Relatively small stable flow ranges experienced on some high Mach number centrifugal compressors, triggered by the inability of stationary vaned diffusers to operate at flows below their maximum recovery point, have focused considerable research effort on the diffuser component. With improved impeller stability, vaned diffusers can be assisted to operate to lower flows where impeller stalling becomes predominant. The results of tests on a small high Mach number centrifugal compressor are presented to suggest that in certain applications the primary factor affecting compressor surge could be the impeller inlet-to-exit velocity diffusion ratio, with specific diffuser types, Mach numbers, and incidence considerations becoming secondary in importance.
This paper describes the results of compressor rig testing with a moderately high specific speed, high inducer Mack number, single-stage centrifugal compressor, with a vaned diffuser, and adjustable inlet guide vanes (IGVs). The results showed that the high-speed surge margin was considerably extended by the regulation of the IGVs, even though the vaned diffuser was apparently operating stalled. Simplified one-dimensional analysis of the impeller and diffuser performances indicated that at inducer tip Mach numbers approaching and exceeding unity, the high-speed surge line was triggered by inducer stall. Also, IGV regulation increased impeller stability. This permitted the diffuser to operate stalled, providing the net compression system stability remained on a negative slope.
PurposeThis paper seeks to evaluate the potential of heat exchanged aeroengines for future Unmanned Aerial Vehicle (UAV), helicopter, and aircraft propulsion, with emphasis placed on reduced emissions, lower fuel burn, and less noise.Design/methodology/approachAeroengine performance analyses were carried out covering a wide range of parameters for more complex thermodynamic cycles. This led to the identification of major component features and the establishing of preconceptual aeroengine layout concepts for various types of recuperated and ICR variants.FindingsNovel aeroengine architectures were identified for heat exchanged turboshaft, turboprop, and turbofan variants covering a wide range of applications. While conceptual in nature, the results of the analyses and design studies generally concluded that heat exchanged engines represent a viable solution to meet demanding defence and commercial aeropropulsion needs in the 2015‐2020 timeframe, but they would require extensive development.Research limitations/implicationsAs highlighted in Parts I and II, early development work was focused on the use of recuperation, but this is only practical with compressor pressure ratios up to about 10. For today's aeroengines with pressure ratios up to about 50, improvement in SFC can only be realised by incorporating intercooling and recuperation. The new aeroengine concepts presented are clearly in an embryonic stage, but these should enable gas turbine and heat exchanger specialists to advance the technology by conducting more in‐depth analytical and design studies to establish higher efficiency and “greener” gas turbine aviation propulsion systems.Originality/valueIt is recognised that meeting future environmental and economic requirements will have a profound effect on aeroengine design and operation, and near‐term efforts will be focused on improving conventional simple‐cycle engines. This paper has addressed the longer‐term potential of heat exchanged aeroengines and has discussed novel design concepts. A deployment strategy, aimed at gaining confidence with emphasis placed on assuring engine reliability, has been suggested, with the initial development and flight worthiness test of a small recuperated turboprop engine for UAVs, followed by a larger recuperated turboshaft engine for a military helicopter, and then advancement to a larger and far more complex ICR turbofan engine.
Test results pertaining to the stalling characteristics of centrifugal compressor impellers with parallel wall vaneless diffusers are presented and studied to correlate the coincidence of stall with a limiting impeller diffusion capability. It is suggested that a modified diffusion factor, to include the effects of meridional curvature, provides improved stall correlation for a wide specific speed range of backswept impeller types. The possibility of applying this diffusion factor to high loading radially bladed impellers is discussed as dependent upon blockage and windage plus recirculation effects. Use of the diffusion factor limit in the preliminary design of most common turbomachinery types, incompressible and compressible, to assess impeller (or rotor) stall is conceivable.
Major design considerations involved in selecting geometries for high-pressure ratio, single-stage centrifugal compressors are discussed. The results of a parametric study are included to indicate optimum regions of design, and to project the potential performance of centrifugal compressors with rotating diffusers. Several different candidate design solutions are evaluated on the basis of providing improved performance at high pressure ratios consistent with maintaining the inherent simplicity and low-cost features of centrifugal compressors.
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