Design Method Of Coanda-jet Blade Profile And Its Application In A Highly Loaded Compressor

To increase both the loading and efficiency of the compressor required by the future high-performance aeroengins or gas turbines,a new flow-separation control technology called "Coanda-jet" is numerically and experimentally studied in this paper.The main contents in this paper can be summarized as follows:(1)The method of constructing the Coanda-jet blade profiles and the main geometric parameters of Coanda-jet blade profile are clarified.The program of constructing Coanda-jet blade profiles which can be applied for all types of blade profiles is developed.(2)The double-circular-arc compressor blade profile is selected as the research target firstly.Design of Experimental(DoE)method is used to analyze the influence of the geometric parameters of the Coanda-jet blade profile.The improvement mechanism with Coanda-jet blade profile to the flow fields is clarified by analyzing the flow fields.(3)Experimental investigation is carried out on a low-speed linear cascade test rig.The experimental results demonstrate the effectiveness of Coanda-jet on reducing the flow loss.(4)A 1.5-stage highly loaded compressor is selected as the second research target to study the effectiveness of Coanda-jet in terms of suppressing the flow separation caused by reducing stator blade number.The various types of control technologies for flow separation are summarized and reviewed in the first part of this paper.Additionally,the difference between the conventional boundary-layer-jet and the Coanda-jet method is clarified.In the second part of this paper,the method of constructing Coanda-jet blade profiles is developed and the main geometric parameters of Coanda-jet blade profile are studied.Only the blade profile on the suction side near the blade trailing edge is modified to keep the incidence angle unchanged.The design parameters of Coanda-jet blade profile are the radius of Coanda surface(R),the intersection point of the blade trailing edge and Coanda surface(?),the intersection point of Coanda surface and the upstram part of blade suction surface(a).The double-circular-arc profile of Zierke&Deutsch is selected as the research target.The influence of the Coanda-jet parameters on the flow fields and the corresponding improvement mechanism are analyzed by means of DoE(Design of Experiments)at a high incidence angle of 5°.The results of Fully Factorial DoE show that the values of R,? and jet mass flow ratio are three main factors of improving the total pressure loss coefficient and the static pressure rise coefficient.? and R influence the effect of Coanda-jet by changing the thickness and slot position of Coanda-jet blade profiles.The smaller the value of ? and the greater the value of R,the blade would be more thick and the position of the slot be located more upstream of the flow separation region.In this way,the suppression of the flow separation would be enhanced accordingly.The jet mass flow ratio could influence the streamwise momentum of boundary layer fluid.The higher the mass flow ratio,the better the flow filed is improved.The neural network algorithm based on genetic algorithm(GA-BP)is utilized to optimize to Coanda-jet blade profile.The optimization goal is to minimize the flow loss caused by the large flow separation on the blade suction side at the high incidence angle.It is found that the optimal design of the Coanda-jet blade proflie could reduce the total pressure loss coefficient by 19.6%and the static pressure rise coefficient can be increased by 5.9%with the jet mass flow ratio of 1%.In the third part of this paper,the aerodynamic performances of the Coanda-jet cascade are tested on a low-speed linear blade cascade facility.The effectiveness of Coanda-jet on reducing the loss of flow field is verified.The flow turning angle and total pressure loss coefficient at the mid-span section were measured along the pitchwise direction at the incidence angles of-3°,0°,3°,5°.The results show that the Coanda-jet only reduced the loss of flow separation at the positive incidence angles.The mass-flow averaged total pressure loss coefficient at the blade mid-span can be reduced by 32%at the incidence angle of 50 with the jet mass flow ratio of 1%.In the fourth part of this paper,the effect of Coanda-jet with three jet mass flow ratios(0.5%,1%,1.5%)in terms of recovering the performance drop of a 1.5-stage highly loaded compressor caused by reducing the stator blade number from 31 to 20 was investigated.Considering the total pressure loss coefficient,the polytropic efficiency and the jet efficiency,it is recommended to use the jet mass flow ratio of 1%.Compared with the original stator blade design with 31 and 20 blades,the stator with Coanda-jet could improve the compressor efficiency and reduce the flow loss at both the peak-efficiency and near-stall conditions.However,the static pressure rise coefficient could be improved only at the near-stall condition when comparing the original stator and Coanda-jet stator with the same blade number of 20.At the near-stall condition,the entropy generation rate countors show that the effect of Coanda-jet on the flow loss depends on the the change of the mixing loss caused by the low momentum fluid in the boundary layer with mian flow,the change of the mixing loss caused by the jet and the low momentum fluid,and the change of the wake mixing loss.The effect of Coanda-jet can be analogized to a "secondary diffuser" that is located downstream of the jet slot.With the increase of the jet mass flow ratio,the ability of the secondary diffuser is enhanced and the static pressure rise coefficient of the stator would be thus increased.To sum up,the effectiveness of Coanda-jet in terms of controlling flow separation is investigated numerically and experimentally in this paper.The results show that both the geometric parameters R,? and the jet mass flow ratio are three main factors for the effect of Coanda-jet.For a two-dimensional double-circular-arc blade profile and a three-dimensional highly-loaded compressor stator,whether the incoming flow is compressible or incompressible,the Coanda-jet could suppress the flow separation,reduce the flow losses,and increase the static pressure rise coefficient effectively at positive high incidence angles.The total pressure loss coefficient that takes the input energy of the jet into account would decreases at first and then increases with the increase of jet mass flow ratio.The static pressure rise coefficient would increase with the increase of jet mass flow ratio.The minimum total pressure loss coefficient can be obtained with the jet mass flow ratio of 1%,and the maximum static pressure rise coefficient can be obtained with the jet mass flow ratio of 1.5%.