Micro tip injection is a stall control technique in which engineers inject small-flowrate yet high-velocity jets into the tip region of axial compressor rotor blades to extend the stable operation range, while keeping the entire compressor characteristic line nearly unchanged. This paper summarizes the related research performed in Chinese Academy of Sciences in the past several years, ranging from understanding the fundamental mechanisms to demonstrating the technology in a laboratory compressor. A brief review in tip injection research indicates that for each compressor, there exists a critical injection momentum ratio, below which the injection flows may work differently from those over the critical value. Detailed casing unsteady static pressure measurements and the related sophisticated data processing proved the existence of self-induced unsteady tip leakage flow, which was also confirmed by unsteady CFD simulations. It is showed that this kind of unsteadiness in tip leakage flow might be responsible to make the auto-correlation coefficient drop significantly as the compressor operated at lower flowrate close to stall limit. This warning signal, which was first successfully used by other research groups with little care taken on its flow mechanism, appears much earlier than the classical modal-wave or spike stall precursors. Micro tip injection was effective if it interacts with the unsteady tip leakage flow long before the stall precursor emerges. As a technology demonstration, a DSP board with a build-in auto-correlation early-stall-warning algorithm and an on-off controller of injection valve worked well to drive a micro tip injection system and successfully delayed the stall in a low-speed compressor with much less jet flows than the continuous micro tip injection.

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