Unsteady 1D Simulation of a Turbocharger Compressor 2009-01-0308
The one-dimensional (1D) modeling of a turbocharged engine requires the availability of the turbine and compressor characteristic maps. This leads to two main problems:
performance maps of the turbocharger device are usually limited to a reduced number of rotational speeds, pressure ratios and mass flow rates. Extrapolation of maps’ data is commonly required;
performance maps are experimentally derived on stationary test benches, while the turbocharger usually operates under unsteady conditions, when coupled to an internal combustion engine (ICE).
To overcome the above problems, in the present paper the flow inside a rotating pipe of a centrifugal compressor is simulated within a 1D modeling approach, with the aim of predicting its characteristic map. The main improvement with respect to the employment of a steady experimental map consists in the absence of data extrapolation and in the possibility of fully characterizing the unsteady operation of the component. In this way it is also possible to handle on a physical point of view surge phenomena (backflows) which may arise particularly at low engine speed and high load.
To this aim, the actual evolution of the blade-to-blade duct profile is specified through the assignment of proper data which define the duct orientation in the space along its curvilinear abscissa. Flow equations are next generalized to include additional terms arising as a consequence of pipe rotation. The whole compressor is schematized as a number of rotating channels in parallel, linked at the impeller inlet and outlet by a constant pressure boundary condition. The velocity triangles are employed to handle the transition between the absolute and the relative motion occurring at the impeller inlet and outlet. The presence of a vaneless diffuser is also considered at the exit section of the rotor blades. The procedure includes some correlations to keep into account slip effects, incidence and distributed losses. The above methodology is utilized to directly compute the stationary map of the device, starting from the specification of its geometry and rotating speed. A comparison with experimental data shows a good agreement with the predicted performance curves. The same procedure can be easily extended to the simulation of a radial turbine, too.