Instantaneous and simultaneous planar velocity field measurements of two phases for turbulent mixing of high pressure sprays

This paper describes the tests of accuracy and the first application of a combined planar visualization technique. Its goal is two-phase flow discrimination, i.e. simultaneous measurements of velocity of droplets and ambient gas in the case of two-phase flow mixing, at the same location and with possible conditioning by “apparent diameter” (AD) of the droplets. It combines the mature techniques of particle image velocimetry (PIV), planar Mie scattering diffusion (PMSD), planar laser-induced fluorescence (PLIF), and it necessitates two synchronized cross-correlation systems, digital image treatment and analysis. This technique was developed with the objective of better describing the mixing between liquid and gaseous phases as in the case of high-pressure spray atomization in quiescent ambient gas. The basic principle of separation is to seed the ambient gas with micrometer particles and to tag the liquid with fluorescent dye. We use digital image treatment and analysis to discriminate between the phases. We use two cross-correlation PIV systems in order to obtain the velocity field of the droplets and gas simultaneously and separately at the same location. The digital image processing for separating the phases involves geometric measurement of droplet shapes. This leads to measurement of droplet parameters close to their real diameter, which could be used for analysis of actual mixing. A synchronized system composed of two CCD cameras is used for image recording, and two Nd:YAG lasers are used for generating pulsed light sheets at times t and t + δt. Tests were performed to check for different sources of errors. The combined technique was applied to measurements in high-pressure spray flow atomizing in a quiescent ambient gas, and first results are presented.     

Simultaneous air/fuel-phase PIV measurements in a dense fuel spray

A new diagnostic has been developed that is capable of obtaining simultaneous two-phase velocity measurements in a gasoline direct-injection fuel spray. This technique utilizes a two-laser (double-pulse) two-camera (double-frame) setup to simultaneously image the injected fuel and entrained air to determine the 2D velocity vector fields of both phases using cross-correlation particle image velocimetry (PIV). The air phase is visualized through fluorescence from seeding particles introduced into the static measurement volume while Mie scattering signals are collected from the fuel droplets. The combination of different laser wavelengths and a spectral signal shift for the air phase allows spectral separation of the signals. Independent timing of the laser pulses permits optimized adaptation of the velocity dynamic range for the two phases to account for the large difference in velocities between air and fuel droplets.     

Probing the velocity fields of gas and liquid phase simultaneously in a two-phase flow

The feasibility of simultaneous measurements of the instantaneous velocity fields of gaseous and liquid phase is demonstrated in a laminar, unsteady two-phase flow. Thus, the instantaneous relative velocity field can be measured in such media. This is achieved by combining Particle Image Velocimetry (PIV) and a gas-phase velocimetry technique, which is based on laser-induced fluorescence (LIF) from a gaseous tracer. The wavelength shift of LIF is exploited to separate it from Mie scattering from the liquid phase. The new technique and the PIV measurement system work independently in this approach. Thus, the measurement accuracy and precision of the new technique can be validated by comparing it to the PIV results in regions of the flow field where the relative velocity vanishes. Received: 18 October 1998/Accepted: 16 October 1999     

A novel PIV technique for measurements in multiphase flows and its application to two-phase bubbly flows

A new experimental procedure for performing simultaneous, phase-separated velocity measurements in two-phase flows is introduced. Basically, the novel particle image velocimetry (PIV) technique is a combination of the three most often used PIV techniques in multiphase flows: PIV with fluorescent tracer particles, shadowgraphy, and the digital phase separation with a masking technique. The combination of these three independent measurement techniques is achieved by shifting the background intensity of a PIV recording to a higher, but uniform gray value level. In order to combine the advantages of these multiphase-PIV methods, a new PIV set-up was developed. With this set-up the velocity distributions of the two phases are measured simultaneously with only one b/w camera. This experimental set-up is aimed at providing a means for characterizing the modification of turbulence in the liquid phase by bubbles. This phenomenon is often called "pseudo-turbulence".     

Spray analysis of a gasoline direct injector by means of two-phase PIV

The hollow-cone spray of a high-pressure swirl injector for a direct-injection spark-ignition (DISI) engine was investigated inside a pressure vessel by means of particle image velocimetry (PIV). As the interaction between the spray droplets and the ambient air is of particular interest for the mixture preparation process, two-phase PIV techniques were applied. To allow phase discrimination, fluorescent seeding particles were used to trace the gas phase. Because of the periodicity of piston engine injection, a statistical evaluation of ensemble-averaged fields to reduce cycle-to-cycle variations and to provide more general information about the two-phase flow was performed. Besides the general spray/air interaction process the investigation of the spray collapse at elevated ambient pressures was the main focus of the study. Future investigations of transient interaction processes require simultaneous techniques in combination with a high-speed camera to resolve the transient interaction phenomena. Therefore, optical filters that attenuate Mie-scattered light and transmit fluorescent light were used to collect both phases on the same image. Consequently, phase separation techniques were employed for data analysis. A masking and a peak separation technique are described and a comparison between the results of an instantaneous two-phase flow field in the spray cone of a DISI injector is presented in the paper.     

Simultaneous planar measurement of droplet velocity and size with gas phase velocities in a spray by combined ILIDS and PIV techniques

A new approach for simultaneous planar measurement of droplet velocity and size with gas phase velocities is reported, which combines the out-of-focus imaging technique ‘Interferometric Laser Imaging Droplet Sizing’ (ILIDS) for planar simultaneous droplet size and velocity measurements with the in-focus technique ‘Particle Image Velocimetry’ (PIV) for gas velocity measurements in the vicinity of individual droplets. Discrimination between the gas phase seeding and the droplets is achieved in the PIV images by removing the glare points of focused droplet images, using the droplet position obtained through ILIDS processing. Combination of the two optical arrangements can result in a discrepancy in the location of the centre of a droplet, when imaging through ILIDS and PIV techniques, of up to about 1 mm, which may lead to erroneous identification of the glare points from droplets on the PIV images. The magnitude of the discrepancy is a function of position of the droplet’s image on the CCD array and the degree of defocus, but almost independent of droplet size. Specifically, it varies approximately linearly across the image along the direction corresponding to the direction of propagation of the laser sheet for a given defocus setting in ILIDS. The experimental finding is supported by a theoretical analysis, which was based on geometrical optics for a simple optical configuration that replicates the essential features of the optical system. The discrepancy in the location was measured using a monodisperse droplet generator, and this was subtracted from the droplet centres identified in the ILIDS images of a polydisperse spray without ‘seeding’ particles. This reduced the discrepancy between PIV and ILIDS droplet centres from about 1 mm to about 0.1 mm and hence increased the probability of finding the corresponding fringe patterns on the ILIDS image and glare points on the PIV image. In conclusion, it is shown that the proposed combined method can discriminate between droplets and ‘seeding’ particles and is capable of two-phase measurements in polydisperse sprays.     

Two-phase PIV method using two excitation and two emission spectra

To be able to characterize the airflow in the presence of liquid droplets in a confined geometry, a new two-phase particle image velocimetry (PIV) method is developed. It is based on a two-colour YAG laser and two different fluorescent dyes dissolved in the gas-phase tracers and droplets. This approach permits to separate the images (and thus the information) of the two phases optically and simultaneously. When experiments need to be carried out in a confined geometry (such as in a wind tunnel) with uniform droplet distribution and high turbulence, which are the case in the present investigation, one should be able to deal with continuous droplet deposition on the lateral walls through which the cameras acquire images. It requires the adaptation of the experimental conditions and the development of a dynamic background subtraction algorithm. The typical results reveal the influence of the presence of liquid droplets on the airflow by comparing single-phase flow field to the air-phase motion in two-phase flow configuration. Furthermore, by analysing the continuous-phase and the discrete-phase properties, some aspects of the interaction between the two phases are shown.     

Simultaneous velocimetry/accelerometry measurements in a turbulent two-phase pipe flow

A two-color digital particle image velocimetry and accelerometry (DPIV and DPIA) measurement technique is described that records the velocity and acceleration fields of both the solid and liquid phases simultaneously. Measurements were taken at turbulent conditions of a vertical pipe flow using glass spheres as the solid phase and fluorescent particles to indicate fluid phase motion. Nd-YAG pulse lasers acted as illumination sources and images were recorded by two monochrome CCD cameras. The two-color aspect of the technique was realized by placing optical filters in front of the cameras to discriminate between the phases. Cross-correlations and auto-correlations were applied to determine velocity and acceleration fields of the two phases. Results showing some of the capabilities of the technique as applied to a two-phase pipe flow experiment are provided. For the condition studied, it was found that there was turbulence suppression due to the solid phase and that the statistics associated with the acceleration probability distribution were different for the solid and fluid phases.     

Simultaneous two-phase PIV by two-parameter phase discrimination

 A flexible and robust phase discrimination algorithm for two-phase PIV employs second-order intensity gradients to identify objects. Then, the objects are sorted into solids and tracers according to parametric combinations of size and brightness. Solids velocities are computed by tracking, gas velocities by cross-correlation. Tests in a fully-developed turbulent channel flow of air showed that the two phases do not contaminate or bias each other's velocity statistics. Error magnitude and valid data yield were quantified with artificial images for three particle sizes (25, 33, and 63 μm), two interrogation area sizes (32 and 64 pixels), and volumetric solids loads from 0.0022% to 0.014%. At the channel centerline, the gas valid data yield was above 98% and the RMS error in gas velocity was less than 0.1 pixels for all variations of these parameters. The solid-to-tracer signal ratio was found to be the major parameter affecting the magnitude of the RMS error. Received: 20 September 2000/Accepted: 2 July 2001 Published online: 29 November 2001     

Local gas and liquid phase velocity measurement in a miniature stirred vessel using PIV combined with a new image processing algorithm

A method which combines standard two-dimensional particle image velocimetry (PIV) with a new image processing algorithm has been developed to measure the average local gas bubble velocities, as well as the local velocities of the liquid phase, within small stirred vessel reactors. The technique was applied to measurements in a gas–liquid high throughput experimentation (HTE) vessel of 45 mm diameter, but it is equally suited to measurements in larger scale reactors. For the measurement of liquid velocities, 3 μm latex seeding particles were used. For gas velocity measurements, a separate experiment was conducted which involved doping the liquid phase with fluorescent Rhodamine dye to allow the gas–liquid interfaces to be identified. The analysis of raw PIV images enabled the detection of bubbles within the laser plane, their differentiation from obscuring bubbles in front of the laser plane, and their use in lieu of tracer particles for gas velocity analysis using cross-correlation methods. The accuracy of the technique was verified by measuring the velocity of a bubble rising in a vertical glass column. The new method enabled detailed velocity fields of both phases to be obtained in an air–water system. The overall flow patterns obtained showed a good qualitative agreement with previous work in large scale vessels. The downward liquid velocities above the impeller were greatly reduced by the addition of the gas, and significant differences between the flow patterns of the two-phases were observed.     

Techniques of PIV in stratified two-phase headers

The stratification of two fluid phases, namely gas and liquid, within flow distribution devices, such as headers, that have side or bottom oriented fluid pipe connections, or discharges, has shown relevance to loss-of-coolant accidents in nuclear power plants. Under critical conditions the gas phase could entrain into the predominantly liquid discharge flow causing the fluid quality to be dramatically affected. This condition is referred to as the onset of gas entrainment (OGE) phenomenon and it occurs at a specific critical liquid height which changes with the Froude number. The liquid velocity field at the OGE is of importance, for example, to theorists who may find a semi-empirical approach to model this phenomenon. Stereoscopic particle image velocimetry (PIV) technique is an excellent candidate for non-intrusively investigating the velocity field. The liquid-phase velocity field was investigated for three discharge Froude numbers at the OGE. It was found that the stereoscopic PIV could be used to extract the velocity field experimentally, yet a high degree of error was found in the region closest to the discharge. The relative error was determined through conservation of mass by comparing the flow rate obtained with the PIV data to that obtained using a flow meter. In summary it was found that the number of image planes used, the resolution of the image planes, and consequently the number of vectors used to calculate the flow rate, all contributed a great deal to the relative error.     

Gas-phase velocity field measurements in sprays without particle seeding

A laser-based technique is presented that can be used to measure the instantaneous velocity field of the continuous phase in sprays and aerosols. In contrast to most well established laser-based velocity measurement techniques, this method is independent of particle seeding and Mie scattering. Instead of that it is based on gaseous flow tracers and laser-induced fluorescence (LIF). Inhomogeneous tracer gas distributions, which are created by an incomplete, turbulent mixing process, are exploited for flow tracing. The velocity field can be measured close to the droplets, because frequency-shifted LIF is separated from Mie scattering by optical filters. Validation tests and results from a water spray in air are given. Accuracy and spatial resolution are discussed in detail. Received: 26 April 1999/Accepted: 16 October 1999     

Vector post-processing algorithm for phase discrimination of two-phase PIV

A simple phase separation method using vector post-processing techniques is evaluated to measure velocity fields in a bubble plume. To provide for validation, fluorescent seeding is used, and two sets of synoptic images are obtained: mixed-phase images containing bubbles and fluorescent particles, and fluid-phase images containing only fluorescent particles. A third dataset is derived by applying a digital mask to remove bubbles from the mixed-phase images. All datasets are processed using cross-correlation particle image velocimetry (PIV). The resulting vector maps for the raw, mixed-phase data contain both bubble and continuous-phase velocity vectors. To separate the phases, a vector post-processing algorithm applies a maximum velocity threshold for the continuous-phase velocities coupled with the vector median filter to identify remaining bubble-velocity vectors and remove them from the mixed-phase velocity field. To validate the phase separation algorithm, the post-processed fluid-phase vectors are compared to PIV results obtained from both the optically separated and digitally masked data. The comparison among these methods shows that the post-processed mixed-phase data have small errors in regions near some bubbles, but for dilute environmental flows (low void fraction and slip velocity approximately equal to the entrained fluid velocity), the algorithm predicts well both instantaneous and time average statistical quantities. The method is reliable for flows having 10% or less of the field of view occupied by bubbles. The resulting instantaneous data provide information on plume wandering and eddy-size distributions within the bubble plume. By comparison among the datasets, it is shown that the patchiness of the vector-post processed and image masked data limit the diameter of identifiable eddy structures to the average distance between bubbles in the image, and that both datasets give identical probability density functions of eddy size. The optically filtered data have better data coverage and predict a greater probability of larger eddies as compared to the other two datasets.     

Pulsed laser measurements of particle and vapour concentrations in a turbulent jet

A turbulent jet of air has been seeded with both particles and a vapour. An excimer pumped dye laser excited visible fluorescence from the biacetyl vapour and Mie scattering from the micron-size particles. It was possible to measure the simultaneous scattering from both phases by using interference filters to separate the signals. It has been found that the biacetyl vapour provides an adequate tracer for measurements of the concentration field in a turbulent flow. Furthermore, the feasibility of making simultanous concentration measurements of two phases in a turbulent flow has been demonstrated.     

Phase discrimination method for simultaneous two-phase separation in time-resolved stereo PIV measurements

A phase discrimination method for two-phase PIV is presented that is capable of simultaneously separating the two phases from time-resolved stereoscopic PIV images taken in a particle-laden jet. The technique developed expands on previous work done by Khalitov and Longmire (Exp Fluids 32:252–268, 2002), where by means of image processing techniques, a raw two-phase PIV image can be separated into two single-phase images according to particle size and intensity distributions. The technique is expanded through the use of three new image processing algorithms to separate particles of similar size (up to an order of magnitude better than published work) for fields of view much larger than previously considered. It also addresses the known problem of noisy background images produced by high-speed CMOS cameras, which makes the particle detection and separation from the noisy background difficult, through the use of a novel fast Fourier transform background filter.     

Simultaneous measurement of particle and fluid velocities in a plane mixing layer with dispersion

 An optical method is presented to measure simultaneously and separately the velocity field of both phases in particle-laden flows. The fluid is seeded with flow markers which are fluorescent at a specific wavelength and thus can be distinguished from the sediment particles by applying an optical filter. The motion of each phase is recorded by two CCD cameras, which are triggered such that a high correlation between subsequent images is guaranteed. The velocity fields are determined by means of least-square matching of a group of particles. The whole set-up was applied to study the sedimentation of particles through a rapidly evolving mixing layer. Received: 14 June 1999/Accepted: 15 January 2000     

煤油液滴直径对两相旋转爆轰发动机流场的影响

为探究煤油液滴不同初始直径对气液两相旋转爆轰发动机流场的影响,假设初始注入的煤油液滴具有均匀直径,考虑雾化破碎、蒸发等过程,建立了非定常两相爆轰的Eulerian-Lagrangian模型,进行了液态煤油/高温空气爆轰的非预混二维数值模拟。结果表明：在初始液滴直径为1～70μm的工况范围,燃烧室内均形成了单个稳定传播的旋转爆轰波;全局当量比为1时,爆轰波前的空气区域大于液滴煤油的蒸气区域,导致波前燃料空气混合不均匀,波前均存在富油区和贫油区,两相速度差导致分离出的空气形成低温条带;当煤油液滴的初始直径较小时,波前的反应物混合过程主要受蒸发的影响,爆轰波可稳定传播;当直径减小至1μm时,煤油液滴在入口处即蒸发,旋转爆轰波表现为气相传播的特性,爆轰波结构平整;当煤油液滴的初始直径较大时,波前的反应物混合过程主要受液滴破碎的影响;对于相同的燃料质量流量,在不同初始煤油液滴直径工况下,煤油液滴最大的停留时间均占爆轰波传播时间尺度的80%以上;爆轰波前燃料预蒸发为气相的占比越高,爆轰波的传播速度越高;初始液滴直径为10～70μm的工况范围内,爆轰波的速度随初始直径的增大先升高后降低。     

Three-component multi-phase velocimetry measurements on a GDI spray using optically efficient fluorescent tracers

Two- and three-component multi-phase air/fuel measurements have been performed on a GDI injector. UV-excitable fluorescent tracers have been used to seed the gas phase, and the naturally occurring droplets in the fuel are the other phase. A high-pressure multi-hole GDI injector was mounted in a rig with a glass barrel to simulate the engine cylinder and provide optical access. Images were obtained under controlled conditions of fuel pressure and injection duration. Flow phase and pulse order have been determined from a single 3CCD colour camera. Suitable corrective processes have been adapted and implemented to account for crosstalk and chromatic aberrations so that the uncertainty of the velocity vectors produced is comparable to that of conventional PIV using 532 nm illumination. Multi-phase air/fuel vector maps have been produced. A second colour camera has been added to obtain stereo velocity measurements providing previously unavailable simultaneous information on the multi-phase (fuel/air) interaction with three velocity components.     

Separated two-phase flow in a nozzle

This work was performed to extend and further test the method of handling separated two-phase flow by studying each phase separately and, particularly, by placing emphasis on the study of the gas phase with interface transport expressions showing the influence of the liquid phase on it. A one-dimensional flow model for accelerating flows was used in conjunction with experimental data to obtain the pressure distribution and velocity distribution in a converging nozzle for several values of flow quality and nozzle inlet stagnation pressure. The results tend to support the use of the model (which includes the assumption that the gas is in critical flow when the two-phase mixture is in critical flow) and give some insight regarding the nature of the liquid distribution near the nozzle throat.     

Atomized non-equilibrium two-phase flow in mesochannels: Momentum analysis

Two-phase pressure drop measurements are very difficult to make while the fluid is in non-equilibrium condition, i.e. while phase change is taking place. This is further complicated when an atomized liquid is introduced in the system at much higher velocity than other components such as liquid layer, vapor core, and entrained droplets. The purpose of this paper is to develop a model to predict the two-phase pressure characteristics in a mesochannel under various heat flux and liquid atomization conditions. This model includes the momentum effects of liquid droplets from entrainment and atomization. To verify the model, an in-house experimental setup consisting of a series of converging mesochannels, an atomization facility and a heat source was developed. The two-phase pressure of boiling PF5050 was measured along the wall of a mesochannel. The one-dimensional model shows good agreement with the experimental data. The effects of channel wall angle, droplet velocity and spray mass fraction on two-phase pressure characteristics are predicted. Numerical results show that an optimal spray cooling unit can be designed by optimizing channel wall angle and droplet velocity.