Search Results (62)

The experimental tests needed for the estimation of the fatigue limit generally require extensive time and many specimens. A valid but not standardized alternative is the thermographic analysis of the self-heating phenomenon. The present work is aimed at using Infrared thermography to determine the fatigue limit in two kinds of Ti-6Al-4V samples obtained by hot rolling: (1) with the standard dog-bone shape (unnotched specimen) and (2) with two opposed semicircular notches at the sides (notched specimen). Uniaxial tensile experiments are performed on unnotched samples, and the surface temperature variation during loading is monitored. The stress corresponding to the end of the thermoelastic stage gives a rough indication of the fatigue limit. Then, fatigue tests at different sinusoidal loads are performed, and the thermographic signal is monitored and processed. The results obtained using lock-in thermography in dissipative mode, e.g., analyzing the second harmonic, showed a sudden change in slope when the applied stress exceeded a certain limit. This slope change is related to the fatigue limit. In addition, the ratio between the fatigue limits obtained for notched and unnotched specimens, e.g., the fatigue strength reduction factor, is consistent with literature values based on the selected geometry. Full article

The escalating heat flux densities in high-performance electronics necessitate superior thermal management. This study enhanced pool-boiling heat transfer, a method offering high heat removal capacity, by leveraging Binder Jetting 3D Printing (BJ3DP) to create complex porous copper structures without the need for chemical treatments. This approach enables a reliable utilization of phenomena like capillarity for improved performance. Three types of porous copper structures, namely Large Lattice, Small Lattice, and Staggered, were fabricated on pure copper substrates and tested via pool boiling of de-ionized and de-gassed water at atmospheric pressure. Compared to a plain polished copper surface, which exhibited a critical heat flux (CHF) of 782 kW/m² at a wall superheat of 18 K, the 3D-printed porous copper surfaces showed significantly improved heat transfer performance. The Staggered surface achieved a conventional CHF of 2342.4 kW/m² (a 199.7% enhancement) at a wall superheat of 24.6 K. Notably, the Large Lattice and Small Lattice structures demonstrated exceptionally stable boiling without reaching the typical catastrophic CHF within the experimental parameters. These geometries continued to increase in heat flux, reaching maximums of 2397.7 kW/m² (206.8% higher at a wall superheat of 55.6 K) and 2577.2 kW/m² (229.7% higher at a wall superheat of 39.5 K), respectively. Subsequently, a gradual decline in heat flux was observed with an increasing wall superheat, demonstrating an outstanding resistance to the boiling crisis. These improvements are attributed to the formation of distinct vapor–liquid pathways within the porous structures, which promotes the efficient rewetting of the heated surface through capillary action. This mechanism supports a highly efficient, self-sustaining boiling configuration, emphasizing the superior rewetting and vapor management capabilities of these 3D-printed porous structures, which extend the boundaries of sustained high heat flux performance. The porous surfaces also demonstrated a higher heat transfer coefficient (HTC), particularly at lower heat fluxes (≤750 kW/m²). High-speed digital camera visualization provided further insight into the boiling phenomenon. Overall, the findings demonstrate that these BJ3DP structured surfaces produce optimized vapor–liquid pathways and capillary-enhanced rewetting, offering significantly superior heat transfer performance compared to smooth surfaces and highlighting their potential for advanced thermal management. Full article

Due to the limitations of the sowing machine performance and rice seed germination rates, missing seedlings inevitably occur after rice is sown in large fields. This phenomenon has a direct impact on the rice yield. In the field environment, the existing methods for detecting missing seedlings based on unmanned aerial vehicle (UAV) remote sensing images often have unsatisfactory effects. Therefore, to enable the fast and accurate detection of missing rice seedlings and facilitate subsequent reseeding, this study proposes a UAV remote-sensing-based method for detecting missing rice seedlings in large fields. The proposed method uses an improved PCERT-DETR model to detect rice seedlings and missing seedlings in UAV remote sensing images of large fields. The experimental results show that PCERT-DETR achieves an optimal performance on the self-constructed dataset, with an mean average precision (mAP) of 81.2%, precision (P) of 82.8%, recall (R) of 78.3%, and F₁-score (F₁) of 80.5%. The model’s parameter count is only 21.4 M and its FLOPs reach 66.6 G, meeting real-time detection requirements. Compared to the baseline network models, PCERT-DETR improves the P, R, F₁, and mAP by 15.0, 1.2, 8.5, and 6.8 percentage points, respectively. Furthermore, the performance evaluation experiments were carried out through ablation experiments, comparative detection model experiments and heat map visualization analysis, indicating that the model has a strong detection performance on the test set. The results confirm that the proposed model can accurately detect the number of missing rice seedlings. This study provides accurate information on the number of missing seedlings for subsequent reseeding operations, thus contributing to the improvement of precision farming practices. Full article

This paper analyzes the possibility of using averaged models to analyze the distribution of power losses in the components of a DC-DC converter including a power module. An electrothermal averaged model of a buck converter including the IGBT module was formulated. This model takes into consideration conduction and switching losses in the mentioned components, the self-heating phenomenon in each component, and mutual thermal coupling between their sub-components. It is designed for SPICE software (version PSPICE A/D 17.4). Its correctness was verified experimentally, and the results obtained were compared with the results of analyses performed with the use of PLECS software and the IGBT module model proposed by the manufacturer. The proposed model’s results show very good accuracy. Through the use of the proposed model, the dependences of the components of power losses and the case temperature of the IGBT module and the inductor on parameters describing the control signal and load of this converter were determined. The distribution of power losses in the converter components was analyzed for selected operating conditions of the buck converter. On the basis of the results obtained, some recommendations were formulated for designers of such DC-DC converters. Full article

Flow-induced vibration (FIV) characteristics are key factors in enhancing heat transfer. However, challenges such as insufficient heat transfer enhancement and the fatigue strength of the tube bundle persist in the context of improving the heat transfer in elastic tube bundle heat exchangers. This study proposes a novel passive heat transfer enhancement paradigm for elastic tube bundles based on externally induced self-excited oscillations of fluid. By constructing a non-contact energy transfer system, the external oscillation energy is directed into the elastic tube bundle heat exchanger, achieving dynamic stress buffering and breaking through the steady-state flow heat transfer boundary layer. A three-dimensional fluid–structure interaction numerical model is established using Star CCM+2021.3 (16.06.008) to conduct a comparative analysis of the flow characteristics and heat transfer performance between the original structure without an oscillator and the improved structure equipped with a fluid oscillator. The results indicate that the improved structure, through the periodic unsteady jet induced by the fluid oscillator, significantly enhances the turbulence intensity of the shell-side fluid, with the turbulent kinetic energy increasing by over 50%. The radial flow area is notably expanded, thereby reducing the thermal resistance of the boundary layer. At cooling fluid velocities of 6 to 9 m/s, the heat transfer capability of the improved structure is enhanced by more than 50%. Compared with the original structure, the new structure, due to the loading of an external oscillation structure, causes the cold air to present a periodic up and down jet phenomenon. This jet phenomenon, on the one hand, increases the heat exchange area between the cold air and the outer surface of the tube bundle, thereby enhancing the heat exchange capacity. On the other hand, the large-area impact of the fluid reduces the thickness of the boundary layer, lowers the thermal resistance and thereby enhances the heat exchange capacity. Furthermore, this improved structure buffers the mechanical vibrations through self-excited oscillations of the fluid medium, ensuring that the stress levels in the tube bundle remain below the fatigue threshold, effectively mitigating the failure risks associated with traditional active vibration strategies. Full article

NiTi alloys and thin film/NiTi composites are extensively utilized in frictional environments, particularly those experiencing extreme temperature fluctuations. Current studies mainly focus on preparing wear-resistant films on NiTi alloy surfaces but neglect the potential impact of temperature-induced phase transitions in the NiTi substrate on thin films’ performance. This study examines the effect of NiTi alloy phase transitions, induced by extreme temperature variations, on the tribological properties of TiN thin films on NiTi substrates. TiN films (1 μm thick) were deposited on NiTi alloy surfaces using magnetron sputtering technology. The transition of the main phase in the NiTi substrate between the R phase and the B19′ phase was achieved via liquid nitrogen cooling (−196 °C) and water bath heating (90 °C). XRD, EDS, SEM, and tribological tests analyzed the phase structure, elemental composition, micromorphology, and tribological behavior. Fatigue wear was identified as the predominant wear mechanism for the TiN films, with minor contributions from oxidative and abrasive wear. Phase transition from the R phase to the B19′ phase in the NiTi substrate induced by temperature change couls reduce the wear rate of the TiN film by up to 41.97% and decrease the friction coefficient from about 0.45 to about 0.25. Furthermore, the shape memory effect of the NiTi alloy substrate, caused by B19′ → B2 phase transition, resulted in the recovery of the TiN thin film wear track depth from 920 nm to 550 nm, manifesting a “self-healing” phenomenon. The results in this study are important and necessary for the provision of thin film/NiTi composites in frictional environments. Full article

A large number of MIG welding tests were carried out on a 3 mm thick 7075 aluminum alloy plate prepared by the self-developed jet forming–extrusion–drawing process of 7075 high-strength aluminum alloy welding wire, and the welding process of the welding wire and the change in the performance of the welded joint after T6 heat treatment were studied. The results show that the self-developed wire has a good forming joint and a wide welding process window: the welding speed is 5–7 mm/s, and the welding current is 100–150 A. The main precipitated phases in the joint were η(MgZn₂), S(CuMgAl₂), Mg₂Si, and Al₁₃Fe₄, which were continuously distributed at the grain boundaries in the form of coarse networks or long strips, which was an important reason for the weak performance of the joints. After the heat treatment of T6, the precipitated phase in the joint was greatly reduced, the element segregation phenomenon was improved, and the residual precipitated phase was mainly Al₁₃Fe₄ and a small amount of insoluble phase Fe and Si, and the recrystallization size of the heat-affected zone was refined. Through heat treatment, the average microhardness of the joint was increased from 110 HV to 150.24 HV, and the tensile strength was increased from 326 MPa to 536 MPa, reaching 97.5% of the strength of the base metal, indicating that the softening phenomenon was significantly improved after heat treatment, and the joint had excellent performance. Full article

During the operation of liquid-phase He-4 (LHe-4) storage tanks, heat leakage changes the thermophysical parameters and phase properties of the LHe-4 in the tanks, resulting in the thermal layering phenomenon. This phenomenon is characterized by the LHe-4 temperature gradient and pressure increase (self-pressurization) phenomena in the tanks. Based on the Layer-by-Layer model, a heat transfer model of a composite adiabatic structure with multilayer insulation and liquid nitrogen screen (LNCS) insulation was established, and the Neumann boundary heat flux of the thermal response model was determined. A numerical simulation model of the thermal response of a liquid helium storage tank was established. The spatial and temporal evolutions of the pressure distribution, natural convection characteristics, thermal stratification characteristics, and self-pressurization characteristics of the LHe-4 tank were investigated. Finally, the self-pressurization thermodynamic model of the LHe-4 storage tank was built based on the isothermal saturation and homogeneous model. It is shown that the predictive performance of the mLee model for the self-boosting characteristics (relative deviation of 14.32%) was significantly improved compared with that of the Lee model (relative deviation of 39.64%). The thermal stratification degree (TSD) of the tank increased with the operation time, with TSDs of 1.023, 1.028, and 1.036 at 1 h, 2 h, and 3 h, which exacerbated the self-pressurization of the tank. The wall surface in contact with the phase interface is a strong evaporation point, so the interfacial mass transfer rate maps show a pattern of high at both ends and low in the middle. Full article

In this investigation, the aqueous corrosion resistance of 9Cr series heat-resistant steel during tempering was investigated. Optical Microscopy (OM), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectrometer (EDS) were used to analyze the effect of tempering temperature on the microstructure and precipitation behavior of precipitates. The heat-resisting steel was heated to 1150 °C for 1 h, and then tempered at different temperatures between 680 °C and 760 °C for 2 h. The microstructure of the heat-resistant steel after tempering was composed of lath-tempered martensite and fine precipitates. The hardness decreased with increasing tempering temperature, ranging from HBW 261 to HBW 193. The aqueous corrosion resistance improved as the tempering temperatures increased from 680 °C to 720 °C but deteriorated at higher temperatures, such as 760 °C, which was obtained by an electrochemical corrosion performance test. The aqueous corrosion resistance was affected by the decrease in dislocation density and the decrease in Cr solution in the tempered martensite. With the increase in the tempering temperature, the aqueous corrosion potential first increases and then decreases, the self-corrosion current density first decreases and then increases, and the polarization resistance first increases and then decreases. Furthermore, the increase in corrosion resistance is attributed to the reduction in dislocation density and chromium depletion in the martensitic structure as the tempering temperature approaches 720 °C. This paper reveals the effect of tempering temperature on the corrosion resistance of 9Cr series heat-resistant steel, which is a further exploration of a known phenomenon. Full article

Inspired by the “Lotus Leaf Effect” in nature, the phenomenon of superhydrophobia has attracted tremendous attention from researchers. Due to their special surface wettability, the superhydrophobic surfaces have been found to have broad potential applications in the fields of marine engineering, medical equipment, and aerospace. Based on the introduction of the principles of wettability, the advantages and disadvantages of various preparation methods for superhydrophobic surfaces were studied and summarized in this paper. The research progress on superhydrophobic surfaces in marine engineering applications was analyzed according to their self-cleaning, anti-corrosion, heat transfer, drag reduction, anti-fouling, ant-icing, and oil/water separation properties. Finally, to advance practical applications, the current challenges associated with superhydrophobic surfaces are highlighted, and potential future development directions are proposed. Full article

Thermal events in lead-acid batteries during their operation play an important role; they affect not only the reaction rate of ongoing electrochemical reactions, but also the rate of discharge and self-discharge, length of service life and, in critical cases, can even cause a fatal failure of the battery, known as “thermal runaway.” This contribution discusses the parameters affecting the thermal state of the lead-acid battery. It was found by calculations and measurements that there is a cooling component in the lead-acid battery system which is caused by the endothermic discharge reactions and electrolysis of water during charging, related to entropy change contribution. Thus, under certain circumstances, it is possible to lower the temperature of the lead-acid battery during its discharging. The Joule heat generated on the internal resistance of the cell due to current flow, the exothermic charging reaction, and above all, the gradual increase in polarization as the cell voltage increases during charging all contribute to the heating of the cell, overtaking the cooling effect. Of these three sources of thermal energy, Joule heating in polarization resistance contributes the most to the temperature rise in the lead-acid battery. Thus, the maximum voltage reached determines the slope of the temperature rise in the lead-acid battery cell, and by a suitably chosen limiting voltage, it is possible to limit the danger of the “thermal runaway” effect. The overall thermal conditions of the experimental cell are significantly affected by the ambient temperature of the external environment and the rate of heat transfer through the walls of the calorimeter. A series of experiments with direct temperature measurement of individual locations within a lead-acid battery uses a calorimeter made of expanded polystyrene to minimize external influences. A hitherto unpublished phenomenon is discussed whereby the temperature of the positive electrode was lower than that of the negative electrode throughout the discharge, while during charging, the order was reversed and the temperature of the positive electrode was higher than that of the negative electrode throughout the charge. The authors relate this phenomenon to the higher reaction entropy change of the active mass of the positive electrode than that of the negative electrode. Full article

In order to determine the junction temperature of semiconductor devices operating at different power supply and cooling conditions, their thermal parameters are needed. This article describes an original measurement set-up enabling the determination of thermal parameters of power semiconductor devices. In contrast to other set-ups described in the literature, this set-up makes it possible to measure thermal parameters which characterize the efficiency of the removal generated due to a self-heating phenomenon, as well as the parameters characterizing mutual thermal couplings. The presented set-up makes use of an indirect electrical method to determine the junction temperature of diodes, bipolar and unipolar transistors and IGBTs. The methods used to measure the self and transfer transient thermal impedances of these devices and the construction of the set-up are described. The influence of selected factors on the accuracy of the measurements is analyzed. Examples of the measurement results of thermal parameters (self and transfer transient thermal impedances) of power semiconductor devices operating at different cooling conditions are presented. The obtained research results are discussed. Full article

Spontaneous detachment from superhydrophobic surfaces can be induced by the coalescence of two or more adjacent droplets. The phenomena have provided implications for the self-removal of droplets in the fields of self-cleaning, anti-icing, and heat transfer. However, many studies focus mainly on the theoretical jumping direction perpendicular to the substrate, although the velocity in the horizontal direction must be involved in practical applications due to various scenarios. This study analyzes numerically the effect of the distribution in ridge structure’s wettability on the performance of coalesced droplet jumping. The jumping dynamics are discussed for varying contact angle ratios and the aspect ratios of the ridge, which are the initial values for the current model. We obtain the height of the jumping and the offset distance in the horizontal direction under the several initial values. In addition, the characteristics of the asymmetric behavior are discussed based on the temporal evolution of the average velocities of the jumping droplets for each direction. Numerical results show that the horizontal offset distance is significantly pronounced at both the high asymmetry in wettability and the high aspect ratio of the ridge geometry. The phenomenon occurs when the droplet detaches from the ridge surface in the retraction process. We determine the role of the distribution within the ridge structure on its wettability, as well as the role of the aspect ratios of the ridge in facilitating the efficient transport of droplets. Full article

Self-excited thermoacoustic instability (SETAI) is an undesirable and dangerous phenomenon in combustion systems. However, its control is difficult, thus greatly limiting the development of combustion technology. Our previous works clarified how the premixed chamber length (L_P) and equivalence ratio (φ) influence SETAI behavior in a symmetrical hedge premixed combustion system. On real-world sites, however, the supply structure or combustion condition in a multi-flame system could be asymmetric due to space limitations or combustion adjustment needs. This paper aims to clarify the SETAI behavior of a combustion system with an asymmetric supply structure or an asymmetric combustion condition. The results indicate that the sound pressure amplitude under strong oscillation can reach 160 dB, which is about 5% of the total pressure. The SETAI state under the asymmetric condition is determined by the coupling between the heat release oscillation and sound pressure oscillation on each side and their cooperation. The asymmetric supply structure leads to asynchronous heat release oscillations between the two sides; it may be that one promotes oscillation and that the other suppresses it, or that both have a promotion effect but with asynchronous action, thus partly canceling each other out to lower the system’s oscillation intensity. This brings an advantage for controlling SETAI, which can be achieved by only changing one side of the structure. The oscillation amplitude can be reduced by 80–90% by appropriately changing one L_P only by ~20%. Under an asymmetric combustion condition with φ differing between the two sides, the heat release oscillation on each side is dependent on the local φ but not the global φ. Consequently, SETAI can also be controlled by changing the distribution but maintaining a constant fuel feeding rate and φ. The concepts identified in this paper demonstrate that SETAI can be effectively controlled by adopting an asymmetric φ distribution or an asymmetric structure of the supply system. This provides a convenient SETAI control approach without affecting the equipment’s thermal performance. Full article

Marangoni bursting describes the spontaneous spread of a droplet of a binary mixture of alcohol/water deposited on a bath of oil, followed by its fast spontaneous fragmentation into a large number of smaller droplets in a self-similar way. Several papers have aimed to describe the physical phenomena underlying this spectacular phenomenon, in which two opposite effects, solutal and thermal Marangoni stresses, play competitive roles. We performed investigations of the Marangoni bursting phenomenon, paying attention to the surface temperature changes during bursting and after it. Fragmentation instabilities were monitored using a thermal camera for various initial alcohol/water compositions and at different stages of the process. We uncovered the role of thermocapillary Marangoni flows within the more viscous oil phase that are responsible for outward and inward shrinking of the periphery circle at the final stage of the phenomenon, enabling a more comprehensive understanding of the thermal Marangoni effect. Simulations of the Marangoni thermocapillary effect in an oil bath by solving coupled Navier–Stokes and heat transport equations using the COMSOL Multiphysics software platform support our experimental observations. Full article