Prediction of vertical flows in large diameter pipes

There is an increasing interest in multiphase flows in large diameter vertical pipes (typically with diameters greater that 100 mm) in the context of hydrocarbon production systems. There are strong indications that flows in such pipes differ greatly from those in smaller diameter pipes on which most of the prediction methodologies are based. In small diameter pipes, an important mechanism for the bubble flow to slug flow transition is the formation of void waves. This research reveal this wave growth and also predict the breakdown points from bubble-to-slug flow transition using Biesheuvel and Gorissen (1990) approximate void wave model based on Harwell small tube bubble flow experiments. As the gas velocity is further increased, the slug flow itself breaks down into churn flow by a process of flooding in the Taylor bubbles. In large diameter pipes, it appears that conventional slug flow does not occur; this is probably due to the fact that there is a size limit on spherical cap bubbles. Thus, this study reviews most of literatures in terms of bubble coalescence and breakup kernels in order to evaluate dynamic bubble size changes by applying population balance model. Unfortunately, these kernels have their own problems to be solved. Therefore we establish a simplified two-group bubble interaction model by taking into account mechanisms of large bubble shearing-off breakup and small bubble coalescence in large bubble wakes, respectively, assuming small bubbles do not coalesce to each other. In large diameter pipes, the bubble/slug and slug/churn transitions appear to be by-passed in favour of a direct transition from bubble to churn flow with increasing gas mass flux. Note that the churn flow studied here is emphasized by a continuous path for the gas phase. This study also describes work aimed at developing a phenomenological understanding of the bubble/churn and churn/annular transition regions in large diameter pipes. Investigation of the liquid transport mechanisms has led to the definition of two new flow regime transition criteria, namely liquid upflow potential and minimum entrained fraction. To estimate the bubble-to-churn flow transition, the liquid upflow potential of a churn flow at the particular local set of gas and liquid flow rates is estimated by using axial view experiments and the existing adiabatic equilibrium data. In churn flow, liquid upflow is achieved by the net upward flow in the film (bearing in mind that both upflow and downflow are occurring in the film, though the net value must be positive) and by droplet transport in the gas core. Once the Kutateladse flooding is reached, suggested by Pushkina and Sorokin (1969), then it is postulated that the transition to churn flow occurs. As the gas velocity is further increased, the flow rate of entrained drops in the gas core decreases to a minimum and then rises again. This minimum is observed to occur at a dimensionless gas velocity approximately equal to one and this serves as a possible criterion for the churn-to-annular flow transition. As a framework for prediction, an existing one-dimensional steady state modelling code (GRAMP2) has been selected. This code takes account of regime changes and predicts void fraction and pressure gradient using phenomenological models. Work on connecting the void wave growth, bubble size evaluation and GRAMP2 code for large diameter pipes will be the main target for the nearly future. In the meantime, CFD simulation is also being undertaken using a finite volume method based the STARCD software in order to numerically predict the evaluations of dynamic bubble size and flow regime changes in large diameter pipes

Real‐Time Software‐Defined Adaptive MIMO Visible Light Communications

Visible light communications (VLC) based on light-emitting diodes (LEDs) merges lighting and data communications in applications of Internet-of-Things and 5G networks. However, phosphor-based white LED has a limited linear dynamic range and limited modulation bandwidth. In practical indoor mobile communications, complex channel conditions change dynamically in real-time, and line of sight (LOS) links may be blocked by obstructions. We propose a real-time software-defined adaptive multi-input multi-output (MIMO) VLC system, that both modulation formats (QPSK,16-QAM,64-QAM, 256QAM) and MIMO reconfigurations (Spatial Diversity and Spatial Multiplexing) are dynamically adapted to the changing channel conditions, for enhancing both link reliability and spectral efficiency. Real-time and software defined digital signal processing (DSP) are implemented by Field Programmable Gate Array (FPGA) based Universal Software Radio Peripheral (USRP) devices. We theoretically analysed and experimentally evaluated nonlinear electrical-optical properties and modulation characteristics of white LEDs. We demonstrated a real-time Single-Carrier 256-Quadrature Amplitude Modulation (QAM) 2×2 MIMO VLC, achieving 1.81% averaged error vector magnitude (EVM), 2×10-5 bit error rate (BER) after 2 m indoor transmission. As an obstacle moved across LOS links, real-time software-defined adaptive MIMO VLC system enhanced average error-free spectral efficiency of 12 b/s/Hz. This will provide high throughputs for robust links in mobile shadowing environments

Monte Carlo Simulation for Polychromatic X-ray Fluorescence Computed Tomography with Sheet-Beam Geometry

X-ray fluorescence computed tomography based on sheet-beam can save a huge amount of time to obtain a whole set of projections using synchrotron. However, it is clearly unpractical for most biomedical research laboratories. In this paper, polychromatic X-ray fluorescence computed tomography with sheet-beam geometry is tested by Monte Carlo simulation. First, two phantoms (A and B) filled with PMMA are used to simulate imaging process through GEANT 4. Phantom A contains several GNP-loaded regions with the same size (10 mm) in height and diameter but different Au weight concentration ranging from 0.3% to 1.8%. Phantom B contains twelve GNP-loaded regions with the same Au weight concentration (1.6%) but different diameter ranging from 1mm to 9mm. Second, discretized presentation of imaging model is established to reconstruct more accurate XFCT images. Third, XFCT images of phantom A and B are reconstructed by fliter backprojection (FBP) and maximum likelihood expectation maximization (MLEM) with and without correction, respectively. Contrast to noise ratio (CNR) is calculated to evaluate all the reconstructed images. Our results show that it is feasible for sheet-beam XFCT system based on polychromatic X-ray source and the discretized imaging model can be used to reconstruct more accurate images