Amir Keshmiri

Effects of Grain and Pore Size on Salt Precipitation During Evaporation from Porous Media

Salt precipitation in saline porous media during evaporation is important in many processes including

RANS AND LES INVESTIGATIONS OF VERTICAL FLOWS IN THE FUEL PASSAGES OF GAS-COOLED NUCLEAR REACTORS

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Assessment of advanced RANS models against large eddy simulation and experimental data in the investigation of ribbed passages with passive heat transfer

CO2 sequestration, soil salinity which is a global problem as well as the preservation of monuments and buildings. In this study, X-ray micro-tomography was used to investigate the evolution of salt precipitation during evaporation to study the effects of particle and pore sizes on salt precipitation patterns and dynamics. The packed beds were saturated with NaCl solution of 3 Molal, and the time-lapsed X-ray imaging was continued for one day to obtain pore- scale information associated with the evaporation and precipitation dynamics and patterns. The results show that the presence of preferential evaporation sites (associated with fine pores) on the surface of the sand columns influences significantly the patterns and dynamics of NaCl precipitation. They confirm the formation of an increasingly thick and discrete salt crust with increasing grain size in the sand column due to the presence of fewer fine pores (preferential precipitation sites) at the surface compared to the sand packs with finer grains. Fewer fine pores on the surface also result in shorter stage-1 precipitation for the columns with larger grain sizes. A simple model for the evolution of salt crust thickness based on this principle shows a good agreement with our experiments. The findings of this study offer new insights about the dynamics and patterns of salt precipitation in drying porous media.

Benchmarking of Three Different CFD Codes in Simulating Natural, Forced, and Mixed Convection Flows

The present article reports RANS simulations of the flow and heat transfer in a 2-dimensional rib-roughened passage. The effects of four different geometrical factors including rib profile, rib pitch-to-height ratio, rib height, and rib width are investigated. The Reynolds number, based on the channel bulk velocity and hydraulic diameter, is 30,000. Two low-Reynolds-number linear EVMs, namely the Menter k-ω-SST model and a variant of Durbins v 2-f formulation, are examined. All computations are undertaken using the commercial CFD code STAR-CD. In comparison with experimental data, it emerges that the v 2-f model generally returns more accurate results than the k-ω-SST closure.

Impact of Using Conventional Inlet/Outlet Boundary Conditions on Haemodynamic Metrics in a Subject-Specific Rabbit Aorta

Coolant flows in the cores of current gas-cooled nuclear reactors consist of ascending vertical flows in a large number of parallel passages. Under post-trip conditions such heated turbulent flows may be significantly modified from the forced convection condition by the action of buoyancy, and the thermal-hydraulic regime is no longer one of pure forced convection. These modifications are primarily associated with changes to the turbulence structure, and indeed flow laminarization may occur. In the laminarization situation heat transfer rates may be as low as 40% of those in the corresponding forced convection case. The heat transfer performance of such ‘mixed’ convection flows is investigated here using a range of refined ReynoldsAveraged-Navier-Stokes (RANS) turbulence models. While all belong to the broad class of Eddy Viscosity Models (EVMs), the various RANS closures have different physical parameterizations and might therefore be expected to show different responses to externally-imposed conditions. Comparison is made against experimental and Direct Numerical Simulation (DNS) data. In addition, Large Eddy Simulation (LES) results have been generated as part of the study. Three different CFD codes have been employed in the work: ‘CONVERT’, ‘STAR-CD’, and ‘ Code_Saturne ’, which are respectively in-house, commercial, and industrial packages. It is found that the early EVM scheme of Launder and Sharma [1] is in the closest agreement with consistentlynormalized DNS results for the ratio of mixed-to-forced convection Nusselt number ( Nu/Nu 0). However, in relation to DNS and experimental data for forced convection Nusselt number, other models perform better than the LaunderSharma scheme. The present investigation has revealed discrepancies between direct-simulation, experimental, and the current LES studies.

Assessment of a common nonlinear eddy-viscosity turbulence model in capturing laminarization in mixed convection flows

The present work is concerned with the modeling of buoyancy-modified mixed convection flows, such flows being representative of low-flow-rate flows in the cores of Gas-cooled Reactors. Three different eddy viscosity models (EVMs) are examined using the in-house code, “CONVERT.” All fluid properties are assumed to be constant, and buoyancy is accounted for within the Boussinesq approximation. Comparison is made against experimental measurements and the direct numerical simulations (DNS). The effects of three physical parameters including the heat loading, Reynolds number, and pipe length on heat transfer have been examined. It is found that by increasing the heat loading, three thermal-hydraulic regimes of “early onset of mixed convection,” “laminarization,” and “recovery” were present. At different Reynolds numbers, the three thermal-hydraulic regimes are also evident. The k-� model of Launder and Sharma was found to be in the closest agreement with consistently normalized DNS results for the ratio of mixed-toforced convection Nusselt number (Nu/Nu0). It was also shown that for the “laminarization” case, the pipe length should be at least “500� diameter” in order to reach a fully developed solution. In addition, the effects of two numerical parameters namely buoyancy production and Yap length-scale correction terms have also been investigated and their effects were found to be negligible on heat transfer and friction coefficient in ascending flows. [DOI: 10.1115/1.4003925]

Enhancing Endothelialisation of Artificial/Engineered Blood Vessels Using Structural Cues

ABSTRACT Rough surfaces are widely used to enhance convective heat transfer by the promotion of higher turbulence levels. The present article reports simulations of the flow and heat transfer in a 2-D rib-roughened passage using a number of advanced Reynolds-averaged Navier-Stokes (RANS) turbulence models including eddy-viscosity models (EVM) and a Reynolds stress model (RSM). Large eddy simulation (LES) is also conducted and results are compared against experimental measurements. In addition, the effects of rib thermal boundary condition on heat transfer are also investigated. In the present work, the blockage ratio of the transversely mounted rectangular ribs is 10% and the rib pitch-to-height ratio of 9 is selected. The Reynolds number, based on the channel bulk velocity and hydraulic diameter, is 30,000. The RANS-based turbulence models investigated here are the k-ω-SST, the v2-f, the ϕ-f, and the elliptic blending RSM. All computations are undertaken using the commercial and industrial CFD codes STAR-CD and Code_Saturne, respectively. Of all the models, the LES predictions were found to be in the best agreement with the experimental data, while the k-ω-SST and EB-RSM returned the least accurate results.

Optimisation of a Novel Spiral-Inducing Bypass Graft Using Computational Fluid Dynamics

In this study, three different CFD codes are assessed in simulating two distinct flow problems with heat transfer. The first case consists of an ascending forced and mixed convection flow, a representative flow in the core of gas-cooled nuclear reactors under “post-trip” conditions, and the second case involves natural convection in an enclosed tall cavity that represents the gas-filled cavities around the nuclear reactor cores. Computations are conducted using the standard k-ω-SST model and the nonlinear k–ε model of Suga. Overall, the results were in satisfactory agreement, and, therefore, the present study represents a successful benchmarking exercise for the nuclear community.