Numerical simulations of gas-liquid two-phase flows in a micro porous structure

The lattice Boltzmann method for two-phase fluid flows is applied to the simulations of gas-liquid two-phase flows in a micro porous structure for various capillary numbers at low Reynolds numbers. The behaviors of the gas-liquid interface and the velocities of the two-phase fluid in the structure are simulated, and the permeability of gas and liquid through the structure are estimated from the calculated results. By changing the void fraction, the contact angle of the interface on walls, and the surface tension, the effect of these properties on the behaviors and the permeability of the two-phase flows in the micro porous structure is investigated. It is found that the permeability of liquid flows depends on the contact angle and it increases for hydrophobic walls. It is also seen that liquid flows are choked in pores for large void fractions and low capillary numbers.     

Local structure of turbulence in flows with large Reynolds numbers

Results are reported on an experimental investigation of the characteristics of fine-scale pulsations of the velocity in several shear flows (mixing layer, boundary layer, planar, axially symmetric, and spatial wakes, and in the return channel of a large wind tunnel) in an interval of definite Reynolds numbers R(lambda) [approximately-equal-to] 70-3000 with respect to the Taylor microscale lambda. The characteristic scales of most of the studied flows are quite large, and the integral scale of the turbulence reaches 5 m. The apparatus had a high resolving power-the ratio of the hot-wire length to the Kolmogorov scale was varied in the range 0.8-2.5. It is shown that the Kolmogorov constant C in the "two thirds" law and the constants C(epsilon) and &mgr; in the energy-dissipation correlation function are not universal and have a systematic dependence on the coefficient of external intermittency. The same constants determined in a completely turbulent fluid are universal within the errors of the measurements.     

Chapter 10 Study of two-phase flows

Chapter 10 Study of two-phase flows     

Modeling two-phase flows in channels and nozzles

The gas dynamics of the flow of a two-phase mixture, consisting of a gaseous phase and polydisperse liquid particles of oxides suspended in it, plays a major role in determining the thrust parameters of rocket engines and the power characteristics of MHD stations based on metallized fuels. A number of monographs and reviews have now been published that reflect the current state in this branch of continuum mechanics. Here we give the results of a numerical investigation of certain features of two-phase streams in combustion chambers, Laval nozzles, and MHD generators.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 71–81, August, 1992.     

Experimental investigation of magnetodynamic processes in two-phase flows

Power plants for use in investigation of flows of products of combustion of metallized plasma-forming fuels in a combustion chamber, Laval nozzle, and MHD channel and for determining the physical characteristics of plasma are described. The results of experimental investigations and numerical calculations of the combustion of fuel in a combustion chamber and flows of combustion products through gas-dynamic channels were employed in the design of the power plants.Scientific-Research Institute of Applied Mathematics and Mechanics at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 92–100, April, 1993.     

Numerical investigation of two-phase flows in plug nozzles

The results of numerical investigations of gas dynamics of two-phase (including vortex) flows in plug nozzles are presented. It is shown that most particles settle on the nozzle walls, which are contoured for ideal-gas flow. A method is studied for decreasing with the help of vorticity the precipitation of particles on the plug contour. Methods are proposed for contouring plug nozzles for two-phase flows.Scientific-Research Institute of Applied Mathematics and Mechanics at Tomsk State University. Academician V. P. Makeev Design Office of Machine Building. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 81–91, April, 1993.     

Phase characterization of experimental gas-liquid two-phase flows

We propose a method to characterize and distinguish flow patterns in experimental two-phase (e.g., gas-liquid) flows. The basic idea is to calculate the instantaneous phase from the signal and to extract scaling behaviors associated with the phase fluctuations. The effectiveness of the method is demonstrated and its applicability is articulated.     

Temperature mapping in a two-phase water-steam horizontal flow

This experimental study focuses on the investigation of a flow boiling that occurs during the cooling process of a high temperature heated horizontal channel. Unpressurized water is used as working fluid. An intrusive thermally instrumented cell enables the phase change to be detected at different positions inside the channel. Alongside, welded thermocouples on the external wall of the channel give information about the influence of the phase change. Several cooling flowrates are tested in order to study their influence on the phase change duration. Finally, experimental results are compared to numerical simulations to determine some characteristic parameters as bulk temperature and heat transfer coefficient.     

Direct numerical simulations of autoignition in turbulent two-phase flows

Three-dimensional direct numerical simulations (DNS) were carried out to investigate the impact of evaporation of droplets on the autoignition process under decaying turbulence. The droplets were taken as point sources and were tracked in a Lagrangian manner. Three cases with the same initial equivalence ratio but different initial droplet size were simulated and the focus was to examine the influence of the droplet evaporation process on the location of autoignition. It was found that an increase in the initial droplet size results in an increase in the autoignition time, that highest reaction rates always occur at a specific mixture fraction ξ_MR, as in purely gaseous flows, and that changes in the initial droplet size did not affect the value of ξ_MR. The conditional correlation coefficient between scalar dissipation rate and reaction rates was only mildly negative, contrary to the strongly negative values for purely gaseous autoigniting flows, possibly due to the continuous generation of mixture fraction by the droplet evaporation process that randomizes both the mixture fraction and the scalar dissipation fields.     

Ordered and disordered patterns in two-phase flows in microchannels

We show that wetting properties crucially control the patterns in two-phase flows of immiscible fluids in microchannels. Ordered patterns, continuously entrained by the flow, are obtained when one phase completely wets the walls, while disordered patterns, intermittently adhering to the channel walls, are unavoidably produced when wetting is partial. A lower limit for the channel sizes capable of generating well structured objects (drops, pears, pearl necklaces, ...) is presented.     

Transient adaptivity applied to two-phase incompressible flows

An anisotropic adaptation process is applied to a three-dimensional incompressible two-phase flow solver. The solver uses a level set/finite element method on unstructured tetrahedral meshes. We show how the level set function can be used to build an anisotropic mesh with good properties. Some computations with a strong transient character and large densities ratios (1/1000) are presented. We show that the efficiency of the computations can be deeply enhanced by mesh adaptations.     

A hybrid scheme for computing incompressible two-phase flows

We propose a hybrid scheme for computations of incompressible two-phase flows. The incompressible constraint has been replaced by a pressure Poisson-like equation and then the pressure is updated by the modified marker and cell method. Meanwhile, the moment equations in the incompressible Navier-Stokes equations are solved by our semidiscrete Hermite central-upwind scheme, and the interface between the two fluids is considered to be continuous and is described implicitly as the 0.5 level set of a smooth function being a smeared out Heaviside function. It is here named the hybrid scheme. Some numerical experiments are successfully carried out, which verify the desired efficiency and accuracy of our hybrid scheme.     

Maximum principle and open systems including two-phase flows

Recently an extension of the principle of maximum, for the variation of the entropy due to irreversibility, has been obtained for the open systems. This result is analysed and a comparision between this one and other thermodynamical variational principles of stability is developed. An application to two-phase flows is done to obtain the pressure drop.     

Evaluation of assumed-PDF methods in two-phase flows using direct numerical simulation

The hypothesis of uncorrelated temperature (T) and vapor-fuel mass fraction (Y_v), frequently made when modeling reaction rates using assumed-PDF models, is examined utilizing transitional databases from direct numerical simulation (DNS) of three-dimensional mixing-layers two-phase (TP) flows with evaporation. Because the databases do not contain chemical reaction, which would further correlate variables, finding here a correlation between T and Y_v is sufficient for invalidating reaction rate modeling of the joint (T, Y_v) probability distribution function (PDF) as a product of the marginal PDFs. The databases comprise four multicomponent fuels, two mass loadings and two free-stream gas temperatures. For comparison, databases for single-phase (SP) flows are also analyzed at two initial Reynolds numbers. The examination is conducted in the mixing layer excluding the free streams and in a more restricted part of the mixing layer constituting its core. The analysis is performed at the DNS and large eddy simulation (LES) scales, and subgrid scale (SGS). To obtain the LES database, the DNS database is filtered, and an evaluation of the examined correlation at the LES and SGS scales is made at two filter sizes. At the DNS scale, T and Y_v are practically uncorrelated for SP flows, showing the weak influence of the perfect-gas equation of state, whereas for TP flows the correlation is strong and increases with mass loading indicating the powerful effect of the phase change. At the LES scale, the findings emulate those at the DNS scale. The fluctuations of the SGS scale are uncorrelated for SP flows, but the product of the marginal PDFs is different from the joint PDF. For TP flows, the fluctuations are correlated and the correlation increases with temperature, casting doubt on current assumed PDFs used to model chemistry in reacting sprays. These results are independent of filter size. The joint PDFs for TP and SP fluctuations are successfully modeled.     

A diffuse-interface method for two-phase flows with soluble surfactants

A method is presented to solve two-phase problems involving soluble surfactants. The incompressible Navier-Stokes equations are solved along with equations for the bulk and interfacial surfactant concentrations. A non-linear equation of state is used to relate the surface tension to the interfacial surfactant concentration. The method is based on the use of a diffuse interface, which allows a simple implementation using standard finite difference or finite element techniques. Here, finite difference methods on a block-structured adaptive grid are used, and the resulting equations are solved using a non-linear multigrid method. Results are presented for a drop in shear flow in both 2D and 3D, and the effect of solubility is discussed.     

平行壁变形导电管磁流体层流数值模拟

采用自主开发的基于 OpenFOAM 环境下的磁流体求解器,对外加横向均匀磁场的导电方管、平行 壁内凹导电管以及平行壁外凸导电管内的磁流体进行了层流数值模拟。在壁面电导率为 0.01、流体雷诺数为 500、 哈特曼数为 500~2000 的条件下,研究了三种导电管中液态金属磁流体速度分布和压降。结果表明：平行壁内凹 和外凸对速度分布具有显著影响;在相同参数条件下,平行壁内凹管的压降大于方管,而平行壁外凸管的压降小 于方管。     

Ultrafast velocity imaging of single- and two-phase flows in a ceramic monolith

Ceramic monoliths, comprising arrays of parallel channels, are increasingly being considered as an alternative to conventional packed beds for chemical processing operations involving both single- and two-phase flows. This paper reports results obtained using a technique based on the rapid acquisition with relaxation enhancement (RARE) pulse sequence in which multiple images are obtained from a single r.f. excitation. The technique is applied to study single- and two-phase flow in a monolith rated at 200 channels per square inch (cpsi). A single image frame, acquired in 156 ms, provides a characterization of the heterogeneity in the magnitude and direction of the flow within the monolith.     

Power-law slip profile of the moving contact line in two-phase immiscible flows

Large-scale molecular dynamics (MD) simulations on two-phase immiscible flows show that, associated with the moving contact line, there is a very large 1/x partial-slip region where x denotes the distance from the contact line. This power-law partial-slip region is verified in large-scale adaptive continuum calculations based on a local, continuum hydrodynamic formulation, which has proved successful in reproducing MD results at the nanoscale. Both MD simulations and numerical solutions of continuum equations indicate the existence of a universal slip profile in the Stokes-flow regime.