Thermal Velocity Distribution Research Articles

Interfacial heat transfer and melt-front evolution at a Fractal Cantor structured interface under various PCM melting conditions

With the aim to investigate heat transfer characteristics of PCM on micro-textured hot surface, this study numerically investigates the heat transfer process of PCM at fractal Cantor structured surfaces using the total enthalpy-based lattice Boltzmann method, which is partly validated by experimental testing. The thermal slip length and liquid velocity distribution are assessed as evaluation criteria. Results show that Fractal Cantor structure can significantly stimulate localized thermal convection, thereby reducing the average thermal slip length from 0.3 mm to approximately 0.14 mm. When the Fourier number exceeds 0.1, both thermal conduction and convection should be considered within the melted layer. With the fractal level increasing, a more uniform melt-front evolution can be observed, consequently, minimizing the total melting time by 500 s with the fractal level of 3. Elevating Ste and gravity can significantly increase the flow rate of liquid PCM by augmenting superheat degree and buoyancy effect, thereby enhancing convective heat transfer strength and reducing thermal slip length. According to the orthogonal design, though the fractal level demonstrates minimal range values for both maximum velocity and Rayleigh number, it elevates the sensitivity to the variation of boundary conditions. Ste yields the most significant enhancement in heat transfer performance.

The necessity of accurate measurements of thermal velocity distribution functions in the solar wind

The necessity of accurate measurements of thermal velocity distribution functions in the solar wind

Simulation of lutetium metal evaporation with considering atomic metastable state

Lutetium-177, one of the current global research hotspots for medical isotopes, can be produced by reactor irradiation of lutetium-176, which can be produced and processed with the assistance of the electron beam physical vapor deposition (EB-PVD). The metal evaporation process is an important part of EB-PVD, which aims to convert solid metal into metal atomic vapor. The distribution of each macroscopic characteristic quantity, such as thermal population distribution, density distribution, velocity distribution, and temperature distribution of metal atomic vapor, will directly affect the utilization of metal vapor atoms, the subsequent deposition process, and coating process. In this paper, two-dimensional model and three-dimensional model of the metal evaporation process are established based on the direct simulation Monte Carlo method, and atomic metastable state is introduced into the collision assumption. The macroscopic characteristic quantities, such as the number density of metal atomic vapor, the motion velocity and the discrete degree of the total motion velocity in the evaporation process with or without considering the lutetium atomic metastable state, are analyzed and compared. The spatial distribution characteristics of each energy level of metal atomic vapor are observed, and the variation characteristics of the macroscopic characteristic quantities in the center with the slit opening size and the evaporation source surface temperature are discussed. The study results show that the number density of metal vapor passing through the beam flow device decreases, the average velocity in the <i>x</i>-direction and <i>z</i>-direction and the discrete degree of the total motion velocity increase, with the atomic metastable state taken into consideration. The beam flow device intensifies the transformation of the atomic energy level of the metal vapor and changes the physical parameters of the metal atomic vapor. At the same time, the slit opening size and the evaporation source surface temperature trigger off the change of the macroscopic characteristic quantity distribution of the metal vapor, including the degree of anomalous protrusion of each macroscopic characteristic quantity above the slit. The results of the three-dimensional simulation based on the two-dimensional model are consistent with the changes in the order of magnitude of data distribution and spatial distribution pattern, which can serve as a reference and guidance for the experimental study of the electron beam metal evaporation process.

The need for accurate measurements of thermal velocity distribution functions in the solar wind

The current state of the art thermal particle measurements in the solar wind are insufficient to address many long standing, fundamental physical processes. The solar wind is a weakly collisional ionized gas experiencing collective effects due to long-range electromagnetic forces. Unlike a collisionally mediated fluid like Earth’s atmosphere, the solar wind is not in thermodynamic or thermal equilibrium. For that reason, the solar wind exhibits multiple particle populations for each particle species. We can mostly resolve the three major electron populations (e.g., core, halo, strahl, and superhalo) in the solar wind. For the ions, we can sometimes separate the proton core from a secondary proton beam and heavier ion species like alpha-particles. However, as the solar wind becomes cold or hot, our ability to separate these becomes more difficult. Instrumental limitations have prevented us from properly resolving features within each ion population. This destroys our ability to properly examine energy budgets across transient, discontinuous phenomena (e.g., shock waves) and the evolution of the velocity distribution functions. Herein we illustrate both the limitations of current instrumentation and why higher resolutions are necessary to properly address the fundamental kinetic physics of the solar wind. This is accomplished by directly comparing to some current solar wind observations with calculations of velocity moments to illustrate the inaccuracy and incompleteness of poor resolution data.

Numerical simulation of flow resistance characteristics in a rod bundle channel under vertical and inclined conditions

Numerical simulation of flow resistance characteristics in a 3 × 3 rod bundle channel under vertical and inclined conditions is carried out. Compared with experimental results, it is verified that Reynolds stress model with enhanced wall treatment can accurately calculate flow parameters in rod bundle. Under vertical conditions, flow boundary layer at high Reynolds number is relatively thinner and radial velocity vector at interface of each sub-channel is basically parallel to dividing line or opposite direction. Mainstream velocity distribution in channel under heating conditions is more uniform and radial secondary flow is more likely to occur than that under adiabatic conditions. Axial velocity near wall is a dominated factor to determine wall shear stress. Under inclined conditions, deviation of overall flow velocity distribution is more obvious and relative intensity of natural convection is more significant at low Reynolds number conditions compared with high Reynolds number conditions. Main mechanism of increase in frictional resistance due to inclination is that natural convection caused by thermal stratification changes flow velocity distribution of each sub-channel. Additionally, natural convection increases radial secondary flow area, broadens wall shear stress distribution and also increases average wall shear stress of cross section.

The bipolar charge plasma spectrometer (BCPS) based on the 2π-field-of-view double-channel electrostatic analyzer.

In this paper, we propose a bipolar charge plasma spectrometer based on the double-channel electrostatic analyzer for simultaneously measuring thermal ions and electrons with a 2π hemispherical field-of-view. Both ions and electrons within the wide field-of-view enter the spectrometer, pass through the variable geometric factor channel, and are then separated by the double-channel electric fields. Two microchannel plates are accommodated at the exit of the analyzer for ion and electron detection. The main performance of the spectrometer has been obtained from on-ground calibration. With the electrostatic deflectors and the cylindrically symmetric structure, the spectrometer provides simultaneous measurements of thermal ion and electron velocity distributions with a shared field-of-view of 360° (azimuth angle) by 90° (elevation angle) and a broad energy range for both ions and electrons. The ion analyzer constant and the electron analyzer constant are 11.1 and 9.7, respectively. The detecting energy range of 33.3-44.4 keV for ions and 29.1-38.8 keV for electrons can be obtained by using the sweeping electrostatic analyzer voltage range of 3-4000 V. The ion and electron energy resolutions are 9.6% and 6.1%, respectively. The variable geometric factor function provides a large geometric factor adjusting range for both ion and electron measurements by two orders of magnitude, which fulfills the requirements of a large dynamic flux range for simultaneous measurements of space thermal plasma in the solar wind and magnetosphere.

Double diffusion in stretched flow over a stretching cylinder with activation energy and entropy generation

Present study accords exploration of double diffusion in an axisymmetric flow of chemically reacting viscous fluid which impulsively starts flowing over a cylinder due to stretched surface. Non-Fourier law is employed to explain the heat flow whereas non-Fick's law of mass flux is employed to delve into the diffusion of mass. Activation energy is also taken into account. Transformed governing equations are solved using efficient mathematical technique called Homotopy Analysis Method (HAM). For the validity of the solution technique convergence table and h curves are plotted. Numerical solution has also been computed by using Shooting method for comparison with HAM. Entropy generation of the system has been investigated. Significant impact of pertinent flow parameters on thermal profile, concentration and velocity distribution are examined graphically. Velocity and thickness of momentum boundary layer increase by increasing curvature. Thermal profile is inversely related with Prandtl number and thermal relaxation time. Concentration profile trickles down by an increase in Schmidt number, mass relaxation parameter, Fitted rate constant and temperature ratio parameter. Most of the studies available in the literature focused on the double diffusion using generalized Fourier and Fick's laws neglecting activation energy and entropy generation which is an inevitable aspect in practical problems now a days. These features have been considered in the present article and it is the novelty of the present research.

Diffusive shock acceleration of cosmic rays in low-Mach galaxy cluster shocks.

Astrophysical shocks are known to accelerate particles to high relativistic velocities. This process requires the particles to repeatedly cross the shock, a process that can only occur if the particle is reflected by the local magnetic field. Such particles are observed on Earth as cosmic rays. This phenomenon has been studied in considerable detail for high-Mach shocks, such as the shocks that occur in colliding stellar winds and supernova explosions, but remains relatively unexplored for low-Mach shocks, such as the shocks of colliding clusters of galaxies. Recent simulations using the particle-in-cell (PIC) method have shown that, depending on the exact Mach number, even low-Mach shocks can accelerate charged particles to the point where they start to deviate from the thermal velocity distribution. However, the computationally intensive nature of the PIC calculations makes it difficult to continue the simulations to determine whether the particles can reach relativistic speeds. We now present new simulations, using a combined PIC and magnetohydrodynamics (MHD) technique. This model, which takes advantage of the computational efficiency of MHD, allows us to simulate a much larger physical volume and study the behaviour of the particles over a longer period of time in order to determine to what extent the acceleration process continues and whether these shocks are capable of contributing to the cosmic ray spectrum.

Simulation of the Thermal Environment and Velocity Distribution in a Lecture Hall

The rational design of heating ventilation and air conditioning systems is an important means to achieve energy conservation and sustainable development. The simulation of air-conditioning systems with finite element methods has gradually become an important auxiliary means of complex airspace design. In this paper, a k-ε turbulence model is used to conduct 3D simulations and optimize the summer air conditioning system of a lecture hall. Various conditions are considered in terms of fresh air temperature and flow rate towards the end to improve comfort. The approach used in this paper could also be used in the future as an auxiliary model to promote the design of other lecture halls and ladder classrooms.

Orbital angular momentum transfer via spontaneously generated coherence

We study the orbital angular momentum (OAM) transfer from a weak Laguerre–Gaussian (LG) field to a weak plane wave in two closed-loop three-level V-type atomic systems. In the first scheme, the atomic system has two non-degenerate upper levels where the corresponding transition is excited by a microwave plane wave. It is analytically shown that the microwave field induces an OAM transfer from an LG field to a generated third field. It is demonstrated that the efficiency of the OAM transfer decreases when the thermal velocity distribution of atoms is considered. In the second scheme, we consider a three-level V-type atomic system with two near-degenerate excited states and study the effect of the quantum interference due to the spontaneous emission on the OAM transfer. It is found that spontaneously generated coherence (SGC) induces the OAM transfer from the LG field to the weak planar field, and the OAM transfer does not occur in the absence of the SGC. The suggested models offer a rather simple method for the OAM transfer that can be used in quantum information processing and data storage.

Destruction of high- n ( n∼300 ) Rydberg atoms in Rydberg-Rydberg collisions

The destruction of high-$n, n\ensuremath{\sim}300$, strontium atoms contained in a hot atomic beam through Rydberg-Rydberg collisions is examined. The Rydberg atoms are initially created, under blockade conditions, in a localized volume and their subsequent motions lead to creation of a string of Rydberg atoms. The Rydberg atoms, however, are formed with a thermal distribution of velocities resulting in Rydberg-Rydberg collisions which lead to their destruction. The experimental data are interpreted using classical trajectory Monte Carlo techniques that simulate the excitation of the Rydberg atoms together with their subsequent motions. Using calculated collisional ionization cross sections and blockade radii, the model yields results in good agreement with experiment. The results highlight the important role collisional destruction can play when attempting to study (long-range) Rydberg-Rydberg interactions in a hot atomic beam.

The effect of a saturated kink on the dynamics of tungsten impurities in the plasma core

We studied the effect of saturated kink-like instabilities on the redistribution of W impurities. A numerical code that follows the exact trajectories of the impurity ions in the total fields and includes collisions was employed. The code is written in CUDA C and runs in graphical processing units, allowing simulations with a large number of particles with modest resources. The total electric and magnetic fields are calculated as the sum of a simple analytical equilibrium (large aspect ratio, circular cross section) plus the fields of the kink mode. Our simulations show that when the W ions have a thermal velocity distribution, with zero mean velocity, the kink has no effect on the W density. However, when we include the plasma rotation and the drag on the impurities, the kink can affect the W density. When the average passing frequency of the W ions is similar to the frequency of the kink mode, the expulsion of W ions from the plasma core is maximized and the W density profile shows a hollow structure. This could have implications for the mitigation of W accumulation.

Quadrupole mass filter operation with dipole direct current and quadrupole radiofrequency excitation.

For mass analysis, a quadrupole mass filter (QMF) usually operates at the upper tip of the first stability region. We introduce a new mode of QMF operation with dipole direct current (dc) and with auxiliary quadrupole excitation. Before experimental investigation of this mode, we have carried out numerical simulations of this process. Based on the analytical description of the equations of ion motion and mapping of stability islands, ion trajectory calculations are used to calculate peak shapes or mass filter transmission contours. Ions are given Gaussian distributions of initial positions in x and y, and thermal initial velocity distributions. The effects of the dipole dc in the y direction and auxiliary quadrupole excitation in the x and y directions are modeled. We find that, with dipole dc excitation, the working area of the first stability region decreases and a new y stability boundary follows a βy line. This allows control of the resolution with the removal of the low-mass tail of a peak thereby improving the isotopic abundance sensitivity by about two orders of magnitude. The operation of a QMF with high resolution (about 5000) and high transmission (20-25%) and with a relatively short sorting time of ions n = 150 radiofrequency (rf) cycles based on the use of dipole dc and quadrupole excitation is shown. A new mode of QMF operation with dipole direct current (dc) and with an auxiliary quadrupole is discussed. Using dc excitation allows control of the resolution with the removal of the low-mass tail of a peak. The method of operation of a quadrupole mass filter with high resolution (about 5000) and high transmission (20-25%) and a relatively short sorting time (150 rf cycles) based on the use of dipole dc and on quadrupole excitation is shown.

Effect of ambient air conditions on PEM fuel cell performance

This paper focuses on the influence of inlet air temperature and relative humidity on the performance of a proton exchange membrane fuel cell (FC) stack with maximum power of 175 W. To control the inlet air temperature, a cooling system was designed and integrated with the fuel cell stack. It was experimentally proven that the lower temperature results in better fuel cell performance. This dependence was found to be non-linear in the range of 15–25 °C. To take into account air humidity and analyze its effects on FC efficiency, an air humidifier was installed at the air inlet. Experiments showed that the increased relative humidity positively affected FC efficiency. Computational fluid dynamics (CFD) simulation of the reactant flow inside the FC stack was conducted, helping to analyze thermal regime, velocity distribution, and migration of hydrogen ions through the membrane on the cathode side. The temperature on the cathode side exceeded operational limits already at 120 W power load. CFD simulation results support the need for an additional cooling system for experimental setup.

Development of an inertial confinement fusion platform to study charged-particle-producing nuclear reactions relevant to nuclear astrophysics

This paper describes the development of a platform to study astrophysically relevant nuclear reactions using inertial-confinement fusion implosions on the OMEGA and National Ignition Facility laser facilities, with a particular focus on optimizing the implosions to study charged-particle-producing reactions. Primary requirements on the platform are high yield, for high statistics in the fusion product measurements, combined with low areal density, to allow the charged fusion products to escape. This is optimally achieved with direct-drive exploding pusher implosions using thin-glass-shell capsules. Mitigation strategies to eliminate a possible target sheath potential which would accelerate the emitted ions are discussed. The potential impact of kinetic effects on the implosions is also considered. The platform is initially employed to study the complementary T(t,2n)α, T(3He,np)α and 3He(3He,2p)α reactions. Proof-of-principle results from the first experiments demonstrating the ability to accurately measure the energy and yields of charged particles are presented. Lessons learned from these experiments will be used in studies of other reactions. The goals are to explore thermonuclear reaction rates and fundamental nuclear physics in stellar-like plasma environments, and to push this new frontier of nuclear astrophysics into unique regimes not reachable through existing platforms, with thermal ion velocity distributions, plasma screening, and low reactant energies.

Numerical Simulation of Waste Heat Boiler for Copper Flash Smelting

Based on the results of a thermal equilibrium, temperature and velocity distributions of the waste heat boiler were simulated. Results show that the gas flow doesn’t rush to the top intensely when the first baffle-board is set in the front of the radiation chamber. Flue gas is so strongly disturbed by the second baffle-board that its residence time is increased. These two baffle-boards enhance the heat transfer intensity in the radiation chamber. The relative error of simulation results to experimental is less than 10%.

Cyclotron resonant scattering feature simulations

Electron cyclotron resonant scattering features (CRSFs) are observed as absorption-like lines in the spectra of X-ray pulsars. A significant fraction of the computing time for Monte Carlo simulations of these quantum mechanical features is spent on the calculation of the mean free path for each individual photon before scattering, since it involves a complex numerical integration over the scattering cross section and the (thermal) velocity distribution of the scattering electrons. We aim to numerically calculate interpolation tables which can be used in CRSF simulations to sample the mean free path of the scattering photon and the momentum of the scattering electron. The tables also contain all the information required for sampling the scattering electron's final spin. The tables were calculated using an adaptive Simpson integration scheme. The energy and angle grids were refined until a prescribed accuracy is reached. The tables are used by our simulation code to produce artificial CRSF spectra. The electron momenta sampled during these simulations were analyzed and justified using theoretically determined boundaries. We present a complete set of tables suited for mean free path calculations of Monte Carlo simulations of the cyclotron scattering process for conditions expected in typical X-ray pulsar accretion columns (0.01<B/B_{crit}<=0.12, where B_{crit}=4.413x10^{13} G and 3keV<=kT<15keV). The sampling of the tables is chosen such that the results have an estimated relative error of at most 1/15 for all points in the grid. The tables are available online at http://www.sternwarte.uni-erlangen.de/research/cyclo.

Features of collisionless turbulence in the intracluster medium from simulated Faraday rotation maps – II. The effects of instabilities feedback

Statistical analysis of Faraday Rotation Measure (RM) maps of the intracluster medium (ICM) of galaxy clusters provides a unique tool to evaluate some spatial features of the magnetic fields there. Its combination with numerical simulations of magnetohydrodynamic (MHD) turbulence allows the diagnosis of the ICM turbulence. Being the ICM plasma weakly collisional, the thermal velocity distribution of the particles naturally develops anisotropies as a consequence of the large scale motions and the conservation of the magnetic moment of the charged particles. A previous study (Paper I) analyzed the impact of large scale thermal anisotropy on the statistics of RM maps synthesized from simulations of turbulence; these simulations employed a collisionless MHD model which considered a tensor pressure with uniform anisotropy. In the present work, we extend that analysis to a collisionless MHD model in which the thermal anisotropy develops according to the conservation of the magnetic moment of the thermal particles. We also consider the effect of anisotropy relaxation caused by the micro-scale mirror and firehose instabilities. We show that if the relaxation rate is fast enough to keep the anisotropy limited by the threshold values of the instabilities, the dispersion and power spectrum of the RM maps are indistinguishable from those obtained from collisional MHD. Otherwise, there is a reduction in the dispersion and steepening of the power spectrum of the RM maps (compared to the collisional case). Considering the first scenario, the use of collisional MHD simulations for modeling the RM statistics in the ICM becomes better justified.

Dark matter density spikes around primordial black holes

We show that density spikes begin to form from dark matter particles around primordial black holes immediately after their formation at the radiation-dominated cosmological stage. This follows from the fact that in the thermal velocity distribution of particles there are particles with low velocities that remain in finite orbits around black holes and are not involved in the cosmological expansion. The accumulation of such particles near black holes gives rise to density spikes. These spikes are considerably denser than those that are formed later by the mechanism of secondary accretion. The density spikes must be bright gamma-ray sources. Comparison of the calculated signal from particle annihilation with the Fermi-LAT data constrains the present-day cosmological density parameter for primordial black holes with masses $M_{\rm BH}\geq10^{-8}M_\odot$ from above by values from $\Omega_{\rm BH}\leq1$ to $\Omega_{\rm BH}\leq10^{-8}$, depending on $M_{\rm BH}$. These constraints are several orders of magnitude more stringent than other known constraints.

Virtual sensors for rooftop unit air-side diagnostics

In this article, virtual sensors for air-side diagnostics are developed and validated for rooftop air conditioners. The virtual air-side sensors require indoor fan differential pressure and speed measurements. Experimental data from laboratory measurements are used to evaluate the accuracy of the virtual sensors. Methodologies for improving the accuracy of air-side temperature sensors often used for outdoor-air economizer diagnostics have also been developed and evaluated. Outdoor-air temperature correction correlations improve single-point temperature inaccuracy caused by exhaust-air recirculation. A model correcting a single-point mixed-air temperature measurement for inaccuracy caused by thermal stratification and non-uniform velocity distributions at the evaporator inlet has also been described and validated. Finally, methods for estimating outdoor ventilation air fraction are presented and compared to measurements.