Boiler Tube Failure Thermohydraulic Analysis
Eskom operates 23 power stations in South Africa with a total capacity of more than 42 GW. It supplies about 95% of all the electricity used in the country. One of its coal-fired power stations was experiencing frequent boiler tube fatigue failures in the hopper section—the bottom part of the boiler—of all six units.The boilers were designed with a complex support beam structure that cradles and surrounds the boiler. Pivoting attachment mechanisms exist between the support beam structure, or buckstays, and the tube wall to allow for thermal expansion while still providing adequate support on all four sides.The boiler can expand up to a meter downwards during a startup sequence. Buckstays join at corner junction locations of the hopper where the slope walls and front/rear walls join. They are connected to each other using hinged members referred to as buckstay connection links.These junctions necessitate the rerouting of the surrounding front/rear wall tubes, leading to discontinuities in tube layout. High tube failure rates were identified at these tube manipulations and the areas were considered to be possible high-stress locations.
Code-to-code comparison for analysing the steady-state heat transfer and natural circulation in an air-cooled RCCS using GAMMA+ and Flownex
- The GAMMA+ and Flownex codes are used in the analyses of the air-cooled RCCS system.
- Radiation heat transfer comprises the bulk of the total rate of heat transfer.
- It is possible to obtain reverse flow through the RCCS standpipes.
- It has been found that the results obtained with the two codes are in good agreement.
- Supporting the safety case.
The system CFD approach applied to a pebble bed reactor
The system CFD code Flownex is used to simulate the transient behaviour of complete thermal-fluid systems such as the Pebble Bed Modular Reactor. In the context of this system simulation, the complexities in modelling the reactor heat transfer is discussed, as well as appropriate methods of dealing with it. The versatility of the system CFD approach is illustrated with three possible topologies for the pebble bed reactor core model. It is shown that the current pebble bed network topology compares very well to other possible network topologies, while having the advantage of predicting the maximum fuel temperature.
Analysis and Optimisation of Mine Water Management
A technique to simulate a tube break in a high-pressure gas/cooling water heat exchanger
The gas cycles of most HTR reject heat to water at some stage. In the helium/water heat exchangers of HTR’s with direct Brayton cycles, the helium is usually at a much higher pressure than the water. If the pressure boundary between the helium and the water fails inside the heat exchanger, the effect on the rest of the water system has to be established. This can be done by using a system simulation code, however, very few system simulation codes has the capability to do gas/liquid interface tracking. This study describes a calculation method with which a gas/liquid heat exchanger tube rupture can be calculated in a simulation code without interface tracking. The course of events after tube rupture is described and appropriate calculation models are derived. The calculation models were implemented in the system simulation software Flownex and used to study a tube rupture on a 5000kPa helium/water heat exchanger. The network solved stable and within reasonable time.
Integrated systems CFD modelling applied to diffusion-bonded compact heat exchangers
Micro-channel heat exchangers consist of a number of plates, containing fluid channels etched in the surface and diffusion bonded together to create a porous core of metal. The primary and secondary sides of the exchanger are formed by connecting the channels on alternating plates to the respective leader pipes. To analyze the thermal response of exchangers during operation, simulation software is used to create a network of numerical models representing the real-life thermal-hydraulics components. The Systems CFD approach uses one-dimensional empirical models for the fluid flow inside the channels and a three-dimensional model for the heat distribution inside the core. Spatial analysis of the geometry gives a connectivity stencil between the one- and three-dimensional models. This stencil implicitly links the equations of the models at matrix level in the numerical solver, with faster convergence in fewer iterations than when the models are coupled explicitly.
1-Way fluid structure interaction modelling methodology for boiler tube fatigue failure
A modeling methodology developed for dealing with fatigue failures on large boiler tube assemblies, as used by power generation industries, is described. Boiler tube fatigue failures are resultant to a coupled combination of fluid flow and heat transfer mechanisms, inducing thermal expansion leading to fatigue failure. A combination of modelling tools is effectively combined for one-way Fluid Structure Interaction, solving for and extracting stress results efficiently. A One Dimensional fluid solver is used to approximate and model the thermal flow components. The study case considered implemented the developed methodology on a quarter boiler hopper section made up of 3 022 tube and membrane structure with a collective length of 4 787 meters. Operating conditions are iteratively adjusted in the one dimensional pipe flow model until a correlation is formed with instrumented data. This validated model enables further use for various postulated plant conditions and operational sequences through transient start-up conditions. The boiler tube temperatures obtained from the one dimensional model are transferred and used as boundary conditions in a full three dimensional finite element analysis where deformations are solved for and stress results obtained due to thermal expansion within the boiler tube walls and the adjacent support structure. The model is used for redesign of sections of the boiler to reduce stress in those areas and subsequently reduce fatigue failures.
Design and successful testing of a physical model of the pebble bed modular reactor
The South African utility Eskom has been investigating the PBMR technology since 1993 for potential application as a power source in South Africa, as well as a viable South African export product. Eskom's partners are the South African Industrial Development Corporation and BNFL.
The South African PBMR is the most advanced within the Generation IV HTGR (High Temperature Gas Cooled) Reactors to be designed for commercial purposes. The intention is to build a 110 MWc class demonstration PBMR at Koeberg near Cape Town, where Africa's only nuclear power plant is situated; and an associated fuel plant at Pelindaba near Pretoria, where fuel for Koeberg used to he manufactured.
The concept allows for additional modules to be added in accordance with demand and to be configured to the size required by the communities they serve. It can operate in isolation anywhere, provided that there is sufficient water for cooling. Dry cooling, although more expensive, is an option that would provide even more freedom of location.
The commercial reactors would be sized to produce about 165 MW each. To maximise the sharing of support systems, however, the PBMR has been configured into a variety of options, such as 2, 4 and 8-pack layouts. These are the most cost effective layouts and allow the plants to be brought on line as they are completed.
The PBMR reactor consists of a vertical steel pressure vessel lined with graphite bricks. It uses silicon carbide coated particles of enriched uranium oxide encased in graphite to form a fuel sphere or pebble, hence its name. The PBMR is based on closed cycle three-shaft recuperative Brayton cycle and it uses helium as the coolant and energy transfer medium.
PBMR fuel is based on a proven high-quality German fuel design consisting of Low Enriched Uranium Triple-coated Isotropic (LEU-TRlSO) particles contained in a molded graphite sphere. A coated particle consists of a kernel of uranium dioxide surrounded by four coating layers.
The PBMR represents a significant advance in nuclear safety. In respect of "conventional" power reactors, the probability of a large release of radioactivity is minimal. In addition, the associated risk to near-by populations is several orders of magnitude less than non-nuclear risks accepted without question in daily lile. In respect of the PBMR, there is no risk of a major release - short of total destruction of the plant by an outside agency. Because the PBMR system is, to a high degree, inherently safe, there is less need for the sophisticated and costly engineered safety systems that surround today's large power reactors. The PBMR therefore also promises significantly cheaper power than its predecessors.
Design of a physical model of the PBMR with the aid of Flownet
The design of a physical model of the power conversion cycle of the PBMR with the aid of the code Flownet is discussed in this paper. The purpose of the physical model is to demonstrate the control strategies and operating procedures of the PBMR and also to demonstrate the accuracy of the simulation code Flownet. Flownet is first used to do component matching and to determine the detail steady-state performance of the system. It is then demonstrated how the code was used to simulate the start-up procedure as well as a load following and a load rejection scenario. The study demonstrates how a micro turbine system can be designed with the aid of a powerful simulation tool in a relatively short period of time at low cost using commercially available turbochargers.
An implicit method for the analysis of transient flows in pipe networks
Existing methods for the analysis of transient flows in pipe networks are often geared towards certain types of flows such as gas flows vis-à-vis liquid flows or isothermal flows vis-à-vis non-isothermal flows. Also, simplifying assumptions are often made which introduce inaccuracies when the method is applied outside the domain for which it was originally intended. This paper describes an implicit finite difference method based on the simultaneous pressure correction approach which is valid for both liquid and gas flows, for both isothermal and non-isothermal flows and for both fast and slow transients. The problematic convective acceleration term in the momentum equation, often neglected in other methods, is retained but eliminated by casting the momentum equation in an alternative form. The accuracy and stability of the method, depending on a time-step weighing factor α, are illustrated by analizing fast transients in a pipeline and simple branching network.
High temperature thermal energy storage utilizing metallic phase change materials and metallic heat transfer fluids
Cost and volume savings are some of the advantages offered by the use of latent heat thermal energy storage (TES). Metallic phase change materials (PCMs) have high thermal conductivity, which relate to high charging and discharging rates in TES system, and can operate at temperatures exceeding 560 °C. In the study, a eutectic aluminium–silicon alloy, AlSi12, is identified as a good potential PCM. AlSi12 has a melting temperature of 577 °C, which is above the working temperature of regular heat transfer fluids (HTFs). The eutectic sodium–potassium alloy (NaK) is identified as an ideal HTF in a storage system that uses metallic PCMs. A concept is presented that integrates the TES-unit and steam generator into one unit. As NaK is highly reactive with water, the inherently high thermal conductivity of AlSi12 is utilized in order to create a safe concept. As proof of concept, a steam power-generating cycle was considered that is especially suited for a TES using AlSi12 as PCM. The plant was designed to deliver 100 MW with 15 h of storage. Thermodynamic and heat transfer analysis showed that the concept is viable. The analysis indicated that the cost of the AlSi12 storage material is 14.7 US$ per kWh of thermal energy storage.
Thermal-hydraulics simulation of a benchmark case for a typical Materials Test Reactor using FLOWNEX
The purpose of this study was to serve as a starting point in gaining understanding and experience of simulating a typical Pool Type Research Reactor with the thermal hydraulic software code Flownex®. During the study the following evaluations of Flownex® were made:
- Assessment of the simplifying assumptions and possible shortcomings built into the software.
- Definition of the applicable modeling methodology and further simplifying assumptions that have to be made by the user.
- Evaluation of the accuracy and compatibility with the Pool Type Research Reactor.
- Comparing the results of this study with similar studies found in the open literature.
For the study the IAEA MTR 10 MW benchmark reactor (IAEA, 1992a) was used. A steady state simulation using Flownex® was done on a single fuel assembly, and this was compared with a model that was developed using the software package EES (Engineering Equation Solver). The results have shown good agreement between the different packages.
After this verification, a steady state simulation of the entire core was done to obtain the characteristics of the reactor operating under normal condition. Finally, transient simulations were performed on various LOFAs (Loss of Flow Accidents). The results of the various LOFAs were compared with studies that were previously done on the IAEA MTR 10 MW reactor.
Network modelling of transient heat exchanger performance
This study investigates the applicability of the thermal-fluid network approach to the modeling of transient heat exchanger performance.
Two different solution algorithms, namely the Implicit Pressure Correction Method (IPCM) and the Runge Kutta method with Trapezoidal Damping (RKTD) for the solution of the one-dimensional governing equations in thermal-fluid network problems are presented. The advantages and disadvantages of two types of numerical discretization schemes used in thermal-fluid network problems are discussed and the discretized one-dimensional governing equations for the staggered grid discretization scheme used in the IPCM and RKTD method is presented. The RKTD method is used as a time integration scheme for the generalized thermal-fluid network solver Xnet. Several test cases are introduced and the basic primitive elements available in Xnet are compared to the commercial thermal-fluid network code, Flownex (which uses the IPCM), for both steady-state and transient conditions.
Two different network topologies are introduced for the discretization of heat exchangers when a network approach is followed and the thermal-fluid network solver Xnet is applied to a basic parallel and counter flow configured pipe-in-pipe type heat exchanger to investigate the effect on the type of the discretization scheme used. The results obtained are compared to primitive element models in Flownex as well as the composite RX element in Flownex.
The extent to which thermal-fluid network solvers are able to predict transient heat exchanger performance are further investigated by modeling a complex shell-and-tube heat exchanger using Xnet and comparing the steady-state and transient results to both a primitive element model in Flownex as well as the composite STX element in Flownex. This contributes to the validation of Flownex’s heat exchanger models by using a different approach than Flownex.
The results showed that the explicit method used in Xnet is capable of solving large arbitrary structured thermal-fluid networks with a high level of accuracy. The result of Flownex compares very well with that of Xnet, which proves (verifies) that the solution algorithm is correctly implemented in both codes. Even though the explicit thermal-fluid network code, Xnet, can accurately predict fast transients, a drawback of this method is the large computational time required to simulate transient heat exchangers with large thermal masses.
Modelling of a gas turbine combustor using a network solver
In this study, a one-dimensional empirical model was developed and integrated with a commercial network solver to predict flow distributions and pressure losses for the combustion chamber of a commercial gas turbine aero engine. The need for such a model arose from practical problems being experienced on the particular gas turbine combustor considered. Results obtained showed that our simplified model is capable of predicting, with reasonable accuracy, the same trends as more detailed numerical models. The advantage, however, is the model’s rapid execution, which allows design modifications and parametric studies to be conducted more simply than before. Moreover, the data obtained from the one-dimensional analysis were also used as boundary conditions for a more detailed three-dimensional model. The results were compared with the measured temperature distribution on the combustor outlet plane, and overall good agreement was obtained.