Flow Visualization Experiments Research Articles

INVESTIGATION OF AERODYNAMIC PERFORMANCE OF A NACA 0021 AIRFOIL MODIFIED WITH CAVITY

In this paper, an experimental investigation was conducted to investigate the effect of cavity on the NACA 0021 airfoil at low Reynolds number. The force measurement and smoke-wire flow visualization experiments were conducted in a low-speed open suction wind tunnel with a test section of 57cm×57cm×100 cm at a Reynolds number of 5.0×104 and 1.0×105. Two different airfoil models produced for the experiments such as base and cavity models. The diameter and location of the cavity was chosen as 0.08C and 80% x/C, respectively. The maximum lift coefficient of the cavity modified airfoil was increased about 6% for both positive and negative angles of attack compared to base airfoil at Reynolds number of 1.0×105. The cavity modified airfoil showed higher lift coefficient values at negative attack angles when compared to base airfoil at Reynolds number of 5.0×104.

Head-neck local ventilation mode for long-narrow mine working face

To improve the thermal health state of workers in mines facing heat hazards and enhance cooling capacity utilization efficiency of mine ventilation, this study proposes a suitable air distribution for mine workers’ local cooling, taking into account the characteristics of long-narrow underground space and workers. The suggested air distribution involves harnessing underground cold air jets along with the mine’s crossflow (mainstream ventilation) to create a localized safeguard airflow around the worker’s head-neck, known as jet ventilation in crossflow (JVIC). The flow visualization experiment identified five flow patterns within a confined space. The study explores the impact of the velocity ratio (R) and confinement scale (C) on the evolution of JVIC flow patterns and presents a parametric description of the resulting flow field. Drawing on the microclimate control scope around mine workers’ head-neck, the study defines the effective cooling zone and the ineffective cooling zone as the boundaries for controlling JVIC air distribution within confined mine spaces. This clarifies the applicability of the air distribution form and introduces an evaluation model for assessing the cooling effect and the efficiency of cooling capacity utilization to manage the non-uniform underground environment.

Research on airfoil surface flow structure testing based on Tuft velocity measurement method

Fluorescent tuft visualization technology, as a practical and convenient technique, has attracted widespread attention. In this study, a novel fluorescent tuft velocity measurement method is developed based on existing technologies. This method involves the recognition of tuft deflection angles, image transformation, block processing, temporal averaging, and post-visualization processing. Additionally, quantitative measurement of flow field velocity is achieved through tuft calibration. The study begins by comparing tufts of different materials, examining both fluorescent and deflection characteristics to determine optimal tuft parameters. The impact of tuft length on deflection characteristics is also investigated. Finally, the cotton tuft with a length of 7 mm and a diameter of about 0.1 mm was obtained as the best tuft sought in this experiment. Subsequently, at Reynolds number Re = 1.7 × 105, airfoil surface flow field visualization and quantitative analysis are conducted, exploring the relationship between airfoil surface flow structure and aerodynamics at different angles of attack. Finally, a comparative study is conducted between tuft velocity results and oil flow visualization experiments, as well as particle image velocimetry experiments, confirming the feasibility of the fluorescent tuft velocity measurement method.

Development of Dehydrogenation System for Liquid Organic Hydrogen Carrier with Enhanced Reaction Rate

Owing to the massive expansion and intermittent nature of renewable power, green hydrogen production, storage, and transportation technologies with improved economic returns need to be developed. Moreover, the slowness of the dehydrogenation reaction is a primary barrier to the commercialization of liquid organic hydrogen carrier (LOHC) technology. The present study focused on increasing the speed of dehydrogenation, resulting in the proposal of a triple-loop dehydrogenation system comprising reaction, heating, and chilling loops. The reactor has a rotating cage containing a packed bed of catalyst pellets, which is designed to enhance both heat and mass transfer by helping to detach precipitated hydrogen bubbles from the catalyst surface. In addition, the centrifugal force aids in isolating the gas phase from the LOHC liquid. A dehydrogenation experiment was conducted using the reaction and chilling loops, which revealed that the average hydrogen production rate during the first hour was 52.6 LPM (liter per minute) from 26.3 L of perhydro-dibenzyl-toluene with 1.5 kg of 0.5 wt% Pt/Al2O3 catalyst. This was approximately 48% more than the value predicted with the reaction kinetics measured with a small-scale plug flow dehydrogenation reactor with less than 1.0 g of 5.0 wt% Pt/Al2O3 catalyst. The concept, construction methods, and results of the preliminary gas infiltration, flow visualization, and reactor pumping experiments are also described in this paper.

Comprehensive Analysis of Flow Dynamics and Pressure Variations in PEM Water Electrolyzers: Experimental and Computational Insights into Gas Diffusion Electrodes

Polymer Electrolyte Membrane Water Electrolyzers (PEMWEs) can produce green hydrogen with zero emission with renewable energy sources integration. The gas diffusion electrodes (GDEs) are one of the main components of PEMWE. In this study, the flow behavior of GDEs for different patterns like square GDE (SGDE), circle GDE (CGDE), equilateral triangle GDE (ETGDE), and rhombic GDE (RGDE) was investigated numerically and experimentally for different flow rates at 100 mL min−1, 200 mL min−1, and 300 mL min−1. In pressure drop (ΔP) and flow visualization experiments, SGDE showed the best performance. Experimental results were compared with each other by validating them in the numerical analysis of multiphase flow conditions. As a result, at 100 mL min−1 flow rate, the flow is directed to the outlet side with 38.96 % less pressure loss in SGDE geometry. In flow visualization experiments, at 100 mL min−1 flow rate, SGDE geometry fills the cell faster than other GDE geometries by 0.437 sec. The results from the ex-situ ink test performance of the GDEs were confirmed in electrochemical single cell PEMWE tests via Pt coated GDEs. Furthermore, SGDE exhibited superior performance in thein-situtests. In the numerical calculation results, when 16 % porosity filter paper was examined, it was concluded that the SGDE geometry was 94.8 % rate successfully carried the liquid phase to all geometry. It is supposed that water transfer inside the cell may be improved by using an SGDE for the anode GDE, and the performance of PEMWE could be increased by providing faster delivery of the oxygen gas and excess water to the outside of the cell.

Flow and clogging behavior of a mixture of particles in a silo

We investigated the clogging behavior observed during the flow of aspherical particles from a silo in the presence of spherical particles of different sizes and proportions using flow visualization experiments and discrete element method simulations. The size of the avalanche, essentially the tendency of clogging, exhibits non-monotonic dependence on the spherical particle volume fraction. For small enough content of spherical particles, the clogging tendency intensifies, whereas it reduces rapidly for high enough spherical particle fractions, with a minimum in between. The non-monotonic behavior is observed to persist over for different spherical particle sizes. The overall behavior is shown to arise due to competing effects between the localized total particle fraction influencing avalanche strength and mean size of the particles exiting the silo, influencing the probability of arch formation.

Elasto-inertial instability in torsional flows of shear-thinning viscoelastic fluids

It is well known that inertia-free shearing flows of a viscoelastic fluid with curved streamlines, such as the torsional flow between a rotating cone and plate or the flow in a Taylor–Couette geometry, can become unstable to a three-dimensional time-dependent instability at conditions exceeding a critical Weissenberg ( $Wi$ ) number. However, the combined effects of fluid elasticity, shear thinning and finite inertia (as quantified by the Reynolds number $Re$ ) on the onset of elasto-inertial instabilities are not fully understood. Using a set of cone–plate geometries, we experimentally explore the entire $Wi$ – $Re$ phase space for a series of nonlinear viscoelastic fluids (with the dependence on shear rate $\dot{\gamma}$ quantified using a shear-thinning parameter $\beta _P(\dot {\gamma })$ ). We tune $\beta _P(\dot {\gamma })$ by varying the dissolved polymer concentration in solution. This progressively reduces shear thinning but leads to finite inertial effects before the onset of elastic instability, and thus naturally results in elasto-inertial coupling. Time-resolved rheometric measurements and flow visualization experiments allow us to investigate the effects of flow geometry, and document the combined effects of varying $Wi, Re$ and $\beta _P(\dot {\gamma })$ on the emergence of secondary motions at the onset of instability. The resulting critical state diagram quantitatively depicts the competition between the stabilizing effects of shear thinning and the destabilizing effects of inertia. We extend the curved streamline instability criterion of Pakdel & McKinley (Phys. Rev. Lett., vol. 77, no. 12, 1996, p. 2459) for the onset of purely elastic instability in curvilinear geometries by using scaling arguments to incorporate shear thinning and finite inertial effects. The augmented condition facilitates predictions of the onset of instability over a broader range of flow conditions, and thus bridges the gap between purely elastic and elasto-inertial curved streamline instabilities.

Mixing and transport of CO2 across a monolayer-covered surface in an open cylinder driven by a rotating knife edge

The dissolution of CO2 into water has attracted much attention for the modeling of air-sea gas exchange, CO2 sequestration into porous aquifers, and gas transport in bioreactors. In nearly all circumstances, the surface of water is covered by surfactants that make the gas-liquid interface behave elastically and exhibit surface (excess) shear viscosity. Here, transport and mixing of CO2 across the interface is studied in an axisymmetric system with inertia. A rotating knife edge at the interface with negligible contact surface area takes advantage of surface shear viscosity to drive a strong overturning flow in the bulk via viscous coupling. When CO2 is dissolved into water, this system has a forced convection nature that enhances the transport of CO2 into water. This transport leads to intense radial gradients in the dissolved CO2 resulting in baroclinic production of azimuthal vorticity which alters the underlying meridional flow and further enhances mixing by breaking down transport barriers. Flow visualization experiments reveal baroclinic instability and numerical simulations detail the hydrodynamics, including the effects of varying the CO2 partial pressure above the water.

Experimental study on flow characteristics of jet ventilation in crossflow in confined mine spaces

The increasing depth of mine excavation presents greater challenges in mine ventilation and in managing cooling energy consumption. Therefore, there is an urgent need for comprehensive research on jet ventilation influenced by low-speed crossflows. This study investigated the impact of flow velocity ratios (R) and jet exit diameters (d) on flow-field distribution and flow characteristics through velocity measurements and smoke flow visualization experiments. The results of the study revealed two distinct types of air lakes formed by jet ventilation in crossflow (JVIC), with one being wall-attached and the other suspended. Notably, a significant secondary flow phenomenon was observed in the near-field near the upper wall. Additionally, the deflection angle (θj) of JVIC decreases as R and d/D increase, leading to the formation and movement of a semi-confined point (SP) and a confined point (CP) in the -x direction. Moreover, the wall confinement effect diminishes the jet’s diffusion and deflection ability in the -z direction, leading to increased penetration in the x direction. Before the formation of the SP, the deflection section of the jet lengthens, followed by a rapid shortening upon its formation. Finally, the study further developed empirical equations for the jet axial trajectory and diffusion width.

Flow visualization experiments of argon injection in a molten salt natural circulation loop

Off-gas systems are implemented in molten salt reactor designs to control the release of gaseous fission products. Two-phase flow in molten salt must be studied to understand how the system will behave in comparison to traditional working fluids like water. Flow visualization experiments and particle image velocimetry measurements were performed for three argon bubble sizes injected into a co-current stream of molten salt in a natural circulation loop facility. Similar bubble sizes were injected in experiments with water to compare the bubble shape, trajectory, and wake flow behavior of the fluids. The bubble region of interest was used to calculate the equivalent diameter and terminal velocity. Results for water showed a wobbling bubble surface and less stable bubble trajectory due to lower surface tension and viscosity compared with molten salt. Particle image velocimetry results demonstrated the increased viscosity of salt dampens turbulent fluctuations for the smaller bubble size. For a cap bubble, turbulent fluctuations were larger and longer lasting than in results for the wake flow of an argon cap bubble in water.

Two-phase flow visualization experiments in variable-aperture fractures: The correlation between flow structure, displacement efficiency and relative permeability

The correlation of flow structure, displacement efficiency and relative permeability for two-phase flow in variable-aperture fractures is critical for many fractured rock applications. A systematic visualization method for two-phase flow experiments was developed and four transparent variable-aperture fracture samples were duplicated by epoxy casting method. A total of 112 times two-phase flow experiments of nitrogen displacing water under various gas flow rates were performed and the corresponding results of images and pressure data during flow process were recorded and analyzed based on optical principles. On the basis of the competition between capillary resistance and inertia force of gas, the flow structure can be generally divided into four states including capillary resistance dominant (CR) state with few flow paths in two lateral sides without fingering flow, competitive (CP) state with the occurrence of clear fingering flow phenomenon, inertia force of gas dominant (IF) state showing more chaotic gas distributions, and steady (SD) state that has few changes in flow paths. The growth of displacement efficiency is greatly affected by the four transition states, in which the increment of displacement efficiency in Frac-1 that has gone through the three states of flow structure approaches 30%, and that of Frac-2 and Frac-3 are about 15%, while Frac-4 with little change of flow structure is less than 10%. As a consequence, the relationship between saturation and relative permeability of gas becomes steeper from Frac-1 to Frac-4 with greater deviation from X, V–C and Corey models due to the smaller variation range of gas saturation, and an improved v-type model is obtained where the exponent n shows a positive correlation to fracture mean aperture.

Molten salt flow visualization to characterize boundary layer behavior and heat transfer in a natural circulation loop

Experimental data for the natural circulation of high-Prandtl number fluids in molten salt systems has not yet been sufficiently studied to ensure safe operation in the event of a loss of forced coolant. A natural circulation loop facility was designed to mitigate major challenges in flow visualization experiments of molten salt. Experiments were conducted with molten salt and water, and four experimental data sets were obtained for each working fluid at increased heater power. Particle image velocimetry (PIV) measurements were performed, and the boundary layer was analyzed as a function of the Prandtl number. Velocity peaks near the walls were found in results for molten salt due to the underdevelopment of the thermal boundary layers, in contrast to the parabolic velocity profile found in results for water. The overall system behavior was characterized using velocity and temperature measurements. Temperature distributions in the loop demonstrated the developing thermal boundary layers at high Prandtl numbers. The wall shear stress was obtained from near-wall PIV measurements to determine the friction factor and a non-dimensional analysis of the boundary layer showed the expected relationship for laminar flow in all test cases. The local and mean Nusselt numbers were calculated and compared between fluids. The local Nusselt number for water stabilized as expected for the laminar flow, while results for salt showed evidence of continually developing flow. Results for the Nusselt number and friction factor were compared with correlations, and the most accurate predictions were identified for the current application.

Subsurface hydrogen storage controlled by small-scale rock heterogeneities

Subsurface porous rocks hold significant hydrogen (H2) storage potential to support an H2-based energy future. Understanding H2 flow and trapping in subsurface rocks is crucial to reliably evaluate their storage efficiency. In this work, we perform cyclic H2 flow visualization experiments on a layered rock sample with varying pore and throat sizes. During drainage, H2 follows a path consisting of large pores and throats, through a low permeability rock layer, substantially reducing H2 storage capacity. Moreover, due to the rock heterogeneity and depending on the experimental flow strategy, imbibition unexpectedly results in higher H2 saturation compared to drainage. These results emphasize that small-scale rock heterogeneity, which is often unaccounted for in reservoir-scale models, plays a vital role in H2 displacement and trapping in subsurface porous media, with implications for efficient storage strategies.

A project-based learning (PBL) on gas dynamic concepts using hydraulic analogy technique

This article suggests the hydraulic analogy technique as a cost-effective approach for flow visualization in fluid mechanics, particularly in dealing with challenges related to supersonic flow, such as shock waves and flow past wedges. The project-based learning approach is adopted to engage students in the study effectively. This active learning method allows students to actively participate in research activities, from setting project goals to designing and executing experiments. The article highlights the positive impact of project-based learning on students’ learning outcomes through internal and external evaluations. The study shows that project-based learning enhances technical competence and encourages a lifelong passion for learning. The cost-effectiveness of the hydraulic analogy method inspires neighbouring academic institutions to consider similar approaches, further promoting the use of practical and affordable solutions for flow visualization experiments.

Experimental and theoretical study on liquid-vapor behavior characteristics near CHF

Liquid-vapor behavior characteristics near CHF conditions are investigated through flow visualization experiments and theoretical analysis. The detailed liquid-vapor behavior near CHF is captured by high-speed camera in this paper. With the increase of heat flux, the periodic vapor layer occurs due to numerous large bubble coalescence. A three-layer structure is observed close to CHF conditions, i.e., liquid film, vapor layer, and liquid core. The physical triggering mechanism of CHF is the complete evaporation at the liquid film wave trough position close to the end of the heating rod. Furthermore, based on the separated flow model and instability analysis, respective prediction models for essential liquid-vapor characteristics, including thickness of vapor layer, interfacial wavelength, and the maximum thickness of liquid film, are developed. The predicted results are in good prediction performance with MAE of 16.9 %, 12.8 %, and 26.1 % compared to experimental results, respectively. The relevant research in this paper provides an important foundation for further development of the mechanistic CHF model.

Analysis of low-speed blade coating flows

From industrial-scale production to small-scale fabrication of functional films, the blade coating method is used widely to apply a uniform thin liquid film on a moving substrate. However, conventional hydrodynamic models are inadequate for laboratory-scale low-speed blade coating, where capillary forces dominate. In this study, the low-speed blade coating of non-evaporative Newtonian fluids was investigated in experimental, computational and analytical approaches. The transient free boundary problem was solved utilizing a two-dimensional finite element method, and a simple viscocapillary model was developed to describe the viscous stress and capillary forces within the puddle, and predict the thickness of the wet film as a function of the speed of the substrate. Comparing the predicted film thickness with the computational results, and a flow visualization experiment of blade coating with silicone oil, respectively, confirmed the model's validity. The study indicates that the proposed model may be a useful tool for optimizing laboratory coating processes, as it provides a greater understanding of the low-speed blade coating system on a laboratory scale.

Delineating the effects of morphological changes on retinal hemodynamics in diabetic human retinas: An invitro investigation

Diabetes-related vascular abnormalities are commonly encountered in the retinal vasculature, the most common being the increased tortuosity and reduced lumen diameter. These morphological changes significantly affect the underlying hemodynamics and remain unexplored. The current study examines the changes produced by the altered morphology on the blood flow velocity and wall shear stress (WSS) using particle streak velocimetry (PSV) experiments and numerical simulations and compares it with the healthy scenario. The diabetic and healthy retinal arteries are extracted from patient-specific fundus images and transparently printed using stereolithography (SLA) in three dimensions. Flow visualization experiments are performed using the blood-mimicking fluid (BMF) with three different flow rates, whose values are evaluated using the similarity analysis. Experimental and simulation results illustrate a significant reduction in the average axial velocity and WSS due to tortuosity with maximum percentage reductions of 10% and 11%, respectively, compared to healthy. Additionally, a strong correlation is observed between a particular artery's local tortuosity and the WSS, with a higher tortuosity index leading to the development of low-shear regions inside the diabetic artery. A 14% diameter reduction in the diabetic arteries leads to approximately 65% and 63% increase in the velocity and WSS, respectively, compared to healthy. Moreover, the experiments also capture the effect of decreased blood viscosity on the WSS in the case of diabetic arteries, illustrating a significant decrease in WSS values. The experiments performed in the current study may help ophthalmologists estimate the induced damage that the arterial morphological changes may create and facilitate an accurate prognosis of retinal vascular diseases.

Thermal phenomena and size effects of Mg powder in combustion process.

Magnesium is a combustible metal that poses various safety risks, including fires and explosions. However, there are limited safety measures available to prevent and respond to potential fires and explosion incidents in the metal industry. In this study, the combustion process of Mg fires was closely examined using infrared thermal imaging, focusing on the effects of Mg powder size. For the experiment, Mg powder was burned by increasing the temperature to approximately 967.4 K using an ignition unit and controller equipped with a tungsten heater. Moreover, combustion velocity measurement experiments for Mg particle sizes of 75, 100, and 150 μm were conducted using the combustion velocity measurement device presented in the NFPA 484 standard. On combustion of Mg, flames are observed; smoke is emitted as demonstrated by thermal and flow visualization experiments. The combustion velocity measurement experiment results demonstrated that the greater the slope value (combustion velocity) for the combustion length over time, the faster is the combustion velocity, with the 75 μm particle size having the fastest combustion velocity. The results of this experiment can be utilized as references for Mg fire control design and to gain a better understanding of the scope of smoke and fire hazard investigation measures.

CHARACTERIZATION OF SEPARATED SHEAR LAYER OF ELEVATED JET IN CROSSFLOW AT LOW-VELOCITY RATIO

The present study reports an experimental investigation of the flow dynamics of separated shear layers of an elevated jet-in-crossflow (EJICF). The current work aims to enhance the understanding of the downwash of jet fluid into the stack-wake region, akin to plume downwash in chimney flows, particularly at a low-velocity ratio. One end of the square stack with the aspect ratio of 9.0 is kept on a flat plate while the jet emerges from the free-end open to crossflow. Flow visualization and particle image velocimetry (PIV) experiments are conducted in a recirculating, low-speed water tunnel for three Reynolds numbers, namely Re = 1000, 2000, and 4000, based on the side of the square stack (<i>D</i>) and the freestream velocity (<i>U</i><sub>∞</sub>). The study mainly focuses on the jet shear layer characteristics and the downstream wake near the free-end formed by the interaction of crossflow and the jet shear layer. The instantaneous flow behavior includes the analysis of the shedding of the jet shear layer in a symmetric plane (<i>z</i> = 0) at a constant velocity ratio of <i>R</i> = 0.5. On the windward side, the jet shear layer reveals the formation of small-scale vortices due to the Kelvin-Helmholtz (KH) instability for Re ≥ 2000. Also, it is found that at a high Reynolds number (Re = 4000), an unstable vortex appears above the jet exit on the upstream side, mentioned as a hovering vortex. The mean and fluctuating flow behavior are investigated using streamline and contour plots of mean velocity and fluctuating components.

Experimental study and modeling of water film thickness in aero-engine under air–water mist flow

The water film not only plays an important role in the mass, momentum, and energy transfer between the air–water-surface but also determines the on-wing washing effect of the aero-engine. In view of this, air–water mist flow visualization experiments have been conducted at different gas velocities and water-to-air ratios in a compressor cascade, and the microscopic water film images have been analyzed to extract the transient water film thickness data by the Matlab code. It was found that the transient water film thickness fluctuation has no obvious association with the gas velocity, and the water film fluctuation is more affected by the water-to-air ratio. As the water-to-air ratio increases, the fluctuation magnitude of the water film thickness increases. The average water film thickness has been studied in relation to gas velocity and water flow rate, i.e., the average thickness of water film decreases with increasing gas velocity and increase with the increasing water flow rate. On the basis of the water film flow equation and taking the droplet collection efficiency into account, i.e., from the perspective of the physical mechanism of water film formed, a new model for predicting the water film thickness of a compressor blade surface under the air–water mist flow condition was proposed and validated. This model predicted, with a root mean square error and the mean absolute percentage error of 11.6% and 9.15%, respectively, under the present experimental flow conditions.