The Role of Computational Fluid Dynamic and Aeroacoustic Simulations in Reducing the Noise of a Forward-Curved Blade Radial Fan

Transcription

1 The Role of Computational Fluid Dynamic and Aeroacoustic Simulations in Reducing the Noise of a Forward-Curved Blade Radial Fan Manoochehr Darvish Bastian Tietjen Stefan Frank darvish@htw-berlin.de tietjen@htw-berlin.de stefan.frank@htw-berlin.de

2 Contents Flow inside radial fans with forward-curved blades Simulation setup (CFD/CAA) Turbulence model & mesh configurations Experimental measurements Noise generation in forward-curved blades fans The importance of the cut-off geometry Methods to reduce the tonal noise of forward-curved blades fans 2

3 Radial fans with forward-curved (FC) blades have some unavoidable flow-separation zones even at the Best Efficiency Point (BEP) inactive active Inactive zone in the rotor can grow up to one third of the rotor width 3

4 STAR-CCM+ Simulation STAR-CCM+ Version 7.04 Simulations start from converged steady solutions RANS (SST k-omega), DES (SST k-omega), LES Non-reflecting inlet/outlet boundaries (Free-Stream) Segregated solver Compressible flow 2 nd order temporal discretization Rotational speed:1000 rpm Number of blades:38 BPF = * 38 ~ 633 Hz Time Step becomes gradually smaller : 1,0.75,0.5,0.25 rotation of the fan-wheel 4

5 STAR-CCM+ CFD/CAA Simulation Monitoring the maximum pressure in the fan discharge Ffowcs Williams-Hawkings (FW-H) receiver is placed near the outlet The whole geometry is assigned to FW-H surface (noise source) 5

6 Different Turbulence Models (Methods) Different Flow Features URANS DES Iso-surface v=20 m/s LES 6

7 16.5M Cells 7 days/rev 12.5M Cells 5 days/rev 26.5M Cells 10 days/rev Detached Eddy Simulation 12.5 million cells 102 M Cells 35 days/rev Accurate prediction of the tonal and broadband noise (up to 1200 Hz) Computational power: 64 core server with 256 GB RAM 7

8 Experimental Noise Measurement According to DIN/ISO

9 The Source of Noise in FC Fans Tonal Noise: Interaction between the impeller blades and the cut-off Blade Passing Frequency (BPF) Broadband Noise: Vortex shedding at the trailing edges & turbulent flow acting on solid surfaces F (Hz) 9

10 Tongueless Design vs. Optimum Design Static Efficiency= 46% Static Efficiency= 35% 10

11 Optimum Design Tongueless Design BPF: 633 Hz Tongueless design: No tonal noise component but louder broadband noise 11

12 How to Reduce the Tonal Noise of FC Fans Unsteady flow leaving the impeller Strong pressure fluctuations at the cut-off Tonal noise generation Reducing the Tonal Noise : Impeller Making the velocity profile more uniform Number of blades blade outlet angle Volute Increasing impellertongue clearance Local noise cancellation at the cut-off Negative effect on the fan performance Employing phaseshift tongues 12

13 The velocity profiles become more uniform by increasing the number of blades

14 Different Number of Blades 30 Blades 38 Blades 48 Blades Different Blade Outlet Angles 38 Blades 38 Blades 38 Blades

15 Changing the Blade Outlet Angle Increasing the outlet angle : Better performance especially in the overload range Slight reduction of the tonal noise 15

16 Changing the Number of Blades Increasing the number of blades: Better performance especially in the overload range Effective reduction of the tonal noise 16

17 Pressure side 30 Blades (BPF=500 Hz) 38 Blades (BPF=633 Hz) 48 Blades (BPF=800 Hz) Suction side 17

18 Designing Phase-Shift Volute Tongues 0.5 BtB 633 Hz (BPF) 1 BtB Stepped tongue geometries with 0.5 and 1 Blade-to-Blade (BtB) height difference 18

19 Designing Phase-Shift Volute Tongues 0.5 BtB 1 BtB Pressure monitors: Phase-shift only in the 0.5 BtB model 19

20 Designing Phase-Shift Volute Tongues Tonal noise at BPF (633 Hz) Reference 0.5 BtB 1 BtB Experiment (db) Pressure-Monitor (db) FW-H (db)

21 Conclusions Tonal noise of the radial fans with forward-curved blades can be reduced by making some geometrical modifications. The geometrical changes which uniformize the velocity profile of the flow leaving the blades have the potential to reduce the tonal noise. Increasing the number of blades not only reduced the tonal noise component but also improved the performance of the fan. Increasing the blade outlet angle can also reduce the tonal noise, yet not as effective as increasing the number of blades. CFD simulations helped to design new volute tongue geometries which generate phase-shift effects and locally cancel the noise at the cut-off. Efficiency of the fan should always remain in focus, since even a simple change in the design of the fan can affect its performance. 21

22 Thank you for your attention!