Challenging Operating Conditions Research Articles

Enhancing Bulb Turbine Performance Assessing Air Injection for Vibration Mitigation and Hydrofoil Cavitation

Small hydro technology is playing a crucial role in advancing sustainable, clean energy policies as part of the global hydro development strategy. Its contribution to social and economic development is becoming more prominent, particularly in ensuring electricity access for rural communities and supporting industrial expansion. The main causes of bulb turbine failures under high operating conditions are frequently attributed to variations in pressure in the draft tubes, which are aggravated when a spinning vortex rope is formed under load operation. Different fluid injection techniques, namely compressed air and water jet injection, address these issues and reduce the negative results of cavitation. The investigation covers the flow visualization on the suction side of a single hydrofoil utilizing a cavitation tunnel and a bulb turbine. This study assesses the effectiveness of compressed air injection in reducing vibration generated by cavitation in bulb turbines. Positive results of the experimental studies suggest a decrease in noise and vibration by air injection that prevents oscillations of the vortex rope. This research also considers how the hydrofoil design of bulb runner blades influences flow characteristics. Hence, it provides knowledge on cavitation structures in diverse cavitation numbers. Different studies that compare the original and modified hydrofoil designs reveal remarkable improvements that may be due to changes in the key parameters of the hydrofoil, such as later cavitation initiation and reduced intensity. To obtain the optimal output of a bulb turbine by considering air injection for vibration reduction and hydrofoil design changes to limit the negative effect of cavitation, the Reynolds number and cavitation number are to be defined. This multidisciplinary approach possesses enormous potential to increase the reliability and efficiency of bulb turbines in challenging operating conditions.

Dynamics Modeling and Motion Evaluation of a Near-Ground Tethered Balloon Cable System under Severe Wind Environments

Weather survival poses a significant challenge for the utilization of tethered balloons. The dynamic modeling of tethered balloon systems presents challenges due to the flexible nature of the cables and the intricate nature of gust forces. The present study introduces a new approach for modeling near-ground tethered balloon systems, which enables the analysis of their dynamic responses and performance evaluation under complex boundary conditions. First, finite cylindrical rigid bodies that are joined together by bushing forces to describe the dynamics of the tethered cable. The properties of the flexible cables under severe bending and translation can be illustrated by the dynamics model. Second, a three-dimensional dynamics model based on the multibody dynamics theory is created to deal with the interaction of the tethered balloon system and flexible cables. The dynamic responses of the tethered balloon system under challenging operating conditions are investigated, focusing on the number of cable segments and the place and direction of gust wind impacts. This model allows for precise assessment and optimization of the system’s overall performance to improve weather resistance. The results show that compared to computational fluid dynamics (CFDs) methods, the multibody system dynamics-based balloon model improved the solution time by 80%, with a pitch angle deviation of only 0.0016°. Moreover, the bushing model effectively reduced cable force and enabled accurate reflection of the system’s motion characteristics.

Dynamic risk evolution analysis for in-situ combustion incidents of offshore heavy oil recovery

In-situ combustion for offshore heavy oil involves significant risk factors related to ignition, gas injection, and corrosion prevention, especially under challenging operating conditions. If not properly managed, these risks can lead to explosions in injection or production wells. This paper proposes a hybrid HDBN-based risk analysis method to estimate the probability of explosions during the three critical phases of in-situ combustion and to diagnose key risk factors contributing to such incidents. The method introduces new bootstrap terms to in-situ combustion and constructs a hybrid DBN to capture the dynamic behavior of explosion incidents. It incorporates the Delphi method and triangular Pythagorean fuzzy theory to address data scarcity and uncertainty through affiliation and uncertainty functions. A key aspect of the proposed method is the graded quantification of damage to the casing due to corrosion and thermal stresses. A case study of a well in Liaodong Bay, Bohai Sea, demonstrates the feasibility of this method. The study captures dynamic risk evolution characteristics during three stages of in-situ combustion and conducts sensitivity analysis to obtain key risk factors such as "bottom high temperature" and "gas corrosion".

Leveraging Optimal Sparse Sensor Placement to Aggregate a Network of Digital Twins for Nuclear Subsystems

Nuclear power plants (NPPs) require continuous monitoring of various systems, structures, and components to ensure safe and efficient operations. The critical safety testing of new fuel compositions and the analysis of the effects of power transients on core temperatures can be achieved through modeling and simulations. They capture the dynamics of the physical phenomenon associated with failure modes and facilitate the creation of digital twins (DTs). Accurate reconstruction of fields of interest (e.g., temperature, pressure, velocity) from sensor measurements is crucial to establish a two-way communication between physical experiments and models. Sensor placement is highly constrained in most nuclear subsystems due to challenging operating conditions and inherent spatial limitations. This study develops optimized data-driven sensor placements for full-field reconstruction within reactor and steam generator subsystems of NPPs. Optimized constrained sensors reconstruct field of interest within a tri-structural isotropic (TRISO) fuel irradiation experiment, a lumped parameter model of a nuclear fuel test rod and a steam generator. The optimization procedure leverages reduced-order models of flow physics to provide a highly accurate full-field reconstruction of responses of interest, noise-induced uncertainty quantification and physically feasible sensor locations. Accurate sensor-based reconstructions establish a foundation for the digital twinning of subsystems, culminating in a comprehensive DT aggregate of an NPP.

In-situ honeycomb spheres for enhanced enzyme immobilization and stability

This paper presents a promising heterogeneous biocatalyst-immobilized cellulase system, which can facilitate the targeted catalytic conversion of biomass to reducing sugars. Magnetic Zeolite Imidazole Framework-8/chitosan (Fe3O4/ZIF-8/CS, FZC) hybrid honeycomb microspheres were synthesized to immobilize cellulase using an in-situ growth strategy. The enhancement of immobilization efficiency induced by each constituent of the composite carrier was explored. This exploration was grounded in varying combinations of components within the honeycomb composite spheres, revealing the indispensability of ZIF-8. Through in-situ assembly, the composite material presents a microsphere with an internal honeycomb structure and multilevel pores. Effective immobilization, temperature-responsive conformational change, and cyclic stability were achieved due to the scaling effect of the channel, extensive specific surface area, and abundant amino group interactions enable immobilized enzymes to maintain excellent performance under challenging operating conditions, demonstrating an enhanced structure-performance relationship. With the obtained immobilized cellulase, we achieved a high load (461.67 mg/g) of cellulase and a significant reducing sugar yield (328.39 mg/g) from corncob waste. In addition, we investigated the enhancement effect of thermal activation on the immobilized enzyme activity of composite carrier. The results showed that the appropriate heat treatment was conducive to the activation of FZC immobilized enzyme with a reducing sugar yield 1.61 times higher than that of the control. The strategy provided in this study promotes the development of immobilized biocatalysts and provides a useful reference for the further design of biocatalytic systems.

Electrochemical instability of room-temperature ionic liquids with LiTFSI at elevated temperature and its consequences in Li/Li-ion based half-cells and full-cells

High-temperature Li-ion batteries capable of operating up to 100 ℃ can replace lithium thionyl chloride primary batteries in specialized industrial applications. Room-temperature ionic liquids (RTIL) offer high thermal stability and can be a promising choice of electrolyte for high-temperature batteries. This work elucidates that high thermal stability alone does not guarantee effective battery operation due to the electrochemical instability observed in RTIL-LiTFSI mixtures at elevated temperatures. Here, we investigate the impact of varying electrochemical stability in RTILs on half-cell, full-cell, and calendar aging. Electrochemical instability of the electrolyte at the cathode interface injects additional electrons and Li-ions into the electrochemical cell, triggering a cascade of detrimental chain reactions that hinder efficient battery performance. We demonstrate that electrochemical information gathered from a full-cell configuration can frequently obscure the presence of electrochemical instability at the cathode surface. To accurately evaluate this instability, it is essential to perform in-depth half-cell studies and other characterizations. Our results indicate that cathode electrolyte interphase (CEI) can reach a thickness of up to 100 nm, with a depth-dependent composition showing higher concentration of inorganic species like LiF and LiNSO near the cathode surface. Further, accelerated calendar aging measurements of such cells at high temperature revealed complete irreversible self-discharge within as little as 14 days. These findings shed light on the critical factors influencing the stability and performance of cells under challenging operating conditions.

Tuning the Actuator Line Method to Properly Modelling Tip Effects in Finite-Length Blades

The Actuator Line Method (ALM) is gaining popularity in wind turbine simulations, as it can better handle some of the challenging operating conditions experienced by modern machines, such as highly turbulent inflows, severe aero-elastic forcing, and complex rotor-to-rotor interactions. However, it still falls behind other medium-fidelity methods such as the Lifting Line Free Vortex Wake (LLFVW) when it comes to resolving tip vortices and their effect on the blade spanwise load profile. The reason for such behavior is still unclear. A recent study suggested that this issue can be solved by reducing the scale of the angle of attack (α) sampling and force insertion towards the tip, without the need of additional corrections. This study builds on these findings to further investigate how the ALM base formulation - in terms of α sampling and force insertion - can be tuned to properly describe tip effects. An in-house ALM tool was employed to simulate a finite, constant-chord, NACA0018 wing, for which high-fidelity blade-resolved CFD (BR-CFD) data are available as benchmark. In the first part of the work, different strategies are outlined, including a novel approach for the de-coupling of the angle of attack sampling step from the force projection one, here called DE-coupled LineAverage (DELA). Their accuracy and sensitivity to ALM numerical settings are assessed at a fixed wing pitch angle of 6°. The analysis is then extended to a wider range of blade pitch angles, benchmarking the new ALM formulation against BR-CFD, ALM with the Dağ and Sørensen correction, and LLFVW in terms of blade loads, tip vortex structure, and computational effort.

Development of multiple input supply based modified SEPIC DC–DC converter for efficient management of DC microgrid

The development of DC microgrids is reliant on multi-input converters, which offer several advantages, including enhanced DC power generation and consumption efficiency, simplified quality, and stability. This paper describes the development of a multiple input supply based modified SEPIC DC–DC Converter for efficient management of DC microgrid that is powered by two DC sources. Here Multi-Input SEPIC converter offers both versatility in handling output voltage ranges and efficiency in power flow, even under challenging operating conditions like lower duty cycle values. These features contribute to the converter's effectiveness in managing power within a DC microgrid. In this configuration, the DC sources can supply energy to the load together or separately, depending on how the power switches operate. The detailed working states with equivalent circuit diagrams and theoretical waveforms, under steady-state conditions, are shown along with the current direction equations. This paper also demonstrates the typical analysis of large-signal, small-signal, steady-state modeling techniques and detailed design equations. The proposed configuration is validated through the conceptual examination using theoretical and comprehensive MATLAB simulation results. Detailed performance analysis has been done for different cases with various duty ratios. Finally, to show the competitiveness, the multi-input SEPIC topology is compared with similar recent converters.

Development and Deployment of a High-Precision Power Factor Measurement Meter

This research article presents the development and deployment of a high-accuracy power factor measurement meter. Power factor measurement is crucial for assessing the efficiency of electrical systems and ensuring optimal power usage. The proposed meter employs advanced signal processing techniques to accurately measure both leading and lagging power factors with high precision. The design incorporates robust hardware components such as ATMega8 micro- controlling chip and sophisticated algorithms to minimize errors and provide reliable measurements even in challenging operating conditions. Experimental results demonstrate the effectiveness of the proposed meter in accurately measuring power factor across a wide range of electrical loads. The developed meter offers significant improvements in accuracy and reliability compared to existing solutions, making it suitable for various industrial and commercial applications where precise power factor measurement is essential for efficient energy management.

IMWMOTE: A novel oversampling technique for fault diagnosis in heterogeneous imbalanced data

In the realm of smart manufacturing, the urgent necessity of surmounting the technical hurdles associated with developing data-driven fault monitoring and intelligent diagnostic technologies for high-end equipment is imminent. In practical engineering applications, the challenging operating conditions and unfavorable environments for data collection often lead to the acquisition of health data that is not only constrained in terms of sample size but also exhibits imbalances and heterogeneity, collectively referred to as Small-Sample, Imbalanced, and Mixed-Class Scenarios. This poses a considerable challenge to the interpretability modeling of faults in high-end equipment, as identified by the scientific community. Prior investigations have highlighted the distinct benefits of resampling techniques in addressing the complexities of imbalanced fault modeling. Nevertheless, the suboptimal performance of current techniques remains a concern, primarily due to ambiguous representations of sample feature heterogeneity and the lack of sophisticated sampling balance strategies. For instance, the absence of tailored strategies and concomitant optimization approaches specifically designed for each heterogeneous feature remains a notable gap. In this study, we undertake a comprehensive analysis of the heterogeneity mechanisms inherent in fault feature data, with a particular emphasis on components like turbine rotor faults. Building upon these insights, we introduce a fault diagnosis framework utilizing the novel oversampling technique based-MWMOTE (IMWMOTE), to address these challenges effectively. This comprehensive framework not only ensures interpretability in classification but also seamlessly integrates an optimization mechanism. Notably, the efficacy and innovation of our proposed approach have been thoroughly verified through an extensive evaluation involving 20 fault diagnosis instances across three distinct categories: turbine rotor, bearings, and gears. The experimental outcomes clearly illustrate that IMWMOTE exhibits remarkable advantages in tackling the intricate challenges associated with imbalanced fault modeling, particularly when dealing with small sample heterogeneity. Given its demonstrated proficiency, the proposed approach holds significant potential in guiding interpretable fault modeling efforts in related domains, thus contributing to the advancement of fault diagnosis techniques.

Experimentally verified thermodynamic framework for corrosion

Utilizing the foundational principles of thermodynamics, this study characterizes material degradation resulting from corrosion, substantiating the outcomes through experimental verification. The proposed methodology establishes a unified corrosion model, effectively correlating entropy generation with measurable parameters, including potential difference, corrosion current, and temperature. The Degradation Entropy Generation (DEG) theorem is applied to establish a linear relationship between entropy generation and the degradation rate through a constant material property known as the degradation coefficient (B). Material degradation, quantified by assessing the material loss, is systematically examined with a focus on low-carbon steel (LCS 1018) and Al6061-T6. Corrosion experiments were conducted using a three-electrode electrochemical corrosion cell immersed in a 3.5 wt% NaCl aqueous solution. The degradation parameter, B, for both materials was assessed through multiple accelerated corrosion tests, where various magnitudes of potentials were applied to specimens with differing exposed areas. The presented results unveil a consistent degradation coefficient of (30±2)×10−9 m3K/J for LCS 1018 and (25±2)×10−9 m3K/J for Al6061-T6. To ascertain the generalizability of these findings under diverse environmental conditions, additional experiments are conducted on LCS 1018, exposing it to varying environmental parameters such as different percentages of NaCl solution, acidic conditions, and elevated solution temperatures. Remarkably, the degradation coefficient remains constant across all tested corrosive environments, attesting to its independence. This study offers a novel approach to understanding material degradation through a thermodynamic lens and demonstrates the applicability of the proposed corrosion model in diverse and challenging operational conditions.

"Breaking down in tears, soaked in sweat, and sick from the heat": Media-based composite narratives of first responders working during the 2021 Heat Dome.

During the summer of 2021, a deadly, unprecedented multiday Heat Dome engulfed western Canada. As a result of this extreme heat event (EHE), emergency dispatchers received an unparalleled increase in incoming 911 calls for ambulance, police, and fire (as first responders) services to attend to hundreds of heat-vulnerable community members succumbing to the heat. With 103 all-time heat records broken during this EHE and indoor temperatures of nearly 40°C, the first responders attending these calls faced extensive job demands and highly challenging operating conditions. Initial investigations have explored the health system-level impacts; however, little has been done to explore the impact on the first responders themselves. Therefore, this study aimed to improve our understanding of EHEs' impacts on the operational capabilities and health of first responders, specifically police, fire, ambulance, and dispatch services. A systematized review and content analysis of media articles published on the 2021 Heat Dome in Canada was conducted (n = 2909), and four media-based composite narratives were developed highlighting police, fire, ambulance, and dispatch services. The Job Demands-Resources (JD-R) model was applied as atheoretical framework for occupational burnout. The media-based composite narratives highlighted that first responders faced record-breaking call volumes, increased mental-health-related claims, and exhaustive heat-related physiological stress. Using the JD-R model as a theoretical framework for occupational burnout, we identified three measures of stressful job demand: work overload (e.g., the surge in call volume, firefighters responding to medical emergencies), emotional demands (e.g., severe medical emergencies, sudden deaths, unresponsive patients, distraught family members), and physical demands (e.g., resuscitation in personal protective equipment, heat-related illness). The experiences described underscore the importance of supporting first responders during work in extreme heat conditions. These findings have important implications for addressing rising rates of burnout during and following public health crises, such as EHEs, a problem that is increasingly being recognized as a threat to the Canadian public healthcare system.

“Rooting” engineering strategy for construction of highly stable, scalable, size-customized oxidized graphene/PA6 ultrafine fiber composite nanofiltration membranes for water purification

Nanofiltration membranes based on graphene oxide exhibit substantial potential for practical applications in water treatment, poised to lead the next generation of separation technology. It is noteworthy that the prominent hydration interaction among graphene–graphene oxide nanosheets stands as the primary factor influencing its stability. Simultaneously, the absence of suitable graphene substrates presents challenges to its large-scale production. In this study, our initial endeavor involved constructing a graphene-based selective layer on a PA6 substrate using a quasi-“rooting” strategy. The graphene selective layer was anchored onto the surface of PA6 fibers (with a diameter size of up to 190.0 mm and a selective layer thickness of approximately 5.0 µm). Experimental results reveal that the permeability of the hybrid graphene oxide/PA6 fiber composite nanofiltration membrane consistently maintained within the range of 7.7 to 14.9 L m⁻² h⁻¹ bar⁻1, with a remarkable dye molecule removal rate of 99.06 %. Even under challenging operating conditions (such as strong acid, strong alkali, polar solvent, oxidation, and the coexistence of salt ions), its separation performance remained stable. The permeability of composite membranes was retained between 10.6 and 12.7 L m⁻² h⁻¹ bar⁻1, and the rejection rate of CR dye reached 95.0 % under harsh conditions. Furthermore, the stability and anti-fouling performance of the composite membrane were significantly enhanced. Deposits on the membrane surface could be effectively removed through physical wiping and repeated water rinsing, maintaining membrane integrity even after vigorous ultrasonic cleaning (120 min, 950 W). Additionally, after undergoing 20 cycles of rinsing with deionized water and a 30-day soak in an aqueous solution, the PA6 substrate and graphene selective layer exhibited robust adhesion. No shedding was observed, and the dye rejection rate of the composite membrane consistently exceeded 95 %. This perfectly illustrates the exceptional structural stability and separation durability of the prepared separation membrane. Our work contributes fundamental design principles and theoretical insights for practical applications.

Enhancing the lifetime and vacuum tribological performance of PVD-MoS2 coatings by nitrogen modification

As a solid lubricant, MoS2 is used for reducing friction and wear in machine elements under challenging operating conditions. In application, service life is thus a decisive factor that depends on the coating. Here, the effect of gaseous nitrogen modification on the performance of physical vapor deposited MoS2 coatings has been studied under vacuum conditions. The coatings were modified chemically or in their structural design by varying the bias voltage alternately. Tribologically induced changes were identified in short-running and long-running intermittent ball-on-disc tests, while the lifetime was evaluated in ball-on-disc tests until coating failure, with a coefficient of friction value exceeding 0.3 defined as the failure criterion. Furthermore, the microstructure was investigated before and after tribological loading. It appeared from scanning electronic microscopic imaging, that nitrogen modification reduced the initial degree of porosity and material compacting occurred after a short tribological loading. Nanoindentation measurements in the wear track of the compacted coating supported these results by a significant increase in indentation hardness and indentation modulus. Overall, the improvement in lubrication performance due to nitrogen modification led to a significant extension of service life, whereas the chemical and structural modification exhibited the highest service life of the coating.

Performance and Durability of Next Generation Automotive Membrane Electrode Assemblies in Automotive Short Stacks

The space for polymer electrolyte membrane fuel cell (PEMFC) systems for the application especially in small cars is a limiting factor. Hence, high power density at high current density is required to fulfill the power demand and space requirements of a fuel cell system when installed into a car. Additionally, the fuel cell operation at elevated temperature supports this requirement facilitating smaller cooling systems. Hence, within the EU funded project GAIA (next Generation AutomotIve membrane electrode Assemblies), a consortium of leading OEMs, industrial partners and academic/research partners/organizations/institutions aimed to develop well beyond state of the art membrane electrode assemblies (MEAs) for high power application at elevated operating temperature.Therefore, electrocatalysts, membranes, ionomers, gas diffusion and microporous layers were developed with respect to minimizing the resistances by improving their interfaces. The potential of the developed next generation MEAs was determined using automotive short stacks aiming a beginning of life power density of 1.8 W/cm2 at 0.6 V. The durability target was defined with less than 10 % voltage loss of the beginning of life performance after 6000 operating hours (extrapolated from a testing time of 1000 h). For verification of the durability, an automotive drive cycle was adapted from real life driving data and applied for durability determination. This drive cycle contained very challenging operating conditions such as e.g., an applied current density up to 3 A/cm2 and an increased operating temperature of 105 °C at the stack outlet.In this study, the project achievements in performance and durability at high power and high temperature operation of the developed next generation automotive MEAs were demonstrated within automotive short stacks. During the projects progression, the application of a newly developed gas diffusion layer with increased degree of graphitization and a microporous layer with large pore structure significantly increased the performance at high current density. The stability of the catalyst coated membrane (CCM) was drastically increased by a membrane with a PBI-X reinforcement developed within the project, which replaced a standard PTFE reinforcement. Additionally, a new ionomer with increased high temperature stability was implemented that inhibited ionomer migration at high temperatures. The addition of a radical scavenger to the catalyst layer inhibited the destruction of the CCM by radicals as observed at high temperature operation with the state-of-the-art material. Finally, besides the improvement of the MEA material properties, the fuel cell operating parameters were optimized in-situ using an automated optimization algorithm to find the optimum operation parameters for the newly designed MEA. This optimization demonstrated a significant contribution to the achievement of the performance and durability target as it allowed the operation of the developed MEA at its “sweet spot”.

Numerical simulation method and structural optimization for shearing capacity of ram blowout preventers

With the increasing demand for energy in the country, there has been a notable rise in the deployment of deep and ultra-deep wells. Consequently, drilling operations have become more complex, with greater intricacies in drilling techniques, challenging operating conditions, and varying geological factors. The size of the drill pipes has grown larger, and higher steel grades are being used, surpassing the design requirements of conventional ram blowout preventers. As a result, issues have arisen, including inadequate shear force of the ram blowout preventer to cut through the drill pipes and the potential for cut debris from the drill pipes to fall and damage the sealing elements, compromising the sealing integrity and posing a risk of blowouts.To address these issues encountered in the field, a shear analysis model for the ram blowout preventer was established based on the Johnson-Cook constitutive model and the Shear Damage fracture criterion. The reliability and accuracy of the simulation were validated through comparative experiments between simulation and laboratory trials. Furthermore, the structure of the ram blowout preventer was optimized to mitigate these challenges. Additionally, the response surface method was employed to optimize the parameters of the novel ram structure for the ram blowout preventer, followed by indoor experiments using the selected parameters. The experimental results demonstrated a significant improvement in both the shear performance and sealing capability of the novel ram blowout preventer compared to the conventional ram blowout preventer design. This advancement provides valuable insights and practical reference for on-site operations.

Packed bed thermal energy storage for waste heat recovery in the iron and steel industry: A cold model study on powder hold-up and pressure drop

Waste heat recovery in the energy intensive industry is one of the most important measures for the mitigation of climate change. The present study examines the integration of a packed bed thermal energy storage for waste heat recovery in the iron and steel industry. Along with the highly fluctuating availability of excess heat the main difficulty of waste heat recovery in industrial processes is the high amount of powder that is transported by the hot exhaust gases. Therefore, the experimental investigations in this study focus on the powder hold-up and pressure drop in a packed bed thermal energy storage that is operated with a gas-powder two phase exhaust gas as heat transfer fluid. The ultimate goal is, to assess its suitability and robustness under such challenging operational conditions. The results indicate, that 98% of the powder that is introduced into the system with the heat transfer fluid during charging accumulates in the packed bed. Remarkably, most of the powder hold-up in the packed bed is concentrated near the surface at which the heat transfer fluid enters the packed bed. When reversing the flow direction of the heat transfer fluid to discharge the storage with a clean single phase gas, this gas is not contaminated with the powder that has been accumulated in previous charging periods. The entirety of these findings reinforces the suitability of packed bed thermal energy storage systems for waste heat recovery in the energy intensive industry.

Material Influence on the Micropitting and Wear Resistance of Nitrided External and Internal Gears

Abstract Due to increasing performance requirements and sometimes tribologically challenging operating conditions in gears (such as those found in wind turbines), alternative heat treatment processes to quenching and tempering or case hardening are becoming more and more important. Nitriding can significantly increase the load-carrying capacity of gears compared to the quenched and tempered initial state. The nitrided surface layer is characterized by a hard compound layer on the surface that is just a few micrometers thick and a diffusion zone that extends deeper into the material. The structure of the compound layer of iron and alloy element nitrides is primarily decisive for the wear behavior of nitrided components. Previous studies on nitrided gears with regard to micropitting and wear focus primarily on a systematic variation of the compound layer on gears made of nitriding steel 31CrMoV9. Results on the influence of the base material on the micropitting and wear resistance of nitrided gears are not yet available. In systematic load and speed stage tests, nitrided gears made from a quenched and tempered steel (42CrMo4) and a material with an alternative alloy composition (34CrNiMo6) were examined in addition to the reference (nitriding steel 31CrMoV9). Both external and internal gear pairings were tested, with the alternative materials showing more favorable tribological behavior in some cases. The results are presented and categorized according to the current state of the art. Finally, recommendations for the practical use of nitrided gears are derived.

Performance of an automotive reversible CO2 heat pump system during periodic frosting-defrosting cycles

Electric vehicles (EVs) have seen rapid growth in the global market share due to the demand for low-emission commuting. However, EVs face the challenge of range anxiety in cold weather due to the capacity degradation of lithium-ion batteries and the increased demands of cabin and battery heating. Heat pump (HP) technology improves the heating efficiency compared to resistive heating, but in cold and humid weather, frost can grow on the outdoor evaporator and reduce the efficiency of the HP system. In this paper, experiments are carried out to assess the impacts of ambient temperature and relative humidity (RH) on the capacity and efficiency of a transcritical CO2 HP system during the continuous frosting-defrosting cycles under some challenging operating conditions (ambient temperature below 0 °C and RH over 80%). Moreover, the mass of frost accumulated during the frosting period, and that of water retained, drained, and vaporized during the defrosting period are measured and analyzed. The results show that when the ambient temperature decreases from 0 to −10 °C, the peak value of the heating capacity in the first frosting or HP mode period reduces from 4.85 to 3.54 kW, and the coefficient of performance (COPhp) drops from 1.61 to 1.47, but the operating time of the first frosting period significantly increases from 32 to 108 min. Similarly, lowering RH from 90% to 80% at 0 °C also helps to extend the operating time of the frosting period in one frosting-defrosting cycle from 32 to 84 min. In addition, with the current commonly used defrost initiation criterion, which is the air-side pressure drop across the outdoor evaporator increasing by five times, the heating capacity and system efficiency drop by less than 10% at the end of one frosting period. This indicates the potential for a longer operating time of the HP system which could reduce the undesirable impact on the passengers’ comfort during the reversed cycle defrosting.

Comparative analysis of thermal and non-thermal discharge modes on ultra-lean mixtures in an optically accessible engine equipped with a corona ignition system

Radio-Frequency corona-based ignition systems are currently being investigated by the engine research community for their capability to ensure robust combustion at challenging operating conditions such as lean mixture and/or high exhaust gas recirculation (EGR) dilution. The low-temperature plasma (LTP) produced by the corona discharge accelerates the early flame development thanks to the production of combustion precursors like radical and excited species. Plasma benefits can be maintained with extensive management of the control parameters, preventing the transition from the non-thermal discharge mode to a thermal one.However, under ultra-lean conditions, where the ignition probability significantly decreases, the LTP discharge may not be suitable for promoting the combustion process because of insufficient thermal energy transfer and scarcity of available fuel, which is essential for radical production. Therefore, it may be necessary to increase the volume in which the plasma interacts to ensure robust ignitions and stable flame propagations. For this reason, the present work aims to assess and quantify the differences, in terms of combustion performance, between a corona igniter operating at LTP mode and with electric arcs occurring, at ultra-lean operating conditions. In arc-mode, the igniter produces volumetric discharges, facilitating extensive mixing of the mixture and resulting in advantageous effects on kernel formation and propagation. Tests were carried out in a single-cylinder optical access engine under low load and low speed, using gasoline as fuel. Close to the leanest stable limit found, the LTP mode shows a lower cycle-to-cycle variability. Above such a threshold, the thermal discharge can guarantee stable combustion events while the LTP discharge leads to unstable working conditions. Indicating, imaging, and pollutant emission analysis are carried out to evaluate the effects of thermal and non-thermal plasma modes on combustion characteristics and their environmental impact.