CHARACTERISTICS OF FAN STALLING BASED ON CORRELATED DIMENSIONS

Different from the previous qualitative analysis of linear systems in time and frequency domains, the method for describing nonlinear systems quantitatively is proposed based on correlated dimensions. Nonlinear dynamics theory is used to analyze the pressure data of a contrarotating axial flow fan. The delay time is 18 and the embedded dimension varies from 1 to 25 through phase-space reconstruction. In addition, the correlated dimensions are calculated before and after stalling. The results show that the correlated dimensions drop from 1. 428 before stalling to 1. 198 after stalling, so they are sensitive to the stalling signal of the fan and can be used as a characteristic quantity for the judging of the fan stalling.

Coupling Effect between Inlet Distortion Vortex and Fan

In the process of inlet/engine matching,large-scale distortion vortexes would be generated due to lip separation,curved duct,shock-wave boundary layer interaction and other factors of inlet,resulting in complex combination distortion of total pressure and swirl,which would affect the stable and efficient operation of fans/compressors of aero-engine.On the basis of main distortion vortex characteristics of a typical S-shape inlet and its theoretical model of velocity and pressure distribution,this paper establishes an inlet boundary condition definition method to generate three-dimensional distortion vortex,and presents a study of coupling effect between steady distortion vortex and NASA Rotor 67.The results show that the fan’s performance curve would move to smaller mass flow direction under the influence of the co-rotating distortion vortex,while fan’s operation curve would change to the opposite direction if the inlet distortion vortex direction is opposite to the fan’s rotating direction.Compared with co-rotating distortion vortex,the efficiency loss caused by counter-rotating distortion vortex would be larger and total pressure ratio loss would be smaller.The vortex core size has significant influence on the stability margin of the fan,but has little influence on the efficiency and total pressure ratio characteristic.As the vortex core size increases,the range of fan’s tip region with high attack angle would become larger and cause the fan to stall in advance.Due to the non-uniform suction of the downstream fan,the inlet distortion vortex accelerates to mix with the surrounding main flow.The total pressure distortion decreases and swirl distortion increases along sections from inlet boundary to the fan leading edge.

Experimental investigation on SPS casing treatment with bias flow

Generally, casing treatment（CT） is a passivity method to enhance the stall margin of fan/compressor. A novel casing treatment based on the small disturbance theory and vortex and wave interaction suggestion is a method combining passive control and active control, which has been proved effective at enhancing the stall margin of fan/compressor in experiment. In order to investigate the mechanism of this kind of casing treatment, an experimental investigation of a stall precursor-suppressed（SPS） casing treatment with air suction or blowing air is conducted in the present paper. The SPS casing treatment is designed to suppressing stall precursors to realize stall margin enhancement in turbomachinery. The experimental results show that the casing treatment with blowing air of small quantity can improve the stall margin by about 8% with about 1% efficiency loss. By contrast, the SPS casing treatment with micro-bias flow does not improve the stall margin much more than that without bias flow, even worse. Meanwhile, the present investigation has also attempted to reveal the mechanism of stall margin improvement with the casing treatment.It is found that the stall margin improvements vary with the modification of the unsteady shedding flow and the unsteady wall boundary impedance. The experimental results agree fairly well with the theoretical prediction using a flow stability model of rotating stall.