Abstract. Thermal management of turbine airfoils is a critical design consideration, but the impact of unsteadiness on heat transfer of attached flow regions has received less attention in the literature. When turbine surfaces are subjected to unsteady zero-mean flow fluctuations, either naturally or artificially, the mean velocity around them is modified due to a nonlinear interaction of fluctuations, known as streaming. In this numerical study, we examine the effect of streaming on heat transfer and skin friction in a simplified model of the flow over a turbine blade. Both heat transfer and skin friction modifications were found to strongly depend on the amplitude and wave speed of the unsteady flow perturbations. Over a wide range of disturbance parameters, skin friction modification was negligible, but a significant effect on heat transfer due to streaming was identified. Moreover, the impact of favorable pressure gradients, which are typical for turbine airfoils, on the streaming phenomena was also considered, and it was found that flow regions of zero-pressure gradient produced the strongest amplification of heat transfer, although the effect of the pressure gradient varied with Strouhal number. Due to its significant effect on wall heat transfer, the streaming phenomenon should be taken into account during the design and measurement of the thermal properties of unsteady systems.

Environmental concerns arising from the increasing use of polluting plastics highlight polylactic acid (PLA) as a promising eco-friendly alternative. PLA is a biodegradable polyester that can be produced through the fermentation of renewable resources. Together with its excellent properties, suitable for a wide range of applications, the use of PLA has increased significantly over the years and is expected to further grow. However, insufficient degradability under natural conditions emphasizes the need for the exploration of biodegradation mechanisms, intending to develop more efficient techniques for waste disposal and recycling or upcycling. Biodegradation occurs through the secretion of depolymerizing enzymes, mainly proteases, lipases, cutinases, and esterases, by various microorganisms. This review focuses on the enzymatic degradation of PLA and presents different enzymes that were isolated and purified from natural PLA-degrading microorganisms, or recombinantly expressed. The review depicts the main characteristics of the enzymes, including recent advances and analytical methods used to evaluate enantiopurity and depolymerizing activity. While complete degradation of solid PLA particles is still difficult to achieve, future research and improvement of enzyme properties may provide an avenue for the development of advanced procedures for PLA degradation and upcycling, utilizing its building blocks for further applications as envisaged by circular economy principles.

Implementation of homogeneous advanced oxidation processes (HAOPs) catalysed by Cu and Ag metals is limited due to high residual concentrations of oxidants and dissolved metals. The proposed post-treatment process starts with recirculation of H₂-enriched HAOP-treated water through the Pt-loaded activated carbon (AC). Hydrogen oxidation reaction (HOR) on Pt results in degradation of oxidants while silver and/or copper ions are reduced and deposited on the catalytic media. The subsequent aeration of the catalyst results in oxygen reduction reaction (ORR) and oxidation of the metals back into ionic forms. This allows the homogeneous metal catalysts to be reused in multiple HAOP cycles.

The capacitive-Faradaic electrochemical structure of the Pt/AC catalyst with Pt acting as a Faradaic electrode and AC as a capacitive electrode plays an important role in the process. The HOR on Pt/AC results in accumulation of electrons in electrical double layer of AC. These electrons can be effectively used in a subsequent H₂-free reduction of metal ions and oxidants. Thus, the degradation of H₂O₂ and S₂O₈²⁻ and reduction of Cu²⁺ and Ag ⁺ ions proceed much faster if the H₂-step of the process is split into two sub-processes: (i) hydrogenation of Pt/AC catalyst in an inert Na₂SO₄ solution to pre-charge the Pt/AC with electrons originating at HOR, followed by (ii) catalytic reduction of oxidants and metal ions by the capacitively stored electrons.

The process was investigated using Ag–S₂O₈²⁻ ([S₂O₈²⁻]₀ = 25 mM; [Ag⁺]₀ = 50 mg/L) and Cu–H₂O₂ ([H₂O₂]₀ = 100 mM, [Cu]₀ = 635 mg/L) solutions. Total degradation of oxidants was achieved within <2 h and complete metals’ recovery was obtained. Several cycles of HAOP experiments were performed for the degradation of 1,4-dioxane (10 mg/L) and phenol (25 mg/L) using Ag– S₂O₈²⁻ and Cu–H₂O₂ HAOPs, respectively. The residual oxidants were completely degraded in all experiments. The reactivation of Pt/AC catalysts inhibited by phenol derivatives was achieved by hydrogenation in Na₂SO₄ solution (i.e., using the pre-charge approach).

Plasma-assisted reforming shows great promise for making ammonia compatible with current engine technology, with minimal modifications required. This study examines the impact of various discharge, flow, and composition parameters on ammonia conversion into hydrogen using a new well-stirred homogeneous plasma reactor. Gas chromatography was used to measure hydrogen and oxygen, while ICCD imaging evaluated plasma homogeneity. Initial mixture composition significantly influences the response to plasma conditions. Relative to the initial ammonia composition, the conversion of pure ammonia yields a lower hydrogen percentage when compared to air-diluted mixtures. Introducing air changes the relationship between pulsed energy deposition and hydrogen production, resulting in a more substantial increase in conversion. Efficiency calculations demonstrate that high ammonia content and high pulse frequencies improve yields, while longer residence times in the reactor result in higher temperatures, also increasing yields. These findings provide insights into the underlying mechanisms of ammonia cracking and reforming in a discharge, and will serve to promote further investigation into plasma reforming strategies.

The impacts of the pulse repetition frequency (PRF), number of pulses, and energy per pulse in a train of nanosecond discharge pulses on the ignition of a flowing lean premixed methane–air mixture are investigated using numerical simulations. A phenomenological plasma model coupled with a compressible reacting flow solver is used for these simulations. The simulation strategy has been well validated by comparing the experimental schlieren and OH planar laser induced fluorescence (PLIF) results with the numerical schlieren (i.e., density gradient) and OH density profiles, respectively. The characteristics of the ignition kernels produced by each discharge pulse and their interaction with each other as functions of the PRF are investigated. Three regimes were defined in the literature based on this interaction of the ignition kernels — fully coupled, partially coupled, and decoupled. This study uses numerical simulations to probe into the constructive and destructive effects, that ultimately determine ignition success, in these different regimes. The complete overlap of kernels and the complete lack of synergy between kernels produced by consecutive pulses are attributed to the success and failure of ignition and flame propagation in the fully coupled and decoupled regimes, respectively. In the partially coupled regime, the convection heat loss driven by the shock-turned-acoustic wave of the next discharge pulse, on the kernel produced by the previous discharge pulse, in addition to diffusion losses, contribute to ignition failure. However, the expansion of the next kernel in a region of higher average temperature and radical concentration created by the previous kernel could help to bridge the gap between the two kernels and result in successful ignition. The important parameters of energy per pulse, number of pulses, and equivalence ratio affect the competition between these constructive and destructive effects, which eventually determines the ignition success in this regime. Finally, the change in the nature of interaction between consecutive kernels from decoupled to partially coupled, at the same frequency but with different energies per pulse, is also shown.

Electricity consumers face challenges in selecting an optimal energy-saving plan, and this is a sustainability problem. To set consumers focus on sustainable energy management, developments around ”Industry 4.0” are needed to achieve an optimal balance between cost and energy consumption with a focus on cutting-edge machine-learning models and smart services introduction. Energy modelling is crucial for confronting the challenges introduced by the energy transition. This research is to promote digital twin (DT) technology in smart energy grids. The hybrid digital twin model is developed based on smart grid end-users’ electrical components. This approach emulates the component specification of the reference smart grid system and encourages a reduction in net energy consumption. Additionally, the application of AI connects smart meters, IoT devices, and the assets of the smart grids to the DT recreation of demand-side end-users for energy efficiency recommendation provision; this setup improves energy management, energy efficiency, and the usage of renewable energy resources. The novelty of this study is that the recent scope of demand-side recommendation schemes has been extended in the direction of cutting-edge Industry 4.0 hybrid DTs. Additionally, a prototype system and its functional characteristics have been proposed with the potential to pave the path for utilization in microgrid environments. Simulation results show a unique level of parallelism between the reference system and the developed model, along with a 36.8% reduction in net energy consumption following implementation of the recommendation advice.

An experimental wind tunnel study was performed to investigate the viability of incompressible turbulent boundary layer skin friction drag modification by temporal acceleration and deceleration, motivated by similar pipe and channel flows studies. The acceleration and deceleration phases were achieved by rapidly opening and closing a set of downstream louver vanes, and it was observed that wind tunnel compressibility effects are substantial despite conditions for incompressibility being met. Wall shear stress and dynamic pressure associated with the deceleration phase, exhibited first-order-type responses, but with significantly different time-constants, leading to an apparent inverse skin friction coefficient response. This was contrasted with pipe and channel flows that exhibit under-damped-type responses. The acceleration phase also exhibited this apparent inverse skin friction coefficient response, but did not subsequently produce the well-known relaminarization undershoot, due to a weaker and insufficiently-sustained acceleration. An equivalent local incompressible pressure gradient parameter was proposed to explain this observation. Cycle-averaged drag reduction can potentially be achieved by quasi-steady acceleration, followed by rapid deceleration for the remainder of the cycle.

This paper investigates mixing and combustion of liquid fuel in a model scramjet combustor with strut-assisted injection adjacent to a cavity flame holder. Fuel/air mixing in a Mach 2.2 cross-flow and stagnation temperature of 1300 K was studied using filtered IR imaging. The insight gained by this method is discussed along with the method limitations. The impact of the strut assisted injection on the mixing of the liquid fuel was seen by the shorter vaporization distance, as the fuel vaporized upstream of the cavity, in contract to a liquid jet that expanded above most of the cavity when the strut was absent. Moreover, the fuel jet was seen to propagate laterally to the tip of the strut, which lead to lateral dispersion of the fuel up the strut height. The combustion experiments were conducted by piloting the cavity with ethylene and were imaged using CH* chemiluminescence. At stagnation temperature of 1300 K thermal choking and stable combustion occurred at equivalence ratio of 0.43, with the flame stabilizing at the strut wake, above the cavity. Intermittent thermal choking occurred at leaner condition. The flame stabilized on the cavity shear layer when the shock train moved downstream of the cavity and the flame stabilized at the strut wake when the shock train moved upstream. Thermal choking with the strut guided injection relative to a combustion only in the cavity without the strut is the result of the observed higher mixing efficiency, that enabled flame spreading into the supersonic flow and higher heat release. At a higher stagnation temperature of 1500 K the shock train moved downstream of the cavity, and the flame stabilized on the cavity shear layer. Finally, it is shown that the combustion efficiency could be further improved by injecting the fuel from the strut side walls.

Nitrogen fixation is a key process that sustains life on Earth. Nitrogenase is the sole enzyme capable of fixing nitrogen under ambient conditions. Extensive research efforts have been dedicated to elucidating the enzyme mechanism and its artificial activation through high applied voltage, photochemistry, or strong reducing agents. Harnessing light irradiation to minimize the required external bias can lower the process's high energy investment. Herein, we present the development of photo-bioelectrochemical cells (PBECs) utilizing BiVO₄/CoP or CdS/NiO photoanodes for nitrogenase activation toward N₂ fixation. The constructed PBEC based on BiVO₄/CoP photoanode requires minimal external bias (200 mV) and suppresses O₂ generation that allows efficient activation of the nitrogenase enzyme, using glucose as an electron donor. In a second developed PBEC configuration, CdS/NiO photoanode was used, enabling bias-free activation of the nitrogenase-based cathode to produce 100 μM of ammonia at a faradaic efficiency (FE) of 12%. The ammonia production was determined by a commonly used fluorescence probe and further validated using ¹H-NMR spectroscopy. The presented PBECs lay the foundation for biotic-abiotic systems to directly activate enzymes toward value-added chemicals by light-driven reactions.

Brownian motors are nanoscale machines that utilize asymmetric physical interactions to generate directed motion in space. The operation mechanism relies on the random motion of nanoscale elements generated by thermal activation. On the other hand, structural superlubricity (SSL) refers to a state of nearly vanishing friction due to structural mismatch between sliding interfaces. Van-der-Waals layered materials, such as graphene are of particular interest in this regard as they exhibit atomically flat surfaces and weak interlayer interaction. In particular, the sliding barrier in these systems turned out to be extremely sensitive to temperature, leading to the observation of thermal lubrication at elevated temperatures. Herein, the unique combination of a carefully designed tilted periodic potential landscape and virtually zero friction in incommensurate 2D layered systems are used to realize a mesoscopic superlubric Brownian machine. In particular, we perform mechanical shearing of superlubric graphite contacts to examine the influence of velocity on friction and adhesion. Our results show that while friction is virtually independent of velocity below 2500 nm*sec-1, the conservative adhesion force increases by ~ 10 % with respect to the lowest measured velocity. Intriguingly, a greater amount of energy can be collected by the system once the retraction velocity is set above the protraction velocity. Our numerical calculations based on force field modelling indicate that a slow adiabatic sliding allows to utilize the available thermal energy to reduce the adhesion in agreement with our experimental observations. As a result, we demonstrate a mesoscale Brownian motor that can harvest thermal energy by adjusting the forward and backward sliding velocities and can pave the way for macroscale directed motion and energy harvesting.