Transient Transport Research Articles

Device Model for a Solid‐State Barocaloric Refrigerator

Solid‐state refrigeration represents a promising alternative to vapor compression cooling systems. Solid‐state devices based on magnetocaloric, electrocaloric, and elastocaloric effects have demonstrated the ability to achieve high‐efficiency, reliable, and environment‐friendly refrigeration. Cooling devices based on the barocaloric (BC) effect—entropy change due to applied hydrostatic pressure, however, has not yet been realized despite the significant promise shown in material‐level studies. As a step toward demonstrating a practical cooling system, this work presents a thermodynamic and heat transfer model for a BC refrigerator The model simulates transient thermal transport within the solid refrigerant and heat exchange with hot and cold thermal reservoirs during reversed Brayton refrigeration cycle operation. The model is used to evaluate the specific cooling power (SCP) and coefficient of performance (COP) of the device comprising nitrile butadiene rubber (NBR) as a representative BC refrigerant. Experimentally validated BC properties of NBR are used to quantify the contribution of different operating parameters including cycle frequency, applied pressure, operating temperatures, and heat transfer coefficient. The results show that a BC refrigerator operating with a temperature span of 2.4 K and 0.1 GPa applied pressure can achieve an SCP of 0.024 W g−1 at 10 mHz cycle frequency and a COP as high as 5.5 at 1 mHz cycle frequency—exceeding that of conventional vapor compression refrigerators. In addition, to identify key refrigerant properties, the effect of bulk modulus, thermal expansion coefficient, heat capacity, and thermal conductivity on device performance are quantified. The results highlight the trade‐off between different material properties to maximize the BC response, while minimizing mechanical work and improving thermal transport. This work demonstrates the promise of solid‐state cooling devices based on soft BC materials and provides a framework to quantify its performance at the device‐level.

Pipe merging for transient gas network optimization problems

In practice, transient gas transport problems frequently have to be solved for large-scale gas networks. Gas network optimization problems typically belong to the class of Mixed-Integer Nonlinear Programming Problems (MINLP). However current state-of-the-art MINLP solvers are not yet mature enough to solve large-scale real-world instances. Therefore, an established approach in practice is to solve the problems with respect to a coarser representation of the network and thereby reducing the size of the underlying model. Two well-known aggregation methods that effectively reduce the network size are parallel and serial pipe merges. However, these methods have only been studied in stationary gas transport problems so far. This paper closes this gap and presents parallel and serial pipe merging methods for transient gas transport problems. An empirical evaluation indicates that the developed methods perform very accurately on a huge set of fine-grained real-world data taken from one of the largest transmission system operators in Europe.

Rapid prediction of transient particle transport under periodic ventilation using a non-uniform state Markov chain model

Predicting transient particle transport is crucial to address the risks posed to human health by indoor contaminants and to improve the design and control of ventilation systems. The Markov chain model with a coarse matrix has proven to be efficient in this regard, demonstrating faster speeds than traditional methods and higher computational accuracy when based on non-uniform states. However, its application under unsteady airflow conditions is limited, and discussions of how to determine non-uniform Markov states and their corresponding transient transition probabilities when the flow field changes are limited. Therefore, this study developed an application method of Markov chain model based on fixed non-uniform states in unsteady airflow fields. First, this model rapidly provides various Markov state schemes based on airflow field velocity and clustering methods. It then selects the optimal scheme through frequency and finally obtains the transition matrix through Lagrangian tracking for particle transport prediction. This model reduces the computational cost of transient particle transport when the wind speed changes while ensuring high computational accuracy. The proposed method was validated using published flow field experiments and simulation data. The computational speed of the proposed method was compared with computational fluid dynamics (CFD), and in the case studies considered in this research, the normalized root-mean-square deviation of the model was less than 10% and the total computation time was more than 75% faster than that of CFD.

RealTimeTransport: An open-source C++ library for quantum transport simulations in the strong coupling regime.

The description of quantum transport in the strong system-reservoir coupling regime poses a significant theoretical and computational challenge that demands specialized tools for accurate analysis. RealTimeTransport is a new open-source C++ library that enables the computation of both stationary and transient transport observables for generic quantum systems connected to metallic reservoirs. It computes the Nakajima-Zwanzig memory kernels for both dynamics and transport in real-time, going beyond traditional expansions in the bare system-reservoir couplings. Currently, several methods are available as follows: (i) A renormalized perturbation theory in leading and next-to-leading order, which avoids the low-temperature breakdown that limits the traditional theory. (ii) Starting from this well-behaved reference solution, a two- and three-loop, self-consistent renormalization-group transformation of the memory kernels is implemented. This allows refined quantitative predictions even in the presence of many body resonances, such as the Kondo enhancement of cotunneling. This paper provides an overview of the theory, the architecture of RealTimeTransport, and practical demonstrations of the currently implemented methods. In particular, we analyze the stationary transport through a serial double quantum dot and showcase for the T = 0 interacting Anderson model the complete time-development of single-electron tunneling (SET), cotunneling-assisted SET, and inelastic cotunneling resonances throughout the entire gate-bias stability diagram. We discuss the range of applicability of the implemented methods and benchmark them against other advanced approaches.

A comprehensive review on the Dynamical behavior of heat and fluid flow mechanism: Thermal performance across different geometries

The prominent novelty of current review is to exhibit the fluctuating and oscillatory convective heat transfer properties along various geometries with magnetic force, variable density, Prandtl number, buoyancy force and magnetic Prandtl number effects. The significance of present work is to illustrate a comprehensive review on transient convective heat transfer. Transient convective heat transfer plays an essential role in various technological and environmental applications, including climate control, structure safety, engines, thermal control, computer heating and cooling, and energy safety. The literature primarily focuses on steady temperature and velocity domains, with few studies exploring time-varying fluid flow in forced, natural, or mixed convective mechanisms with different methods. In current review, justification of results was performed by using oscillating stokes conditions directly in partial differential models. The similarity variables and stream functions are used in literature but in current review, the primitive variable transformation is used with implicit form of finite difference method and Gaussian elimination technique through FORTRAN and Tecplot-360 programming tools. The governing model is reduced into steady, real and imaginary form to explore steady and oscillating convective heat transmission. Transient flows have gained significance due to their ability to achieve large oscillating convective heat transfer rates. Compared with steady movement, oscillating flows exhibit higher velocity variations along the heated surface. Periodic flow produces an improved transient surface heat transmission rate than steady flow. Predicting and controlling transients in thermal exchangers requires a concept of transient forced convection heat transport. Transient convective thermal transmission solutions are important for ensuring the effectiveness and reliability of electrical components, nuclear and thermal power stations, heat-generating engines, steam turbines, condensation systems, catalytic converters, heat shielding for spacecraft, electrical devices, internal combustion engines, air conditioning and refrigerator components, cooling networks for batteries, generators, and transformers. It is found that the frequency of convective heat transport enhances through periodic and oscillating stokes variables. It was depicted that the higher amplitude in convective heat transfer was displayed for each choice of magnetic force parameter around two angles π/4 and π of circular magnetized surface. It was depicted that the maximum transient convective heat transmission was reported along vertical angle π/2 with buoyancy force, Prandtl and magnetic Prandtl values.

PIN2-mediated self-organizing transient auxin flow contributes to auxin maxima at the tip of Arabidopsis cotyledons.

Directional auxin transport and formation of auxin maxima are critical for embryogenesis, organogenesis, pattern formation, and growth coordination in plants, but the mechanisms underpinning the initiation and establishment of these auxin dynamics are not fully understood. Here we show that a self-initiating and -terminating transient auxin flow along the marginal cells (MCs) contributes to the formation of an auxin maximum at the tip of Arabidopsis cotyledon that globally coordinates the interdigitation of puzzle-shaped pavement cells in the cotyledon epidermis. Prior to the interdigitation, indole butyric acid (IBA) is converted to indole acetic acid (IAA) to induce PIN2 accumulation and polarization in the marginal cells, leading to auxin flow toward and accumulation at the cotyledon tip. When IAA levels at the cotyledon tip reaches a maximum, it activates pavement cell interdigitation as well as the accumulation of the IBA transporter TOB1 in MCs, which sequesters IBA to the vacuole and reduces IBA availability and IAA levels. The reduction of IAA levels results in PIN2 down-regulation and cessation of the auxin flow. Hence, our results elucidate a self-activating and self-terminating transient polar auxin transport system in cotyledons, contributing to the formation of localized auxin maxima that spatiotemporally coordinate pavement cell interdigitation.

Transient analytical solution for coupled water–gas transport in unsaturated soil cover of landfill

A new analytical solution for coupled water–gas transport in unsaturated landfill cover system is proposed, which can consider transient diffusive-advective transport of gas under transient water transport condition. The proposed analytical solution is first verified against the laboratory measurements and numerical simulations. Using the proposed solution, parametric studies are conducted on the coupled transient behaviors of water–gas transport. The results show that ignoring the coupling effect can lead to significant error in gas transport results under various conditions, e.g., the surface gas emission flux is underestimated by about 28.3%∼79.8% due to ignoring the spatial and temporal variations of water content induced by transient water transport. The influencing parameters (i.e., initial saturation degree, rainfall patterns, and evaporation rate) all have significant impact on the coupled water–gas transport behaviors. The magnitude of the error induced by ignoring the coupling effect increases with increasing initial saturation degree and increasing evaporation rate (e.g., for initial saturation degree increasing from 0.35 to 0.95, the accumulated percolation at cover bottom increases by 10.9 times and the gas emission flux at cover surface decreases by 64.7%). The results highlight that it’s imperative to consider the effect of transient water transport when one needs more accurate assessment of gas transport results in the cover. This analytical solution can also be utilized to aid landfill cover design and to verify other numerical models.

Numerical simulations of temperature loads on multilayer Laue lenses

We present numerical simulations of the heat loads on novel diffractive X-ray optics, known as multilayer Laue lenses, exposed to high-intensity X-ray beams produced by an X-ray free electron laser (XFEL). These lenses can be used to focus XFEL beams to nanometer spots. The temperature distribution within the lens and temperature evolution as a function of incident pulse frequencies were calculated for two different lens geometries and several material pairs and material ratios of the MLLs. Simulations considered the special pulse structure of European XFEL with X-rays being delivered in pulse trains. After defining the geometric model, computational grid, material properties, and boundary conditions, a grid sensitivity study was carried out. We solved the transient heat energy transport equation in solids for mixed boundary conditions. The results of these simulations will help select materials and lens geometry for future XFEL experiments.

Multi-Phase flow simulation for transient analysis of ballast drainage behavior and shoulder cleaning effect

This paper presents multi-phase flow simulations to analyze the transient transport of rain droplets through rail ballast and its dependence on fouling conditions and shoulder cleaning. The entire process consisting of random rain droplet injection, droplet migration through the air, falling on the ballast surface, drainage through the void spaces between ballast aggregates, and running off the ballasts are modeled. A computational fluid dynamics (CFD) model is developed, in which the volume of fluid (VOF) method and Darcy-Forchheimer's equation are employed to capture the gas–liquid mixture interface and consider fouling-induced flow resistance, respectively. The model is implemented as an interFoam solver in the OpenFOAM CFD tool. A method for determining trapped moisture inside the fouled and moderately fouled ballast layers is also presented. Parametric simulations are conducted to investigate the effect of fouling conditions, more specifically, Fouling Index (FI), Fouling Profile (FP), and shoulder cleaning on ballasts' drainage performance and time-dependent moisture accumulation and penetration. The results indicate that severe congestion in fouled ballast is responsible for higher moisture accumulation, slower penetration, and poorer drainage of ballast. Shoulder cleaning is an effective measure to reopen flow passage and restore drainage capability, in particular, for the fouled ballast.

Monte Carlo / Dynamic Code (MC/DC): An accelerated Python package for fully transient neutron transport and rapid methods development

Monte Carlo / Dynamic Code (MC/DC): An accelerated Python package for fully transient neutron transport and rapid methods development

Modeling transient edge plasma transport with dynamic recycling

The work presents numerical simulation studies of the role that dynamic plasma recycling on the main wall and divertor target surfaces plays in transient edge plasma transport phenomena, such as edge localized modes (ELMs). The studies are performed by coupling the edge plasma transport code UEDGE [Rognlien et al., J. Nucl. Mater. 196–198, 347 (1992)] and the wall reaction–diffusion transport code FACE [Smirnov et al., Fusion Sci. Technol. 71, 75 (2017)]. The two-dimensional, time-dependent, two-way coupling of the codes, in a realistic tokamak geometry, is accomplished using the Integrated Plasma Simulator framework [Elwasif et al., in 18th Euromicro Conference on Parallel, Distributed and Network-Based Processing (PDP 2010), Pisa, Italy (IEEE, 2010), pp. 419–427] for all modeled material plasma boundaries. The simulations show that dynamic plasma recycling has substantially different characteristics on the main wall and on the divertor plates. It is demonstrated that during an ELM cycle the outer wall can dynamically absorb and release a number of particles comparable to that expelled by the ELM from the core plasma, by far exceeding the dynamic retention capacity of the divertor surfaces. The resulting evolution of the edge and divertor plasma conditions during an ELM cycle is analyzed.

Comparison of the Climatic Characteristics of Ozone Valley Over the Tibetan Plateau and the Rocky Mountains

AbstractThis study compares the ozone valleys over the Tibetan Plateau (TP) and the Rocky Mountains (RM) using the ERA5 reanalysis data set. The dynamical transport of the ozone over these two regions is analyzed using the Lorenz circulation decomposition method. The ozone content valley over TP is observed around 200–50 hPa (upper troposphere and lower stratosphere, or UTLS), and that over RM is around 300–100 hPa. It is shown that the TP ozone content is smaller than that over RM. By analyzing the spatiotemporal distribution of the ozone content and the general circulation, the anticyclone over Southern Asian (SAH) plays a significant role in existence of the TP ozone valley, and the ozone content flux reaches its maximum in July. Large‐scale terrain and related general circulation determine the ozone valley appearance. Further analysis suggests that stationary transport has a larger impact on the ozone valley formation than the transient transport. The transport by the zonal circulation nearly cancels out most of that by the meridional circulation, due to the fact that the zonal transport magnitude is nearly equal to the meridional transport. The transport center over the RM is much weaker than that over the TP. Furthermore, the contrasts between transient and stationary transports are less evident over RM than over TP. The eddy‐driven stationary ozone transport flux significantly impacts the development of the two low ozone centers across these large terrains.

Dynamic Effect of Overscreening on Electrochemical and Thermal Properties of EDLC Electrodes during Charging/Discharging Cycles

Overscreening was commonly observed during charging/discharging cycles of electric double layer capacitors(EDLCs). This study presented a two-dimensional continuum electrochemical thermal model with a distribution function for concentration considering overscreening to investigate how overscreening affected the dynamic formation of electric double layer (EDL) near electrodes in macroscale. The results indicated that ion distribution was co-affected by EDL formation and overscreening near the electrode. Correspondingly, layered space charge density distribution resulted in a thicker diffuse layer, whose thickness was determined by the sum of co-ion and counter-ion diameters. Additionally, the diffuse layer capacitance was proportional to the reciprocal of counter-ion diameter, while the total capacitance was proportional to a linear combination of counter-ion and the larger ion diameters. Moreover, the shift from endothermic to exothermic for the total heat generation rate was dominated by heat of mixing, and was caused by the local ion mixing due to overscreening, resulting in local entropy increase. The results of this study can be used to further investigate the effect of overscreening on ion transient transport dynamic and heat generation rates.

Modeling analysis of pulsatile non-Newtonian blood flow in a renal bifurcated artery with stenosis

A transient transport flow model is adopted in a renal bifurcated artery with different stenosis cases to examine the flow phenomena when the non-Newtonian blood flow regime within the three-dimensional space domain is chosen as a pulsatile nature. The nonlinear governing equations are solved using the Finite Element Method with suitable initial-boundary conditions. During numerical computation, distinct hemodynamic parameters including Wall shear stress, Reynolds number, power law index are evaluated based on different stenosis severities (0%, 25%, and 50%) and stenosis lengths (0mm, 0.5mm, and 1mm). For 0%, 25%, and 50% stenosis, the approximate values of velocity and pressure profiles are estimated at t = 5s at various positions, and achieved the highest velocities that were around 0.07m/s, 0.08m/s, and 0.13m/s respectively. The unsteady response of the stress due to stenosis-induced shear on the outside wall is described as a result of stenosis at t = 0.1s, 0.2s, 5s. The flow is severely restricted by constriction, which increases wall movement and produces stress, depending on the amount of stenosis. As pulsatile flow time increases due to the presence of stenosis, the wall shear stress decreases gradually. The impact of Reynolds numbers on pressure profiles and velocity profiles are displayed for Re = 100, 300, 500, and 700, and found that both velocity and pressure outlines increase significantly. In order to identify the flow phenomena, the influences of the power law index for n = 0.1, 0.3568, 0.5 are presented on the velocity and pressure profiles. Shear-thinning flow occurs for 0<n<1, pseudoplastic flow occurs for n = 0.1 and Bingham plastic flow occurs for n = 0.5. The modeling investigations of blood flow via arteries could assist in the diagnosis and treatment of arterial illnesses; as well as expand the knowledge about cardiovascular physiology and lead the development of revolutionary medical interventions to enhance patient treatment outcomes.

Investigating the protective performance of turnout gears for firefighters under diverse exposure conditions: Effect of age and body segments

Present work investigates the impact of different exposures (flame, 50/50 convection and radiation, and radiation alone) on thermal performance of firefighters' clothing. The protective performance is evaluated across various body segments for firefighters of different ages. Transient heat transport equations are numerically discretized using finite difference approach. The computational domain comprises of a three-layer fabric assembly, a single-layer skin, and a microclimate between the fabric and skin. Heat transfer in the porous fabric assembly results from the interaction of both conduction and incident radiation penetrating through confined hot air within the fabric. The energy equation of fabric employs a two-flux model, encompassing self-emission originating from absorbed incident radiation in the encapsulated hot air. A nonlinear bioheat transfer model, accounting for age and body segment, incorporates temperature-dependent thermal conductivity, blood perfusion rate, skin thickness, and heat generation from metabolism, shivering, and external work. Results found that first and second-degree burn durations decrease by 30.85 % and 37.92 %, 26.73 % and 29.76 %, and 24.23 % and 26.63 % for flame, 50/50 convection-radiation and pure radiation heat exposure respectively as firefighter’s age from 25 to 65 years. The protective attire excels the best for the torso against direct flame but is the least effective for shielding arms from radiant heat. Moreover, the transition time from first to second-degree burn injury decreases by 64.77 %, 65.73 %, and 66.03 % for arms, and by 70.93 %, 66.29 %, and 67.22 % for legs in flame, 50/50 convective-radiative, and purely radiant heat exposures, respectively, with a 100 % reduction in skin conductivity coefficient.

A numerical research on transient transport of high-temperature particles associated with air gouging process

The high-temperature particles produced by air gouging processes are common pollutants in cleaning workshops, which could endanger workers’ health. In this research, a two-way coupled Eulerian-Lagrangian method based on discrete phase modelling (DPM) was used to investigate the transport characteristics of high-temperature particles generated from air gouging processes. The particle group in the movement was divided into core zone and boundary zone, and the kinetic mechanism of particles in different zones was analysed. The results show particles in the boundary zone are more susceptible to the vortex entrainment effect than particles in the core zone, and some particles could be moved from the vortex diffusion zone to the vortex recirculation zone. The smaller the particle size, the better the followability to the hot airflow. The slower the temperature decay of particles, the more significant the thermophoresis effect, which could enhance the diffusion behaviour of the small particles. By analysing the average residence time of particles in the breathing zone, the influence of different factors on the average vertical diffusion height [Formula: see text] of particles is summarized, and a nonlinear regression prediction model of [Formula: see text] is established. The results can contribute to health risk assessment and industrial ventilation design.

Modulation of the interfacial thermal resistances of the w-AlN/Graphene/3C-SiC interface by nanoscale nonplanar feature structures

Revealing the heat transport mechanism in typical heterostructures is crucial for designing GaN power chips with better engineering heat transfer performance. Herein, the effect of nanoscale nonplanar feature structures (NNFSs) on the interfacial thermal resistances (ITRs) of w-AlN/bilayer graphene/3C-SiC heterostructures is investigated systematically by molecular dynamics. The results indicate that the increase of the NNFSs numbers contributes to the decrease of the ITRs when the height is lower than the cutoff distance of the interaction. Multiple analyses indicate that, although the interactions at two van der Waals sub-interfaces (w-AlN/graphene and graphene/graphene) slightly weaken the phonon transfer, the increase of the NNFSs numbers enlarge the overlaps of phonon density of state (PDOS), which provides more phonon transfer channels. However, when the height is higher than the cutoff distance, the ITRs increase significantly due to the double decrease of the interactions and the overlaps of PDOS for the w-AlN/graphene sub-interface. These phenomena are further verified by the transient transport of energy. Besides, the NNFSs with random distribution also contribute to the reduction of the ITRs. We believe that the results of this work will provide significant guidance for improving the thermal transport performance of GaN power chips.

Cyclic Thermal and Structural Testing of a Hot Particle Storage Bin

Thermal energy storage is a key element in concentrating solar energy systems. In 2017 a Roadmap toward a third-generation system that could meet the SunSHOT goals of 0.06 $/kWe recommended increasing temperatures to &gt; 700° C for heat transfer media to in-crease thermal efficiencies and lower levelized costs of heat. In 2021, the U.S. Department of Energy selected the particle pathway, G3P3, to build a 1 MWt prototype solar tower with 6 MWh thermal energy storage at the NSTTF in Albuquerque, NM. Of the primary components, the falling particle receiver, and particle-to-sCO2 heat exchanger have been demonstrated at the 250 kWt capacity. The storage component is now being demonstrated as part of the G3P3-USA and G3P3-KSA pilot plants. Storage bin liner materials have been demonstrated by KSU in 2016 and 2019. A flowing particle storage container was demonstrated in 2020 by KSU. The testing presented in this work will be the first to test the particle to wall interactions with mono-lithic refractory insulation, and to validate a transient thermal transport model with the unique kinetics of bulk solids in funnel-flow where cooler particles near the walls flow inward toward a hot central flow channel. This work will also de-risk and characterize the specific design of the G3P3-USA storage bin.

Influence of reduced pressure on arc and weld pool properties in TIG welding

In this article, transient transport phenomena in the arc and weld pool for different ambient pressures during tungsten inert gas welding were reported. Arc plasma and weld pool behaviour vary with ambient pressure because of changes in arc thermophysical properties. The maximum arc temperature decreases with decreasing ambient pressure because of reduced plasma electrical conductivity. Decreased arc mass density results in a lower maximum weld pool surface temperature and a decrease in maximum metal vapour content as ambient pressure decreases. Arc pressure decreases with decrease in ambient pressure, resulting in shallower penetration. Calculated arc temperature distribution and weld cross-section are compared with experiments to validate the modeling method for simulating heat and mass transfer phenomena in low ambient pressure welding.

Investigating arc and molten metal transport phenomena in gas metal arc welding with Ar–CO2 gas mixtures using a numerical method

This paper presents a numerical investigation of the transient transport phenomena of the arc and molten metal during gas metal arc welding (GMAW) using shielding gas mixtures ranging from 100% Ar + 0% CO2 to 80% Ar + 20% CO2. The thermophysical parameters of the Ar–CO2 mixtures, considering the presence of metal vapor, were calculated as a function for a temperature range of 1000–30 000 K. The influence of metal vapor content and CO2 proportion on the thermophysical properties of the mixed gas was discussed in detail. As the CO2 content increased from 0 to 20%, the shape of the arc changed from a bell to a cone due to the increase in mass density, specific heat, and thermal conductivity. The maximum arc temperature and velocity decreased with increasing CO2 content, resulting in larger droplets and a lower droplet transfer frequency. Although the change in electrical conductivity did not affect the arc shape, it did influence the arc temperature by altering the distribution of current density. Experiments of droplet transfer and arc behavior were carried out, and the results showed that the simulated droplet size, transfer frequency, and arc temperature distribution agreed well with the experimental values. These findings could serve as a theoretical tool for better understanding the underlying physical mechanisms of the GMAW process using different shielding gases, ultimately aiming to achieve high weld quality.