Ejector Tip Injection for Active Compressor Stabilization

Abstract

In propulsion industry there is an ongoing need to significantly reduce SFC of jet engines resulting in cost reduction and lower emissions. Since the design of most of the engine components is at the limit of today’s technology level further gain of improvement on short term is to be achieved by implementation of new system concepts. Especially the stall safety margin in compression system design holds high potential for the optimization of the overall engine system. Once a reliable and effective stall control system becomes available an extension of present operating range is likely to be achieved by moving the steady operating line towards the stability limit and to intervene only in critical situations. At the Institute of Jet Propulsion at the University of the Federal Armed Forces in Munich, Germany a Larzac 04 twin-spool turbofan engine has already been equipped and tested with an adequate active stabilization system of the low pressure compressor for research purposes. Those investigations revealed a strong dependency of the achievable stabilization effect and the amount and momentum of the injected air mass flow. For flying applications this mass flow has to be delivered by carried on means. Therefore it always penalizes the propulsion efficiency. In the given configuration, redirected air from the last stage of the high pressure compressor is used for injection. Usage of this bleed air directly influences the propulsion efficiency of the engine. In order to optimize the mass flow needed for stabilization, the existing injection system was redesigned to utilize ejector pumps. With this configuration a comparable stabilizing effect could be realized with less redirected air mass flow. In fact the open ejector pump configuration showed an even higher performance at maximum injection rate than the closed injection before. Therefore further investigations with this system focused on the effect of additional flow ports to the engine intake as they are necessary for an ejector pump and their basic influence on the operation stability of the low pressure compressor (LPC). In combination with the already existing stall detection algorithm of the institute a very promising system for increasing the available operating range in turbo compressors could be achieved.

Ejector tip injection system for active aerodynamic compressor stabilization part I: design and experiment

“…In the conventional tip injection designs, the pressurized HPC or shop air is injected directly into the tip region of the compressor. The studies by Kefalakis and Papailiou (2006), Bindl et al (2009a) and Stößel et al (2014) showed that a higher injection mass flow rate is advantageous for the tip injection. Therefore, ejector tip injection utilizes the pressurized air as primary mass flow, which is is injected into an ejector geometry inside the EIS.…”

“…This system only activates the air injection, if a stall event is directly imminent, which helps to increase the overall efficiency of the jet engine. In order to increase the efficiency of the tip injection, Stößel et al (2014) extended the existing injection system with an ejector. With this modification, a surge margin increase between 5 % and 60 % (depending on injection mass flow and operating point) was observed.…”

“…The application of the ejector effect can increase both effectivity and efficiency of tip air injection (cf. Stößel et al (2014)). In order to maximize these gains and to prove the substantial benefits expected from ejector tip injection, a totally new ejector injection system (EIS) has been designed from scratch.…”

Ejector tip injection system for active aerodynamic compressor stabilization part II: CFD investigations

“…Niehuis et al (2012)). Stößel et al (2014) have proven experimentally that deploying the ejector effect in the previously described manner can significantly improve impact and efficiency of the entire air injection concept. Based on these findings, a further advanced Ejector Injection System (EIS) has been developed by Kern et al (2017) and is currently under investigation.…”

Active flow control in an axial compressor for stability improvement: on the effect of flow control on stall inception

“…In parallel Active control technics, and more precisely discrete tip injection, have shown their effectiveness on the improvement of the compressor stability by moving away the surge line (Suder et al 2001;Nie et al 2002;Kefalakis and Papailiou 2007;Li et al 2014). Some successful applications have also been demonstrated on real aircraft engines (Stößel et al 2013(Stößel et al , 2014. Understanding their effects is thus of first interest but surprisingly, few works have investigated the mechanisms involved when active flow control is applied and how it affects the flow patterns in the tip gap while extensive work in this domain has been done with casing treatments (Takata and Tsukuda 1977;Smith and Cumpsty 1983;Fujita and Takata 1984;Cumpsty 1989).…”