Wetted Surface Area Research Articles

Optimising hydrofoils using automated multi-fidelity surrogate models

ABSTRACT Lifting hydrofoils are gaining importance, since they drastically reduce the wetted surface area of a ship, thus decreasing resistance. To attain efficient hydrofoils, the geometries can be obtained from an automated optimisation process. However, hydrofoil simulations are computationally demanding, since fine meshes are needed to accurately capture the pressure field and the boundary layer on the hydrofoil. Simulation-based optimisation can therefore be very expensive. To speed up the fully automated hydrofoil optimisation procedure, we propose a multi-fidelity framework which takes advantage of both an efficient low-fidelity potential flow solver dedicated to hydrofoils and a high-fidelity RANS solver enhanced with adaptive grid refinement and dedicated foil-aligned overset meshes, to attain high accuracy with a limited computational budget. Both solvers are shown to be reliable for automatic simulation, and remarkable correlation between potential-flow and RANS results is obtained. Two different multi-fidelity frameworks are compared for a realistic hydrofoil: only RANS based and potential-RANS based. According to the optimisation results, the drag is able to be reduced by 17% and 8% in these frameworks, within a realistic time frame. Thus, industrial optimisation of hydrofoils appears possible. Finally, critical areas of future improvement regarding the robustness and efficiency of the optimisation procedure are discussed in this study.

Influence of Electrolyte Saturation on the Performance of Li-O2 Batteries.

Electrolyte saturation can strongly affect the Li-O2 battery performance. However, it is unclear to what extent saturation reduction will impact the battery capacity. In this study, we investigated the influence of electrolyte saturation and distribution within a porous positive electrode on the deep discharge-charge capacities and cycling stability. The study used both models and experiments to investigate the change of electrolyte distribution, double-layer capacitance, ohmic resistance, and O2 concentration in the positive electrode at different electrolyte saturations. Results revealed that electrodes with 60% electrolyte saturation achieved almost the same maximum discharge (6.38 vs 6.76 mAh/cm2) and charge (5.52 vs 5.65 mAh/cm2) capacities with fully saturated electrodes. The partially wet positive electrode (40% saturation) obtained more cycles than the electrode with 100% saturation before the discharge capacity dropped below the cutoff point. However, the electrode with 40% saturation had a low average charging efficiency of 88.76%, whereas the fully saturated electrode obtained 98.96% charging efficiency. Moreover, the fully wet positive electrode had the lowest overpotential during cycling (1.26-1.39 V). The measured electrochemically active surface areas showed that even 40% saturation could sufficiently wet the positive electrode surface and obtain a double-layer capacitance (18.12 mF) similar to that with 100% saturation (20.4 mF). Furthermore, a considerable increase in O2 concentration at wetted surface areas was observed for the electrolyte saturation of less than 60% due to the significantly higher O2 diffusivity in the gas phase.

Study on resistance performance of hexagonal hull form with variation of angle of attack, deadrise, and stern for flat-sided catamaran vessel

Abstract The flat-sided hull vessel was designed with the purpose of simplifying hull production by eliminating the requirement for fairing, bending, and assembly of curved panel lines. Nevertheless, implementing the flat hull concept may marginally increase the hull resistance. The objective of the study is to investigate the impact of the hexagonal hull concept on resistance characteristics when altering the deadrise angle, angle of attack, and stern angle. Additionally, the resistance performances of the proposed hexagonal hulls were compared to evaluate the influence of the hull design geometry parameter. The contribution of this work is to identify the configuration of the design parameters to achieve an ideal level of resistance in a hexagonal catamaran hull design. The findings indicate that a greater angle of attack could potentially enhance wave generation and increase the resistance. Despite the increase in the wave resistance coefficient caused by the increment in deadrise angle, the overall resistance was reduced due to the significant reduction in wetted surface area resulting from the greater deadrise angle. As a result, the decrease in friction resistance was more pronounced than the increase in wave resistance. At last, the stern angle can cause a substantial oscillation in the curved line, resulting in a large increase in wave generation and overall resistance, particularly at high speeds.

Numerical Analysis and Experimental Validation of Trimaran and Pentamaran Resistance at Various Separation Distances

Trimaran and pentamaran are multihull types with an odd number of hulls, namely three and five hulls, generally consisting of one center hull and two or four side hulls smaller than the center hull, which can reduce ship resistance. The trimaran and pentamaran have a more complex phenomenon than the monohull, because of the interaction between the main hull and the side hull, which causes interference caused by changes in flow velocity, pressure changes, and wave interactions generated by each hull. The objective of this study is to analyze the resistance of trimaran and pentamaran NPL-4b models with transom-symmetrical hull and separation distances, specifically S/L ratios of 0.2, 0.3, and 0.4. Since a more limited base of experience exists for multihull ships, experimental or numerical modeling techniques are essential for designers. Numeric investigations were conducted using Numeca software, and experiments were performed in a towing tank. Both methods follow ITTC procedures. Furthermore, the numerical analysis with CFD simulation modifies the Navier-Stokes equation using the turbulence model k-ꞷ SST to generate the RANS equation so that unsteady fluid flow problems can be calculated. It can be implemented in predicting resistance in Froude numbers (Fr) 0.2 to 0.6 at the systematic series of trimaran and pentamaran hull shapes at the model scale. The simulation results were compared to experimental data to validate the resistance of the ships. Overall, good resistance was produced by the trimaran and pentamaran in the C configuration at an S/L ratio of 0.4 with a total resistance coefficient CT of 6.60 x 10-3 and 7.37 x 10-3, respectively. The two results are in good agreement, both methods have a discrepancy of 3.70 %. Fluctuations influence the resistance in the wetted surface area (WSA), wave interactions between hulls, ship velocity, and wave propagation in the aft hull.

Effects of Homogeneous and Inhomogeneous Roughness on the Ship Resistance

Ship hull roughness can significantly increase the ship resistance. The roughness caused by biofouling attached to the ship hull is not uniform but has a random distribution. The purpose of this study is to investigate how the inhomogeneous surface roughness distribution affects the ship resistance and the various resistance components. The KRISO container ship (KCS) is considered as a case study. To model the inhomogeneous surface roughness, the ship hull is divided into three segments with equal wetted surface area. Combinations of three roughness heights, denoted as P, Q, and R with ks values of 125 μm, 269 μm, and 425 μm, respectively, are considered to obtain homogeneous and inhomogeneous roughness arrangements (PPP, QQQ, RRR, PQR, PRQ, QPR, QRP, RPQ, and RQP). CFD method is utilized in this study, utilizing RANS equations and k-ω SST turbulence model. A VoF method is used to model the free surface. CFD simulation results show that for the homogeneous roughness, the total resistance coefficient CT increases with increasing ks (PPP < QQQ < RRR), as expected. For the inhomogeneous roughness, the friction resistance coefficient CF increases in the order PQR < PRQ < QPR < QRP < RPQ < RQP, consistent with results from earlier studies. In all the cases, the friction resistance (CF) is the dominant component of the total resistance. As Re increases from 2.2 x 109 to 2.7 x 109, the percentage of the friction resistance decreases, while the percentage of the wave resistance increases. The viscous-pressure resistance decreases slightly as Re increases from 2.2 x 109 to 2.7 x 109.

Enhancing Efficiency in Deep-V Planing Hull Ships: A Study on The Design of Spray Strips to Minimize Resistance

Spray strips are deflectors installed on the hull to decrease the wetted surface area (WSA). By reducing the WSA, the overall resistance of the ship is lowered because it lessens the friction of water flowing against the hull. The purpose of this research is to predict the function of spray strips in an effort to improve ship performance. In this study, the numerical methodology employs the Finite Volume Method (FVM) along with the Reynolds-averaged Navier-Stokes equation (RANS) for resolving fluid dynamics issues. The modeling of the two-phase flow involves the utilization of both the Volume of Fluid (VOF) model and the Eulerian model. The results of this study show that spray strips can reduce the total resistance by about 2.7%. Increasing the number of spray strips can reduce the drag shear value by 3.5%. Spray strips are effective in reducing total drag when the Froude number (Fr) is greater than 1 or when the vessel is in the planing phase. The profile shape of spray strips with dimension 2:1 shows the best performance on ship resistance.

In Situ Quantification of a Wetted Surface Area during Scanning Electrochemical Cell Microscopy Using Retraction Curves

In Situ Quantification of a Wetted Surface Area during Scanning Electrochemical Cell Microscopy Using Retraction Curves

Enhancing the productivity of pyramid solar still utilizing repurposed finishing pads as cost-effective porous material

The primary objective of the current research is to augment the potable water yield of the solar still (SS). This objective is achieved by integrating a used brushed refinishing pad (BRP) as a porous material (PM) in the absorber basin. This integration maximizes the wet surface area due to the pad's porous nature, thereby enhancing photothermal absorption, which is the process where a material absorbs light and converts it into heat. An experimental investigation was conducted to analyze the effect of porous materials in a rectangular pyramid solar still (RPSS) regarding potable water yield and feasibility. The results were compared with those of the conventional solar still (CSS), revealing a noteworthy enhancement in the production of potable water with RPSS-PM, showing a 47.7 % increase on day one and a 48.1 % increase on the second day with a mere 10 % rise in basin water temperatures. The energy efficiency of RPSS-PM improved significantly by about 9 % on both days compared to CSS. From an economic perspective, this system's payback period (PBP) was 5.2 months, compared to 6.1 months for the CSS. Furthermore, the cost per liter (CPL) of potable water produced by the RPSS-PM was 16.6 % lower than that of the CSS. This innovative approach holds great potential for effectively addressing challenges related to water scarcity.

A model of racing kayak surge motion a numerical approach with realistic oscillating propulsion forces

This study presents a novel forward dynamics model of racing kayak surge motion. The model, driven by adjustable oscillating propulsion forces, replicates true paddle stroke forces. It is based on Newton’s second law of motion, with the net force comprising of propulsion force, skin friction drag, wave-making drag, form drag, and air drag. The total mass includes the kayak, paddler, and added mass. The fourth-order Runge–Kutta method is used to solve for speed and distance. The model incorporates a unique hull formulation for wetted surface area estimation, Michell’s integral for wave-making drag, and a log wind profile with allometric scaling for air drag. It accounts for varying stature height, body mass, kayak dimensions, wind speed, and stroke propulsion profiles. The model is a significant advancement in its domain, with broad applicability for the future. It aligns with existing data and analyses, but validation data is limited.

Measuring the Electrolyte Distribution in Silver Gas Diffusion Electrodes during CO2 Electroreduction Using Operando Synchrotron Tomography

Electrochemical CO2 reduction (eCO2R) has emerged as a promising technology to support the shift away from fossil fuels in the chemical industry. When coupled with renewable energies it allows for an overall carbon-negative process, while enabling CO2 as a carbon source for the production of commodity chemicals. Efficient eCO2R requires gas diffusion electrodes (GDEs) that provide a large wetted surface area inside the porous system, while also preserving gaseous diffusion pathways for CO2 mass transport through their hydrophobicity. Under the high cathodic overpotential in eCO2R, electrowetting diminishes the hydrophobicity of the GDE and can lead to excessive flooding of the porous system [1]. This severely hinders the CO2 mass transport and thus gives rise to the competing hydrogen evolution reaction (HER), lowering the Faradaic efficiency of the process. Therefore, understanding the flooding behavior of GDEs is of major interest for further development of the process.Mathematic models can be applied here to describe the flooding state under the influence of electrowetting and elucidate its influence on the gas transport through the GDE. The validation of such models requires direct experimental measurements of the electrolyte saturation during eCO2R, which can be accomplished with the use of synchrotron radiation [2].In this work, experimental results for the direct measurement of the electrolyte saturation during eCO2R in sprayed silver GDEs are presented. In a first, these measurements were obtained in operando synchrotron tomography experiments, utilizing a high beam energy to penetrate through the 3 mm diameter GDE in the in-plane direction. The results indicate a higher beam absorption towards the gas side of the GDE, as well as higher absorption at increased current density. Key to the interpretation of these results is the disentanglement of the effects of concentration increase due to migration in the electric field and the actual increase in electrolyte saturation. The experimental results are used to validate a continuum model for a sprayed silver GDE in eCO2R which includes the effect of electrowetting on flooding behavior by introducing capillary pressure – saturation and contact angle – potential correlations [3]. A good agreement between model and tomography results is reached, showing an interplay between concentration increase and flooding effects.[1] Bienen, F., Paulisch, M. C., Mager, T., Osiewacz, J., Nazari, M., Osenberg, M., Ellendorff, B., Turek, T., Nieken, U., Manke, I., & Friedrich, K. A., Investigating the electrowetting of silver‐based gas‐diffusion electrodes during oxygen reduction reaction with electrochemical and optical methods. Electrochemical Science Advances (2022) e2100158.[2] Hoffmann, H., Paulisch, M. C., Gebhard, M., Osiewacz, J., Kutter, M., Hilger, A., Arlt, T., Kardjilov, N., Ellendorff, B., Beckmann, F., Markötter, H., Luik, M., Turek, T., Manke, I., & Roth, C., Development of a Modular Operando Cell for X-ray Imaging of Strongly Absorbing Silver-Based Gas Diffusion Electrodes. Journal of The Electrochemical Society 169 (2022) 044508.[3] Osiewacz, J., Löffelholz, M., Turek, T., Modeling the Influence of Electrolyte Distribution in Silver Gas Diffusion Electrodes for CO2 Electroreduction. ECS Meeting s 01 (2023) 1727. Figure 1

Experimental study of a high-speed suspension hexamaran in calm water

This research proposed a high-speed hexamaran with a suspension system composed of springs and joints as a new type of multihull. To investigate its hydrodynamic characteristics in calm water, numerous model tests for two programs were carried out for the volumetric Froude number from 0.6 to 7.68. In one program, known for flexibility, there is a hexamaran equipped with a suspension system, while in the other program, known for rigidity, there is a hexamaran with no suspension system, but clamps instead of springs. Motion parameters like trim angle, dynamic sinkage, wetted surface, hydrodynamic lift, and resistance were determined, and the flow field was further analyzed. Based on the results of comparison experiments, it can be concluded that the suspension system of the hexamaran can change the ratio of each resistance component in the total resistance, improve the motion attitude and increase the wetted surface area at high speeds, which can avoid porpoising behavior and improve longitudinal stability. The high-speed hexamaran with a suspension system performs better than that with no suspension system when traveling at high speeds.

Statistical modeling and multi-objective optimization of a flat tubular indirect evaporative cooler for enhanced performance

The flat tubular indirect evaporative cooler complements the conventional round tubular indirect evaporative cooler owing to its superior wetting surface area and enhanced heat transfer capacity. Given the multitude of influencing parameters, achieving a comprehensive analysis and optimization of the flat tubular indirect evaporative cooler demands a significant number of experimental tests or numerical simulations. Consequently, this study seeks to employ statistical techniques for constructing a precise and expeditious performance prediction model for the flat tubular indirect evaporative cooler. Nine pivotal parameters were selected to evaluate their impact on the cooling performance of the flat tubular indirect evaporative cooler. A numerical model was developed and verified experimentally. The response surface method was applied to establish polynomial regression equations for the cooling performance prediction of flat tubular indirect evaporative coolers. In addition, this work proposed a multi-objective optimization strategy for the proposed cooler. By innovatively using weighting factors to assign different weights to response values, multi-objective optimization with preferences was performed for the flat tubular indirect evaporative cooler with different demands to increase the applicability of the proposed cooler. The optimization results showed that when only the optimized wet-bulb efficiency was considered, the wet-bulb efficiency can reach up to 81.1%. When optimizing both the wet-bulb efficiency and the coefficient of performance simultaneously, the coefficient of performance can be up to 58.0, and the wet-bulb efficiency was 64.7%.

Small sea with high traffic - what is the biofouling potential of commercial ships in the Baltic Sea

Despite the Baltic Sea being one of the most intensive shipping regions in the world the potential magnitude of the biofouled hulls in this region is unknown. This study estimated the biofouling load to Baltic Sea Region (BSR) based on the wetted surface area (WSA) method with regard to country, ship type and donor bioregion. WSA flux reached 656 km2, of which 86% was associated with ships operating inside and 14% was WSA flux brought by ships from outside of the Baltic Sea. Most of the WSA was transported to Swedish, Finnish and Danish ports as well. The highest WSA flux was assigned to roll-on/roll-off, passenger and general cargo ships. The high biofouling potential in BSR indicates a potential high risk to the environment and, therefore there is an urgent need for appropriate guidelines to be introduced into daily use by the commercial shipping community.

Design and Planning of Mine Drainage Systems at PT. Pertama Mina Sutra Perkasa, Puger District, Jember Regency, East Java, Indonesia

PT. Pertama Mina Sutra Perkasa, a limestone mining company, operates in Grenden Village, Puger District, Jember Regency, East Java Province. Utilizing an open mining system, the company faces challenges related to mine drainage. Based on rainfall analysis from 2013 to 2020. the planned rainfall rate is 25.938 mm/hour, with a rainfall intensity of 5.712 mm/hour, resulting in a runoff water discharge of 0.599 m³/second. The water discharge entering the well at the site is 2159.08 m³/hour. The designed drainage channel has a discharge capacity of 3.109 m³/second, a wet surface area of 0.5 m², a channel bottom slope of 0.03%, a hydraulic radius of 0.25 m, a Manning roughness coefficient of 0.011. a wet cross-section circumference of 2 m, and a flow depth of 0.5 m. The planned well dimensions are 60 m in length, 20 m in width, and 7 m in depth, with a volume of 8400 m³. The current sludge settling pond has three compartments with a total capacity of 393.75 m³. The proposed design for the sludge settling pond includes a length of 60.6 m, a width of 15 m, a depth of 7 m, and a volume of 5454 m³. The actual mud dredging frequency is once every 2 days, while the proposed plan extends this interval to 9 days using a Doosan 340DX type excavator. This comprehensive drainage system design aims to efficiently manage runoff and sedimentation, ensuring sustainable mining operations

Optimization design and experimental research on cooling performance of U-shaped latticework cooling structure with perforations used for gas turbine blade

Latticework cooling structure provides great structural strength and well heat transfer enhancement commonly used for internal cooling of turbine blades. However, traditional rectangle latticework cooling structure raises friction loss badly and reduces cooling efficiency of turbine blades due to the increase of wetted surface area. A new latticework cooling structure with U-shaped sub-channel and perforations on raffles was proposed, aiming to promote heat transfer and reduce friction loss for latticework cooling structure. Numerical simulation, optimization method and experimental test are conducted to investigate heat transfer and fluid flow characteristics of this new latticework cooling structure. Optimization design based on response surface approximation model and non-dominated sorting genetic algorithms-Ⅱ was conducted and objectives were to maximize heat transfer and minimize friction loss. The optimal U-shaped latticework cooling structure with perforations was obtained and experimental research was conducted. Characteristics of heat transfer and friction loss between the optimal U-shaped latticework cooling structure with perforations and the rectangle latticework cooling structure were compared and analyzed. The experiment results showed that the overall heat transfer and thermal–hydraulic performances of the optimal U-shaped latticework cooling structure with perforations are 21.6–37.0% and 27.9–46.7% respectively higher than those of the rectangle latticework cooling structure while friction loss performance of the former is 16.3–29.2% lower than that of the latter as Reynolds number increases from 5000 to 50000. Moreover, empirical formulas for the optimal U-shaped latticework cooling structure with perforations and the rectangle latticework cooling structure are obtained. The present study indicates that the U-shaped latticework cooling structure with perforations not only maintains the superior heat transfer performance, but also reduces friction loss, which is mainly due to the perforation ribs reduce flow distance for part of coolant and also provide an impingement effect for the heated wall and enhance flow interactions and flow mixing.

Prediction of the impact of biofouling roughness on a full-scale planing boat performance using CFD

This study investigates the crucial role of surface roughness caused by biofouling in contributing to increased drag in fast planing vessels. Through rigorous computational fluid dynamics (CFD) simulations using a full-scale reference hull, the impact of surface roughness on planing hull performance is examined. The results show that heavy slime roughness on the hull surface causes a significant increase in total resistance, ranging from 10% at low speeds to 80% at high speeds. Skin friction was identified as the most affected component of resistance, with additional changes in residual resistance. Although the change in residual resistance is relatively small, they give rise to dynamic phenomena such as variations in pressure distribution, wave elevation, and wave making formation. The study also explores the influence of surface roughness on dynamic lift forces, observing an increase in upward friction forces but ultimately concludes that overall hull performance was hampered due to the disproportionately higher increase in resistance caused by roughness (biofouling). Changes in dynamic trim, dynamic sinkage, and dynamic wetted surface area due to this roughness are also discussed.

Experimental investigation of blade-shaped riblets for drag reduction on UAV applications

This study presents an experimental investigation of blade-shaped riblets for drag reduction in unmanned aerial vehicle (UAV) applications. UAVs have gained significant attention since they can perform various missions, including surveillance, reconnaissance, and package delivery. However, their aerodynamic performance, specifically the high drag associated with their exposed surfaces, remains a key challenge for enhancing their efficiency and extending their flight endurance. To address this issue, riblet geometries are proposed as a potential solution, which can reduce the turbulent skin friction drag by up to 8%. The experimental investigation involves wind tunnel testing of blade-shaped riblets, with various spacing-to-height (s/h) ratios and constant groove cross-sectional area (Ag). The riblets are designed for application on the wing, empennage, and fuselage surfaces of a UAV. The investigations are performed on a flat plate for various flow conditions, including different freestream velocities, to evaluate the drag reduction effectiveness of the riblet configuration. The drag force is measured using a force balance system and flow visualization techniques are employed to assess the position where the boundary layer has transitioned to fully turbulent. The results demonstrate the drag-reducing effect of blade-shaped and trapezoidal riblets and the different performances observed for the various s/h ratios. The cases with s/h=1 result in the smallest drag coefficients, while the cases with s/h=2 have significantly increased drag values, compared to the smooth flat plate, due to the increased wetted surface area. These findings highlight the potential of riblets as an effective drag-reduction technique for UAV applications, enabling increased endurance and/or enhanced payload capacity.

Numerical Study of Effects of Inhomogeneous Roughness on the Ship Friction Resistance

The cleanliness of a ship hull is crucial for energy saving. Biofouling, which can cause microbiological corrosion, can also increase ship resistance. If left unchecked, this can lead to inefficient energy use and increased emissions, contributing to global warming. Ship resistance is a critical factor in ship design, which affects ship powering. The analysis of friction resistance due to biofouling on a ship hull requires an examination of the distribution of roughness height (ks), which is randomly distributed. In this study, an analysis of friction resistance coefficient (CF) due to inhomogeneous roughness is carried out with the help of CFD simulations with a full-scale model at 19 kn and 24 kn. The roughness of three ship segments, namely aft-hull, midship, and fore-hull, varies with ks values of 125 μm (P), 269 μm (Q), and 425 μm (R), respectively, while maintaining the same wetted surface area (S) in each segment. According to the simulation results, the RQP model produces the highest CF of 2.154 x 10-3, while the PQR model produces the smallest CF of 2.119 x 10-3 each at speed 24 knots. The CF contributes an average of 72.2% CT.

Systematic Assessment of Electrode Wettability for Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFBs) are a promising energy storage technology due to their advantages such as long-life cycle and scalability. In these systems, porous electrodes such as carbon paper and carbon felts have been used because of their desirable properties such as good acid resistance and low cost. However, these materials may have poor reversibility and wettability, which can negatively affect the system performance. Recently, different activation methods have been proposed to enhance physico-chemical properties and overcome these limitations. Among different methods, surface treatments have been commonly used in the literature [1-5] since they can introduce desirable functional groups (e.g., oxygen, nitrogen) and enhance electrochemically active surface area (ECSA), which is believed to be the main mechanisms for electrode performance improvement. Although it is suggested that higher electrode performance can be achieved by enhancing wetted or active surface area [6], there is still a disparity in how to assess relevant wetting properties, since wettability assessment is often neglected and performance assessment is usually done based on VRFB cycling test results, which is costly and time consuming.Static contact angle (CA) is commonly used to characterize electrode wettability and evaluate the effect of different treatments on the electrode surface. However, this technique offers a limited understanding and superficial analysis, since samples are either classified as hydrophobic or hydrophilic, showing no correlation to electrochemical performance. Moreover, CA measurements are not adequate for porous electrodes, since porosity and surface roughness can impact on contact angle results. In the present work, we propose to use the method of standard porosimetry (MSP) and cyclic voltammetry (CV) to evaluate the impact of surface treatments on electrode wettability from a complete physical (pore size distribution), chemical (functional group density), and electrochemical (electrochemically active surface area) perspective. These tools offer richer information than basic CA measurements and only take a few hours to be performed, thus reducing the assessment time and cost compared to VRFB assembly and cycling tests.Preliminary results show that both MSP and CV techniques can provide useful information regarding physical and chemical properties that can correlate electrode wettability to electrochemical performance. While MSP results can provide electrode morphology (pore size distribution, porosity, specific and wetted surface area), CV measurements can provide surface chemistry information (functional groups density) and electrochemically active surface area (capacitance). Thus, both tools can be used synergistically to investigate how different treatments impact both physical and chemical properties and how wetting properties impact electrode performance. Achieving high performance is essential to increase system energy density and decrease both capital and operational cost, making VRFBs more competitive and suitable for grid-scale storage of renewable energy, having positive environmental impacts and facilitating sustainable transition. Acknowledgements This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI), British Columbia Knowledge Development Fund (BCKDF), Western Economic Diversification Canada (WD), Canada Research Chairs (CRC), and the National Research Council of Canada (NRC). Technical support from Elizabeth Fisher and Jonas Stoll is also acknowledged.

Вторичные течения Прандтля 2-го рода в пространственно-неравновесном турбулентном течении

In this work, a numerical study of turbulent flow and heat transfer in a plane channel was carried out. The features of secondary flows of Prandtl's 2nd kind, which arise in the vicinity of longitudinal ribs on the channel walls, are studied under conditions of spatial inhomogeneity of the flow. Simulations of four turbulent flows were carried out: flows in a smooth channel uniform in length, a uniform ribbed channel, as well as flows in a smooth and ribbed channel with an average velocity varying along the length. It is found that the presence of a rib in a homogeneous channel in the considered case significantly changes the distribution of turbulent flow characteristics over the channel section. These changes are due to the action of the emerging secondary flow with a maximum velocity of 5.2% of the average flow rate. In this case, changes in the velocity and thermal characteristics are similar. Both friction and heat transfer increase by about 10% due to the increased wetted surface area. The inhomogeneity of the flow, which is provided by the organization of blowing and suction through the upper wall of the channel, leads to more noticeable changes. In areas where the flow slows down, the fluid moves under conditions of an unfavorable pressure gradient. The intensity of turbulent pulsations in these places increases noticeably. In the presence of ribs, this leads to a significant increase in secondary flows. Both in the smooth and in the ribbed channel, an increase in the Reynolds analogy coefficient to a value of 1.07 was recorded, which is 6% higher than the uniform flow index. In general, based on the results of this work, we can conclude that the longitudinal ribbing is hardly capable of significantly changing the thermal-hydraulic properties of a flat surface in a turbulent flow, at least without taking special optimization measures. The spatial inhomogeneity of the flow, on the contrary, due to the different effects on the velocity and temperature profiles, has a certain potential for designing heat exchange devices with more suitable thermal-hydraulic properties.