| The flow instabilities represented by rotating stall and surge are the biggest menaces for aeroturbo engine running safety. When flow instabilities occur, it will not only degrade the engineperformance (power, s.f.c, etc.). More seriously, it might cause an abrupt flameout, or intensevibration of the compressor blades possibly inducing blades rupture and destroy the whole engine.Therefore, it is necessary to study more on the following two aspects. First, the instabilities controltechnology, i.e. exploring the most effective and shortcut engineering way to capture the instablesignal well and truly, sequentially, to control the engine getting rid of the instable operation conditionrapidly. Secondly, the stabilities extending technology, i.e. searching for effective method to controlinstabilities, not only to control the engine running away from the instable condition, but also toenlarge the surge margin, to have more clearance of the engine instable operation line.Hence, by analysis and absorbing the thought and methods in this field domestic and abroad,based on the relevant researches on turboshaft engine, some investigations have been carried on flowinstabilities online predicting and stabilities enhancement technology for turboshaft engine. In thispaper, the following aspects are mainly studied:Research on the flow unsteady characteristics. Based on the analysis of pressure signals obtainedfrom the sensors located on the casing of compressors using time domain, wavelet, andauto-correlation methods, the characteristic and its developing process of the flow instable signal hasbeen abtained. With the found parameter criterion representing the stall foreboding, an on line stallpredicting method based on the auto-correlation has been promoted. Several exciting rerults havebeen revealed by detail analysis in this paper. Stall incepted by spikes can only be detected by timedomain analysis before1～2rotor revolutions, while the spots in wavelet can be found before tens tohundreds revolutions. The amplitude of auto-correlation coefficient of the wall pressure drops rapidlywhen the compressor is close to the stall point (about7%～9%compared to the lager flow rateworking condition), which is consistent with the wavelet spots appearing at the16times frequency ofthe rotor revolution. Both are all caused by the breakage of the compressor casing pressure sawtoothperiodicity when the flow encounters instability.Flow instabilities prediction technology. Grounded on the flow unsteady characteristics of thepressure signal from the rotor tip casing, an instable signal estimation criterion by monitoring thethreshold of auto-correlation declining (exampled as0.87) while it reaches at certain probability (setas30%) has been established. Based on the truly stall precursor detection and modern DSP technology,a set of online instabilities prediction system has been developed and demonstrated on a highsub-sound velocity axial compressor test rig, whose results showed that the stall precursor alarming before stall175rotor revolution (about1.28s) earlier, which brings the pre-warning time muchforward compared with the conventional prediction method.Stabilities enhancement technology: Blade tip injection experiment research on a specific axialcompressor whose front stages tending to unsteady at low speed has been implemented. Resultsshowed that the compressor stability has been improved (7.1%with0.256%mass flow rate injection)and the efficiency has been increased (about1%) correspondingly. Application suggestions have beenproposed, i.e, blow injecting along0°axial line, with the nozzle installed close to the rotor front tip.Both the wavelet and auto-correlation analysis results disclose, for the first time, that the reason ofenhancing stabilities in axial compressor is that the end-wall pressure micro turbulence can besuppressed by jet injection, hence the pressure periodicity can be improved efficiently and the flowstability can be broadened.Demonstration of flow control stabilities enhancement has been applied on the centrifugaldiffuser of a combined compressor, in which the jet holes are drilled on the casing where the instableflow arises more likely. With certain amount of mass flow rate injection (1.5%,2.0%,2.5%of theprime flow mass), the inlet flow field has been improved effectively, featured by the surge abundantenhanced by (0.6～2.3)%, while the efficiency raised by (0.4～2.5)%of the combined compressor.Surge monitoring and control device development. The fluctuant magnitude (40～85)%andfrequency (0.5～10)Hz of the turboshaft engine compressors outlet pressure have been found bystatistically analysis of the tests results under different rotation speeds. Taking the threshold value ofcompressor surge fluctuating pressure, a specific novel surge judging device is designed. When surgeoccurs, a step signal feeds out by the surge judging circuit, to startup the digital acquisition and recordsystem, on the mean time, sound and light alarm, compressor retreats from the surge synchronously.In this way, the purpose of measure the surge line correctly and eliminating surge rapidly are allachieved. This online surge monitoring and control device has been applied on the compressor andturoshaft engine test rigs, where fast response (less than100ms) and high alarm accuracy (better than59%) have been demonstrated.All of the above research results in this dissertation lay a solid foundation and a technical supportfor the design and development of flow stability online management for China next generationturboshaft engine. |
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