Flow Visualization Experiments Research Articles

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.

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.

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.

Development and analysis of thermal flow field: a new experimental approach

A new approach to experimental investigation of thermal flow field produced along an isothermal plate of finite dimensions in free convection is presented. The experimental procedure is based upon a flow visualization technique developed earlier by author. The contribution of this paper apparently is that a physical grid placed in the light path of a photographic flow visualization system enhances the ability to discern growth of thermal flow field produced due to temperature difference between a body and its surroundings. The results obtained from flow visualization experiment are verified with a correlation predicted by earlier published work.

Effect of aero-shaped vortex generators on NACA 4415 airfoil

A parametric study was carried out to investigate the effect of aero-shaped vortex generators (AsVGs) on NACA 4415 airfoil by comparing conventional vortex generators (CVGs). Force and flow visualization experiments were carried out for the CVGs and AsVGs at Re = 1.4 × 105. Six different configurations were selected for the positioning of the CVGs to investigate the best one. The best configuration was applied to the Aero-shaped VGs type, which is generated with the S1223 airfoil model. The results indicated that even if AsVGs did not show better performance to delay the stall angle when compared with the CVGs, in some positions they produced the same stall performance as the CVGs. Moreover, aero-shaped VGs increased the lift coefficient (CL) more than the conventional ones in the pre-stall region. At x/c = 0.1, CL and CD improve by up to 107.7% and 23.39% compared to baseline, respectively. At α = 18° and Re = 0.8 × 105, AeM5 at x/c = 0.1 increased the CL by 102.45% compared to the baseline model. While the CVGs increased the lift coefficient (CL) and postponed the stall (α) angle up to 4° according to the baseline model, the CVGs also caused an overall increase in the drag coefficient (CD). AsVGs decreased the drag coefficient (CD) more than conventional ones. Lift to drag ratio (L/D) of AsVGs is higher than that of the CVGs. The results of the surface oil flow visualization showed that both CVGs and AsVGs segmented the laminar separation bubble (LSB) and formed attached flow zones on the airfoil surface. As a result, AsVGs can be used as an alternative to CVGs in flow control applications.

Vortex dynamics of accelerated flow past a mounted wedge

This study is concerned with the simulation of a complex fluid flow problem involving flow past a wedge mounted on a wall for channel Reynolds numbers Rec = 1560, 6621, and 6873 in uniform and accelerated flow medium. The transient Navier–Stokes (N–S) equations governing the flow have been discretized using a recently developed second order spatially and temporally accurate compact finite difference method on a nonuniform Cartesian grid by the authors. Almost all the flow characteristics of a well-known laboratory experiment of Pullin and Perry [“Some flow visualization experiments on the starting vortex,” J. Fluid Mech. 97(2), 239–255 (1980)] have been captured by our numerical simulation, and we provide a qualitative and quantitative assessment of the same. Furthermore, the influence of the parameter m, controlling the intensity of acceleration, has been discussed in detail along with the intriguing consequence of non-dimensionalization of the N–S equations pertaining to such flows. The simulation of the flow across a time span significantly greater than the aforesaid lab experiment is the current study's major achievement. Meanwhile, a grid independence study performed in the process confirmed that our simulation is devoid of any under-resolution or numerical artifact. For the accelerated flow, the onset of shear layer instability leading to a more complicated flow toward transition to turbulence has also been aptly resolved. The quality of our simulation is validated by the close similarity of our simulation to the high Reynolds number experimental results of Lian and Huang [“Starting flow and structures of the starting vortex behind bluff bodies with sharp edges,” Exp. Fluids 8(1–2), 95–103 (1989)] for the accelerated flow across a typical flat plate. All three steps of vortex shedding, including the exceedingly intricate threefold structure, have been captured quite efficiently.

Simulation of supersonic jet flow past a blunt body in a laboratory experiment using computer vision

The complex flow of a supersonic jet (Mach number 2.4–2.5) around the blunt aerodynamic body (consisting of a cylinder with a diameter of 16 mm and a truncated cone) was studied using the high-speed shadowgraph technique and computer vision-based visual data processing. The problem considered is directly related to the rocket and space systems stages safe separation problem in a sufficiently dense atmosphere. For systems with a sequential arrangement of stages, their separation often occurs under the rocket engine jet action. The occurrence of lateral forces in violation of axial symmetry can lead to overturning of the discarded stage and undesirable interaction with the outgoing stage. The goals of the paper were to study the pulsation characteristics of the incoming supersonic jet streamlining the body, as well as the pulsations of the bow shock wave in both axisymmetric and asymmetric flows. The recording frame rate in the experiments was up to 775 000 fps. Computer vision and various machine learning approaches were used to process a large number of shadowgraph images. Canny edge detection and Hough transform, as well as a convolutional neural network (CNN) based on YOLOv2 architecture, were used to automatically track the position of the bow shock relative to the body. Jet sound generation was studied - the amplitude dependence on the distance from the nozzle and the evolution in time. The position of the bow shock relative to the streamlined body and flow pulsations over time were analyzed. Bow shock pulsation spectra at zero angle of attack were compared to those at a small angle of attack (α = 2.5°–3°). The power spectral density was also estimated. The slope of the spectra for the zero angle of attack case was found to be horizontal and the spectra for the small angle of attack case had a slope close to −5/3. The strongest of the measured bow shock frequencies were found to be close to those of the jet mixing layer pulsations. The hardware signal spectrum was also measured and this signal can be considered as noise. The described approach makes it possible to obtain the physical characteristics of the flow at any point of interest. It was also shown that the automation of flow visualization experiments using computer vision techniques allows to increase the speed of data processing and obtaining new physical information, which may be important for engineers developing aircraft and spacecraft technologies.

Experimental and numerical simulations to examine the mechanism of nozzle geometry affecting cavitation water jets

The geometry of the cavitation nozzle is the major determinant of the cavitation process and cleaning effect. To better service the cleaning industry, it is necessary to understand the process by which nozzle geometry impacts cavitation. This research uses experimental flow visualization and large eddy simulation (LES) to study the cavitation effects of three different nozzle configurations (Cylindrical, organ-pipe and converging–diverging nozzles). The shedding frequency of the cavitation cloud was determined through power spectrum analysis (PSD). The collapse range of the cavitation cloud produced was measured using the frame difference technique (FDM). A cleaning experiment was carried out to remove the tube scale with the aim of assessing the performance of the nozzle. The findings indicate that under the impact of the re-entrant jet, the cavitation cloud was distorted and fractured, and large shear stresses were produced at the fracture sites. The cylindrical nozzle creates a flow field with more vapour, velocity, and Reynolds stress than the other two nozzles. The findings from the cleaning experiments indicate that the utilization of appropriate nozzle geometry and an optimal standoff distance can significantly enhance the efficiency of tube descaling. This paper can help choose the right nozzle geometry and standoff distance for the cavitation water jet cleaning process, offer crucial technical support for the efficient cleaning within tubes.