Search Results (532)

Radioactive metal oxides are highly radioactive, hygroscopic spent fuel reprocessing products generally stored in container-sealed dry storage. During the storage process of metal oxides, a large amount of heat is generated due to radioactive decay, and helium is produced by α-decay, which leads to an increase in the temperature and pressure of the storage container. In order to ensure the safety of the radioactive metal oxides in the long-term storage process, computational fluid dynamics simulations are used to investigate the effects of storage conditions on the temperature and pressure of the container. Based on a large amount of simulated temperature data under different storage conditions, a power function is used to construct a mathematical model of ventilation speed, ventilation temperature, stack density, loading volume, heating power, water content, and cumulative helium mass versus metal oxide temperature to obtain a safe, reliable, and economical storage method. The results show that reducing the loading volume and increasing the density of metal oxides, increasing the ventilation speed, and lowering the ventilation temperature are beneficial to the heat transfer and cooling in the dry storage process; increasing the density of metal oxides and lowering the water content of metal oxides and increasing the ventilation temperature and speed are beneficial to avoid the high pressure inside the container. Based on the optimized storage conditions, the temperature peak in the storage process occurs near 25 years, and its temperature reaches 527.6 K. The mathematical model of storage temperature constructed in this study has high computational accuracy, and the maximum relative error of storage temperature is less than 1.80%. Full article

Energy storage stations (ESSs) need to be charged and discharged frequently, causing the battery thermal management system (BTMS) to face a great challenge as batteries generate a large amount of heat with a high discharge rate. Supercritical carbon dioxide (SCO₂) is considered a promising coolant because of its favorable properties, including non-flammability, high dielectric strength and low cost for the BTMS. The heat of a battery can be absorbed to a great extent if there is a small temperature rise because as the fluid temperature approaches a pseudo-critical temperature, the specific heat capacity of SCO₂ reaches its peak. In this study, a periodic model of the unit BTMS is established, and a numerical simulation is implemented to investigate the effects of different boundary conditions on the heat dissipation of a battery pack. The flow and heat transfer characteristics of SCO₂ in the liquid cold plate (LCP) of a battery pack with an extreme discharge rate are revealed. The results show that SCO₂ is more preferably used as a coolant compared to water in the same conditions. The maximum temperature and the temperature difference in the battery pack are reduced by 19.22% and 79.9%, and the pressure drop of the LCP is reduced by 40.9%. In addition, the heat transfer characteristic of the LCP is significantly improved upon increasing the mass flow rate. As the operational pressure decreases, the pressure drops of SCO₂ decrease in the LCP. Overall, the maximum temperature and the temperature difference in the battery pack and the pressure drops of the LCP can be effectively controlled by using a coolant made out of SCO₂. This study can provide a reference for the design of BTMSs in the future. Full article

This paper provides an optimization analysis of the hydrothermal liquefaction (HTL) process of sewage sludge to biocrude oil in a continuous plug-flow reactor. The increase in flow rate led to enhanced swirling flow, which significantly improved convective heat transfer. The composition and yield of biocrude oil produced in the process (HTL) can vary significantly depending on the type of feedstock used. Using process simulation principles and a kinetic model, this study thoroughly evaluated the mass and energy balance of the HTL reaction, considering heat and mass momentum exchange in a multiphase system. Therefore, it is useful to use a transient flow model to determine the influence of process parameters on optimization. A parametric study with multiphase profiles along the reactor axis allowed tracing of interphase flow trajectories for optimal conditions in order to maximize the process efficiency of biocrude oil production. Through optimization of process parameters, there was a significant improvement in the conversion of sewage sludge to biocrude oil in the continuous HTL process. The optimal conditions were where the reaction mass maintained in the liquid phase enabled the stabilization of process parameters, preventing evaporation and heat loss by increasing the energy process efficiency. Full article

This study establishes a numerical simulation model based on heat and mass transfer theory to reflect the variations in temperature and humidity conditions within a tunnel. It analyzes the impact of high-temperature fissure water, humid porous media, and drainage methods on the temperature and humidity distribution in a tunnel. The results indicate the following: (1) When the area of the humid porous media increases from 150 m² to 300 m², the relative humidity (RH) of the air in the tunnel rises from 52.7% to 55.8%, but the impact on air temperature (T_a) is minimal. (2) The heating and humidification effects of hot water in a drainage ditch on the airflow cannot be overlooked. Meanwhile, the hot water transfers heat to the surrounding rock, with heat transfer predominantly driven by the surrounding rock convection. Compared to a drainage pipe, the heat transfer amount increases by 44.9%, and RH rises by 9.3%. (3) For every increase of 5 °C in water temperature (water volume of 90 m³/h), the ventilation outlet T_a linearly increases by 0.15 °C, and the rate of increase in RH accelerates with rising water temperature. (4) Covering a drainage ditch with a cover plate can reduce RH by 12.3%, while spraying a 10 cm insulation layer on the tunnel walls can significantly lower T_a by 0.66 °C. These findings provide a potential solution for the application of insulation materials in reducing the thermal hazards of deep high temperatures. Full article

This paper addresses the numerical simulation of unsteady, non-isothermal ventilation in a dead-end mine working of a potash mine excavated using a borer miner. During its operations, airflow can become unsteady due to the variable operating modes of the borer miner, the switching on and off of its motor cooling fans, and the movement of a shuttle car transporting ore. While steady ventilation in a dead-end working with a borer miner has been previously studied, the specific features of air microclimate parameter distribution in more complex and realistic unsteady scenarios remain unexplored. Our experimental studies reveal that over time, air velocity and, particularly, air temperature experience significant fluctuations. In this study, we develop and parameterize a mathematical model and perform a series of numerical simulations of unsteady heat and mass transfer in a dead-end working. These simulations account for the switching on and off of the borer miner’s fans and the movement of the shuttle car. The numerical model is calibrated using data from our experiments conducted in a potash mine. The analysis of the first factor is carried out by examining two extreme scenarios under steady-state ventilation conditions, while the second factor is analyzed within a fully unsteady framework using a dynamic mesh approach in the ANSYS Fluent 2021 R2. The numerical results demonstrate that the borer miner’s operating mode notably impacts the velocity and temperature fields, with a twofold decrease in maximum velocity near the cabin after the shuttle car departed and a temperature difference of about 1–1.5 °C between extreme scenarios in the case of forcing ventilation. The unsteady simulations using the dynamic mesh approach revealed that temperature variations were primarily caused by the borer miner’s cooling system, while the moving shuttle car generated short-term aerodynamic oscillations. Full article

In the process of boiling heat transfer, the microlayer is not only a crucial medium for enhancing heat transfer but also directly determines the heat flux distribution, dry zone expansion, and overall heat transfer efficiency through its morphological evolution and evaporation behavior. Building on this, this study employs the Lattice Boltzmann Method (LBM) with a single-component multiphase model to numerically simulate the evaporation process of microlayers on non-isothermal walls. The results show that, due to the uneven velocity distribution in the flow field, the microlayer exhibits significant contraction behavior during evaporation, particularly at the three-phase contact point, where velocity differences lead to fluid accumulation and the formation of a “cap-like” structure. The initial growth of the dry zone follows a linear trend, but its growth rate gradually decreases as the microlayer thickness increases, while near-wall density effects result in residual thickness within the dry zone. Additionally, the microlayer height first increases and then decreases over time, accompanied by a noticeable time lag. Heat flux analysis reveals that, during the formation of the dry spot, the lowest heat flux occurs at the three-phase contact point, followed by a sudden increase. A cold air ring forms above the dry zone, expanding and splitting as it moves with the dry spot. Higher temperatures promote microlayer evaporation, with the evaporation volume exhibiting nearly linear growth and the total fluid mass decreasing linearly. Full article

Annular flow channels, which are distinct from circular pipes, represent a complex flow structure widely applied in fields such as food engineering and petroleum engineering. Discovering the internal flow patterns is conducive to the study of heat and mass transfer laws, thereby playing a crucial role in optimizing flow processes and selecting equipment. However, the mechanism underlying the influence of annular turbulent flow on macro-pressure drop remains to be further investigated. This paper focuses on the roughness of both inner and outer pipes, as well as positive and negative eccentricities. Numerical simulation is employed to study the microscopic characteristics of the flow field, and the numerical model is validated through indoor experimental measurements of pressure drop laws. Further numerical simulations are conducted to explore the microscopic variations in the flow field, analyzed from the perspectives of wall shear force and turbulence characteristics. The results indicate that an increase in inner pipe roughness significantly enhances the wall shear force on both the inner and outer pipes, and vice versa. In the concentric case, wall shear force and turbulence characteristics exhibit central symmetry. Eccentricity leads to uneven distributions of velocity, turbulence intensity, and shear force, with such unevenness presenting axial symmetry under both positive and negative eccentricities. Additionally, eccentricity demonstrates turbulence drag reduction characteristics. This study enhances our understanding of the mechanism by which annular turbulent flow influences pressure drop. Furthermore, it offers theoretical backing for the design and optimization of annular space piping, thereby aiding in the enhancement of the performance and stability of associated industrial systems. Full article

In recent years, packed-bed systems for large-scale applications have emerged as a highly promising design for Thermal Energy Storage systems because of their high thermal efficiency and economic feasibility. Large-scale application systems typically include packed-bed thermal energy stores as essential components, enabling effective integration with renewable energy and processed heat. The packed-bed systems investigated in this paper utilise Magnesia as the storage medium and optimised parameters, which have previously been identified through research involving charging and discharging cycles of both the hot and cold storage systems when air is the heat transfer fluid. This includes solid particle diameters of 0.004 m, a material porosity of 0.2, an aspect ratio of 1 for the storage tank, and a mass flow rate of 13.7 kg/m³. This paper aims to present a comparative analysis of the influence of alternative heat transfer gases, namely air, argon, carbon dioxide, helium, hydrogen, and nitrogen, on the performance of Pumped Thermal Energy Storage hot and cold storage systems. The performance of the six gases in the storage system was evaluated using an axisymmetric model simulated with COMSOL Multiphysics 5.6 software, with the total energy stored and the capacity factor serving as key performance indicators. The results revealed that carbon dioxide gas was the most promising heat transfer fluid and that the packed bed could be operated efficiently over 72% and 76% of its range for hot and cold systems, respectively. Hydrogen, nitrogen, and air performed similarly but less adequately than carbon dioxide and had operating ranges of 55% and 75% for hot and cold storage. Helium and argon had the poorest performance, with optimal charging and discharging rates corresponding to 50% and 66%. Full article

In simulations of elastic flow using the lattice Boltzmann method (LBM), the steady-state behavior of the flow at low capillary numbers is typically poor and prone to the formation of bubbles with inhomogeneous lengths. This phenomenon undermines the precise control of heat transfer, mass transfer, and reactions within microchannels and microreactors. This paper establishes an LBM multiphase flow model enhanced by machine learning. The hyperparameters of the machine learning model are optimized using the particle swarm algorithm. In contrast, the non-dominated sorting genetic algorithm (NSGA-II) is incorporated to optimize bubble lengths and stability. This results in a coupled multiphase flow numerical simulation model that integrates LBM, machine learning, and the particle swarm algorithm. Using this model, we investigate the influence of elastic flow parameters on bubble length and stability in a T-shaped microchannel. The simulation results demonstrate that the proposed LBM multiphase flow model can effectively predict bubble elongation rates under complex conditions. Furthermore, multi-objective optimization determines the optimal gas–liquid two-phase inlet flow rate relationship, significantly mitigating elastic flow instability at low capillary numbers. This approach enhances the controllability of the elastic flow process and improves the efficiency of mass and heat transfer. Full article

The porous solar receiver (PSR) is a promising technology in advanced high-temperature applications. However, the non-uniform distribution of concentrated solar flux (CSF) and the dense pore structure lead to localized overheating and significant thermal losses for the PSR. This review focuses on the optimization strategies to enhance the thermal performance of the PSR, including porosity parameters, spectral selectivity, geometric configurations, and optical windows. Furthermore, mitigation strategies for addressing localized high temperatures in the PSR were thoroughly discussed, including methods for homogenizing CSF and improving the velocity of heat transfer fluid (HTF). Additionally, a numerical simulation and experimental measurements were introduced and evaluated. Additionally, the paper emphasizes the need to optimize the macroscopic geometry of OPSRs to improve their flow and heat transfer performance, thereby enhancing their practical value. It also suggests designing PPSRs that integrate adjustments for HTF mass velocity, CSF, optical window load, and reflection losses. Consequently, future studies should focus on developing efficient simulation and validation methods to advance the practical application of PSRs. Full article

Fluidized bed reactors (FBRs) are integral to various industries due to their exceptional capability in facilitating efficient gas–solid interactions, resulting in superior mixing and heat and mass transfer. This research delves into the impact of temperature on fluidization dynamics, particularly focusing on the collisional properties of particles within the bed. The investigation builds upon foundational research, notably Geldart’s classification of fluidization regimes and recent advancements in high-temperature experimental techniques, such as High-Temperature Endoscopic-Laser particle image velocimetry/digital image analysis. To explore these temperature effects, a coupled Discrete Element Method and Computational Fluid Dynamics (cfd–dem) model was employed. This approach enables a detailed examination of gas–particle and particle–particle interactions under varying temperature conditions. The simulations in this study explore the friction coefficient, as well as changes in both tangential and normal restitution coefficients, which affect the fluidization behavior. These changes were systematically analyzed to determine their influence on minimum fluidization velocity and bubble formation. The numerical results are compared with experimental data from high-temperature fluidization studies, highlighting the necessity of incorporating inter-particle forces to fully capture the observed phenomena. The findings underscore the critical role of particle collisional properties in high-temperature fluidization and suggest the potential increasing role of inter-particle forces. Overall, this paper provides new insights into the complex dynamics of fluidized beds at elevated temperatures, emphasizing the need for further experimental–numerical research to enhance the reliability and understanding of these systems in industrial applications. Full article

In fourth-generation district heating systems (DHSs), the supply temperature of modern heat sources such as heat pumps and waste heat can potentially be reduced by mixing in hot water from combustion-based producers, thereby increasing efficiency and facilitating integration into networks with unrenovated buildings. However, this approach introduces the risk of thermo-hydraulic oscillations driven by mixing dynamics, transport delays, and mass flow adjustments by consumers. These oscillations can increase wear and cost and may potentially lead to system failure. This study addresses the asymptotic stability of multi-supplier DHSs by combining theoretical analysis and practical validation. Through linearization and Laplace transformation, we derive the transfer function of a system with two suppliers. Using pole-zero analysis, we show that transport delay can cause instability. We identify a new control law, demonstrating that persisting oscillations depend on network temperatures and low thermal inertia and enabling stabilization through careful temperature selection, thorough choice of the slack supplier, or installation of buffer tanks. We validate our findings using dynamic simulations of a nonlinear delayed system in Modelica, highlighting the applicability of such systems to real-world DHSs. These results provide actionable insights for designing robust DHSs and mitigating challenges in multi-supplier configurations by relying on thoughtful system design rather than complex control strategies. Full article

Models were developed to study the heat-mass transfer and stress–strain process in carrots during hot air drying. The distribution and variation in temperature, moisture content, strain, and stress of the samples were investigated at different drying temperatures. The results showed that the models developed could be used to simulate the hot air drying process of carrots; the maximum weighted absolute percentage errors were 9.01%. The difference between the heat flux and vapor diffusion flux in the regions led to a non-uniform temperature and moisture content distribution, which resulted in non-uniform strain, causing stress within the carrots. The value of the thermal strain and stress was small compared to that of the moisture strain and stress. The thermal stress and moisture stress increased first and then decreased; the peak values of thermal stress and moisture stress occurred in the middle period of the whole drying process. When the hot air drying temperature was higher, the peak value of stress was higher. These results are helpful for understanding the drying mechanism and optimizing operating conditions in carrot drying. Full article

A new kinetic equation for the adsorption and desorption of H₂ on graphite is derived based on the adsorption and desorption equilibrium rates obtained from the molecular dynamics. These rates are proportional to the activity in the gas and the adsorbed phase and thus do not obey Langmuir kinetics. The new equation offers a new route for understanding experimental results. It is used to simulate the kinetics under different thermodynamic conditions, both isothermal and non-isothermal. The characteristic times of adsorption and desorption are in good agreement with the data from the literature. The relation between the kinetics and the mass flow equation is discussed within the framework of the non-equilibrium thermodynamics of heterogeneous systems. Finally, expressions for the transport coefficients are proposed for both the transfer of mass and the coupling between the mass and heat fluxes. Full article

This study explores the thermodynamic and kinetic properties of CeO₂ nano-fuels, with a particular focus on the influence of nanoparticle additives on the diffusion and thermal conductivity of C14-based fuel systems. Using molecular dynamics simulations and the COMPASS force field, we model the interactions between C14 molecules and CeO₂ nanoparticles, varying nanoparticle size and concentration. Our results reveal that the inclusion of CeO₂ nanoparticles leads to significant enhancements in both thermal conductivity (increasing by 9.8–23.6%) and diffusion coefficients (increasing by approximately 140%) within the 20 °C to 100 °C temperature range. These improvements are attributed to the interactions between nanoparticles and fuel molecules, which facilitate more efficient energy and mass transport. Notably, nanoparticles with smaller sizes (0.2 nm and 0.5 nm) exhibit more pronounced effects on both the thermodynamic and kinetic properties than larger nanoparticle analogs (20 nm and 50 nm). The study also highlights the temperature-dependent nature of these properties, demonstrating that nanoparticle additives enhance the fuel’s thermal stability and diffusion behavior, particularly at elevated temperatures. This work provides valuable insights into the optimization of nano-fuel systems, with potential applications in enhancing the performance and efficiency of diesel combustion and heat transfer processes. Full article