Search Results (115)

Quasicrystalline coatings based on Al₆₅Cu₂₀Fe₁₅ are of increasing interest as potential alternatives to conventional wear-resistant materials due to their unique structural and tribological properties. This study explores the influence of air pressure during high-velocity oxy-fuel (HVOF) spraying on the phase composition, morphology, and wear behavior of Al₆₅Cu₂₀Fe₁₅ coatings deposited on U8G tool steel. Coatings were applied at a fixed spraying distance of 350 mm using three air pressures (1.9, 2.1, and 2.3 bar), with constant propane (2.0 bar) and oxygen (2.1 bar) supply. X-ray diffraction analysis identified the formation of Al₇₈Cu₄₈Fe₁₄ and Al_0.5Fe_1.5 phases, while scanning electron microscopy revealed a dense, uniform microstructure with low porosity and homogeneous element distribution across all samples. Tribological testing using the ball-on-disk method showed wear track widths ranging from 853.47 to 952.50 µm, depending on the air pressure applied. These findings demonstrate that fine-tuning the air pressure during HVOF spraying significantly influences the structural characteristics and wear resistance of the resulting quasicrystalline coatings, highlighting their promise for advanced surface engineering applications. Full article

Hydrogen is a promising fuel in the current transition to zero-net CO₂ emissions. However, most practical combustion equipment is not yet ready to burn pure hydrogen without adaptation. In the meantime, blending hydrogen with natural gas is an interesting option. This work reports a computational study of the performance of swirl-stabilized natural gas/hydrogen flames in a novel combustion chamber design. The combustor employs an air-staging strategy, introducing secondary air through a top-mounted plenum in a direction opposite to the fuel jet. The thermal load is fixed at 5 kW, and the effects of fuel composition (hydrogen molar fraction ranging from zero to one), excess air coefficient (λ = 1.3, 1.5 or 1.7), and primary air fraction (α = 50–100%) on the velocity, temperature, and emissions are analysed. The results show that secondary air changes the flow pattern, reducing the central recirculation zone and lowering the temperature in the primary reaction zone while increasing it further downstream. Secondary air improves the performance of the combustor for pure hydrogen flames, reducing NO emissions to less than 50 ppm for λ = 1.3 and 50% primary air. For natural gas/hydrogen blends, a sufficiently high excess air level is required to keep CO emissions within acceptable limits. Full article

The atomization performance of methanol fuel plays a crucial role in enhancing methanol engine efficiency, contributing to the decarbonization of the shipping industry. The droplet microscopic characteristics of methanol spray were experimentally investigated using a single-hole direct injection injector in a constant volume chamber. The particle image analysis (PIA) system equipped with a slicer was employed for droplet detecting at a series of measurement positions in both the dense spray region and dilute spray region, then the spatial distributions of droplet size and velocity were examined. Key findings reveal distinct atomization behaviors between dense and dilute spray regions. Along the centerline, the methanol spray exhibited poor atomization, characterized by a high concentration of aggregated droplets, interconnected liquid structures, and large liquid masses. In contrast, the spray periphery demonstrated effective atomization, with only well-dispersed individual droplets observed. Droplet size distribution analysis showed a sharp decrease from the dense region to the dilute region near the nozzle. In the spray midbody, droplet diameter initially decreased significantly within the dense spray zone, stabilized in the transition zone, and then exhibited a slight increase in the dilute region—though remaining smaller than values observed at the central axis. Velocity measurements indicated a consistent decline in the axial velocity component due to air drag. In contrast, the radial velocity component displayed irregular variations, attributed to vortex-induced flow interactions. These experimentally observed droplet behaviors provide critical insights for refining spray models and enhancing computational simulations of methanol injection processes. Full article

In this study, the cooling performance of fuel cell electric vehicles (FCEVs) with regard to thermal derating is investigated. Particularly in hot climate conditions, low operating temperature of the fuel cell stack and hence low temperature difference to the environment can result in thermal derating of the fuel cell stack. Experimental investigations on a production vehicle with a fuel cell drive (Hyundai Nexo) are conducted to analyze the influence of climatic boundary conditions and a dynamic driving scenario on the thermal management system of the vehicle. Therefore, a new method based on energy balances is introduced to indirectly measure the average cooling air velocity at the cooling module. The results indicate that the two high-power radiator fans effectively maintain a high cooling airflow between a vehicle speed of approximately 30 and 100 $\mathrm{k}\mathrm{m}/\mathrm{h}$, leading to efficient heat rejection at the cooling module largely independent of vehicle speed. Furthermore, this study reveals that the efficiency of the fuel cell system is notably affected by ambient air temperature, attributed to the load on the electric air compressor (EAC) as well as on cooling system components like cooling pump and radiator fans. However, at the stack level, balance of plant (BoP) components demonstrate the ability to ensure ambient temperature-independent performance, likely due to reliable humidification control up to 45 °C. Additionally, a new method for determining thermal derating of FCEVs on roller dynamometer tests is presented. A real-world uphill drive under ambient temperatures exceeding 40 °C demonstrates derating occurring in 6.3% of the time, although a worst case with an aged stack and high payload is not investigated in this study. Finally, a time constant of 50 $\mathrm{s}$ is found to be suitable to correlate the average fuel cell stack power with a coolant temperature at the stack inlet, which gives information on the thermal inertia of the system observed and can be used for future simulation studies. Full article

To investigate the physical phenomena interactions between airstream and liquid injection or droplets within a complex multi-stage swirl flow field, this study investigated the flow field and spray characteristics in a central stage direct injection combustor with a variety of optical diagnostic techniques, including using time-resolved particle image velocimetry (PIV) to measure the swirl flow field, using time-resolved planar Mie scattering (PMie) to measure the spray pattern, and using a laser particle size analyzer (LPSA) to measure the spray droplet size and its distribution. The results indicate that the lip recirculation zone (LRZ) and the swirl jet zone (SJZ) significantly influence droplet spatial and size distribution characteristics, such as spray penetration, cone angle, and droplet size. Due to the unique characteristics of the dual-stage swirl atomizer, the spray cone angle and penetration do not increase monotonically with the gas Weber number (${We}_g$). For the pilot stage, at a constant ${We}_g$, both the spray cone angle and penetration increase with higher fuel injection velocity. At different fuel injection velocities, the spray penetration increases with rising ${We}_g$. When the fuel injection velocity is low, the cone angle initially increases and then decreases as ${We}_g$ grows. The results about the effect of ${We}_g$ on droplet size distribution further support this conclusion. The Sauter mean diameter (SMD) of the main and pilot stage decreases with increasing relative pressure drop of air until reaching a stable state. The aerodynamic shear of the swirling airstream is sufficient to promote thorough fuel atomization, ensuring that the SMD remains low at the whole operating condition. Therefore, for the dual-stage swirl atomizer investigated in this study, good atomization can be achieved under low operating conditions, which provides a theoretical foundation and data support for the improvement and design of a low-emission, high-performance atomizer. Full article

The effects of microstructure, density, and porosity of a FeCoNiCrAl high-entropy alloy (HEA) coating, fabricated using an internal diameter high-velocity air fuel (ID-HVAF) torch (model: i7 ID), on the isothermal oxidation behavior were investigated. This study pioneers the use of the ID-HVAF i7 ID system for HEA bond coat manufacturing, achieving a highly dense microstructure because of its low-operating spray temperature technique. To elucidate these effects, the microstructure and chemistry of the coating, the growth of the thermally grown oxides (TGOs), the phase transformation of alumina, and the oxidation rate were investigated at different temperatures. After 50 h at 1000 °C, 1100 °C, and 1150 °C, a dense, uniform, and thin alumina TGO layer (1.8 μm) was observed. The results demonstrate that the oxidation resistance of the HEA coating is enhanced because of the dense microstructure achieved via HVAF-i7, characterized by low porosity and uniform phase distribution, which contribute to improved barrier properties against oxygen diffusion. The growth of the TGO layer is controlled, resulting in a dense and continuous TGO layer. However, with increasing temperature and time, the alumina TGO layer becomes spalled, which is attributed to the absence of reactive elements. Overall, this study reveals that the FeCoNiCrAl HEA exhibits significant potential for enhancing oxidation resistance at high temperatures. Full article

Aircraft gas turbine blades operate in aggressive, generally oxidizing, atmospheres. A solution to mitigate the degradation and improve the performance of such components is the deposition of thermal barrier coatings systems (TBCs). High-velocity air fuel (HVAF) is a very efficient process for coating deposition in TBC systems, particularly for bond coats in aerospace applications. However, its low-temperature variant has received little attention in the literature and could be a promising alternative to limit oxidation during spraying when compared to conventional methods. This study has the main objective of analyzing how the geometry of the low-temperature HVAF gun influences the microstructure and the in-flight oxidation of MCrAlX coatings. To that end, a low-temperature HVAF torch is used to deposit MCrAlX coatings on a steel substrate with different nozzle lengths. In-flight particle diagnosis is used to measure the MCrAlX particle velocity, and to correlate to the nozzle geometry and to analyze its influence on the final coating. The microstructure of the coatings is assessed by scanning electron microscopy (SEM) and the material oxidation is analyzed and measured on a field emission scanning transmission electron microscope (FE-STEM) equipped with focused ion beam (FIB) and by Energy Dispersive Spectroscopy (EDS). Full article

Increasing demand to deposit dense and oxidation-resistant bond coats requires reliable and efficient deposition techniques. High-Velocity Air-Fuel (HVAF), among other thermal spray processes, is showcasing consistent potential to optimize spraying techniques and deposition strategies for depositing NiCoCrAlY coatings. NiCoCrAlY coatings are sensitive to high-temperature oxidation, and preserving the aluminum reservoir in the bond coats is of the highest priority to potentially resist oxidation during thermal cycling. Contrary to the existing literature on comparing carbide-based HVAF deposition with other processes, this work investigates the specific role of nozzle configurations. It primarily focuses on in-flight particle characteristics using diagnostic tools and the corresponding inflight particle oxidation of NiCoCrAlY feedstock. This work details individual splat and coating characteristics, revealing the significant influence of nozzle configurations. A comprehensive understanding of process–material–microstructure correlations was established using a commercially available NiCoCrAlY coating system. Comprehensive discussions on nozzle configurations over various feedstock powder characteristics were carried out in this work. Advanced characterization techniques were employed to assess the in-flight particle oxidation and coating microstructure using focused ion beam (FIB), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Full article

Methane–hydrogen–air mixtures present a viable alternative to conventional fuels, reducing CO₂ emissions while maintaining high energy density. This study numerically investigates their combustion characteristics in millimeter-scale reactors, focusing on flame stabilisation and combustion dynamics in confined spaces. A species transport model with volumetric reactions incorporated a detailed kinetic mechanism with 16 species and 41 reactions. The simulations employed a laminar flow model, second-order upwind discretisation, and SIMPLE algorithm for pressure–velocity coupling. The key parameters analysed include equivalence ratio, hydrogen volume fraction, inlet velocity, and gas pressure and their impact on fuel conversion efficiency and heat release was evaluated. The results indicate that hydrogen enrichment enhances flame stability and combustion efficiency, with optimal performance over 40% hydrogen content. Additionally, increased outlet pressure raises flame temperature by 15%, while larger reactor diameters reduce heat losses, improving combustion efficiency by 20%. Emissions of CO decrease significantly at higher hydrogen fractions, demonstrating the potential for cleaner combustion. These findings support the integration of methane–hydrogen mixtures into sustainable energy systems, providing insights for designing efficient, low-emission micro-combustors. Full article

The stability of hydrogen-fueled flames in afterburner systems is crucial for advancing clean energy technologies but is challenged by intense turbulence and flow variability. This study uniquely integrates advanced particle image velocimetry (PIV) techniques to investigate the flow dynamics around a V-gutter flame holder fueled with 100% hydrogen. Detailed velocity measurements were conducted to analyze the standard deviation of V_y, average V_y, average V, and uncertainty of V_y, as well as the mean swirling strength and mean vorticity profiles across multiple horizontal and vertical lines. The results reveal significant flow variability and turbulence intensity near the flame holder, with standard deviation peaks of up to 12 m/s, indicating zones of high turbulence and potential flame instability. The mean swirling strength, peaking at 850,000 [1/s²], and vorticity values up to 5000 [1/s] highlight intense rotational motion, enhancing fuel–air mixing and flame stabilization. The average V_y remained stable near the centerline, ensuring balanced flow conditions, while lateral deviations of up to −10 m/s reflect vortical structures induced by the flame holder geometry. Low uncertainty values, typically below 1 m/s, validate the precision of the PIV measurements, ensuring a reliable representation of the flow field. By providing a detailed analysis of turbulence structures and their impact on hydrogen combustion, this study offers novel insights into the interplay between flow dynamics and flame stability. These findings not only advance the understanding of hydrogen-fueled afterburner systems but also demonstrate the critical role of rotational flow structures in achieving stable and efficient combustion. By addressing key challenges in hydrogen combustion, this study provides a foundation for designing more robust and environmentally sustainable combustion systems, contributing to the transition toward clean energy technologies. Full article

Power generation is an important part of air vehicle energy management when developing long-endurance and reusable hypersonic aircraft. In order to utilize an air turbine power generation system on board, fuel-based rotating cooling has been researched to cool the turbine’s rotor blades. For fuel-cooling air turbines, each blade corresponds to a separate cooling channel. All the separate cooling channels cross together and form a distributary cavity and a confluence cavity in the center of the disk. In order to determine the flow characteristics in the distributary and confluence cavities, computational fluid dynamics (CFD) simulations using the shear–stress–transport turbulence model were carried out under the conditions of different rotating speeds and different mass flow rates. The results showed great differences between non-rotating flow and rotating flow conditions in the distributary and confluence cavities. The flow in the distributary and confluence cavities has rotational velocity, with obvious layering distribution regularity. Moreover, a high-speed rotational flow surface is formed in the confluence cavity of the original structure, due to the combined functions of centrifugal force, inertia, and the Coriolis force. Great pressure loss occurs when fluid passes through the high-speed rotational flow surface. This pressure loss increases with the increase in rotating speed and mass flow rate. Finally, four structures were compared, and an optimal structure with a separated outlet channel was identified as the best structure to eliminate this great pressure loss. Full article

Oxygen present in the High Velocity Air-Fuel (HVAF) process can react with the in-flight metallic particles and cause their oxidation. A grown brittle oxide shell on metallic micro-size particles can reduce their deposition efficiency and impair the coating’s final deposited properties/microstructure. In the current study, the oxide growth of MCrAlY particles, where M stands for Nickel (Ni) and Cobalt (Co), during their flight in the HVAF process has been numerically modeled and validated with experimental single-particle depositions. A thorough theoretical oxide layer growth background is also presented. The utilized oxidation development follows the Mott–Cabrera theory for very thin films, which uses the particle surrounding temperature and oxygen partial pressure to track and describe the oxide growth. The obtained results provide a good correlation between the HVAF system design, the operating conditions, and surface oxidation phenomena observed using focus ion beam scanning electron microscope (FIB/SEM) analysis on collected particles. Furthermore, the particle’s degree of oxidation in HVAF is compared to High Velocity Oxy-Fuel (HVOF) to demonstrate the influence of combustion processes on oxidation level. Full article

High-alkali Zhundong coal presents significant challenges for power generation, due to its propensity for fouling and slagging. This study investigates a retrofit of a 300 MW tangentially fired boiler with the integration of a slag-tap chamber to improve combustion performance. Computational fluid dynamics (CFD) simulations are employed to examine the influence of this modification on combustion dynamics and the effects of Zhundong coal blending ratios on heat and mass transfer. The results demonstrate that the retrofit facilitates stable airflow recirculation, optimizing combustion efficiency with a peak temperature of 2080 K in the combustion chamber. The flue gas temperature decreases to approximately 1650 K upon exit, which can be attributed to the slag catcher cooling. The integration of the liquid slagging chamber significantly mitigates slag formation, while enhancing oxygen and CO₂ distribution throughout the furnace. As the blending ratio of Zhundong coal increases, oxygen concentrations rise in the bottom burner region, indicating improved air–fuel mixing. With a 30% Zhundong coal ratio, the combustion chamber temperature increases by 3%, and flow velocity in the upper and middle furnace sections decreases by 15%, leading to enhanced combustion intensity. This retrofit demonstrates substantial improvements in combustion stability, slagging control, and the efficient utilization of high-alkali coal. Full article

The paper presents a comparative study of the erosion wear resistance of WC-10Co4Cr, Cr₃C₂-25NiCr and martensitic stainless steel (SS) coatings deposited onto an AlSi7Mg0.3 (Al) alloy substrate by high-velocity air‒fuel (HVAF) spraying. The influence of the abrasive type (quartz sand or granite gravel), erodent attack angle, thickness, and microhardness of the coatings on their and Al substrate’s wear resistance was comprehensively investigated under dry erosion conditions typical for fan blades. The HVAF-spraying process did not affect the Al substrate’s structure, except for when the near-surface layer was 20‒40 μm thick. This was attributed to the formation of a modified Al-Si eutectic with enhanced microhardness and strength in the near-substrate area. Mechanical characterization revealed significantly higher microhardness values for the cermet WC-10Co4Cr (~12 GPa) and Cr₃C₂-25NiCr (~9 GPa) coatings, while for the SS coating, the value was ~5.7 GPa. Erosion wear tests established that while Cr₃C₂-25NiCr and SS coatings were more sensitive to abrasive type, the WC-10Co4Cr coating exhibited significantly higher wear resistance, outperforming the alternatives by 2‒17 times under high abrasive intensity. These findings highlight the potential of HVAF-sprayed WC-10Co4Cr coatings for extending the service life of AlSi7Mg0.3-based fan blades exposed to erosion wear at normal temperatures. Full article

Prescribed burning is a cost-effective method for reducing hazardous fuels in pine- and oak-dominated forests, but smoke emissions contribute to atmospheric pollutant loads, and the potential exists for exceeding federal air quality standards designed to protect human health. Fire behavior during prescribed burns influences above-canopy sensible heat flux and turbulent kinetic energy (TKE) in buoyant plumes, affecting the lofting and dispersion of smoke. A more comprehensive understanding of how enhanced energy fluxes and turbulence are related during the passage of flame fronts could improve efforts to mitigate the impacts of smoke emissions. Pre- and post-fire fuel loading measurements taken during 48 operational prescribed burns were used to estimate the combustion completeness factors (CC) and emissions of fine particulates (PM_2.5), carbon dioxide (CO₂), and carbon monoxide (CO) in pine- and oak-dominated stands in the Pinelands National Reserve of southern New Jersey. During 11 of the prescribed burns, sensible heat flux and turbulence statistics were measured by tower networks above the forest canopy. Fire behavior when fire fronts passed the towers ranged from low-intensity backing fires to high-intensity head fires with some crown torching. Consumption of forest-floor and understory vegetation was a near-linear function of pre-burn loading, and combustion of fine litter on the forest floor was the predominant source of emissions, even during head fires with some crowning activity. Tower measurements indicated that above-canopy sensible heat flux and TKE calculated at 1 min intervals during the passage of fire fronts were strongly influenced by fire behavior. Low-intensity backing fires, regardless of forest type, had weaker enhancement of above-canopy air temperature, vertical and horizontal wind velocities, sensible heat fluxes, and TKE compared to higher-intensity head and flanking fires. Sensible heat flux and TKE in buoyant plumes were unrelated during low-intensity burns but more tightly coupled during higher-intensity burns. The weak coupling during low-intensity backing fires resulted in reduced rates of smoke transport and dispersion, and likely in more prolonged periods of elevated surface concentrations. This research facilitates more accurate estimates of PM_2.5, CO, and CO₂ emissions from prescribed burns in the Pinelands, and it provides a better understanding of the relationships among fire behavior, sensible heat fluxes and turbulence, and smoke dispersion in pine- and oak-dominated forests. Full article