Contribution Of Convection Research Articles

Decomposition of the drag force in steady and oscillatory flow through a hexagonal sphere pack

We investigate steady and oscillatory flow through a hexagonal close-packed arrangement of spheres in the framework of the volume-averaged momentum equation. We quantify the friction and pressure drag based on a direct numerical simulation dataset. Using the pressure decomposition of Graham (J. Fluid Mech., vol. 881, 2019), the pressure drag can be further split up into an accelerative, a viscous and a convective contribution. For the accelerative pressure, a closed-form expression can be given in terms of the potential flow solution. We investigate the contributions of the different drag components to the volume-averaged momentum budget and their Reynolds number scaling. For steady flow, we find that the friction and viscous pressure drag are proportional to $Re$ at low Reynolds numbers and scale with $Re^{1.4}$ for high Reynolds numbers. This is close to the steady laminar boundary layer scaling. For the convective pressure drag, we find a cubic scaling at low and a quadratic scaling at high Reynolds numbers. The Reynolds stresses have a minor contribution to the momentum budget. For oscillatory flow at low and medium Womersley numbers, the amplitudes of the drag components are similar to the steady cases at the same Reynolds number. At high Womersley numbers, the drag components behave quite differently and the friction and viscous pressure drag are relatively insignificant. The drag components are not in phase with the forcing and the superficial velocity; the phase lag increases with the Womersley number. This suggests that new models beyond the current quasisteady approaches need to be developed.

Interaction of end-gas autoignition and cold wall in closed chamber

The presence of cold walls in spark-ignition (SI) engines could induce the interaction with end-gas autoignition. This interaction is numerically studied in the current work considering detailed chemistry and transport, with emphasis on near-wall flame structures and dynamics, as well as autoignition-affected wall heat flux. Two alternative fuels, hydrogen, and dimethyl ether (DME), are considered. In hydrogen/air mixtures, autoignition first develops in the end-gas and leaves an unburnt near-wall region. The combustion mode in this region then sequentially behaves as spontaneous ignition, laminar flame, and low-to-intermediate temperature reaction. Correspondingly, the contribution of diffusion and convection in the energy budget respectively increases and decreases as the reaction front moves towards the wall. End-gas autoignition also introduces strong pressure waves propagating back-and-forth, which subsequently lead to multiple local maxima on the temporal evolution of wall heat flux. Consequently, the maximum wall heat flux no longer changes monotonically with both the wall temperature and the quenching distance. Furthermore, the role of low-temperature chemistry (LTC) is studied using DME/air mixtures, which exhibits typical multi-stage heat release and Negative Temperature Coefficient (NTC) behaviors. It is also noted that, the influence of pressure wave is minimal when the unburnt mixtures have relatively low energy density, such as at off-stoichiometric conditions and/or lower initial pressures. Specifically: consistent to distinct three-stage homogeneous ignition at certain fuel-lean conditions (ϕ = 0.25) and intermediate initial temperature (typically around T0 = 600–750 K), in the corresponding 1D case three-stage end-gas autoignition respectively evolves into the near-wall cool, warm and hot flame; in the case at relatively low initial pressures, only the first-stage autoignition occurs in the end-gas, and as such LTC slightly increases the wall heat flux and advances flame quenching. As the reactivity of unburnt mixtures becomes higher, LTC and HTC respectively introduce strong and weak knocking combustion. Multiple weak and strong local maxima are observed in the temporal evolution of wall heat flux, both of which changes non-monotonically with the wall temperature.

Diffusion mechanisms of convective instability in liquid and gas mixtures

AbstractA review of publications on stability of mechanical equilibrium of multicomponent mixtures in gases and liquids under non‐isothermal and isothermal conditions is given. Studies where diffusion in binary and multicomponent mixtures led to the occurrence of instability of mechanical equilibrium of the system have been analyzed. The main attention is paid to the description of approaches predicting the areas where there are increasing convective disturbances, as well as the determination of the spectrum of thermophysical parameters where the contribution of convection to heat and mass transfer is negligible. At the same time, the problems determining the role of diffusion, thermodiffusion, and double diffusion mechanisms in the separation of mixture components at the boundary of convective instability were given priority in the description. The results of investigations related to the study of the evolution of flows arising due to instability at the initial stage of mixing were also discussed.

Numerical study on forced ignition and flame propagation in a counterflow of nitrogen-diluted hydrogen versus air

Hydrogen is widely perceived as the next generation of clean energy due to its zero carbon emissions. However, there is an escalating concern regarding hydrogen safety. A deeper understanding of the ignition characteristics of hydrogen in inhomogeneous environments is imperative for enhancing our physical comprehension and conducting accurate risk assessments. The purpose of this study is to delve into the ignition phenomena of hydrogen and air in a non-premixed counterflow. Transient two-dimensional simulations are performed, considering detailed chemistry and transport. The focus is placed on the effects of strain rate, mixture stratification, and ignition position on ignition and flame propagation. The first part studies the ignition process at varying strain rates while the ignition position is fixed at the stagnation point. The results show that a low strain rate has a minimal impact on ignition outcome, whereas a high strain rate hinders ignition by reducing mixing layer thickness and residence time. As a result, the minimum ignition energy (MIE) barely changes at a relatively low strain rate but rises rapidly as the strain rate approaches the extinction limit. Subsequently, ignition is studied at different positions along the symmetry axis, where the flow field and mixture composition vary simultaneously. It is demonstrated that ignition is more feasible on the lean side than on the stoichiometric and rich sides, and a high strain rate can expand the ignitability limit of the mixture fraction under extreme off-stoichiometric conditions. The separate contributions of forced convection and mixture inhomogeneity are further assessed through simulations with and without counterflow and stratified mixture. The results suggest that the significance of the forced convection is contingent upon the timescale competition between flow and ignition, while the mixture stratification determines the evolution of the ignition kernel into either an outwardly propagating premixed flame or a triple flame. The MIEs at premixed and non-premixed conditions are examined, and the influence of fuel stratification is analyzed. In the end, a regime diagram is presented, illustrating the comprehensive roles of strain rate and mixture fraction during ignition. This study provides invaluable insights into the non-premixed ignition of hydrogen in complex flow conditions.

Flame spread over convex and inclined flat surfaces

Convex surface is a general configuration in various building structures. Most studies for flame spread only investigated the flat surface type, while the flame spread over convex surface has not been addressed yet. A comparative study of convex and inclined flat surfaces was therefore performed by designing different curvatures (reflected by central angle φ) and flat-slope inclination angles (denoted by θ). The thermally thin PMMA sheet with a fixed horizontal projection length of 500 mm was chosen. The average path flame spread rate presents an exponential growth with increasing φ and θ. Moreover, the average horizontal flame spread rates of convex and inclined flat surfaces are close. For convex surface, radiant heat flux received by the sample rear increases with increasing φ. Also, two peaks of radiant heat flux occur at φ ≥ 45°. However, convective heat flux increases suddenly when θ ≥ 22.5°, in which convective heat transfer was the main mechanism. In addition, with increasing φ and θ, overall burnout rate increases for convex surface, but has an increasing first and then decreasing tendency for flat surface. The convection contribution on the pyrolysis zone is more obvious for inclined flat surface, in comparison to convex surface.

Diapycnal Mixing and Double Diffusion over the Continental Slope in the Northern Sea of Japan in the Warm Half-Year

This study investigates double diffusion processes responsible for diapycnal mixing in the Primorye current in the northern Sea of Japan. The results of processing and analysis of a long time series of data from regular Aqualog-profiler measurements of thermohaline characteristics and current velocity over the continental slope between the horizons of 60 and 420 m from April to October 2015. Using the Osborne-Cox parameterization (1972) and Gregg’s (1989) parameterization, we estimated heat and salt diffusivities, as well as the buoyancy flux. To reveal the processes of double diffusion, the Turner angle was estimated. We estimated contributions of diffusive convection and salt fingers into the buoyancy flux. Due to double diffusion, the intense vertical mixing covered the layer from 80 to 170 m. From mid-spring to early May, diffusive convection penetrated to a horizon of 250 m, and then a process like salt fingers played a leading role. Shear instability caused by the weakly nonlinear interaction of internal waves prevailed in the underlying layers.

Experimental and numerical investigation of thermally induced convection along a high-temperature borehole heat exchanger

Thermally induced convection in the vicinity of borehole heat exchangers (BHE) in porous media affects the heat transfer behaviour between the BHE and the surrounding ground and thereby their performance for building climatization, heat extraction and heat storage. This study presents a combined experimental–numerical analysis of the qualitative and quantitative impacts of convection on heat transfer for a laboratory-scale BHE analogue over a wide temperature range. For this purpose, a grouted coaxial high-temperature BHE was installed within a fully saturated porous sand medium in a cylindrical container of 1.4 m3 volume under spatially extensive temperature monitoring. Four transient heat charging/discharging experiments were performed at different charging temperatures of 30, 50, 70, and 90 °C and analysed for the onset and magnitude of convection. After minimal calibration, a numerical model accurately reproduced the experimental data of all experiments and helped quantify the contribution of convection to the total heat transfer at each temperature level. For heat charging at 30 °C, the heat transfer was dominated by conduction. At higher temperatures, convection increased the total heat-transfer rate by up to 35% in the experimental setup. In contrast, for heat discharge, due to thermal stratification in the sand medium, convection reduced the heat transfer rate and heat recovery by up to −30% and −35%, respectively. Rayleigh numbers, buoyant flow velocities, and the centre of heat were derived from the monitoring and simulation data and consistently indicated the onset of convection for the experimental setup at a BHE operating temperature of 50 °C. The Rayleigh number was found to be closely correlated with the magnitude of the convective flow. Overall, the results suggest that the performance of BHEs in cooling applications may benefit from thermal convection, while the performance of BHEs for thermal energy storage may deteriorate markedly at high operating temperatures.

Tidal dissipation due to the elliptical instability and turbulent viscosity in convection zones in rotating giant planets and stars

ABSTRACT Tidal dissipation in star–planet systems can occur through various mechanisms, among which is the elliptical instability. This acts on elliptically deformed equilibrium tidal flows in rotating fluid planets and stars, and excites inertial waves in convective regions if the dimensionless tidal amplitude (ϵ) is sufficiently large. We study its interaction with turbulent convection, and attempt to constrain the contributions of both elliptical instability and convection to tidal dissipation. For this, we perform an extensive suite of Cartesian hydrodynamical simulations of rotating Rayleigh–Bénard convection in a small patch of a planet. We find that tidal dissipation resulting from the elliptical instability, when it operates, is consistent with ϵ3, as in prior simulations without convection. Convective motions also act as an effective viscosity on large-scale tidal flows, resulting in continuous tidal dissipation (scaling as ϵ2). We derive scaling laws for the effective viscosity using (rotating) mixing-length theory, and find that they predict the turbulent quantities found in our simulations very well. In addition, we examine the reduction of the effective viscosity for fast tides, which we observe to scale with tidal frequency (ω) as ω−2. We evaluate our scaling laws using interior models of Hot Jupiters computed with mesa. We conclude that rotation reduces convective length-scales, velocities, and effective viscosities (though not in the fast tides regime). We estimate that elliptical instability is efficient for the shortest period Hot Jupiters, and that effective viscosity of turbulent convection is negligible in giant planets compared with inertial waves.

Effect of urbanization on gust wind speed in summer over an urban area in Eastern China

Previous studies have extensively examined effects of urbanization on mean wind speed, but few studies were aimed at gust wind speed, while large wind gust could cause safety and economic hazard to a variety of activities. In this study, the effect of urbanization on the gust wind speed in Nanjing, China is assessed using the Weather Research and Forecasting (WRF) model with a parameterization of the gust wind speed. The WRF simulations are run for the summer period of 2013 with the underlying surface before and after the urbanization. The results indicate that although the mean wind speed is reduced, the gust wind speed in the urban areas is increased significantly due to the enhanced friction velocity and less atmospheric stability induced by the urbanization, while the contribution of deep convection is relatively small. The gust wind speed increases more in the nighttime (0.6–0.9 m s−1) than in the daytime (less than 0.3 m s−1), since the turbulence is enhanced more in the nighttime than in the daytime after the urbanization. The probability distribution shows that the increase of gust wind speed is mainly between 0.0–0.5 m s−1 in the urban areas. In different urban land categories, the increase of the gust wind speed is larger in the commercial or industrial areas than in high-intensity and low-intensity residential urban areas. Averagely, the gust wind speed in the entire city after the urbanization increases by 0.02, 0.36 and 0.19 m s−1 for the daytime, nighttime and daily mean, respectively.

Analytical solution of phosphate kinetics for hemodialysis

Chronic kidney diseases imply an ongoing need to remove toxins, with hemodialysis as the preferred treatment modality. We derive analytical expressions for phosphate clearance during dialysis, the single pass (SP) model corresponding to a standard clinical hemodialysis and the multi pass (MP) model, where dialysate is recycled and therefore makes a smaller clinical setting possible such as a transportable dialysis suitcase. For both cases we show that the convective contribution to the dialysate is negligible for the phosphate kinetics and derive simpler expressions. The SP and MP models are calibrated to clinical data of ten patients showing consistency between the models and provide estimates of the kinetic parameters. Immediately after dialysis a rebound effect is observed. We derive a simple formula describing this effect which is valid both posterior to SP or MP dialysis. The analytical formulas provide explanations to observations of previous clinical studies.

A passive control approach for simulating thermally enhanced Jeffery nanofluid flows nearby a sucked impermeable surface subjected to buoyancy and Lorentz forces

Owing to the latest progression in the rheology of non-homogeneous composite media, viscoelastic materials have nowadays gained outstanding interest from experimenters because of their practical uses in different scientific areas (e.g., lubrication processing, blood vascular systems, polymer treatment, and petrochemical industry). Keeping in mind these vital applications, a realistic passive control approach has been applied in this numerical scrutinization to explore the aspects of radiating viscoelastic nanofluids during their MHD mixed convective flows nearby a sucked impermeable surface in the existence of an exponentially lessening heat generation. By invoking the constitutive rheological equations of Jeffery’s model and including the convective mass transport contribution along with the thermophoresis and Brownian diffusive mechanisms, the mathematical formulation of the flow model has been derived properly under the boundary layer assumptions. After a specific numerical treatment of the resulting boundary layer equations, several tabular and graphical demonstrations have been drawn accordingly. In this respect, it is demonstrated that Jeffery’s and Deborah’s numbers exhibit dissimilar behaviors toward the nanofluid motion and the resulting transport phenomena. As important findings, it is noticed that the convective heating and radiative heat transfer mechanisms enhance considerably the heat exchange rate with a slight weakening in the drag forces at the vertical surface. Whilst, the suction process shows an advantageous impact on these quantities of interest with a feeble perturbation in the surface temperature and nanoparticles concentration.

THE THERMAL CONDUCTIVITY OF MOLTEN MIXTURES OF СeCl<sub>3</sub>–MCl (M = Li, Na, K, Cs) SYSTEMS

The paper presents experimental data on the thermal conductivity of molten salt mixtures СeCl3–MCl, where M = Li, Na, K, Cs. The concentration of cerium trichloride varies from 0.25 to 0.75 mole percent in 0.25 increments. The initial salts of alkali metal chlorides were certified by DSC. The obtained values of melting temperatures are in good agreement with the literature data. Anhydrous cerium trichloride was obtained from cerium(IV) oxide in 2 stages: preparation of cerium crystalline hydrate and removal of water of crystallization. The measurements were carried out by the stationary method of coaxial cylinders in a nickel device in the temperature range individually selected for each composition. The relative measurement error does not exceed 5%. In this work, the convective and radiative contributions to heat transfer were estimated. The value of the product of Prandtl and Grashof numbers is less than 1000, which confirms the absence of convection. The calculated radiative contribution to heat transfer does not exceed 2.4%. The thermal conductivity of all investigated melts increases with increasing temperature. The concentration dependences of molten mixtures of cerium and alkali metal chlorides were obtained. The thermal conductivity decreases upon passing from Li to Cs, which is due to an increase in the radius of the alkali metal cation and, as a consequence, an increase in the interionic distance.

Numerical modeling for evaluation of the thermal resistance of reflective airspaces with and without defects

The thermal resistances (R-values) of airspaces depends on the emittance of all surfaces around an airspace, dimensions, heat-flow direction, and the temperatures of the bounding surfaces. Assessing the energy performance of building envelope components and fenestration systems requires accurate results for the R-values of any enclosed spaces. The evaluation of reflective insulation R-values has evolved to include use of computational fluid dynamics and surface-to-surface radiation to quantify convective and radiative contributions to the heat transfer across airspaces of all types. This paper compares an advanced and validated model for calculating enclosed airspace R-values with the widely-used ISO 6946 and airspace R-values in the ASHRAE Handbook-Fundamentals. The impact of construction and installation defects on the thermal performance of reflective insulation has not been previously evaluated. In this research, an advanced model was used to evaluate a construction defect and dimensional aspect ratios that one-dimensional methods do not address. Imperfect installation and defects that result in air movement into or through a reflective insulation assembly reduces the thermal resistance of the assembly. Additionally, the amount of thermal resistance reduction depends on the amount and temperature of invasive air or the size of internal defects that allows natural convection inside the reflective airspace. In this study, these performance issues are evaluated quantitatively using computer simulation techniques. The differences in results obtained using methods that are currently being used to evaluate the R-value and the advantages of the advanced method for evaluating the reflective insulation performance for different applications are discussed. For the case considered in this study, the results showed that the failure to achieve parallel surfaces results in less than a 5% decrease in thermal resistance. Also, the results showed that internal air gaps between airspaces result in negligible loss in R-value unless air gaps that allow circulation between airspaces are created.

Investigation of the effect of aspect ratio on heat transfer from a heated vertical wall to phase change material in a rectangular enclosure

This work is an experimental investigation of the relative contributions of conduction and convection to the melting rate for a phase change material. Experiments were conducted in a rectangular enclosure where heat was transferred from a vertical heated wall to the PCM (Polyethylene glycol (PEG 1500)). PCM is a high energy density material that can be used to reduce the size of thermal energy storage system (TES) significantly and store the energy at constant temperature. Six different enclosures were used for the study of increasing aspect ratio to show how the melting process is influenced by PCM thickness. Performance was recorded using both thermocouples and digital photos to record the melt temperatures and progression of the melt interface with time. In practice it is difficult to measure directly the heat transferred from the heated wall. Therefore, for a given time period the percentage of PCM melted was measured and this was used to calculate the heat transfer rate for the given time period. A sandwich type energy balance was then conducted across the full section of the block to confirm the calculated rate was correct.The results show that natural convection dominates the heat transfer from the hot wall for aspect ratios less than 6.2 shortly after heating was initiated. The results also show that conduction heat transfer from the hot wall becomes the dominant mode during the whole melting process when the aspect ratio rises above 6.2 (width = 25 mm).

Technique for calculating streamwise local fluxes in general flows

This paper presents a new technique for calculating microscopic local fluxes that provides a robust and accessible way to obtain the streamwise fluxes in general flows. Flux calculation is one of the fundamental elements in molecular simulations. Several-decades-long research efforts on the subject made significant progresses since the pioneering work of Irving and Kirkwood [J. Chem. Phys, (1950) 18, 817–829]. However, some issues remain regarding the fluxes in the flow direction. The streaming velocity (SV) and the fluxes independent of SV are difficult to manage during computation. For example, the local fluxes and SV that vary in more than one direction are not well represented despite being prevalent in the microscopic phenomena such as nanofluidics, biotechnologies, and energy conversions. The technique presented in this paper is based on the instantaneous microscopic conservation equations and depends only on the total fluxes that include the convective contributions. This allows a calculation of all the fluxes at the end of a simulation run, with no complication arising in terms of defining and collecting SV, pressure tensor, and heat flux vector. Widely-used methods such as the method of planes and volume-average method were adapted, respectively. This study then investigated a planar shear flow in a highly confined channel, where a flow is unknown at the beginning of a simulation, and the controversial streamwise heat flux with no temperature gradient was present. The flow and fluxes were structured, and the streamwise heat flux evolved significantly and slowly over a few tens of nano-seconds even with steady boundary conditions.

Conjugate heat and mass transfer at heterogeneous burning surface of AP/HTPB/Al composite propellant at 2–10 MPa

Conjugate heat and mass transfer at heterogeneous burning surface has significant effect on combustion characteristics of AP/HTPB/Al composite propellant. In the present study, the conjugate heat and mass transfer characteristics of AP/HTPB/Al composite propellant are studied experimentally and numerically based on formulated combustion model and experimental facilities. With the increase of pressure and Al content, flame temperature increases significantly and leads to stronger heat transfer to the burning surface. With the content of Al varying from 18 wt% to 9 wt%, the total heat transfer coefficient decreases for all the sampled positions of the burning surface with a maximum decrease of 12% at the AP/HTPB interface. It is found that heat convection accounts for more than 97% of the total heat transfer coefficient. However, with pressure increasing from 2 MPa to 10 MPa, the total heat transfer coefficient and the respective contribution of heat convection and radiation vary slightly. Owing to the heterogeneous sandwich-like structure of the propellant, the temperature distribution at the burning surface of propellant is highly non-uniform. Therefore, heat transfer characteristics vary significantly at different positions of the burning surface. The heterogeneity of the burning surface is exacerbated by the non-uniform distribution of heat transfer coefficients at different positions of the burning surface. The burning rate of the propellant varies from 0.5 mm/s to 9.5 mm/s for different positions of the burning surface at 10 MPa. The results of micro-scale flame structure show that the mass transfer of fine Al particles in the combustion of AP/HTPB/Al composite propellant can be basically divided into four stages, i.e., migration, melting, agglomeration and oxidation/combustion. Coral reef-like aggregate structure is formed by some of the fine Al particles before oxidation and combustion. Increasing pressure is conducive to avoiding formation of large Al agglomerates and enhancing mass transfer at the heterogeneous burning surface.

Applications of triadic hybridized-cross nanomaterials suspended in engine oil using quadratic and linear convection with magnetic dipole

This work comprises the contribution of buoyancy forces, linear and quadratic convection on cross Nano fluid under the effects of dipole effects with chemical reaction. The flow of a cross model (CM) with mass and thermal transport has been modeled in Cartesian coordinates for the two-dimensional fluid flow problem. Temperature gradient effects have been incorporated in mass transport and concentration gradient impacts in thermal transport phenomenon with chemical reaction. The governing laws are derived under boundary layer principle (BLP) in the conformation of coupled nonlinear partial differential equations. Afterwards, coupled partial differential equations are converted into a scheme of ordinary differential equations (ODEs) by incorporating the similarity transformation. The nonlinear modeled equations are solved numerically with the aid of finite element scheme as well as analysis of grid independent is reported based on the reliability of the utilized approach. Several graphs and tables have been prepared to observe the bearing of multiple emerging parameters regarding velocity, temperature as well as concentration fields. Comparison has been made to confirm the efficiency of a finite element scheme. It is perceived that higher values of a Schmidt number control the fluid concentration. Moreover, velocity is diminished against higher values of Weissenberg parameter.

Vorticity dynamics of fully developed leading-edge vortices on revolving wings undergoing pitch-up maneuvers

In nature, birds and insects usually execute pitch-up maneuvers, which is either an active perching or a passive response against gusts. During such maneuvers, their wings flap with a concomitant nose-up rotation around an axis, and thus, both vortex structures and aerodynamic forces of the wings are influenced. This research focuses on the impact of pitch-up maneuvers on the evolution and underlying vorticity dynamics of a fully developed leading-edge vortex (LEV), which has received limited interest in previous research. Based on data obtained from numerical simulations, an analysis of vortex dynamics and vorticity transport is conducted at different pitch rates and pitch axis locations. Our findings show that an increase in pitch rate and a shift of pitch axis toward the trailing edge can both terminate the growth of LEV and then initiate its movement toward the trailing edge via strong downward convection. However, the contributions of spanwise convection and vortex stretching (or compression) are distinct in these two scenarios, leading to different lift generations. Other vortex-tilting-based mechanisms, e.g., the planetary vorticity tilting and the dual-stage radial-tangential vortex tilting, are reduced during pitch-up maneuvers. Moreover, a rapid pitch-up around the leading edge is encouraged to maximize the lift during the maneuver, although this should be accompanied by constraints in flight height and kinetic energy when being applied to guide the perching of bio-inspired flapping wing micro air vehicles.

Inference of main ion particle transport coefficients with experimentally constrained neutral ionization during edge localized mode recovery on DIII-D

The plasma and neutral density dynamics after an edge localized mode are investigated and utilized to infer the plasma transport coefficients for the density pedestal. The Lyman-Alpha Measurement Apparatus (LLAMA) diagnostic provides sub-millisecond profile measurements of the ionization and neutral density and shows significant poloidal asymmetries in both. Exploiting the absolute calibration of the LLAMA diagnostic allows quantitative comparison to the electron and main ion density profiles determined by charge-exchange recombination, Thomson scattering and interferometry. Separation of diffusion and convection contributions to the density pedestal transport are investigated through flux gradient methods and time-dependent forward modeling with Bayesian inference by adaptation of the Aurora transport code and IMPRAD framework to main ion particle transport. Both methods suggest time-dependent transport coefficients and are consistent with an inward particle pinch on the order of 1 m s−1 and diffusion coefficient of 0.05 m2 s−1 in the steep density gradient region of the pedestal. While it is possible to recreate the experimentally observed phenomena with no pinch in the pedestal, low diffusion in the core and high outward convection in the near scrape-off layer are required without an inward pedestal pinch.

Theoretical Evaluation of the Persistence of Transverse Phonons across a Liquid-like Transition in Superionic Conductor KAg3Se2

A fundamental understanding of unusual atomic dynamics and collective modes in superionic conductors has been of great importance in improving renewable energy technologies such as thermoelectric energy conversion and electrolytes for rechargeable batteries. In this work, we have performed an in-depth theoretical study of the evolution of lattice dynamics across the superionic transition of KAg3Se2 by analyzing the thermally driven current fluctuations. We show that the diffuson-like phonons induced by the strong anharmonicity of the Ag sublattice dominate thermal transport below the superionic transition, resulting in nearly temperature-independent ultralow thermal conductivity in the normal ordered state. We find that the contributions of convection and conduction–convection interactions to thermal conductivity increase significantly due to the liquid-like flow of Ag atoms. We further demonstrate that the dynamic disorder is too slow to completely suppress the propagation of long-wavelength transverse phonons in the superionic state of KAg3Se2, while the propagation of Ag-dominated short-wavelength transverse phonons near the Brillouin zone boundary breaks down. These results provide new physical insights into the complex atomic dynamics of superionic conductors.