The Role of Computational Fluid Dynamic and Aeroacoustic...

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

[email protected]

[email protected]

[email protected]

Contents

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

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

3

Radial fans with forward-curved (FC) blades have some

unavoidable flow-separation zones even at the Best Efficiency

Point (BEP)

active

inactive

Inactive zone in the rotor can grow up to one third of the

rotor width

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

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

2nd order temporal discretization

Rotational speed:1000 rpm

Number of blades:38

Time Step becomes gradually smaller : 1°,0.75°,0.5°,0.25°

rotation of the fan-wheel

4

BPF = 16.667 * 38 ~ 633 Hz


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)


6

Different Turbulence Models (Methods) Different Flow Features

URANS

DES

LES Iso-surface v=20 m/s


7

16.5M Cells 7 days/rev



Detached Eddy Simulation

102 M Cells 35 days/rev


12.5 million cells

Accurate prediction of the tonal and broadband noise (up to 1200 Hz)

Computational power: 64 core server with 256 GB RAM

8

Experimental Noise Measurement According to DIN/ISO 5136


9

F (Hz)

Tonal Noise: Interaction between the impeller blades and the cut-off

Broadband Noise: Vortex shedding at the trailing edges & turbulent flow acting on solid surfaces

Blade Passing Frequency (BPF)


The Source of Noise in FC Fans

10

Static Efficiency= 35%

Static Efficiency= 46%

Tongueless Design vs. Optimum Design


11

Optimum Design Tongueless Design

Tongueless design: No tonal noise component but louder broadband noise

BPF: 633 Hz



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

Local noise cancellation at the cut-off

Employing phase-shift tongues

Increasing impeller-tongue clearance

Negative effect on the fan performance

13

0°

90°

270°

180°

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


14

30 Blades

38 Blades

48 Blades

Different Blade Outlet Angles

160° 165° 170°

Different Number of Blades

38 Blades

38 Blades

38 Blades



Changing the Blade Outlet Angle

Increasing the outlet angle : Better performance especially in the overload range Slight reduction of the tonal noise


Changing the Number of Blades

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

17

Pressure side

Suction side

30 Blades (BPF=500 Hz)




18

Designing Phase-Shift Volute Tongues


Stepped tongue geometries with 0.5 and 1 Blade-to-Blade (BtB) height difference

0.5 BtB

1 BtB

633 Hz (BPF)

19



0.5 BtB

1 BtB

Pressure monitors: Phase-shift only in the 0.5 BtB model

20

Tonal noise at BPF (633 Hz) Reference 0.5 BtB 1 BtB

Experiment (dB) 62 57 62

Pressure-Monitor (dB) 64 62 64

FW-H (dB) 59 55 59



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.


Thank you for your attention!

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