Shaojuan Geng

A Summary of Stall Warning and Suppression Research With Micro Tip Injection

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.Copyright

Numerical Study on the Response of Tip Leakage Flow Unsteadiness to Micro Tip Injection in a Low-Speed Isolated Compressor Rotor

A numerical study on the unsteady tip leakage flow with discrete micro tip injection from casing shroud in a low-speed isolated axial compressor rotor is presented. The main target is to clarify the flow mechanism of how the stall control measures act on the tip leakage flow typified by its self-induced unsteady flow characteristics. At operating condition near stall point, a series of calculations have been carried out for different axial position of injector and different injected mass flow rate. The computation results of flow field near rotor tip region show that under the influence of injected flow, the transient pressure distribution fluctuates along blade chord on both pressure and suction sides with respect to the relative position of injector and rotor. The pressure difference across the pressure and suction sides of compressor blade changes correspondingly, thus introduces a forced flow unsteadiness interacting with the unsteady tip leakage flow. When the injection is relatively strong and able to meet the tip leakage flow at its origination, the self-induced unsteadiness of tip leakage flow can be suppressed completely. In most cases, both frequency components of the self-induced unsteadiness and forced-induced unsteadiness are co-existing. The corresponding transient flow contours show that a local high pressure spot appears near blade pressure side, which moves downstream and shifts the tip leakage flow trajectory with less or without touching the neighboring pressure surface of the blade. Based on this understanding of discrete tip injection as force-induced flow unsteadiness, the numerical results are also analyzed to optimize the effect of injection in changing the route of tip leakage flow trajectories and therefore the chance of stability improvement of the compressor rotor.Copyright

Decoupled scheme based on the Hermite expansion to construct lattice Boltzmann models for the compressible Navier-Stokes equations with arbitrary specific heat ratio

A decoupled scheme based on the Hermite expansion to construct lattice Boltzmann models for the compressible Navier-Stokes equations with arbitrary specific heat ratio is proposed. The local equilibrium distribution function including the rotational velocity of particle is decoupled into two parts, i.e., the local equilibrium distribution function of the translational velocity of particle and that of the rotational velocity of particle. From these two local equilibrium functions, two lattice Boltzmann models are derived via the Hermite expansion, namely one is in relation to the translational velocity and the other is connected with the rotational velocity. Accordingly, the distribution function is also decoupled. After this, the evolution equation is decoupled into the evolution equation of the translational velocity and that of the rotational velocity. The two evolution equations evolve separately. The lattice Boltzmann models used in the scheme proposed by this work are constructed via the Hermite expansion, so it is easy to construct new schemes of higher-order accuracy. To validate the proposed scheme, a one-dimensional shock tube simulation is performed. The numerical results agree with the analytical solutions very well.

Evolution of Pressure Signature Dominated by Unsteady Tip Leakage Flow

This paper deals with the detailed numerical analyses of diverse manifestations of unsteady features induced by periodical oscillation of tip leakage flow under different operating points for the cases with uniform and hub distorted inlet conditions. The characteristics evolutions of pressure signature near rotor tip region during compressor throttling process are studied and compared with the experimental results. Monitors circumferentially arranged and aligned with blade chord are imposed to collect static pressure signals. Analysis methods of coordinate transformation between the rotor relative and absolute stationary reference frames, fast Fourier transform and frequency band pass filter are used. The modulated frequency features along blade chord in two reference frames are analyzed. Typically for the dominated frequency components, the circumferential propagation characteristics are studied, such as propagation speed and mode orders. And the unified evolution trends of modulated frequency relation for dominant components between two reference frames and circumferential propagation features during throttling process are summarized. A critical mass flow point is found to distinguish the different change trend of the characteristics of tip leakage flow unsteadiness. Based on the different speeds between circumferential propagation of tip leakage flow unsteadiness and revolution of compressor rotor, the fluid dynamic reason for the decrease of autocorrelation coefficient of pressure signals from transducer mounted on compressor casing is elucidated. All the results are helpful to further unveil the initiation mechanism of stall inception.Copyright