Fan noise measurement

For many years it has been known that the aerodynamic perfor­mance of a fan is dependent on the ductwork connections at­tached to the fan inlet and/or outlet. If the fan is to develop its maximum pressure capability, then air must be presented to its inlet as a symmetrical and substantially fully developed velocity profile. In like manner, outlet ducting should permit the recovery of excess kinetic energy in the uneven velocity pressure at the discharge plane and its conversion to useful static pressure further along this duct.

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подпись: po*i»a«vna tor f an loadng Fan noise measurement

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Figure 14.16 Arrangement of test ducting for measurement on in-duct and free field sound power levels

подпись: figure 14.16 arrangement of test ducting for measurement on in-duct and free field sound power levels Fan noise measurementThe form of the inlet connection can have a significant bearing on the aerodynamic and acoustic performance, according to how the fan is ducted. Thus, a spigot may be ideal for a unit at­tached to its system via a flexible connection. If, however, the
fan is unducted, and drawing its air from free space, the spigot will lead to the formation of a “vena contracta” with correspond­ing reduction in fan pressure and flow and an increase in noise. In such a case a bellmouth at entry will render any losses negligible.

Free outlet ducted outlet free outlet ducted outlet

подпись: free outlet ducted outlet free outlet ducted outletIt is only of recent years that these performance differences have been recognised in test standards and four installation categories defined in ISO 5801.

Code A: free inlet Code B: free inlet Code C: ducted inlet Code D: ducted inlet

ISO 13347 and ISO 5136 have determined parallel test meth­ods for noise, the ducting arrangements being shown in Figure 14.16. In similar manner, fan sound levels used to be consid­ered a fixed quantity (Figure 14.17) dependent only on the posi­tion of the operating point on the fan’s aerodynamic character­istic. Inlet and outlet sound power levels in open spaces around the fan inlet/outlet were calculated according to classical for­mulae using end reflection corrections. Research in the 1970s

Fan noise measurement

Fan noise measurement

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Figure 14.17 Typical shape of sound power level characteristics

By Baade suggested that this approach was no longer valid but it is only recently that differences in fan sound power levels, ac­cording to how a unit is ducted, have even begun to be recog­nised by industry.

We now have considerable experimental evidence to support the theory that the sound generated and radiated or transmitted by a fan, is dependent on the acoustic loading at its inlet or out­let. Hence the cross-sectional area, length and geometry of any ducting will all have an effect on the sound power levels mea­sured.

For each of the installation categories specified above, there will, therefore, be a definitive inlet and outlet sound power. An example of these differences is shown in Figure 14.18 for a mixed flow fan.

Additionally, noise will be radiated from the fan casing, to which will be added the noise from any external motor and transmis­sion. It will thus be seen that there are a number of noise levels that may be specified for any particular flow and rotational speed.

But even this is not the end of the story, for Bolton in 1986 also showed that outlet in-duct sound power levels measured in an anechoically terminated duct, changed when the open ended inlet duct was altered in length (Figure 14.19).

Not all researchers (see Bibliography, Section 14.15) in the field are convinced that the differences in these various levels are in­capable of resolution. Whilst sound power spectra in the plan wave mode, determined by in-duct tests are invariably higher than those obtained under free field or reverberant room condi­tions, it is claimed that the differences can be attributed to the reflection of the sound waves at the fan inlet/outlet when the duct is removed.

Tests, however, have produced results where the differences cannot be explained by end reflections, alone. The change in acoustic loading on the inlet side due to removal of the anechoic duct leads to a reduced total (i. e. logarithmic addition of inlet and outlet) sound power output of the fan. Such an effect is not thought to be present on the outlet side.

Conversely, in the frequency range of higher order modes, in-duct sound power levels have been shown to be lower than those measured under free field conditions. It is thought by some that this may be explained by inaccuracies in the terms for “modal correction” and “flow velocity correction" contained in the standards.

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Figure 14.18 Inlet and outlet sound power differences for a 315 mm mixed flow fan at 2850 rev/min and max efficiency (0.53 to 0.54 m3/s)

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