Browsing by Author "Farah, Amjad"

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Assessment of FLUENT CFD code as an analysis tool for SCW applications
(2012-08-01) Farah, Amjad; Harvel, Glenn; Pioro, Igor
Chosen as one of six Generation‒IV nuclear-reactor concepts, SuperCritical Water-cooled Reactors (SCWRs) are expected to have high thermal efficiencies within the range of 45 ‒ 50% owing to the reactor‟s high pressures and outlet temperatures. The behaviour of supercritical water however, is not well understood and most of the methods available to predict the effects of the heat transfer phenomena within the pseudocritical region are based on empirical one-directional correlations which do not capture the multi-dimensional effects and do not provide accurate results in regions such as the deteriorated heat transfer regime. Computational Fluid Dynamics (CFD) is a numerical approach to model fluids in multidimensional space using the Navier-Stokes equations and databases of fluid properties to arrive at a full simulation of a fluid dynamics and heat transfer system. In this work, the CFD code, FLUENT-12, is used with associated software such as Gambit and NIST REFPROP to predict the Heat Transfer Coefficients at the wall and corresponding wall temperature profiles inside vertical bare tubes with SuperCritical Water (SCW) as the cooling medium. The numerical results are compared with experimental data and 1-D models represented by existing empirical correlations. Analysis of the individual heat-transfer regimes is conducted using an axisymmetric 2-D model of tubes of various lengths and composed of different nodes count along the heated length. Wall temperatures and heat transfer coefficients were analyzed to select the best model for each region (below, at and above the pseudocritical region). To neutralize effects of the rest of the tube on that region, smaller meshes were used were possible. Two turbulent models were used in the process: k-ε and k-ω, with many variations in the sub-model parameters such as viscous heating, thermal effects, and low-Reynolds number correction. Results of the analysis show a fit of ±10% for the wall temperatures using the SST k-ω model in the deteriorated heat transfer regime and less than ±5% for the normal heat transfer regime. The accuracy of the model is higher than any empirical correlation tested in the mentioned regimes, and provides additional information about the multidimensional effects between the bulk-fluid and wall temperatures. Despite the improved prediction capability, the numerical solutions indicate that further work is necessary. Each region has a different numerical model and the CFD code cannot cover the entire range in one comprehensive model. Additionally, some of the trends and transitions predicted are difficult to accept as representation of the true physics of SCW flow conditions. While CFD can be used to develop preliminary design solutions for SCW type reactors, a significant effort in experimental work to measure the actual phenomena is important to make further advancements in CFD based analysis of SCW fluid behaviour.
CFD determination of fluid and geometry related localized heat transfer phenomena for supercritical water flow
(2017-04-01) Farah, Amjad; Harvel, Glenn; Pioro, Igor
The proposed concept of Supercritical Water-cooled Reactor (SCWR) as part of the Generation IV International Forum aims to improve the thermal efficiency over current power plants by utilizing cooling water at pressures and temperature above the critical point. At supercritical conditions, however, the properties of the fluid can vary rapidly in response to changes in temperature and pressure, and without a phase change. One example is the specific heat, which exhibits a sharp peak at a point defined as the pseudocritical temperature. Computational Fluid Dynamics (CFD) is a numerical approach to model fluids in multidimensional space using the Navier-Stokes equations and databases of fluid properties to arrive at a full simulation of a fluid dynamics and heat transfer system. Turbulence models employed in CFD are a set of equations that determine the turbulence transport terms in the mean flow equations. They are based on hypotheses about the process of turbulence, and as such require empirical input in the form of constants or functions, in order to achieve closure. This work is conducted to further develop an understanding of supercritical water (SCW) flow by analyzing the flow- and geometry-dependent localized phenomena under supercritical conditions using CFD turbulence models. The numerical study employed the Realizable k-ε and the SST k-ω turbulence models. The created meshes are three dimensional to capture the multi-dimensional effects of SCW heat transfer phenomena. In the first part of the study, the turbulent Pr number effect on SCW heat transfer characteristics is determined by analyzing changes in fluid properties such as temperature profiles, turbulence intensity, and velocity in response to varying the turbulent Pr values in the CFD models. This investigation has shown the energy turbulent Pr to have the most effect on improving SCW heat transfer simulation results under the deteriorated heat transfer regime, by affecting the turbulence production in the fluid due to buoyancy forces. Buoyancy forces were also studied in downward flow under the same conditions and were shown to reduce the deterioration in heat transfer observed in upward flow. The second part involved an investigation of fluid property effects in complex geometries to determine important flow parameters that capture localized flow phenomena effects. Two geometries are considered: an annular channel with helical fins, and a tube with a sudden area change. The helicity of the first geometry did not appear to induce additional turbulence in the flow, compared to bare geometries. On the other hand, the sudden area change introduced large levels of turbulence, and while it dissipated quickly, it did show an enhancement in the heat transfer and lowered the outlet wall temperatures. These results can be used as a design input for SCWR fuel geometry design. As a result, this study contributes to the understanding of the SCW heat transfer fundamentals under normal and deteriorated regimes in bare and complex geometries, and identifies the areas of improvement in the related experimental work. Significant experimental work is needed to verify the findings