A Two-Dimensional, Finite-Difference Model of the Oxidation of a Uranium Carbide Fuel Pellet

The oxidation of spent uranium carbide fuel, a candidate fuel for Generation IV nuclear reactors, is an important process in its potential reprocessing cycle. However, the oxidation of uranium carbide in air is highly exothermic. A model has therefore been developed to predict the temperature rise, as well as other useful information such as reaction completion times, under different reaction conditions in order to help in deriving safe oxidation conditions. Finite difference-methods are used to model the heat and mass transfer processes occurring during the reaction in two dimensions and are coupled to kinetics found in the literature

Modelling and simulation of a novel Electrical Energy Storage (EES) Receiver for Solar Parabolic Trough Collector (PTC) power plants

In this paper, the mathematical modelling of a novel Electrical Energy Storage (EES) Receiver for Solar Parabolic Trough Collector (PTC) is presented. The EES receiver is essentially a Heat Collecting Element (HCE) with built in storage in the form of thermal batteries such as the Sodium Sulphur (NaS) battery. The conceptual design and mathematical models describing the operation of the receiver are presented along with important results of model validation. When held under adiabatic conditions (a primary indicator of model validity), results were highly consistent with established PTC, models and with National Renewable Energy Laboratory (NREL, USA) experimental data for existing SCHOTT PTR-70 and Solel UVAC3 receiver tubes, currently being used in existing PTC power plants

Implications of hydrodynamics on the design of pulsed sieve-plate extraction columns: A one-fluid multiphase CFD model using the volume of fluid method

Presented is a detailed assessment of dispersive mixing and turbulence characterisation of an industrially representative pulsed sieve-plate extraction column (PSEC) obtained using multiphase CFD modelling. The system consists of a 150 mm diameter column with two perforated plates dispersing a 30 vol% dodecane/tributyl phosphate mixture in 3 M nitric acid. Operational conditions were chosen to examine pseudo steady-state dispersion regime operation. Three-dimensional transient flow calculations were performed using large eddy simulation (LES), coupled with the volume of fluid method. This study finds that LES is effective at capturing the different scales of turbulence present within PSECs, and their operational influence. Explicit analysis of the hydrodynamics established that the sieve-plates drive dispersive mixing through their influence on the resulting turbulent flow and flow structures. Furthermore, the standard round-hole sieve-plate design is found to perform poorly at producing and distributing the types of flow and turbulence beneficial to droplet size reduction

A coupled level set and volume of fluid method with a re-initialisation step suitable for unstructured meshes

This paper presents a coupling method of the level set and volume of fluid methods based on a simple local-gradient based re-initialisation approach that evaluates the distance function depending on the computational cell location. If a cell belongs to the interface, the signed distance is updated based on a search in the neighbouring cells and an interpolation procedure is applied depending on the local curvature or the sign of the level set function following [41]. The search algorithm does not distinguish between the upwind and downwind directions and hence it is able to be used for cells with an arbitrary number of faces increasing the robustness of the method. The coupling with the volume of fluid method is achieved by mapping the volume fraction field which is advected from the isoface evolution at a subgrid level. Consequently, the coupling with the level set approach is utilised without solving the level set equation. This coupled method provides better accuracy than the volume of fluid method alone and is capable of capturing sharp interfaces in all the classical numerical tests that are presented here

Zonal modelling of a counter-current spray drying tower

A generic multi-zonal modelling methodology for spray drying towers is proposed to provide relatively quick (seconds) simulations on desktop computers for decision making on changing process parameters for automatic process control. A multi-zonal model comprises combinations of all or some of six differing zones: plug-flow, semi-plug-flow and well-mixed zones in either co- or counter-current flow. This approach is demonstrated by predicting the dried powder characteristics of a detergent powder from a pilot-scale counter-current spray drying tower. The types, sizes and locations of the different zones are obtained by a detailed analysis of predictions from a previous 3-dimensional Computational Fluid Dynamics (CFD) simulation of gas and particle flow dynamics and drying kinetics within the tower. The multi-zonal model consists of seven zones – five in counter-current and two in co-current flow. The trends of the predicted gas temperature profiles are close to those from the CFD results. The particle exit temperature, moisture content and residence times over the full particle size distribution (PSD) range are very close to the CFD values unlike the previous simulated results from our plug-flow model. The outcome clearly demonstrates that the prerequisite to have a sound conceptual model of spray drying towers, such as the multi-zonal model developed here, is a detailed knowledge of the gas and particle flow fields within the tower

CFD modeling and performance evaluation of multipass solar air heaters

This article investigates the impacts of flow configurations on the thermal performance of a solar heater system. Recycled aluminum cans (RACs) have been utilized as turbulators with a double pass single duct solar air collector. The CFD software of COMSOL Multiphysics V5.3a is used to model three designs: Cocurrent (model A), countercurrent (model B), and U-shape (model C). The numerical results reveal that the U-shape design offers a greater thermal performance of 5.4% and 6.5%, respectively, compared with the cocurrent and countercurrent flow models. Furthermore, an outdoor experiment is performed based on the numerical modeling of flow configurations. The experimental setup is examined for three configurations of model C, namely, solar air heater (SAH) without RAC model C-I (plain model), SAH with in-line RAC layout (model C-II), and SAH with staggered RAC layout (model C-III). We found the double pass single duct solar air collector (model C) design is in a good agreement with the experimental data, and model C-III has a better thermal efficiency of 60.2%, compared to those of model C-II, 53.1%, and model C-I, 49.4%

CFD modelling of a pilot-scale counter-current spray drying tower for the manufacture of detergent powder

A steady-state, three-dimensional, multiphase CFD modelling of a pilot-plant counter-current spray drying tower is carried out to study the drying behavior of detergent slurry droplets. The software package ANSYS Fluent is employed to solve the heat, mass and momentum transfer between the hot gas and the polydispersed droplets/particles using the Eulerian-Lagrangian approach. The continuous phase turbulence is modelled using the differential Reynolds stress model. The drying kinetics is modelled using a single droplet drying model[1] which is incorporated into the CFD code using user-defined functions. Heat loss from the insulated tower wall to the surrounding is modelled by considering thermal resistances due to deposits on the inside surface, wall, insulation and outside convective film. For the particle-wall interaction, the restitution coefficient is specified as a constant value as well as a function of particle moisture content. It is found that the variation in the value of restitution coefficient with moisture causes significant changes in the velocity, temperature and moisture profiles of the gas as well as the particles. Overall, a reasonably good agreement is obtained between the measured and predicted powder temperature, moisture content and gas temperature at the bottom and top outlets of the tower; considering the complexity of the spray drying process, simplifying assumptions made in both the CFD and droplet drying models and the errors associated with the measurements

CFD analysis of a one-pass photovoltaic/thermal air system with and without offset strip fins

The focus on managing PV panel temperature has undergone a remarkable development in the last two decades. Specifically, in countries with moderate weather temperature and high insolation, the problem of keeping the PV cell temperature in an optimal range has been managed by use of PV/T collectors. In this work, a single pass PV/T collector using laminar air flow has been assessed. Two PV/T collector designs are utilised, one with and one without offset strip fins. COMSOL Multiphysics v5.3a has been used for the analysis of the thermal and electrical performances. Two assumptions were implemented in order to reduce the computational time from 95 hours to 7 hours, namely ignoring radiative effects between the fins and the wall channels, and representing thin layers as 2D boundaries, whilst ensuring a high level of conformity (4%),. Monocrystalline silicon PV cells were used with a power temperature coefficient of 0.41%. A validation against work in the literature was made, showing a good consistency. The objective of this work is to verify the performance of the air PV/T collector with offset strip fins compared to an unfinned air PV/T collector. The results reveal that the use of offset strip fins has a noticeable impact on both the electrical and thermal efficiencies of the system. In addition, the maximum combined efficiency (ηC o ) for the finned PV/T system is 84.7% while the unfinned PV/T system is 51.2%

Mathematical modelling of the pre-oxidation of a uranium carbide fuel pellet

Uranium carbide is a candidate fuel for future nuclear reactors. However, for it to be implemented in a closed fuel cycle, an outline for its reprocessing is necessary. One proposed method is to oxidise the uranium carbide into uranium oxide which can then be reprocessed using current infrastructure. A mathematical model describing the heat and mass transfer processes involved in such an oxidation has been constructed. The available literature was consulted for reaction coefficients and information on reaction products. A stable and convergent numerical solution has been developed using a combination of finite-difference approximations of the differential equations. Completion times of approximately 3-30h are predicted given a spherical pellet with a radius of 9.35mm under varying initial conditions. The transient temperature distribution throughout the system is predicted, with a maximum temperature of 1458°C observed from an initial temperature of 500°C at an oxygen concentration of 3.15mol m‾³

Simplified heat transfer model for highly active raffinate contained in buffer storage tanks

The research develops a simplified heat transfer model that allows prediction of the temperature response of the radioactive contents of a buffer storage tank at Sellafield, UK. The model emanates from an energy balance around a singular tank with comprehensive analysis of the heat transfer phenomena. A converged solution has been demonstrated by comparing different numerical solution techniques. The model has been validated using measurements made at Sellafield indicating strong agreement in trends. The results indicate, whilst storing a full tank of radioactive liquor, the vessel contents will not reach its boiling point of 95 °C but will instead reach an equilibrium temperature of 89 °C. The results also indicate that the model provides more accurate predictions compared with current operational assessments. Simulations from the model revealed the tanks can safely store material for approximately 67% longer than is predicted by current practice. The model has been developed in Microsoft Excel ® and can be run with minimal computational power on any machine. It includes a user-friendly interface allowing operators to generate results from a range of scenarios by running simulations within minutes, and provide operations personnel with more information to influence decisions surrounding industrial-scale problems