Development of a One-Dimensional Dynamic Gas Turbine Secondary Air System Model—Part II: Assembly and Validation of a Complete Network

In the first part of this paper the equations and results for the transient models developed for the SAS components in isolation have been thoroughly explained together with the assumptions made and the limitations that arose subsequently. This second part explains the work carried out to couple the individual components into a single network with the aim of assembling a dynamic model for the whole engine air system. To the authors’ knowledge the models published hitherto are only valid for steady or quasi steady state. It is then the case that the differential equations that govern the fluid movement are not time discretised and thus can be solved in a relatively straightforward fashion. Unlike during transients, the flow is not supposed to reach sonic conditions anywhere within the network and most important, flow reversal cannot be accounted for. This study deals with the mathematical apparatus utilised and the difficulties found to integrate the single components into a network to predict the transient operation of the air system. The flow regime — subsonic or supersonic — and its direction have deemed the choice of the appropriate numerical and physical boundary conditions at the components’ interface for each time step particularly important. The integration is successfully validated against a known numerical benchmark — the De Haller test. A parametric analysis is then carried out to assess the effect of the length of the pipes that connect the system cavities on the pressure evolution in a downstream reservoir. Transient flow through connecting pipes is dependent on the fluid inertia and so it takes a certain time for the information to be transported from one end of the duct to the other. As it would be expected, the system with a longer pipe is found to have a longer settling time. Finally, the work concludes with the analysis of the flow evolution in the secondary air system during a shaft failure event. This work is intended to continue to address the limitations imposed by some of the assumptions made for an extended and more accurate applicability of the tool.