3. COMPONENT DEVELOPMENT & PERFORMANCE ON TEST RIG
3.1 COMPRESSOR
The development goals for both compressor stages were:
・to reach full load pressure ratios of 4.2 for the TPS..D and 4.7 for the TPS..E by using an aluminum alloy with an acceptable lifetime
・to reach the highest possible specific volume flow rate combined with state of the art efficiencies for safe operation on typical propeller and constant speed operating lines
In order to reach these challenging targets the development of the compressor stage had to be supported by powerful design tools. The main dimensions of the compressors were determined based on a simplified one-dimensional analysis program. The final designs of the flow channels and blades were then optimized using both a quasi-three dimensional and a three-dimensional viscous aerodynamic computer code simultaneously. At the same time comprehensive three-dimensional finite element calculations were carried out to determine the stresses throughout the entire impeller. All designs were then tested and optimized on a special compressor test rig. The resulting impellers feature design principles such as backward curved main and splitter blades manufactured on modern five axis high speed milling machines.
Fig.4 shows a typical compressor map of the TPS57E compared with a VTR251 compressor map for an identical operating line.
The progress made in the achievable compressor pressure ratio with the TPS57E can be clearly seen along the plotted engine operating line.
3.2 TURBINE
The increase of compressor pressure ratio, an expected high level of turbocharger efficiency and the necessity to use the turbocharger in combination with pulse supercharging place very challenging demands on the turbine with regard to expansion ratio, efficiency, volume flow rate and mechanical reliability. In order to consider these boundary conditions correctly, a new one-dimensional computer code for radial und mixed flow turbines was developed to evaluate the main dimensions of the new turbine stage.
This code included loss correlations for all turbine components and an optimization procedure. As for the compressor, a quasi-three-dimensional aerodynamic computer code was used to determine the flow channels and the blading. To evaluate the stress levels and the eigenfrequencies the entire turbine wheel was modelled using three-dimensional finite elements. To satisfy all the requirements in an optimal way a mixed flow design was chosen for the turbine. The overall design consists of a turbine inlet volute, a nozzle ring, a turbine impeller and a short diffuser as illustrated in Fig.2. The resulting efficiency level of this design, compared with the VTR251 turbine design, is shown in Fig.5.
Fig.5: Turbine efficiency vs. turbine expansion ratio for a TPS 57 D/E and a VTR251 in relation to nozzle ring area
The efficiencies are valid for the largest turbine trim and two different nozzle ring specifications. The small nozzle ring specification is suitable for an engine which runs at constant speed and load - the more efficient the match point the better the performance. The large nozzle ring specification allows applications for variable loads, e.g. for a variable speed marine propulsion engine which needs an acceptable part load performance. The TPS..D/E turbine design shows an improved efficiency level, is well suited for matching different engine applications and has the required expansion ratio capability.
