Thermo, Volume 4, Issue 4 (December 2024) – 6 articles

In this paper, a reliable model of the viscosity in liquids in the dual model of liquids (DML) framework is developed. The analytical expression arrived at exhibits the correct T–dependence Arrhenius-like exponential decreasing trend, which is typical of Newtonian simple fluids. The model is supported by the successful comparison with both the experimental values of the viscosity of water, and with those related to the mechano-thermal effect in liquids under low-frequency shear, discovered a few years ago, for which the first-ever theoretical interpretation is given by the DML. Moreover, the approach is even supported by the results of numerical models recently developed, that have shown that dual liquid models, such as the DML, provides very good agreement with experimental data. The expression of viscosity contains terms belonging to both the subsystems constituting the liquid, and shows an explicit dependence upon the sound velocity and the collective vibratory degrees of freedom (DoF) excited at a given temperature. At the same time, the terms involved depend upon the Boltzmann and Planck constants. Finally, the physical model is coherent with the Onsager postulate of microscopic time reversibility as well as with time’s arrow for macroscopic dissipative mechanisms. Full article

A kinetic model based on the two-stage semi-global multi-reaction model of Grioui was developed using the TG and DTG curves for the by-products of Kambala and Ayous. These two tropical species are widely used in the Republic of Congo. The TG and DTG curves were obtained through thermogravimetry at five different heating rates (3, 7, 10, and 20 K/min) up to a final temperature of 800 °C under a nitrogen atmosphere. The thermal decomposition of both species started at similar temperatures, but the profiles exhibited notable differences. Kambala showed a distinct profile with two peaks at approximately 500 °C and 700 °C, which upon further investigation were found to correspond to ash decomposition. Additionally, the shoulder present in Ayous between 250 °C and 300 °C, attributed to hemicelluloses degradation, was absent in the DTG curves for Kambala. The kinetic model for Ayous was formulated in three steps, while the model for Kambala consisted of four steps. Both models accurately predicted the thermal degradation of the wood species, and the resulting kinetic parameters aligned with those reported in the literature. Full article

Due to modern comfort demands and global warming, heating, ventilation, and air conditioning (HVAC) systems are widely used in many homes and buildings. However, HVAC based on the Vapor Compression System (VCS) is a major energy consumer, accounting for 20–50% of a building’s energy consumption and responsible for 29% of the world’s CO₂ emissions. Dew-point evaporative coolers offer a sustainable alternative yet face challenges, e.g., dew point and wet bulb effectiveness. Given the above, dew point evaporative cooling systems may find a place to dethrone conventional air conditioning systems. This research aims to design a dew point evaporative cooler system with better performance in terms of dew point and wet bulb effectiveness. In terms of methodology, a heat exchanger as part of a counter-flow dew point cooling system was designed and analyzed using COMSOL simulations under different representative climatic, geometric, and dimensional conditions, taking into account turbulent flow. Next, our model was compared with other cooling systems. The results show that our model performs similarly to other cooling systems, with an error of around 6.89% in the output temperature at low relative humidity (0–21%). In comparison, our system is more sensitive to humidity in the climate, whereas heat pumps can operate in high humidity. The average dew point and wet bulb effectiveness were also higher than reported in the literature, at 91.38% and 147.84%, respectively. In addition, there are some potential limitations of the simulations in terms of the assumptions made about atmospheric conditions. For this reason, the results cannot be generalized but must be considered as a starting point for future research and technology development projects. Full article

Here, we investigate the performance of phase-change materials (PCMs) in the passive thermal control of space habitats. PCMs are able to absorb and release large amounts energy in the form of latent heat during their (typically, solid-to-liquid) phase transition, which makes them an ideal choice for passive temperature control. In this study, a conceptual design of an igloo-shaped habitat is proposed. A scaled model for laboratory experiments is manufactured via 3D printing, using tap water as the PCM. The setup is used to conduct experiments and analyze PCM performance, based on temperature measurements inside and outside the habitat. Results demonstrate the effectiveness of PCMs in increasing thermal inertia and stabilizing the habitat interior temperature around the melting temperature, confirming that PCMs can be a suitable alternative for passive thermal control. The present study holds significant interest for the future of space exploration, with the emerging need to design habitats that are capable of accommodating astronauts. Full article

Global efforts aiming to shift towards renewable energy and smart grid configurations require accurate unit commitment schedules to guarantee power balance and ensure feasible operation under different complex constraints. Intelligent systems utilizing hybrid and high-level techniques have arisen as promising solutions to provide optimum exploration–exploitation trade-offs at the expense of computational complexity. To ameliorate this requirement, which is extremely expensive in non-interconnected renewable systems, radically different approaches based on enhanced priority schemes and Boolean encoding/decoding have to take place. This compilation encompasses various mappings that convert multi-valued clausal forms into Boolean expressions with equivalent satisfiability. Avoiding any need to introduce prior parameter settings, the solution utilizes state-of-the-art advancements in the field of artificial intelligence models, namely Boolean mapping. It allows for the efficient identification of the optimal configuration of a non-convex system with binary and discontinuous dynamics in the fewest possible trials, providing impressive performance. In this way, Boolean mapping becomes capable of providing global optimum solutions to unit commitment utilizing fully tractable procedures without deteriorating the computational time. The results, considering a non-interconnected power system, show that the proposed model based on artificial intelligence presents advantageous performance in terms of generating cost and complexity. This is particularly important in isolated networks, where even a-not-so great deviation between production and consumption may reflect as a major disturbance in terms of frequency and voltage. Full article

Textile heaters are made from knitted conductive yarns integrated into their fabric, making them stretchable, washable, breathable and suitable for close-to-skin wear. However, the non-zero resistance in the lead wires causes non-uniform power distribution, which presents a design challenge. To address this, the electrical performance of the heaters is modeled as an n-ladder resistor network. By using the finite difference method, simple, closed-form expressions are derived for networks with their power source connected to input terminals A₁B₁ and A₁B_n, respectively. The exact results are then used to derive approximations and design criteria. The solutions for the ladder networks presented in this paper apply to a wider class of physical problems, such as irrigation systems, transformer windings, and cooling fins. Full article