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3.3 Extra dynamics A gas turbine consists of different components and when all components are put together some extra dynamics arise. The flow model of the T100 in figure 7 above gives a more detailed view of how the turbine works. Air is taken from the outside and flows around the generator into the compressor in axial direction. The air is compressed and leaves in radial direction. The high- pressure air (blue pipes) is fed through the recuperator where it is preheated with the exhaust gas. Now at a much higher temperature (pink pipes) it is mixed with the fuel gas in the combustion chamber and burned. The flue gas enters the turbine radially and leaves in an axial direction (red pipes). The flue gas exchanges heat with the colder air in the recuperator and then leaves the figure. The gas/water heat exchanger (not shown in the figure) is placed directly after the recuperator. In an ideal machine the turbine would be a purely algebraic component with no dynamics involved. Instead, when we include the turbine rotor and the turbine diffuser (the part directly after the turbine marked red in the figure above), we get some extra dynamics. The turbine rotor can store thermal energy and when the temperature of the exhaust gas decreases, due to a load change, the rotor acts as small thermal reservoir with a limited amount of energy. The rotor speed is very high, so that the energy stored in the rotor is easily transferred to the gas, thus acting as a filter on the temperature dynamics. Due to the geometric design, the turbine diffuser serves as an extra heat exchanger, although a very poor one. The extra heat exchanger has a considerably smaller effect than the recuperator, but along with the turbine rotor they have an interesting impact on the dynamics. The actual effect can be seen in the verification section. 3.4 Bypass The T100 CHP microturbine produces heat through the gas/water heat exchanger. The amount of heat produced is directly linked to the amount of electricity produced. In some cases the owner wants to produce less heat than electricity. If too much heat is transferred to the water, the water might boil and evaporate causing damage to the heat exchanger. Therefore the amount of heat transfer must be controlled in a way that does not interfere with the electricity production. The solution is a bypass system, where some or all exhaust gases are diverted around the gas/water heat exchanger in a similarly way as a valve works. Due to time and the low priority given to bypass operations, there is no complete verified model of the bypass function in this thesis. 4. Simulation Tools In this chapter I will describe the tools I have used in the modelling of the microturbine. There are different layers of tools. First there is the simulation language, the language the model is written in, e.g. Modelica. Then there are libraries e.g. ThermoFluid that contain complete submodels. And last there is a simulation program e.g. Dymola, that contains a graphical user interface, different solvers, plot functions and parameter settings. 4.1 Selecting the tools Thermodynamic models are often large and complex. They consist of volumes, flows but also electronic components. There exist numerous simulation languages and programs e.g. Fortran, Matlab/Simulink and Modelica/Dymola to just mention a few of them. To make things more complicated there are several different simulation programs used within Turbec AB for different purposes and applications. For static simulations, a program called Dynamic Systems Analyzer v2.0 (DSA) is used. DSA is a program developed at Volvo Aero Corporation AB. With version 2.0 only static simulations are possible. A very detailed model of the T100 with all components 15PDF Image | Modelling of Microturbine Systems
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