Sidharth Paranjape

Fast classification of two-phase flow regimes based on conductivity signals and artificial neural networks

On-line identification of flow regimes is important in two-phase flow because hydrodynamics and adequate operation of multiphase systems are highly dependent on the flow pattern. This work describes the application of an artificial neural network (ANN) to process the signals measured by a conductivity probe and classify them into their corresponding flow regimes. Experiments were performed in an adiabatic air–water upward two-phase flow rig. Some statistical parameters of the cumulative probability density functions (CPDF) of the bubble chord length were used as the inputs to the ANN. Different ANN configurations were evaluated to optimize the characteristics that best suit the specific ANN application. The results demonstrate good agreement with the visual flow map identification, even for reduced temporal conductivity signals.

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.

Interfacial structures and regime transition in co-current downward bubbly flow

The vertical co-current downward air-water two-phase flow was studied under adiabatic condition in round tube test sections of 25.4-mm and 50.8-mm ID. In flow regime identification, a new approach was employed to minimize the subjective judgment. It was found that the flow regimes in the co-current downward flow strongly depend on the channel size. In addition, various local two-phase flow parameters were acquired by the multi-sensor miniaturized conductivity probe in bubbly flow. Furthermore, the area-averaged data acquired by the impedance void meter were analyzed using the drift flux model. Three different distributions parameters were developed for different ranges of nondimensional superficial velocity, defined by the ration of total superficial velocity to the drift velocity

Flow Regime Identification in Large Diameter Pipe

Air-water vertical two-phase flow experiments were performed in a 0.15 m diameter and 4.4 m long test section. Superficial liquid velocity was varied from 0.05 m/s to 2.0 m/s and superficial gas velocity was varied to obtain the area averaged void fraction range of 0.1 to 0.7. Exit pressure was close to the atmospheric pressure. In order to study the development of flow structure over the length of test section, area averaged void fraction was measured using impedance meters at four different measuring ports. Pressure drop was also measured between these ports. Since the temporal variation of void fraction signal obtained from the impedance meter and its distribution are characteristic of the flow regime, a Cumulative Probability Distribution Function (CPDF) of the void fraction signal was utilized for the identification of flow regime at each port. The CPDFs of the impedance probe void fraction signal were supplied as an input to the Kohonen Self Organized neural network or the Self Organized Map (SOM) for the identification of the patterns by employing self organized neural network technique. The three flow regimes identified by the neural network are subjectively named as bubbly flow, cap-bubbly flow and cap-turbulent flow.Copyright

Effect of Thermal Stratification on Full-Cone Spray Performance in Reactor Containment for a Scaled Scenario

The results of an experimental study on the nuclear reactor containment spray system are presented. Depending on the initial conditions, the spray nozzle configuration and flow rates, the spray may cause higher hydrogen concentration during depressurization due to steam condensation, or it may erode the hydrogen stratification by enhanced mixing. To investigate these phenomena, the tests are performed using a full-cone spray nozzle in PANDA facility at Paul Scherrer Institut, Switzerland. Temporal evolution and spatial distribution of the fluid temperature and the fluid concentrations are measured using thermocouples and mass spectrometers. Two tests are performed with initial vessel wall temperatures of 105°C and 135°C, which create condensing and non-condensing environments respectively. The different initial conditions lead to different density stratifications. The effect of these different density stratification on the flow patterns and mixing of gases in the vessels due to the action of the spray is revealed by these tests.© 2014 ASME

Impedance-Based Void Fraction Measurement and Flow Regime Identification in Microchannel Flows

Electrical impedance of a two-phase mixture is a function of void fraction and phase distribution. The difference in the electrical conductance and permittivity of the two phases can be exploited to measure electrical impedance for obtaining void fraction and flow regime characteristics. An electrical impedance meter is constructed for the measurement of void fraction in microchannel two-phase flow. The experiments are conducted in air-water two-phase flow under adiabatic conditions. A transparent acrylic test section of hydraulic diameter 780 micrometer is used in the experimental investigation. The impedance void meter is calibrated against the void fraction measured using analysis of images obtained with a high-speed camera. Based on these measurements, a methodology utilizing the statistical characteristics of the void fraction signals is employed for identification of microchannel flow regimes.Copyright

Two‐phase Flow Regime Identification Combining Conductivity Probe Signals and Artificial Neural Network

Important aspects of the hydrodynamics and thus, the correct identification of the flow regime could enhance safety and overall performance in multiphase flow systems. Several works on flow regime identification have been carried out in the past. Most of them consist, in a first stage, on measuring certain flow parameters that can be used as good flow regime indicators and, then, developing a flow regime map using these indicators. In this work, a vertical two‐phase flow loop facility was used, whereby local conductivity signals were recorded and utilized for the development of an Artificial Neural Network (ANN) based method for the flow regime classification. The experimental database consists of a total number of 125 test cases covering a wide range of situations in the loop working area. Each experiment flow regime was identified by visual inspection, and classified into bubbly (B), cap‐bubbly (CB), slug (S), churn turbulent (CT) or annular (A). The bubble chord length cumulative probability function (...

Local Liquid Velocity in Vertical Air-Water Bubbly Downward Flow

Local characteristics of the liquid phase in downward air-water bubbly and slug flows were investigated in a 50.8-mm inner-diameter round pipe. A laser Doppler anemometry 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. The comparisons between the liquid flow rates measured by the magnetic flow meter and those obtained from the local measurements showed good agreements. In addition, based on the LDA measurements and the data acquired by the local conductivity probes, the local relative velocity distribution, the distribution parameter and the drift velocity in the drift-flux model were obtained for the current downward flow.© 2003 ASME

Interfacial Structures and Regime Transition in Co-Current Downward Bubbly Flow

The vertical co-current downward air-water two-phase flow was studied under adiabatic condition. In view of studying the effect of flow area on interfacial structure and regime transition, round tubes of 25.4-mm ID and 50.8-mm ID were employed as test sections. The flow regime map was constructed for each test section by employing a newer approach. Unlike the conventional flow visualization method, the present approach minimizes the subjective judgment in determining the flow regimes. It was found that the flow regime in the co-current downward flow strongly depend on the channel size. The local two-phase flow parameters were acquired by the multi-sensor miniaturized conductivity probe in bubbly flow. They include: the local time-averaged void fraction, interfacial area concentration, bubble velocity and bubble Sauter mean diameter. The area-averaged data acquired by the impedance void meter were analyzed by the drift flux model. Three different distributions parameters were developed for the different ranges of non-dimensional superficial velocity, defined by the ration of total superficial velocity to the drift velocity. The new correlations can be applied to a co-current downward two-phase flow in a wide range of flow regime spanning from bubbly to annular flow.Copyright

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