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very thin so the main obstacle for the heat flow will be the convection from wall to fluid and not from one edge of the wall to the other edge. This means that the middle term of equation (5.5.7) can be neglected. This does not mean however that the wall is of no importance. The mass of the wall is very important for dynamic reasons, since heat will be stored in the wall. As a summary, we can see that important factors for the efficiency of a heat exchanger are the surface area A, the convection heat transfer coefficient α, the mass flow and the specific heat capacity of the mediums. 5.6 Pipes and pressure loss models All pipes in the model consist of a control volume and a flow model. The flow characteristics such as pressure drop are included in the flow model. There are many variables concerning the pressure loss for a simple pipe such as friction, bends, inlets, outlets and height differences. It is a task beyond the scope of this thesis to do a detailed pressure loss model of a complicated machine like a gas turbine. In Gustafsson (1998), detailed pressure loss equations for a recuperator are derived. In the end Gustafsson needed to do manual adjustments of the temperature measurements to calibrate the model to experiment data. In the equations there are also some theoretical values that need to be estimated. Another source, Thomas (1999), was chosen to get pressure loss equations for a normal pipe. (5.6.1) where C is a constant factor due to friction, bends and inlets/outlets. The constant C can be measured or estimated in a certain design point. Then we can choose a standard pressure loss model from the ThermoFluid library and modify it to deal with large thermal changes. (5.6.2) where dp0, mdot0 and ρ0 are constant pressure loss, mass flow and density at a known design point and together they form the constant factor C. This is an easy-to-use formula that gives reasonable results. Since my thesis emphasizes the model of the entire system, there is no time to create and tune a more detailed pressure loss as in Gustafsson (1998). 5.7 The heat exchanger model The two heat exchangers in the T100 microturbine, the recuperator and the gas/water heat exchanger are both modelled in the same way, except for the medium models used. In the ThermoFluid library, there are complete pipe models with heat conduction and with air or flue gas as medium. The pipe model inherits code for each important property, as e.g. pressure drop, geometry and medium, see figure 12. The heat exchangers are then modelled as two parallel pipes with a wall in between. There is no difference in the model for counterflow or parallel flow, since the direction of the flow is decided by the pressure, which is originally set by the user. The wall is modelled as a simple dynamic wall with a homogenous temperature throughout the wall. It inherits code from classes as e.g. geometry and heat conduction. The pipes and the wall are then connected with heat connectors so that the heat equations of each component are linked together. m& 2 dp = ρ C d p = m& 2 ρ 0 d p m&02 ρ 0 36PDF Image | Modelling of Microturbine Systems
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