Saturated flow boiling in small- to micro- diameter metallic tubes: Experimental results and modeling

Some results of a long-term study of flow boiling patterns, heat transfer rates and pressure drop of R134a at pressures of 6-14 bar in five vertical stainless steel tubes of internal diameter 4.26, 2.88, 2.01, 1.1 and 0.52 mm are presented in this paper. The flow regimes in the 4.26 mm to 1.1 mm tubes were identified as dispersed bubble, bubbly, slug, churn, annular and mist flows. As the diameter was reduced, progressively slimmer vapour slugs, a thinner liquid film around the vapour slug and a less chaotic vapour-liquid interface in churn flow were observed. Confined flow appeared first in the 2.01 mm tube. Dispersed bubble flow was not observed in the smallest tube (0.52 mm) for the range studied in runs in which wavy film flow occurred. The heat transfer coefficients in tubes ranging from 4.26 mm down to 1.1 mm increased with heat flux and system pressure, but did not change with vapour quality for low quality values. At higher quality, the heat transfer coefficients decreased with quality, indicating local dryout. The heat transfer characteristics of the 0.52 mm tube were different from those in the larger tubes. The data fell into two groups that exhibited different influences of heat flux below and above a heat flux threshold. The pressure drop and heat transfer results were compared with existing correlations but with some limited success. Recent progress on mechanistic models for heat transfer along with comparisons and recommendations are included in the paper

Quench front progression in a superheated porous medium: experimental analysis and model development

In case of severe accident in a nuclear reactor, the fuel rods may be highly damaged and oxidized and finally collapse to form a debris bed. Removal of decay heat from a debris bed is a challenging issue because of the difficulty for water to flow inside. Currently, IRSN has started experimental program PEARL with two experimental facilities PRELUDE and PEARL, to investigate the reflood process at high temperature, for various particle sizes. On the basis of PRELUDE experimental results, the thermal hydraulic features of the quench front have been analysed and the intensity of heat transfers was estimated. From a selection of experimental results, a reflooding model was improved and validated. The model is implemented in the code ICARE-CATHARE developed by IRSN which is used for severe accident reactor analysis

The XDEM Multi-physics and Multi-scale Simulation Technology: Review on DEM-CFD Coupling, Methodology and Engineering Applications

The XDEM multi-physics and multi-scale simulation platform roots in the Ex- tended Discrete Element Method (XDEM) and is being developed at the In- stitute of Computational Engineering at the University of Luxembourg. The platform is an advanced multi- physics simulation technology that combines flexibility and versatility to establish the next generation of multi-physics and multi-scale simulation tools. For this purpose the simulation framework relies on coupling various predictive tools based on both an Eulerian and Lagrangian approach. Eulerian approaches represent the wide field of continuum models while the Lagrange approach is perfectly suited to characterise discrete phases. Thus, continuum models include classical simulation tools such as Computa- tional Fluid Dynamics (CFD) or Finite Element Analysis (FEA) while an ex- tended configuration of the classical Discrete Element Method (DEM) addresses the discrete e.g. particulate phase. Apart from predicting the trajectories of individual particles, XDEM extends the application to estimating the thermo- dynamic state of each particle by advanced and optimised algorithms. The thermodynamic state may include temperature and species distributions due to chemical reaction and external heat sources. Hence, coupling these extended features with either CFD or FEA opens up a wide range of applications as diverse as pharmaceutical industry e.g. drug production, agriculture food and processing industry, mining, construction and agricultural machinery, metals manufacturing, energy production and systems biology

Development of an optimised integrated underbalanced drilling strategy for cuttings transport in gas-liquid flow through wellbore annuli.

Although understanding the relationship between gas-liquid two-phase fluid flows and the effects of the major drilling variables is critical to optimising underbalanced drilling (UBD) operations, to date, this has been an area of limited research and knowledge. This study contributes to the limited knowledge base by: 1) determining the key operational drilling parameters which shape the gas-liquid two-phase multiphase flow behaviour characteristics during UBD operations, 2) evaluating the most critical operational issues that have impacted the implementation of global UBD programmes, and 3) investigating the Newtonian and non-Newtonian gas-liquid two-phase flow patterns which affect the wellbore hydraulics and cuttings transport efficiency during UBD operations. Thus, this study developed a rigorous integrated strategy for maximising the efficiency of UBD for the transport of cuttings in gas-liquid two-phase flow through wellbore annuli. An experimental approach was applied to analyse and evaluate the relationship between the gas-liquid two-phase flow patterns and the major operational drilling parameters (gas and liquid flowrates, fluid rheology, inner pipe rotation, pipe inclination angle, pipe eccentricity and solid particle size and density) and to investigate their influence and interaction on the fluid flow dynamics and solids transport mechanisms in horizontal and inclined annuli. Experimental results revealed that drilling fluid flowrate along with fluid flow pattern are the most prominent parameters that strongly influence the cuttings transport efficiency within wellbore annuli. Annuli cleaning requirements for a concentric annulus was found to be lower than that required for an eccentric annulus for both Newtonian and non-Newtonian fluids. Pipe inclination angle was shown to affect hole cleaning, with the degree of its effect being significantly influenced by the drilling fluid properties, prevailing gas-liquid fluid flow pattern and cuttings transport mechanism. Moreover, inner pipe rotation was observed to improve cuttings transport in both horizontal and inclined eccentric annuli to varying extents. Experimental evidence was supplemented with a theoretical approach. Flow pattern dependent multi-layered mathematical models applicable for any level of pipe eccentricity were used for the different cuttings transport mechanisms existing in the different fluid flow patterns (dispersed bubble, bubble, and slug), offering a unique method to evaluate cuttings transport efficiency and wellbore hydraulics performance for UBD operations. A favourable comparison was observed between the experimental data and proposed flow pattern dependent multi-layered mathematical models with an error margin of ±15%. This research has generated new knowledge and created value through mapping the factors influencing particle transport and by evaluating the fluid-particle dynamics (fluid forces, gas-liquid fluid flow patterns and particle transport mechanisms) for flow in wellbore annuli. It has further identified and evaluated the effect of gas-liquid two-phase fluid flow patterns on fluid-particle transport dynamics which results in areas of preferential flows and stagnation zones. It also proposed a systematic solution to the governing equations for the simultaneous flow of gas-liquid two-phase fluids and solid particles in wellbore annuli. Overall, the mapping of the major operational drilling parameters and their influence and interdependencies on wellbore dynamics and cuttings transport efficiency in the context of gas-liquid fluid systems, provides a tool for the prediction of cuttings transport mechanism, determination of the stationary bed height, and calculation of the annuli pressure losses. Therefore, wellbore pressure evaluation and management and hole cleaning requirements for UBD operations can be addressed

Modelling of flows in vertical pipes and its application to multiphase flow metering at high gas content and to the prediction of well performance

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