Thermo, Volume 6, Issue 3 (September 2026) – 14 articles

Accurate prediction of coupled heat-transfer and fluid-flow phenomena is essential for the thermal design, performance assessment, and optimization of heat exchangers (HXs). Among key HXs, concentric tube heat exchangers (CTHXs) are widely used in thermal energy systems, where their performance is governed by the coupled interaction of fluid flow and heat transfer. Although computational fluid dynamics (CFD) provides detailed insights into these transport phenomena, its high computational cost limits its applicability in design optimization and real-time monitoring applications. To overcome this limitation, the present study proposes a physics-guided neural network (PGNN) for the accurate and efficient prediction of CTHX thermo-hydraulic performance, including the overall heat-transfer coefficient (U) and the pressure drops of the cold (ΔP_c) and hot (ΔP_h) streams. The PGNN introduces correlation-based physical guidance through established Nusselt number, overall thermal-resistance, and Darcy–Weisbach pressure-drop relations. Accordingly, the proposed framework is a correlation-guided PGNN rather than a residual-based physics-informed model, because the local conservation-equation residuals are not explicitly enforced during training. For comparison, a standard artificial neural network (ANN) with the same architecture and input parameters was also developed. Both models were trained on a dataset generated from 1575 CFD simulations covering a wide range of operating and geometric conditions, including the Reynolds and Prandtl numbers of both fluids, inner and outer tube diameters, and inlet temperatures. A comprehensive error analysis demonstrates the superior predictive capability of the PGNN over the ANN under various flow and geometric conditions. On the unseen test dataset, the PGNN achieved mean absolute percentage errors of 2.03%, 1.09%, and 1.11% for predicting U, ΔP_c, and ΔP_h, respectively. The proposed PGNN therefore provides a reliable, high-fidelity, and computationally efficient alternative to CFD, supporting the analysis, optimization, and operation of thermal energy systems. Full article

This study addresses the energy demand profiles of commercial buildings by developing an optimal dispatch strategy for a regional high-efficiency distributed energy system integrating electricity, cooling, and storage through source–load coordination. The spatiotemporal distribution characteristics of cooling, heating, and electrical loads are analyzed and an integrated energy system model is established, comprising gas internal combustion engine, a lithium bromide absorption chiller/heater, gas-fired boiler, centrifugal chillers, and an ice storage system. Taking into account seasonal electricity pricing policies and meteorological variations in Shanghai, a load grading system and a time-of-use (TOU) pricing response mechanism are constructed, leading to the development of operational strategy portfolios for different typical scenarios. A multi-objective optimization dispatch model is formulated with the dual aims of minimizing operating costs and maximizing energy efficiency. The results indicate that, compared to a fixed operational mode, the optimized strategy achieves average CO₂ emission reduction rates of 16.4% in summer, 25.2% in non-summer periods, and 20.9% annually. Additionally, annual grid electricity purchases are reduced by 10.2%, with a static investment payback period of 11.37 years. This research provides an intelligent, practically applicable operational solution for distributed energy systems in commercial buildings, effectively overcoming the limitations of traditional approaches in terms of flexibility and economic performance. Full article

Ventilated façades are increasingly used in building renovations, often containing non-renewable and CO₂-emissions-intensive cement-based materials. Renewable biomass-based materials offer a more sustainable alternative with a high amount of sequestered CO₂. However, water uptake is a critical factor in exterior applications. This study investigates the water resistance of high-density particleboards made of wheat straw (WS), grey alder (GA), and softwood (SW) for façade-related exterior applications. Two general board types were produced from each biomass using (1) steam explosion (SE) treatment and (2) the addition of birch-bark-derived suberinic acids (SAs) as the bio-based binder. In addition, the influence of conventional and mold hot pressing was investigated. The particleboards were coated with four types of innovative finishes, comprising (1) purified SA, (2) SA + chitosan (SH), (3) SA + earth pigment (SP), and (4) SHP. The water resistance of the particleboards was evaluated using an internal bonding (IB) test after 2 h of boiling and by measuring the water drop contact angle. FTIR analysis was performed to identify differences between the board varieties and to explain the obtained results. Only two board varieties (GASA and SWSA) fulfilled the Type P5 EN 312 water resistance requirement (0.15 N/mm²), achieving IB values of 0.81 ± 0.23 N/mm² and 0.22± 0.07 N/mm², respectively. In turn, the coatings used did not significantly increase the static contact angle compared to the reference board. Although the results of this study confirm the inherent moisture sensitivity of engineered particleboards, two board varieties demonstrate promising potential for façade-related exterior applications. Full article

Decarbonizing industrial heating in cold climates remains challenging due to high thermal demand and strong seasonal variability. While the existing literature predominantly relies on steady-state or isolated component analyses, this study introduces a novel, multi-scale dynamic modeling framework. This framework evaluates the annual transient performance of an integrated renewable architecture. The proposed system couples a building-envelope Transpired Solar Collector (TSC) with a series-connected Photovoltaic Thermal/Solar Thermal (PVT-ST) array. Computational Fluid Dynamics (CFD) is employed to resolve the localized convective heat transfer within the TSC. Subsequently, a data-driven clustering methodology scales these transient results into a comprehensive annual system-level simulation featuring sensible Thermal Energy Storage (TES). The results demonstrate robust performance under Canadian winter conditions. The TSC maintains stable thermal efficiencies between 50% and 60%, peaking at over 64%. Annually, the integrated dual-source system delivers 229.7 MWh of useful thermal energy to offset primary fossil fuel consumption. Furthermore, the analysis identifies 128.76 MWh of seasonal surplus capacity. This underscores the critical necessity of dynamic TES integration. Ultimately, this framework establishes a highly defensible, predictive methodology for designing and implementing synergistic solar thermal networks for industrial decarbonization. Full article

Conventional continuum thermodynamics is characterized by a classical form of the energy law and the second law of thermodynamics in the form of the Clausius–Duhem inequality. This thermodynamic framework fails to capture certain features in material response as, e.g., length scale effects, temperature waves in rigid heat conductors and diffusion phenomena. Typically, such material characteristics are associated with pronounced non-localities in time and space. To tackle these issues, non-conventional thermodynamic approaches might be appropriate. The employment of non-conventional thermodynamics is very attractive, as it enables the extension of the applicability of conventional thermodynamics in a simple way. A specific non-conventional thermodynamic framework has previously been proposed as a generalization of irreversible thermodynamics. Energy supply effects were neglected in this work. However, energy supply terms may become important when discussing thermodynamical consistency of many physical models. The present paper extends the applicability of the proposed non-conventional thermodynamic framework by accounting for energy supply densities and demonstrates its capabilities by addressing Cattaneo’s heat conduction law—known for predicting temperature waves—and thermo-diffusion coupling theories. It is shown that, within the adopted thermodynamics, the considered physical models are thermodynamically consistent and that the resulting field theories admit formulations within a variational framework for rate problems; in this sense, the models are properly formulated. Full article

Tables giving gas properties at low pressure enable the efficient analysis of processes in which the gas is approximated as thermally perfect but not calorically perfect. In addition to specific enthalpy and specific internal energy, thermally perfect gas tables include special functions that depend only on temperature—relative pressure and relative specific volume. These functions may be used to determine pressure, volume, and temperature of thermally perfect gases undergoing hypothetical isentropic processes. However, the definitions of these functions included in widely used thermodynamics textbooks are vague, inconsistent, or incorrect. The intent of this work is to discuss common inaccuracies in the definitions and the tabulated values of relative pressure and relative specific volume. The origins of the tabulated data used in many engineering thermodynamics textbooks are reviewed and consistent definitions are proposed. A table listing thermally perfect gas properties for air at low pressure based on the proposed definitions is presented. Full article

Thermal energy storage plays a crucial role in meeting human energy demands and is particularly essential for many solar energy applications. Among the various storage methods, phase change materials (PCMs) have attracted significant attention because their thermal performance can be greatly influenced by the material properties, physical characteristics, and the geometry of the encapsulating container. In this paper, the melting process of phase change materials (PCMs) within an elliptical enclosure using the finite volume method is analyzed. Gallium is selected as a PCM with a low Prandtl number. A physical model employing the enthalpy porosity formulation is elaborated to describe the coupling between natural convection and the melting process of PCMs. Numerical simulations are performed to examine the influence of the aspect ratio (n = b/a), ranging from 1 to 4, and inclination angles from 0° to 90° of the elliptical enclosure on the melting process. It has been found that the use of the elliptical capsule can reduce the melting process time. For a Rayleigh number of ${10}^6$, the melting time decreases as the aspect ratio increases from 1 (circle) to 4. The horizontal orientation (θ = 0°) is found to be the most efficient, with a melting rate higher than that observed for inclined positions (30°, 45°, 60°, and 90°). For a low Rayleigh number of ${10}^4$, the inclination angle has an imperceptible effect on the phase change. Empirical correlations are proposed to relate the Nusselt number to the Rayleigh number, with coefficients adapted to different Fourier numbers and geometric parameters. Full article

The increasing penetration of variable renewable energy sources (RESs) poses significant challenges to power system flexibility and reliability, particularly in systems with high solar generation. At the same time, existing Concentrating Solar Power (CSP) plants in Europe face declining economic viability due to high capital costs and the expiration of incentivized tariff schemes. This study proposes and evaluates a novel approach to repurpose CSP plants as flexible energy assets through the integration of cold thermal energy storage (CTES) within a Thermally Integrated Power-to-Heat-to-Power Energy Storage (TI-PTES) framework. The proposed system combines an ice/water-based cold storage with a CO₂-based refrigeration cycle to enhance the efficiency of the CSP steam cycle by reducing condenser temperatures, while also enabling temporal shifting of electricity consumption. A techno-economic optimization model based on PyPSA is developed to determine the optimal sizing and operation of the storage and refrigeration system under realistic load and electricity price conditions representative of the Spanish market. Results show that the integration of cold storage significantly alters system operation, shifting the chiller from a continuous demand-following mode to an intermittent, high-intensity regime. This leads to a reduction in annual operating expenditures by approximately 32% and an increase in annual profit and net present value (NPV), despite higher capital investment. While hourly net revenue becomes more volatile, with negative values during charging periods, cumulative annual performance improves due to effective temporal optimization. However, the absence of strong electricity price arbitrage and negative price signals limits the revenue potential of the storage system, which primarily acts as a cost-reduction mechanism. The findings demonstrate that cold thermal storage can successfully reposition CSP plants as flexible, value-generating assets in modern electricity systems. The proposed concept offers a promising pathway for extending the operational lifetime of existing CSP infrastructure while supporting higher integration of renewable energy sources. Full article

This numerical study investigates the thermo-hydraulic and thermodynamic performance of a rectangular-channel heat exchanger incorporating isothermal circular tubes, with particular emphasis on geometric design strategies suitable for compact thermal systems. Two configurations with identical total heat transfer surfaces are analyzed: baseline geometry comprising four aligned tubes (G1) and an optimized geometry consisting of eight tubes arranged in two parallel rows (G2) maintaining the same exchange surface. Laminar forced convection is also considered. For the baseline configuration, results show a pronounced thermal shadowing effect, leading to a reduction of nearly 50% in the heat transfer contribution of downstream tubes.In contrast, optimized geometry significantly improves flow redistribution and temperature field uniformity. An optimal inter-row spacing, equal to 0.1, is identified as a robust design parameter, maximizing the total average Nusselt number. At this spacing, all heated surfaces actively contribute to heat transfer, resulting on an overall heat transfer enhancement of approximately 20–40% compared to the baseline configuration. Entropy production analysis shows that increasing Re strongly intensifies thermal irreversibility, while viscous irreversibility exhibits a moderate increase. The impact of nanoparticles addition, carried out on the optimal configuration of G2, shows that heat transfer increases by about 8% for a nanoparticle concentration of 4% at high Re values, with an insignificant change in the Bejan number. The present findings demonstrate that geometric optimization represents a more effective and energetically sustainable enhancement strategy than nanofluid addition for compact tubular heat exchangers. Full article

Additive Manufacturing (AM) for jewelry applications is increasingly adopting Binder Jetting (BJ) to overcome the fusion-related limitations associated with precious metals, including unstable melt pools, excessive reflectivity, and high thermal conductivity. In this context, the present review establishes a thermophysical and manufacturability-oriented framework that redefines thermal management beyond localized melt-pool stabilization toward the furnace-scale control of densification kinetics, shrinkage evolution, atmosphere-assisted sintering, and viscoplastic deformation. Particular emphasis is placed on gold-, silver-, and platinum-based jewelry alloys, with a specific focus on the thermal, mechanical, and chemical phenomena governing Binder Jetting sintering. During consolidation, low-density green bodies (~40–65% relative density) must transform into highly dense components through extensive volumetric shrinkage and gravity-driven deformation, creating major challenges in dimensional fidelity and surface quality. The review further examines predictive viscoplastic constitutive models (SOVS/ROH), reversed-deformation compensation strategies, and atmosphere-engineering approaches for oxide reduction, pore-pressure regulation, and residual-porosity control. By linking thermophysical consolidation, dimensional fidelity, polishability, and jewelry-grade manufacturability within a hierarchical framework, this review provides a structured basis for the development of high-precision and low-waste precious-metal additive manufacturing. Full article

This experimental study investigates the performance and sustainability of a modified double-slope solar still (MDSSS) integrated with a combined water channel to enhance evaporation rates. The integration of the water channel ensures uniform water flow and enhanced heat distribution across the basin surface, thereby improving thermal performance. Experiments were conducted using three types of feed water, groundwater, saline water, and domestic wastewater, to assess the system’s versatility and effectiveness in various water desalination applications. Under identical meteorological conditions, thermal parameters, distillate yield, energy efficiency, and sustainability were analyzed. The results revealed that incorporating the water channel significantly increased evaporation and condensation rates compared to the conventional double-slope solar still (DSSS) configuration. Also, the performance of an MDSSS was evaluated under various water qualities, including physical, chemical, and biological parameters. The experiment begins at half the optimal water depth for water quality, with the remaining half passing through an open-channel attachment into the solar still basin. The modified system effectively reduced pollutants, achieving a 98.18% reduction in chemical oxygen demand in groundwater, complete salt removal from saline water, and a 96.67% reduction in sewage water. Full article

This study investigated the thermophysical properties of mangaba (Hancornia speciosa) and guavira (Campomanesia adamantium) pulps at different soluble solid concentrations (9.0 to 13.5 °Brix) and temperatures (0 to −25 °C). Using mathematical models and experimental data, properties such as density (ρ), apparent specific heat capacity (c_p), thermal conductivity (k), and thermal diffusivity (α) were estimated. The results showed that all properties were strongly influenced by temperature and concentration. Density and apparent specific heat capacity increased with °Brix and temperature, while thermal conductivity and diffusivity were higher in samples with greater moisture content. These results provide useful information for the design, simulation, and optimization of freezing and storage processes for native Cerrado fruit pulps, contributing to their technological valorization and potential use in frozen food products. Full article

An experimental setup has been developed that enables the conversion of a complex stream of polydisperse droplets generated by an ultrasonic dispenser into a stream of nearly identical droplets falling through a vertical channel. The fall of droplets of an aqueous NaCl solution in this channel, filled with heated dry air, is studied. Water from the droplets evaporates quickly, and crystals of a solid salt crust form on their surface. At a later stage of the process, the remaining solution is removed from the droplet using a jet of water vapor that passes through the pores of the polycrystalline crust. It was first observed that some of the drying droplets suddenly shifted to one side under the influence of the reactive force generated by the vapor jet. Images obtained using a scanning electron microscope show that the salt particles formed have a diameter of around 25 µm, are slightly porous, and consist of numerous crystals. It has been proven that these particles do not have a central cavity. The use of seawater and the role of salt particles in protecting against thermal radiation from fires are briefly discussed. Calculations based on Mie theory have shown that the contribution of light scattering by thin-walled hollow sea salt particles formed above the ocean surface during relatively slow evaporation of seawater droplets can be significant to the ocean’s heat balance. Full article

The present paper is motivated by challenges in the design of the surface insulation in borehole thermal energy storage (BTES). A case study of a BTES with nine borehole heat exchangers (BHEs) in a cold climate is considered. Transient numerical modeling of the storage charging phase is performed by solving the three-dimensional heat equation using the finite difference method. Heat conduction through the insulation cover is simulated in accordance with Fourier’s law. A parametric study is conducted with respect to the prescribed heating setpoint temperatures in the BHEs and to the geometry of the insulation cover. The thermal analysis shows that the efficiency of the storage volume is strongly dependent on the heat transfer through the upper boundary. The insulation layer affects the minimum temperature reached within the BTES, with the influence of insulation thickness being most pronounced at thicknesses up to 10 cm. Furthermore, it is demonstrated that the lateral extension of the insulation cover has a greater impact on storage capacity gains than increasing its thickness, and that these energy gains expand progressively over time. Under cold ambient conditions, effective seasonal storage requires managing sharp ambient thermal gradients via a wider peripheral coverage of the insulation layer to offset vertical conductive losses. Full article