Owen C. Jones

3-D turbulence structure and phase distribution measurements in bubbly two-phase flows

Turbulent bubbly air/water two-phase up and down flows in a circular test section were investigated. Important flow quantities such as local void fraction, liquid velocity and the Reynolds stresses were measured using both single-sensor and three-sensor hot-film anemometer probes. For up flows, it was found that the bubbles tended to migrate toward the wall and thus the void fraction profile showed a distinct peak near the wall. In contrast, for down flows, it was found that the bubbles tended to migrate toward the center of the pipe causing void “coring”. It was also found that the observed wall peaking and coring phenomena, and thus the radial void distribution in up and down flows, could be predicted by considering the turbulence structure of the continuous phase and lateral lift force acting on the dispersed phase (i.e. the bubbles). All Reynolds stress components were measured using a special 3-D conical probe. In two-phase flows, the normal Reynolds stress components (i.e. u2, v2and w2) showed nearly flat profiles in the core region (r/R < 0.8) and, except near the wall, the turbulence structure was more anisotropic compared to single-phase flows. Normally, the presence of the bubbles increased the level of turbulence in the flow. However, because the bubbles in turbulent two-phase flow enhance dissipation as well as promoting the production of turbulence kinetic energy, it was found that for higher flow rates the presence of bubbles suppressed the level of turbulence.

Phase distribution in bubbly two-phase flow in vertical ducts

Abstract The lateral phase distribution in bubbly flows in vertical ducts was analyzed using a three-dimensional two-fluid model. The constitutive relations of the model are based on analytic and experimental information on the behavior of a single bubble, and on the assumption of linear superposition of shear-induced and bubble-induced turbulence. The experiments chosen to test the model include available data for pipes and new data obtained in an isosceles triangular duct. While most of the data could be reproduced satisfactorily by the model, some could not. This is attributed to certain physical mechanisms that are still not well understood and therefore were not included in the constitutive relations.

Development of a k-ε Model for Bubbly Two-Phase Flow

An extension of the k-[epsilon] model for bubbly two-phase flow is proposed and tested against experimental data. The basic assumption made is that the shear-induced turbulence and bubble-induced turbulence may be linearly superposed. This assumption results in a model with two time constants that matches both homogeneous two-phase turbulence data (Lance and Bataille, 1991) and pipe data (Serizawa, 1986). The coefficients of the single-phase k-[epsilon] model have not been modified and only one additional coefficient is required: the virtual volume coefficient of the bubbles, which may be determined from first principles. This model not only agrees with the data trends, but it also predicts the turbulence suppression which has been measured for high Reynolds number bubbly air/water flows in pipes.

Phase distribution in complex geometry conduits

Abstract Bubbly air/water two-phase flow data have been taken as an isosceles triangle using hot film probes. It was found that a 3-D two-fluid model was able to predict these data and those taken previously in circular conduits. It appears that mechanically-based CFD predictions of bubbly two-phase flows is possible for many cases of practical concern.

Impedance imaging relative to gas-liquid systems

Abstract Determining interfacial area density in two-phase, gas-liquid flows is one of the major elements impeding significant development of predictive tools based on the two-fluid model. Currently, these models require coupling of liquid and vapor at interfaces using constitutive equations which do not exist in any but the most rudimentary form. This paper describes the theory of impedance imaging of two-phase mixtures and summarizes bench-type experiments utilized in its development, testing, and feasibility evaluation.

Nucleation and flashing in nozzles—1. A distributed nucleation model

Abstract It is well known that both the number and size of bubbles must be accurately determined for the initial calculation of flashing void development downstream of flashing inception in ducts, nozzles and restrictions. This paper presents a new method of accurately determining both for small geometries with water, which results in accurate calculation of the downstream void development. A wall cavity model is described for use in the calculation of nucleation rates and bubble number densities at flashing inception, and subsequently in the calculation of the void development downstream of minimum area zones in nozzles. The model is based on the physics of the nucleation phenomena in flashing and considers transient conduction to be the sole means of heat transfer from the superheated liquid to the vapor bubble. The activation criterion developed for site nucleation is one-sided, due to the uniform superheat, rather than two-sided as in boiling. A figure of merit for the particular fluid solid combination is then determined which yields the minimum nucleation surface energy per site. Characteristic site nucleation frequencies and the number densities of nucleation sites of given sizes are then obtained from the data, providing the first link between a surface-characteristic-based nucleation and evaporation model and global behavior. Throat void fractions for all data found in the literature are , confirming earlier assumptions. A bubble transport equation is used to predict the number density and size of bubbles at the throat. Throat superheats are then calculated for all throat superheats up to ≈100 K and expansion rates between 0.2 bar/s to over 1 Mbar/s, with a standard deviation of 1.9 K. This extends previous correlations by more than 3 orders of magnitude. As a result, flow rates can be calculated to within 3% of measured values using a combination of single-phase theory and accurate calculation of the throat pressure under critical conditions. This provides a valuable consistency check to independent critical flow predictions.

Nucleation and flashing in nozzles—2. Comparison with experiments using a five-equation model for vapor void development

Abstract A quasi-one-dimensional, five-equation, homogeneous, nonequilibrium model has been developed and utilized on a microcomputer to calculate the behavior of flowing, initially subcooled, flashing water systems. Equations for mixture and vapor mass conservation, mixture momentum conservation, liquid energy conservation and bubble transport were discretized and linearized semi-implicitly, and solved using a successive iteration Newton method. Closure was obtained through simple constitutive equations for friction and spherical bubble growth, and a new nucleation model for wall nucleation in small nozzles combined with an existing model for bulk nucleation in large geometries to obtain the thermal nonequilibrium between phases. The model described was applied to choked nozzle flow with subcooled water inlets based on specified inlet conditions of pressure and temperature, and vanishing inlet void fraction and bubble number density. Good qualitative and quantitative agreement with the experiment confirms the adequacy of the nucleation models in determining both the initial size and number density of nuclei, and indicates that mechanical nonequilibrium between phases is not an important factor in these flows. It is shown that bulk nucleation becomes important as the volume-to-surface ratio of the geometry is increased.

Turbulent bubbly two-phase flow data in a triangular duct

Abstract Multidimensional bubbly two-phase flow data was obtained in an isosceles triangular duct having D h = 40 mm and L / D = 73. The data include local measurements of phase distribution, velocity and Reynolds stresses at 10,000 Re 1 bubble = 1250. The measurements were performed with single sensor and X-sensor cylindrical hot film probes of 0.025 mm diameter. These small probes have good bubble penetration characteristics so they are capable of the simultaneous measurement of the liquid phase instantaneous velocity and the phase indicator function. These data are useful for the validation of multidimensional two-fluid CFD models.

Statistical Analysis of Turbulent Two-Phase Pipe Flow

The autocorrelation functions and the power spectral density functions of the axial turbulence fluctuations in the liquid phase were determined. The high frequency content of the power spectrum in bubbly two-phase pipe flow was significantly larger than in single-phase pipe flow

INVESTIGATION OF ELECTRICAL IMPEDANCE IMAGING RELATIVE TO TWO-PHASE, GAS-LIQUID FLOWS

Abstract The determination of interfacial area density in two-phase, gas-liquid flows is one of the major elements impeding significant development of predictive tools based on the two-fluid model. Currently, these models require coupling of liquid and vapor at interfaces using constitutive equations which do not exist in any but the most rudimentary form. Work described herein represents the progress towards the development of electrical impedance imaging (EII), sometimes called electrical impedance computed tomography (EICT) which may ultimately be utilized for nonintrusive determination of interfacial structure and evolution in such flows.