Search

This paper proposed a numerical modelling framework aiming to couple both Volume-Of-Fluid (VOF) model and MUltiple-SIze-Group (MUSIG) model to handle the formation of large-scale free surface, bubble entrainment and bubble dispersion in hydraulic oscillating jumps. To consider all essential physics, the modelling framework resolves and couples the flow structure of three different fluids (i.e. continuous air and water and dispersed air bubbles) using the Eulerian–Eulerian multi-fluid approach. To model the air entrainment at the jump toe, a sub-grid scale air entrainment model was also implemented within the framework. To evaluate the capability of the proposed model, model predictions were validated against experimental data of ( Chachereau and Chanson, 2010 ). Comparisons between predicted and measured results are in satisfactory agreement demonstrating the potential of the proposed methodology. Discussions on the drawbacks and deficiencies of the current model are also included. [ABSTRACT FROM AUTHOR]

Abstract: Modeling subcooled flow boiling in vertical channels at low pressures requires not only the consideration of the dynamic behaviors of two-phase flow and bubbles undergoing coalescence, breakup and condensation in the bulk subcooled liquid but also the characterization of the single-phase and local boiling heat transfer phenomena in the near-wall region. The focus of this paper is the assessment of the heat flux partitioning model in handling the latter physics of subcooled flow boiling. In order to achieve closure to the model, the current prevailing approach has always been the utilization of empirical correlations particularly for the active nucleation site density, bubble departure diameter and bubble departure frequency. A comprehensive survey of existing empirical correlations is presented in the first part of this paper to assess the performance of these empirical models. Selected combinations of empirical correlations are compared and validated against axial and local radial experiments encompassing a wide range of different mass and wall heat fluxes and inlet subcooling temperatures for subcooled flow boiling at low pressures. Based on the comparisons made against the axial and local distributions of void fraction and bubble Sauter diameter, not one single combination of empirical correlations has shown the propensity of providing satisfactory predictions covering the entire axial and local conditions. For the modeling of subcooled flow boiling at low pressures to become an effective predictive tool, it must therefore be complemented with further consideration of first principal models of the underlying physical phenomena. [Copyright &y& Elsevier]

Highlights: [•] Analyze gas–liquid flow in medium and large pipe under various flow regimes. [•] Identifying relative merits and capabilities of DQMOM. [•] Comparison of DQMOM with ABND and homogeneous MUSIG model. [•] Numerical results validation against gas–liquid flow experiments. [Copyright &y& Elsevier]

Abstract: Gas–liquid flows are commonly encountered in industrial flow systems. Numerical studies have been performed to assess the performances of different population balance approaches – direct quadrature method of moments (DQMOMs), average bubble number density (ABND) model and homogeneous MUlti-SIze-Group (MUSIG) model – in tracking the changes of gas void fraction and bubble size distribution under complex flow conditions and to validate the model predictions against experimental measurements from medium- and large-sized vertical pipes. Subject to different gas injection method and flow conditions, bubble size evolution exhibited a coalescence dominant trend in the medium-sized pipe; while bubble break-up was found to be dominant in large-sized pipe. The two experiments were therefore strategically selected for carrying out a thorough examination of existing population balance models in capturing the complicated behaviour of bubble coalescence and break-up. In general, predictions of all the different population balance approaches were in reasonable agreement with experimental data. More importantly, encouraging results have been obtained in adequately capturing the dynamical changes of bubbles size due to bubble interactions and transition from wall peak to core peak gas void fraction profiles. As a compromise between numerical accuracy and computational time, DQMOM has performed rather well in capturing the essential two-phase flow structures within the medium- and large-sized vertical pipes when compared to those of ABND and homogeneous MUSIG models. From a practical perspective, the ABND model may still be considered as a more viable approach for industrial applications of gas–liquid flow systems. [Copyright &y& Elsevier]

Abstract: Three-fluid model along with two-group average bubble number density equations have been applied for the classification of bubbles in vertical gas–liquid flow. The current study focused on (i) consideration of three-fluid model transport equations governing the conservation of mass and momentum, (ii) formulation of two-group averaged bubble number density equations, (iii) classification of bubble interaction between spherical bubbles (Group-1) and cap bubbles (Group-2), (iv) consideration of source and sink terms via the proposal of Hibiki and Ishii (Nuclear Engineering and Design 202, 39–76) in the averaged bubble number density equations and (v) assessment by means of the experimental data sets at the bubbly-to-cap flow transition. Reasonable agreement has been achieved between the measured and predicted local and axial distributions of void fraction, interfacial area concentration (IAC) and volume equivalent bubble diameter. This preliminary assessment demonstrated the capability of the current approach in capturing the interfacial transport of bubbly to cap flow transition. [Copyright &y& Elsevier]

Abstract: This paper presents a numerical study on the drag reduction mechanism created by a ventilated partial cavity and its associated effects by the downstream dispersed microbubbles. A semi-empirical approach is introduced to model the discrete interface of the ventilated cavity and its complex gas leakage rate induced by the local turbulent shear stress. Based on the Eulerian–Eulerian two-fluid modeling framework, a population balance approach based on MUltiple-SIze-Group (MUSIG) model is incorporated to simulate the dynamical effects of bubbly flow along the test body. Particular attention is also directed to grasp a better understanding of the size evolution of microbubble and its associated effects on drag reduction. Model predictions are validated against three experimental measurements carried out in a high-speed water tunnel by and . Close examination of the flow structures, gas void fraction distributions and its resultant density ratio provides valuable insights on the complex physical phenomenon, helping to consolidate idea to maximize the drag reduction for ventilated cavitating vehicles. [Copyright &y& Elsevier]

Abstract: The dispersion of bubbles into a down-liquid flow in a vertical pipe is investigated. At low flow rates, the intended design of a swarm of discrete bubbles is achieved. At high flow rates, a ventilated cavity is nonetheless formed, which is attached close to the gas sparger. Behind this ventilated cavity, three different flow regimes characterize the complex bubbly flow field downstream of the down-liquid flow: vortex region with high void fraction, transitional region and pipe flow region. In this study, a numerical model that solved the entire development of the gas–liquid flow including the extended single-phase liquid region upstream to the wall-jet and recirculating-vortex zones in order to allow a more realistic determination of the boundary conditions of the down-liquid flow was adopted. Coupling with the Eulerian–Eulerian two-fluid model to solve the respective gas and liquid phases, a population balance model was also applied to predict the bubble size distribution in the wake right below the cavity base as well as further downstream in the transitional and fully-developed pipe flow regions. The numerical model was evaluated by comparing the numerical results against the data derived from theoretical, numerical and experimental approaches. Prediction of the Sauter mean bubble diameter distributions by the population balance approach at different axial locations confirmed the dominance of breakage due to the high turbulent intensity below the ventilated cavity which led to the generation of small gas bubbles at high void fraction. Further downstream, the coalescence effect dominated leading to merging of the small bubbles to form bigger bubbles. [Copyright &y& Elsevier]

Abstract: Gas–liquid bubbly flows with wide range of bubble sizes are commonly encountered in many industrial gas–liquid flow systems. To assess the performances of two population balance approaches – Average Bubble Number Density (ABND) and Inhomogeneous MUlti-SIze-Group (MUSIG) models – in tracking the changes of gas volume fraction and bubble size distribution under complex flow conditions, numerical studies have been performed to validate predictions from both models against experimental data of and measured in the Forschungszentrum Dresden-Rossendorf FZD facility. These experiments have been strategically chosen because of flow conditions yielding opposite trend of bubble size evolution, which provided the means of carrying out a thorough examination of existing bubble coalescence and break-up kernels. In general, predictions of both models were in good agreement with experimental data. The encouraging results demonstrated the capability of both models in capturing the dynamical changes of bubbles size due to bubble interactions and the transition from “wall peak” to “core peak” gas volume fraction profiles caused by the presence of small and large bubbles. Predictions of the inhomogeneous MUSIG model appeared marginally superior to those of ABND model. Nevertheless, through the comparison of axial gas volume fraction and Sauter mean bubble diameter profiles, ABND model may be considered an alternative approach for industrial applications of gas–liquid flow systems. [ABSTRACT FROM AUTHOR]

Abstract: The phenomenon of drag reduction by the injection of micro-bubbles into turbulent boundary layer has been investigated using an Eulerian–Eulerian two-fluid model. Multiple-size group (MUSIG) based on population balance models, which resolve a wide range of bubble sizes taking into account the bubble break-up and coalescence have been used for this purpose. The simulated results are compared against the experimental findings of Madavan et al. [1984. Reduction of turbulent skin friction by micro-bubbles. Physics of Fluids 27, 356–363] and also other numerical studies explaining the sophisticated phenomena of drag reduction. For the two Reynolds number cases considered, the buoyancy with the plate on the bottom configuration is investigated, as from the experiments it is seen that buoyancy seem to play a role in the drag reduction. Numerical model employed in the investigation comprises of a micro-bubble laden flow wherein two independent sets of Reynolds averaged Navier–Stokes (RANS) transport equations were used to describe both the phases of the flow. The shear stress transport (SST) turbulence model is used as the turbulent closure for the primary phase and a zero equation turbulence model is used for the micro-bubbles. Change in the mean streamwise velocity profiles, void fraction, turbulence modification and other results are presented and discussed with corresponding change in the gas injection rates. The complex mechanism of drag reduction are scrutinised and explained in context to our numerical findings. Special attentions have been also devoted to divulge the effect of bubble coalescence and break-up caused by random collision and turbulent impact. Numerical results showed good agreement for the skin-friction coefficients against experimental data throughout various air injection rates. The MUSIG model was found to be one of the best candidates to resolve the bubble dynamics in micro-bubble-induced drag reduction problems. [Copyright &y& Elsevier]

Abstract: In this paper, the improved heat flux partitioning model based on first principal models of the underlying physics in determining the active nucleation site density, bubble size and bubble frequency is evaluated for subcooled flow boiling in vertical heated channels at low pressures. Salient features of this model include the fractal approach in determining the active nucleation site density, the force balance model in determining bubble diameters at sliding and lift-off as well as bubble frequency through the consideration of growth and waiting times and the additional heat flux at the heated wall due to surface quenching of sliding bubbles. Model predictions, covering a wide range of different mass and heat fluxes and inlet subcooling temperatures, are compared against local and axial measurements. In comparison to the measured data and selected combinations of empirical correlations such as described in Part 1, the current model predictions clearly demonstrate the effect of the subcooling effect on the activation of nucleation sites at the heated wall. The selected combinations of empirical correlations consistently under-estimate the wall superheat temperature while the current model yields predictions that agree very well with experimentally measured temperatures. Bubble sliding along the heat wall and its influence on heat partitioning and surface quenching heat flux has been ascertained to play an important role during the subcooled flow boiling process. [Copyright &y& Elsevier]

Abstract: In consideration of the practical importance regarding the application of gas–liquid flow in a vertical pipe and the quest towards the development of more robust physical models to accurately predict the essential interfacial parameters of the two-phase flow, comprehensive analysis of the characteristics and phase distribution patterns of such a flow have been performed on both experimental measurements and theoretical predictions. In this first part, analysis of experimental data in a large diameter pipe with an inner diameter of 195.3mm via the wire-mesh senor measuring technique was presented. The experiments were performed at the TOPFLOW facility of Helmholtz-Zentrum Dresden-Rossendorf. In the present paper, measurements of local interfacial parameters which included the void fraction, volume equivalent bubble diameter, bubble size distribution and interfacial velocities were discussed. Test points covering flow regimes from bubbly to cap to slug to churn-turbulent flow were selected to investigate the flow of different bubble shapes and sizes and the significant bubble coalescence and break-up mechanism throughout the large vertical pipe. The radial and axial evolutions of the local flow structure were interpreted in terms of the classifications of different groups of bubbles (Group-1 and Group-2). In addition, the phase distribution patterns were analyzed through the concept of skewness, which essentially identified two fundamental patterns, namely, wall peak and core peak. In general, Group-1 bubbles being smaller spherical bubbles have shown to exhibit a wall peaking distribution while Group-2 bubbles being larger non-spherical bubbles corresponded to a core peaking distribution. The classification of bubbles that have been performed in this present study can be employed for the development of bubble coalescence and break-up mechanistic kernels and other interfacial force closure models for a two bubble group approach in the context of computational fluid dynamics. [Copyright &y& Elsevier]

A novel Large Eddy Simulation (LES) based fire field model that incorporates pyrolysis modelling has been developed. This model is specifically designed for flame propagation of wildland fire scenarios. It uniquely embraces the radiation heat feedback from the flame, gaseous combustion and soot products towards the fuel bed surface. It also considers the detailed chemical kinetics for combustion, primary soot incipient and oxidant for soot formation, turbulent microscopic fuel–air mixing which are fully coupled, interactive and non-linear. Numerical simulation has been performed to study the effect of slope angle on the flame propagation characteristic of pine needle fuel beds. The fire spread rate and temperature predictions are within 12% accuracy in comparison to experimental data. Owing to the unbalanced air entrainment drew by the flame combustion for the inclined slope angle cases, it can be observed from the visualised flame that it was tilted to the unburned portion of the board. This behaviour strongly promotes the radiative heat transfer from the flame onto the fuel bed, which led to a rapid increase in pyrolysis rate thus accelerated the overall flame spread on the board surface. This physical phenomenon was successfully captured by the pyrolysis model and was found to be significantly more accurate in predicting the fire spread rate for slope angles higher than 20° (error of 11.12%) when compared to empirical flame tracking methods (error of 87.19%). [ABSTRACT FROM AUTHOR]

A novel in-house computation code based on large eddy simulations (LES) incorporating fully coupled subgrid-scale (SGS) turbulence, combustion, soot and radiation models for turbulent reacting flows in compartment fires has been developed. It uniquely embraces the detailed reaction mechanisms for the chemical processes involved during combustion, which provides a comprehensive description of the fuel oxidation processes. Furthermore, it gives a more complete representation of the generation of intermediate chemical by-products including toxic gases such as carbon monoxide and irritant gases such as soot. Since the modelling of hydrocarbons by-products are enabled when considering the full chemical profile, the formation of soot particles is correlated to the concentration of main soot incipient acetylene, which provides an appropriate representation of nucleation, surface growth processes. The LES code has been validated with experimental fire tests results for both ventilated and under-ventilated fires in compartment for confirmation of its robustness. The importance of incorporating the detailed reaction mechanisms in compartment fire simulations has been confirmed by comparing with experiments. For under-ventilated fires in compartment, the chemical kinetics become increasingly important since the combustion efficiency drops significantly involving generation of intermediate chemical species. It is discovered that species concentrations especially CO 2 and CO are more accurately predicted by the detailed scheme comparing to the multi-step scheme, since the formation of hydrocarbons and nitrogen oxides are considered. In general, the simulation incorporating detailed kinetics showed an averaged improvement of 9.2% and 81.7% in the prediction of CO 2 /CO ratio and volume fraction of soot respectively. This also improves the replication of the flame structure as the fire is chemically-driven within the combustion zone. [ABSTRACT FROM AUTHOR]

• Characterisation of polymer pyrolysis process via Reactive Molecular Dynamics. • Three polymers were considered: HDPE, PMMA and HIPS. • Detailed pyrolysis kinetics and char formation were in good agreement with experiment. • PE, PMMA, and HIPS mainly form alkane group combustible gases (i.e. mainly C1-C3). • Char formation kinetics for the selective polymers has been characterized. One of the major limiting factors in fire modelling involving solid pyrolysis of polymer materials is the fundamental understanding of thermal degradation process from solid fuel to gas volatiles. This also brings along other challenges such as the characterisation of the parent combustion fuel in the gas-phase chemical reaction process. To bridge the knowledge gap, this article proposes a methodology applying molecular dynamics (MD) simulation as a tool to characterise the thermal degradation process of polymer composites, especially the emission of volatile and toxic gas species. The method was applied to three common engineering polymers: i) high-density polyethene (HDPE), ii) polymethyl methacrylate (PMMA), and iii) high-impact polystyrene (HIPS). Based on the modelling results, the chemical distribution of the fully-decomposed chemical compounds was realised for the selected polymers. The chemical composition and charring kinetics were validated against thermogravimetry data via experimental measurement. Numerical simulations demonstrated good agreement with the thermogravimetric analysis experiments. It was found that all HDPE, PMMA, and HIPS formed fuel gas with alkane group (i.e. mainly C1-C3) that acted as the combustible source. Furthermore, the composition of char formations for the selective polymers can be predicted by the MD simulation through analysing the accumulation of pure carbon chain compounds. In this study, MD simulation identified the detailed decomposition process from solid to gas phases, which could further act as the precursors of combustible fuel gases in combustion models, and significantly enhance the reliability of toxic gas, charring, and smoke particulate predictions. [ABSTRACT FROM AUTHOR]