Map-Based and 1D Simulation of a Turbocharger Compressor in Surging Operation 2011-24-0126
One-dimensional (1D) models are commonly employed to study the performances of turbocharged engine. Manufacturers' provided steady turbomachinery maps are usually utilized, although they operate in unsteady conditions as a consequence of pressure pulses propagating into the intake and exhaust systems. This may lead to some inaccuracies in the engine-turbocharger matching calculations, which may be solved through the introduction of proper time-delays (virtual pipe corrections). These drawbacks, however, became more relevant when engine operates under low speed and high load conditions, or during a transient maneuver, because of possibilities of compressor surging. This phenomenon can't be opportunely described employing classical manufacturers' maps: first of all, the flow through the compressor is strongly unsteady in these conditions; moreover, the information about both the unstable and reverse flow regions of compressor map is unavailable, and a somewhat arbitrary map's extrapolation technique must be utilized.
In this paper two numerical procedures have been presented to predict automotive turbocharger compressor performance in surging operation. Firstly, a recently proposed approach is utilized to directly compute the stationary map of the compressor for direct and reverse flow conditions. The detailed 3D geometry of the compressor is reconstructed starting from a reduced set of geometrical data measured on the compressor wheel. Then, the 1D meanline evolution of blade angles, cross section and equivalent diameter are deduced from the 3D geometrical model. The steady 1D flow equations are finally solved along the meanline of stationary and rotating channels constituting the compressor device. Proper flow loss correlation holding for direct or reverse flow conditions are included in the model, which is applied to predict the extended steady maps of a turbocharger compressor. With reference to stable operating regions, the theoretically derived maps are compared to available manufacturer data and, after a limited loss coefficients tuning, a good agreement has been obtained.
The steady procedure, however, can be utilized under unsteady flow conditions, too. Unsteady terms in flow equations are now accounted for, and mass and energy storages are allowed to occur within each stationary and rotating channel. In this way, compressor operation can be described with a quasi-steady method employing the extended map, or in a fully unsteady approach, as well. In the quasi-steady case, a proper virtual pipe correction is introduced, too. The two methodologies are tested to analyze their potential in characterizing compressor operation under surging conditions. To this aim, a reference compressor is connected to a outlet duct and a plenum. Compressor speed and downstream circuit are managed in order to promote deep surging conditions. Results obtained through the applications of the two methods are discussed in terms of pressure and temperature waves, mass flow rate evolution at compressor inlet and outlet, and Surging loop frequency, as well.