Tested control methods
6.3 Casing and volute tongue redesign
This section is dedicated to the modification of the volute geometry in order to improve the flow conditions in general and therefore increase the overall efficiency and reduce the noise generated. It is important to refer that many studies point that the sound emission are the lowest in the range of maximum efficiency of the fan [15] and therefore the volute geometry will be redesigned for better efficiency. This was done in partnership with Garcia [24] who was in charge of all the CFD simulations.
Through CFD analysis, that will be explained with more detail in section 7.1, it was possible to predict the pressure distribution inside the volute and outlet duct (Fig. 6.7). Through the analysis of this figure, it is evident that the total pressure near the volute tongue is substantially high, since the flow runs to the side wall away from the volute tongue. This leads to the formation of a leakage flow region in the vicinity of the volute tongue and therefore to a deterioration of the overall efficiency of the fan and to the generation of acoustic noise. Therefore, the volute tongue geometry will be object of modifications in an attempt to reduce the high pressure fluctuations at this zone.
(a) P1. (b) P2.
Figure 6.7: Total pressure at middle meridian section (original volute design), at motor speeds P1 (left) and P2 (right).
Likewise, an increased total pressure along the side walls of the volute is also recognised. Thus, the volute radius of curvature will also be increased so that a better guidance of the flow turning from the axial into the radial direction is accomplished. This, as explained in section 5, leads to improvements of the flow conditions in general and, as a consequence, to the reduction of the noise generation.
This way, we firstly proceeded to the modification of the volute curvature. On a study performed by Kitadume et al. [7] a concept of casing design is proposed in such a way that the mean velocity in the cross section is the same at any azimuth angle at the design condition of the fan. The expression here proposed to determine the curvature radius of the volute (Rv) is:
Rv= (R2+d) exp (ψtanαv), (6.1) where the variables presented can be visualised in Fig. 2.2(a); also note thatθc≤ψ≤360o.
To design the scroll casing with the minimum curvature possible some constraints have to be taken into account: the limited space on the exterior parallelepiped box that accommodates the casing and the fixed outlet discharge duct position. On the other hand, notice that it is possible to move the rotor axis in the x-direction, which has an effect on the volute tongue angleθc.
In a study by Frank et al. [51], an optimization of the efficiency of a 40 bladed sirocco fan with similar characteristics as the one being here studied was performed. It was concluded that the optimum volute angle was αv = 5o. Due to the fact that no other volute with a different geometry was available, the aforementioned volute angle was used on the redesign process of the scroll. Finally, defined that the cut-off clearance would remain the same, it was possible to fit the redesigned volute inside the exterior box.
The volute tongue redesign was based on trial and error. After several CFD simulations performed by Garcia [24] in order to avoid flow separation at this zone, it was possible to obtain the final volute tongue geometry. A detailed sketch of the new volute design is shown on Fig. 6.8.
The CFD analysis containing the information of the total pressure at middle meridian section of the blower for the new volute geometry is shown in Fig. 6.9. Comparing the total pressure distributions for the case of the original volute (Fig. 6.7) and redesigned one (Fig. 6.9) a significant reduction is observed on the total pressure along the side walls of the volute and outlet duct, as a result of a better guidance of the flow. Furthermore, the volute tongue redesign allowed to substantially decrease the pressure fluctuations near this zone and consequently downstream near the duct wall due to a more uniform flow though the tongue.
New acoustic experiments were carried out using the redesigned volute in the anechoic chamber.
These experiments were performed in the same way as the previous ones described in section 4.
First, it was measured the OASPL and the sound spectra in 1/3 octave-bands radiated by the housed impeller for both motor speeds. The complete results of these measurements made at 10, 20, 40 and 80 cm away form the volute are shown in Appendix C.4 (Fig. C.5). For an easier comparison between the results obtained with the original and redesigned volutes, the measurements for both cases with microphone positioned at 40 cm from the volute are presented in Fig. 6.10.
Figure 6.8: Sketch of the redesigned volute.
(a) P1 (b) P2
Figure 6.9: Total pressure at middle meridian section (modified volute design), at motor speeds P1 (left) and P2 (right).
A reduction on the OASPL around 6-7 dB was obtained for the measured positions at the lowest motor rotational speed in comparison with the analogous case with the original volute (see subsection 4.2.5). This reduction is more noticeable for low-frequency bands, more specifically for frequencies less
or equal than 200 Hz. This phenomena of diminished low-frequencies noise emissions was previously explained in section 5.2 and it can be justified by the decreased flow separations and turbulent flow conditions. Since the reduction was more intense at low frequencies, the reduction in the OASPL in dB(A) was not as large as in dB, but it is still noteworthy a drop around 3-4 dB(A) at the measured points.
Figure 6.10: Sound spectra of the housed impeller with the original and redesign volutes without the outlet duct, 40 cm away from the volute.
For the highest motor speed we can also notice a significant low-frequency noise reduction, with OASPLs reductions around 4-5 dB or 3 dB(A) for the microphone positions mentioned above.
For both rotational speeds, reductions on the noise emissions at twice line frequency are also noted, which could be related to the fact that the mountings of the redesigned volute to the motor are slightly different from the ones of the original volute.
Next, the outlet duct was attached to the volute and new experiments were performed. The 1/3 octave band spectra measured for both motor operating positions are shown in Appendix C.4 (Fig. C.6).
In order to compare the sound spectra with the analogous case with the original volute take a look at Fig. 6.11.
Figure 6.11: Sound spectra of the housed impeller with the original and redesign volutes with the outlet duct, 40 cm away from the volute.
A pronounced noise reduction of the noise radiated for frequencies below 200 Hz at both motor
speeds is again noticed, with an OASPL reduction of 3-10 dB or 1-4 dB(A) that increases with the proximity to the noise source.
In addition, these changes on the volute geometry also led to an increase in the overall efficiency of the blower around 42% [24]. Hence, it seems quite reasonable to assume that both modifications on the volute tongue and volute scroll geometries had a positive effect on the efficiency and acoustic emissions; still, testing both changes separately could give more information about how each parameter is affected by each modification.