Thermo, Volume 5, Issue 4 (December 2025) – 8 articles

This article presents experimental results for motive nozzles with profiled supersonic parts of parabolic, hyperbolic, and elliptical shapes, compared to conical nozzles with unprofiled supersonic parts. This study examined the effect of nozzle geometry and profile on thermodynamic and flow parameters of the vaporization process. The measured parameters included outlet pressure, flow velocity, and mass vapor content, along with dimensionless efficiency indicators, such as relative outflow velocity and the velocity coefficient. Graphical dependencies of these parameters on the relative initial underheating, (1 − ε_s0), were obtained. This parameter represents the ratio of the pressure difference between inlet and saturation conditions (at inlet temperature) to the inlet pressure. The results show that profiled nozzles operate effectively over a wider range of (1 − ε_s0) = 0.20–0.45, compared to conical unprofiled nozzles. The vaporization constant for profiled nozzles remained at b_n ≈ (2/3)^0.5 along their length. The velocity coefficients for profiled designs were 4–6% higher, and the volumetric vapor content at the outlet was also greater, indicating a more efficient vaporization process. Overall, the findings demonstrate that profiling the supersonic section of a motive nozzle improves the operating range, flow characteristics, and vaporization quality compared to conventional conical designs. Full article

Biogas has been identified as a sustainable resource of renewable and clean energy because of its social, economic, and environmental benefits. In this work, the analysis of a biogas combined cycle power plant coupled with a carbon capture and utilization (CCU) technology and an organic Rankine cycle (ORC) was considered. The integrated process was subjected to a multi-objective assessment considering energy, economic, environmental, and safety items. The CCU system was taken to produce syngas as a value-added product, and the use of different working fluids for the ORC, namely, R1234yf, R290, and R717, was also examined. Such working fluids were selected to represent options with varying environmental and inherent safety implications. It was shown that the integration of the CCU and ORC components to the biogas cycle plant can provide significant benefits that include a 48.65 kt/year syngas production, a decrease in carbon capture energy penalty by 33%, and a reduction in e-CO₂ emissions above 80% with respect to the stand-alone power plant. Comparison with conventional technologies also showed important environmental benefits. The analysis of inherent safety showed that the selection of working fluids for the ORC can have a significant impact on the process risk. From the set of working fluids considered in this work, R717 provided the best choice for the integrated system based on its lowest operational risk and the highest electricity production (355 kWe). The multi-objective approach used in this work allowed the quantification of benefits provided by the integration of CCUs and ORCs with respect to the base process within an overall economic, sustainability, and inherent safety assessment. Full article

This study systematically investigates the variation in the ceiling flame tilt angle in an immersed tube tunnel under the combined effect of longitudinal ventilation and sidewall smoke extraction. The experimental program considers different longitudinal velocities, various sidewall smoke exhaust rates and multiple relative distances between the fire source and the sidewall exhaust outlet, aiming to comprehensively reveal the flame tilt angle under multi-factor coupling conditions. Experiments were carried out in a reduced-scale tunnel model (6.64 m long, 0.96 m wide and 0.5 m high). A porous gas burner supplied a steady heat release, with its distance from the sidewall exhaust outlet systematically varied. Results indicate that the flame tilt angle decreases as the distance between the fire source and the sidewall exhaust outlet increases. A theoretical model was developed to predict the flame tilt angle by incorporating both the sidewall smoke exhaust rate and the relative fire source–exhaust distance. The model accounts for mass loss due to smoke extraction, estimated from the local longitudinal velocity distribution. Predictions from the proposed model agree well with the experimental data. Full article

The decarbonization of hard-to-defossilize sectors, such as international maritime transport, requires innovative, and at times disruptive, energy solutions that combine efficiency, scalability, and climate benefits. Therefore, power-to-liquid (PtL) routes have stood out for their potential to use low-emission electricity for the production of synthetic fuels, via electrolytic hydrogen and CO₂ capture. However, the high energy demand inherent to these routes poses significant challenges to large-scale implementation. Moreover, PtL routes are usually at most neutral in terms of CO₂ emissions. This study evaluates, from a thermo-energetic perspective, the optimization potential of an e-methanol synthesis route through integration with a biomass oxy-fuel combustion process, making use of electrolytic oxygen as the oxidizing agent and the captured CO₂ as the carbon source. From the standpoint of a first-law thermodynamic analysis, mass and energy balances were developed considering the full oxygen supply for oxy-fuel combustion to be met through alkaline electrolysis, thus eliminating the energy penalty associated with conventional oxygen production via air separation units. The balance closure was based on a small-scale plant with a capacity of around 100 kta of methanol. In this integrated configuration, additional CO₂ surpluses beyond methanol synthesis demand can be directed to geological storage, which, when combined with bioenergy with carbon capture and storage (BECCS) strategies, may lead to net negative CO₂ emissions. The results demonstrate that electrolytic oxygen valorization is a promising pathway to enhance the efficiency and climate performance of PtL processes. Full article

Hydrogen-powered unmanned aerial vehicles (UAVs) offer significant advantages, such as environmental sustainability and extended endurance, demonstrating broad application prospects. However, the hydrogen fuel cells face prominent thermal management challenges during flight operations. This study established a numerical model of the fuel cell thermal management system (TMS) for a hydrogen-powered UAV. Computational fluid dynamics (CFD) simulations were subsequently performed to investigate the impact of various design parameters on cooling performance. First, the cooling performance of different fan density configurations was investigated. It was found that dispersed fan placement ensures substantial airflow through the peripheral flow channels, significantly enhancing temperature uniformity. Specifically, the nine-fan configuration achieves an 18.5% reduction in the temperature difference compared to the four-fan layout. Additionally, inlets were integrated with the fan-based cooling system. While increased external airflow lowers the minimum fuel cell temperature, its impact on high-temperature zones remains limited, with a temperature difference increase of more than 19% compared to configurations without inlets. Furthermore, the middle inlet exhibits minimal vortex interference, delivering superior thermal performance. This configuration reduces the maximum temperature and average temperature by 9.1% and 22.2% compared to the back configuration. Full article

The acceleration of climate change and increasing weather-related disasters require more active utilization of renewable energy. To maximize the use of renewable energy, energy storage is an essential part. Liquid piston gas compressors have recently drawn attention because of their applicability to compressed air-based energy storage. Aqueous foam can be used to enhance the efficiency of liquid piston gas compression by boosting heat transfer. To validate the effectiveness of the combination of liquid piston and aqueous foam in a multi-stage compression system, which can contribute to higher efficiency, the present work performed experimental study at various pressure levels. Compressions were performed with and without aqueous foam at three different initial pressure levels of 1, 2, and 3 bars. For each cycle of compression, a pressure ratio of 2 was used, and the impact of pressure levels on compression efficiency was measured. With the use of foam, isothermal efficiencies of 91.4, 88.2, and 86.6% were observed at 1, 2, and 3 bar(s), which improved by 2.2, 2.1, and 1.3% compared to the baseline compressions. To identify the cause of the effectiveness variations, the volume changes in the foam at the different pressure levels were visually compared. In higher-pressure tests, a significant reduction in the foam amount was observed, and this change may contribute to the decreased effectiveness of the technique. Full article

Infrared thermography for human skin temperature measurement, when calibrated with standard blackbodies, suffers from errors due to the mismatch in emissivity between a blackbody and human skin. This study introduces a novel calibration method utilizing a human skin-like gradient radiation source to enhance measurement accuracy. A custom radiation source with six temperature points and skin-like emissivity was developed. Thermal imagers were calibrated using this source, and their performance was compared against traditional blackbody calibration. The proposed method reduced the calibration error to 0.04 °C, a significant improvement over the 0.15 °C error obtained with blackbody calibration. Calibration with a skin-like radiation source proves superior to the blackbody method, enabling high-accuracy (less than 0.1 °C) human skin temperature measurement for improved fever screening. Full article

This study evaluates the impact of using group concept maps in the teaching of Applied Thermodynamics in the Bachelor’s Degree in Industrial Electronics and Automation Engineering. The methodology consisted of selecting topics with a high conceptual load, collaboratively creating concept maps, and subsequently evaluating them by both students and teaching staff. Students achieved average scores above 7/10 in the concept map activity, with teacher and student evaluations averaging 7.8 and 7.3, respectively. Knowledge assessment via pre- and post-tests revealed a 20% increase in concept comprehension. For example, in the topic of Principles of Thermodynamics, the percentage of correct answers on the most complex question increased from 13% in the Pre-Test to 40% in the post-test. In the topic of Refrigeration Cycles, some questions showed an improvement from 18% to 25%. The students’ perception of the activity was positive, with an average satisfaction rating of 6.9 out of 10. Furthermore, most students acknowledged that the activity helped them stay engaged with the subject matter and identify errors in their own learning. The high participation in the activity, despite its low impact on the final grade, demonstrates the students’ strong motivation for this study approach. Therefore, the implementation of concept maps not only facilitated the understanding of key concepts but also promoted critical reflection and collaborative learning, establishing itself as an effective strategy in the teaching of Applied Thermodynamics. Full article