Jennifer Uhle

Drift-flux model for downward two-phase flow

Abstract In view of the practical importance of the drift-flux model for two-phase flow analysis in general and in the analysis of nuclear-reactor transients and accidents in particular, the distribution parameter and the drift velocity have been studied for downward two-phase flows. The constitutive equation that specifies the distribution parameter in the downward flow has been derived by taking into account the effect of the downward mixture volumetric flux on the phase distribution. It was assumed that the constitutive equation for the drift velocity developed by Ishii for a vertical upward churn-turbulent flow determined the drift velocity for the downward flow over all of flow regimes. To evaluate the drift-flux model with newly developed constitutive equations, area-averaged void fraction measurement has been extensively performed by employing an impedance void meter for an adiabatic vertical co-current downward air–water two-phase flow in 25.4-mm and 50.8-mm inner diameter round tubes. The newly developed drift-flux model has been validated by 462 data sets obtained in the present study and literatures under various experimental conditions. These data sets cover extensive experimental conditions such as flow system (air–water and steam–water), channel diameter (16–102.3 mm), pressure (0.1–1.5 MPa), and mixture volumetric flux (−0.45 to −24.6 m/s). An excellent agreement has been obtained between the newly developed drift-flux model and the data within an average relative deviation of ±15.4%.

Interfacial area transport equation: model development and benchmark experiments

Abstract The interfacial area transport equation dynamically models two-phase flow regime transitions and predicts continuous changes of the interfacial area concentration along the flow field. It replaces the flow regime-dependent correlations for the interfacial area concentration in thermal-hydraulic system analysis. In the present study, detailed formulation of the interfacial area transport equation is presented along with its evaluation results based on the detailed benchmark experiments. In view of model evaluation, the equation is simplified into one-dimensional steady state one-group interfacial area transport equation. The prediction made by model agrees well with the experimental data obtained in round pipes of various diameters. The framework for the two-group transport equation and the necessary constitutive relations are also presented in view of bubble transport of various sizes and shapes.

Interfacial area of bubbly flow in a relatively large diameter pipe

This paper presents the local measurement results of interfacial parameters in a vertical air–water upward two-phase bubbly flow. The measured parameters are local time-averaged void fraction, interfacial area concentration, bubble Sauter mean diameter, and interfacial velocity. The measurements are performed in a 101.6-mm inner diameter pipe by four-sensor conductivity probes. These data are further used to evaluate the one-dimensional, steady state, one-group interfacial area transport equation (IATE), which can serve as one of the constitutive relations for the two-fluid model. The agreement between the predictions from the transport equation and the experimental measurements is reasonably good. The resulting deviation is carefully examined. As a conclusion, the two-group IATE is recommended for general gas–liquid two-phase flow in various flow regimes.

LDA measurement in air–water downward flow

Abstract Local characteristics of the liquid phase in air–water downward flow were investigated in a 50.8 mm inner-diameter round pipe. A laser Doppler anemometry (LDA) system was used to measure axial liquid velocity and its fluctuations. To reduce the measurement uncertainty, the experiments were performed in flow conditions with low void fraction. Titanium dioxide particles with a mean diameter of 2 μm were used as seeding particles to enhance the data rate. Benchmark experiment in the single-phase liquid flow was first carried out to ensure good performance of the LDA system in the current setup. A total of 13 flow conditions were examined in air–water two-phase experiment. By applying a special setup of the LDA system, it was found that no further signal discrimination process was required to obtain the liquid velocity in the present low void fraction conditions. The comparisons between the liquid flow rates measured by the magnetic flow meter and those obtained from the local measurements showed good agreements, with differences less than 6.0%. The measurement results demonstrated that the presence of the bubbles tended to flatten the liquid velocity radial profile, and the maximum liquid velocity might occur off the pipe centerline, in particular at relatively low flow rates. Furthermore, the axial liquid velocity fluctuations were quite uniform in the radial direction. No significant turbulent reduction in the two-phase downward flow was observed in the current experimental flow conditions.

Flow Regime Identification of Co-Current Downward Two-Phase Flow With Neural Network Approach

Flow regime identification for an adiabatic vertical co-current downward air-water two-phase flow in the 25.4 mm ID and the 50.8 mm ID round tubes was performed by employing an impedance void meter coupled with the neural network classification approach. This approach minimizes the subjective judgment in determining the flow regimes. The signals obtained by an impedance void meter were applied to train the self-organizing neural network to categorize these impedance signals into a certain number of groups. The characteristic parameters set into the neural network classification included the mean, standard deviation and skewness of impedance signals in the present experiment. The classification categories adopted in the present investigation were four widely accepted flow regimes, viz. bubbly, slug, churn-turbulent, and annular flows. These four flow regimes were recognized based upon the conventional flow visualization approach by a high-speed motion analyzer. The resulting flow regime maps classified by the neural network were compared with the results obtained through the flow visualization method, and consequently the efficiency of the neural network classification for flow regime identification was demonstrated.Copyright

Parallel applications of the USNRC consolidated code

The United States Nuclear Regulatory Commission has developed the thermal-hydraulic analysis code TRAC-M to consolidate the capabilities of its suite of reactor safety analysis codes. One of the requirements for the new consolidated code is that it supports parallel computations to extend code functionality and to improve execution speed. A flexible request driven Exterior Communication Interface (ECI) was developed at Penn State University for use with the consolidated code and has enabled distributed parallel computing. This paper reports the application of TRAC-M and the ECI at Purdue University to a series of practical nuclear reactor problems. The performance of the consolidated code is studied on a shared memory machine, DEC Alpha 8400, in which a Large Break Loss of Coolant Accident (LBLOCA) analysis is applied for the safety analysis of the new generation reactor, AP600. The problem demonstrates the importance of balancing the computational for practical applications. Other computational platforms are also examined, to include the implementation of Linux and Windows OS on multiprocessor PCs. In general, the parallel performance on UNIX and Linux platforms is found to be the most stable and efficient.

Study of Interfacial Structures: Bubbly Flow in 1.27 cm Diameter Pipe

The objective of the present research is to study the flow regime map, the detailed interfacial structures, and the bubble transport in an adiabatic air-water two-phase flow mixture, flowing upward through a vertical round pipe having 1.27 cm. inner diameter. The flow regime map is obtained by processing the characteristic signals acquired from an impedance void meter, using a self-organized neural network. The local two-phase flow parameters are measured by the state-of-the-art four-sensor conductivity probe at three axial locations in the pipe. The measured local parameters include void fraction (α), interfacial area concentration (ai ), bubble frequency (fb ), bubble velocity (Ub ) and bubble Sauter mean diameter (Dsm ). The radial profiles of these parameters and their development along the axial direction reveals the structure of the two phase mixture and the bubble interaction mechanisms.Copyright

Local Interfacial Structure in Downward Two-Phase Bubbly Flow

The local interfacial structure for vertical air-water co-current downward two-phase flow was investigated under adiabatic conditions. A multi-sensor conductivity probe was utilized in order to acquire the local two-phase flow parameters. The present experimental loop consisted of 25.4 mm and 50.8 mm ID round tubes as test sections. The measurement was performed at three axial locations: L/D = 13, 68 and 133 for the 25.4 mm ID loop and L/D = 7, 34, 67 for the 50.8 mm ID loop, in order to study the axial development of the flow. A total of 7 and 10 local measurement points along the tube radius were chosen for the 25.4 mm ID loop and the 50.8 mm ID loop, respectively. The experimental flow conditions were determined within bubbly flow regime. The acquired local parameters included the void fraction, interfacial area concentration, bubble interface frequency, bubble Sauter mean diameter, and interfacial velocity.Copyright