FAN-01 Benchmark: Low pressure Axial Fan in a short Duct

Benchmark case: Axial Fan Measurements All experimental investigations were made in a standardized inlet test chamber according to ISO 5801, see figure 2. The test chamber was built as an anechoic chamber with absorbing walls, ceiling and floor, to enable aeroacoustic measurements. The fan’s operating point was carefully adjusted with the butterfly damper (decrease volumetric flow) or the auxiliary fan (increase volumetric flow). The volumetric flow was determined with a standardized bellmouth inlet, see figure 2. The fan was installed in a short duct with an inlet nozzle on the suction side and a diffusor on the pressure side to resemble a realistic test setup, see figure 1. torque meter motor cm x z y all dimensions in mm ∅500 390 top view test chamber side ambient Figure 1: Experimental setup - detail view. x y z splitter-type silencer butterfly damper auxiliary fan test fan bellmouth inlet flow straightener absorber 2320 1320 2445 all dimensions in mm Figure 2: Standardized inlet test chamber. 1 The fan was driven by a motor outside the duct. Torque and rotational speed were measured with a precision torque meter, see figure 1. Measurement details Information on the used measurement equipment and measuring times are shown in this sections. A detailed description of the experimental setup and the measurement positions is given in [1]. LDA LDA system 2-component LDA probe, type 2D FiberFlow (Dantec Dynamics) BSA P80 burst spectrum analyzer (Dantec Dynamics) BSA Flow software v5.20 (Dantec Dynamics) Measurement time 12 min or 2.5×106samples per position Wall pressure fluctuations Type differential pressure transducers XCS-093-1psi D (Kulite Semiconductor Products Measurement time 30 s with a sampling frequency of 48 kHz Microphones Type 1/2 inch free-field microphones 4189-L-001 (Br¨uel & Kjær Measurement time 30 s with a sampling frequency of 48 kHz Microphone array microphones Type 1/4 inch array microphones 40PH-Sx (G.R.A.S. Sound & Vibration) Measurement time 30 s with a sampling frequency of 48 kHz References [1] Zenger, F., Junger, C., Kaltenbacher, M., and Becker, S., ”A Benchmark Case for Aerodynamics and Aeroacoustics of a Low Pressure Axial Fan,” SAE Technical Paper 2016-01-1805, 2016, doi: 10.4271/201601-1805. [2] International Organization for Standardization, ”ISO 5801:2007 Industrial Fans - Performance Testing Using Standardized Airways”, 2007. Results This section includes the results for the fan aerodynamic and acoustic characteristic curve, flow field on the suction and pressure side, wall pressure fluctuations and sound field. Fan characteristic curve The aerodynamic characteristic curve is shown in figure 3. The fan reaches a maximum volumetric flow of ˙ V≈2.1 m s−1for ∆pts = 0 and a maximum pressure difference of ∆pts = 250 Pa for ˙ V≈0.2 m s−1. The total-to-static efficiency is 53 % at the design volumetric flow of ˙ V= 1.4 m s−1. Figure 4 shows the acoustic characteristic curve of fan N1UG. At the design volumetric flow, the overall sound power level is LW= 87.3 dB. Stall onset at lower volumetric flows can be detected by the rise in LWat 1.1 m3s−1with a maximum of LW= 93 dB at ˙ V= 1 m s−1. Flow field Figure 5 shows the time averaged values of the meridional velocity on the suction and pressure side for a volumetric flow of 1.4 m3s−1. 2 volume t ric flow in m3 / s t o t alt o-s t a t ic pressure d ifference pts in Pa total-to-static efficiency ts 00.5 11.5 2 0 50 100 150 200 250 300 350 400 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 pts N1UG tsN1UG Figure 3: Fan aerodynamic characteristic curve. volumetric flow m3/s sound power level in dB 00.5 11.5 2 75 80 85 90 95 N1UG Figure 4: Fan acoustic characteristic curve. r/r d uct cmin m/s 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 2 4 6 8 10 12 14 suction side pressure side Figure 5: Time averaged values of meridional velocity on suction and pressure side. 3 On the suction side, the meridional flow is accelerated around the hub at r/rduct = 0.5 and then decreased towards the duct wall at r/rduct = 1. On the pressure side, the peak velocity is at r/rduct = 0.84 with 12 m s−1. Towards the hub, it decreases to 10 m s−1and towards the duct wall to 3.5 m s−1. Figure 6a shows the ensemble averaged values for meridional velocity cm(contour) and circumferential velocity cθand radial velocity cr(vector field) on the suction side with fan N1UG in the background. Rotational direction is clockwise. Corresponding to figure 5, the meridional velocity decreases from hub to tip. The maximum is reached during the blade passage near the hub. The circumferential velocity is also maximal near the hub, short after the blade passage. Near the tip, a small region with flow against the rotational direction during the blade passage can be observed. The radial velocity shows high values during the blade passage at the midspan region. Accordingly, figure 6b shows the ensemble averaged values for the pressure side. Here, the meridional velocity cmhas the highest values in between two blade passages. Due to the motion of the fan, a circumferential component is superimposed on the pressure side. The values are small in the same region as with high meridional velocity. Radial velocity is rather constant, only near the hub, a region with negative values can be observed. aSuction side: ensemble averaged meridional velocity cm. bPressure side: ensemble averaged circumferential velocity cθ. Figure 6: Ensemble averaged velocity values. The ensemble averaged turbulent kinetic energy on the suction and pressure side is shown in figures 7a and 7b. On the suction side, values are low, except during the blade passage where the turbulent kinetic energy reaches values of k= 35 m2/s2. On the pressure side, turbulent kinetic energy is high at the blade tips and at the hub region. 4 aSuction side: ensemble averaged turbulent kinetic energy k. bPressure side: ensemble averaged turbulent kinetic energy k. Figure 7: Turbulent kinetic energy. Wall pressure fluctuations Spectra of wall pressure fluctuations (superposition of acoustic and hydrodynamic fluctuations) for selected transducers are shown in figure 8. Note that y-axis values are in dB. fre q uenc y in Hz pressure level in d B, rel. 2e-5 Pa 102103104 40 60 80 100 120 140 transducer 02 transducer 07 transducer 13 Figure 8: Spectra of wall pressure fluctuations. Tonal components can mainly be seen at the blade passing frequency (BPF) at 225 Hz and harmonics. The curve for transducer 2 shows the highest broadband values, being axially positioned very close to the blade tip during the blade passage. Elevations can also be observed at 338 Hz (1.5·BPF) and 1.5·338 Hz = 507 Hz. This effect does also occur in the sound power spectrum. Sound field The sound power spectrum, calculated from seven 1/2 inch free-field microphones is shown in figure 9. Tonal components can be seen at the BPF (225 Hz) and harmonics. Additional tonal components in the range from 7 kHz to 8.5 kHz are caused by the frequency converter of the drive motor. Similar to the spectrum of wall pressure fluctuations of transducer 2, subharmonic peaks can be observed at 338 Hz and 507 Hz. The amplitude of the highest peak is even above the amplitude of the BPF, suggesting a dominant effect. It is expected that this is caused by an interaction of the gap flow with the fan blades. 5 frequency in Hz sound power level in dB 102103104 40 60 80 N1UG Figure 9: Sound Power Spectra. Beamforming results for 2 kHz, 4 kHz and 5 kHz third-octave band are displayed in figures 10 to 12. On low pressure axial fans, leading edge noise is expected to be prevailing in the low-frequency region, whereas trailing edge noise is mainly anticipated in the high-frequency region. Futhermore, more sound sources are anticipated at the blade tip, as there is a higher circumferential velocity and consequently a higher Mach number than at the hub region. Looking at the sound map at 2 kHz, this can be confirmed, as sound sources are mainly located at the leading edge and at the blade tip (with high circumferential velocity). At 4 kHz, sound sources are distributed over the whole region from leading to trailing edge. Hence the transition from leading to trailing edge noise takes place in this frequency band. At 5 kHz, sound sources are more dominant on the trailing edge than on the leading edge. Figure 10: Sound map, 2 kHz third-octave band. 6 Figure 11: Sound map, 4 kHz third-octave band. Figure 12: Sound map, 5 kHz third-octave band. 7