High-speed Particle Image Velocimetry Research Articles

The role of flow confinement on turbulent kinetic energy transfer across premixed flames

The interaction between turbulent flows and premixed flames remains a significant topic of interest for improved modeling efforts and the advancement of clean and efficient combustion systems. Due to the challenging nature of the problem, the majority of studies related to turbulence-flame interactions have been conducted with unconfined numerical simulations or unconfined experimental burners. This paper describes the effects of flow confinement on the dynamics of turbulent reacting flows. An experimental premixed bluff-body combustion facility is operated under confined and unconfined conditions to understand the subsequent impact on the evolution of the turbulent reacting flow field across the flame. High-speed particle image velocimetry and chemiluminescence diagnostics are simultaneously employed to identify the flame front and capture turbulent velocity fields. The results show that the unconfined flame follows the classical description of turbulence evolution across the flame, in which turbulent fluctuations are dampened on the product side of the flame. In contrast, the opposite trend is observed for the confined configuration, and turbulent fluctuations are augmented by the reaction. The difference between the two cases is attributed to flow expansion and mean pressure gradients, which alters the balance between flame-generated turbulence and viscous dampening effects. Ultimately, the results provide a new perspective of premixed turbulent reacting flows and simultaneously highlight the importance of acquiring data in environments that better replicate practical combustion systems.

Modeling Sand Particulate Flows Through Angled Apertures and Surfaces in Concentrated Solar-Thermal Power Receivers

Abstract Concentrated solar-thermal power towers are increasingly migrating toward sand-based particle receiver designs to improve thermal efficiency and reach temperatures above 1000 °C for air Brayton power cycles and supercritical CO2 power cycles. However, utilizing sand affords complexities in modeling particle flow characteristics due to variable geometric shape, size, and composition. Thus, the objective of this study is to model the particle flow characteristics of sand through variable angled chevron shapes and receiver hoppers to help formulate robust modeling for flow dynamics. Treating the chevrons as an array of apertures, a novel method of calculating sand particle mass flowrate across angled apertures and surfaces is developed. Employing high-speed photography and particle imaging velocimetry techniques, our results incorporate the impact of effective angles upon velocity, residence time, and breakage profiles of falling sand particles. This study determines that a Beverloo equation incorporating effective angles for velocity and aperture size effectively predicts mass flowrate through chevrons, which can serve as a reference for future particulate flow modeling in this field. Furthermore, increasing hopper and chevron tip angles resulted in a more significant decrease in particle diameter and curtain opacity after sand flow trials.

Flow induced by the single-bubble chain depending on the bubble release frequency

In the present study, we experimentally investigated the flow induced by the in-chain bubbles (2.4–2.8 mm in size) while varying the bubble release frequency (fb=0–31 Hz), using high-speed two-phase particle image velocimetry and shadowgraphy. Along the streamwise (vertical) direction, we identified the transition of in-chain bubble dynamics and associated liquid flow. That is, the released bubbles tend to follow the straight path initially but transition to the oscillating (zigzag or spiral) trajectories. In particular, in the developing regime (near the nozzle), the coherent behavior of bubble chain generates the rib-like oblique jet flow, deviating from the rise path, even after the onset of the path instability of each bubble. As the flow develops, the scattering of rising path along the lateral direction becomes significant, and the liquid flow shows the similarity to the typical single-phase jet flow, which becomes more obvious at fb > 8 Hz. In this regime, with increasing the fb, the oscillating motion of bubbles changes to the flattened spiral one, and the deviation (fluctuation) of the lateral amplitude increases resulting from the enhanced flow-induced wobbling of bubble shape. Finally, we developed an analytical model to predict the evolution of time-averaged vertical velocity profile of the liquid, which considers the contributions of (i) local void fraction and (ii) evolution of upstream flow, and validated successfully with the measurement.

Wake-vortex evolution behind a fixed circular cylinder with symmetric jets

In this study, we investigated the wake-vortex dynamics behind a circular cylinder with an active symmetric jet control on its leeward surface by using high-speed particle image velocimetry (PIV) via wind tunnel tests. The experimental investigations were performed at a Reynolds number (Re) of 3.33 ×104. Steady slot jets were acted symmetrically into the near-wake region from the leeward surface of the cylindrical model. The azimuthal position of the slot jets was varied from 0°to 90°from the center-line. The proper orthogonal decomposition (POD) method was used as a tool to study the effect of modifications of active jets on the periodic vortex shedding and their dynamic interactions in the cylinder wake. Both dynamic evolutions and time-averaged results in the near-wake region were presented and analyzed to demonstrate the control effects of the symmetric jets. The experimental results indicated that the unsteady wakes of the cylinder were stabilized and attenuated when the azimuthal angle of the jets was set to 0°and 90°, whereas for other azimuthal angles, the symmetric jets were oblique to the sheared layers and their control effects were limited.

Experimental investigation of the flow-spray field in a realistic concentric staged high-temperature-rise combustor

The present work investigated the flow and spray fields in a realistic concentric staged high-temperature-rise (HTR) combustor in isothermal conditions using high-speed (10 kHz) particle image velocimetry (PIV) and 10 Hz kerosene planar laser-induced fluorescence (kerosene-PLIF). The two stages of the combustor, including a pilot stage and a main stage, were operated both individually and jointly to investigate their corresponding flow fields and the interactions between the flow from the two stages. From the time-averaged flow field results, strong coupling between the pilot and main swirling flows has been identified, leading to the formation of a primary recirculation zone (PRZ) and a lip recirculation zone (LRZ). From the instantaneous flow fields, the presence of a precessing vortex core (PVC) was observed when the two stages were flowing jointly, and the dominant PVC unsteady motions located in the inner shear layer of the pilot swirling was identified using snapshot proper orthogonal decomposition (snapshot POD) with dominant frequencies at 1400 and 1515.4 Hz. Comparing the flow field with the spray field indicates that the unsteady flow motion, in addition to the mean bulk flow, plays a key role in the understanding of the spray distribution. Noticeably, it is shown in present study that the dominant unsteady flow motions properly captured from the POD analyses show good agreement with some primary features of the spray distribution, which can be valuable to unraveling the complex flow-spray coupling in the HTR combustor operated under realistic conditions.

Experimental Investigation of the Effects of Fuel Injection on the Cycle-to-Cycle Variation of the In-Cylinder Flow in a Direct-Injection Engine Using High-Speed Particle Image Velocimetry Measurement

Experimental Investigation of the Effects of Fuel Injection on the Cycle-to-Cycle Variation of the In-Cylinder Flow in a Direct-Injection Engine Using High-Speed Particle Image Velocimetry Measurement

Time-averaged flow characteristics of pulsed swirling coaxial jets with annular blockage

An experimental study was conducted to investigate the mixing and time-averaged turbulence characteristics of pulsed swirling coaxial jets with annular blockage. The study was conducted using a custom-made device by pulsing the central jet using acoustic excitation. A high-speed camera was used to identify the flow structures. The mixing characteristics and the turbulence properties of the flow were studied through a hot-wire anemometer and high-speed particle image velocimetry (PIV) techniques. Flow visualization, mean velocity profiles, vorticity and Reynolds stress fields, and Lagrangian time and length scales were used to investigate the turbulence and mixing characteristics of the flow. The flow was excited under two distinct conditions. At the lower pulsation intensity, mixing was shown to improve near the exit because of the entrainment induced during the growth of the periodic vortex rings. At the higher pulsation intensity, mixing was shown to improve further downstream due to increased jet spreading caused by the breakup of the vortex rings. Investigation of the ensemble average flow field showed that the structure of the unexcited flow was altered because of the excitation. At the higher pulsation intensity, a stagnation point was formed on the centerline which was not observed in the unexcited flow and excitation at the lower pulsation intensity. Within a single period, the position of the stagnation point moved cyclically along the centerline. In comparison with the naturally evolving jets, excitation at the higher pulsation intensity has reduced the mean axial velocity allowing better mixing of the jets in the downstream area.

Characterization of flow field and combustion dynamics in a novel pressurized side-wall quenching burner using high-speed PIV/OH-PLIF measurements

A novel, pressurized side-wall quenching (SWQ) burner test rig enabling investigations on flame-wall interactions (FWI) of a fully premixed V-shaped flame at conditions mimicking some characteristics of practical devices, e.g. increased pressures and Reynolds numbers, is introduced within this study. Two operating cases featuring turbulent flow conditions at atmospheric and 3 bar operating pressure are defined and characterized based on velocity fields and flame front positions provided by simultaneous high-speed particle image velocimetry (PIV) and planar laser-induced fluorescence of the OH radical (OH-PLIF). For non-reacting conditions, profiles of mean velocities and mean velocity fluctuations are measured in addition to near-wall flow fields at locations where FWI occurs for chemically reacting conditions. For reacting conditions, near-wall flows are characterized in the vicinity of the quenching zone. Based on measured fluctuations of the streamwise velocity component, the turbulent flame is classified at the transition from the wrinkled to the corrugated flamelet regime. Using the derived flame front positions, the instantaneous flame-wall interaction is classified into SWQ-like, head-on quenching (HOQ)-like and multi-zone quenching events and the transition between these scenarios is identified by means of OH-PLIF image sequences. A statistical analysis of the flame front topology is performed based on the flame brush and deduced frequency distributions. Statistically significant differences between both operating cases are confirmed by chi-squared homogeneity tests.

A ‘virtual sensing’ framework for µECM/hybrid laser- µECM for monitoring interelectrode gap conditions

Micro-ECM (µECM) has capability to process wide class of conductive materials ranging from metals, cermets to bulk metallic glasses and shape memory alloys. The process efficiency and speed is further enhanced by hybridization of micro-ECM process with laser which can act as a localized heat source or a cutting energy source depending on the fluence on the workpiece surface. The hybridization of micro-ECM with laser facilitates machining of materials/alloys with conductivity variations and promotes their uniform dissolution. The process mechanism involves several multiphysics phenomena occurring in the interelectrode gap (IEG) which is of the order of 20-50 µm. The technological limitations of putting sensors in this small IEG limits the monitoring of physical quantities (such as temperature, hydrogen and oxygen gas volume fraction, electrolyte conductivity, current density on workpiece surface) during the process. However, these physical quantities can be effectively estimated by multiphysics simulations.At first, the experimental experience with different sensors (temperature sensor, current sensor, high speed imaging and particle image velocimetry (PIV)) is presented for µECM/hybrid laser-µECM process and their limitations are discussed. Subsequently, this work proposes a ‘virtual sensing’ approach enabled by multiphysics simulations. A detailed multiphysics model is developed in Comsol® software which models several phenomena occurring in µECM/hybrid laser-µECM process such as temperature-dependent electrolyte properties, hydrogen and oxygen gas generation, heat generation due to joule heating and laser heating, laminar fluid flow and electric currents. The properties are evaluated at 5 different (virtual sensor) locations inside the IEG, more specifically on the workpiece surface, tool surface, in the center of IEG, just at the exit of IEG and at a specified location. These virtual sensors are implemented in the Comsol® software by employing a dynamic point probe which probes the model-solution at a defined point in a moving geometry. The evolution of the aforementioned parameters in the IEG is monitored and supported by fundamental explanations as well as observational experiments. This research is a first step towards facilitating predictive sensing of ECM and allied multiphysics processes thereby supporting process design and minimizing experimental efforts by virtual process monitoring.

Spontaneous runaway of fast turbulent flames for turbulence-induced deflagration-to-detonation transition

One of the fundamental mechanisms for detonation initiation is deflagration-to-detonation transition (DDT). This research experimentally explores the runaway condition for highly turbulent fast flames before DDT, which are characterized by extremely high turbulent flame speeds. Such fast turbulent flames experience increased effects of compressibility and may develop a runaway acceleration combined with a pressure buildup that leads to a turbulence-induced DDT (tDDT) mechanism that has been recently reported. The flame dynamics and the associated reacting flow field are characterized using simultaneous high-speed particle image velocimetry, OH* chemiluminescence, pressure measurements, and schlieren imaging. We study the flow-field conditions for runaway acceleration of fast turbulent flames and effects of compressibility on the evolution of these flames. The locally measured turbulent flame speed is found to be greater than that of a Chapman–Jouguet deflagration speed, which places the flame in the runaway transition regime that would eventually lead to a detonation.

Modification of subcritical cylinder flow with an upstream rod

In the current research, a small circular rod is placed upstream of the main cylinder. This is used to change the incoming flow and, thus, impact the vortex shedding process of the wake as well as the resultant aerodynamic forces. The experiments are carried out in a wind tunnel with a subcritical Reynolds number of Re = 32 000. The sectional pressure distribution around the main cylindrical model is obtained to calculate unsteady aerodynamic forces. The experimental results show that the gap ratio markedly affects the aerodynamic forces exerting on the main cylinder. The optimal gap ratios for drag reduction are found to be G/D = 1.5 and 2.0, where the average drag and unsteady lift forces exerting on the main cylinder are suppressed. In addition to pressure measurements, we also apply the high-speed particle image velocimetry system to obtain flow characteristics of the cylinder wake as well as the gap flow. Two different modes of gap flow are found, i.e., the cavity mode and the wake impingement mode, and the transition value between them is around G/D = 1.5. Moreover, at the appropriate gap ratio, the wake vortex shedding pattern of the main cylindrical model changes from antisymmetric to symmetric.

Endoscopic high-speed particle image velocimetry for flow and turbulence measurements in multi-cylinder engines

Endoscopic high-speed particle image velocimetry for flow and turbulence measurements in multi-cylinder engines

An experimental/numerical investigation of non-reacting turbulent flow in a piloted premixed Bunsen burner

In this paper, an experimental study of the non-reacting turbulent flow field characteristics of a piloted premixed Bunsen burner designed for operational at elevated pressure conditions is presented. The generated turbulent flow fields were experimentally investigated at atmospheric and elevated pressure by means of high-speed particle image velocimetry (PIV). The in-nozzle flow through the burner was computed using large-eddy simulation (LES), and the turbulent flow field predicted at the burner exit was compared against the experimental results. The findings show that the burner yields a reasonably homogeneous, nearly isotropic turbulence at the nozzle exit with highly reproducible boundary conditions that can be well predicted by numerical simulations. Similar levels of turbulence intensities and turbulent length scales were obtained at varied pressures and bulk velocities with turbulent Reynolds numbers up to 5300. This work demonstrates the burner’s potential for the study of premixed flames subject to intermediate and extreme turbulence at the elevated pressure conditions found in gas turbine combustors.Graphical abstract

The preferential accumulation and the settling velocity of small heavy particles in Taylor–Couette flows

ABSTRACT In a gap between two concentric cylinders, we discover that the small heavy particle settling velocity (Vs) is greater than its associated terminal velocity (Vt) in featureless turbulence that is generated by counter-rotating inner and outer cylinders simultaneously. The peak enhancement of Vs occurs at St × (Vt/u′) ∼ 1, where u′ and St are, respectively, the r.m.s. turbulence fluctuation velocity and the Stokes number defined as the ratio between the particle response time to the Kolmogorov turbulence time. The present (Vs − Vt)/u′ data as a function of St measured by the high-speed particle imaging velocimetry are compared with previous experimental results obtained from different turbulent flow configurations in an attempt to gain further understandings of the complicated interactions between small heavy particles and featureless turbulence with negligible mean velocities. Moreover, it is found that small heavy particles are preferentially accumulated around the edges of an array of counter-rotating turbulent Taylor vortices with the inner cylinder rotation alone when a time ratio between particle response and Taylor vortices is near unity. These experimental results are interesting that may deserve to disseminate in our fluid mechanics community.

Attenuation of vortex street by suction through the structured porous surface

We experimentally investigated the attenuation of the von Kármán vortex street behind a circular cylinder by the use of active suction through a structured porous surface in wind tunnel tests. The Reynolds number Re, based on the outer diameter of the cylinder D, is set to be 1.0×104. The structured porous surface of the test model is made of resin by the three-dimensional printing technique to obtain well-organized porous structures. The active suction control effectiveness is quantified by a non-dimensional suction coefficient CQ, which is determined by the suction flow rate Q, the porous structure of the cylinder's surface, and the free stream velocity U∞. A high-speed particle image velocimetry measurement system is utilized to acquire global and detailed wake flows behind the baseline and controlled cylinders. In addition to directly perceived time-averaged characteristics, flow analysis methods including proper orthogonal decomposition, spectral analysis, and linear stability analysis are employed to study the underlying nature of cylinder wakes with and without distributed suction control. Experimental results show that multi-scale coherent structures in the cylinder wake are homogenized and the near wake is regularized under the control of suction. With a proper CQ, the alternating vortex street behind the circular cylinder is found to be greatly attenuated and the vortex shedding mode completely switched.

Effects of flexibility and motion parameters on a flapping foil at zero freestream velocity

To investigate the influence of the flexibility and motion parameters of flapping foils on their hydrodynamic characteristics, this paper experimentally investigated five different flexibility models in a hydrostatic water tank over a wide range of motion parameters. With the help of a biomimetic flapping foil propulsion test platform, the combination of a pitching motion and heaving motion of the flapping foil is realized. Meanwhile, a six-axis force/moment sensor, a high-speed camera and particle image velocimetry (PIV) are used to record the mechanical parameters during the motion and visualize the flow field. The mechanical data are homogenized and normalized to obtain the variation of foils’ hydrodynamic properties with flexibility and motion parameters. The experimental images are processed to obtain the motion trajectories and the flow field information. The experimental results show that the highly flexible foil performs well at low frequencies; its vortices are a little stronger and slightly farther apart laterally but closer together longitudinally, and appear to persist farther downstream than those of the other models. Thus, flexibility plays an essential role in thrust stability and speed stability. Furthermore, the flap trajectory changes from haphazard to a Fig. 8 shape and ultimately into an arc as the stiffness increases.

On the coupling mechanism of rain–wind two-phase flow induced cable vibration: A wake-dynamics perspective

Large amplitude rain–wind induced vibration (RWIV) of stay cables of long-span cable-stayed bridges has been a major concern in the past three decades, yet its excitation mechanism has not been clarified thoroughly. Recent numerical simulations and experimental work showed that when the RWIV of a cable is developing, the dominant frequency of its unsteady lift forces is notably decreased, i.e., the conventional Strouhal law of cylinder flow is obeyed. In some experimental studies, however, the small-scale vortical structures, which are believed to be closely associated with the low-frequency fluctuating components, are visualized qualitatively in the cable wake during the RWIV. In the present work, the first experimental observation of the dynamic wake behind a cable during the RWIV is presented. The RWIV is reappeared based on a stay cable model supported by springs in wind tunnel tests. The dynamic characteristics of the water rivulet on the upper surface of the cable are recorded by a high-speed camera, and the detailed wake flow of the cable is captured by employing the high-speed particle image velocimetry technique. We focus on the low-frequency synchronization between the cable vibration, upper-rivulet movement, and the wake dynamics to investigate the excitation mechanism of the RWIV. A three-phase (gas–liquid–solid) coupling scenario is finally proposed to explain this interesting phenomenon as low-frequency resonance between fluids and the cable structure.

Bubble dynamics and induced flow in a subcooled nucleate pool boiling with varying subcooled temperature

We experimentally investigate the dynamics of a vapor bubble and subsequent flow induced in a bulk liquid, generated by a subcooled nucleate pool boiling. We focus on the dependency of bubble lifetime (e.g., growth, departure, rise and collapse) on the subcooled temperature. While varying the subcooled temperature (ΔTsub) as 2−15∘C, the temporal variation of bubble dynamics and liquid flow induced in its vicinity are measured simultaneously using the high-speed two-phase particle image velocimetry. In general, the bubble departure diameter and growth time tend to decay exponentially with increasing ΔTsub, and it is possible to classify the regimes further. In a highly subcooled condition (ΔTsub≥8∘C), the bubble starts to shrink (condense) before departing from the wall, while it grows without shrinkage in a less subcooled condition (ΔTsub<8∘C). We modify and combine the existing bubble growth models to predict this phenomenon, which shows a good agreement with the measured data. When ΔTsub<8∘C, the departing bubble (2.5−3.1 mm in size) rises vertically with a significant deformation driven by the liquid inertia. However, the smaller bubble (1.5−2.2 mm) in a highly subcooled condition dissipates rapidly (much shorter lifetime) with a less deformation, owing to the enhanced condensation rate and surface tension. The rising velocity and aspect ratio of the bubble in a less subcooled condition fluctuate following the liquid flow induced by the bubble deformation. Counter-rotating vortical structure around a rising bubble develops larger for the less subcooled condition, which convects further from the wall than the cases of a highly subcooled condition.

Relative Effects of Velocity- and Mixture-Coupling in a Thermoacoustically Unstable, Partially Premixed Flame

Abstract Combustion instability, which is the result of a coupling between combustor acoustic modes and unsteady flame heat release rate, is a severely limiting factor in the operability and performance of modern gas turbine engines. This coupling can occur through different pathways, such as flow-field fluctuations or equivalence ratio fluctuations. In realistic combustor systems, there are complex hydrodynamic and thermo-chemical processes involved, which can lead to multiple coupling pathways. In order to understand and predict the mechanisms that govern the onset of combustion instability in real gas turbine engines, we consider the influences that each of these coupling pathways can have on the stability and dynamics of a partially premixed, swirl-stabilized flame. In this study, we use a model gas turbine combustor with two concentric swirling nozzles of air, separated by a ring of fuel injectors, operating at an elevated pressure of 5 bar. The flow split between the two streams is systematically varied to observe the impact on the flow and flame dynamics. High-speed stereoscopic particle image velocimetry, OH planar laser-induced fluorescence, and acetone planar laser-induced fluorescence are used to obtain information about the velocity field, flame, and fuel-flow behavior, respectively. Depending on the flow conditions, a thermoacoustic oscillation mode or a hydrodynamic mode, identified as the precessing vortex core, is present. The focus of this study is to characterize the mixture coupling processes in this partially premixed flame as well as the impact that the velocity oscillations have on mixture coupling. Our results show that, for this combustor system, changing the flow split between the two concentric nozzles can alter the dominant harmonic oscillation modes in the system, which can significantly impact the dispersion of fuel into air, thereby modulating the local equivalence ratio of the flame. This insight can be used to design instability control mechanisms in real gas turbine engines.

Fluid Dynamics Behind a Circular Cylinder Embedded With an Active Flapping Jet Actuator

Abstract The effects of an active flapping jet actuator on the wake flow dynamics behind a circular cylinder in wind tunnel tests were investigated. An active flapping jet actuator was embedded in the cylinder in advance to invoke a spontaneous flapping jet into the cylinder's wake. The experiment, which was performed in a wind tunnel with a Reynolds number of Re = 1.99 × 104, was based on the oncoming wind speed, cylinder diameter, and kinematic viscosity of the air at the laboratory's temperature. The flow-field structures behind the cylinder model with different dimensionless jet momentum coefficients, Cu, were obtained using the high-speed particle image velocimetry technique. The proper orthogonal decomposition (POD) method was used to represent the variation of the POD mode energy, mode coefficients, and the reconstructed spreading vorticity. The dynamic temporal evolution and time-averaged results in the near wake region of the cylinder with and without active flapping-jet control were calculated and analyzed to illustrate the rich phenomena produced by, and the control effect of, the flapping jet. For Cu values up to 0.0554, the periodic vortex shedding was pushed to farther wakes. Meanwhile, the time-averaged wake changed considerably, and the distributions of the turbulent kinetic energy and Reynolds shear stress decreased significantly. A data-driven dynamic mode decomposition method was used to extract the coherent structure of the wake of the cylinder embedded with the flapping jet actuator. The Strouhal number of the main mode of the Cu = 0.0865 case was different from the natural case.