Showing papers on "Thermal equilibrium published in 2021"

Effects of half-sinusoidal nonuniform heating during MHD thermal convection in Cu–Al 2 O 3 /water hybrid nanofluid saturated with porous media

Abstract: The intent of this study is to demonstrate an approach for augmenting heat transfer through porous media subjected to nonuniform heating during the magnetohydrodynamic flow of a hybrid nanofluid of Cu–Al2O3/water. The efficacy of such a heating technique is examined utilizing a classical flow geometry consisting of a square cavity. The heating is made at the bottom following a half-sinusoidal function of different frequencies, along with the presence of a uniform magnetic field. The thermal conditions of the cavity, particularly at the bottom wall, drive thermo-hydrodynamics and associated heat transfer. Furthermore, the addition of different types of nanoparticles to the base liquid in order to boost the thermal performance of conventional fluids and mono-nanofluids is a current technique. The coupled nonlinear governing equations are solved numerically in dimensionless forms adapting the finite volume approach, the Brinkman–Forchheimer–Darcy model, local thermal equilibrium and single-phase model. The study is conducted for wide ranges of parametric impacts to analyze global heat transfer performance. The results of this study reveal that the multi-frequency spatial heating during hybrid nanofluid flow can be utilized as a powerful means to improve the thermal performance of a system operating under different ranges of parameters, even with the presence of porous media and magnetic fields. In addition to different heating frequencies, the variations in amplitude (I) and superposed uniform temperature ( $$\theta_{\text{os}}$$ ) to half-sinusoidal heating are also examined thoughtfully in the analysis for different concentrations of Cu–Al2O3 nanoparticles. Compared to the base liquid, the hybrid nanofluid can contribute toward higher heat transfer.

Thermal equilibration between singlet and triplet excited states in organic fluorophore for submicrosecond delayed fluorescence

Abstract: In any complex molecular system, electronic excited states with different spin multiplicities can be described via a simple statistical thermodynamic formalism if the states are in thermal equilibrium. However, this ideal situation has hitherto been infeasible for efficient fluorescent organic molecules. Here, we report a highly emissive metal-free purely organic fluorophore that enables thermal equilibration between singlet and triplet excited states. The key to this unconventional excitonic behavior is the exceptionally fast spin-flipping reverse intersystem crossing from the triplet to singlet excited states with a rate constant exceeding 108 per second, which is considerably higher than that of radiative decay (fluorescence) from the singlet excited state. The present fluorophoric system exhibits an emission lifetime as short as 750 nanoseconds and, therefore, allows organic light-emitting diodes to demonstrate external electroluminescence quantum efficiency exceeding 20% even at a practical high luminance of more than 10,000 candelas per square meter.

Hydrodynamic study of hyperon spin polarization in relativistic heavy ion collisions

Abstract: We perform a systematic study of the spin polarization of hyperons in heavy ion collisions using the music hydrodynamic model with a multiphase transport preequilibrium dynamics. Our model calculations nicely describe the measured collision energy, centrality, rapidity, and ${p}_{T}$ dependence of the $\mathrm{\ensuremath{\Lambda}}$ polarization. We also study and predict the global spin polarization of ${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}$ and ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}}$ as a function of collision energy, which provides a baseline for the studies of the magnetic moment, spin, and mass dependence of the spin polarization. For the local spin polarization, we calculate the radial and azimuthal components of the transverse $\mathrm{\ensuremath{\Lambda}}$ polarization and find specific modulating behavior which could reflect the circular vortical structure. However, our model fails to describe the azimuthal-angle dependence of the longitudinal and transverse $\mathrm{\ensuremath{\Lambda}}$ polarizations, which indicates that the hydrodynamic framework with the spin Cooper-Frye formula under the assumption of thermal equilibrium of the spin degree of freedom needs to be improved.

Long-ranged velocity correlations in dense systems of self-propelled particles

Abstract: Model systems of self-propelled particles reproduce many phenomena observed in laboratory active matter systems that defy our thermal equilibrium-based intuition. In particular, in stationary states of self-propelled systems, it is recognized that velocities of different particles exhibit non-trivial equal-time correlations. Such correlations are absent in equivalent equilibrium systems. Recently, researchers found that the range of the velocity correlations increases with increasing persistence time of the self-propulsion and can extend over many particle diameters. Here we review the initial studies of long-ranged velocity correlations in solid-like systems of self-propelled particles. Then, we demonstrate that the long-ranged velocity correlations are also present in dense fluid-like systems. We show that the range of velocity correlations in dense systems of self-propelled particles is determined by the combination of the self-propulsion and the virial bulk modulus that originates from repulsive interparticle interactions.

Combined Heat and Mass Transfer and Thermodynamic Irreversibilities in the Stagnation-point Flow of Casson Rheological Fluid Over a Cylinder With Catalytic Reactions and Inside a Porous Medium Under Local Thermal Nonequilibrium

Abstract: The transport of heat and mass from the surface of a cylinder coated with a catalyst and subject to an impinging flow of a Casson rheological fluid is investigated. The cylinder features circumferentially non-uniform transpiration and is embedded inside a homogeneous porous medium. The non-equilibrium thermodynamics of the problem, including Soret and Dufour effects and local thermal non-equilibrium in the porous medium, are considered. Through the introduction of similarity variables, the governing equations are reduced to a set of non-linear ordinary differential equations which are subsequently solved numerically. This results in the prediction of hydrodynamic, temperature, concentration and entropy generation fields, as well as local and average Nusselt, Sherwood and Bejan numbers. It is shown that, for low values of the Casson parameter and thus strong non-Newtonian behaviour, the porous system has a significant tendency towards maintaining local thermal equilibrium. Furthermore, the results show a major reduction in the average Nusselt number during the transition from Newtonian to non-Newtonian fluid, while the reduction in the Sherwood number is less pronounced. It is also demonstrated that flow, thermal and mass transfer irreversibilities are significantly affected by the fluid’s strengthened non-Newtonian characteristics. The physical reasons for these behaviours are discussed by exploring the influence of the Casson parameter and other pertinent factors upon the thickness of thermal and concentration boundary layers. It is noted that this study is the first systematic investigation of the stagnation-point flow of Casson fluid in cylindrical porous media.

Nonequilibrium Lattice Dynamics in Monolayer MoS2.

Abstract: The coupled nonequilibrium dynamics of electrons and phonons in monolayer MoS2 is investigated by combining first-principles calculations of the electron-phonon and phonon-phonon interactions with the time-dependent Boltzmann equation. Strict phase-space constraints in the electron-phonon scattering are found to influence profoundly the decay path of excited electrons and holes, restricting the emission of phonons to crystal momenta close to a few high-symmetry points in the Brillouin zone. As a result of momentum selectivity in the phonon emission, the nonequilibrium lattice dynamics is characterized by the emergence of a highly anisotropic population of phonons in reciprocal space, which persists for up to 10 ps until thermal equilibrium is restored by phonon-phonon scattering. Achieving control of the nonequilibrium dynamics of the lattice may provide unexplored opportunities to selectively enhance the phonon population of two-dimensional crystals and, thereby, transiently tailor electron-phonon interactions over subpicosecond time scales.

Comparison of the ion-to-electron temperature ratio prescription: GRMHD simulations with electron thermodynamics

Abstract: The Event Horizon Telescope (EHT) collaboration, an Earth-size sub-millimetre radio interferometer, recently captured the first images of the central supermassive black hole in M87. These images were interpreted as gravitationally-lensed synchrotron emission from hot plasma orbiting around the black hole. In the accretion flows around low-luminosity active galactic nuclei such as M87, electrons and ions are not in thermal equilibrium. Therefore, the electron temperature, which is important for the thermal synchrotron radiation at EHT frequencies of 230 GHz, is not independently determined. In this work, we investigate the commonly used parameterised ion-to-electron temperature ratio prescription, the so-called R-$\beta$ model, considering images at 230 GHz by comparing with electron-heating prescriptions obtained from general-relativistic magnetohydrodynamical (GRMHD) simulations of magnetised accretion flows in a Magnetically Arrested Disc (MAD) regime with different recipes for the electron thermodynamics. When comparing images at 230 GHz, we find a very good match between images produced with the R-$\beta$ prescription and those produced with the turbulent- and magnetic reconnection- heating prescriptions. Indeed, this match is on average even better than that obtained when comparing the set of images built with the R-$\beta$ prescription with either a randomly chosen image or with a time-averaged one. From this comparative study of different physical aspects, which include the image, visibilities, broadband spectra, and light curves, we conclude that, within the context of images at 230 GHz relative to MAD accretion flows around supermassive black holes, the commonly-used and simple R-$\beta$ model is able to reproduce well the various and more complex electron-heating prescriptions considered here.

Mixed thermomagnetic convection of ferrofluid in a porous cavity equipped with rotating cylinders: LTE and LTNE models

Abstract: In the present study, the impressions of the MHD and porous medium on mixed convection of Fe3O4-water nanofluid in a cavity with rotating cylinders in three different temperature cases with local thermal equilibrium and local thermal non-equilibrium approaches were studied. The effect of the presence of cylinders in three different temperature cases to improve the heat transfer rate was investigated. A finite volume method was used to solve equations. The Richardson, Hartmann, and Darcy numbers ranges are 1 ≤ Ri ≤ 100, 0 ≤ Ha ≤ 30, 0.001 ≤ Da ≤ 0.1, respectively. The volume fraction of nanoparticles varies in the range of 0–3%. The results show that the use of porous media has a beneficial effect on increasing the heat transfer rate, but the combination of the porous medium and the magnetic field can increase or decrease the heat transfer. Also, the most effective and highest heat transfer rate was occurred at Da = 0.01 and Da = 0.1, respectively. In addition, when the cylinders are cold or hot, the highest and lowest heat transfer rates occur, respectively. Finally, it was concluded that the magnetic field could control the fluid flow inside the cavity.

The effect of the magnetic field on the damping of slow waves in the solar corona

Abstract: Context. Slow magnetoacoustic waves are routinely observed in astrophysical plasma systems such as the solar corona, and they are usually seen to damp rapidly. As a slow wave propagates through a plasma, it modifies the equilibrium quantities of density, temperature, and the magnetic field. In the corona and other plasma systems, the thermal equilibrium is comprised of a balance between continuous heating and cooling processes, the magnitudes of which vary with density, temperature and the magnetic field. Thus the wave may induce a misbalance between these competing processes. Its back reaction on the wave has been shown to lead to dispersion, and amplification or damping, of the wave.Aims. This effect of heating and cooling misbalance has previously been studied in the infinite magnetic field approximation in a plasma whose thermal equilibrium is comprised of optically thin radiative losses and field-aligned thermal conduction, balanced by an (unspecified) heating process. In this work we extend this analysis by considering a non-zero β plasma. The importance of the effect of the magnetic field in the rapid damping of slow waves in the solar corona is evaluated and compared to the effects of thermal conduction.Methods. A linear perturbation under the thin flux tube approximation is considered, and a dispersion relation describing the slow magnetoacoustic modes is found. The dispersion relation’s limits of strong non-adiabaticity and weak non-adiabaticity are studied. The characteristic timescales were calculated for plasma systems with a range of typical coronal densities, temperatures, and magnetic field strengths.Results. The number of timescales characterising the effect of the misbalance is found to remain at two, as with the infinite magnetic field case. In the non-zero β case, these two timescales correspond to the partial derivatives of the combined heating and cooling function with respect to constant gas pressure and with respect to constant magnetic pressure. The predicted damping times of slow waves from thermal misbalance in the solar corona are found to be of the order of 10–100 min, coinciding with the wave periods and damping times observed. Moreover, the slow wave damping by thermal misbalance is found to be comparable to the damping by field-aligned thermal conduction. The change in damping with plasma-β is complex and depends on the coronal heating function’s dependence on the magnetic field in particular. Nonetheless, we show that in the infinite field limit, the wave dynamics is insensitive to the dependence of the heating function on the magnetic field, and this approximation is found to be valid in the corona so long as the magnetic field strength is greater than approximately 10 G for quiescent loops and plumes, and 100 G for hot and dense loops.Conclusions. A thermal misbalance may damp slow magnetoacoustic waves rapidly in much of the corona, and its inclusion in our understanding of slow mode damping may resolve discrepancies between the observations and theory relying on compressive viscosity and thermal conduction alone.

Shock–multicloud interactions in galactic outflows – II. Radiative fractal clouds and cold gas thermodynamics

Abstract: Galactic winds are crucial to the cosmic cycle of matter, transporting material out of the dense regions of galaxies. Observations show the coexistence of different temperature phases in such winds, which is not easy to explain. We present a set of 3D shock-multicloud simulations that account for radiative heating and cooling at temperatures between $10^2\,\rm K$ and $10^7\,\rm K$. The interplay between shock heating, dynamical instabilities, turbulence, and radiative heating and cooling creates a complex multi-phase flow with a rain-like morphology. Cloud gas fragments and is continuously eroded, becoming efficiently mixed and mass loaded. The resulting warm mixed gas then cools down and precipitates into new dense cloudlets, which repeat the process. Thus, radiative cooling is able to sustain fast-moving dense gas by aiding condensation of gas from warm clouds and the hot wind. In the ensuing outflow, hot gas with temperatures $\gtrsim 10^6\,\rm K$ outruns the warm and cold phases, which reach thermal equilibrium near $\approx 10^4\,\rm K$ and $\approx 10^2\,\rm K$, respectively. Although the volume filling factor of hot gas is higher in the outflow, most of the mass is concentrated in dense gas cloudlets and filaments with these temperatures. More porous multicloud layers result in more vertically extended outflows, and dense gas is more efficiently produced in more compact layers. The cold phase is not accelerated by ram-pressure, but, instead, precipitates from warm and mixed gas out of thermal equilibrium. This cycle can explain the presence of high-velocity H\,{\sc i} gas with $N_{\rm H\,{\scriptstyle I}}=10^{19-21}\,\rm cm^{-2}$ and $\Delta v_{\rm FWHM}\lesssim37\,\rm km\,s^{-1}$ in the Galactic centre outflow.

Bubble wall velocities in local equilibrium

Abstract: It is commonly expected that a friction force on the bubble wall in a first-order phase transition can only arise from a departure from thermal equilibrium in the plasma. Recently however, it was argued that an effective friction, scaling as $\gamma_w^2$ (with $\gamma_w$ being the Lorentz factor for the bubble wall velocity), persists in local equilibrium. This was derived assuming constant plasma temperature and velocity throughout the wall. On the other hand, it is known that, at the leading order in derivatives, the plasma in local equilibrium only contributes a correction to the zero-temperature potential in the equation of motion of the background scalar field. For a constant plasma temperature, the equation of motion is then completely analogous to the vacuum case, the only change being a modified potential, and thus no friction should appear. We resolve these apparent contradictions in the calculations and their interpretation and show that the recently proposed effective friction in local equilibrium originates from inhomogeneous temperature distributions, such that the $\gamma_w^2$-scaling of the effective force is violated. Further, we propose a new matching condition for the hydrodynamic quantities in the plasma valid in local equilibrium and tied to local entropy conservation. With this added constraint, bubble velocities in local equilibrium can be determined once the parameters in the equation of state are fixed, where we use the bag equation in order to illustrate this point. We find that there is a critical value of the transition strength $\alpha_{\rm crit}$ such that bubble walls run away for $\alpha>\alpha_{\rm crit}$.

Direct Observation of Shallow Trap States in Thermal Equilibrium with Band‐Edge Excitons in Strongly Confined CsPbBr 3 Perovskite Nanoplatelets

Abstract: DOI: 10.1002/adom.202001308 low trap state density of perovskites. Confinement of the bulk 3D structure into materials of reduced dimensionalities, such as nanoplatelets (NPls) (2D), nanorods (1D) or quantum dots (0D), has also widened the range of applications to photodetection,[2–4] lasing,[5] and lightemitting devices.[6–8] Compared to their bulk counterparts, perovskite nanocrystals (PNCs) offer greatly enhanced photoluminescence (PL) due to the strong electron–hole interaction, that is, large exciton binding energy,[9] induced by quantum confinement. One of the key features that has been ascribed to PNCs for efficient optoelectronic devices is their defect tolerance.[10,11] Indeed, the majority of the intrinsic defects in hybrid lead halide perovskites have been considered to be shallow trap states, with energies close to or within the energy bands.[12] Therefore, PNCs do not require postprocessing surface passivation, as conventional quantum dots do, to obtain high photoluminescence quantum yields (PLQYs).[13,14] Although most of the studies focused their attention on large, weakly confined, cubic PNCs,[15] the size of the PNCs could be further reduced to match the strong quantum confinement conditions (at least one dimension smaller than the exciton Bohr radius). In this case, the surface-to-volume ratio greatly increases, leading to an abundance of surface defects. In PNCs, surface defects are usually passivated by the use of organic capping ligands during synthesis.[16,17] The ligands play a role not only in the PNC PLQYs but also in their sensitivity to moisture and oxygen.[18,19] However, ligand binding to the surface is highly dynamic, which could lead to rapid desorption of the capping ligands, resulting in the formation of undercoordinated lead atoms, which act as surface traps.[20–25] Some reports have already shown that postsynthetic thiocyanate surface treatment[26,27] or inorganic passivation via the addition of ZnBr2 enables efficient trap suppression and a PLQY near unity. Since the PL tracks only the excited state population, following its dynamics is one of the easiest ways to understand the processes occurring in PNCs. However, standard techniques, such as time-correlated single-photon counting (TCSPC), have a limited time resolution of, at best, a few tens of picoseconds,[29] while exciton trapping usually occurs within the first ps after photoexcitation.[27,30] Therefore, femtosecond broadband fluorescence upconversion spectroscopy (FLUPS) is Lead halide perovskites exhibit great potential for light-emitting devices. Enhanced photoluminescence (PL) is obtained in perovskite materials of reduced dimensionalities due to the large exciton binding energy. However, as the nanocrystal size is reduced, the surface-to-volume ratio increases, leading to an abundance of surface defects. Here, a fast PL decay, 3–10 ps, is observed in quasi-1D CsPbBr3 perovskite nanoplatelets using broadband fluorescence upconversion spectroscopy. This decay is attributed to reversible trapping of band-edge excitons into dark states that lie close to the band edge. A simplified model is proposed to further confirm the presence of shallow traps and to fit the data obtained by ultrafast spectroscopy for multiple samples. Finally, the presence of deep trap states in aged nanoplatelets is revealed, likely arising from desorption of the organic capping ligands from the surface. Exciton trapping into these states is slower, 20–30 ps, but leads to a decrease in the photoluminescence quantum yield. These results may not only explain the extended luminescence lifetimes that have been reported for perovskite nanocrystals but also demonstrate the potential of combining ultrafast transient absorption and fluorescence up-conversion to obtain a full description of the spectroscopic properties of the material.

Event Shape and Multiplicity Dependence of Freeze-Out Scenario and System Thermodynamics in Proton+Proton Collisions at Using PYTHIA8

Abstract: Recent observations of QGP-like conditions in high-multiplicity pp collisions from ALICE experiment at the LHC warrant an introspection whether to use pp collisions as a baseline measurement to characterize heavy-ion collisions for the possible formation of a Quark-Gluon Plasma. A double differential study of the particle spectra and thermodynamics of the produced system as a function of charged-particle multiplicity and transverse spherocity in pp collisions would shed light on the underlying event dynamics. Transverse spherocity, one of the event shape observables, allows to separate the events in terms of jetty and isotropic events. We analyse the identified particle transverse momentum ( ) spectra as a function of charged-particle multiplicity and transverse spherocity using Tsallis nonextensive statistics and Boltzmann-Gibbs Blast-Wave (BGBW) model in pp collisions at using PYTHIA8 event generator. The extracted parameters such as temperature ( ), radial flow ( ), and nonextensive parameter ( ) are shown as a function of charged-particle multiplicity for different spherocity classes. We observe that the isotropic events approach thermal equilibrium while the jetty ones remain far from equilibrium. We argue that, while studying the QGP-like conditions in small systems, one should separate the isotropic events from the spherocity-integrated events, as the production dynamics are different.

Thermalization of a quantum Newton's cradle in a one-dimensional quasicondensate

Abstract: We study the nonequilibrium dynamics of the quantum Newton's cradle in a one-dimensional (1D) Bose gas in the weakly interacting quasicondensate regime. This is the opposite regime to the original quantum Newton's cradle experiment of [Kinoshita et al., Nature 440, 900 (2006)], which was realized in the strongly interacting 1D Bose gas. Using finite temperature $c$-field methods, we calculate the characteristic relaxation rates to the final equilibrium state. Hence, we identify the different dynamical regimes of the system in the parameter space that characterizes the strength of interatomic interactions, the initial temperature, and the magnitude of the Bragg momentum used to initiate the collisional oscillations of the cradle. In all parameter regimes, we find that the system relaxes to a final equilibrium state for which the momentum distribution is consistent with a thermal distribution. For sufficiently large initial Bragg momentum, the system can undergo hundreds of repeated collisional oscillations before reaching the final thermal equilibrium. The corresponding thermalization timescales can reach tens of seconds, which is an order of magnitude smaller than in the strongly interacting regime.

Evidence for a thermally driven charge-density-wave transition in 1T-TaS2 thin-film devices: Prospects for GHz switching speed

Abstract: We report on the room-temperature switching of 1T-TaS2 thin-film charge-density-wave devices, using nanosecond-duration electrical pulsing to construct their time-resolved current–voltage characteristics. The switching action is based upon the nearly commensurate to incommensurate charge-density-wave phase transition in this material, which has a characteristic temperature of 350 K at thermal equilibrium. For sufficiently short pulses, with rise times in the nanosecond range, self-heating of the devices is suppressed, and their current–voltage characteristics are weakly nonlinear and free of hysteresis. This changes as the pulse duration is increased to ∼200 ns, where the current develops pronounced hysteresis that evolves nonmonotonically with the pulse duration. By combining the results of our experiments with a numerical analysis of transient heat diffusion in these devices, we clearly reveal the thermal origins of their switching. In spite of this thermal character, our modeling suggests that suitable reduction of the size of these devices should allow their operation at GHz frequencies.

Coexistence of active Brownian disks: van der Waals theory and analytical results.

Abstract: At thermal equilibrium, intensive quantities like temperature and pressure have to be uniform throughout the system, restricting inhomogeneous systems composed of different phases. The paradigmatic example is the coexistence of vapor and liquid, a state that can also be observed for active Brownian particles steadily driven away from equilibrium. Recently, a strategy has been proposed that allows to predict phase equilibria of active particles [Solon et al., Phys. Rev. E 97, 020602(R) (2018)2470-004510.1103/PhysRevE.97.020602]. Here we elaborate on this strategy and formulate it in the framework of a van der Waals theory for active disks. For a given equation of state, we derive the effective free energy analytically and show that it yields coexisting densities in very good agreement with numerical results. We discuss the interfacial tension and the relation to Cahn-Hilliard models.

Contacts, equilibration, and interactions in fractional quantum Hall edge transport

Abstract: We study electron transport through a multichannel fractional quantum Hall edge in the presence of both interchannel interaction and random tunneling between channels, with emphasis on the role of contacts. The prime example in our discussion is the edge at filling factor 2/3 with two counterpropagating channels. Having established a general framework to describe contacts to a multichannel edge as thermal reservoirs, we particularly focus on the line-junction model for the contacts and investigate incoherent charge transport for an arbitrary strength of interchannel interaction beneath the contacts and, possibly different, outside them. We show that the conductance does not explicitly depend on the interaction strength either in or outside the contact regions (implicitly, it only depends through renormalization of the tunneling rates). Rather, a long line-junction contact is characterized by a single parameter which defines the modes that are at thermal equilibrium with the contact and is determined by the interplay of various types of scattering beneath the contact. This parameter - playing the role of an effective interaction strength within an idealized model of thermal reservoirs - is generically nonzero and affects the conductance. We formulate a framework of fractionalization-renormalized tunneling to describe the effect of disorder on transport in the presence of interchannel interaction. Within this framework, we give a detailed discussion of charge equilibration for arbitrarily strong interaction in the bulk of the edge and arbitrary effective interaction characterizing the line-junction contacts.

Effect of Discrete Breathers on the Specific Heat of a Nonlinear Chain

Abstract: A nonlinear chain with sixth-order polynomial on-site potential is used to analyze the evolution of the total-to-kinetic-energy ratio during development of modulational instability of extended nonlinear vibrational modes. For the on-site potential of hard-type (soft-type) anharmonicity, the instability of $$q =\pi $$ mode ( $$q = 0$$ mode) results in the appearance of long-living discrete breathers (DBs) that gradually radiate their energy and eventually the system approaches thermal equilibrium with spatially uniform and temporally constant temperature. In the hard-type (soft-type) anharmonicity case, the total-to-kinetic-energy ratio is minimal (maximal) in the regime of maximal energy localization by DBs. It is concluded that DBs affect specific heat of the nonlinear chain, and for the case of hard-type (soft-type) anharmonicity, they reduce (increase) the specific heat.

Thermal quantum correlations in the two-qubit Heisenberg XYZ spin chain with Dzyaloshinskii-Moriya interaction

Abstract: We investigate the quantum correlations of a two-qubit XYZ Heisenberg spin-1/2 chain model with Dzyaloshinskii–Moriya interaction. The two-qubit system is considered in thermal equilibrium. The var...

The solar corona as an active medium for magnetoacoustic waves

Abstract: The presence and interplay of continuous cooling and heating processes maintaining the corona of the Sun at the observed one million K temperature were recently understood to have crucial effects on the dynamics and stability of magnetoacoustic (MA) waves. These essentially compressive waves perturb the coronal thermal equilibrium, leading to the phenomenon of a wave-induced thermal misbalance (TM). Representing an additional natural mechanism for the exchange of energy between the plasma and the wave, TM makes the corona an active medium for MA waves, so that the wave can not only lose but also gain energy from the coronal heating source (similarly to burning gases, lasers and masers). We review recent achievements in this newly emerging research field, focussing on the effects that slow-mode MA waves experience as a back-reaction of this perturbed coronal thermal equilibrium. The new effects include enhanced frequency-dependent damping or amplification of slow waves, and effective, not associated with the coronal plasma non-uniformity, dispersion. We also discuss the possibility to probe the unknown coronal heating function by observations of slow waves and linear theory of thermal instabilities. The manifold of the new properties that slow waves acquire from a thermodynamically active nature of the solar corona indicate a clear need for accounting for the effects of combined coronal heating/cooling processes not only for traditional problems of the formation and evolution of prominences and coronal rain, but also for an adequate modelling and interpretation of magnetohydrodynamic waves.

A study of the natural convection of water-AA7075 nanoliquids in low-porosity cylindrical annuli using a local thermal non-equilibrium model

Abstract: Natural convection in nanoliquid-saturated porous cylindrical annuli due to uniform heat and mass influxes from the solid cylinder and effluxes from the outer hollow cylinder is investigated analytically. The Darcy model and the modified version of the Buongiorno two-phase model are used, and local thermal non-equilibrium between the phases is assumed. A nanoliquid-saturated porous medium made up of glass balls with a dilute concentration of AA7075 alloy nanoparticles well-dispersed in water is considered. Out of three types of annuli considered, shallow annuli provide the best heat transport and tall annuli show the worst performance. The presence of a dilute concentration of nanoparticles significantly enhances the heat transport in the system. Of nine nanoparticle shapes considered, lamina-shaped nanoparticles enhance heat transport the most. Heat transport is enhanced in the case of heat-and-mass-driven convection compared to the case of purely heat-driven convection. The results for a rectangular enclosure are obtained as a particular case of the present study. Two asymptotic routes that take us to the results of thermal equilibrium are shown. The vanishing limit of the concentration Rayleigh number yields the result for a single-phase model. Results for the base-liquid-saturated porous medium form a limiting case of the present study. We conclude that a shallow cylindrical annulus saturated with water-AA7075 lamina-shaped alloy nanoparticles is best suited for heat transfer due to its high effective thermal conductivity in comparison with that of other shaped nanoparticles and a tall rectangular enclosure saturated by water is best suited for heat storage applications.