54 Microturbine Performance Improvements 541 Turbomachinery Performance

The efficiency of turbomachinery components such as radial compressor and turbine stages continues to improve with time as fluid dynamic designers employ new design methods and technologies. For example, the blading of a modern radial compressor stage is typically designed using a set of straight-line elements. These lines are arranged so as to create three-dimensional blade shapes called ruled surfaces. This approach can create fairly complex surfaces and, if designed correctly, produces relatively efficient stage designs. But new techniques are becoming available, including three-dimensional CFD analysis capability, which allow the designer to define more complex blades shapes (called sculpted surfaces). This offers opportunities for even greater stage performance.

Figure 5.14 illustrates the current state of the art in compressor performance using current design techniques. MTs use radial compressor stages that generally fall into the specific speed range of 0.7 to 1.0. As shown, stage efficiencies of 87 to 89% could potentially be achieved for certain types of compressors in this specific speed range. However, compressors typically used in MTs are more likely to only reach efficiency level percentages in the low 80s. Part of the reason for this lies in the relatively small size (typically a few inches in diameter) of the compressors used in MTs. Although state-of-the-art efficiencies might be possible with large diameter designs, several factors significantly limit the efficiency MT-scale compressors can reach. For example, Reynolds number effects impose aerodynamic limits, and clearance ratios are relatively large due to practical limits in the bearings. In addition, the tight cost constraints imposed on MT in order to meet market price targets dictate the use of high-volume/low-cost manufacturing techniques. For example, compressor impellers are cast rather than precision machined to reduce cost. However, cast parts exhibit larger tip clearances, looser dimensional tolerances, and other characteristics that significantly reduce compressor performance. Therefore, there are practical limits to improving turbomachinery performance. Only relatively small overall system gains can

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Non-Dimensional NASA SDecific SDeed. Nc

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FIGURE 5.14

Compressor efficiency improvements.

be achieved as long as the turbomachinery components are well matched to the operating requirements of the MT in the first place.

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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