1,721,002 research outputs found
Simulation of natural convective flow in an experimental reactor cavity cooling system facility
Get PDFMEng (Nuclear Engineering), North-West University, Potchefstroom Campus, 2017The very high temperature reactor (VHTR) has many safety features. One of these features is the reactor cavity cooling system (RCCS). This system is intended to remove decay heat from the reactor cavity during upset conditions. The Korea Atomic Energy Research Institute (KAERI) constructed a facility that represents a ¼ scale model of the RCCS of a VHTR. The preliminary testing on the facility has been completed and a simulation model has been set up for the facility, using the system code GAMMA+. GAMMA+ was intended to be used to simulate the phenomena in gas-cooled reactors, particularly the PMR200 (under development by KAERI).
This study aims to simulate the facility using the 1D CFD program Flownex SE and compare the results with the results obtained with GAMMA+. The Flownex simulation was set up as close as possible to the GAMMA+ model by using the same initial- and boundary conditions. The fluid and surface temperatures, as well as the mass flow rates in the riser tubes, were compared to determine the agreement of the results. The results show very good agreement. There are differences in the philosophies of the programs, as well as some differences in the calculation of the fluid properties. The small differences in the results are attributed to these factors.
The mixed convection regime was found to be present and therefore the relevant correlations were used to calculate the heat transfer. The convection heat transfer coefficient had to be calculated based on a Nusselt number which is a combination of the forced and free convection Nusselt numbers.
The mixed convection regime can either increase or decrease the amount of heat that is transferred. In this particular study, the heat transfer was impeded, since the forced convection and free convection was orientated in the same direction while in the flow was in the turbulent regime. This was due to a laminarizational effect that the mixed convection regime can have on the boundary layer.Master
Numerical analysis of the flow distribution within packed columns using an explicit approach
Get PDFThesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2012.Existing correlations developed to account for pressure drop and velocity distribution
in packed beds are not ideal for beds with low aspect ratios. This study investigated
a method to model flow distribution through packed columns by
performing numerical analysis using an explicit approach. Fixed random packed
beds for column–to–sphere diameter ratios of 1.39Master
Pressure drop through a packed bed
Get PDFThesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2007.The importance of the development of PBMR technology for the generation of electricity in South Africa is undeniable. Part of the development includes simulation models to predict operating and transient thermal-fluid behaviour of the reactor core. With regard to thermal-fluid simulations pressure drop correlations are very important, and must be validated experimentally. The High Pressure Test Unit (HPTU) was designed, built and successfully commissioned to provide a facility capable of producing the range of experimental results required. Two types of pressure drop tests are performed on the HPTU, namely integrated and separate effects tests.
In this study a data processing methodology is established that is used to convert raw experimental data into meaningful results. The data processing methodology includes criteria for the prediction of steady state conditions and an uncertainty analysis to investigate the total uncertainty in the Euler number and the friction factor for packed beds. The data processing methodology was implemented and used to estimate the uncertainty in the desired variables. The methodology proved to be successful, and the estimated uncertainties were within the desired range and confidence interval. The repeatability of the results proved to be excellent, which further supports that the tests were successfully conducted.
The final results were compared with relevant correlations identified from a literature survey. The results from the separate effects tests could not be predicted by any correlations obtained from the literature and the Euler numbers were found to be significantly lower. The reason for the large deviation from existing correlations seems to be the result of an inherent characteristic of the packing arrangement of the beds. The results from the integrated effects tests were predicted reasonably well by correlations from the literature. Methods of including the influence of the walls in the prediction of the pressure drop showed that the walls could play an important role in the pressure drop through annular packed beds.
This study showed that the integrity and quality of the data obtained from the HPTU is high and that the results can be used with confidence in further research of pressure drop through packed beds.Master
Comparison of heat transfer models at the pebble, gas and reflector interface in the PBMR
Get PDFThesis (M.Sc. Engineering Sciences (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2010.It is a great challenge in the design of the PBMR to accurately predict gas flow and heat transfer in the reactor. Understanding the heat transfer at the core-reflector interface in particular is a very important aspect as the reactivity of the control rods housed in the reflectors is highly temperature dependent. It is also very important because the core-reflector interface is on the critical path for heat removal during accident conditions. PBMR has developed an OECD/NEA coupled neutronic/thermal-hydraulic benchmark to aid in the understanding of the different modelling approaches currently employed at PBMR. A comparison of THERMIX-KONVEK and DIREKT results showed large temperature differences at the core-reflector interfaces. Further investigation showed that these differences are as a result of the numerical methods used i.e. Cell-Centred (CC) vs. Vertex-Centered (VC). The present study extended this comparison to Star-CD (CC) and Flownex (VC) which are also used to simulate the reactor at PBMR. An ID MATLAB program that mimics the CC and VC numerical methods was verified against Star-CD and Flownex. This program was then used to model an ID version of the OECD/NEA benchmark. Results were compared with DIREKT and THERMIX-KONVEK. Although the results compared well, there were significant errors at the core-reflector interfaces. The findings of this study were that different numerical methods will predict different temperatures, heat fluxes and (temperature-dependent) sink terms. It was also shown that in addition to the differences resulting from numerical methods, differences were seen between Star-CD and DIREKT and Flownex and THERMIX-KONVEK in the region of the core-reflector boundary. In general, for complicated simulations like that of the pebble bed, the numerical basis of software used to simulate the problem needs to be understood for the problem to be correctly modelled.Master
Characterisation of long range radiation heat transfer in packed pebble beds
Get PDFThesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2012Due to its importance in the safety case of high temperature gas-cooled nuclear reactors, the effective thermal conductivity in packed pebble beds has been extensively studied by various researchers and several correlations have been developed. The correlations, mostly based on a unit cell approach, can model the total heat transfer through packed beds with reasonable accuracy. However, these correlations are typically highly empirical and do not discriminate between short and long-range radiation phenomena. It therefore does not specifically address the long-range radiation in detail at higher temperatures and in cases with large temperature gradients through the bed. It also does not discriminate between the pebble-to-pebble radiation in the bulk region and the pebble-to-reflector radiation in the near-wall regions. Long range radiation in such cases becomes important and it greatly influences the temperature distribution and heat flux. A need therefore arose to study long range radiation in packed pebble beds in more detail. Using the Computational Fluid Dynamics (CFD) program Star-CCM+, different unstructured and structured beds were studied in an effort to characterise the long range radiation phenomenon. Long range radiation in the bulk regions and near-wall regions of the beds was quantified through the use of view factors. Based on data from the characterisation of long range radiation, a new model is proposed in which long range radiation is simplified and predicted through a so-called Spherical Unit Nodalization (SUN) approach. This model was validated using CFD. The new model can now form the basis for the development of a specific term in the effective thermal conductivity which may be used in correlations to represent the effects of long range radiation in more detail.Master
Pressure formulation and adaptive control of numerical algorithms for transient flow in pipe networks
Get PDFThesis (PhD (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2013Fluid flow network simulation codes are commonly used as a design and analysis tool for many engineering problems such as gas distribution networks, power plants and heat pumps. Two formulations of conservation of momentum have been widely applied in fluid flow network simulation models namely those based on static pressure and those based on total pressure. The total pressure formulations are convenient in that they eliminate the difficulties associated with the calculation of the convective terms and components such as pipe junctions are treated in a straightforward manner based on total pressure losses. However, the different formulations of total pressure for compressible and incompressible flow require different formulations of the momentum conservation equation, which is inconvenient for implementation in a generic network simulation code. In this thesis a united total pressure formulation is first derived which is valid for all fluids and therefore eliminates the inconvenience of switching between the compressible and incompressible formulations. A non-iterative method for the solution of the non-isothermal discretised equations based on the total pressure formulation is then introduced and consistency is illustrated. The method appears to be very stable for subsonic flows, while rapid steady state convergence is observed. A systematic comparison is also done with traditional static pressure based methods and the similarities and differences between the two formulations are illuminated. The different time scales involved in the simulation of transient flow in fluid networks are problematic when conventional fixed time step methods are used for time-wise integration. The time scales associated with acoustic and kinematic wave phenomena as well as storage effects can differ by orders in magnitude. This thesis also presents a simple adaptive time step algorithm which can be readily used in conjunction with all the commonly used first order methods for fluid flow networks. Two test problems are selected to demonstrate the efficiency and savings obtained with this procedure. The adaptive time step algorithm correctly selects appropriate time steps for all phenomena and significant computational savings are observed for accurate integration. In addition, a procedure is implemented which automatically selects the appropriate integration method. The resulting algorithm is a fully adaptive algorithm which switches between a fully implicit method and a semi-implicit method. Two test problems are once again used to demonstrate the efficiency and savings. The fully adaptive algorithm correctly selects appropriate methods for all phenomena and significant additional computational savings are observed.Doctora
Application of the rate form of the equation of state for the dynamic simulation of thermal-hydraulic systems
Get PDFMIng (Nuclear Engineering), North-West University, Potchefstroom Campus, 2014The modelling of multi-phase water flow is an important modern-day design tool used by engineers to develop practical systems which are beneficial to society . Multi-phase water flow can be found in many important industrial applications such as large scale conventional and nuclear power systems, heat transfer machinery, chemical process plants, and other important examples.
Because of many inherent complexities in physical two-phase flow processes, no generalised system of equations has been formulated that can accurately describe the two-phase flow of water at all flow conditions and system geometries. This has led to the development of many different models for the simulation of two-phase
flow at specific conditions. These models vary greatly in complexity. The simplest model that can be used to simulate two-phase flow is termed the homogeneous equilibrium (HEM) two-phase flow model. This model has been found useful in investigations of choking and flashing
flows, and as an initial investigative model used before the formulation of more complex models for specific applications. This flow model is fully de ned by three conservation equations, one each for mass, momentum and energy. To close the model, an equation of state (EOS) is required to deliver system pressure values. When solving the HEM, a general practice is to employ an equation of state that is derived from a fundamental expression of the second law of thermodynamics. This methodology has been proven to deliver accurate results
for two-phase system simulations. This study focused on an alternative formulation of the equation of state which was previously
developed for the time dependent modelling of HEM two-phase flow systems, termed the rate form of the equation of state (RFES). The goal of the study was not to develop a new formulation of the EOS, but rather to implement the RFES in a transient simulation model and to verify that this implementation delivers appropriate results when compared to the conventional implementation methodology. This was done by formulating a transient pipe and reservoir network model with the HEM, and closing the model using both the RFES and a benchmark EOS known to deliver accurate system property values. The results of the transient model
simulations were then compared to determine whether the RFES delivered the expected results. It was found that the RFES delivered sufficiently accurate results for a variety of system transients, pressure conditions and numerical integration factors.Master
Modelling the effective thermal conductivity in the near–wall region of a packed pebble bed
Get PDFThesis (PhD (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2010Inherent safety is claimed for gas-cooled pebble bed reactors, such as the South African Pebble Bed Modular Reactor (PBMR), as a result of its design characteristics, materials used, fuel type and physics involved. Therefore, a proper understanding of the mechanisms of heat transfer, fluid flow and pressure drop through a packed bed of spheres is of utmost importance in the design of a high temperature Pebble Bed Reactor (PBR). In this study, correlations describing the effective thermal conductivity through packed pebble beds are examined. The effective thermal conductivity is a term defined as representative of the overall radial heat transfer through such a packed bed of spheres, and is a summation of various components of the overall heat transfer. This phenomenon is of importance because it forms an intricate part of the self-acting decay heat removal chain, which is directly related to the PBR safety case. In this study standard correlations generally employed by the thermal fluid design community for PBRs are investigated, giving particular attention to the applicability of the correlations when simulating the effective thermal conductivity in the near-wall region. Seven distinct components of heat transfer are examined namely: conduction through the solid, conduction through the contact
area between spheres, conduction through the gas phase, radiation between solid surfaces, conduction between pebble and wall, conduction through the gas phase in the wall region, and radiation between the pebble and wall surface. The effective thermal conductivity models are typically a function of porosity in order to account for the pebble bed packing structure. However, it is demonstrated in this study that porosity alone is insufficient to quantify the porous structure in a randomly packed bed. A new
Multi-sphere Unit Cell Model is therefore developed, which accounts more accurately for the porous structure, especially in the near-wall region. Conclusions on the applicability of the model are derived by comparing the simulation results with measurements obtained from various experimental test facilities. This includes the PBMRs High Temperature Test Unit (HTTU) situated on the campus of the North-West University in Potchefstroom in South Africa.
The Multi-sphere Unit Cell Model proves to encapsulate the impact of the packing structure in a more fundamental way and can therefore serve as the basis for further refinement of models to simulate the effective thermal conductivity.Doctora
Characterisation of thermal radiation in the near-wall region of a packed pebble bed
Get PDFMSc (Mechanical Engineering), North-West University, Potchefstroom Campus, 2015The heat transfer phenomena in the near-wall region of a randomly packed pebble bed are important in the design of a Pebble Bed Reactor (PBR), especially when considering the safety case during accident conditions. At higher temperatures the contribution of the radiation heat transfer component to the overall heat transfer in a PBR increases significantly. The wall effect present in the near-wall region of a packed pebble bed affects the heat transfer in this region.
Various correlations exist to predict the effective thermal conductivity through a packed pebble bed, but not all of the correlations consider the contribution of radiation and some are only applicable to the bulk region. Experimental research has been done on the heat transfer through a packed pebble bed. However, most of the results are case specific and cannot necessarily be used to validate models or simulations to predict the effective thermal conductivity of a pebble bed. The objective of this study is to develop a methodology that uses experimental work together with Computational Fluid Dynamics (CFD) simulations to predict the effective thermal conductivity in the near-wall region of a randomly packed pebble bed, and to separate the conduction and radiation components of the effective thermal conductivity. The proposed methodology inter alia includes experimental tests and the calibration of a CFD model to obtain numerical results that correlate well with the experimental results. To illustrate the proposed methodology the newly constructed Near-wall Effect Thermal Conductivity Test Facility (NWETCTF) was used to gather experimental results for the temperature and heat transfer distribution through a randomly packed pebble bed. Two identical but separate experimental tests were performed and the results of the two tests were in good agreement. From the experimental results the effective thermal conductivity was derived. The effect of the near-wall region on the heat transfer and the significance of radiation at higher temperatures are evident from the results. Recommendations were made for future experimental work with the NWETCTF from the findings of the investigation. A numerically packed pebble bed that is representative of the experimental pebble bed was generated using the Discrete Element Method (DEM) and a CFD model was set up for the heat transfer through the pebble bed using STAR-CCM+.. The CFD results showed trends similar to that of the experimental results. However, some discrepancies were identified that must be addressed in future studies by calibrating the CFD model. The effective thermal conductivity for the numerical simulation was determined using the CFD results and the conduction and radiation components were separated.Master
Effective material usage in a compact heat exchanger with periodic micro–channels
Get PDFThesis (MIng (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2013All modern High Temperature Reactors (HTR) thermal cycles have one thing in common: the use of some form of heat exchanger. This heat exchanger is used to pre-heat or cool the primary loop gas, from where the secondary power generation cycle is driven. The Compact Heat Exchanger (CHE) type offers high heat loads in smaller volumes. Various studies have been done to improve the heat transfer in the flow channels of these CHEs but little focus has been placed on the thermal design of surrounding material in such a heat exchanger. The focus of this study is on the effective material usage in a CHE. Three test cases were investigated (trapezoidal, serpentine and zigzag layouts with semi-circular cross-sections) all under the same boundary conditions. Computational Fluid Dynamics (CFD) was used to simulate these test cases and the results were evaluated according to four factors, the volume ratio, heat spots, temperature difference and the combined enhancement factor. From the results it was concluded that the zigzag layout performs best when evaluated according to the volume ratio and the temperature difference and gave the best overall enhancement factor. The serpentine layout performed the worst when evaluated according to the enhancement factor.Master
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