Front. Energy Power Eng. China 2008, 2(4): 448–452 DOI 10.1007/s11708-008-0088-0

RESEARCH ARTICLE

Bo LIU, Weimin HOU, Changyou MA, Yangang WANG, Qiang ZHOU

Measurement and analysis of tip clearance unsteady flow spectrum in axial-flow fan rotor

E

Higher Education Press and Springer-Verlag 2008

Abstract The dynamic pressure measurement device and test technology are described in this study. The tip clearance unsteady flow development from the inlet to the outlet of an axial-flow rotor was revealed by analyzing pressure frequency spectrum acquired from measuring the unsteady pressure field of the tip endwall. The experiment provides test basis for thoroughly understanding the tip clearance unsteady flow and building interaction models of tip clearance flow and main flow.

scientists at home and abroad in recent years [1–4]. In this paper, the endwall surface pressure fluctuation at the tip clearance was measured with a high speed dynamic pressure measurement system by embedding dynamic pressure sensors on the case of a single stage axial fan test bed. By analyzing the spectrum of the pressure fluctuation, the tip clearance flow rule is obtained, and associated with the aerodynamic performance and the flow stability of the fan.

Keywords axial flow fan, tip clearance flow, unsteady flow spectrum properties

2 Test equipment

1

Introduction

The fan complex channel shape determines that its flow is a complex 3D flow. For many years, the fan interior flow has been studied under certain simplifications of time and spatial structure, and the theory of the sole fluid part steady flow has been summarized. However, the fan aerodynamic design based on the theory of steady flow has become increasingly unable to meet the needs of modern fan development towards attaining high load, high efficiency, and low noise. Therefore, it has become the current international trend of fan research to study the characteristics of fan interior unsteady flow, to master the turbulence field structure and the mechanism of energy loss, to improve the existing theory of fan steady aerodynamic design, and to propose the use of unsteady aerodynamic theory which is more in line with the actual fluid. Generally, since certain tip clearance exists between the axial fan blades and its case, the leakage flow through the gap is inevitable, which has drawn a lot of attention from

This experiment is conducted on a small axial flow fan test stand, as shown in Fig. 1, whose basic parameters are shown in Table 1. The fan’s case is made of transparent acrylics. The rotor is driven by an AC inverter motor, which can attain a rotor speed of 0–3000 r/min stepless speed regulation. There is a back-pressure regulating valve disc at the fan exit, whose angle can be adjusted by the handwheel to change the fan exit area and control the outlet flow, so as to change the fan exit back-pressure.

Fig. 1

Axial fan test section

Translated from Compressor, Blower & Fan Technology, 2007, (5): 12–15 [译自: 风机技术]

3 Measurement system

Bo LIU (*), Weimin HOU, Changyou MA, Yangang WANG, Qiang ZHOU School of Power and Energy, Northwestern Polytechnical University, Xi’an 710072, China E-mail: [email protected]

As shown in Fig. 2, the entire measurement system is composed of dynamic pressure sensors, signal conditioning modules, data acquisition modules, triggers and a computer.

Tip clearance unsteady flow spectrum Table 1

449

Fan basic parameters

item

parameter

item

parameter

rotor speed range/ r?min21 rotor blade count Z stator vane count

0–3000

case inside diameter/mm rotor hub-tip ratio tip clearance/mm

380

6 13

0.37 3.55

3.1 Selection and installation of dynamic pressure sensors An XCO-080-5D transducer produced by the American Kulite company is selected to measure the unsteady flow dynamic pressure distribution in the rotor tip clearance. The transducer is only 2 mm in diameter so that it facilitates the distribution of more axial measurement points. Furthermore, the transducer has a natural frequency of 300 kHz. When the fan operates at the max speed of 3000 r/min, the unsteady flow’s pulse frequency of the tip clearance is also 300 Hz; therefore, it can capture 10 or higher-order harmonic characteristics, which completely meets the test requirements. As shown in Fig. 3, the dynamic pressure sensors is installed in the case by the axial uniform distribution of points method, located in front of the rotor blade tip front A, the blade front B, the middle blade tip C, the trailing edge of the blade tip D, and the rear of the blade tip’s trailing edge E. When measuring, the same Kulite highfrequency sensors are used to eliminate the potential measurement difference from different sensors [4]. In order to facilitate the installation and dismantling, the sensors are stuck in a protection package, and then installed in the transparent acrylics case. The sensors’ spacing is Dh, and has the following relations with the blade tip’s chord length b and the setting angle by: Dh~b0 =2, b0 ~b tan by ,

ð1Þ

Fig. 2

Fig. 3 Sensor installation location 1—casing; 2—sensor protected package; 3—dynamic pressure sensor; 4—rotor blade; 5—wheel hub

where, b9 is the projection of the blade tip chord length in the meridian. 3.2

Signal conditioning module

The sensor signal is usually quite weak. The sensitivity of the sensors used in this test is only about 0.003 mV/Pa. The pressure of the fan at maximum fan speed is no more than 1000 Pa; therefore, before A/D conversion, the mV or several tens of mV voltage signals must be magnified to 0–10 V by the signal conditioning module, otherwise the accuracy of the final gathered digital signal can not be guaranteed. Moreover, because the fan is driven by a frequency conversion motor whose power supply is frequency conversion power, the fans in operation will have such serious high-frequency electromagnetic interference that the measured true signal is disturbed. To eliminate the interference from the motor and the frequency inverter effectively, not only measures should be taken to shield the noisy signals (e.g. the motor shell grounded, the signal wires shielded, and the sensor positive and negative power

Measurement system

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supplying and so on), but signal conditioning for low pass filter should also be adopted to effectively eliminate the high-frequency signal interference. A US PRESTON 8300XWB signal conditioner is used, which can accurately amplify the gathered signal to 500 times, and can simultaneously perform signal low-pass filtering. The conditioner’s enlargement factor is 1000, and the low-pass cut-off frequency is 10 kHz. After the experimental confirmation, the selected parameters of the conditioner can meet the experimental requirements. 3.3

Data acquisition

A WaveBook/516E signal acquisition system from US IOtech Company is selected, whose sampling frequency on the ports is up to 1 MHz, with an accuracy of 16 bits. The acquisition system supports the external trigger method, and an E6B2-CWZ1X encoder made by the Omron Corporation of Japan is set at the tail side of the motor as a trigger, accomplishing phase-locked sampling, and ensuring that the system can collect the pressure signal of all measuring points at the identical circumferential position. Before the test, the entire measurement system is calibrated. The data acquisition module will automatically convert the gathered pressure voltage of the sensors into the pressure signal of the flow.

Equation (2) transforms the pressure waveform data into dimensionless units. pi ~ where n{1 P

p~

i~0

n

pi {p , StdDev

pi , StdDev~

ð2Þ

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi un{1 uP 2 u t i~0 ðpi {pÞ n

,

where p is dimensionless pressure; p expresses the average pressure signal; StdDev is the standard deviation of the pressure signal. 2) FFT transformation From the Fourier formula, the dimensionless timedomains signal is transformed into the dimensionless frequency-domains signal, and the frequency spectrum characteristics (includes the amplitude-frequency and the phase frequency) are obtained. The powerful signal processing functions from the LabWindows/CVI software are used to make the postprocessing software of the corresponding data in order to process the original signal data. An illustration of this is shown in Fig. 4.

5 Discussion of the results 4

Data processing

Table 2 gives the mean value p and the standard deviation StdDev of the pressure at five measuring points, A, B, C,

1) Dimensionless units

Fig. 4

Data analysis platform

Tip clearance unsteady flow spectrum

451

Table 2 Average and standard deviation of pressure fluctuation at measurement points measurement point

average/Pa

standard deviation/Pa

A B C D E

254 26 17 107 180

122.5 304.3 330.2 145.3 107.7

D, and E, whose time-domains pressure waveform are not converted into the dimensionless value, when the fan’s speed is 3000 r/min and the exit is fully open. The symbol  p reflects the static pressure of the end-wall flow, and StdDev reflects the fluctuation level of the pressure of the end-wall flow. From Table 2, it can be seen that the end-wall flow enters the tip clearance from the front of the rotor blades, and that  p is negative for the suction. As the blades work on the air, the air static pressure gradually increases; so this section is an exhaust process. But actually the StdDev value increases first, reaching its maximum at point C, and then decreases. This indicates that, affected by the rotor, the unsteady effect of the tip clearance flow is quite obvious. Figure 5 shows the time-domain dimensionless waveform of the pressure at all measuring points, while the rotor speed n is 2500 r/min and the outlet disc valve is full open. It is shown in Fig. 5 that the dynamic pressures at the 5 points present certain periodicity, whose cycle is precisely the interval that the two adjacent blades do across the sensor. There are other pulsation components in the cyclical pressure waveforms, which show that the flow is extremely complex in the tip clearance. It is

Fig. 5 Dimensionless pressure fluctuation time-domain waveform for measurement points (a) Point A; (b) point B; (c) point C; (d) point D; (e) point E

noteworthy that the pressure waveform at point A also presents the obvious periodicity located at the front of the blade tip. This shows that the circumferential non-uniform pressure field in the rotor spreads toward the upstream of the flow, which results in the fact that even the well-distributed flow is no longer circumferential, even at the front of the blade channel. In addition, compared with other points, the pressure waveform periodicity at point E is the worst. It is known from Fig. 5 that the time-domain chart can only give a little information. But the time-domain pressure waveform is performed by fast Fourier transformation to obtain the frequency spectrum characteristic of the corresponding pressure fluctuation, as shown in Fig. 6. There are higher peaks at 300 Hz on the five points A, B, C, D, and E, namely the first-order spectrum. This peak frequency fp is only related to the rotor speed n and the number of the rotor blade Z, namely fp 5 Zn/60, which is equal to the frequency that the blades pass, and is identical with Ref. [5]. Moreover, there are still some higher-order spectrums of the second-order, the third-order, and so on. The frequencies of the various spectra are just multiples of the frequency of the first-order, but their peaks become smaller and smaller. Comparing points A, B, C, D, and E, the peaks of the various order spectra at point C are higher than that of the other points, and there is also the fifthorder spectrum. This shows that the flow pulse at point C is bigger and the circumferential periodicity is fine. But there

Fig. 6 Amplitude-frequency spectrum of dimensionless pressure fluctuation on measurement points (a) Point A; (b) point B; (c) point C; (d) point D; (e) point E

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is only first-order spectrum at point E, whose peak is lower than that of the other points, and there are also some other peaks in Fig. 5(e). The reason for this is that the flow between the rotor and the guide vane mixes with the secondary flow caused by the trailing flow of the rotor blade, and that there are mutual interferences between the rotor blades and the guide vane, and so on. So when the greater pulsation flow of the middle of the blade tip arrives in the area, the flow pulse decreases gradually, and the periodicity is bad, which is consistent with Fig. 5.

6

Conclusions

1) A set of dynamic pressure measurement equipment and the correlative measurement technology have been used successfully to investigate the unsteady flow in the blade tip clearance of an axial fan. 2) The pulsation frequency of the unsteady flow in the tip clearance is directly proportional to the rotor speed and the number of rotor blades. 3) Because the unsteady effects from the rotor rotation spreads upstream, the flow of the front of the rotor is not uniform, its pulsation is larger and has the obvious circumferential periodicity; in the middle of the tip clearance, the unsteady pulsation of the flow is big and the circumferential periodicity is fine; and because of the influence of

the secondary flows on the rotor trailing flow and the mutual interference between the rotor and the guide vane, when the flow leaves the tip clearance, the flow pulsation quantity decreases, the periodicity is not obvious, and the flow becomes very complex. Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 50476071).

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