Cross-flow Conditions Research Articles

Enhancement of flying wing aerodynamics in crossflow at high angle of attack using dual synthetic jets

To address the asymmetric flow field of the flying wing under cross-flow conditions at high angles of attack, dual synthetic jet actuators (DSJAs) are positioned at the leading edge of the windward side. DSJ control effectively affects the flow separation structure on the windward side, thereby enhancing leading-edge suction. This leads to both lift enhancement and drag reduction. Furthermore, it strengthens the stabilizing roll moment and improves lateral static stability. Additionally, the effective suppression of the separation zone significantly reduces the aerodynamic load fluctuations of the flying wing after control. In terms of excitation frequency, low-frequency excitation more effectively generates lift, transfers energy downstream, promotes momentum transfer, and results in an 8.2% increase in lift. Moreover, the DSJ excitation exhibits fast response characteristics, with the entire flow establishment process taking only 190 ms. Therefore, through DSJ control, the asymmetric flow under lateral conditions can be effectively corrected, expanding the flight envelope and enhancing the maneuverability and reliability of the aircraft.

Effect of rib width on the thermo-electro-mechanical behavior of solid oxide fuel cells

In this study, a three-dimensional multi-physics coupled model is employed to investigate the effects of the rib width of the cathode-side interconnect on the flow characteristics, electrochemical performance, temperature distribution and stress distribution of solid oxide fuel cells (SOFCs) under cross-flow conditions based on a fixed gas channel-electrode interface area. The results show that as the rib width is reduced, there is a marked improvement in the uniformity of gas distribution and gas flow velocity within the channels, leading to an increase in the output power density of the SOFC stack, a reduction in the temperature gradient of the cell and a reduction in the thermal stress carried by the cell. However, if the ribs are of insufficient width, the pressure drop within the channels will be significantly increased, reducing the static pressure head of the gas and leading to a decrease in the gas flow velocity within the electrodes. This, in turn, will result in an increase in concentration polarization and higher manufacturing costs. The simulation results facilitate a more detailed comprehension of the internal reaction processes of SOFCs, thereby offering valuable guidance for the structural optimization of interconnects.

Breakup dynamics in a pressure-swirl injector for urea-water solution applications: A computational study

The co-optimization of in-cylinder combustion and after-treatment technology has become a major aspect in engine design and development, with the goal of meeting the increasingly restrictive emission regulations in the transportation industry. Selective Catalytic Reduction is a robust technology to control the emission of NOx, and the injection of urea in water solution is the exhaust tailpipe is a key aspect of its operation. The proposed work uses high-fidelity Computational Fluid Dynamics to characterize the atomization dynamics of the liquid jet in relevant cross-flow conditions. The study focuses on a commercial low-pressure (9 bar) pressure-swirl injector which is characterized in its internal geometry through high-resolution X-ray micro-computational tomography. The internal two-phase flow has been modeled according to the volume-of-fluid approach in a large eddy simulation framework and validated against near-nozzle X-ray radiography measurement. Moreover, characterizing the breakup dynamics for the swirling hollow cone formation, and assessing the influence of the cross-flow in the breakup dynamics was completed. The results have been reported proposing Re-Oh maps and probability density functions of the spray kinematics. Higher cross-flow momentum generates an increase in the jet intact length and a reduction of the liquid droplet diameters. The axial momentum of the jet is affected by the cross-flow already in the near-nozzle region, determining a relevant deviation of the spray velocities. This work aims to inform the initialization of Eulerian-Lagrangian spray models through the assignment of droplet kinematics and static one-way coupling between volume-of-fluid results and Lagrangian spray parcels, to be used for system-size domain simulations.

Film Superposition Characteristics With Laidback Fan-Shaped Holes Under the Influence of Internal Crossflow

Abstract In the serpentine passage of actual rotor blades, the coolant at the inlet of the film hole has an internal crossflow. The internal crossflow significantly influences the film cooling effectiveness of multi-row film holes. This paper presents a numerical investigation of the superposition characteristics of multiple rows of laidback fan-shaped holes under the influence of internal crossflow. The numerical simulations utilize the RANS method considering the effects of blowing ratio, crossflow velocity ratio, arrangement patterns, and row-to-row spacing of the internal crossflow on the superposition characteristics of film holes. The results indicate that film cooling effectiveness exhibits a distinctly asymmetric distribution under crossflow conditions. At different blowing ratios, there exists an optimal crossflow velocity ratio, at which the laterally averaged film cooling effectiveness is maximized. The crossflow-to-jet velocity ratio (VRi) is utilized to comprehensively evaluate the impact of blowing ratios and crossflow velocity ratios, and the range of high area-averaged film cooling effectiveness is accurately captured. Regarding the study of arrangement patterns and row-to-row spacings, it has been found that under identical coolant mass flowrates, the staggered arrangement is better than the inline arrangement in both laterally averaged film cooling effectiveness and cooling uniformity. Meanwhile, reducing the row-to-row spacing leads to an improvement in laterally averaged film cooling effectiveness. Finally, an assessment of the Sellers model's applicability under crossflow conditions revealed that it demonstrates good applicability in cases of staggered arrangement with lower blowing ratios.

Investigation of the Discharge Coefficient for Laidback Fan-Shaped Holes on Turbine Blades Under Internal Crossflow Condition

Abstract The discharge coefficient of laidback fan-shaped holes on turbine blades under internal crossflow conditions is investigated using experimental and numerical simulation methods. The study is conducted on a linear cascade, with single rows of film holes set on both the pressure and suction surfaces of the test blade, coolant supplied by an internal crossflow channel. The internal crossflow Reynolds number ranged from 39,100 to 78,200, and the pressure ratio ranged from 1 to 1.5. This research considered the effects of internal crossflow Reynolds numbers and hole length-to-diameter ratio on the discharge coefficient of laidback fan-shaped holes. The findings are as follows: internal crossflow increases the inlet loss of film holes and flow separation within the holes, leading to a significant reduction in the discharge coefficient of film holes. Meanwhile, film holes on the pressure surface of the blade exhibit greater sensitivity to crossflow. Under different hole length-to-diameter ratio conditions, the length of the cylindrical section is the primary factor affecting the discharge coefficient, and the magnitude of the discharge coefficient depends on the extent of flow separation within the cylindrical section. The sensitivity of the discharge coefficient to variations in the cylindrical section length decreases once the cylindrical section length exceeds 2.8D for the pressure surface and 2.0D for the suction surface, where the inlet separation region reattaches to the wall. Under plenum conditions, there are significant differences in the discharge coefficients of film holes between the suction surface and pressure surface, especially at pressure ratios below 1.05.

Mixing and ignition characteristics of laser-induced solid fuel pyrolysis under wide-range variation of crossflow

A theoretical model has been developed to elucidate the thermal initiation laser ignition process of solid fuel. This model comprehensively explores both the heating process of solid fuel and the mixing process of gas-phase pyrolysis gases with crossflow conditions. A two-dimensional transient numerical model has been established and validated through experimental investigations across a spectrum of crossflow fluxes (ranging from 20 to 150 kg/m2/s) and pressures (ranging from 0.15 to 1.83 MPa). The study involves a quantitative analysis of the influence of key macroscopic parameters, including laser energy levels, combustion pressure, and flow rates, on essential mixing characteristics. The solid fuel heating process is segmented into two distinct phases: a preheating phase and a pyrolysis phase, with the latter exhibiting considerably higher heating rates. Notably, a direct linear relationship is observed between the laser energy levels and the rate of heating. It reveals that the dynamics of pyrolysis product mixing are primarily determined by the solid fuel pyrolysis rate and the prevailing crossflow conditions. The dimensionless penetration depth, which measures the extent of pyrolysis product mixing with crossflow, demonstrates an exponential positive correlation with pyrolysis rates and oxidizer flow rates while displaying a negative correlation with combustion pressure. This analysis includes the quantitative determination of proportionality constants and power exponents within the correlations. Finally, the study delves into the intricate interplay between crossflow conditions and the criteria for laser ignition, specifically ignition temperature and the flammable lower limit.

Study on the spray characteristics of transverse jets with elliptical nozzles in supersonic crossflows using the volume of fluid–discrete phase model

The atomization characteristics of liquid jets injected transversely into a supersonic crossflow significantly affect the performance of scramjet engines. Existing research on this topic has mainly focused on circular nozzles, while the influence of nozzle geometry, particularly elliptical nozzles, has received relatively limited attention. Therefore, this study employs a numerical simulation method coupling the volume of fluid and discrete particle model to investigate the breakup and atomization characteristics of transverse liquid jets from elliptical nozzles with different aspect ratios under supersonic crossflow conditions, as well as the total pressure loss. The simulation model is validated against experimental data obtained from a pulse wind tunnel, including measurements of the liquid jet penetration depth and the Sauter mean diameter (SMD). The results indicate that for elliptical nozzles with an aspect ratio (AR) greater than 1, columnar breakup occurs earlier, and the columnar breakup length is shorter compared to circular nozzles. As the AR increases, the jet penetration depth decreases, while the spray expansion angle increases. Furthermore, the SMD of the atomized spray field from the circular nozzle is larger than that from the elliptical nozzles, and the SMD of the spray field is smallest for an elliptical nozzle with AR of 4. Finally, the elliptical nozzles exhibit a higher total pressure recovery coefficient, indicating reduced total pressure loss in the combustion chamber. This reduction in pressure loss is expected to improve the thrust performance of the scramjet engine.

Velocity And Concentration Measurements Of Supersonic Underexpanded Jets

The release of high-speed gas jets due to a tank leakage can pose a substantial safety hazard, as it can give rise to flammable, explosive, or toxic mixtures. Hence, understanding jet dispersion in real-world conditions is crucial for designing effective safety measures. Existing targeted or traceable studies provide velocity field data, but limitations in source-driven-pressures, nozzle geometries, or cross-flow conditions restrict their representativeness under real conditions. Indeed, an exhaustive experimental dataset for supersonic underexpanded gas jet dispersion under the influence of an Atmospheric Boundary Layer (ABL) is currently lacking. As part of a collaboration with the CEA, the von Karman Institute is continuing research efforts towards the development of high quality nonintrusive, optical measurement techniques for the characterization of the velocity and the concentration of a supersonic underexpanded jet submitted to different cross-flow conditions. The research explicitly targets measurements far from the release point within the self-similar region. Such measurements have already been tried on a supersonic jet without cross-flow and with a uniform velocity low-speed cross-flow [2], but never under the influence of an ABL. By filling this gap, the objective of the present work is to further improve the dispersion characterization by combining different techniques, such as Laser Mie Scattering and Light Extinction Spectroscopy (LES). To evaluate the dispersion within the self-similar region up to x/D ≈ 100, a significant FOV 1 is required. Therefore, Large-Scale Particle Image Velocimetry (LS-PIV) has been adapted and successfully applied to measure the velocity field. Prominent results have been obtained and comparable to standard PIV measurements. The pronounced variation in velocity magnitude observed while transitioning from the potential core to the self-similar region requires collecting multiple datasets to cover the broad spectrum of velocities comprehensively. A Mie scattering theory-based technique has been used to retrieve a relative concentration field. It is a non-intrusive technique that operates on the same experimental images produced by the LS-PIV campaign, assuming that light scattered by particles within a specific volume is proportional to the number of particles within this volume. The algorithm for concentration retrieval was adapted from and consequently improved for the present case. Discrepancies in the scaled concentration evolution among different nozzle diameters highlighted the necessity for refining the technique. This emphasizes accurately determining the reference concentration in a region free from intricate phenomena such as multiple scattering. The absolute and independent concentration measurement is done via Light Extinction Spectroscopy (LES). The LES technique measures the transmittance spectrum T of a collimated light beam passing through the particle-laden flow. This method has successfully been applied to several types of flows with similar seeding particles and concentration quantities (e.g. PSD 2 ). The extrapolation of absolute concentration parameters relies on the regularized solution of an ill-conditioned inverse problem. This solution is derived using the transmittance recorded during the test as the primary input parameter. The system to solve exhibits high sensitivity to small perturbations, making it challenging to employ numerical techniques confidently. Due to the substantial impact of minor perturbations in initial conditions on the system solution, a meticulous adaptation of the technique to the demands of compressible flows is necessary and still ongoing. The setup consists of a Deuterium-Tungsten Halogen UV-NIR light source emitting a divergent beam of light towards a 90◦ off-axis parabolic mirror, which collimates and deviates it into a beam to the jet. After passing through the jet, the resulting attenuated light beam goes into a collector mirror, redirecting it into a composite grating spectrometer UV-NIR with a 200 − 1100 nm wavelength range with 0.5 nm optical resolution. The results will examine the dispersion pattern and feature in terms of velocity and concentration of a supersonic underexpanded jet produced by an upstream tank with total pressure ranging from 5 to 9 bar without cross-flows. The latter will provide a foundation for successive studies on supersonic underexpanded jets subjected to an ABL.

Large-Eddy vs. Reynolds-Averaged Navier–Stokes Simulations of Flow and Heat Transfer in a U-Duct with Unsteady Flow Separation

Large-eddy simulation (LES) and Reynolds-Averaged Navier–Stokes (RANS) equations were used to study incompressible flow and heat transfer in a U-duct with a high-aspect-ratio trapezoidal cross section. For the LES, the WALE subgrid-scale model was employed, and its inflow boundary condition was provided by a concurrent LES of incompressible fully-developed flow in a straight duct with the same cross section and flow conditions as the U-duct. LES results are presented for turbulent kinetic energy, Reynolds stresses, pressure–strain rate, turbulent diffusion, turbulent transport, and velocity–temperature correlations, with a focus on how they are affected by the U-turn region of the U-duct. The LES results were also used to assess three commonly used RANS models: the realizable k-ε with the two-layer model in the near-wall region, the two-equation shear-stress transport model, and the seven-equation stress-omega Reynolds stress model. Results obtained show steady and unsteady RANS to incorrectly predict the effects of unsteady flow separation. The results obtained also identified the terms in the RANS models that need to be modified and suggested how turbulent diffusion should be modeled when there is unsteady flow separation.

Experimental investigation of rear flexible flaps interacting with the wake dynamics behind a squareback Ahmed body

We have conducted an experimental study on the use of rear flexible vertical flaps as adaptive solutions to reduce the drag of a squareback Ahmed body, and on the fluid–structure interaction mechanisms at the turbulent wake. To that aim, wind tunnel experiments were conducted to compare the performance of various configurations including the baseline body, the body with rigid flaps and with flexible flaps. These configurations were tested under different aligned and cross-flow conditions. The results reveal that the flexible adaptive devices effectively reduce the drag within for low values of the dimensionless stiffness quantified through the Cauchy number, Ca. Thus, the two-dimensional deformation of the flexible flaps, which undergo progressive inwards reconfiguration (with an averaged tip deflection angle of Θ≃4°), reduces the bluffness of the flow separation at the body base, thus shrinking the recirculation region. This reconfiguration leads to increased base pressure, resulting into a 8.3% decrease in the global drag, CD, under aligned conditions. Similar drag reductions are observed under yawed conditions.Two regimes are identified in terms of the coupled fluid–structure dynamics. For low Ca, the passive reconfiguration of the flaps include small amplitude, periodic oscillations corresponding to the first free deformation mode of a cantilevered beam. Alongside these weak oscillations, the flaps are deformed guided by the changes in the value of the horizontal base pressure gradient, depicting bi-stable behavior which is caused by the synchronization between the Reflectional Symmetry Breaking (RSB) mode, typically present in the wake of three-dimensional bluff bodies, and the flaps deformation. For higher values of Ca, the flexible flaps deflect inwardly by about Θ≃20° on average, but exhibit vigorous oscillations combining the first and second free deformation modes of a cantilevered beam. These large amplitude oscillations excite the flow separation at the model’s trailing edges, leading to significant fluctuations in the separated shear layers and a consequent 31% increase in the global drag. Under yawed conditions, the flaps responses for large values of Ca are different due to the asymmetry of the corresponding recirculation region.

Effect of the Key Geometry and Flow Parameters on Discharge Coefficient of Laidback Fan-Shaped Hole Under Coolant Crossflow Condition

Abstract A laidback fan-shaped hole is commonly used due to its superior lateral film coverage. Its discharge coefficient is significantly influenced by internal crossflow owing to its complex geometrical structure. In this paper, the authors numerically investigate the flow mechanisms of the laidback fan-shaped hole under the influence of internal crossflow. The numerical simulations utilize the validated SST k–ω turbulence model, with the Reynolds number of internal crossflow ranging from 20,000 to 160,000 and the ratio of pressure ranging from 1 to 1.6. The results show that the different orientations of internal crossflow cause varying degrees of in-hole separation that led to a discrepancy in the discharge coefficient. The larger the Reynolds number of the crossflow is, the more drastic the change in the discharge coefficient. Furthermore, a comparison between the results obtained with and without internal crossflow has shown that the length of the cylindrical section is the primary factor determining the discharge coefficient of the laidback fan-shaped hole. The magnitude of the discharge coefficient depended on the extent of flow separation within the cylindrical section. Additionally, the numerical simulations obtained the discharge coefficient under a high internal crossflow Reynolds number of internal crossflow and a wall with a constant thickness and compared it with the predictions of a low-dimensional model of the discharge coefficient (based on our previous experimental data). The discrepancy between the results is within 10%, thus verifying the scalability of the low-dimensional model.

Metal-organic framework-intercalated graphene oxide nanofiltration membranes for enhanced treatment of wastewater effluents

Graphene oxide membranes (GO) hold immense potential in the field of water purification. However, when applied directly to real wastewater effluents, pure GO membranes suffer from drawbacks such as fouling sensitivity and limited stability. To address these challenges and unlock the full potential of GO membranes, novel nanocomposite membranes have been developed by the intercalation of GO with nanoparticles of ZIF-8 (a type of zeolitic imidazolate framework). The prepared GO/ZIF-8 (GZ) nanocomposite membranes have exhibited enhanced hydrophilicity and exceptional water purification capabilities. Specifically, the GZ membranes have demonstrated a permeance enhancement of over two-fold when compared to the pristine GO reference membrane. This enhancement is coupled with anti-fouling performance and competitive rejection rates for both salts and organic pollutants. GZ membranes have been effectively employed for the purification by cross-flow filtration of 3 industrial wastewater effluents. They have shown improved separation performance compared to the pristine GO reference membrane, and high stability under cross-flow conditions. The origin of the high performances of the GZ membrane has been clarified using structural and morphological analyses. This work highlights the significant progress made in the field of water treatment using graphene-based membranes.

Validation and Assessment of a Hybrid VOF-Lagrangian Numerical Methodology for Turbulent Liquid Fuel Jets in High-Speed Crossflow

Abstract For gas turbine combustors, injecting liquid fuel in crossflow can achieve superior mixing characteristics to improve performance and reduce emissions. Robust numerical design tools are needed to accelerate the development of low-emissions technologies. Atomization modeling is often facilitated by using the Lagrangian approach rather than the more complex and computationally expensive Volume-of-Fluid (VOF) approach. However, most Lagrangian breakup models rely on empirical constants that must be fully calibrated for jets in crossflow and representative conditions. A hybrid approach could deliver a better compromise between accuracy and computational cost. This study proposes and validates a novel numerical methodology for coupling the VOF and Lagrangian approaches using a stochastic breakup model with Adaptive Mesh Refinements for turbulent liquid fuel jets in crossflow under more representative gas turbine conditions. The predictions were validated in the near and far-field regions using datasets at high pressures (1–8 bar), Weber numbers (720–1172), and momentum flux ratios (6–33). The predictive capabilities and computational cost were also compared to the Lagrangian approach coupled with large eddy simulations (LES) and with the unsteady Reynolds-Averaged Navier–Stokes (RANS) methodology previously developed and validated by the authors. The effect of different VOF-Lagrangian transition criteria on the computational cost was also assessed and recommendations were provided for further improvements. The overall LES predictions were significantly improved by the proposed hybrid methodology. Although it tends to underpredict the spray trajectory and Sauter Mean Diameter compared to the URANS methodology, it better captures the diameter in the wake region and the droplet velocities.

Inducing Deep Sweeps and Vortex Ejections on Patterned Membrane Surfaces to Mitigate Surface Fouling.

Patterned membrane surfaces offer a hydrodynamic approach to mitigating concentration polarization and subsequent surface fouling. However, when subjected to steady crossflow conditions, surface patterns promote particle accumulation in the recirculation zones of cavity-like spaces. In order to resolve this issue, we numerically subject a two-dimensional, patterned membrane surface to a rapidly pulsed crossflow. When combined with cavity-like spaces, such as the valleys of membrane surface patterns, a rapidly pulsed flow generates mixing mechanisms (i.e., the deep sweep and the vortex ejection) and disrupts recirculation zones. In only four pulses, we demonstrate the ability of these mechanisms to remove over half of the particles trapped in recirculation zones via massless particle tracking studies (i.e., numerical integration of the simulated velocity field). The results of this work suggest that when combined with a rapidly pulsed inlet flow, patterned membrane surfaces can not only alleviate concentration polarization and the surface fouling that follows but also reduce the need for traditional cleaning methods that require operational downtime and often involve the use of abrasive chemical agents.

Effect of wall-impingement distance on fuel adhesion characteristics of split injection spray under cross-flow condition

In direct-injection spark ignition (DISI) engines, high-pressure fuel injectors introduce fuel into the cylinder, causing the spray to come into contact with and adhere to the cylinder wall; this is known as fuel adhesion. Over time, fuel adhesion contributes to carbon deposition on the cylinder wall, which affects its thermal conductivity and consequently diminishes engine combustion efficiency. This study investigates the influence of different impingement distances and cross-flow velocities on fuel adhesion under the triple injection strategy. The fuel adhesion propagation and side-view spray are measured using refractive index matching (RIM) and Mie scattering, respectively. The findings demonstrate that high cross-flow velocity promotes the fuel adhesion shape to be elongated strips. In the early stage, the growth rate of the fuel adhesion area increases with an increase in cross-flow velocity. In the later stage, the decrease rate in the fuel adhesion area initially increases with an increase in the cross-flow velocity; however, when the critical velocity threshold (20 m/s) is exceeded, the decrease rate in the fuel adhesion area tends to stabilize. The average fuel adhesion thickness then accordingly decreases with the increase in the cross-flow velocity and impingement distance in the later stage. In addition, the cross-flow promotes the volatilization of spray and fuel adhesion, thereby decreasing the fuel adhesion mass over time. In the context of carbon neutrality, this study underscores the importance of optimizing fuel injection conditions to reduce emissions and fuel consumption.

Effect of film hole diameter and attack angle on the film cooling performance of leading edge with double swirl chamber under crossflow condition

The film cooling characteristics of the blade leading edge with a double swirl chamber are studied experimentally under crossflow(CF) conditions. Aerodynamic and structural parameters such as CF intensity, blowing ratio(BR, 0.5–1.5), film hole diameter(df, 2–4 mm), and attack angle(αa, 0-16°) are considered comprehensively in the experiment. The computational simulation method was introduced to reveal the mechanism of CF affecting the cooling performance. The results present that introducing the CF condition at the same BR condition led to BR redistribution in the film holes, significantly improving the cooling protection near the C1 film holes for the df=3 mm and df=4 mm cases. Unlike the adverse effects of CF on the heat transfer in previous studies, the results present that CF under low BR conditions can help avoid the mainstream intrusion of the film holes. In the case of αa=8° and 16°, The film cooling effectiveness performs the highest in the CF0-BR1.0 condition at s/df>8, and the equivalent film coverage can be obtained under the CF1-BR0.5 condition. CF at αa=8° similarly avoids the mainstream intrusion under the small BR, but the CF reduces η near the C2– film holes. In contrast, the CF has little effect on the cooling performance around the C2– film holes in the case of αa=16° under BR=0.5. The area-averaged film cooling effectiveness of CF2-BR0.5 improves by 85 % relative to CF0-BR0.5 at df=3 mm case. When df=4 mm, the area-averaged film cooling effectiveness enhancement of the CF1-BR1.0 condition is about 87.43 % higher than that of CF0-BR1.0.

Characterization of High-Pressure Hydrogen Leakages

Abstract Reduction of gas turbine (GT) carbon emissions relies on a strategy for fueling the engines with pure or blended hydrogen. The major technical challenges to solve are (i) the adjustments to the engine and in particular the combustion chamber and (ii) a series of issues to solve to guarantee safe operations. In fact, compared to natural gas, hydrogen fueling implies higher risks of explosion in case of leak in the turbine enclosure and a more careful design of the ventilation system. Thus, a deeper comprehension of hydrogen leak scenarios is needed to adjust the safe design strategy of the enclosure. To this aim, a series of numerical investigations was carried out to understand how different methane–hydrogen blends (from pure methane to pure hydrogen) behave when leaking from a pipeline with fuel pressure that span from 1.5 to 4.5 MPa. The different fuel blends' leaks in form of underexpanded jets were studied under different cross-flow ventilation conditions, with ventilation velocity spanning from 0 m/s to 5 m/s. When compared to pure methane, the outcome is a three times longer penetration distance for pure hydrogen axisymmetric flammable clouds, whereas in cross-flow conditions a more complex three-dimensional behavior was found, potentially opening a safety-related concerns discussed in the paper.

Adaptation of conical liquid sheet and spray morphologies to cross-flowing gas

A variety of industrial devices use liquid injection into the traverse stream of air. Ambient flow changes the spray characteristics, which can alter a system's function. Two different spill-return pressure-swirl atomizers (SRA) were investigated under cross-flow. One of the tested atomizers generates a conical liquid sheet with shorter breakup lengths due to reduced internal air-core stability. Phase Doppler anemometer (PDA) was used for 2D velocity and droplet size measurements, along with high-speed visualization to record instantaneous spray behaviour. A wide range of operating parameters was tested, e.g., inlet pressure (pl) ranging from 0.25 MPa to 1 MPa, spill-to-feed ratio (SFR) from 0 to 0.9, and cross-flow velocity from 0 to 32 m/s. As the cross-flow velocity increases, the spray starts to tilt and the breakup length (lb) shortens as a result of increased aerodynamic forces. This reduces the differences between atomizers. Moreover, the droplet diameter depends on the lb, yet no semi-empirical model was found to accurately predict the droplet sizes. A map of breakup modes was established, and a transition point from long- to short-wave mode was identified. The Linear Stability Analysis of the liquid sheet breakup was used and compared with a semi-empirical approach for the first time for the SRA and cross-flow conditions. The results imply that considering only quiescent ambient conditions during atomizer development might not lead to the optimal design.

A comprehensive heat transfer investigation for impingement/effusion cooling under crossflow conditions

Impingement/effusion cooling is regarded as a highly efficient cooling structure and has been widely adopted in various energy-dependent scenarios. It often exhibits crossflow phenomena due to design or concurrent factors. However, quantifying the individual heat transfer contributions of each cooling structure in impingement/effusion cooling under crossflow conditions remains unclear. In this study, the pressure-sensitive paint and temperature-sensitive paint techniques were adopted to separately evaluate the performance of effusion and impingement cooling under crossflow conditions for a comprehensive heat transfer investigation. Different coolant flow rate (normalized as Rejet = 10,000 to 25,000) and crossflow flow rates (normalized as CMR = 0 to 0.3) were examined. In effusion cooling, cooling effectiveness generally decreased with higher Rejet but improved with increased CMR, except for Rejet = 10,000 and 15,000 with CMR ≥ 0.25, where excessive crossflow coolant negatively affected cooling effectiveness. When comparing cooling effectiveness at the same blowing ratio, higher Rejet with crossflow consistently outperformed, with the maximum improvement reaching 11.55 %. In impingement cooling, the Nusselt number (Nu) increased with Rejet due to enhanced impingement momentum. Crossflow had a detrimental effect at Rejet = 10,000 but showed improvements at Rejet = 15,000 to 25,000. With increasing Rejet, heat transfer performance became less sensitive to varying CMR. Compared to cases without crossflow, the Nu increments at CMR = 0.25 were -5.01 %, 4.03 %, 3.53 %, and 2.64 % for Rejet = 10,000 to 25,000, respectively. It was revealed that crossflow was influential on the impingement cooling, meanwhile, highly influential on the effusion cooling. This study intended to provide a more comprehensive understanding of impingement/effusion cooling and assistance in designing more efficient and tailored cooling strategies for high-performance energy systems.

Air Interactions of Magnetically Driven Plasma Discharges in Crossflow

Motivated by the use of magnetohydrodynamic devices in fluid dynamic flow control research, the current work experimentally studied the actuation mechanism and the resulting aerodynamic interactions of magnetically driven low-current arc-plasma discharges. The investigation was conducted in the context of boundary-layer interactions in low-speed crossflow conditions through the use of coaxial and v-shaped geometries. Time-averaged velocity field measurements showed that the toroidal region of vorticity induced by the coaxial geometry in quiescent air resulted in the formation of a horseshoe vortex in crossflow. Similarly, the v-shaped actuator resulted in the formation of streamwise vortices. The resulting boundary layers had s-shaped streamwise velocity profiles as measured in the wall-normal direction characteristic for the flow downstream of a conventional vortex generator pair, due to the three-dimensional mixing induced by coherent vortex structures. A simplified model based on intermolecular momentum transfer through collisions is proposed to explain the fundamental discharge–air interactions and the induced flowfields. These results inform the applications of various magnetically driven discharges in the field of aerodynamic flow control.