Enhancing low-temperature isothermal convective drying of waste municipal sewage sludge with wood-derived biochar in sequential drying cycles.

Multi-scale analysis of solute dispersion in free and forced convection flow between two parallel plates filled with a porous medium

An efficient three-step subgrid stabilized method for the steady natural convection equations

Thermo-hydraulic coupling effects of natural convection, forced convection, and fractures on heat extraction in closed-loop geothermal systems

Optimization of natural convection heat sinks with inverted trapezoidal fins and elliptical perforations

Freshwater scarcity is a global challenge, and solar-driven interfacial evaporation (SDIE) technology has received widespread interest for its sustainability. However, vapor accumulation and salt deposition significantly reduce its performance. This study describes a solar evaporator with a Janus wettability grooved structure. This design not only allows for efficient double-sided evaporation but also provides a localized heat environment that greatly improves natural convection, effectively enhancing vapor diffusion. Under 1 sun irradiation, the optimized evaporator (JGE-60°) had an evaporation rate of 2.27 kg m-2 h-1. Under forced convection (4 m/s) conditions, its surface temperature dropped below ambient, reversing heat loss and increasing the evaporation rate to 5.96 kg m-2 h-1. Owing to the asymmetric wettability of its dual-sided structure, salt is selectively directed and deposited on the shaded side. This enables stable operation for 30 h in a 20 wt % brine while achieving a salt collection rate of 193.9 g m-2 h-1. This work simultaneously addresses two critical bottlenecks─vapor diffusion suppression and salt accumulation─through a simplified structural design, providing an important scientific and applied paradigm for evaporator design.

Natural convection analysis in a safety pool of an SMR using three-dimensional component-scale thermal hydraulics technique

Optimization of Thermal Energy Storage in Solar Systems Using Phase Change Materials: Design, Simulation, and Applications

Analysis of Thermal Stratification and Inclined Magnetic Field Influence on Magnetohydrodynamic Flow in Porous Media

Enhancing the thermal efficiency of pin fin heat sinks is a key concern in engineering, especially for managing heat in electronic components. Performance is heavily influenced by factors such as fin geometry, spacing, orientation, fluid flow behavior, and available heat transfer surface area. This study explores the combined effects of porous media, graphene (carbon-based nanoparticles)-water radiative nanofluids, and externally mounted V-shaped ribs on the natural convection performance of pin fin heat sinks. The combined influence of radiative nanofluid flow, V-shaped ribbed pin fins, and porous medium permeability on natural convection heat sink efficiency has never been examined in a single study. The enhancement strategy starts with modifying Classic Pin Fins (CPF) by incorporating standard transverse ribs, resulting in a configuration referred to as Pin Fins with Classic Ribs. For further thermal improvement, these ribs are substituted with V-shaped ribs to get Pin Fins with V-shaped Ribs (PFVSR), and the impact of varying the opening angle ( γ ) is analyzed to determine optimal conditions. To intensify heat removal, the entire heat sink system is embedded in a porous domain saturated with a radiatively active graphene-based nanofluid. Numerical simulations are performed in COMSOL Multiphysics to assess the roles of several key parameters. Because of increased buoyancy driven flow and better nanofluid circulation, numerical simulations show that the PFVSR configuration increases the average Nusselt number by 16.21% when compared to the CPF model. Heat transfer is further improved by 4% when the rib opening angle is optimized. The average Nusselt number rises by 45.86% when the heat sink is embedded in a high-permeability porous medium (Da = 10 2 ) as opposed to the low permeability instance (Da = 10 − 4 ), demonstrating the crucial role permeability plays in natural convection augmentation. Moreover, adding thermal radiation ( R d = 1) speeds up the flow field and boosts heat transfer efficiency by 15.84%. These findings show that a high-performance, compact heat sink design can be achieved by the combination of geometric alteration, porous media, radiative effects, and nanofluids.

This study evaluated the efficiency of a passive thermal homogenization device (aluminum tube) designed to optimize the temperature profile in a thermoelectric cooler (TEC) box. An experimental approach was considered, comparing a standalone TEC1-12706 Peltier module with a system integrated with an aluminum tube. Both configurations operated under natural convection with passive thermal management on the TEC hot side. The introduction of the aluminum tube led to a temperature reduction of 10°C inside the tube compared to 5°C in the rest of the enclosure, creating two distinct thermal zones. The results showed that the COP value increased from 35% to 250%, with an average gain of 80%, compared to the module without the aluminum tube. Although the absolute Coefficient of Performance (COP) remains lower than that of active systems, the incorporation of the aluminum tube significantly mitigates thermal stratification within the storage medium. These results underscore the potential of this approach for sustainable cooling applications in energy-constrained regions, primarily due to reduced mechanical complexity and enhanced system robustness.

Boiling heat transfer enables extremely high heat fluxes at small temperature differences, but the underlying mechanisms remain difficult to resolve due to the dynamic interplay of microscale interfacial processes and macroscopic heat transfer. In this work, synchronized optical–thermal measurements of pool boiling on mechanically ground-and-polished copper surfaces were conducted to directly correlate near-wall temperature transients with boiling regime transitions. Embedded micro-thermocouples captured repeatable near-wall temperature transients associated with boiling regime transitions, while synchronized video recording confirmed regime transitions from natural convection to nucleate and film boiling. Critical heat flux (CHF) values of 105.1–132.0 W/cm2 were measured at transition wall superheats of 25.3–31.4 °C for all three tested samples. The largest performance change occurred after the first boiling cycle, with wall superheat and CHF increasing significantly, whereas subsequent cycles produced smaller shifts. On cooling, film-to-nucleate collapse occurred at wall superheats around 4–17 °C higher than in the forward transition, confirming a pronounced hysteresis effect. Scanning electron microscopy analysis revealed substantial roughening after three boiling cycles, while energy-dispersive x-ray spectroscopy showed the oxygen-to-copper x-ray intensity ratio nearly doubled (0.08–0.17), indicating oxide layer growth. These surface modifications (conditioning) explain the observed performance evolution: roughening enhanced CHF, while oxidation introduced thermal resistance, elevating wall superheat. Taken together, these results demonstrate that boiling hysteresis and cycle-dependent CHF evolution are governed by coupled morphological and chemical transformations of the surface. The integrated optical, thermal, and microstructural approach provides direct evidence linking interfacial dynamics to surface conditioning in pool boiling.

This study investigates the complex interplay between standing wave acoustic streaming and natural convection in a differentially heated enclosure via a hybrid lattice Boltzmann method and finite difference solver. Covering Rayleigh numbers from 103 to 105 and various acoustic amplitudes, results reveal a non-monotonic heat transfer modulation driven by the competition between acoustic inertia and thermal buoyancy. In the conduction-dominated regime (Ra = 103), acoustic streaming acts as a powerful mixer, increasing the average Nusselt number by up to 400%. In contrast, the convection-dominated regime (Ra = 105) exhibits a counter-intuitive suppression, where strong acoustic forcing reduces heat transfer efficiency by approximately 41.6%. This suppression arises from antagonistic hydrodynamic interactions, where counter-rotating acoustic vortices disrupt the global buoyancy circulation. Additionally, strong thermo-acoustic coupling at high amplitudes induces a symmetry-breaking bifurcation via baroclinic torque, resulting in a robust asymmetric flow topology. A dimensionless thermo-acoustic velocity ratio is proposed as a unified criterion to predict the enhancement-to-suppression transition, offering key guidelines for acoustic thermal management.

Cryo-compressed hydrogen (CcH2) storage tanks onboard fuel cell electric trucks represent a promising technology for decarbonization of the road transport sector. In order to allow pressure control in the tank, an inner heat exchanger is needed that heats the hydrogen inside the tank and increases the pressure when a minimum pressure level is reached. In this study, the heat transfer from the heat exchanger into the tank is investigated. As the heat exchanger is considered as a horizontal smooth cylinder within the cylindrical tank, the heat transfer is characterized by turbulent natural convection in a concentric-cylindrical annulus with large gap width and large ratio of outer to inner diameter. Further, high temperature difference, increased pressure and cryogenic hydrogen result in a challenging scenario for simulation. Different heat transfer correlations are compared with two-dimensional, steady-state CFD simulation results. Best agreement is achieved with a modified form of the correlation by Kuehn and Goldstein (1978). The mean fluid temperature is defined differently than in the original correlation and is now determined with an energy balance and a separate consideration of fluid properties at the inner and outer walls. The modified correlation shows a mean absolute relative error from the CFD results of 6.9 % with standard deviation of 5.2 % . The calculated convective heat transfer coefficient at the internal heat exchanger surface ranges from α = 96 W/(m 2 K) t o 424 W/(m 2 K) , indicating a high natural convection heat transfer in the CcH2 storage tank. Furthermore, the influence of Boussinesq approximation on the Grashof number is investigated and found to be not significant. • Parametric study of turbulent natural convection in large-gap concentric cylinders. • Focus on high temperature differences in cryo-compressed hydrogen atmospheres. • Steady-state, two-dimensional CFD simulations and comparison of heat transfer results with empirical correlations from literature. • Modification of Kuehn and Goldstein (1978) correlation based on iteratively solved energy balance. • Mean absolute relative error between modified Kuehn and Goldstein (1978) correlation and CFD results of 6.9%

Natural convection and flow structure evolution during solid–liquid phase change in a horizontal annulus

ABSTRACT The enhancement of performance and safety in Li‐ion batteries strongly depends on effective cooling strategies. This study presents a comprehensive, time‐dependent numerical analysis of six alternative configurations of battery thermal designs for an electric racing car. Starting from a benchmark configuration relying on natural convection only, the other configurations incorporate more complex cooling systems, including liquid cooling, forced air convection, and phase change materials (PCM), either individually or in hybrid arrangements. Three‐dimensional computational fluid dynamics (CFD) simulations were performed on a 40‐cell lithium iron phosphate battery pack to evaluate transient temperature evolution under realistic racing operating profiles at ambient temperatures of 20, 25°C and 30°C. To identify the optimal design, a multi‐objective optimization framework is considered and solved through a weighted‐sum scalarization, combining four dimensionless normalized indicators representative of thermal efficiency and structural compactness. The results show that purely passive cooling is insufficient, whereas hybrid liquid–PCM configurations markedly reduce over‐temperatures and improve cell temperature uniformity. An original optimization procedure identifies as optimal a hybrid liquid–PCM solution capable of balancing thermal performance and system compactness while exhibiting robust thermal response over a wide ambient temperature range from 5°C to 35°C. The proposed approach provides a quantitative framework for balancing competing design requirements for high‐performance battery thermal management systems (BTMS).

Abstract Effective thermal management is crucial for maintaining the safety, performance, and lifetime of lithium-ion batteries (LIBs) in electric vehicles (EVs), particularly during high-current charge-discharge cycles that generate significant heat. This paper presents an experimental investigation and dimensionless theoretical analysis of a single-phase static immersion cooling system for cylindrical battery modules, a configuration rarely explored in the literature. Three dielectric fluids with different thermophysical properties, namely deionized water, RT22HC, and mineral oil, were tested in three cooling configurations: pure immersion, immersion with heat pipes (natural convection), and immersion with heat pipes combined with forced convection. A total of 24 cylindrical heat sources were tested at heat loads of 10–50 W, and key dimensionless parameters such as Rayleigh (Ra), Grashof (Gr), and Nusselt (Nu) numbers were calculated. Results show that increasing the heat load strengthens the natural convection due to buoyancy and increases the Nu value, with deionized water providing the most effective cooling. The integration of heat pipes and forced convection reduces the battery temperature by up to 34.22% (deionized water), which mathematically lowers the Nu value but significantly reduces the total thermal resistance of the system. This study is the first to validate the classical laminar Nu-Ra correlation (exponent = 0.246) for vertical cylindrical cells in a static immersion system, linking experimental findings and dimensionless modeling. The results provide practical guidance in fluid selection and configuration optimization for compact, passive, and energy-efficient battery thermal management systems in EV and stationary energy storage applications.

Natural convection in U-shaped cavities with nanofluids: A comprehensive review of heat transfer mechanisms and applications

Abstract This study investigates three‐dimensional (3D) natural convection and entropy generation within a cubic cavity containing a finned pipe subjected to a linearly decreasing temperature profile in the transversal direction. The cavity is filled with Carbon Nanotube (CNT)‐Al 2 O3/water hybrid nanofluid, and its boundaries are insulated except for cold‐maintained walls. External and internal Rayleigh numbers (10 3 ≤ Ra E ≤ 10 5 , 10 3 ≤ Ra I ≤ 10 6 ), fins’ lengths (0.1≤ 𝐿 𝑓 ≤ 0.3), and nanoparticle volume fractions (0 ≤ ϕ ≤ 0.045) are the main examined key parameters. Numerical simulations using the 3D Finite element method (FEM) reveal the interplay between buoyancy‐driven convection, fin geometry, and nanoparticle‐enhanced heat transfer. It was shown that raising the nanoparticles concentration and the internal Rayleigh number increases the entropy generation and significantly improves heat transfer. Furthermore, the convective circulations and heat removal are intensified when using longer fins whose influence on irreversibilities depends on the strength of buoyant forces. This work is a contribution in the advancement of thermal systems design that enhances excess heat dissipation and minimizes the entropy generation in order to boost the overall thermal performance.

Constructal law, discriminated dimensional analysis and boundary layer similarities: Fluid flow and natural convection