessed between subvillages (hamlets), using multiple geospatial and (spatio)temporal autocorrelation and hotspot analyses. One to three hotspots were identified each year, often persisting for 1–5 successive years with a modal radius of ~500m. The relative risk of having VL was 5–86 times higher for inhabitants of hotspots, compared to those living outside hotspots. Hotspots harbour significantly more households from the two lowest asset quintiles (as proxy for socio-economic status). Overall, children and young adelescents (5–14 years) have the highest risk for VL, but within hotspots and at the start of outbreaks, older age groups (35+ years) show a comparable high risk.Conclusions/SignificanceThis study demonstrates significant spatiotemporal heterogeneity in VL incidence at subdistrict level. The association between poverty and hotspots confirms that VL is a disease of ‘the poorest of the poor’ and age patterns suggest a potential role of waning immunity as underlying driver of hotspots. The recommended insecticide spraying radius of 500m around detected VL cases corresponds to the modal hotspot radius found in this study. Additional data on immunity and asymptomatic infection, and the development of spatiotemporally explicit transmission models that simulate hotspot dynamics and predict the impact of interventions at the smaller geographical scale will be crucial tools in sustaining elimination.

Properties of hot-spot emission in a warm plastic-shell implosion on the OMEGA laser system

A warm plastic-shell implosion is performed on the OMEGA laser system. The measured corona plasma evolution and shell trajectory in the acceleration phase are reasonably simulated by the one-dimensional lilac simulation including the nonlocal and cross-beam energy transfer models. The results from analytical thin-shell model reproduce the time-dependent shell radius by lilac simulation and also the hot-spot x-ray-emissivity profile at stagnation predicted by spect3d. In the spect3d simulations within a clean implosion, a U-shaped hot-spot radius evolution can be observed with the Kirkpatrick-Baez microscope response (the photon energy is from 4 to 8 keV). However, a fading-away hot-spot radius evolution is measured in OMEGA warm plastic-shell implosion because of mixings. To recover the measured hot-spot x-ray emissivity profile at stagnation, a nonisobaric hot-spot model is built and the normalized hot-spot temperature, density, and pressure profiles (normalized to the corresponding target-center values) are obtained.

Symmetric cumulants as a probe of the proton substructure at LHC energies

We present a systematic study of the normalized symmetric cumulants, NSC(n,m), at the eccentricity level in proton-proton interactions at s=13TeV within a wounded hot spot approach. We focus our attention on the influence of spatial correlations between the proton constituents, in our case gluonic hot spots, on this observable. We notice that the presence of short-range repulsive correlations between the hot spots systematically decreases the values of NSC(2,3) and NSC(2,4) in mid- to ultra-central collisions while increases them in peripheral interactions. In the case of NSC(2,3) we find that, as suggested by data, an anti-correlation of ε2 and ε3 in ultra-central collisions, i.e. NSC(2,3)<0, is possible within the correlated scenario while it never occurs without correlations when the number of gluonic hot spots is set to three. We attribute this fact to the decisive role of correlations on enlarging the probability of interaction topologies that reduce the value of NSC(2,3) and, eventually, make it negative. Further, we explore the dependence of our conclusions on the number of hot spots, the values of the hot spot radius and the repulsive core distance. Our results add evidence to the idea that considering spatial correlations between the subnucleonic degrees of freedom of the proton may have a strong impact on the initial state properties of proton-proton interactions [1].

X-Ray Observations of Magnetar SGR 0501+4516 from Outburst to Quiescence

Abstract Magnetars are neutron stars having extreme magnetic field strengths. Study of their emission properties in quiescent state can help understand effects of a strong magnetic field on neutron stars. SGR 0501+4516 is a magnetar that was discovered in 2008 during an outburst, which has recently returned to quiescence. We report its spectral and timing properties measured with new and archival observations from the Chandra X-ray Observatory, XMM-Newton, and Suzaku. We found that the quiescent spectrum is best fit by a power-law plus two blackbody model, with temperatures of kT low ∼ 0.26 keV and kT high ∼ 0.62 keV. We interpret these two blackbody components as emission from a hotspot and the entire surface. The hotspot radius shrunk from 1.4 km to 0.49 km since the outburst, and there was a significant correlation between its area and the X-ray luminosity, which agrees well with the prediction by the twisted magnetosphere model. We applied the two-temperature spectral model to all magnetars in quiescence and found that it could be a common feature among the population. Moreover, the temperature of the cooler blackbody shows a general trend with the magnetar field strength, which supports the simple scenario of heating by magnetic field decay.

An investigation of the hot spot formation mechanism for energetic material

The shock-induced pore collapsing and hot spot formation processes of plastic bonded explosives are simulated by molecular dynamics. After shock loading, the temperature field, pressure field, particle velocity field, energy field, plastic work field, and plastic temperature field are calculated by using the virtual grid method. A set of microscopic parameters about the hot spot are evaluated, including the pore collapsing time, pore collapsing speed, plastic work, and hot spot radius. The physical models to describe the energy dissipation and temperature relaxation behaviors of the hot spot are developed. We find that the hot spot formation consists of three steps: pore collapsing, work-heat transition, and temperature relaxation. The pore collapsing speed is proportional to the piston speed, and the temperature relaxation time is proportional to the square of the hot spot radius.

The physics of long- and intermediate-wavelength asymmetries of the hot spot: Compression hydrodynamics and energetics

The effect of asymmetries on the performance of inertial confinement fusion implosions is investigated. A theoretical model is derived for the compression of distorted hot spots, and quantitative estimates are obtained using hydrodynamic simulations. The asymmetries are divided into low (ℓ&amp;lt;6) and intermediate (6&amp;lt;ℓ&amp;lt;40) modes by comparison of the mode wavelength with the hot-spot radius and the thermal-diffusion scale length. Long-wavelength modes introduce substantial nonradial motion, whereas intermediate-wavelength modes involve more cooling by thermal losses. It is found that for distorted hot spots, the measured neutron-averaged properties can be very different from the real hydrodynamic conditions. This is because mass ablation driven by thermal conduction introduces flows in the Rayleigh–Taylor bubbles that results in pressure variations, in addition to temperature variations between the bubbles and the neutron-producing region. The differences are less pronounced for long-wavelength asymmetries since the bubbles are relatively hot and sustain fusion reactions. The yield degradation—with respect to the symmetric case—results primarily from a reduction in the hot-spot pressure for low modes and from a reduction in burn volume for intermediate modes. A general expression is found relating the pressure degradation to the residual shell energy and the flow within the hot spot (i.e., the total residual energy).

Simultaneous X-ray and radio observations of the radio-mode-switching pulsar PSR B1822−09

We report on simultaneous X-ray and radio observations of the radio-mode-switching pulsar PSR B1822-09 with ESA's XMM-Newton and the WSRT, GMRT and Lovell radio telescopes. PSR B1822-09 switches between a radio-bright and radio-quiet mode, and we discovered a relationship between the durations of its modes and a known underlying radio-modulation timescale within the modes. We discovered X-ray (energies 0.2-1.4 keV) pulsations with a broad sinusoidal pulse, slightly lagging the radio main pulse in phase by 0.094 +/- 0.017, with an energy-dependent pulsed fraction varying from ~0.15 at 0.3 keV to ~0.6 at 1 keV. No evidence is found for simultaneous X-ray and radio mode switching. The total X-ray spectrum consists of a cool component (T ~ 0.96 x 10^6 K, hot-spot radius R ~ 2.0 km) and a hot component (T ~ 2.2 x 10^6 K, R ~ 100 m). The hot component can be ascribed to the pulsed emission and the cool component to the unpulsed emission. The high-energy characteristics of PSR B1822-09 resemble those of middle-aged pulsars such as PSR B0656+14, PSR B1055-52 and Geminga, including an indication for pulsed high-energy gamma-ray emission in Fermi LAT data. Explanations for the high pulsed fraction seem to require different temperatures at the two poles of this orthogonal rotator, or magnetic anisotropic beaming effects in its strong magnetic field. In the X-ray skymap we found a harder source at only (5.1+/- 0.5 )arcsec from PSR B1822-09, which might be a pulsar wind nebula.

Hot spot formation and stagnation properties in simulations of direct-drive NIF implosions

We investigate different proposed methods of increasing the hot spot energy and radius in inertial confinement fusion implosions. In particular, shock mistiming (preferentially heating the inner edge of the target's fuel) and increasing the initial vapor gas density are investigated as possible control mechanisms. We find that only the latter is effective in substantially increasing the hot spot energy and dimensions while achieving ignition. In all cases an increase in the hot spot energy is accompanied by a decrease in the hot spot energy density (pressure) and both the yield and the gain of the target drop substantially. 2D simulations of increased vapor density targets predict an increase in the robustness of the target with respect to surface perturbations but are accompanied by significant yield degradation.

Density Gradient Effect on the Non-Linear Spectrum of the Rayleigh–Taylor Instability

Nonlinear spectrum of the deceleration phase Rayleigh–Taylor instability (RTI) has been investigated in the inertial confinement fusion. Growth rate of the deceleration phase RTI has been expressed well in the form of $$ \upgamma_{\text{k}} =\upalpha_{2} \sqrt {\frac{\text{kg}}{{1 + {\text{kL}}_{\text{m}} }}} -\upbeta_{2} {\text{ku}}_{\text{a}} , $$ where α and β are constants. g, k, L m and u a are the target acceleration, the wave number, the finite density gradient scale length and the ablation velocity, respectively (Betti et al. in Phys Plasmas 5:1446, 1998). Analytically obtained results indicate that the mass power spectrum can be described as an inverse power law with an approximate spectral index of 2.3 in the limit of $$ {\text{kL}}_{0} \ll 1 $$ , where k is the wave number and L 0 is the ablation front thickness and we have shown that power-law slope for kL m ≫ 1 is the most that it is changing with k −4. Furthermore, it has been found that nonlinear power spectrum decreases slowly by increasing of the hot spot radius. Our obtained value is in agreement with theoretical findings (Keskinen et al. in Phys. Plasmas 14:012705, 2007).

Improvement of non-isobaric model for shock ignition

In this paper, improved relations of total fuel energy, fuel gain, hot-spot radius and total areal density in a non-isobaric model of fuel assembly have been derived and compared with the numerical results of [J. Schmitt, J.W. Bates, S.P. Obenschain, S.T. Zalesak, D.E. Fyfe, Phys. Plasmas 17, 042701 (2010); S. Atzeni, A. Marocchino, A. Schiavi, G. Schurtz, New J. Phys. 15, 045004 (2013)] and several simulations performed by MULTI-1D radiation hydrodynamic code for shock ignition scenario. Our calculations indicate that the approximations made by [M.D. Rosen, J.D. Lindl, A.R. Thiessen, LLNL Laser Program Annual Report, UCRL-50021-83, pp. 3–5 (1983); J. Schmitt, J.W. Bates, S.P. Obenschain, S.T. Zalesak, D.E. Fyfe, Phys. Plasmas 17, 042701 (2010)] for the calculation of burn-up fraction are not accurate enough to give results consistent with simulations. Therefore, we have introduced more appropriate approximations for the burn-up fraction and total areal density of the fuel that are in more agreement with simulation results of shock ignition. Meanwhile, it is shown that the related formulas of the non-isobaric model for total fuel energy, fuel gain and also hot-spot radius cannot determine the model parameters independently, but improved model choose a better selection and less restrictions on determination of the parameters for the non-isobaric model. Such derivations can be used in theoretical studies of the ignition conditions and burn-up fraction of the fuel in shock ignition scenario.

Using penumbral imaging to measure micrometer size plasma hot spots in Gbar equation of state experiments on the National Ignition Facility.

We have developed an experimental platform for absolute equation of state measurements up to Gbar pressures on the National Ignition Facility (NIF) within the Fundamental Science Program. We use a symmetry-tuned hohlraum drive to launch a spherical shock wave into a solid CH sphere. Streaked radiography is the primary diagnostic to measure the density change at the shock front as the pressure increases towards smaller radii. At shock stagnation in the center of the capsule, we observe a short and bright x-ray self emission from high density (∼50 g/cm(3)) plasma at ∼1 keV. Here, we present results obtained with penumbral imaging which has been carried out to characterize the size of the hot spot emission. This allows extending existing NIF diagnostic capabilities for spatial resolution (currently ∼10 μm) at higher sensitivity. At peak emission we find the hot spot radius to be as small as 5.8 +/- 1 μm, corresponding to a convergence ratio of 200.

Scattering of carbon ions in the material of the protective membrane of a fast ignition, indirect compression target without cone

AbstractIt is shown that scattering of the laser-accelerated carbon ions in the material of the protective membrane of a fast ignition, indirect compression target without cone can result in a significant increase in the hot spot radius and will complicate significantly or even prevent the effective shaping of the bombarded region of the compressed fuel.

Studies on the robustness of shock-ignited laser fusion targets

Several aspects of the sensitivity of a shock-ignited inertial fusion target to variation of parameters and errors or imperfections are studied by means of one-dimensional and two-dimensional numerical simulations. The study refers to a simple all-DT target, initially proposed for fast ignition (Atzeni et al 2007 Phys. Plasmas 7 052702) and subsequently shown to be also suitable for shock ignition (Ribeyre et al 2009 Plasma Phys. Control. Fusion 51 015013). It is shown that the growth of both Richtmyer–Meshkov and Rayleigh–Taylor instability (RTI) at the ablation front is reduced by laser pulses with an adiabat-shaping picket. An operating window for the parameters of the ignition laser spike is described; the threshold power depends on beam focusing and synchronization with the compression pulse. The time window for spike launch widens with beam power, while the minimum spike energy is independent of spike power. A large parametric scan indicates good tolerance (at the level of a few percent) to target mass and laser power errors. 2D simulations indicate that the strong igniting shock wave plays an important role in reducing deceleration-phase RTI growth. Instead, the high hot-spot convergence ratio (ratio of initial target radius to hot-spot radius at ignition) makes ignition highly sensitive to target mispositioning.