Research On Supersonic Vaned Diffuser Design Methodology

In order to collect the high kinetic energy from the outlet of high performance centrifugal compressor,complete the high-efficiency transformation from high kinetic energy to pressure energy,the flow mechanism of blade diffuser at the outlet of centrifugal compressor under supersonic speed condition was studied in this paper.The variation rules of supersonic flow field under blade parameters changing was obtained.The relationship between unique flow angle and blade parameter was established by modifying Levine's formula.Based on the relationship a Laval-like nozzle S-type vane diffuser with contraction of semi-vaned region and expansion of blade passage was successfully designed.The flow reason of its high performance was obtained by comparing with the traditional design vaned diffuser.The main conclusions are as follows:1.The performance of vaneless diffuser has a maximum value at whole speed map.The maximum value is under the subsonic region.Under the supersonic condition,the performance of diffuser decreases gradually with the increase of Mach number.The Mach number decreases gradually with the increase of radius,the static pressure rise increases gradually,the inlet flow angle increases gradually in the front section of the inlet,and the increment increases with the increase of Mach number.This is due to the conservation of mass and angular momentum,the flow area first decreases and then increases under supersonic condition.2.In the supersonic flow field,there are usually three shock systems with two oblique shocks and one normal shocks.After passing through the leading edge of the blade,the diffuser accelerates gradually in the semi-vaned region of the suction surface,and the static pressure decreases gradually until it becomes subsonic after the normal shocks.the boundary layer separates under the interference of normal shock wave.A pair of vortices are formed on the suction side,and the flow channel is extruded,and the flow to the pressure side is formed on the solid wall side.The main flow is extruded on the pressure side,and the diffuser's diffusivity is further weakened.With the increase of back pressure,the diffuser pressure coefficient,total pressure recovery coefficient and static pressure ratio gradually increase,and the diffuser performance increases.A detached shock wave is formed after the forward shock wave is pushed out of the throat.The overflow causes the extruded shock wave to disappear,the shock wave structure of the diffuser changes,the Mach number and flow rate of the inlet decrease,and the inlet gas flow angle increases.The diffuser pressure coefficient and total pressure recovery coefficient continue to increase,while the static pressure ratio decreases.3.The upstream flow field must be stabilized under the wave structures of expansion wave + shock wave(compression wave),so the structure will form the unique inlet angle and attack angle by affecting the inlet flow angle and attack angle.The flow field of convex diffuser in semi-vaneless region must be in flow state with negative attack angle.Oblique shock wave + expansion wave structure will be formed in semi-vaneless region.Oblique shock wave will affect upstream to reduce the Mach number of inlet,increase the gas flow angle,and expand wave will accelerate the flow and decrease the gas flow angle.Concave diffuser will be in flow state with positive attack angle,forming expansion wave + compression wave structure.Expansion wave will affect upstream to increase inlet Mach number,the flow angle decreases,and the compression wave slows down the flow and increases the flow angle.The disturbance generated by the pressure profile can not be transmitted forward,so it has no effect on the flow field structure and flow pattern before the throat,but only on the flow in the blade passage,so it has little effect on the overall performance of the diffuser.The change of diffuser inlet caused by upstream impeller speed is only reflected in Mach number.The unique flow angle does not change with the change of speed.The larger the Mach number of inlet,the larger the static pressure ratio.However,the smaller the total pressure recovery coefficient,the lower the pressure coefficient,and the smaller the diffuser comprehensive performance.4.A Levine's method for solving radial flow is corrected by supplementing the radial flow equations.The relationship under radial flow statement between the unique inlet angle and the blade parameters is established.Based on the method,a new design method for supersonic vaned diffuser was proposed.Under the new diffuser shape,the supersonic flow filed is under the flow state with positive attack angle.Large wedge angle can be achieved at a small thickness diffuser shape,which let the diffuser has strong diffusing capacity and circulation area capacity.The influence of the radius ratio of inlet and outlet and the equivalent expansion angle on the vane diffuser is studied.The best equivalent expansion angle is between 4 °~5,the vane diffuser has the best pressure coefficient,maximum static pressure ratio,total pressure recovery coefficient and comprehensive performance.When the radius is between 1.15 and 1.2,the diffuser has the best total pressure recovery coefficient and comprehensive performance.Caused by the smaller diffuser length,the pressure coefficient and the maximum static pressure are smaller.The diffuser has the longest diffuser passage,the best pressure coefficient and the maximum static pressure ratio between 1.25 and 1.3,however,the cumulative friction loss leads to the increase of total pressure loss and the smaller maximum comprehensive performance.The pressure coefficient and the maximum static pressure ratio increase with the increase of radius,while the total pressure recovery coefficient decreases with the increase of radius.5.Compared with the traditional vaned diffuser,the S type diffuser has the lowest total pressure loss,the strongest backpressure resistance,the highest pressure coefficient and the best comprehensive performance.The position of normal shock wave that closes the rear part of diffuser under the same static pressure ratio gives the S-type diffuser the strongest backpressure resistance.When the normal shock wave is pushed to the throat and the blade channel becomes the full diffusion channel,the S-type diffuser has the highest pressure coefficient and the best diffuser performance.The boundary layer on the pressure side affected by the normal shock wave is less separated,the total pressure loss is further reduced,and the comprehensive performance is further improved.