Abstract:
This research theme is originated from a development project that is going on in
South Africa, for the design and construction of a closed cycle gas turbine plant using
gas-cooled reactor as the heat source to generate 115 MW of electricity. South African
Power utility company, Eskom, promotes this developmental work through its
subsidiary called PBMR (Pebble Bed Modular Reactor). Some of the attractive features
of this plant are the inherent and passive safety features, modular geometry, small
evacuation area, small infrastructure requirements for the installation and running of the
plant, small construction time, quick starting and stopping and also low operational
cost.
This exercise is looking at the operational aspects of a closed cycle gas turbine,
the finding of which will have a direct input towards the successful development and
commissioning of the plant. A thorough understanding of the fluid dynamics in this
three-shaft system and its transient performance analysis were the two main objectives
of this research work. A computer programme called GTSI, developed by a previous
Cranfield University research student, has been used in this as a base programme for the
performance analysis. Some modifications were done on this programme to improve its
control abilities. The areas covered in the performance analysis are Start-up, Shutdown
and Load ramping. A detailed literature survey has been conducted to learn from the
helium Turbo machinery experiences, though it is very limited. A critical analysis on
the design philosophy of the PBMR is also carried out as part of this research work.
The performance analysis has shown the advantage, disadvantage and impact of
various power modulation methods suggested for the PBMR. It has tracked the effect of
the operations of the various valves included in the PBMR design. The start-up using a
hot gas injection has been analysed in detail and a successful start region has been
mapped. A start-up procedure is also written based on this. The analysis on the normal
and emergency load rejection using various power modulation devices has been done
and it stress the importance of more control facilities during full load rejection due to
generator faults.
A computational fluid dynamics (CFD) analysis, using commercial software, has
been carried out on some geometry of the PBMR design to find out whether its flow
characteristic will have any serious impact on the performance on the cycle during the
load control of the plant. The analysis has demonstrated that there will not be much
impact on the performance, during load control using pressure level changes, from this
geometry. However, some locations in the geometry have been identified as areas where
the flow is experiencing comparatively high pressure losses. Recommendations, which
include modification in the physical design, were made to improve this.
The CFD analysis has extended to a cascade to compare the flow behaviour of
Air and Helium with an objective of using air, being inexpensive, to test the helium
flow characteristic in a test rig to simulate the behavioural pattern of helium in the
PBMR pressure vessel. The specification of a hypothetical test rig and the necessary
scaling parameters has been derived from this exercise. This will be useful for designing
test rigs during the developmental and operational stage of the PBMR project.