Modal Analysis of Hood Fan using EDM Modal

The experimental object in this modal test is a hood cross-flow fan with an overall structure that includes a roulette wheel, rotor frame and cross-flow blades that are composed of metal materials. The fan is hung using bungee cords to imitate free-free boundary conditions.

The experimental goal is to study the fan axial mode, natural frequency, mode shape, damping ratio and other mode parameters.

The hardware system used for this modal test is the Spider-80X vibration analyzer, a PCB lightweight single-axis accelerometer, and a PCB modal impact hammer. EDM Modal software was used for the modal testing and analysis workflow.

Table 1. Test systems and software

System Component	Model	Specification	Comment
Data acquisition and analysis systems	Spider-80X	8 channels	Crystal Instruments
Force hammer	086C03	500 N	PCB
Acceleration sensor	Single-axis 333B30	2×100 mV/g	PCB
Software	EDM Modal	9.0.1.11	Crystal Instruments

The geometric model of the structure is built using EDM Modal and a total of 52 measurement points were laid out to represent the measurement grid as shown below.

Figure 2. Geometric model of the structure

A roving excitation with one fixed accelerometer was used to execute this modal test without inducing any mass loading effect. One complete row of the FRF matrix as illustrated below with this approach.

Figure 3. Schematic Diagram of the Roving Excitation Hammer Impact Test

The modal data selection tab overlaps the measured FRFs and assists in observing the resonances and anti-resonances within the desired frequency range as shown below.

The latest Poly-X modal parameter recognition algorithm (p-LSCF) is used and the first six modal results (i.e., frequency, damping and mode shape information) are analyzed.

The Modal Assurance Criterion (MAC) is used to check the consistency and correlation between the modes. The diagonal elements of the MAC matrix of this modal experiment are close to 1 and the values of non-diagonal elements are small. Users can conclude that the vectors of each order are better orthogonal and the decoupling between the modes is better.

Figure 7. Modal Assurance Criterion (MAC) Chart

Information regarding the first 6 modes is provided in the following table:

    Modal order
    Frequency (Hz)
    Damping ratio (%)
    Comment
  


    #1
    258.602
    0.835
    Circumferential bending and twisting
   
Composite vibration
  

    #2
    299.204
    0.918
    Radial vibration
  

    #3
    387.525
    0.315
    Circumferential bending vibration
  

    #4
    441.7
    0.89
    Circumferential bending and torsion
   
Composite vibration
  

    #5
    567.872
    0.82
    Roulette wheel local vibration is dominant
  

    #6
    782.438
    0.638
    Circumferential bending vibration
  

The experimentally measured 6th-order mode has complex vibration characteristics. The #1 and #4 modes are represented by circumferential bending and torsional composite vibrations from the mode shape. The #2 mode is characterized by radial vibration, and there is a relative movement between the wheel and the impeller. The #3 and #6 modes are manifested as bending vibrations without torsional vibration. The #5 mode is expressed as the local vibration mode of the roulette wheel.

Through the modal analysis of the cross-flow fan, the structural design can be optimized. Since the fan is driven by an electric motor, it is easy to cause torsional resonance, so it is particularly necessary to pay attention to the composite modes with torsional vibration of #1 and #4 in order to avoid the resonance frequency; from #5 local mode, it can be seen from the mode shape that the relative deformation is larger, and the stress may be larger, and it is recommended to strengthen the stiffness of the roulette mechanism.

Each mode shape can be found in the following pictures: