M. Fairweather

Numerical modelling of particle-laden sonic CO2 jets with experimental validation

The characteristics of the particle distribution, evolution and movement in a sonic jet release of carbon dioxide (CO2) from a high pressure reservoir are investigated. The motivation is to numerically model the sonic jet with particles, using the hitherto unknown initial particle distribution measured herein, and hence understand and numerically reproduce the experimentally observedparticle behaviour downstream of the Mach shock, including turbulence characteristics and level of agglomeration. We employ a Reynolds-averaged Navier-Stokes scheme with adaptive mesh refinement (AMR), combined with a Lagrangian particle tracker and particle distribution function. The model is seeded at the nozzle with the experimentally measured particle distribution and exploited to reproduce the observed characteristics of the jet. These releases are designed to be representative of a sonic CO2 release into the atmosphere and so provide data to help interpret how accidental or operational releases from the transport aspect ...

Prediction of turbulent non-premixed hydrogen flames using second-order conditional moment closure modelling

The advent of increasingly stringent emissions legislation inevitably leads to the requirement for more accurate modelling of pollutant formation in practical combustion applications. Previous limited success in modelling species such as NO, using first-order conditional moment closure (CMC) models indicates the need for more advanced modelling techniques. Here, a method of including higher-order chemistry within a one-dimensional, parabolic CMC framework is investigated, and applied to the prediction of three hydrogen jets of varying degrees of helium dilution. Interaction of the combustion model with both the k-e and Reynolds stress turbulence models is examined. Results are encouraging and found to be in line with expectations. Suggestions are made in light of this to account for anomalous predictions of nitrous radical formation.

Analysis of kinetic mechanism performance in conditional moment closure modelling of turbulent, non-premixed methane flames

This paper presents results obtained from the application of a first-order conditional moment closure approach to the modelling of two methane flames of differing geometries. Predictions are based upon a second-moment turbulence and scalar-flux closure, and supplemented with full and reduced chemical kinetic mechanisms, ranging from a simple 12-step to a complex 1207-step mechanism. Alongside analysis of the full kinetic schemes performance, is an appraisal of the behaviour of their derivatives obtained using mechanism-reduction techniques. The study was undertaken to analyse the practicality of incorporating kinetic models of varying complexity into calculations of turbulent non-premixed flames, and to make comparison of their performance. Despite extensive studies of the predictive ability of such schemes under laminar flame conditions, systematic evaluations have not been performed for turbulent reacting flows. This paper reflects upon the impact that selection of chemical kinetics has upon subsequent calculations and concludes that, although application of reduced schemes is more than adequate to reproduce experimental data, selection of the parent mechanism is of paramount importance to the prediction of minor species. Although widely used schemes are well documented and validated, their performances vary considerably. Thus, careful consideration must be made to their application and origins during the evaluation of combustion models.

Large-Scale Validation of a Numerical Model of Accidental Releases from Buried CO2 Pipelines

The work presented in this paper concerns a number of experiments and simulations performed as part of National Grids COOLTRANS research programme, initiated in order to address knowledge gaps relating to the safe design and operation of onshore pipelines for transporting dense phase carbon dioxide (CO2) from industrial emitters in the UK to storage sites offshore. Such pipelines are considered to be the most likely method for the transportation of captured CO2. The research presented here describes further developments of a state-of-the-art multi-phase heterogeneous discharge and dispersion model capable of predicting fluid dynamic and phase phenomena in releases from high pressure pipelines of CO2 into air. Model validation is included against a number of field-scale experiments considering various vertical releases of CO2 into free air. The model is also used to simulate a puncture release in a buried pipeline and the results near the crater edge are compared to field-scale experimental data. Model predictions are found to describe the experimental observations very well, with a high level of agreement between the two. The study demonstrates the advantages of using a model for addressing accidental releases of CO2 that includes shock-capturing methods and complete three-phase formulations. Such models are required to predict the physical and thermodynamic properties of CO2 in order to accurately predict the details of the discharge and dispersion phenomena of interest in risk assessments. Carbon capture and storage refers to a set of technologies designed to reduce carbon dioxide emissions from large point sources of emission such as coal-fired power stations, in order to mitigate greenhouse gas release. CCS technology, or sequestration, involves capturing CO2 and then storing it in a suitable storage facility, instead of allowing its release to the atmosphere, where it contributes to climate change. Necessary transportation can be achieved in different ways, but it is commonly acknowledged that high pressure pipelines transporting liquid CO2 will be the most reliable and cost effective choice. Their safe operation is of paramount importance as the inventory would likely be several thousand of tonnes, and CO2 poses a number of issues upon release due to its physical properties; it is a colourless, odourless asphyxiant which sinks in air and has a tendency to solid formation upon release with subsequent sublimation. It is directly toxic in inhaled air at concentrations around 5% and is likely to be fatal at concentrations around 10%. Predicting the correct fluid phase during the discharge process in the near-field is of particular importance given the very different hazard profiles of CO2 in the gas and solid states. A state-of-the-art multi-phase heterogeneous discharge and dispersion model capable of predicting both the near- and far-field fluid dynamic and phase phenomena in such CO2 releases was presented at ESCAPE 22. Predictions are based on solution of the density-weighted forms of the transport equations for mass, momentum and total energy. Closure of this equation set is achieved using a compressibility-corrected k- turbulence model. Solutions are obtained of the time-dependent forms of the descriptive equations and the integration of the equations is performed by a shock-capturing conservative, upwind second-order accurate Godunov numerical scheme. The fully-explicit time-accurate cell-centred finite-volume Godunov method is a predictor-corrector procedure, where the predictor stage is spatially first-order, and used to provide an intermediate solution at the half-time between time-steps. This solution is then subsequently used at the corrector stage for the calculation of second-order fluxes that lead to a second-order accurate cell-centred solution. A Harten, Lax, van Leer Riemann solver is employed to calculate fluxes at cell boundaries. A non-ideal equation of state with additional formulations to accurately predict solid phase properties is implemented to describe the physical and thermodynamic characteristics of CO2. The paper will describe further developments of this model in terms of its ability to accurately predict both gaseous and solid phases, and its validation against field scale experimental data from a vertical release of CO2 into free air. The model has also been used to simulate a puncture release in a buried pipeline and the results near the crater edge will again be compared to field scale data. This work forms part of the case studies performed in the National Grid COOLTRANS Research Programme. National Grid has initiated this research programme to address knowledge gaps relating to the safe design and operation of onshore pipelines for transporting dense phase CO2 from industrial emitters in the UK to storage sites offshore.

Particle-Interaction Effects in Turbulent Channel Flow

Large eddy simulation and a discrete element method are applied to study the flow, particle dispersion and agglomeration in a horizontal channel. The particle-particle interaction model is based on the Hertz-Mindlin approach with Johnson-Kendall-Roberts cohesion to allow the simulation of Van der Waals forces in a dry air flow. The influence of different particle surface energies on agglomeration, and the impact of fluid turbulence, are investigated. The agglomeration rate is found to be strongly influenced by the particle surface energy, with most of the particle-particle interactions taking place at locations close to the channel walls, aided by the higher concentration of particles in these regions.

A Numerical Case Study of Packed Columns

This paper presents results concerning the validation of a recently developed packing algorithm. The basic ethos of this algorithm, known as DigiPac, is to use three- dimensional methods to digitise particle shapes, and to use this digital information directly in computations of how particles pack together without further conversion or the need for modelling. A variety of simulations of packed columns, comprising mono, binary and ternary mixtures of spherical particles, together with shell-side containers, have been undertaken and the results compared to existing experimental data with good agreement being found. The ultimate aim of this work is to develop the packing algorithm as a design tool for use in optimising the performance of packed bed systems, and to enable the characterisation of any particle population and prediction of particle behaviour in packing, segregation and mixing. Particulate beds are commonly encountered in chemical and allied engineering fields, with the application of packed bed systems covering a wide range of areas including heterogeneous catalysis, solids handling, heat recovery, absorption, filtration and distillation. The design and performance prediction of such systems depends greatly on mathematical models that describe the behaviour of fluid flow, heat and mass transfer, and the pressure drop of the fluid through the bed. The models themselves are to a great extent dependant on accurate experimental data describing transport parameters such as effective thermal conductivity coefficients, wall heat transfer coefficients and dispersion coefficients. In turn, these parameters are sensitive to the structural properties of packed beds, namely the global and local voidages. The significance and magnitude of bed voidage is influenced by a number of factors, including the geometry of the particles and container, the method of particle loading and the treatment of the packing matrix during and after deposition. To design effective models of fixed-bed systems a variety of problems must therefore be addressed. Amongst those that merit investigation, the present study assesses the ability of DigiPac to predict the global and local voidage of beds containing mixed-sized particles. These systems occur by design, and in situations when particle breakage occurs, for example, during catalyst dumping.

Computational modelling of packed bed systems

Abstract Understanding the way particles pack within beds, and fluids flow through them, is of interest to a wide range of unit operations. This paper described a novel digitalisation approach that is capable of predicting the way in which particles of any shape pack into a container of any geometry. Prediction of fluid flow through such beds is also demonstrated through the use of lattice Boltzmann simulations. Both methods are shown to be capable of providing detailed information on the structure and flow within packed beds, with predictions being in reasonable agreement with available data. Overall, the combination of techniques provides a powerful modelling capability for packed beds that is of value to the improved design of a wide range of unit operations.

Inertial Particle Behaviour in Turbulent Fluid Flow

Abstract The influence of particle inertia, shape and orientation, and fluid turbulence intensity on the behaviour of particles in a channel flow is investigated using large eddy simulation coupled to a Lagrangian particle tracking technique that solves the Newton-Euler equations of rigid particle motion. The statistical moments of the particle velocity, the probability density function of the hydrodynamic acceleration and particle distributions within the flows are all used to characterise the particle-turbulence behaviour in these flows.

Non-spherical particle translation and orientation in wall-bounded turbulence

The translational and orientational dynamics of non-spherical particles in a turbulent channel flow at shear Reynolds number Reτ=300 is studied using an Eulerian-Lagrangian method coupled to an Eulerian rotational equation. Results show that particle dispersion to and away from the wall differs with shape, while the particle orientation exhibits several distinctive states depending on the particle shape andinitial position and orientation. There is a transition from one state to another with simulation time.

Measurement and RANS modelling of large-scale under-expanded CO2 releases for CCS applications

The deployment of a complete carbon-capture and storage chain requires a focus upon the hazards posed by the operation of CO2 pipelines, and the consequences of accidental release must be considered as an integral part of the design process. Presented are results from the application of a shock-capturing numerical scheme to the solution of the Favre-averaged Navier-Stokes fluid-flow equations, coupled with a compressibility-corrected turbulence model, and a novel equation of state for CO2. Comparisons are made with a series of as-yet unreported experimental observations of field-scale, high-pressure CO2 releases. The effects of corrections to the solenoidal turbulence energy dissipation are tested, with conclusions drawn, and recommendations made for future developments.