The traditional method to draw automotive-turbocharger compressor maps is based on equivalent chocking conditions. In this way, theoretically, the compressor chocking conditions would be exactly evaluated with the new ambient (inlet) conditions. Unfortunately, chocking is not the worst working condition for turbochargers in piston engine applications. In addition, most available compressor maps are interpolated from very few CFD (Computational Fluid Dynamics) or experimental data. The result is that many designers convert the map into a “row” one with the volumetric max flow on the x-axis. However, even this approach has many limitations, since compressor performance depends on Mach and Re numbers. To clarify the concept, a simplified CFD method to draw the compressor map is introduced in this paper. An example, based on a true turbocharger, shows the limitations of most interpolated-maps that can be found in literature. This initial “raw” map has a volumetric flow rate in input (x-axis) and an absolute pressure ratio in output (yaxis). The islands of constant efficiency are then calculated by assuming that the diffuser has a unitary efficiency. Then a new method based on invariants is introduced to calculate the new map with different input ambient conditions [1]. It is based on the ambient sound speed. This method is then corrected in this paper by introducing more accurate values for density and Mach speed. In particular the correction due to the air moisture content is particularly critical. The new invariant map obtained in this way takes into account of variations in inlet air for Re and Mach numbers. The method is valid for automotive and aerospace applications up to 3,000m. Unfortunately, for higher altitudes, even this new method shows its limitations, with the necessity to recalculate the maps with CFD simulation. In fact, rarified air and lower inlet temperature reduce compressor performance in term of efficiency and compression ratio. On the contrary, turbines tend to transfer more power to the shaft. In this way, the compressor to turbine match is far from ideal.
OFF-DESIGN PERFORMANCE OF AUTOMOTIVE DERIVED CENTRIFUGAL COMPRESSORS
Piancastelli L.
;Cassani S.
2020
Abstract
The traditional method to draw automotive-turbocharger compressor maps is based on equivalent chocking conditions. In this way, theoretically, the compressor chocking conditions would be exactly evaluated with the new ambient (inlet) conditions. Unfortunately, chocking is not the worst working condition for turbochargers in piston engine applications. In addition, most available compressor maps are interpolated from very few CFD (Computational Fluid Dynamics) or experimental data. The result is that many designers convert the map into a “row” one with the volumetric max flow on the x-axis. However, even this approach has many limitations, since compressor performance depends on Mach and Re numbers. To clarify the concept, a simplified CFD method to draw the compressor map is introduced in this paper. An example, based on a true turbocharger, shows the limitations of most interpolated-maps that can be found in literature. This initial “raw” map has a volumetric flow rate in input (x-axis) and an absolute pressure ratio in output (yaxis). The islands of constant efficiency are then calculated by assuming that the diffuser has a unitary efficiency. Then a new method based on invariants is introduced to calculate the new map with different input ambient conditions [1]. It is based on the ambient sound speed. This method is then corrected in this paper by introducing more accurate values for density and Mach speed. In particular the correction due to the air moisture content is particularly critical. The new invariant map obtained in this way takes into account of variations in inlet air for Re and Mach numbers. The method is valid for automotive and aerospace applications up to 3,000m. Unfortunately, for higher altitudes, even this new method shows its limitations, with the necessity to recalculate the maps with CFD simulation. In fact, rarified air and lower inlet temperature reduce compressor performance in term of efficiency and compression ratio. On the contrary, turbines tend to transfer more power to the shaft. In this way, the compressor to turbine match is far from ideal.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
