This study experimentally investigated the effect of showerhead hole injection angle on film cooling effectiveness over a turbine vane surface. A linear cascade was designed based on the 50 % span profile of a high pressure turbine vane, and the test vanes were manufactured using additive manufacturing. Coolant was supplied independently through separate internal channels to the showerhead, pressure side, and suction side holes. The showerhead injection angle was set to 0°, 30°, and 60°, and a vane without showerhead holes was also tested to isolate the effect of hole presence. Film cooling effectiveness was measured using the pressure-sensitive paint method under a mainstream inlet Mach number of 0.109 and average blowing ratios of 1.0 and 2.0. The results show that the showerhead hole configuration influences not only the leading edge region but also the downstream film cooling effectiveness on the vane surface. Smaller injection angles reduce film cooling effectiveness in the downstream region, even in the absence of showerhead coolant injections. Under showerhead-only injection conditions, increasing the blowing ratio enhances coolant lift-off, while increasing the injection angle promotes lateral redistribution of coolant toward the shroud side and upper span region. Under full injection conditions, higher film cooling effectiveness is observed in the upper span region, indicating that coolant distribution varies with the showerhead injection angle. Within the tested range, the 30° case generally provided the highest area-averaged FCE under most full-injection conditions, whereas lower and higher injection angles result in reduced performance due to non-uniform distribution or excessive spanwise redistribution. These results demonstrate that the showerhead hole configuration affects vane cooling performance through modification of coolant distribution along the vane surface.
This study investigates evaporation-driven deposition of ternary mixture droplets laden with graphene nanoplatelets (GNPs) on polydimethylsiloxane (PDMS) substrates to achieve uniform films suitable for flexible and wearable device applications. Deposition patterns obtained from single-component, binary, and ternary mixtures were compared to elucidate the mechanisms governing uniform deposition patterns. While deionized water droplets produced substantial inner patterns and binary mixtures exhibited only partial suppression, the ternary mixture containing water, ethanol, and surfactant consistently yielded highly uniform deposition patterns with reduced rim accumulation and enhanced interior particle coverage. This uniformity is attributed to the coupled effects of volatility contrast and interfacial tension differences, which are consistent with inferred Marangoni-assisted particle redistribution during evaporation. Concentration-dependent experiments revealed distinct deposition regimes ranging from non-uniform film formation to uniform films across different ethanol-surfactant combinations. To quantitatively interpret these transitions, a modified contact-line pinning model was employed to evaluate the ratio of the characteristic contact-line recession-onset time to the characteristic particle-migration time during evaporation. The model provided a semi-empirical framework for rationalizing the experimentally observed pattern transitions. Furthermore, increasing droplet volume improved deposition homogeneity and suppressed cracking, as confirmed by field-emission scanning electron microscopy and three-dimensional profilometry.
The atomization characteristics of liquid jets in crossflow (LJIC) under sub-atmospheric pressure conditions are critical to the performance of high-speed propulsion systems. This study systematically investigated the effects of operating parameters on the primary breakup and droplet size distributions of the LJIC under sub-atmospheric pressure conditions. Experiments were conducted at inlet air pressures of 40–100 kPa and velocities of 52.08–190.95 m/s, corresponding to the gaseous Weber numbers Weg of 15.51–435.47, the gaseous Reynolds numbers Reg of 138,804–1,066,277, and the liquid–gas momentum flux ratios q of 12.58–4372.87. Planar Mie scattering and a laser particle size analyser were used to obtain information on atomization. A novel spray image-processing method based on two-dimensional Fast Fourier Transform (2D-FFT) spatial frequency spectral analysis was proposed to identify the breakup regime. The “Band Intensity (BI)” value was calculated by integrating the intensity of a specific frequency band from the frequency domain of the images. A larger BI corresponded to a sharper, more irregular gas–liquid interface, indicating a less developed breakup state. Subsequently, a primary breakup regime map under sub-atmospheric pressure was constructed. The observed transition boundary from surface breakup to column breakup was qualitatively consistent with the literature, with a higher intercept. This difference in intercept can be primarily attributed to lower liquid turbulence level and lower Reg, which tended to weaken surface breakup at the same q and Weg under sub-atmospheric pressure conditions. Furthermore, decreasing Reg suppressed leeward-side surface breakup, whereas the windward side remained largely unaffected. In the far field, the Reg had a negligible influence on Sauter Mean Diameter (SMD).
An extensive experimental investigation of geometry-induced flow separation and reattachment of a turbulent boundary layer at nearly-zero pressure gradient was conducted in the Large Water Tunnel Braunschweig on a flat-plate configuration featuring a 25° backward-facing ramp. The examined ramp Reynolds number, based on the freestream velocity and on the ramp height, was 8,800. Lagrangian particle tracks and surface temperature maps were highly resolved in space and time via 3D Shake-The-Box and temperature-sensitive paint, respectively, whereas the time-averaged surface pressure distribution was measured by means of pressure taps. This enabled a description of the evolution of the attached boundary layer approaching the ramp, of the shear layer, of the recirculation zone, of flow reattachment, and of the recovery region further downstream. The time-averaged map of the celerity of propagation of temperature fluctuations was determined via analysis of the time-resolved surface temperature maps, while time- and spanwise-averaged distributions of flow velocity, acceleration and Reynolds stresses were obtained via bin-averaging of the Lagrangian particle tracks. The distributions of temperature fluctuation celerity and flow velocity allowed the identification of the time-averaged reattachment location, which was found to be in mutual, reasonable agreement for the two different methods, and also in agreement with previous work on 25° backward-facing ramps. Furthermore, the instantaneous flowfield and surface temperature data enabled capturing the arrangement and evolution of the turbulent flow structures and of their wall thermal traces in the aforementioned flow regions. Spectral analysis of the fluctuating wall temperature and velocity fields showed consistent evolution of these flow features; in particular, in the attached flow regions populated by streamwise-oriented structures, reasonable agreement was found for the spanwise wavenumbers corresponding to the maxima of the respective power spectra.
In the pursuit of affordable, sustainable and decentralized energy production, there is an increasing interest in achieving wind energy integration within urban environments. However, the low aerodynamic efficiency of urban wind turbines and their noise generation constitute a major challenge. This work proposes the use of a passive flow control mechanism, trailing edge serrations (TES) to improve vertical axis wind turbine (VAWT) blades and to advance in this context. The DU06-W200 airfoil (used in most of the literature designs) has been tested in a wind tunnel with and without serrations to obtain aerodynamic coefficients and noise levels, for a comprehensive range of angles of attack and Reynolds numbers, representative of urban VAWTs. An extensive, multidisciplinary analysis has been carried out, correlating the aerodynamic and aeroacoustic behavior of both configurations. The results show significant improvement of the lift-to-drag ratio of the airfoil even in detached flow conditions. Additionally, the results confirm the effectiveness of TES as a noise reduction strategy achieving global noise reductions of up to 5 dB. Furthermore, VAWT performance, evaluated using the Double-disk Multiple Streamtube (DMST) model, shows significant enhancement using TES. These findings support the application of TES to VAWTs, especially in urban environments.
Inspired by previous works focusing on Laser-Induced Fluorescence (LIF) thermometry in aqueous solutions, we have characterised the thermal sensitivity of different dyes in order to identify potential dye combinations to improve the Two Colors/Two Dyes LIF (2c-2d-LIF) method. The objective of this work is to improve temperature measurement accuracy in a multiphase flow, and more precisely in an aqueous cavitating flow where evaporation and condensation phenomena can take place. Experimental constraints in such flow require performing pH-independent measurements on a large temperature range and getting a high enough sensitivity to allow the measurement of small temperature variations. As a result, in contrast with previous works, those investigations focus on a large temperature range −10<T<80∘C. Under the excitation wavelength of 532 nm and for pH=9, by considering two dyes with opposite (positive and negative) temperature sensitivities and low spectral conflict, our results show that Kiton Red 620 mixed with Fluorescein Disodium provides the highest sensitivity ever reached compared to the previous dye associations proposed in the literature. Depending on the dye concentration ratio, the sensitivity can reach up to 4.05%/∘C for −10<T<80∘C and up to 5.12%/∘C for −10<T<20∘C. This method has been evaluated for the thermal mapping inside a cavitating flow around a rectangular body placed in a microchannel.
This study develops a new momentum-dissipation system for bubble column reactors (BCRs). The operation of BCRs primarily depends on various two-phase flow factors, including geometric configurations (e.g., gas injection), operational conditions (e.g., temperature, pressure, and liquid and gas flow rates), and gas–liquid mass transfer. Among these, the bubble injection geometry plays a particularly important role, influencing parameters such as the gas void fraction and interfacial area. In this study, a bubbly flow rig was built, primarily featuring a bubble-injection mechanism that induces swirl in gas–liquid bubbly flows. Different open-channel configurations were designed to dissipate momentum, which is essential for reducing the liquid’s rotary motion while promoting bubble swirl within the static liquid. This allows for measurement of the liquid-phase velocity field and a qualitative assessment of bubble–liquid slip behavior. PIV technique was applied to single and two-phase systems in a laboratory setting with water and air serving as the working fluids. The experiments assessed the impact of four fins with different geometries, including simple holes and open channels, on velocity vectors across the horizontal plane and, consequently, on liquid momentum. Velocity fields with swirl gas injection were also compared to those without air injection to evaluate the behavior of interacting fluids. Results show that both 90° and 180° open channels produce significantly greater momentum dissipation than the straight and simple hole configurations, with the 90° open channel achieves the highest dissipation overall. Two-plane PIV was used to experimentally map the hydrodynamics of a swirl-injection bubble column reactor equipped with a collar with fins, providing new insight and design guidance for swirl-driven multiphase reactors.
To investigate the impact of combustor configuration on the propagation characteristics of a rotating detonation wave (RDW) fueled by solid fuel-rich gas,this study employs ambient-temperature CH4/CO/H2 gas mixtures as fuel to simulate the primary combustion products of solid propellants, with ambient-temperature air as the oxidizer. By varying the exit blockage ratios of annular and hollow combustors, four combustor configurations with distinct structures were designed. Rotating detonation experiments were conducted across an equivalence ratio (ER) range of 0.48 to 1.79. Results demonstrate three distinct RDW propagation modes within different combustors: single-wave mode, two-wave collision mode, and periodic oscillation mode. Under the tested conditions, the maximum RDW velocity of 1829.9 m/s occurred at an ER of 1.02 in the hollow combustor without blockage. Conversely, the maximum average peak pressure of 3.1 MPa was observed at an ER of 0.59 in the hollow combustor with blockage. Compared to the annular combustor, the hollow combustor configuration exhibited enhanced RDW propagation velocity, average peak pressure, and wave stability. For identical chamber widths, implementing a larger blockage ratio reduced RDW propagation velocity but increased both average peak pressure and stability. Furthermore, the minimum ignition delay time of 3 ms was recorded in the annular combustor without blockage, indicating its superior ignition performance. Significantly, both annular and hollow combustors exhibited substantial degradation in ignition capability when equipped with larger blockage ratios.
Gas–liquid two-phase flows generated by in situ chemical reactions provide a controlled platform for investigating the fundamental physics of bubble nucleation, growth, interactions, and transport under confined conditions. By systematically varying the reactant concentration while maintaining fixed hydrodynamic and thermal conditions, we directly visualise transitions from a nucleation-dominated regime, characterised by dense microbubble trains, to a growth-dominated regime featuring sparse, large, and confined bubbles that significantly obstruct the channel. Quantitative analysis reveals a non-monotonic trend in bubble number density with increasing concentration, reflecting a competition between nucleation activation and growth-driven local depletion of dissolved gas. The confinement, deformation, and hydrodynamic disturbances we observe share key phenomenological features with cavitation development in narrow passages—specifically, the transition from many small nuclei (inception-like) to a few large, flow-blocking cavities (developed cavitation). However, we explicitly note that our chemically driven system operates on slower timescales (tens to hundreds of milliseconds) and does not replicate inertial collapse, shock waves, or microsecond dynamics. Therefore, this platform serves as a complementary, quasi-static model for studying nucleation competition, confinement effects, and growth-dominated flow resistance under well-controlled conditions. By studying the roles of chemical supersaturation and geometric confinement in bubble behaviour, we propose a mechanistic framework for future research on coupled thermal and acoustic effects in cavitation. The results have implications for the design of gas-evolving microreactors, lab-on-a-chip systems for gas evolution, as well as for predictive models of cavitation onset and growth in thermofluidic applications.
This study investigates gas–liquid two-phase flow in wide rectangular microchannels with constant microscale depth and different widths, with emphasis on extending the operating window of slug flow to higher volumetric flow rates. Experiments were performed in acrylic microchannels with an average depth of approximately 0.66 mm and widths of 5, 10, and 15 mm using air with distilled water and silicone oils of 1, 3, and 5 cSt. Flow regime maps were constructed from visual observations, and the liquid phase velocity field in slug flow was measured using Micro-PIV. For the fully wetting air-silicone oil systems, six regimes were identified: bubble, bubble-slug, slug, unstable slug, annular, and churn flow. In contrast, only unstable slug, annular, and churn regimes were observed for air–water, due to partial wetting of the acrylic walls, gas-wall contact, and the absence of a continuous liquid film around the gas plugs. Increasing channel width shifted the slug-family regimes toward higher total flow rates and extended the unstable slug operating window for all silicone-oil systems. Stable slug flow was also extended for the 1 cSt and 3 cSt oils, whereas the 5 cSt oil showed earlier transition to unstable slug flow in the widest channel because of stronger viscous resistance to lateral gas plug expansion. Existing maps which are defined based on hydraulic diameter showed poor agreement with the present high aspect ratio data. Therefore, force based empirical transition maps were developed using an area-based characteristic length, (Lc=A). A (Γ-θ) map classified the four main regime families with 92 % accuracy, while a (Γ-ψ) map separated bubble-slug, slug, and unstable slug regimes with 88.8 % accuracy. Micro-PIV measurements revealed plug slip, transverse interfacial motion, and back tail circulation within the liquid film. A continuity-based analysis showed that local reverse flow must occur near the minimum film thickness location, supporting the experimentally observed circulation.
In this paper, a color camera with a trichromatic mask is used to measure three-dimensional flow fields. The trichromatic mask separates imaging light paths through red, green and blue filters, and the image sensor of the color camera records the image superposition of the three perspectives of the tracer particles. Three view images are obtained by separating images through RGB channels, and color crosstalk correction is applied to improve image accuracy. The Shake-The-Box (STB) method is chosen to realize frame to frame particle tracking based on the images from three perspectives of tracer particles at high densities, and reconstruct the continuous-time three-dimensional Lagrangian trajectory. By artificially generating the flow field and its corresponding digitally synthesized images, the effects of key parameters such as particle density, aperture spacing of the trichromatic mask and camera magnification factor on particle tracking performance and computational efficiency were evaluated. The feasibility of the proposed method was demonstrated by the successful reconstruction of the three-dimensional flow field around a cylinder at a Reynolds number of 101.7.
The convective flow field in a rectangular fin perforated according to the first three steps of the Sierpinski carpet was studied experimentally using PIV-2D. In addition, the resulting surface temperature distribution on the fins was analyzed using infrared thermography. The effects of perforations, fractal iteration step, and Grashof number on the temperature and velocity fields were examined to better understand heat transfer behavior in these fins. The experimental results showed that, regardless of the fractal iteration, increasing the Grashof number enhances buoyancy forces and thereby increases the vertical velocity. Meanwhile, the horizontal velocity component is predominantly affected by the fractal perforations. In addition, the highest temperature gradients were observed in the periphery of the perforations. More specifically, the perforation manufactured in the first fractal step has a greater effect on the convective flow, altering its path and reducing the temperatures at the upper and lower edges of the large perforation. In contrast, the small perforations soften both the convective flow and the temperature distribution in the fin.
Heat-transfer measurements of jets impinging on rough surfaces are scarce because conventional heated-thin-foil methods confine high-resolution measurements to smooth walls. This study presents an infraredthermography-based gradient sensor that enables spatially resolved wall-heat-flux measurements on thermally thick plates with embedded, statistically homogeneous roughness. The method is validated against direct numerical simulation and established smooth-wall datasets for Reynolds numbers in the range 5000 <= Re <= 30000 and nozzle-to-plate distances in the range 2 <= H/D <= 5, showing good accuracy and repeatability. Using two roughness scales (k99/D = 0.04 and 0.12), we provide the first systematic dataset of local Nusselt-number distributions for turbulent impinging jets on rough surfaces at Reynolds numbers relevant for practical applications. Roughness is shown to have little effect at low Re, but a pronounced heat-transfer enhancement appears once a combined threshold in roughness height and Reynolds number is exceeded. For the larger roughness, strong augmentation is already observed for Re >= 10000, especially near r/D approximate to 1 where wall-jet shear is highest. At higher Reynolds numbers, roughness appears to fundamentally modify the radial heat-transfer pattern: the secondary peak, typically observed for smooth-wall impingement, disappears, giving way to a bell-shaped Nusselt profile, likely governed by altered near-wall flow dynamics. These findings offer new insight into heat transfer of rough-wall jet impingement at high Re and establish a validated measurement technique suitable for future studies integrating heat transfer, flow-field diagnostics, and wall-shear measurements.
Understanding scalar mixing requires determining the scalar dissipation rate in its full form, without relying on simplifying assumptions especially in shear flows. Approaches that use a constant turbulent Prandtl number Pr-1 and derive the mechanical to thermal time scale ratio under a local isotropy assumption can lead to dissipation estimates that are either over or under-predicted, and may distort its spatial distribution when applied in numerical models. To address this, an experimental investigation was carried out in a fully developed turbulent plane wake behind a slightly heated plate, where temperature acts as a passive scalar. The kinetic-energy and temperature-variance dissipation rates, epsilon(k) and epsilon(theta), were obtained directly from the corresponding energy budgets without invoking simplifying hypotheses. Measurements of Reynolds shear stress and heat flux were found to be consistent with the self-preserved forms of the momentum and enthalpy equations. All terms in the energy budgets were carefully measured, except for the pressure-diffusion term, which was estimated using Lumley's model. Particular attention was given to obtaining the third-order velocity moment vw(2), essential for evaluating the turbulent-diffusion term in the turbulent kinetic-energy equation. The study also demonstrated the importance of the temperature self-similarity variable xi(theta), which strongly influences the convection term C-theta and the overall scalar-variance budget. The distributions of the turbulent Prandtl number Pr-t and the mechanical-to-thermal time-scale ratio r commonly assumed constant in numerical simulations were shown to vary significantly across the wake. However, r, tends toward a constant value of approximately 2 for xi greater than 0.5.
We investigate the process of sheet fragmentation via perforations for shear-thinning liquids. Nucleation of holes has been reported to be a common feature in case of complex liquids like emulsions, suspensions, aerated liquids, which is attributed to the intrinsic inhomogeneities or defects in the form of dispersed phase of known physical character. Gels, being a class of complex fluid, however, do not exhibit the obvious inhomogeneity when at rest or at low strain rate. Combining experimental observations and analogy with other similar phenomena, we show that the gels behave like a colloidal suspension containing dispersed micro-gel particles when a continuous strain is applied. Under certain flow conditions of Weber number and above a critical gellant concentration, the local sites of micro-gel particles create surface tension gradient triggering the nucleation of holes. The growth of the holes is shown to follow the Taylor-Culik law. Depending upon the flow conditions, the complex interactions between the expanding holes and the sheet form web of ligaments breaking into a variety of drop sizes. Further, a systematic analysis of droplet statistics is presented to demonstrate the spatial segregation of droplets, which is governed by the complex hole dynamics and the rheology of the gels.
The article presents the results of studies of the characteristic features of heat transfer by rapeseed oil/water emulsions that accompany powerful heat release on their surface. Two methods have been employed to provide short-term penetration into the region of superheated states of the dispersed phase (water) preceding an explosive boiling-up of the emulsion: the pulse heating of a wire probe and the heating of a droplet in a gas flow. The experiments have been performed in wide ranges of variables, including the probe/gas temperature (350-950 K), the heating rate (102; 105-106 K/s), the water content (0-10 wt%), and the pressure in probe experiments (from 0.1 MPa to 5p/pc), where pc is the critical liquid-vapor pressure of the substance. Based on the primary data of the pulse experiment, the heat transfer coefficient as a function of time, KT(t), has been determined, depending on the water content and the reduced pressure p/pc, as well as an approximation for the critical parameters (pc, Tc) of rapeseed oil. The fundamental effect of explosive boiling-up on the change in the coefficient KT(t) has been revealed. The results contribute to the development of fuel technologies based on micro-explosive fragmentation of composite liquids and to verification of the choice of coolant for micro-devices, the action of which is accompanied by powerful heat generation.
An experimental investigation is conducted to evaluate the wind loading on a solar panel during a sudden wind direction change. The scaled solar panel has an aspect ratio of unity, an angle of attack of 30 degrees, and ground clearance of 0.1 chord lengths. The chord Reynolds number is 50000. Direct lift measurements are facilitated through a load cell, and three-component planar particle image velocimetry is used to capture tip vortex development. A sudden wind direction change is modeled by yaw rotations from 0 degrees to 30 degrees and from 30 degrees to 0 degrees with respect to the incoming flow direction over 3.8 convective units. Peak transient lift coefficients are approximately 10% of steady state levels immediately following the yaw rotation. The lift coefficient exhibits significant hysteresis between the two yaw rotation directions, which is attributed to the differences in tip vortex development. These results provide valuable insight for optimization of support structure design and flow control strategies for solar panel installations to account for transient wind direction changes in current major global solar energy producing nations.
We experimentally investigate drag reduction in flow past a circular cylinder using two symmetrically mounted dielectric barrier discharge (DBD) plasma actuators. Genetic programming (GP) and reinforcement learning (RL) are employed to optimize three control parameters, namely actuation frequency, duty cycle, and phase difference, allowing systematic exploration of effective actuation regimes. With parameters optimized by GP and RL, the maximum drag reduction rate (DR) increases from 23% to 31% relative to steady actuation, while reducing the actuation energy consumption. Time-resolved particle image velocimetry (TR-PIV) reveals that plasma actuation disrupts the shear layers and promotes the formation of small-scale vortical structures, which in turn weaken coherent vortex shedding and reduce drag of the cylinder. Furthermore, the results indicate that higher DR is achieved when the induced vortical structures remain spatially persistent and coherent. Notably, the actuation frequency primarily determines the size of the induced vortical structures, which in turn governs the drag reduction. The duty cycle mainly controls the actuation intensity and the strength of its harmonics, thereby modulating the wake structure and the resulting drag reduction.
Hydrogen bubble dynamics on microelectrodes strongly influence the performance and durability of electrochemical devices, yet the combined effects of electrode diameter, electrolyte concentration, and surface texturing remain insufficiently understood. In this study, we systematically investigate the growth, detachment, and regime transitions of electrolytically generated hydrogen bubbles on untextured (polished) and laser-textured platinum microelectrodes with diameters of 50, 100, and 200 mu m in sulfuric acid solutions with concentrations between 0.05 and 1.00 mol L-1. Synchronized high-speed imaging and potentiostatic measurements are used to identify four reproducible regimes: (I) isolated bubble growth without coalescence, (II) limited lateral coalescence, (III) a single coalesced bubble, and (IV) a pinned single coalesced bubble. A regime map constructed in the parameter space of applied potential, electrolyte concentration, and electrode diameter reveals that increasing H2SO4 concentration and decreasing electrode diameter promote earlier onset of coalescence and pinning. The detachment diameter increases with applied potential under most conditions, while the current-density response reflects a balance between increasing driving force and bubble-induced surface shielding. For the 200 mu m electrode, an optimal operating window is identified at intermediate electrolyte concentration (similar to 0.5 mol L-1), where high current density can be sustained to higher potentials before the onset of persistent pinning. In addition, an empirical, diffusion-inspired correlation is proposed for the detachment diameter on the untextured 200 mu m electrode, providing a compact description of its dependence on potential and electrolyte concentration within the monotonic regime. Surface texturing significantly modifies bubble dynamics by reducing nucleation overpotential and delaying the onset of persistent pinning by similar to 2.3 V, resulting in higher current densities at high applied potentials. Overall, the results establish a unified framework linking bubble dynamics, operating conditions, and electrochemical performance, and provide practical guidance for the design of efficient microstructured electrochemical systems.
