Experimental Study On Unsteady Performance And Flow Field Of Pulsating Backpressure Centrifugal Compressor

Centrifugal compressors are widely used in vehicle and aircraft power,chemical industry,energy transport and transformation.It is very important to improve the performance of centrifugal compressors.In related applications,when combined with reciprocating machinery or coupled with detonation combustion mode,the compressor is in a strong pulsating backpressure environment,and its performance and flow field will be significantly different from that under constant backpressure condition.It is of great significance to improve the performance of the power plant and realize energy saving and emission reduction to carry out the test and theoretical research on the centrifugal compressor under pulsating backpressure.The inlet flow field of a compressor is the key to influence downstream flow structure and aerodynamic stability.In this paper,the performance and internal flow field of centrifugal compressors under pulsating backpressure are measured.The main work includes the following contents: Firstly,the unsteady test rig for centrifugal compressor is designed and constructed,an automatic flow field measurement traversing system suitable for high frequency pulsating backpressure has been developed,which make the probe step inside the compressor in time,overcoming the difficulties caused by centrifugal compressor’s compact structure,high speed and pulsating pressure of high frequency.Secondly,the performance and flow field distribution of the compressor under pulsating backpressure are proved.The results show that under pulsating backpressure,the compressor system shows a significant filling and empting effect,and the pressure ratio curves present hysteresis loops around the steady conditions,their size and shape are related to the time-averaged mass flow rate and pulsating frequency respectively;the compressor efficiency decreased significantly compared with the steady state,up to about 8 %.The intensity of pulsation gradually weakens during propagation,leading to great pressure difference between leading edge of main blades and splitters.There are obvious differences in the velocity value and phase of different measuring points at inlet of the compressor.Further analysis of the flow field at the compressor inlet shows that the velocity of measured surface’s upper half is significantly lower than that of the lower half under pulsating conditions,and the distortion intensity is positively correlated with mass flow rate.The inlet velocity oscillates greatly in the period,and the velocity of rising and falling edges of average mass flow rate in the same hysteresis loop are different.Under pulsating conditions,inlet flow angles of the compressor showed obvious circumferential asymmetric distortion at about 85% blade height,the proportion of the strong distortion region of the flow angle at the blade height is negatively correlated with the frequency during pulsating period.Finally,the mechanism of the flow field distortion at the compressor inlet is revealed through numerical analysis: The non-uniform distribution of the circumferential pressure at the outlet of the impeller propagates upstream of the compressor,resulting in the distortion of compressor inlet velocity and flow angle,as well as inlet flow field differences at the same instantaneous mass flow rate of hysteresis loops,the distortion intensity is positively correlated with the slope of the circumferential pressure change.