1、2009 ASHRAE 443This paper is based on findings resulting from ASHRAE Research Project RP-1223.ABSTRACTIn this study, noise performance was measured experi-mentally for two propeller fans subjected to systematic vari-ation of inlet flow components. The inlet conditions were intended to simulate insta
2、llations of fans typically encountered in the field. The acoustic performance penalties associated with the various appurtenances are presented in this paper. INTRODUCTIONLittle information exists for accurately predicting the aerodynamic and acoustical response of small propeller fans to common app
3、urtenances at the fan inlet, which are referred to as system effects. Hence ASHRAE RP-1223 was initiated to experimentally measure air and sound performance of propeller fans with systematic variation of conditions at the inlet plane of the fan. The intent was to simulate typical “in the field” inst
4、allations of the fans, such as mitered elbows mounted at various angles at the fan inlet plane, inlet duct contractions of various area ratios, and walls perpendicular to the fan axis and located at various distances from the inlet. The tests were conducted in accordance with ANSI/AMCA 210/ASHRAE 51
5、 (1999). A complete description of the test program, including the experimental apparatus and test proce-dure, was provided in Young et al. (2008a). Aerodynamic system effects, in the form of dimensionless loss coefficients, are presented in Young et al. (2008b). In the present paper, the acoustical
6、 data resulting from this test program are presented.Characteristic System Curve EquationsAerodynamic performance data obtained for the 610-mm (24-in) diameter fan at 100% and 70% of the speed are presented in Figure 1; it is noted that at free delivery the 100% nominal fan speed was 908 rpm. All ot
7、her data are available in Darvennes et al. (2008). Superimposed on each graph are system curves calculated at approximately peak efficiency and at free delivery. These curves were added to facilitate the acoustics data reduction and were calculated from(1)where PT,C denotes total corrected pressure,
8、 A represents a constant, and QT,C signifies the total corrected flow rate. The constant A was determined from measured data and was used to interpolate the frequency sound data for the peak efficiency values. There were possibly four constants to be obtained, two for the free delivery point of the
9、100% and 70% performance curves, and two for the peak efficiency for the 100% and 70% performance curves. For most cases, the constants for the free delivery system curves, were the same. The constants for the peak efficiency case were evaluated by plotting the two perfor-mance curves and determinin
10、g which best captured the peak efficiency point. ACOUSTICS DATA REDUCTION EQUATIONSThe sound power level of the test fan and appurtenance was calculated using the comparison method, per AMCA Standard 300 (1996) using the following(2)where LWdenotes the sound power level, LWRindicates the sound power
11、 level of the reference sound source (R.S.S.), PTC,AQTC,2=LWLWRLPQMLPM+=Acoustic System Effects of Propeller Fans Due to Inlet InstallationsC. Darvennes, PhD M.N. Young, PhD, S. Idem, PhDMember ASHRAEC. Darvennes and S. Idem are professors in the Department Mechanical Engineering at Tennessee Tech U
12、niversity, Cookeville, TN. M.N. Young is an engineer with the Tennessee Valley Authority, Knoxville, TN.LO-09-041 (RP-1223) 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009, vol. 115, part 2. For personal us
13、e only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.444 ASHRAE TransactionsLPQMrepresents the recorded sound pressure level of the R.S.S. and room background as measured over the normal microphone pa
14、th, and LPMis the recorded sound pressure level of fan and room background as measured over the normal microphone path. Interpolation of the sound power levels at peak efficiency was performed using the intersection point between the system curve and the performance curve. Sound levels were normaliz
15、ed according to AMCA Standard 300 (1996) by the fan pressure and flow rate, as needed, using,(3)where NLWdenotes the normalized sound power levels. The measured sound power levels with appurtenances were compared to the sound power level of the baseline case at peak efficiency and free delivery for
16、both the 100% and 70% cases. The purpose was to determine the acoustical effect of the individual appurtenance by means of the following:(4)In this instance LW,ACTUALdenotes the sound power level of the appurtenance only, LW,APPURTENANCE represents the measured sound power level for appurtenance and
17、 test fan installed, and LW,BASELINEsignifies the measured sound power level for the baseline case.ACOUSTIC RESULTSThe noise data for the 610-mm (24-in) and 914-mm (36-in) diameter fans at 100% and 70% of the speeds are summa-rized in Figures 2 through 12. Peak efficiency data are shown for all case
18、s, while the free delivery data are presented for the 610-mm (24-in) fan with the contractions only; all other free delivery data are available in Darvennes et al. (2008). Figure 2 shows the effect of the contraction ratio of 1.0 at peak efficiency at 100% and 70% tip speeds for the 610-mm (24-in) d
19、iameter fan, by subtracting the baseline sound levels from the sound levels with obstructions. Power levels are shown, as well as normalized power levels. The auxiliary fan generated high sound levels at some frequencies, which made some data points unusable. Therefore, sound data are also presented
20、 wherein the power levels have not been corrected for the background noise of the auxiliary fan. Comparing corrected to uncorrected power levels shows that the auxiliary fan noise has a minimal effect on the overall installation effects, within experimental error. Similar data are available for the
21、914-mm (36-in) in Darvennes et al. (2008). Therefore, this approach was taken for each appurtenance for the 610-mm (24-in) and 914-mm (36-in) diameter fans.Figures 3 through 10 portray the effect of the appurte-nances for the 610-mm (24-in) and 914-mm (36-in) diameter fans by subtracting the baselin
22、e sound levels from the sound levels measured with obstructions placed near the entrance plane of the fan. These figures illustrate the effect of the indi-vidual appurtenances and their installation for peak efficiency at 100% and 70% tip speeds and in one instance for free deliv-ery also. The data
23、are neither normalized for flow rate or pres-sure nor corrected for auxiliary fan noise. The blade passing frequencies for the 610-mm (24-in) diameter fan for 100% and 70% tip speeds are 91 and 64-Hz, respectively. The 914-mm (36-in) diameter fans blade passing frequencies are 79 and 56-Hz.NLWLW10 Q
24、TC,()20 PTC,()loglog=LWACTUAL,LW APPURTENANCE,LW BASELINE,=Figure 1 610-mm (24-in) fan, baseline flow data, and efficiency curves at 100% and 70% fan speeds and system curves at peak efficiency and at free delivery.ASHRAE Transactions 445Figure 3 depicts the effects of the contractions for the 610-m
25、m (24-in) diameter fan at peak efficiency and free delivery for 100% and 70% tip speeds. At 100% tip speed, the contrac-tions both aid and hinder the noise levels of the fan at similar frequencies. The area ratio of 1.5 affects the noise data the least. There are similar trends amongst the contracti
26、ons. Figures 3 c and d show that, at 70% tip-speeds, the area ratio of one has the most detrimental effect on noise levels while the area ratio of 1.5 has the most positive effect on the noise levels. The effects of the contractions show similar trends amongst the various area ratios that both aid a
27、nd hinder the noise levels for both 100% and 70% tip speeds for free delivery. There is not a noticeable difference in the overall A-weighted decibel difference. It is less than 3 dB in all instances. One explanation for the drop in noise level in the 63-Hz band at peak efficiency and free delivery
28、could be that the overall length of the appur-tenance, which measured 1.2-m (4-ft), corresponded to a wavelength. This same explanation could be used to explain the drop in the 125-Hz band for both peak efficiency and free delivery at both 100% and 70% tip speeds. The area ratios changed at the midp
29、oint in the length of duct, such that each Figure 2 610-mm (24-in) 4-in) fan, effect of the contraction ratio of 1, on the sound power level at peak efficiency and (a) 70% tip speed (b)100% tip speed. Fan Lw represents the sound power level of the fan, corrected for the noise of the auxiliary fan; F
30、an+Aux Lw represents the combined sound level of the test fan and the auxiliary fan (no correction for the auxiliary fan noise); Fan NLw represents the noise of the test fan, corrected for the noise of the auxiliary fan, and normalized by the test fan pressure and flow rate; and Fan+Aux NLw represen
31、ts the combined sound level of the test fan and the auxiliary fan (no correction for the auxiliary fan noise) and normalized by the test fan pressure and flow rate.446 ASHRAE Transactionssection was 0.6-m (2-ft) in length. This 0.6-m (2-ft) length corresponds to wavelength in the 125-Hz band.Sound m
32、oves as a pressure wave. When the length of the duct corresponds to an odd multiple of wavelengths, the sound pressure wave sees the end of the duct as infinite imped-ance and is reflected back in the duct. When the pressure wave is an even multiple of wavelengths, the pressure wave sees the end of
33、the duct as zero impedance and the pressure wave is transmitted out the end, without the duct affecting the noise levels. For any values between zero and infinity, the duct has an effect on the noise levels that are transmitted. The drops at the lower frequencies correspond to the end reflection los
34、s that is most pronounced for small ducts at low frequencies. This reflection loss can be as much as 8 dB at 63-Hz and gradually approaches zero dB as the frequency becomes higher, from Harris (1998).Figure 4 displays the effects of the elbow for the 610-mm (24-in) diameter fan at peak efficiency fo
35、r 100% and 70% tip speeds. The orientation of the elbow does not significantly affect the noise levels of the fan. In all cases, the elbow has an overall detrimental effect on the noise levels, having an overall A-weighted decibel difference of up to 5 dB at peak efficiency for both 100% and 70% tip
36、 speeds. An explanation for the increase in noise levels at low frequencies could be due to turbulence generated at the elbow since there were no turning vanes. The addition of turning vanes would allow for a decrease in turbulence being generated. Harris (1998) states that elbows tend to attenuate
37、higher frequencies, as much as 10 dB at 4000-Hz. An explanation for the drop in the 250-Hz band at both 100% and 70% tip speeds could be attributed to the length of the straight section of duct preceding the elbow, assuming the length of the straight section is measured from the centerline of the el
38、bow. This would correspond to a wavelength. This same assumption would also explain the drop in the 100-Hz band that corresponds to a wavelength for the 100% tip speeds. Figure 5 describes the effects of the wall for the 610-mm (24-in) diameter fan at peak efficiency for 100% and 70% tip speeds. The
39、 separation distance of 0.25D has the most detri-mental effect on the noise levels. The wall increases the overall A-weighted level by as much as 6 dB at 0.25D. As the distance of the wall is increased from the fan, the effect of the wall Figure 3 610-mm (24-in) fan, effect of the contractions at pe
40、ak efficiency (a, c) and free delivery (b, d) for 100% tip speed (a, b) and 70% tip speed (c, d).ASHRAE Transactions 447decreases and becomes relatively constant as can be seen with overall A-weighted values of less than 3 dB. A possible expla-nation for the drop in the 100-Hz band at 100% tip speed
41、 could be that it corresponds to the blade passing frequency of 91-Hz and standing waves are present between the constructed wall and the fan. One explanation in the significant drop of noise levels as the wall distances are 0.75D could be that the wall insertion loss is more significant than the no
42、ise level increase caused by its presence.Figures 6 through 9 illustrate the effect of the appurte-nances on the 914-mm (36-in) fan. There is a drop in the 160-Hz band in the majority of the figures. This drop is associated neither with the blade passing frequencies of the test and auxil-iary fans n
43、or with appurtenance lengths corresponding to an odd number of quarter wavelengths. Its most likely explana-tion is an electrical grounding problem when taking the base-line data. Consequently, the overall A-weighted levels are presented with and without the 160-Hz band included.Figure 6 represents
44、the effect of the contractions for the 914-mm (36-in) diameter fan for peak efficiency at 100% and 70% tip speeds. The effect of the contractions shows similar trends amongst the various area ratios and tip speeds, i.e., a significant increase in noise levels at low frequencies and a decrease at mid
45、 and high frequencies. The overall A-weighted Figure 4 610-mm (24-in) fan, effect of the elbow at peak efficiency for 100% tip speed (a) and 70% tip speed (b).448 ASHRAE Transactionsdecibel difference for the area ratios is less than 2 dB, such that the overall effect is negligible. The drop in the
46、100-Hz band could be explained by the individual lengths of the area ratios measured, 0.9-m (3-ft), which corresponds to a wavelength.Figure 7 illustrates the effect of the elbows on the 914-mm (36-in) diameter fan at peak efficiency for 100% and 70% tip speeds. The orientation of the elbow does not
47、 have a signifi-cant effect on the noise levels. The 0 orientation has the worst effect on the fan noise levels while the 45 and 90 orientations aid in the reduction of noise levels at free delivery. The overall A-weighted decibel difference for all orientations of the elbow is less than 3 dB, such
48、that the overall effect is negligible. As stated earlier, the increase in decibel difference at low frequen-cies could be attributed to turbulence occurring at the elbow and elbows attenuate higher frequencies by as much as 10 dB at 4000-Hz.Figures 8 and 9 depict the effects of the wall for the 914-
49、mm (36-in) diameter fan at peak efficiency for 100% and 70% tip speeds with and without the Insertion Loss of the wall removed from the data. Due to physical constraints of the room, the wall was placed in close proximity of the micro-phone path. The physical size of the wall with respect to the reverberant chamber also had an effect on the sound data. The reverberant chamber measured 9-m (29-ft) in length and Figure 5 610-mm (24-in) fan, effect of the wall at peak efficiency for 100% tip speed (a) and 70% tip speed (b).ASHRAE Transactions 449roughly 6-m (20-ft) in h
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