Digital holography for micro-droplet diagnostics

Digital in-line holography and digital image plane holography (DIPH) are applied for the study of the droplets generated by a micro-dispensing device. This micro-dispenser provides droplets whose diameter can be changed by varying the pressure applied to the reservoir containing the substance to be atomized. The droplet diameters measured with both techniques show good agreement in the range between 10 and 20 psi. The diameter increases with the pressure and so does the dispersion. Droplet velocity has been measured with DIPH at 10 and 20 psi. It was found that the velocity increases linearly with the droplet size.     

Three-component velocity field measurement in confined liquid flows with high-speed digital image plane holography

In the last years, several techniques have been developed for the measurement of the three velocity components in a fluid plane or volume. Techniques as stereoscopic particle image velocimetry (SPIV) or tomographic PIV need a complex set-up and present serious restrictions when applied to confined liquid flows. Other like digital holographic PIV has some limitations in the particle concentration that can be measured. In this work, high-speed digital image plane holography has been applied for the measurement of the three velocity components in a complex geometry brain aneurysm model, using a two-cavity high-speed laser, one double frame camera and normal visualization, like in regular PIV. A portable and compact system has been built for adapting the high-speed laser short coherence length to the measurement of larger areas.     

Fixed stream-tube method for solving two-phase plane flow problems and its theoretical analysis

The fixed stream-tube method widely adopted in engineering field for giving an approximate solution to the two-dimensional problems of two-phase flow through porous media is summarized and an improvement has been made in this paper. Its core part, i.e., the fluid displacement within a one-dimensional stream tube with variable cross-sectional area under a given pressure difference across the tube is thoroughly studied. The existence and uniqueness of solution are proved, the exact solution, numerical solution and its convergence, stability analyses are given in this paper.     

Use of holography in particle image velocimetry measurements of a swirling flow

 Particle Image Velocimetry (PIV) is now a well established experimental technique to measure two components of the velocity in a planar region of a flow field. This paper shows how its proven capabilities can be further extended by using holographic recording to register the particle displacements. Among other unique characteristics, holography enables the acquisition of multiple images on a single plate, and the recording of three dimensional images. These features are used to circumvent some of the limitations of conventional PIV. Some of these possibilities are demonstrated in this study by applying the technique to a high Reynolds number swirling flow using a lens-less off-axis orthogonal recording geometry. Received: 25 February 1998/ Accepted: 2 September 1998     

Difference scheme for two-phase flow

A numerical method for two-phase flow with hydrodynamics behavior was considered. The nonconservative hyperbolic governing equations proposed by Saurel and Gallout were adopted. Dissipative effects were neglected but they could be included in the model without major difficulties. Based on the opinion proposed by Abgrall that “a two phase system, uniform in velocity and pressure at t = 0 will be uniform on the same variable during its temporal evolution“, a simple accurate and fully Eulerian numerical method was presented for the simulation of multiphase compressible flows in hydrodynamic regime. The numerical method relies on Godunov-typescheme, with HLLC and Lax-Friedrichs type approximate Riemann solvers for the resolution of conservation equations, and nonconservative equation. Speed relaxation and pressure relaxation processes were introduced to account for the interaction between the phases. Test problem was presented in one space dimension which illustrated that our scheme is accurate, stable and oscillation free.     

Bounding principles for two-phase flow systems

For two classes of multiphase flow problems, upper and lower bounding principles are constructed for the rate of dissipation of mechanical energy as the result of viscous forces both in the bulk fluid and in the phase interface. These principles are developed for simple classes of non-linear constitutive equations for the bulk stress tensor and for the surface stress tensor. The integral mechanical energy balance relates these bounds to quantities that are subject to direct experimental evaluation.     

Transient flow redistribution in annular two-phase flow

A model is described for the prediction of transient flow redistribution in vertical annular two-phase flow. The model is based on an analysis of the local parameters controlling the flow and takes account of the diffusive motion of entrained droplets and the delay time for change in the wave structure on the film. Comparisons are made with experimental results on inlet effects and it is shown that the wall injection experimental results can be described by the model. The jet injection results are not fitted by the model and it is shown that some additional deposition mechanism must be present.     

Curved laser-sheet for conformal surface flow diagnostics

Three-dimensional surface illumination using curved laser-sheet techniques is introduced for optical flow measurements in conformal curved surfaces. The illumination method is applicable to many different optical-based flow measurement techniques, with this paper focusing on application to flow visualization and particle image velocimetry. A brief discussion and example of curved laser-sheet generation is given followed by an example of the technique applied to PIV of low Reynolds number transitional flow around a low-pressure turbine blade.     

Vortices in bubbly two-phase flow

The properties of a von Kármán vortex street are examined theoretically for bubbly flow around a rectangular obstacle. The time-dependent, two-dimensional Navier-Stokes equations describing each field are coupled through local pressure equilibration, a phenomenological momentum exchange term and a new representation of the virtual mass acceleration terms. Bubble fragmentation and coalescence are represented by the effect of relaxation to local Weber number equilibrium in a transport equation for the time and space variations of bubble-number density. Turbulence is represented by an eddy viscosity model. High-speed computer results for an air-water system agree well with published data for downstream gas accumulation in the vortices. Variations in Strouhal frequency with upstream void fraction are discussed in terms of bubble sublayer growth along the sides of the obstacle and the resulting movement of the flow separation streamline. Satellite eddy formation is observed, and the alteration of street characteristics by bubble migration is examined.     

Nonspherical particles in two-phase flow

We bave examined the trio-phase fiow of a gas with dispersed, nonspherical, rigid particles. We describe a model for the motion of a single ellipsoid in the fiuid, including the degrees of freedom associated with translation, lift, orientation and angular velocity. We assume there is no interaction between the particles, and give the Liouville equation for the distribution function of the particles. A computer code bas been developed to simulate the two-dimensional behavior of such a fiow. We demonstrate, in the case of a high-speed jet impinging on an obstacle, that the force exerted on the obstacle by the particles depends strongly on the eccentricity.     

Flooding in vertical two-phase flow

The phenomenon of flooding limits the stability of a liquid film falling along the walls of a tube in which a gas is flowing upward. This paper describes a series of experiments to observe and photograph the flooding process. Flooding in a vertical perspex tube was observed by viewing axially along the tube from the top, and flooding on the walls of a stainless steel rod supported centrally within a perspex tube was observed by viewing normally through the tube walls. The resulting observations show that flooding occurs as a result of the formation and motion of a large disturbance wave on the surface of the liquid film. A simple theoretical treatment of flooding is presented, based on the observations of the flooding disturbance. The resulting equations enable the calculation of the gas velocity and the liquid wave height at flooding, and the predictions are in good agreement with experimental observations reported here and elsewhere.     

Drops in annular two-phase flow

Drops, one of the forms in which liquid is present in annular gas—liquid flow, are formed from the wall film, carried by the gas or vapour and redeposited. During this time they exert a strong influence on many important parameters of both flow and heat transfer. The available information on the creation, size and velocity, and removal of drops is identified and reviewed.

This review shows that there is an extensive literature on drops and the associated topic of waves in annular gas—liquid flows. In spite of the large number of papers that have been published, there are still some fundamental questions which remain unanswered and there are large gaps in the parameter ranges to be considered.     

Probabilistic determination of two-phase flow regimes in horizontal tubes utilizing an automated image recognition technique

Probabilistic two-phase flow map data is experimentally obtained for R134a at 25.0, 35.0, and 49.7°C, R410A at 25.0°C, mass fluxes from 100 to 600 kg/m²-s, qualities from 0 to1 in 8.00, 5.43, 3.90, and 1.74 mm I.D. single, smooth, adiabatic, horizontal tubes in order to extend probabilistic two-phase flow map modeling techniques to single tubes. A new web camera based flow visualization technique utilizing an illuminated diffuse striped background was used to enhance images, detect fine films, and aid in the automated image recognition process developed in the present study. This technique has an average time fraction classification error of less than 0.01.     

气固两相流介尺度LBM-DEM模型

LBM-DEM耦合方法通常是指一种颗粒流体系统直接数值模拟算法,即是一种不引入经验曳力模型的计算方法,颗粒尺寸通常比计算网格的长度大一个量级,颗粒的受力通过表面的粘性力与压力积分获得,其优点是能描述每个颗粒周围的详细流场,产生详细的颗粒-流体相互作用的动力学信息,可以探索颗粒流体界面的流动、传递和反应的详细信息及两相相互作用的本构关系,但其缺点是计算量巨大,无法应用于真实流化床过程模拟。本文针对气固流化床中的流体以及固体颗粒间的多相流体力学行为,建立了一种稠密气固两相流的介尺度LBMDEM模型,即LBM-DEM耦合的离散颗粒模型,实现在颗粒尺度上流化床的快速离散模拟。该耦合模型采用格子玻尔兹曼方法(LBM)描述气相的流动和传递行为,离散单元法(DEM)用于描述颗粒相的运动,并利用能量最小多尺度(EMMS)曳力解决气固耦合不成熟问题,以提高其模拟精度。通过经典快速流态化的模拟,验证了介尺度LBM-DEM耦合模型的有效性。模拟结果表明介尺度LBM-DEM模型是一种探索实验室规模气固系统的有力手段。     

Drop sizes in annular two-phase flow

Drop sizes in annular flow have been measured using a diffraction technique. Several series of experiments were carried out to determine the effect of gas velocity, drop concentration, film flow rate and tube diameter on drop size. Film flow rate and tube diameter have been found to have very little influence on the sizes of drops produced. An empirical equation which describes the drop sizes is presented.     

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.