Effect Of Tandem Diffuser On Performance Of Centrifugal Compressor At Low Flow Rate

Coriolis force and centrifugal force has a great influence on the internal flow in the centrifugal compressor impeller.The flow developed near the impeller outlet is not uniform,and usually shows the ?jet wake? structure at outlet of centrifugal impeller.After entering the diffuser,a local flow separation of passage caused by this non-uniform flow,especially in the off design conditions.As a major diffusion part for the compressor,the geometric modeling of the diffuser and the matching with the centrifugal impeller have an important influence on the internal three-dimensional flow and efficiency.The stage polytropic efficiency and stable working range can be simultaneously increased by using suitable tandem cascade.In this paper,the tandem cascade diffuser is used in a centrifugal compressor stage,which is one of the series of LB56150 KY108 designed by Shenyang Blower Works.By using CFD software NUMECA,the influence of the geometric parameters of the tandem cascade diffuser on the performance of the centrifugal compressor stage is studied.In the first part of this paper,the effect of relative circumferential position of the front and the rear diffuser vanes on the internal flow structure and the performances of the compressor stage are analyzed.The relative circumferential position is modified by changing the angle in a pitch.And numerical simulation of five compressor stage models are carried out with different relative circumference position among location 1~location 5.The conclusions are as follows: The stage performance reaches best as the relative circumference position situates location 2 in low mass flow rate condition.The proper relative circumference position improves the stable operating condition,optimizes the distribution of radial velocity and flow angle at diffuser outlet and decreases the variation degree of circumferential velocity and radial velocity at impeller outlet.The rear blade close to the front vane suction surface side is beneficial to improve the flow structure,and the flow blockage is deterioration when the rear vane locates the middle of diffuser flow passage.In the second part of this paper,the effect of total bend angle of the front and the rear diffuser vanes on the internal flow structure and the performances of the compressor stage are analyzed.The total bend angle is defined as the angle between the tangent of mean camber line of the front vane at the leading edge and the tangent of mean camber line of the rear vane at the trailing edge.The results show that the proper increase of total bend angle greatly improves polytropic efficiency in low flow rate condition,of which the total bend angle increases 10 degrees is obtained best performance.The total bend angle optimizes airflow pattern at the rear vane passage,levels the distribution of radial velocity at diffuser outlet and enhances diffusion capability.However,the excessive large total bend angle causes great loss in flow passage and leads to the deterioration of compressor stage.In the third part of this paper,the effects of chord-length ratios of the front and rear diffuser vanes on the internal flow structure and the performances of compressor stage are studied.The numerical simulation of seven tandem cascade diffuser model with different chord-length ratios shows that the polytropic efficiency and total pressure ratio increase as the chord-length ratio increases at high mass flow rate conditions.Once the flow rate is under the design flow rate,there is the right chord length ratio to obtain the best efficiency.However,the excessive and minimize chord-length ratios leads to the deterioration of the stable operating condition.In the last part of this paper,relative circumferential position,total bend angle and chord-length ratio are considered comprehensively in order to study the impact of each parameter on the performance of centrifugal compressor stage.The results reveal that the polytropic efficiency and total pressure ratio are greatly improved by using multi parameter optimization scheme in low flow rate condition.For one parameter optimization scheme,the total bend angle has significant influence on the performance of centrifugal compressor in the low flow rate.The multi parameter optimization scheme decreases the separation vortex on the suction surface in the rear vane,optimizes the distribution of radial velocity at diffuser outlet and increases the static pressure recover coefficient.