Ali Heidari

Dispersion of carbon dioxide from vertical vent and horizontal releases—A numerical study:

Numerical simulations of far-field carbon dioxide dispersion were conducted for a vertical vent release and a horizontal release from a shock tube. These scenarios had also been studied experimentally at field scale commissioned by National Grid. This work and the experiments both form part of the National Grid dense phase CO2 pipeLine TRANSportation (COOLTRANS) research programme. All tests involved releases of dense phase CO2 into an atmospheric flow. The dispersing plumes were subjected to transient wind conditions where both the direction and magnitude of the wind fluctuated with time. As part of the COOLTRANS research programme, the far-field dispersion simulations started from source terms derived from the near-field simulations conducted by the University of Leeds and outflow simulations conducted by University College London. The numerical model used for the far-field simulations is based on OPENFOAM, which is an object-oriented open source computational fluid dynamics toolbox. A dedicated solver CO2FOAM has been developed within the framework of OPENFOAM for simulating dispersion from dense phase CO2 releases. This has included the implementation of the homogeneous equilibrium method for fully compressible two-phase flow, treatment of the transient atmospheric boundary conditions and the time-varying inlet boundary conditions. The experimental measurements were supplied to the authors after the predictions were completed and submitted to National Grid. Hence, the validation reported here is indeed “blind.” While further fine tuning of the model and validation is still underway, the relatively good agreement between the predictions and measurements in the present study has demonstrated the potential of CO2FOAM as an effective predictive tool for far-field CO2 dispersion in the context of pipeline transportation for carbon capture and storage.

Alteration in refractive index profile during accommodation based on mechanical modelling.

The lens of the eye has a gradient refractive index (GRIN). Ocular accommodation, which alters the shape of the lens in response to visual demand, causes a redistribution of the internal structure of the lens leading to a change in the GRIN profile. The nature of this redistribution and the consequence of change in the GRIN profile are not understood. A modelling approach that considers how the GRIN profile may change with accommodation needs to take into account optical and mechanical parameters and be cognisant of individual variability in the shape and size of lenses. This study models the normalised axial GRIN profile during accommodation using reduced modelling and incorporating finite element analysis to connect inhomogenous mechanical characteristics of the lens to optical performance. The results show that simulated stretching changes the length of the plateau but does not alter the cortical gradient, which supports clinical findings. There is a very small change to the accommodated and non-accommodated profiles when normalised, yet this yields measurable changes in aberrations with around 11% and almost 13% difference in spherical aberration and astigmatism respectively. The results can be used in reconstruction of the refractive index and for investigating gradual changes with age.

A Computational Fluid Dynamic Investigation of Inhomogeneous Hydrogen Flame Acceleration and Transition to Detonation

Gas explosions in homogeneous reactive mixtures have been widely studied both experimentally and numerically. However, in practice and industrial applications, combustible mixtures are usually inhomogeneous and subject to vertical concentration gradients. Limited studies have been conducted in such context which resulted in limited understanding of the explosion characteristics in such situations. The present numerical investigation aims to study the dynamics of Deflagration to Detonation Transition (DDT) in inhomogeneous hydrogen/air mixtures and examine the effects of obstacle blockage ratio in DDT. VCEFoam, a reactive density-based solver recently assembled by the authors within the frame of OpenFOAM CFD toolbox has been used. VCEFoam uses the Harten–Lax–van Leer–Contact (HLLC) scheme fr the convective fluxes contribution and shock capturing. The solver has been verified by comparing its predictions with the analytical solutions of two classical test cases. For validation, the experimental data of Boeck et al. (1) is used. The experiments were conducted in a rectangular channel the three different blockage ratios and hydrogen concentrations. Comparison is presented between the predicted and measured flame tip velocities. The shaded contours of the predicted temperature and numerical Schlieren (magnitude of density gradient) will be analysed to examine the effects of the blockage ratio on flame acceleration and DDT.

Modeling liquid fuel cascades with OpenFOAM

Evaporating liquid cascades resulting from gasoline and liquefied natural gas tanks overfilling or rupture of elevated pipes create a source of flammable vapor cloud. Such phenomena were responsible for the formation of the large fuel vapor cloud, the ignition of which resulted in the large scale explosion, in Buncefield [Buncefield Major Incident Investigation Board, Explosion Mechanism Advisory Group Report, 2007] on December 11, 2005 at the Hertfordshire Oil Storage Terminal, an oil storage facility located by Hemel Hempstead in Hertfordshire, England. Despite its significance, there lacks adequate models treating the underlying physics of this phenomenon. The present study numerically analyses fuel cascades which are considered as a droplet‐laden system. Consideration is given to vapor production inside the cascade due to droplets evaporation and breakup. The solver used here is a modification of the sprayFoam solver which is present in the open source computational fluid dynamics (CFD) toolbox OpenFOAM® [OpenFOAM 2.3.0, Available at http://www.openfoam.com]. The fuel droplets evaporate during their motion and create a cloud of flammable vapor. In order to capture the characteristics of the hazardous phenomena, the CFD model needs to address the underlying physics with adequate submodels. In the present study, the multiphase flow is simulated with a combined Eulerian‐Lagrangian approach. The governing equations of the gas phase represent the conservation equations of mass, momentum, and energy including the source terms arising from the interaction with the droplets. The Reynolds Averaged Navier–Stokes simulation approach was used for its computational efficiency. The Large‐Eddy Simulation would be more robust in handling the interaction of the droplets with the flow but it would require more computational resource. The particulate phase is simulated through a Lagrangian deterministic or stochastic tracking models to provide particle trajectories and particle concentration. Particular emphasis is given to the effect of impingement of droplets to account for the effect of splashing in the impact region. The study involves developing robust and accurate modeling approaches for the instabilities and aerodynamic breakup in the cascade which contribute to the formation of the cloud, air entrainment, and fuel impingement on deflector plates. Suitable submodels have been implemented in OpenFOAM® to facilitate the study. The predictions are compared with the experimental measurements and CFD predictions previously conducted by Atkinson and Coldrick [Research Report 908, 2012] from the Health and Safety Laboratory, an agency of the Health and Safety Executive (HSE). The present predictions are found to better capture the interaction between the droplets and the gas phase. Improved agreement with the experimental measurements in the gasoline fuel cascades has also been achieved.