Fluid dynamics and thermal physics of two-phase flows: Problems and achievements

The problem and specific aspects of studying two-phase flows laden with solid particles, droplets, and bubbles are considered. The main characteristics of two-phase flows and methods for their modeling are reported. The results of experimental and computation-theoretical investigations of two-phase flows of different types are described.     

Two-phase cryogenic flow meters: Part I – Realization of the calorimetric method

While operating with multicomponent flows, one needs to determine such characteristics as temperature, pressure, mass flow rate and component composition – mass quality, and void fraction. The main attention is given to measurement of mass flow rate of the two-phase cryogenic flows which can be used for superconducting accelerators, refueling hydrogen system for space and liquid natural gas industry, in particular. On the one hand, a two-phase flow is one of the simplest cases of multi-component flows which can be observed in cryogenics. On the other hand, this is rather sophisticated problem in cryogenics to create two-phase flow meters and estimate their metrological characteristics. This problem is discussed. Two methods are suggested – calorimetric and pressure drop ones. Features of the calorimetric method are discussed in this part.     

A Digital Special-Purpose Signal Processor for Two-Phase Flow Real-Time Analysis

In the statistical study of two-phase flow real-time control of experiments is often needed. Furthermore, in industrial processes involving boiling units control or two-phase gas-liquid transport control, real-time signal processing is required to check the status of two-phase flows. An instrument is described for the measurement of the fundamental characteristic parameters of two-phase flows. The measurement technique employs extensively high speed digital computation. It is based on a discrimination of the probe output signal into two signals, representing, respectively, a "bubble" flow and a slug flow. The parameters are measured on averaging times ranging from 10 ms to 104 s. Measurement errors and limitations of this high-resolution measurement technique are pointed out. Its utility in evaluating the fundamental statistical parameters of two-phase flows is demonstrated.     

Application of radio frequency method to measurements in cryogenics

This paper reports the specific features of the radio frequency method used to measure characteristics of cryogenic flows. The techniques for determination of the void fraction and quality of two-phase flows and the mean integral thermodynamic characteristics of single-phase flows, e.g. temperature and density, are described. The design and basic parameters of sensors with channels of round and annular cross-section are discussed. Possible combinations of these sensors with other devices for the measurement of two-phase and single-phase medium flow rates are shown. Information about the measuring system as well as measurement results for helium and nitrogen are presented. The sensor characteristics for hydrogen and methane have been estimated theoretically.     

Effects of higher wall roughness on dense particle–laden dispersion behaviors

To analyze the effects of higher wall roughness on dense particle–laden dispersion behaviors under reduced gravity environments, a dense gas–particle two-phase second-order-moment turbulent model are developed. In this model, the wall roughness function and the kinetic theory of granular flows are coupled and closed. Anisotropy of gas–solid two-phase stresses and the interaction between gas–particle are fully considered using two-phase Reynolds stress model and the two-phase correlation transport equation. Numerical simulation test is validated by Sommerfeld and Kussin (2003) experiments data with higher wall roughness 8.32 μm. Predicted results showed that the particle concentration distribution, particle fluctuation velocity, particle temperature and particle collision frequency are greatly affected by higher wall roughness, as well as particle Reynolds stress and interactions between gas and particle turbulent flows are redistributed. Under microgravity conditions, particle temperature and collision frequency are greatly less than those of earth and lunar gravity.     

The Influence of the Gravity Force on Longwave Marangoni Patterns in Two-Layer Films

An Euler–Euler two-fluid model based on the second-order-moment closure approach and the granular kinetic theory of dense gas-particle flows was presented. Anisotropy of gas-solid two-phase stress and the interaction between two-phase stresses are fully considered by two-phase Reynolds stress model and the transport equation of two-phase stress correlation. Under the microgravity space environments, hydrodynamic characters and particle dispersion behaviors of dense gas-particle turbulence flows are numerically simulated. Simulation results of particle concentration and particle velocity are in good agreement with measurement data under earth gravity environment. Decreased gravity can decrease the particle dispersion and can weaken the particle–particle collision as well as it is in favor of producing isotropic flow structures. Moreover, axial–axial fluctuation velocity correlation of gas and particle in earth gravity is approximately 3.0 times greater than those of microgravity and it is smaller than axial particle velocity fluctuation due to larger particle inertia and the larger particle turbulence diffusions.     

Vertically stratified flows in microchannels. Computational simulations and applications to solvent extraction and ion exchange

In this paper, we describe the conditions under which two immiscible fluids flow atop one another (viewed perpendicular to the plane on which the channel is inscribed) in a shallow microfluidic channel. First, we predict the behavior of a two-phase system using fluid dynamic simulations with water-butanol and water-chloroform as model systems. We numerically model the effect of various physical parameters, such as interfacial surface tension, density, viscosity, wall contact angle, and flow velocity on the type of flow observed and find that interfacial surface tension and viscosity are the parameters responsible for formation of vertically stratified, side-by-side, or segmented flows. As predicted by numerical simulations, a water-chloroform system never assumes a vertically stratified configuration, but a water-butanol system does when the two liquids flow at sufficiently high flow velocities. In actual experiments, we test conditions under which potentially useful two-phase systems form stable vertically stratified flows. We also demonstrate that compared to side-by-side flow schemes, shorter diffusion paths are achievable, and thus, the system can be used at higher flow rates to obtain the same performance. We then apply such findings to practical analytical problems, such as solvent extraction and ion exchange.     

Lagrange-Euler description of discontinuous flows of a two-phase reacting medium

Novel methods for calculating hydrodynamic behavior are used to analyze the propagation of shock waves in two-phase reacting mixtures. New data are reported on the detailed structure of shock-wave two-phase flow, and criteria are proposed to determine the conditions for the initiation of combustion, which is important for explaining the characteristic features of such flows.     

Calculation of two-phase adiabatic flow in a pebble bed

Experimental and theoretical data for two-phase flows in pebble beds are generalized. The dependence of pressure loss on the main characteristics of flow is treated, as well as the correlation between the steam quality and void fraction and the conditions of similarity of water-vapor and water-gas flows. Analysis of these dependences reveals that the gas phase distribution, which is uniform over the cross section of the pebble bed, may be stable and unstable.     

Numerical modeling of droplet deposition in channels of intricate shape

Numerical modeling of two-phase flows in channels of intricate configuration is conducted. Problems of the stability of the proposed algorithm for highly subsonic flows are studied. The effect of various physicomechanical parameters on the flow structure in corrugated channels is considered.     

Fringe probing of gas-liquid interfacial film in a microcapillary tube

An optical diagnostic technique has been developed to measure the gas-liquid interfacial film thickness in microcapillary two-phase flows. The spatial frequencies from the multiscattering measured with a CCD camera are used to determine the slug diameter and film thickness. It is found that, with an optimized optical orientation angle, the spatial frequency method shows great accuracy in the measurements. To demonstrate the capability of the newly developed method, a validation experiment was conducted in water-air and water-honey mixture-air two-phase flows. We measured the spatial frequency variations when the microbubble and slug were pulsating by utilizing a highly accurate signal processing technique and a five-point interpolation method. This newly developed optical method is easy to implement, and it will be a useful technique for two-phase flow measurements.     

Numerical simulation of unsteady cavitating flows using a homogenous equilibrium model

 Numerical simulations of two-dimensional cavity flows around a flat plate normal to flow and flows through a 90^∘ bent duct are performed to clarify unsteady behavior under various cavitation conditions. A numerical method applying a TVD-MacCormack scheme with a cavitation model based on a homogenous equilibrium model of compressible gas-liquid two-phase media proposed by the present authors, is applied to solve the cavitating flow. This method permits the simple treatment of the whole gas-liquid two-phase flow field including wave propagation and large interface deformation. Numerical results including detailed observations of unsteady cavity flows and comparisons of predicted results with experimental data are provided. Received: 5 August 2002 / Accepted: 6 January 2003     

Studies of instability of two-phase flows in parallel channels

Experiments were carried out to determine boundaries of unsteady-state flows of a two-phase water-air mixture in three parallel vertical channels. It is shown that under certain conditions of parallel operation of individual channels self-sustained fluctuations of the pressure drop can appear and that interchannel interations can lead to unstable two-phase flow regimes with stable hydrodynamic characteristics of the individual channels and the whole system. It is shown that these fluctuations are caused by nonlinear characteristics of the two-phase flow.     

Instabilities of two-phase flows in short rectangular microchannels

Two-phase (liquid-gas) flows in a short horizontal slit channel of rectangular cross section with heights (thicknesses) from 100 to 500 μm have been experimentally studied using the laser-induced fluorescence and schlieren photography methods. It is established that the formation of various two-phase flow regimes and the transitions between different regimes are determined by instabilities of the liquid-gas flow in the side parts of a channel. In a 100-μm-thick channel, a frontal instability has been observed during the liquid-gas interaction in the region of liquid output from the nozzle.     

Augmentation of two-phase heat transfer and pressure drop of refrigerants in horizontal tubesAugmentation du transport de chaleur biphasé et de la chute de pression des frigorigènes dans des tubes horizontaux

Passive methods to improve the heat transfer characteristics of two-phase refrigerant flows by altering the tube-side geometry are reviewed and categorized for use in performing system studies and defining future research efforts. The information presented includes available correlations and data for the prediction of the two-phase, heat transfer coefficient and the pressure drop in six different geometries under various combinations of operating conditions and refrigerant selection.     

Augmentation of two-phase heat transfer and pressure drop of refrigerants in horizontal tubes

Passive methods to improve the heat transfer characteristics of two-phase refrigerant flows by altering the tube-side geometry are reviewed and categorized for use in performing system studies and defining future research efforts. The information presented includes available correlations and data for the prediction of the two-phase, heat transfer coefficient and the pressure drop in six different geometries under various combinations of operating conditions and refrigerant selection.     

Calculation of the Boundary Layer of a Two-Phase Medium

Results are given of the calculations of the turbulent boundary layer of a two-phase mixture consisting of a gas and spherical particles 10^–6 m in size. The problem formulation is presented, and the concentration of particles on the wall and the heat flux to the wall are determined. The two-phase mixture is treated as Newtonian liquid, and the gas and solid particles which form this mixture are calculated as continuous flows. The calculation of a two-phase flow in the boundary layer is performed with due regard for the effect of the concentration of solid particles on the viscosity and thermal conductivity of a binary mixture in the presence of the forces of interfacial interaction.     

Variant of a volume-of-fluid method for surface tension-dominant two-phase flows

The capabilities of the volume-of-fluid method for the calculation of surface tension-dominant two-phase flows are explained. The accurate calculation of the interface remains a problem for the volume-of-fluid method if the density ratios of the fluids in different phases are high. The simulations of bubble growth is performed in water at near critical pressure for different degrees of superheat using combined level-set and volume-of fluid (CLSVOF) method. The effect of superheat on the frequency of bubble formation was analyzed. A deviation from the periodic bubble release is observed in the case of superheat of 20 K in water. The vapor-jet-like columnar structure is observed. Effect of heat flux on the slender vapor column has also been explained.