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Studies on part load controlled cooling air supplies in stationary gas turbines

The design of new stationary gas turbines and development of upgrades for existing respectively is facing challenges regarding part load operation. The demand for high overall efficiency and compliance with legal requirements depend on the design of cooling air circuits among others. The design of an optimized secondary air distribution at both base load and part load as well as the consideration of ambient conditions requires conceptual studies and hence appropriate models.

This paper introduces the holistic model of a literature based generic stationary gas turbine, which essentially couples a gas turbine performance synthesis model with a more detailed secondary air system (SAS) network model. Extended with additional models such as evaluation of blade and vane material temperatures Tmat, it allows for comparative off design studies with uncontrolled and controlled turbine cooling air circuits. The presented studies here first focus on margins of Tmat with base load condition as benchmark. The subsequent exploitation of these margins is limited by the fundamental requirements of hot gas ingestion at common rim seal configurations. Either way, the reduction of cooling air at part load is beneficial in terms of fuel flow reduction: vane cooling air control results in up to 0.12% of fuel flow reduction at part load operation.

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An integrated systems CFD simulation of a pebble bed reactor

The theoretical basis of a systems CFD model of a pebble bed reactor is discussed. This model is employed to simulate the thermal-fluid phenomena of the reactor core. The formulation of the fundamental equations results in a collection of one-dimensional elements that can be used to construct a network model of the reactor. One preliminary test is discussed to illustrate the application of the model.

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Verification and validation of a thermal hydraulic code used to model the PBMR

Flownex is a general-purpose thermal-fluid network analysis code used as a tool in the design of the Pebble Bed Modular Reactor. The National Nuclear Regulator requires the use of benchmarks to Verify and Validate (V&V) such a design code. This paper covers the verification and validation of selected fundamental components and aspects in Flownex. Attention is also given to the integration of these components to verify and validate their use in arbitrary structured thermal-fluid networks.

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A finite volume-based network method for the predicition of heat, mass and momentum transfer in a pebble bed reactor

In this paper a new control volume based network scheme for the prediction of heat, mass and momentum transfer in a pebble bed reactor is presented. The discretization scheme takes detail effects such as convection, radiation and conduction heat transfer into account and also provides for several types of reactor flow elements. The network representation of the reactor is solved as an integral part of the overall thermal-fluid network for the entire power conversion unit. The new control volume based network scheme facilitates the setting up of the reactor network taking detail effects such as complex radiation and effective pebble bed-solid interface conduction into account. The model is used as a benchmark for the current element-based network scheme and it is also used to investigate the effect of more accurate radiation discretization on the temperature profile in the reactor. Excellent agreement has been found with the current element-based reactor model.

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Dynamic systems CFD simulation code for the modeling of HTGR power plants

In this paper the development of a dynamic systems CFD simulation code for the modeling of HTGR power plants will be discussed. The basic methodology of the numerical method is known as an implicit pressure correction method (IPCM) will be presented. The method is capable of solving the pressure distribution, mass flow rate and temperature through an arbitrarily structured, physically realistic thermal-fluid system. The different component models used in the modeling of a typical HTGR power plant will also be presented. A loss of load incident for a three shaft recuperative Brayton Cycle will be modeled and the results obtained from the simulation will be discussed to demonstrate the utility of the code. The simulation will be used to show both the steady-state and dynamic ability of the method.

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Verification and validation of the HTGR systems CFD code Flownex

Regulatory requirements prescribe extensive V&V of computer codes that are used in the design and analysis of accident conditions in nuclear plants. Flownex is a dynamic systems CFD code used as the primary thermal-fluid simulation code by the PBMR. Stringent quality assurance processes have been implemented to ensure that the code conforms to the set standards. These processes include the comparison of Flownex with analytical results and experimental data. Analytical solutions are used to verify Flownex's element models so as to ensure that the basic theory is correctly implemented. Comparison with experimental and plant data is a very important feature of the V&V program to validate that the chosen theory is fit for purpose. For this, validation data from the PBMM is used. In addition to the PBMM experimental data Flownex is compared to a number of small thermal-fluid experiments in which certain specific component phenomena is validated.

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Flownet Nuclear Architecture, Implementation and Verification & Validation

Flownet is a general network analysis code used by PBMR (Pty) Ltd for the thermalfluid design and analysis of the PBMR plant. It is based on an implicit pressure correction method (IPCM) that solves the continuity, momentum and energy equations (including rotating element dynamics) in large arbitrary structured networks for both steady-state and dynamic analysis. In order to facilitate maximum code re-use and maintainability it was re-developed in an object-oriented paradigm, using C++, within a strict quality system. This paper presents the architectural design, model development, implementation, and verification and validation philosophies used during the development of Flownet. Finally it comments on results obtained with this software package.

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The analysis of the pebble-bed modular reactor thermal fluid cycle using a network code

The PBMR power plant is currently being developed by PBMR (Pty) Ltd. This HTGR plant is based on a three-shaft Brayton cycle. Engineers are faced with major challenges when carrying out the thermal-fluid design of the plant. System performance predictions must be done for both steady-state and transient conditions. The complexity associated with the thermal-fluid design of the cycle requires the use of a variety of analysis techniques and simulation tools. These range from simple one-dimensional models that do not capture all the significant physical phenomena to large-scale three-dimensional CFD codes that, for practical reasons, can not simulate the entire plant as a single integrated model. An approach that has gained wide acceptance is the network approach - models of standard components are developed that can be interconnected in any arbitrary way. This paper gives an overview of one of the most prominent codes that provide a suitable compromise, namely Flownex.

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A comprehensive reactor model for the integrated network simulation of the PBMR power plant

The theoretical basis and conceptual formulation of a comprehensive reactor model to simulate the thermal-fluid phenomena of the PBMR reactor core and core structures is given. Through a rigorous analysis the fundamental equations are recast in a form that is suitable for incorporation in a network code. This formulation of the equations results in a collection of one-dimensional elements (models) that can be used to construct a comprehensive multi-dimensional network model of the reactor.

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Start-up of the Three-shaft Closed Loop Brayton Cycle Model of the PBMR Power Plant

The PBMR is currently being developed by the South African utility ESKOM, as a new generation nuclear power plant. This so-called high temperature gas-cooled reactor plant is based on a three-shaft, closed-loop, recuperative, inter-cooled Brayton cycle with Helium as the working fluid. In order to demonstrate the envisaged PBMR control methodologies, a model of the plant was built. The conceptual design of the plant was done with the aid of Flownet, a thermal-fluid simulation software package that has the ability to simulate the steady-state and transient operation of the integrated system. This paper describes the differences and similarities between the so-called Pebble Bed Micro Model (PBMM) and the actual PBMR. It provides the layout and overall specifications of the plant and describes the approach followed in the design of the start-up system. It gives the results of the actual start-up event including a comparison between the measured and simulated conditions.

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Operation and simulation of a three-shaft, closed-loop, Brayton Cycle model of the PBMR power plant

The Pebble Bed Modular Reactor (PBMR) is currently being developed by the South African utility ESKOM, as a new generation nuclear power plant. This so-called high temperature gas-cooled reactor plant is based on a three-shaft, closed-loop, recuperative, intercooled Brayton cycle with Helium as the working fluid. In order to demonstrate the envisaged PBMR control methodologies, a model of the plant was built. The conceptual design of the plant was done with the aid of Flownet, a thermal-fluid simulation software package that has the ability to simulate the steady-state and transient operation of the integrated system. This paper describes the results of the various tests performed on the plant to evaluate the different control methodologies as well as preliminary comparisons between measured and simulated results.

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Thermal-Fluid Comparison of Three- and Single-Shaft Closed Loop Brayton Cycle Configurations for HTGR Power Conversion

With the resurgence of the high temperature gas-cooled reactor (HTGR) various closed loop Brayton cycle configurations are currently being investigated. The most notable being the PBMR, the GT-MHR and the GTHTR300. These systems are all in different phases of design and optimisation and the developers are advocating different performance parameters and arguments for and against specific cycle layouts. In order to further this debate, this paper presents a comparison of the three- and single-shaft versions of the pre- and inter-cooled, recuperative Brayton cycle configurations based on detailed steady-state and transient thermal fluid simulations. The results show that although cycle efficiency and specific power of the two configurations compare well at steady-state full power operation, the transient response shows important differences that will impact directly on the system design.

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