Expanding the choke margin of a mixed flow compressor stage for a micro gas turbine engine

Abstract

In an attempt to reduce engine frontal area, while maintaining a high single stage pressure ratio, mixed flow compressor stages are frequently used in micro gas turbine (MGT) engines. The expansion of the choke margin of such a mixed flow compressor is presented. The use of a crossover diffuser configuration in a mixed flow compressor stage has displayed superior performance results compared to legacy diffuser configurations, especially when geometric restrictions are enforced. A disadvantage of a crossover diffuser configuration is that it typically displays an inferior operating range compared to legacy diffuser configurations. In an attempt to expand the choke margin of a MGT mixed flow compressor, the use of tandem and splitter vane crossover diffuser configurations was evaluated. It was found that a low solidity first vane row configuration provided a 3% increase in choke margin. A splitter vane crossover diffuser configuration provided a 5.9% increase in choke margin. A tandem vaned diffuser with a reduced first row vane number provided a 7.8% increase in choke margin.

Upgrading the Compressor Stage of the CAT250TJ Micro Gas Turbine Engine

“…To expand the operating range of the upgraded CAT250TJ_BL compressor, selected operating range expansion methods discussed in Van Eck [15] are employed. These are discussed below.…”

“…3) allows the boundary layer to be re-energised, better controlling the boundary layer at various operating points and therefore the overall operating range. Van Eck [15] numerically investigated various tandem vane crossover diffuser configurations and their effect on compressor design point performance and operating range. It was shown that a low solidity front vane configuration displayed a marked increase in choke margin.…”

“…A 70-75 configuration provided superior exit tangential velocity results. The designation 70-75 refers to a tandem vane diffuser having a front vane length 70% relative to the total diffuser vaned [12] length and a second vane relative tangential shift of 75% [15].…”

“…Van Eck et al [16] evaluated methods to increase the choke margin of a mixed flow compressor fitted with a crossover diffuser. Various splitter vaned, tandem vaned and combinations of these were presented and these provided a marked increase in choke margin.…”

“…It is important to note that the current study forms part of a larger PhD study [15] evaluating the effect of various crossover diffuser configurations on the performance of the compressor stage of an MGT engine. The current paper is in fact the culmination of this study, where the findings of the larger study is tested on a real-world application, i.e.…”