Effect Of Special Vane Diffuser On Internal Flow And Stage Performance Of Centrifugal Compressor

The diffuser is the key stationary component of the centrifugal compressor and it is responsible for converting fluid kinetic energy into pressure energy.The flow separation caused by the adverse pressure gradient seriously affects the performance of centrifugal compressor stage.It is easy to cause the unstable flow such as rotating stall and surge,which seriously affects the normal operation of the system.Further study of the flow mechanism inside the diffuser is helpful to develop a vane diffuser with high efficiency and wide operating range.In this paper,the structure improvement of vane diffuser in the LB56150 KY108 centrifugal compressor provided by Shenyang Blower Works Group Co.,Ltd.is carried out,including changing the circumferential position and the height(equal height and non-equal height along the flow direction)of blade in the vane diffuser.The computational fluid dynamics method was taken to analyze the internal flow field.The following aspects of work are mainly carried out in this paper:The effects of non-axisymmetric blade arrangements on stage performance and internal flow were investigated.Five equidistant positions along the circumferential direction in a single passage of the diffuser are selected for the positions of the non-axisymmetric blade.The flow characteristics of the centrifugal compressor stage with non-axisymmetric vane diffuser were analyzed under different operating conditions.The numerical results show that compared with the traditional axisymmetric vane diffuser,the non-axisymmetric vane diffuser made an increase of 1.46%~4.62% and 1.7%~3.2% for the stage efficiency and stage pressure ratio respectively under the low flow rate condition.The proper circumferential position made the distribution of radial velocity and flow angle on the diffuser outlet more uniform and the flow stability is better;the distribution of radial velocity on the impeller outlet is more uniform,the velocity gradient is smaller and the flow loss is reduced.The optimum circumferential relative position of the blade exits which maximizes the stable operating range of the centrifugal compressor stage.The effects of half-height vane diffuser on stage performance and internal flow were investigated.The diffuser models with full height blade,shroud side 0.1b,hub side 0.1b,shroud side 0.3b and hub side 0.3b(b=36.6mm,diffuser width)respectively is selected to analyze the internal flow of the centrifugal compressor through numerical simulation.The analysis focuses on characteristics such as flow uniformity,stable operating range and internal flow losses.Theresults showed that compared with the full-height blade,the entropy of the flow in the diffuser with half-height blade is much lower,the flow loss is improved,and the flow loss on the shroud side is smaller than that on the hub side.As the diffuser blade height increases,the diffuser outlet flow is more unstable.The 10% shroud side diffuser outlet flow angle is more uniform and the flow loss is smaller.A new type vane diffuser with different height at leading edge and trailing edge of blade is proposed,and analyze the effect of the new vane diffuser on the stage performance and internal flow loss under different combinations.The results show that the stage efficiency is increased by1.2%~2.4% under the low flow rate condition,and 0.4%~1.2% under the designed flow rate condition,and the efficiency is increased by 0.2%~0.5% under the high flow rate condition.The stage efficiency and stable operating range of the compressor have been improved.The total pressure loss of the new vane diffuser is reduced compared to the full-height vane diffuser.Influenced by the wall surface of shroud and hub,the maximum value of the total pressure loss is concentrated at the upper and lower wall surfaces;the total pressure loss near the hub decreases rapidly,while it decreases slowly inside the passage,and then increases rapidly when approaching the shroud side.Due to the change of the distance between the blade and the hub,the circumferential flow of the fluid comes into being,which forcing the fluid on the hub side to converge.The existence of the gap improves the organization of fluid,the radial velocity distribution at the outlet of the diffuser becomes more uniform,and the flow conditions also get improved.Based on the above optimum value of diffuser structure improvement,the structure of non-axisymmetric blade,semi-high diffuser and new vane diffuser is optimized.The results show that the efficiency of centrifugal compressor stage is improved under full flow condition.The static pressure recovery coefficient and total pressure loss coefficient of the combined scheme and the optimal position 3 of the non-axisymmetric blade are better than those of the original model under the condition of low flow rate.The static pressure recovery coefficient of the combined scheme is increased by 12% and the total pressure loss is reduced by 21%.Under the design conditions,the static pressure recovery factor of the combined scheme is reduced by2% due to the cutting of the vane diffuser blade,but the total pressure loss in the diffuser runner is reduced by 6% compared with the original model.With the increase of flow coefficient,the static pressure recovery coefficient of the combined model decreases slightly,but the total pressure loss is suppressed.Under different working conditions,the radial velocity at the diffuser outlet of the combined scheme decreases.