Planar Laser Induced Fluorescence Particle Image Velocimetry Research Articles

Characterizing the development of gravity-driven slug flows using high-speed imaging and PIV-PLIF techniques

Although developing gravity-driven slug flow frequently occurs in oil and gas, and energy systems, its development dynamics remain underexplored; this gap, in turn, has left the underlying relationships between flow evolution and transport phenomena in these applications inadequately characterized as well. The present study experimentally investigates the spatiotemporal-spectral development of gravity-driven air–water and CO2-water slug flows in a vertical 25.4 mm ID pipe. Enhanced flow visualization techniques, utilizing high-speed imaging and particle image velocimetry-planar laser induced fluorescence (PIV-PLIF), were employed to determine the behaviors of gas and liquid phases and interactions at four positions along the pipe axis. A machine vision-based algorithm was employed to extract slug unit cell characteristics and instantaneous void fraction signals, allowing for a comprehensive statistical analysis of gas phase behavior across the flow domain. A novel algorithm was also developed to preprocess raw PIV-PLIF images, facilitating phase discrimination and noise reduction before PIV cross-correlation analyses are conducted. The results showed a logarithmic growth in the lengths of Taylor bubbles, liquid slugs, and slug unit cells along the pipe, with liquid slugs constituting nearly 60 % of slug units downstream. Taylor bubble length distributions correlated well with log-normal fits, while liquid slug and unit cell lengths transitioned from log-normal patterns upstream to near-normal distributions downstream with broader and less peaked shapes. Taylor bubble velocities and appearance frequencies of the flow structures declined exponentially along the pipe, with Taylor bubble velocities showing narrower and more peaked near-normal distributions downstream. Instantaneous void fraction signals exhibited fewer, wider peaks and troughs with reduced small-amplitude oscillations downstream. The analysis of the signals indicated a complete bubbly-to-slug transition at Z/D=30. Gas-liquid phase interactions, classified as almost-zero, weak, and strong, impacted liquid phase velocity profiles and the behavior of Taylor bubbles, with minimum stable liquid slug lengths of 8–9 D and a wake length of 1.8 D observed. Empirical correlations were developed to represent the spatiotemporal-spectral aspects of flow development, with spectral parameters, particularly liquid slug frequency, identified as the most reliable indicators of the fully developed region, predicting entrance lengths of 114.0 D and 113.4 D for air–water and CO2-water, respectively. Gas density was found to strongly influence flow characteristics and transition, accelerating the approach to the fully developed region.

Scalar transport in flow past finite circular patches of tall roughness

Simultaneous particle image velocimetry-planar laser-induced fluorescence (PIV-PLIF) measurements are conducted on the flow around and downstream of patches of tall cylinders with various solidity ϕ≡Nc(d/D)2, where Nc is the number of cylinders in a patch, d and D are the diameters of the cylinder and the patch, respectively. Results suggest that the bleeding and scalar concentration in the wake and above the patches are dependent upon ϕ. Recirculation bubble appears at downstream of the patch boundary, limiting the flow from convecting downstream and forcing it (and the scalar) to escape vertically from the top of the patch. Above the wake, a shear layer generated by the velocity difference between the free flow and the wake itself shields the near-wake region. The net effect of the solidity on the wake topology is the appearance of an elongated patch as ϕ decreases and a taller patch as ϕ increases. In terms of scalar transport, both advective and turbulent vertical scalar transport are enhanced as ϕ increases. High advective vertical scalar transport CV is concentrated around the canopy (above the canopy due to vertical bleeding and at the TE due to high scalar concentration), while high turbulent transport c′v′¯ largely coincides with the shear layer. Octant analysis of the conditionally averaged fluctuating components (u′, v′, and c′) shows an enhanced mixing inside the shear layer and entrainment of turbulent flows between the shear layer, the wake region below and the freestream above this layer. In terms of eddy diffusivity models, the complexity of the eddy diffusivity constant increases with ϕ.

Experimental investigation of fountains in homogeneous fluid with coexistent temperature and concentration effects using PIV/PLIF

Fountains formed due to the injection into homogeneous fluids with the combined effect of temperature and concentration occur widely in nature and in environmental improvement and industrial applications such as brine water discharge, living sewage disposal, release of waste hot gases into the atmosphere from chimneys, solar desalination discharge, and sewage wastewater discharge for agriculture irrigation. In this study, the behavior of round fountains with salt water as a homogeneous ambient fluid in a transparent glass tank over 0.25 ≤ FrT ≤ 2.0, 0 ≤ N ≤ 5.0, and 47 ≤ Re ≤ 824 was studied using particle image velocimetry/planar laser-induced fluorescence (PIV/PLIF). The recorded PIV/PLIF images provide a better understanding of the transient behavior and intrinsic laws of buoyant jets in an ambient fluid and optimally use their transport mechanisms and entrainment characteristics in environmental improvement and industrial applications. Scaling for the initial maximum fountain height at the development stage, the obtained scaling correlation from the experimental results is consistent with previous results from direct numerical simulation results.

Time-averaged velocity and scalar fields of the flow over and around a group of cylinders: a model experiment for canopy flows

We conduct a well-controlled model experiment for a wide variety of canopy flows. Examples of these include engineering flows such as wind flow, dispersion of scalars through and over urban areas, and the convective heat transfer in many heat exchangers, as well as natural canopies such as flows through terrestrial or aquatic vegetation. We aim to shed the light on fundamental flow and transport phenomena common to these applications. Specifically, the characteristics of mean flow and scalar concentration characteristics of a turbulent boundary layer flow impinging on a canopy, which comprises a cluster of tall obstacles (this can also be interpreted as a porous obstruction). The cluster is created with a group of cylinders of diameter $d$ and height $h$ arranged in a circular patch of diameter $D$ . The solidity of the patch/obstruction is defined by $\phi$ (the total planar area covered by cylinders), which is systematically varied ( $0.098 \leq \phi \leq 1$ ) by increasing the number of cylinders in a patch ( $N_c$ ). A point source is placed at ground level upstream of the patch and its transport over and around the patch is examined. Time-averaged velocity and scalar fields, obtained from simultaneous planar particle image velocimetry-planar laser-induced fluorescence (PIV-PLIF) measurements, reveal that the characteristics of wake and flow above porous patches are heavily influenced by $\phi$ . In particular, we observe that the horizontal and vertical extent of the wake and scalar concentration downstream of the patches decreases and increases with $\phi$ , respectively. Here, the recirculation bubble is shifted closer to the trailing edge (TE) of the patches as $\phi$ increases, limiting the flow from convecting downstream, decreasing the scalar concentration and virtually ‘extending’ the patch in the streamwise direction. As the bubble forms in the TE, vertical bleeding increases and hence the concentration increases above the patch where the cylinders appear to ‘extend’ vertically towards the freestream.

Experimental Observation of Flow Reversal in Thin Liquid Film Flow Falling on an Inclined Plate

A customized particle image velocimetry/planar laser induced fluorescence (PIV/PLIF) experimental method, aiming to capture transient hydrodynamics of solitary waves of inertia-dominated falling liquid films, is presented in this paper. A novel PIV/PLIF technique, which only uses one camera to capture simultaneously both particle image and fluorescence, and meanwhile a post-processing imaging method is also developed, which is able to simultaneously measure both internal velocity field in film and its topology. To validate the fidelity of the novel PIV/PLIF technique, a comparison between experimental results of streamwise velocity profile and film thickness and that of the Nusselt’s prediction at low Reynold number is carried out, and in addition, integral continuity is checked for transient wavy film, both of which shows that they are in good agreement. Based on experimental velocity fields and film topology, pressure distribution inside film is derived with the Poisson equation. Considering characteristics of traveling waves, the experimental results are presented respectively in spatial mode and temporal mode. In spatial mode, capillary wave dynamics are demonstrated out of velocity field, film topology and pressure distribution, which reveals that flow reversal occurs at capillary troughs. In temporal mode, the mechanism of flow reversal at capillary troughs is scrutinized on the basis of high-frequency velocity sampling and the derived pressure gradient. It is shown that flow reversal at capillary troughs is triggered due to occurrence of positive pressure gradient at the back side of the capillary wave crest, rather than the trough upstream as stated by the previous researchers. By elucidating the dynamics of flow reversal, mechanisms for the upper limit of Reynold number with respect to flow reversal underneath capillary wave trough were proposed, which might be the gradually saturated deceleration from the capillary curvature and shorten deceleration duration determined by the wave speed and capillary wave length. Our results should be of interest for optimization of the mass transport model of falling liquid films and shed light on the revealing of flow reversal mechanism.

Macroscopic Spray Behavior of a Single-Hole Common Rail Diesel Injector Using Gasoline-Blended 5% Biodiesel

This research studies the macroscopic spray structure from a single-hole common rail diesel injector using gasoline-blended 5% biodiesel for use in compression ignition engines. To reduce the NOX/PM trade-off emissions, researchers are investigating gasoline used in compression ignition engines, called gasoline compression ignition. As a result that gasoline is injected directly into the combustion chamber, its spray field has a significant effect on combustion and emissions. Due to its low lubricity, gasoline is blended with biodiesel 5%, as a lubricity enhancer, to prevent the failure of the high-pressure injection system. The macroscopic spray structures of this gasoline blend were investigated Schlieren photography and planar laser-induced fluorescence-particle image velocimetry. Injection pressure was handled by a conventional common rail system, while ambient pressure was controlled by supplying nitrogen into the constant-volume combustion chamber. The effects of injection pressure and ambient pressure on the gasoline spray elucidated by Planar laser-induced fluorescence coupled with particle-image velocimetry (PLIF-PIV) imagery and comparisons with variations in neat diesel spray. In addition, the flow field of gasoline spray that formed vortexes and vorticity was characterized. The results show that the injection pressure and back pressure had the same effects on the gasoline spray structure, in terms of the penetration tip and cone angle, as on the diesel spray. However, the injection pressure had a greater effect on the diesel spray than the gasoline at low ambient pressure due to the occurrence of cavitation. Moreover, the images show the remarkable turbulent structure of gasoline spray and indicate air entrainment at the spray tip region.

Quantitative visualization of temperature field and measurement of local heat transfer coefficient over heat exchanger elements in sinusoidal oscillating flow

The flow and heat transfer in the near field of simplified heat exchanger elements (two cylinders in a tandem arrangement and a longitudinal finned tube) immersed in oscillatory flow were experimentally investigated. The effects of cylinder spacing on heat transfer in the tandem arrangement were examined. The ambient flow was a sinusoidal planar oscillating flow. A synchronous PIV-PLIF (Particle Image Velocimetry- Planar Laser Induced Fluorescence) technique was used to simultaneously capture the time-dependent velocity and temperature fields. In the tandem cylinder configuration, a higher temperature drop in the gap region between the cylinders was observed for the larger cylinder spacing, which yielded to a higher heat transfer rate. For the finned tube configuration, both the velocity and temperature fields featured two recirculation zones at the bases of the attached longitudinal fins. These recirculation bubbles contained flow particles with low velocity and high temperature which led to a lower heat transfer rate in these zones. The heat transfer characteristics of heat exchanger elements in oscillating flows were found to be significantly different from those in steady flows. The results were compared to the heat transfer from a single circular cylinder under identical conditions.

Liquid-phase turbulence measurements in air-water two-phase flows over a wide range of void fractions

This paper focuses on liquid-phase turbulence measurements in air-water two-phase flows over a wide range of void fractions and flow regimes, spanning from bubbly, cap-bubbly, slug, to churn-turbulent flows. The measurements have been conducted in two test facilities, the first one with a circular test section and the second one with a rectangular test section. A particle image velocimetry-planar laser-induced fluorescence (PIV-PLIF) system was used to acquire local liquid-phase turbulence information, including the time-averaged velocity and velocity fluctuations in the streamwise and spanwise directions, and Reynolds stress. An optical phase separation method using fluorescent particles and optical filtration technique was adopted to extract the liquid-phase velocity information. An image pre-processing scheme was imposed on the raw PIV images acquired to remove noise due to the presence of bubble residuals and optically distorted particles in the raw PIV images. Four-sensor conductivity probes and high-speed images were also used to acquire the gas-phase information, which was aimed to understand the flow interfacial structure. The highest area-averaged void fraction covered in the measurements for the circular and rectangular test sections was about 40%.

Simultaneous velocity and concentration measurements in the near field of a turbulent low-pressure jet by digital particle image velocimetry-planar laser-induced fluorescence

The main purpose of this work is to develop a method for simultaneous measurement of velocity and passive scalar concentration by means of digital particle image velocimetry and planar laser-induced fluorescence. Details of the implementation of the method are given, and the technique is applied to measurements of concentration and velocity in the centre-plane of a liquid jet with a Reynolds number of 6,000. The measurements are compared with large eddy simulations. Mean velocities and concentrations, fluctuating velocities and concentrations, and correlation between fluctuating velocities and concentrations are analysed for the first six diameters downstream of the jet exit. The general agreement between measured and simulated results was found to be good, in particular for mean quantities. Mean profiles are also found to be in good agreement with other experimental work on jets reported in the literature. The “whole-plane” measurement method was found to be very useful for detailed comparisons of turbulent statistics with simulated data. The inadequacy of models for turbulent mass transport based on the standard gradient diffusion concept is demonstrated through the experimental data.

Combined planar laser-induced fluorescence-particle image velocimetry technique for velocity and temperature fields

Combined use of particle image velocimetry (PIV) with planar laser-induced fluorescence (PLIF) was developed to measure both velocity and scalars such as temperature and concentration in thermo-fluid flow. PLIF, which uses a temperature-sensitive fluorescent dye excited by a laser light sheet, is a whole-field temperature diagnostic. The technique was applied to a thermal stratified flow and a plane impinging jet. The time evolutions of vortical and thermal structures were successfully obtained in order to understand the mechanism of the turbulent heat transfer in these flows.