Particle Image Velocimetry Results Research Articles

Effect of swirl cup’s secondary swirler on flow field and ignition performance

In a gas turbine engine combustor, highly swirling combustion is usually adopted to stabilize flame and reduce pollutant emissions. Swirl cup, as an air blast atomizer, is widely used to provide a uniform presentation of fuel droplets to the combustor dome. This paper investigated the effect of secondary swirler on the flow field downstream of the swirl cup using particle image velocimetry (PIV). Three swirl cups’ non-reacting flow field were studied: case A, B and C with secondary swirler vane angle 53°, 60° and 68° respectively. Detailed mean and transient velocities and vorticity in the center plane were obtained. From the PIV results, a sharp contrast flow field was obtained for case A to other two cases due to the lower secondary swirling intensity. The recirculation zone is collapsed in disorder for the case A. Ignition tests of the three cups were completed in a single cup combustor. In general, the ignition performance increases with the increasing of the secondary swirling intensity. For case A, the ignition performance is very unstable and has much randomness and there is no clear lean ignition boundary can be generated. This work can further understand the swirl behavior and ignition mechanism.

Application of PIV measurement techniques to study the characteristics of flame–acoustic wave interactions

The present paper introduce the development of the experimental setup and measurement methodologies to study the structure of acoustic wave and the interaction between the acoustic fields and flame flow inside a square tube by using a time-resolved Particle Image Velocimetry (PIV). The design of signal synchronisation system, the selection of seeding particles, the test of seeding methods and the statistical analysis of the PIV data were introduced. The titanium dioxide (TiO2) was selected as the seeding particle for the PIV. The accuracy of synchronisation system was checked by a simple experiment. A time-resolved PIV results showed that the acoustic velocity difference was less than 2.8% at specific phase angle over 1000 excitation cycles at an excitation frequency of 385Hz. For the case of hot flame gas, the largest difference is 4.4% over 100 excitation cycles at an excitation frequency of 10Hz. Results proves that the present experimental system has high reliability to measure and analyse the characteristics of flame–acoustic interactions.

3D and 2D experimental views on the flow field of gas-evolving electrode cold model for electrolysis magnesium

In the magnesium electrolysis process, chlorine gas bubbles release at the surfaces of anode and affect electrolyte flow patterns. This paper presents an experimental apparatus to simulate the flow field induced by chlorine gas evolution at the gas-evolving electrodes of magnesium electrolysis cell. The three-dimensional flow structures were determined by using volumetric three-component velocimetry (V3V) technique, which has the ability to capture the out-of-plane velocity component. The three-dimensional flow structures in the region with a depth about 120mm can be obtained. To achieve this, approximately 15,000 three-dimensional velocity vectors were detected in the flow measurements and constituted the three-dimensional flow field, which eliminated the perspective error caused by the out-of-plane motion in Particle Image Velocimetry (PIV) method. In experiments, comparisons are made between the V3V and PIV results. The in-of-plane velocities data obtained by V3V technique have the same trend with the PIV results, and V3V provides more details in the third direction for the flow field accurately.

Experimental and numerical study of external performance and internal flow of a molten salt pump that transports fluids with different viscosities

Molten salt centrifugal pump is used to deliver high temperature molten salt, whose performance is highly affected by the fluid viscosity. In order to investigate the external performance and internal flow of molten salt pump for different viscous fluids, especially reveal the effect of viscosity on unsteady performance, the unsteady flow in a molten salt pump was researched when the pump handles the fluids with different viscosities by numerical simulation to quantitatively clarify the effects in this article. The external performance estimated using the unsteady flow model was compared with experimental results. By taking water as the working fluid, the velocity profiles near the volute tongue were validated by the results of particle image velocimetry (PIV) measurements. It shows that the numerical simulation results agree well with the experimental results. The shaft power increases with the increase of density and viscosity. Compared with delivering water, the head and efficiency show less change for delivering low viscosity fluids (μ<0.044Pa·s), while for delivering high viscosity fluids (μ>0.044Pa·s), they all decrease significantly. Flow steadiness in the molten salt pump is mainly affected by the viscosity and the interference effect of blade. An increasing viscosity of fluid reduces the fluctuation in flow variables. The results of the study have reference significance on the development of high-performance molten salt pump and stable operation.

The effect of aspect ratio on the wake of the Ahmed body

This paper seeks to further elucidate the wake of the Ahmed body by investigating how the time-averaged flow structures vary with frontal aspect ratio. High-resolution particle image velocimetry results are provided for eight different width Ahmed geometries at $$\text {Re}_{\sqrt{FA}}=3\times 10^4$$ . It is shown that the narrower the body, the greater the downwash over the back slant, meaning the flow remains more attached. At a critical aspect ratio ( $$=1.9$$ ), the flow downstream changes. The separation over the back slant is shown to be affected by the , and this in turn has a significant effect on the circulation in the c-pillar vortices.

An experimental investigation of overexpanded jets with chevrons

Results are presented for experiments investigating the impact of chevrons on overexpanded and subsonic jet plumes and associated acoustic radiation. Faceted, bi-conic convergent-divergent nozzles with design Mach numbers equal to 1.51 and 1.65 are used in the experiments. A design space of nine chevrons with a range of penetrations, lengths and widths are investigated. Low-penetration chevrons are shown to have limited impact on broadband and shock-associated noise for all jet Mach numbers investigated. High-penetration chevrons produce significant peak-noise reduction (relative to the baseline nozzle) in the peak-jet-noise direction with greater noise reduction for subsonic than for supersonic exhausts. High-penetration chevrons are found to increase broadband-shock-associated noise for the highest Mach number jet investigated. While particle image velocimetry results indicate chevrons introduce axial vorticity at all jet conditions, the associated mixing is limited for supersonic exhausts relative to that for subsonic exhausts due to the strong and periodic radial velocity components associated with the jet-shock-cell structure. The introduction of chevrons modifies the near-nozzle shock-cell structure and the impact of internally (within the nozzle) generated shocks on the shock-cell structure generated in the jet plume. The modifications in the near-nozzle shock-cell structure can lead to increased broadband-shock-associated-noise levels for some operating conditions.

CFD investigation and PIV validation of flow field in a compact return diffuser under strong part-load conditions

The internal flow fields in a compact return diffuser under strong part-load conditions are investigated both numerically and experimentally. For numerical simulation, three-dimensional unsteady Reynolds-Averaged Navier-Stokes equations are solved on high-quality structured grids in conjunction with the shear stress transport k-ω turbulence model by employing the computational fluid dynamics (CFD) software ANSYS-Fluent 14.5. For flow field measurements, a special test rig is designed and the two-dimensional particle image velocimetry (PIV) measurements are conducted in the diffuser midplane to capture the complex flow field and for validation of the CFD results. The analysis of the results has been focused on the flow structure in the diffuser, especially under part-load conditions. The detailed comparison between CFD and PIV results is performed. Vortical flow and recirculation flow patterns in the diffuser are captured and analyzed. Large flow separation and backflow appear under the part-load flow conditions. This paper provides a good data set for developing as well as evaluating the accuracy of various CFD models for capturing the complex flow field in a compact return diffuser used with multistage pumps.

Numerical Simulation of Dynamic Process under Low Pressure Casting Based on Two Phase Flow

It has been proved that the mold filling under low pressure casting can be described by the second order nonlinear differential equations characterized by damping oscillations. To obtain higher accuracy of the velocity field, the mold filling process with damping oscillations should be considered. A computational model taking the liquid level droping in the crucible into account based on two phase flow for filling process of damping oscillations is presented in this paper. Hydraulic simulation with the particle image velocimetry was adopted to verify the numerical simulation result. It is proved that the nemuerical simulation results used the presented model perfecly match with the particle image velocimetry results. Liquid level droping in crucible is the main reason for damping oscillations in mold filling. Velocity oscilations can be eliminated when the melt flow across a sudden expansion section. The detailed flow field based on the presented model can provide guidance to optimize the process parameters.

Computational Study on flow Structures of Quench Zone in a Rich-quench-lean Trapped-vortex Combustor

Abstract The quench zone is one of key issues for a rich-quench-lean(RQL)/trapped-vortex combustor(TVC). The characteristics of flow structures of quench zone were investigated computationally with the correction by the particle image velocimetry(PIV) results. RQL/TVC combines the potential advantages of the low emission staged combustion technology of RQL with TVC. TVC has been a very promising novel combustor concept offering improvements in lean blow-out, altitude relight, and operating range. Besides, compared to conventional combustors, TVC has a potential to decrease nitrogen oxides emissions. By utilizing quench devices with different sizes of holes under the same blockage ratio in mainstream position, this paper describes the characteristics of flow structures of quench zone in a RQL/TVC. The results show that, through the quench devices, the mainstream flow quickly mixes with the flow from the cavity zone under variable mixing levels which are related to the changing of different sizes of holes. The effects of turbulent intensity and unmixedness level were examined and discussed.

Cavity dynamics of smooth sphere and golf ball at low Froude numbers, part 2: PIV analysis

In this work, water entry developments of smooth sphere and golf ball were investigated using particle image velocimetry (PIV). Through the attempts of use of PIV, velocity contour plots were generated. PIV results were compared to raw images of water entry profiles and challenges with regard to experimental errors were discussed.

Flow pattern evaluation of the internal circulation gasifying principle

Gasification is a key process for the thermochemical conversion of coal for both energetic utilization and its application as a chemical feedstock. In this regard low grade coal with high-ash content gains in importance. Because of its high availability, production share is increasing. Therefore, adapted gasifiers of novel design are required to meet the demands associated with converting high-ash coals. The internal circulation (INCI) principle is a multi-stage gasification system with the ability to efficiently convert high-ash feedstock.The present paper introduces the test-scale COORVED reactor, which was designed according to the INCI principle. Particle image velocimetry (PIV) and computational fluid dynamics (CFD) simulations are conducted for ambient conditions in a cold-flow test facility to demonstrate the flow pattern according to the INCI principle. The evidence of a recirculation cell is provided by PIV experiments. CFD simulations show similar results in terms of overall flow pattern and size of the recirculation cell. After the intensive investigations of the flow pattern in the cold-flow test facility, first PIV results were obtained in the COORVED reactor. The complex accessibility of the reactor raised the necessity of the validation of the elaborate measurement setup. The reliability of the measurement results is proven by generating a laminar velocity profile at ambient conditions, whose results are comparable to analytical solutions.

Boundary Layer Heat Transport in turbulent Rayleigh‐Bénard Convection in Air

AbstractWe study the convective heat transfer in a large‐scale Rayleigh‐Bénard (RB) experiment which is called the “Barrel of Ilmenau”. We present the results of flow visualization and Particle Image Velocimetry (PIV) measurements of the near wall flow field in a plane perpendicular to the surface of the heated bottom plate. The experiment was run in a smaller rectangular inset that was placed inside the larger barrel. The Rayleigh number amounts to Ra = 1.4 × 1010. The aspect ratios were Γx = 1 in flow direction and Γy = 0.26 perpendicular to the vertical flow plane. The measurements have been undertaken using a 2 W continuous wave Laser in combination with a light sheet optics and various cameras. Due to the slender geometry of the cell, the mean wind is confined in one direction where the Laser light sheet is aligned parallel. The flow was seeded with droplets of 1...2 µm size generated using an ordinary fog machine. Flow visualization as well as the PIV data clearly show the intermittent character of the boundary layer flow field that permanently switches between “laminar” and “turbulent” phases. (© 2014 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Reproducing tornadoes in laboratory using proper scaling

Experimentally simulated tornado-like vortices are related to field tornadoes in order to: (i) establish proper kinematic and dynamic scaling and (ii) attempt to determine a relationship between laboratory parameters and the Enhanced Fujita Scale (EF-Scale). Data from recent in-situ Doppler radar campaigns are analyzed using the Ground-Based Velocity Track Display (GBVTD) method and a unique dataset of three-dimensional axisymmetric tornado flow fields is generated. In parallel, Particle Image Velocimetry (PIV) results of the most recent experimental simulations of tornado vortices performed in the model WindEEE Dome (MWD) are analyzed and then compared with the GBVTD-retrieved full-scale data. Based on these comparisons, the swirl ratio of the full-scale tornadoes, as well as the length and velocity scaling ratios of the simulated tornadoes are identified. It is concluded that the MWD apparatus can generate tornado-like vortices equivalent to EF0 to low-end EF3 rated tornadoes in nature.

Computational Fluid Dynamics (CFD) modelling of transfer chutes: Assessment of viscosity, drag and turbulence models

Computational Fluid Dynamics (CFD) has been successfully applied to evaluate potential dust emissions from bulk material transfer chutes. The implementation of appropriate models and modelling parameters is shown to be critical to the overall accuracy of the simulation results. This paper presents the influence of different models on the CFD simulation of transfer chutes and follows from an earlier study that details the influence of model parameters. The aim of this paper is to offer guidance to select models, and provide a better understanding of their influence in order to evaluate the most appropriate viscosity model, drag model and turbulence model for this application. A two-phase three-dimensional Euler–Euler model in commercial CFD software ANSYS FLUENT has been selected to model the granular and air flow in the transfer chute. The simulated air velocity profiles are discussed by comparing with each other and against experimental data obtained from Particle Image Velocimetry (PIV) results. The simulated particle velocity distributions were compared with results obtained using a well-established continuum method which was developed by Roberts. Furthermore, the air mass flow rates were analysed to evaluate the influence of different models. The results show that the granular viscosity model had a strong influence on the predictions of both air and particle flow. It was observed that both the drag model and turbulence model had limited influence on the outlet air velocities. The results also indicate that the Di-Felice drag model and SST k–ω turbulence model provide solutions closer to the experimental values than the other models investigated.

HIGH-RESOLUTION PIV MEASUREMENTS FOR REAR-END AND HEAD-ON COLLISIONS OF TWO SOLITARY WAVES

The flow visualization technique was applied to the collision of two solitary waves propagating in the same and opposite directions. Measurements of the velocity and trajectory of the solitary waves were conducted using a particle image velocimetry (PIV) system consisting of an 8-W ND:YAG laser and a high-speed CCD camera. In the solitary- wave interaction tests, we set up three kinds of velocity fields-smaller, taller, and compound waves in the rear-end collision tests, and right-running, left-running, and colliding waves in the head-on collision tests. Instantaneous and spatial surface profiles were measured using the image thresholding method in which the boundary plane between the air and water can be detected as the interface having the maximum luminance value. The measured run-up elevation of the colliding wave was compared with the theoretical elevation using a third-order perturbation solution. Based on a Eulerian-Lagrangian algorithm, the PIV result was applied to the particle tracking process that occurred in the two- dimensional plane.

Low-frequency unsteadiness of vortex wakes over slender bodies at high angle of attack

A type of flow unsteadiness with low frequencies and large amplitude was investigated experimentally for vortex wakes around an ogive-tangent cylinder. The experiments were carried out at angles of attack of 60–80° and subcritical Reynolds numbers of 0.6–1.8×105. The reduced frequencies of the unsteadiness are between 0.038 and 0.072, much less than the frequency of Karman vortex shedding. The unsteady flow induces large fluctuations of sectional side forces. The results of pressure measurements and particle image velocimetry indicate that the flow unsteadiness comes from periodic oscillation of the vortex wakes over the slender body. The time-averaged vortex patterns over the slender body are asymmetric, whose orientation is dependent on azimuthal locations of tip perturbations. Therefore, the vortex oscillation is a type of unsteady oscillation around a time-averaged asymmetric vortex structure.

High fidelity phase locked PIV measurements analysing the flow fields surrounding an oscillating piezoelectric fan

Piezoelectric fans have been studied extensively and are seen as a promising technology for thermal management due to their ability to provide quiet, reliable cooling with low power consumption. The fluid mechanics of an unconfined piezoelectric fan are complex which is why the majority of the literature to date confines the fan in an attempt to simplify the flow field. This paper investigates the fluid mechanics of an unconfined fan operating in its first vibration frequency mode. The piezoelectric fan used in this study measures 12.7mm × 70mm and resonates at 92.5Hz in air. A custom built experimental facility was developed to capture the fan's flow field using phase locked Particle Image Velocimetry (PIV). The phase locked PIV results are presented in terms of vorticity and show the formation of a horse shoe vortex. A three dimensional A2 criterion constructed from interpolated PIV measurements was used to identify the vortex core in the vicinity of the fan. This analysis was used to clearly identify the formation of a horse shoe vortex that turns into a hairpin vortex before it breaks up due to a combination of vortex shedding and flow along the fan blade. The results presented in this paper contribute to both the fluid dynamics and heat transfer literature concerning first mode fan oscillation.

Computational and experimental studies of the flow field near the beam entrance window of a liquid metal target

After the first world liquid metal target has been successfully operated at the SINQ facility at the Paul Scherrer Institut (PSI) for 6 months. The idea of having a reliable target with a bypass flow for cooling the beam entrance window, but with the bypass flow not driven by a separate pump, was examined within the project called LIMETS (Liquid Metal Target for SINQ). In designing of liquid metal targets, turbulence modelling is of high importance due to lack in methods for measuring the spatial distribution of flow and turbulence characteristics. In this study, validation of different turbulence models were performed in water model with hemispherical geometry using particle image velocimetry (PIV) technique. Two components of water flow velocity in plexiglas container with inner radius of 88mm were measured in different cross sections, with the velocities varying from 1 to 10m/s. Numerical calculations using large eddy simulation (LES) approach and Reynolds averaged Navier–Stokes (RANS) models were carried out to validate their applicability and study performance issues. Mean velocity and turbulence kinetic energy data were used for comparison of PIV and calculation results. Reasonable agreement was obtained for mean velocity data, with some discrepancies due to the limited length of the inlet tube. However, several discrepancies in turbulence characteristics were found in numerical results, especially in RANS model calculations.

Fluidically Enhanced Chevrons for Supersonic Jet Noise Reduction

An experimental and computational investigation of a combination of fluidic injection and chevrons (fluidically enhanced chevrons) for supersonic jet noise reduction was performed. Previous studies have shown that chevrons underperform in overexpanded flow due to reductions in effective penetration. Consequently, this study focuses on improvements gained by fluidic enhancement in the overexpanded regime. Acoustic results indicated the fluidically enhanced chevrons outperform the chevrons, with additional overall sound pressure level reductions of 2 dB in the upstream direction and nearly 1.5 dB in the downstream direction. Spectral results indicated that primary benefits were in shock and low-frequency noise. A small high-frequency penalty was the only observed detriment. Time-averaged velocity and turbulence quantities as well as shock cell spacing, were computed by large-eddy simulation and then compared to particle imaging velocimetry and shadowgraph results. These flowfield measurements indicated that the fluidically enhanced chevrons modify the flowfield by reducing the shock cell spacing and shock strength and increasing the upstream jet half-width, peak and integrated turbulence values, and shear-layer thickness. These modifications were caused by the introduction of streamwise vortices with over four times the magnitude of the unenhanced chevrons. Noise reductions were attributed to flowfield modifications through the use of several theoretical models, such as Powell’s screech formula.

Effects of Plasma Aerodynamic Actuation on Corner Separation in a Highly Loaded Compressor Cascade

This paper reports experimental results on the effects of plasma aerodynamic actuation (PAA) on corner separation control in a highly loaded, low speed, linear compressor cascade. Total pressure loss coefficient distribution was adopted to evaluate the corner separation control effect in wind tunnel experiments. Results of pressure measurements and particle image velocimetry (PIV) show that the control effect of pitch-wise PAA on the endwall is much better than that of stream-wise PAA on the suction surface. When both the pitch-wise PAA on the endwall and stream-wise PAA on the suction surface are turned on simultaneously, the control effect is the best among all three PAA types. The mechanisms of nanosecond discharge and microsecond discharge PAA are different in corner separation control. The control effect of microsecond discharge PAA turns out better with the increase of discharge voltage and duty cycle. Compared with microsecond discharge PAA, nanosecond discharge PAA is more effective in preventing corner separation when the freestream velocity increases. Frequency is one of the most important parameters in plasma flow control. The optimum excitation frequency of microsecond discharge PAA is 500 Hz, which is different from the frequency corresponding to the case with a Strouhal number of unity.