Journal of Thermal Science Vol.15, No.4 289―295

Received: April 2006 Yutaka Ohta: Professor Dr. Eng

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DOI: 10.1007/s11630-006-0289-y

Aerodynamic Performance and Noise Characteristics of a Centrifugal Compressor with Modified Vaned Diffusers

Yutaka OHTA Yasuhiko OKUTSU Takashi GOTO Eisuke OUTA

Department of Mechanical Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan

Improvement of aerodynamic performance and reduction of interaction tone noise of a centrifugal compressor with vaned diffusers are discussed by experiments and visualization techniques using a colored oil-film method. The focus of the research is concentrated on the leading edge shape of diffuser vanes that are deeply related to the generation mechanism of the interaction tone noise. The compressor-radiated noise can be reduced by more than ten decibels by using modified diffuser vanes which have 3-D tapered shapes on both pressure and suction sur-faces of the leading edge. Furthermore, by adopting the proposed modified diffuser vanes, the secondary flow which is considered to be an obstruction of diffuser pressure recovery can be suppressed, and also the pressure decrease observed in the throat part of the diffuser flow passage is reducible. Thus, the proposed diffuser vanes show a favorable result for both noise and the aerodynamic performance of the centrifugal compressor, and offer a few basic guidelines for the diffuser vane design.

Keywords: centrifugal compressor, vaned diffuser, tapered diffuser vanes, noise reduction. CLC number:TK474.8+2 Document code: A Article ID: 1003-2169 (2006) 04-0289-07

Introduction

When a centrifugal compressor is equipped with a vaned diffuser, high pressure-rise characteristics can be obtained, whereas the generating noise level increases remarkably. Therefore, developing a technology for re-ducing the noise level and improving the working envi-ronment of the compressor circumference is expected from a practical point of view. Since the noise, which is called an interaction tone noise (ITN), is considered to be caused by direct impingement of the impeller-discharged flow on the diffuser vane, the leading edge geometries of the diffuser vane play an important role in discussing the noise generation mechanisms. Although the interaction between impeller-discharged flow and diffuser vanes is an obvious and powerful noise source, surprisingly little research has been reported on the ITN [1-5]. Furthermore, the frequency of the ITN coincides with that of the

blade-passing frequency components, abbreviated to BPF components, so the noise could dominate the overall noise level if sufficient care is not taken.

The authors have already shown by experiments and also by flow visualization technique that the leading edge shape of the diffuser vane may play an important role in both the noise generation and the compressor perfor- mance, and the tapered diffuser vanes which have three-dimensional tapered shapes on both pressure and suction surfaces of the diffuser leading edge can reduce the noise level of the ITN effectively without any slight influence on the compressor performance [6].

In the present paper, therefore, based on the previous results by the authors, research attention is focused on the effects of tapered diffuser vanes on the noise level of the ITN and also on the aerodynamic performance and flow field of the centrifugal compressor with vaned diffusers. Three types of 3-D tapered diffuser vanes are designed

290 Journal of Thermal Science, Vol.15, No.4, 2006

Nomenclature BBT diffuser vane height at leading edge (m) Z number of impeller blades CP pressure recovery coefficient Greek letters D diameter (m) φ flow coefficient Lep tapered portion on pressure side (%) ψT total pressure-rise coefficientLes tapered portion on suction side (%) Subscripts N rotational speed (r/min) 1 impeller inlet P static pressure (Pa) 2 impeller outlet PT total pressure (Pa) 3 diffuser inlet Q volume flow rate (m3/s) 4 diffuser outlet V number of diffuser vanes IE measuring location at 0.96D2

and used in the experiments. Total pressure-rise coeffi-cients and generated noise data are acquired precisely by making the flow coefficient into an experimental pa-rameter, and a flow structure within a diffuser blade-to- blade passage is visualized by the colored oil-film method. While the tapered vane can reduce the noise level of the ITN remarkably, the compressor performance falls especially in the off-designed low-flow region. However, if a tapered portion on both pressure and suc-tion surfaces is limited within a 30% chord from the leading edge, pressure-rise characteristics higher than the original 15-vaned diffuser can be obtained, and reduction of ITN by more than 8 decibels can be attained.

Furthermore, detailed measurements of the surface pressure fluctuations on the shroud surface of the diffuser wall are conducted in order to investigate the effects of the tapered shape on the pressure recovery characteristics of the vaned diffusers. By using the proposed tapered diffuser vanes, high-pressure recovery within the diffuser passage can be obtained and also the secondary flow which is considered to be an obstruction of diffuser pressure recovery can be suppressed. Also, the sudden drop of the static pressure recognized in the throat part of

the original 15-vaned diffuser can be reduced, and the pressure fluctuation level which is relevant to the gener-ated noise can also be attenuated.

Experimental Procedure Experimental apparatus The tested compressor and measuring system used in

the experiments are shown in Fig. 1. The compressor is a low-specific-speed centrifugal type and is designed based on a turbocharger for marine diesel engines. The flow enters the compressor through an axial suction pipe and leaves downstream in an annular collecting chamber ending with a circular pipe. The inlet and outlet diame-ters of the unshrouded impeller are 248 and 328 mm, respectively. The number of main and splitter blades is 7 each. Two types of diffusers are used in the experiment. One is a channel diffuser, and the number of wedge-type vanes is 15. The vanes are located between two parallel diffuser walls 26.14 mm apart from each other. The other is a vaneless diffuser. Each diffuser has an identical me-ridional profile, as shown in Fig. 1. The specifications of the tested centrifugal compressor and dimensions of the impeller and vaned diffuser are listed in Table 1. Refer to

Fig. 1 Experimental apparatus and measuring system.

Yutaka OHTA et al. Aerodynamic Performance and Noise Characteristics of a Compressor with Modified Vaned Diffusers 291

Table 1 Design performance of tested compressor Tested centrifugal compressor

Rotational speed N 7000 r/min Mass flow rate G 1.64 kg/s Pressure ratio P5/P0 1.1 Impeller Number of blades (Main + Splitter) Z 14( 7+7 )

Inlet diameter D1 248 mm Outlet diameter D2 328 mm Exit blade height B2 26.14 mm Diffuser Number of vanes V 15 Leading edge diameter D3 360 mm Trailing edge diameter D4 559 mm Exit vane height B4 26.14 mm

the related report [6] for further details of the tested compressor.

Three types of tapered diffuser vanes, type A, B and C, are designed and utilized for the experiment. Their forms are all three-dimensional and asymmetric as shown in Fig. 2. The diffuser vane height BBT at the leading edge is ex-perimentally decided at 40% which is considered to be the most effective for both the compressor performance and the overall noise level [6]. By attaching the three types of modified vanes on the vaneless diffuser, noise, performance and unsteady pressure measurements are carried out under various conditions.

Fig. 2 Three types of tapered diffuser vanes.

Measuring method At the outset of the noise research, the compressor ro-

tational speed is limited to 6000 r/min. The operation point is set by a butterfly valve installed at the outlet duct and the flow coefficient is changed in the restricted range between 0.14 and 0.28. The volume flow rate can be cal-culated by using an orifice flow meter and a thermo- couple installed at the outlet duct end. The compressor is installed in an anechoic chamber, and the background noise level is sufficiently lower than the compressor noise level.

The sound pressure level of the compressor noise is measured using a B&K 4133 condenser microphone at a location 0.3 m apart from the inlet bellmouth. The pres-sure fluctuation data are acquired at 6 locations on the shroud side of the impeller exit allocated along the im-peller periphery, and 19 locations on the diffuser shroud

surface, as schematically shown in Fig. 1. The arrange-ment of the measuring location contains three rows (in-dicated by A, D and G) in a radial direction, and one line which passes along the mid-pitch of the diffuser vane. Kulite XCQ-062-25A pressure transducers are used mounted flush to the diffuser shroud surface. Each of the analogue signals is transferred to a computer boarded with a 24-bit/133 MHz A/D converter and an FFT ana-lyzer after amplifying with a B&K NEXUS 2690A con-ditioning amplifier.

Moreover, the flow structure in the vicinity of the dif-fuser vane surface is visualized by a colored oil-film method. The oil-film which contains mixed titanium di-oxide with light oil and oleic acid, which is colored by blue and red dyes, is applied on the pressure- and suc-tion-side passage of the vaned diffusers, respectively.

Experimental Results and Discussion

Characteristics of interaction tone noise Tyler and Sofrin [7] investigated the mechanism of the

rotor-stator interaction in an axial flow compressor and showed that the contribution of the ITN to the overall noise level is considered to be much larger than that of the BPF noise. In the case of the tested compressor, therefore, almost all the discrete tone noise is surely gen-erated by the interaction between the impeller-discharge flow and diffuser vanes, and the m number of the pres-sure pattern, the so-called lobe pattern, can be expressed as,

, 1, 2, , 1,0,1, .= + = = −L L Lm nZ kV n k (1) The expression for the smallest value of m in Eq.(1)

gives −1 ( 1n = , 1k = − ), which is interpreted as a 1-lobe pattern, rotating in the opposite direction to that of the impeller, and its speed is Z times the impeller rotational speed. The frequency of the ITN, therefore, coincides with that of the BPF noise.

Power spectra of the compressor-radiated noise meas-ured at the location 0.3 m apart from the inlet bellmouth are shown in Fig. 3. Since the noise spectra radiated from the compressor with 15-vaned, vaneless, types A, B and

Fig. 3 Typical power spectra of compressor noise

292 Journal of Thermal Science, Vol.15, No.4, 2006

C diffusers are very similar to each other and are overlaid on the figure, the peak levels of the ITN are indicated by small arrows. The ITN appears most remarkably in the power spectrum and dominates the overall noise level, as usually expected. The discrepancy level of the ITN be-tween the cases of vaned and vaneless diffusers is more than 40 decibels. By using three types of tapered diffuser vanes, not only the sound pressure level of the ITN but also that of the broadband noise component can be de-creased remarkably.

Effects of tapered diffuser vanes The characteristics of the compressor- radiated noise

and total pressure-rise characteristics are shown in Fig.4 and Fig.5, respectively. The flow and total pressure-rise coefficients used in these figures are defined as follows:

2 2 2 2 22 2 2

,/ 2

TT

PQD B N D N

φ ψπ ρπ

= = . (2)

The effects of tapered diffuser vanes on ITN and over-all noise level are significant. Especially in cases of type A and B with large tapered portions on the vane surface, the noise level of ITN can be attenuated by more than 20 decibels in the designed flow operation. Moreover, the overall noise level is also decreased, so that it indicates a near value compared with the result of vaneless diffuser installation. However, as one can see from the result shown in Fig.5, by setting a large tapered portion on the vane surface (cases of type A and B), the total pres-sure-rise coefficient declines clearly in the low-flow op-eration, and the stable operation range of the compressor becomes narrow. A new style diffuser vane, type C, which is proposed as a means for improving this per-formance decrement, can improve the total pressure-rise characteristics higher than that of the original 15-vaned diffuser installation in all flow ranges between φ =0.14 and 0.28, although the amount of ITN reduction remains at 8.3 dB. In the next step of the experiment, the unsteady pressure fluctuation on the shroud surface of the vaned diffuser is measured in order to investigate the effect of the tapered shape on the flow structure within the diffuser passage.

Pressure recovery characteristics The pressure recovery coefficient calculated from the

results of static pressure measurements along the mid- pitch line 3 on the shroud surface of the diffuser is indi-cated in Fig.6. In the experiment, the performance of the vaned diffuser is evaluated by using the pressure recov-ery coefficient which is defined as follows [8]:

22( ) / .IE ICp P P Vρ= −

E (3) In this expression, the values of the flow velocity IEV

and static pressure IEP measured at the impeller exit

with vaneless diffuser installation are used in order to avoid the influence of the diffuser vanes.

The above results seem to indicate that the pressure recovery coefficients Cp of the tapered vane diffusers type A and B, which have large tapered portions on the vane surface, show much lower values in general than that of the original 15-vaned diffuser in a low-flow re-gion of φ = 0.16. On the other hand, by using the tapered vane type C which has a small tapered portion on the vane surface, a pressure recovery coefficient much higher than that of the original 15-vaned diffuser can be ob-tained. Particularly, the effect is remarkable between the leading edge and mid-chord of the diffuser vane. In the high-flow region, however, the pressure recovery coeffi-cients show much higher values in cases of compressors with vaneless and tapered-vane diffuser installation. This is because the impeller-discharge flow impinges on the forefront part of the diffuser vane and the vane acts only as a resistance passage by increasing the impel-ler-discharge flow angle in the high-flow operation. Fur-thermore, a sharp decline of the static pressure observed in the throat part of the original 15-vaned diffuser can also be suppressed by using tapered diffuser vanes.

Furthermore, even though the static pressure between location F and G, near the trailing edge of the diffuser vane, shows a tendency to decrease in the case of the original 15-vaned diffuser installation as indicated by small arrows in the figure, the decline of the pressure is suppressed and further pressure-rise characteristics can be acquired by using the presented tapered diffuser vane type C. This effect is examined by a flow visualization technique using the oil-film method and the results are indicated in Fig. 7. These pictures show the flow pattern on the shroud surface of the diffuser passage at the maximum efficiency operation (φ =0.24). At the flow field near the trailing edge of the original 15-vaned and type A diffuser vanes, a flow separation as well as curved streamlines indicated by arrows in the figure can be con-firmed on the pressure surface. These secondary flows are considered to be the causes of the pressure drop de-scribed previously and are not observed in fact when the tapered diffuser vanes type B and C are used.

In the case of the low-flow operation (φ =0.16), a blue dye applied on the pressure-side of the diffuser passage passes through the tapered portion and runs into the suc-tion-side as clearly shown in Fig.8. Probably this circula-tion flow is considered to be the cause of the low-pres- sure recovery between locations A and D, which is ob-served in cases of vaneless and tapered-vane diffuser installation. By using the tapered-vane type C, the leak-age flow which passes the tapered portion is much re- duced and the circulation flow can be suppressed signifi-cantly. The pressure recovery coefficient, therefore, indi-cates a marked increase within the diffuser passage

Yutaka OHTA et al. Aerodynamic Performance and Noise Characteristics of a Compressor with Modified Vaned Diffusers 293

Fig. 4 Characteristics of compressor radiated overall and interaction tone noise.

Fig. 5 Total pressure-rise characteristics of modified vaned diffuser.

between A and D, as indicated in Fig. 6. As shown in these figures, the presented tapered dif-

fuser vane type C not only can reduce the noise level of ITN, but can also obtain high-pressure recovery charac-teristics within diffuser passages.

Unsteady pressure fluctuation in diffuser passage Unsteady pressure fluctuation within the diffuser pas-

sage is measured for the preliminary investigation about the generation and propagation mechanisms of the ITN. As an example of measured data, the radial distributions of the pressure waveforms measured at the suction (posi-tion 1) and pressure (position 5) sides of the diffuser vanes are indicated in Fig. 9.

In the results measured at line A of the vane leading edge, pressure fluctuation generated at each blade pas-sage can be clearly confirmed in all the waveforms. However, in the cases of type A, B and C which have

Fig. 6 Pressure recovery coefficient along the streamline within the diffuser passage (mid-pitch line 3).

Fig. 7 Flow visualization of diffuser shroud surface by oil-film method (φ = 0.24).

Fig. 8 Flow visualization of diffuser shroud surface by oil-film method (φ = 0.16).

294 Journal of Thermal Science, Vol.15, No.4, 2006

Fig. 9 Radial distribution of pressure waveforms measured at shroud suface of vane diffuser surface.

tapered portions on the vane surface, the amplitudes of the pressure fluctuation induced by impeller-diffuser in-teraction are remarkably small. This tendency is particu-larly notable in measuring point 5 located on the pressure side of the vane. With respect to the change between line A and D, the fluctuation is attenuated in position 5 and amplified in position 1 to the contrary. Variation of the complicated pressure fluctuation pattern observed in the front part of the diffuser has suggested that the flow within the diffuser passage is very complicated. In the measurement results of tapered vane type A and B, the pressure fluctuations are suppressed and small because line D exists in a tapered part of the diffuser vane.

At line G near the trailing edge of the diffuser vane, pressure fluctuation in the cases of tapered diffuser vanes type A, B and C are attenuated and show a tendency similar to that of the vaneless diffuser installation. In the case of the original 15-vaned diffuser installation, how-ever, the pressure fluctuation level measured at position 5 on the pressure side is still large, and it is shown that the noise is not generated only from the leading edge of the diffuser vanes. Further detailed measurements and/or numerical simulation is necessary to investigate the ef-fects of diffuser vane passages on the compressor- gener-ated noise.

Conclusions

Three types of tapered diffuser vanes which have 3-D asymmetric tapered shapes on their leading edge were designed, manufactured and utilized for the experiment in order to develop a low-noise and highly efficient cen-trifugal compressor with vaned diffusers. The conclu-sions are summarized as follows:

(1) When the leading edge of the diffuser vane is processed into a tapered shape, not only the noise level of the interaction tone but also that of the broadband noise component can be reduced significantly in almost all flow regions. Furthermore, in a high-flow region, a de-cline in the pressure generated in the throat part of the diffuser passage can be suppressed.

(2) The suction-side slitting of a tapered portion is made deep, and also the tapered length is set at less than 30% chord, and the pressure recovery near the diffuser trailing edge can be increased in all flow regions.

(3) The presented tapered diffuser vane type C turned out to be effective not only in noise reduction but also in improvement of the compressor performance.

(4) The results of pressure measurement within the diffuser passage suggest that the discharged flow is much complicated so that the blade passing frequency compo-

Yutaka OHTA et al. Aerodynamic Performance and Noise Characteristics of a Compressor with Modified Vaned Diffusers 295

nents are not generated only from the leading edge of the diffuser vanes.

Acknowledgement

This investigation was supported by a Grant-in-Aid for Scientific Research through grant number 17560162 from Japanese Society for the Promotion of Science.

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