Explore the vast range of titles available.

Steam generation using solar energy provides the basis for many sustainable desalination, sanitization, and process heating technologies. Recently, interest has arisen for low-cost floating structures that absorb solar radiation and transfer energy to water via thermal conduction, driving evaporation. However, contact between water and the structure leads to fouling and pins the vapour temperature near the boiling point. Here we demonstrate solar-driven evaporation using a structure not in contact with water. The structure absorbs solar radiation and re-radiates infrared photons, which are directly absorbed by the water within a sub-100 μm penetration depth. Due to the physical separation from the water, fouling is entirely avoided. Due to the thermal separation, the structure is no longer pinned at the boiling point, and is used to superheat the generated steam. We generate steam with temperatures up to 133 °C, demonstrating superheated steam in a non-pressurized system under one sun illumination. Solar steam generation is limited by fouling of solar converters, and the steam temperature is usually pinned to 100 °C. Here, both limitations are overcome in a system utilizing a solar absorber and light down-converter to achieve radiative heating, which does not require physical contact between absorber and water.

The melting of a solid, like other first-order phase transitions, exhibits an intrinsic time-scale disparity: The time spent by the system in metastable states is orders of magnitude longer than the transition times between the states. Using rare-event sampling techniques, we find that melting of representative solids—here, copper and aluminum—occurs via multiple, competing pathways involving the formation and migration of point defects or dislocations. Each path is characterized by multiple barrier-crossing events arising from multiple metastable states within the solid basin. At temperatures approaching superheating, melting becomes a single barrier-crossing process, and at the limit of superheating, the melting mechanism is driven by a vibrational instability. Our findings reveal the importance of nonlocal behavior, suggesting a revision of the perspective of classical nucleation theory.

The superheating process is a unique grain refining method found only in aluminum-containing magnesium alloys. It is a relatively simple method of controlling the temperature of the melt without adding a nucleating agent or refining agent for grain refinement. Although previous studies have been conducted on this process, the precise mechanism underlying this phenomenon has yet to be elucidated. In this study, a new approach was used to investigate the grain refinement mechanism of aluminum-containing magnesium alloys by the melting superheating process. AZ91 alloy, a representative Mg-Al alloy, was used in the study, and a rapid solidification process was designed to enable precise temperature control. Temperature control was successfully conducted in a unique way by measuring the temperature of the ceramic tube during the rapid solidification process. The presence of Al8Mn5 and Al10Mn3 particles in non-superheated and superheated AZ91 ribbon samples, respectively, manufactured by the rapid solidification process, was revealed. The role of these Al-Mn particles as nucleants in non-superheated and superheated samples was examined by employing STEM equipment. The crystallographic coherence between Al8Mn5 particles and magnesium was very poor, while Al10Mn3 particles showed better coherence than Al8Mn5. We speculated that Al10Mn3 particles generated by the superheating process may act as nucleants for α-Mg grains; this was the main cause of the superheating grain refinement of the AZ91 alloy.

The micro-ORC systems are widely considered a reliable solution for domestic power production from renewable sources. The investigation of the optimal operating conditions to maximize system efficiency is an interesting challenge. In this study, a preliminary experimental campaign has been carried out on a biomass-fired micro-ORC system. The system is designed for stationary applications for domestic users, with a gear pump, a scroll expander and R245fa as the working fluid. The performance characterization of the micro-ORC under steady-state conditions has been obtained varying the water flow rate in the condenser at constant pump and expander speeds. The temperature of the hot source (thermal oil) is the maximum achievable in each operating condition. The temperature at the expander inlet and the condenser and evaporator pressure strongly influence the system performance. The increase in water flow leads to a decrease in the condenser pressure and a reduction of the superheating degree of the organic fluid. The system reaches the maximum electric power output of approximately 2565 W with a water flow rate of about 20 l/min. The highest electrical efficiency increases as the refrigerant flow rate decreases and reaches the highest value of 8.1% for the minimum investigated water flow rate.

The model of a single flow channel of a plate heat exchanger (PHE) has been established, and the condensation characteristics of vapor with different saturation temperatures and vapor superheating levels have been investigated. The results show that with a higher vapor saturation temperature, the average surface heat transfer coefficient (HTC) increases. The HTC of saturated steam at a saturation temperature of 348.15 K is about 104.1% higher than that at a saturation temperature of 323.15 K. The influence of vapor superheating is weaker compared to that of the saturation temperature. With the increase of vapor superheat, the surface average HTC first increased and then decreased. The HTC of superheated vapor with a vapor superheating of 10 °C is about 0.72% higher than that of saturated vapor. In industrial applications, it is appropriate to control the vapor inlet superheating to be between 5-10°C.

The recent restrictions on the use of refrigerants with high Global Warming Potential (GWP) have pushed the research to consider alternative solutions, such as the HydroFluoroOlefins, as possible replacements. Even though they are generally mildly flammable, their GWP is significantly lower compared to those of the HydroFluoroCarbons. This paper presents experimental measurements of R1234ze(E) and R134a heat transfer coefficients during condensation inside a brazed plate heat exchanger. Experimental tests during condensation heat transfer have been conducted considering different degrees of superheating, subcooling and outlet vapour quality. The condensation temperature varies between 34.6 °C and 42.3 °C and the refrigerant mass flux is between 9 kg m −2 s −1 and 49 kg m −2 s −1 . The results showed that the heat transfer coefficients measured with R134a are between 4% and 8% higher than those of R1234ze(E). Complete condensation experiments showed that an increase in the liquid subcooling degree significantly reduces the thermal performance at low refrigerant mass velocities. In some cases, the plate area occupied by liquid refrigerant reached almost 30% of the overall heat transfer area, thus decreasing by 2.7 times the average heat transfer coefficient when passing from 3 K to 8 K subcooling degree. A comparison with the predictions of some empirical models is also presented to assess which one can better predict the experimental data.

The effect of melt superheating treatment on the solidification microstructure and mechanical properties of the γ ’ phase precipitation-strengthened K424 superalloy was investigated. Differential scanning calorimetry (DSC) experiments were conducted to explore the influence of melt treatment temperature on the undercooling of the superalloy. Additionally, pouring experiments were carried out to assess how alterations in both the temperature and duration of melt treatment impacted the grain size, secondary dendrite arm spacing (SDAS), elemental segregation, and mechanical properties of the alloy. Metallographic analysis, scanning electron microscopy, energy dispersive spectroscopy (EDS) and Thermo-Calc software were employed for microstructure characterization. The test specimens were subjected to tensile testing at room temperature and stress rupture testing at 975 °C under 196 MPa. The findings reveal that appropriate melt treatment conditions result in decreased grain size, refined SDAS, minimized elemental segregation, and significant improvements in mechanical properties. Specifically, the study demonstrates that a melt treatment at 1,650 °C for 5 min results in the smallest average grain size of 949 µm and the smallest SDAS of 25.38 µm. Furthermore, the room temperature tensile properties and creep resistance are notably affected by the melt treatment parameters. It is shown that specific melt treatment conditions, such as holding at 1,650 °C for 5 min, result in superior room temperature strength and extended stress rupture life of the K424 superalloy, while a balance between strength and stability is achieved at 1,600 °C with a holding time of 10 min. These findings offer guidance for optimizing the melt treatment parameters for the K424 superalloy, laying a foundation for further investigations.

Crystallisation kinetics play a fundamental role in controlling conduit dynamics and eruptive style. The degree of superheating is critical in controlling crystallisation kinetics; however, its effect is still debated and has an unclear impact on eruption dynamics. Here, we investigate how superheating influences clinopyroxene nucleation in tephritic magmas from the 2021 Tajogaite eruption (La Palma, Spain) through both in situ and ex situ view experiments. Our findings show that superheating delays nucleation by dissolving pre-existing nuclei, thereby inhibiting crystallisation upon return to subliquidus conditions. Using a numerical model, we investigate how different nucleation delays resulting from different degrees of superheating affect magma ascent dynamics. Depending on the initial thermodynamic conditions and on the pre-eruptive history of magma, an increased nucleation delay can significantly reduce crystal content during ascent, lowering magma viscosity and affecting eruptive style. These findings highlight the critical role of pre-eruptive thermal histories in controlling eruptive style, and provide constraints for refining experimental protocols and numerical models, with direct implications for improving volcanic hazard assessment and eruption forecasting. This study shows that heating magma above its crystallisation temperature delays crystal formation, reducing crystal content during ascent and controlling volcanic eruption style and intensity.

This contact line vicinity model is conceived as a subgrid model for the DNS of bubble growth in boiling. The model is based on the hydrodynamic multiscale theory and is suitable for the partial wetting case. On the smallest length scale (distance from the contact line) ∼ 100 nm, the interface slope is controlled by the Voinov angle. It is the static apparent contact angle (ACA) that forms due to evaporation, similarly to previous models neglecting the contact line motion. The calculation of the Voinov angle is performed with the generalized lubrication approximation and includes several nanoscale effects like those of Kelvin and Marangoni, vapor recoil, hydrodynamic slip length and interfacial kinetic resistance. It provides the finite values of the heat flux, pressure and temperature at the contact line. The dynamic ACA is obtained with the Cox-Voinov formula. The microscopic length of the Cox-Voinov formula (Voinov length) is controlled mainly by the hydrodynamic slip. The integral heat flux passing through the contact line vicinity is almost independent of the nanoscale phenomena, with the exception of the interfacial kinetic resistance and is mostly defined by the dynamic ACA. Both the dynamic ACA and the integral heat flux are the main output parameters of the subgrid model, while the local superheating and the microscopic contact angle are the main input parameters. The model is suitable for the grid sizes > 1 µ m.

Purpose. Development of an energy-technological scheme and investigation of the thermodynamic efficiency of atomic-hydrogen electroaccumulation systems based on a small modular reactor with a hydrogen-oxygen energy storage and generation system. Methodology. The study presents a methodology for designing and calculating an energy storage system with hydrogen steam superheating using electrolysis modules for hydrogen and oxygen generation. Findings. A systematic analysis of the energy balance of a power installation based on a small modular reactor with hydrogen steam superheating has been conducted. The energy storage system with hydrogen steam superheating was calculated using an electrolysis module with a capacity of 12 m3/h and a specific energy consumption of 4 kW · h/m3 of hydrogen. An assessment of the gas storage system volume at a pressure of 150 atmospheres was performed, and a sectional design variant of the storage system was proposed, equipped with an expander-compressor unit for more complete gas extraction from the storage system. The use of a turbo-compressor operating within a pressure range of 0.5–3.5 MPa, driven by an expander turbine operating within a pressure range of 15.0–3.5 MPa and mounted on the same shaft as the compressor, ensures more complete gas utilization and reduces the volume of the storage system. Originality. A methodology for the creation and thermodynamic analysis of autonomous atomic-hydrogen energy-technological complexes based on a small modular reactor and a high-pressure electrolyzer for energy accumulation and peak power generation has been proposed. This approach enables an increase in efficiency from 28 to 34.8 % during peak load periods. Practical value. The use of hydrogen and oxygen for steam superheating before the turbine in a power plant with a 45 MW SMR ensures an energy return coefficient of 0.55 with a daily energy accumulation of 190 MWh.