Experimental Study of a Radial Turbine Using Pitch-Controlled Guide Vanes … · 2019. 8. 1. ·...

Hindawi Publishing CorporationInternational Journal of Rotating MachineryVolume 2006, Article ID 17379, Pages 1–7DOI 10.1155/IJRM/2006/17379

Experimental Study of a Radial Turbine Using Pitch-ControlledGuide Vanes for Wave Power Conversion

Manabu Takao,1 Yoshihiro Fujioka,1 Hiroki Homma,1 Tae-Whan Kim,2 and Toshiaki Setoguchi3

1 Department of Mechanical Engineering, Matsue National College of Technology, 14-4 Nishiikuma-cho, Matsue-shi,Shimane 690-8518, Japan

2 Department of Architectural Facility, Doowon Technical College, 678 Janwon-ri, Ansung-si, Kyongki 456-890, Korea3 Department of Mechanical Engineering, Saga University, 1 Honjo-machi, Saga-shi, Saga 840-8502, Japan

Received 13 August 2005; Accepted 2 September 2005

In order to develop a high-performance radial turbine for wave power conversion, a radial turbine with pitch-controlled guidevanes has been proposed and manufactured in the study. The proposed radial turbine has been investigated experimentally bymodel testing under steady and sinusoidal flow conditions. Then, the experimental results have been compared with those of theconventional radial turbine for wave power conversion, that is, a radial turbine with fixed guide vanes. As a result, the runningcharacteristics of the proposed radial turbine under steady and sinusoidal flow conditions were clarified and the effect of diffusersetting angle of guide vane on the turbine characteristics was presented. Furthermore, it seems that the proposed radial turbine ismuch superior to the conventional radial turbine.

Copyright © 2006 Manabu Takao et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. INTRODUCTION

The performance of radial flow turbines having low rota-tional speed and low noise level, which can be used for wavepower conversion using the oscillating water column (OWC)principle, has been studied by a number of authors (Kanekoet al. [1, 2]; McCormick et al. [3]; McCormick and Cochran[4]; Veziroglu [5]). It was found that the efficiency of ra-dial turbines using reaction-type rotor blading was extremelylow (Kaneko et al. [1, 2]). On the other hand, the efficiencyof impulse blading is higher according to the studies (Mc-Cormick et al. [3]; McCormick and Cochran [4]). How-ever, detailed performance characteristics of impulse-typeradial turbines were not found in the literature. In an at-tempt to fill this gap, performance characteristics were mea-sured on turbines (508.8 mm rotor diameter) with differentguide vane geometries by the authors (Setoguchi et al. [6];Takao et al. [7]). Performance was also measured for flowradially inward and outward through the turbine, which ismade possible by an oscillatory flow rig. As a result, it wasclarified by the experiment that the turbine efficiency of im-pulse blading was not so high because there are large differ-ences between the absolute outlet flow angle and setting an-gle of the downstream guide vane, and the downstream guidevane does not work as a diffuser (Setoguchi et al. [6]; Takaoet al. [7]).

In order to overcome the above drawback and enhancethe performance of the radial turbine, the authors have pro-posed a radial turbine with pitch-controlled guide vanesfor wave power conversion. The proposed radial turbinehas been investigated experimentally by model testing understeady and sinusoidal flow conditions. Then, the experimen-tal results have been compared with those of a radial turbinewith fixed guide vanes.

2. EXPERIMENTAL APPARATUS AND PROCEDURE

The test rig consists of a large piston-cylinder, one end ofwhich is followed by a settling chamber as shown in Figure 1.The radial turbine’s axial entry/exit is attached to the settlingchamber as shown in Figure 2. The piston can be driven backand forth inside the cylinder by means of three ball screwsthrough three nuts fixed to the piston. All three screws aredriven by a DC servomotor through chain and sprockets. Acomputer controls this motor and hence the piston velocityto produce any airflows (intermittently for short periods).The test turbine rotor shaft is coupled to the shaft of a ser-vomotor generator through a torque transducer. The motor-generator is electronically controlled such that the turbine-shaft angular velocity is held constant at any set value. Theflow rate through the turbine Q, whether it is inhalation(flow from the atmosphere into the rig) or exhalation (flow

2 International Journal of Rotating Machinery

32

1

7 89

10

411

6 5

PC

12

PC

1 Wind tunnel2 Piston3 Ball screw4 Servomotor5 D/A converter6 Servopack

7 Settling chamber8 Radial turbine9 Torque transducer10 Servomotor generator11 Pressure transducer12 A/D converter

Figure 1: Test apparatus.

PCController

Driver

Duct

Airflow

Shaft

Timingpulley

Settlingchamber

Shroudcasing

Timing beltRotation

Airflow

Steppingmotor

Bearing

To torquetransducer and

servomotorgenerator

DiskInner guide

vane

RotorOuter guide

vane

Airflow

Figure 2: Radial turbine with pitch-controlled guide vanes.

from the rig to the atmosphere), is measured by Pitot tubesurvey. The radial flow velocity vR at mean radius rR in theturbine is calculated from Q = ARvR, where AR is the flowpassage area at mean radius (= 2πrRh). In a typical test,for a particular turbine geometry, the volumetric flow rateQ, pressure difference between settling chamber and atmo-sphere Δp, turbine torque To, and turbine angular velocityω are all recorded. Thereby, data for one flow coefficient φdefined in (3) are obtained. Data for a range of flow coeffi-cients are collected by varying flow rate or turbine angularvelocity. Tests were performed with turbine shaft angular ve-locities ω up to 68.1 rad/s and flow rates Q up to 0.275 m3/s.The Reynolds number based on the blade chord was approx-imately 3× 104 at conditions corresponding to the peak effi-ciency of the turbine. The measurement uncertainty in effi-ciency is about ±1%.

0.5

θdoθno

Airflow

R24.8

67◦

5024.9

Sg3737 θdi

θni

Rotation

24.990◦

R20.750

Airflow

R217.4

Unit: mm

Exhalation

Inhalation

Figure 3: Configuration of turbine.

The radial turbine shown in Figure 2 was tested at a con-stant rotational speed in steady or sinusoidal airflow. Thesign and magnitude of the torque of the motor generatorare servo-controlled so as to hold the turbine speed constanteven if the flow velocity is varying with time. The part ofshroud casing and the part of disk covering the inner guidevane to the exit are flat and parallel to each other. The heightof flow path of the turbine h (gap between the shroud cas-ing and the disk) is 44 mm. The flow passage from inlet toinner guide vane entry has been shaped such that the flowarea is constant along this passage. The turbine system hasguide vanes before and behind the rotor so as to operate ef-ficiently in reciprocating airflow. They are set by pivots onthe shroud casing wall as shown in Figure 2. The pivots arelocated at the end of the guide vane chord close to the rotor.The pitch angles of guide vanes are controlled by the step-ping motors, timing pulleys, and timing belts. Each cascadeof outer and inner guide vanes changes the pitch angle simul-taneously when the airflow direction changes. These guidevanes rotate between nozzle setting angle (upstream side ofthe rotor) and diffuser setting angle (downstream side), thatis, θni and θdi for inner guide vanes, and θno and θdo for outerguide vanes, as shown in Figures 3 and 4, where VR and Tare the maximum velocity and period of sinusoidal airflow,respectively.

The geometries of the guide vane are shown in Figure 3.The guide vane consists of a straight line and circular arc. De-tails of the guide vane are given by chord length of 50 mm;solidity of inner guide vane at rR of 1.15; and solidity ofouter guide vane of 1.16. The nozzle setting angle is only 15◦

for both the airflow directions, that is, θni = θno = 15◦. Inorder to clarify the effect of the diffuser setting angle on theturbine characteristics, the range of θdi is from 20◦ to 60◦

for the inner guide vanes in the case of inhalation, and the

Manabu Takao et al. 3

vR

VR

0

−VR

Exhalation

Inhalation

T/2

T

t

(a)

θo

θdo

θno

t

(b)

θi

θdi

θni

t

(c)

Figure 4: Behavior of pitch-controlled guide vanes. (a) Radial flowvelocity. (b) Pitch angle of outer guide vane. (c) Pitch angle of innerguide vane.

range of θdo is from 30◦ to 90◦ for the outer guide vanes inthe case of exhalation. The geometry of rotor blade is shownin Figure 5 and is the same as that used in previous stud-ies (Setoguchi et al. [6, 8, 9]; Takao et al. [7]). The bladeprofile consists of a circular arc on the pressure side andpart of an ellipse on the suction side. The ellipse has semi-major axis of 125.8 mm and semi-minor axis of 41.4 mm.Detailed information about the blade profile is as follows:chord length of 54 mm; tip clearance of 1 mm; mean radiusof rR = 217.4 mm; blade inlet (or outlet) angle of 60◦; thick-ness ratio of 0.3. The blade is oriented such that the bladeprofile is tangent to a radial line at the maximum thicknesspoint on the suction side which can be seen clearly for thesecond blade from the left in Figure 3.

Sr 16.160 ◦

R0.5R30.2

54

R0.5

60◦ta 125.8

Blade height:43 mm

Solidity at meanradius: 2.02

Number ofblades: 51

ta/Sr = 0.4

41.4

Unit: mm

Figure 5: Rotor blade.

3. EXPERIMENTAL RESULTS AND DISCUSSIONS

3.1. Turbine characteristics under steadyflow conditions

As the first step to an analysis of the new radial turbine,the turbine characteristics under steady flow conditions havebeen clarified in this section. Experimental results on therunning characteristics of the turbine are expressed in termsof the torque coefficient CT , input coefficient CA, and effi-ciency η, which are all plotted against the flow coefficient φ.The various definitions are

CT = To{ρ(vR2 + UR

2)ARrR/2} ,

CA = ΔpQ{ρ(vR2 + UR

2)ARvR/2} = Δp{

ρ(vr2 + UR

2)/2} ,

(1)

where ρ and UR are density of air, and rotational speed atrR, respectively. Efficiency, which is the ratio of shaft poweroutput to pneumatic power input, can be expressed in termsof the coefficients mentioned above:

η = ToωΔpQ

= CTCAφ

. (2)

The flow coefficient is defined as

φ = vRUR

. (3)

Figures 6 and 7 show the effect of diffuser setting angleof the inner guide vane on the turbine characteristics understeady flow conditions. The pitch angle is set at a particu-lar value because tests are performed under steady flow con-ditions in the experiment. In Figure 6(c) the solid line rep-resents the efficiency of the radial turbine with fixed guidevanes which has the optimum setting angle (θi = θo = 25◦)(Setoguchi et al. [6]; Takao et al. [7]). When the flow di-rection is from atmosphere to settling chamber (i.e., inhala-tion) the inner guide vane is downstream of the rotor and itworks as a diffuser. Consequently, the torque coefficient CT is


8

6

4

2

0

−2

CT

0 0.5 1 1.5 2 2.5

φ

θdi = 20◦θdi = 30◦

θdi = 40◦θdi = 50◦

θdi = 60◦

(a)

25

20

15

10

5

0

−5

CA

0 0.5 1 1.5 2 2.5

φ

θdi = 20◦θdi = 30◦

θdi = 40◦θdi = 50◦

θdi = 60◦

(b)

0.5

0.4

0.3

0.2

0.1

0

η

0 0.5 1 1.5 2 2.5

φ

θdi = 20◦θdi = 30◦

θdi = 40◦θdi = 50◦

θdi = 60◦Fixed GV

(c)

Figure 6: Effect of diffuser setting angle of inner guide vane on tur-bine characteristics (inhalation, θno = 15◦). (a) Torque coefficient.(b) Input coefficient. (c) Efficiency.

0.5

0.45

0.4

0.35

0.3

ηp

0 20 40 60 80

θdi (deg)

Figure 7: Effect of diffuser setting angle of inner guide vane on peakefficiency (inhalation, θno = 15◦).

independent of θdi as shown in Figure 6(a), whereas the in-put coefficient CA decreases with increasing θdi for θdi ≤ 40◦in Figure 6(b). Then, CA increases with θdi for θdi ≥ 40◦.Combining the above results and (2), it is evident that thehighest efficiency occurs for the highest value of θdi = 40◦and its value is approximately 0.45 when the flow is “fromatmosphere” (Figures 6(c) and 7). Moreover, it can be ob-served from Figure 6(c) that the efficiency of the proposedradial turbine is higher than that of the conventional radialturbine (i.e., the radial turbine with fixed guide vanes) by0.15.

Conversely, Figures 8 and 9 show the effect of diffusersetting angle of the outer guide vane on the turbine char-acteristics under steady flow conditions. When the flow di-rection is from chamber to atmosphere (i.e., exhalation), theouter guide vane is downstream of the rotor and it worksas a diffuser. Hence, the torque coefficient CT is indepen-dent of θdo as shown in Figure 8(a). Regarding CA-φ char-acteristics, in Figure 8(b) CA decreases slightly with increas-ing θdo for θdo ≤ 60◦ and CA-φ characteristics in the caseof θdo ≥ 60◦ are almost the same. As a result, η increaseswith θdo ≤ 60◦ and remains in a stable situation at around0.33 (Figures 8(c) and 9). Its efficiency is higher than that ofthe conventional radial turbine by 0.03. Moreover, looking atthe efficiency curves in Figures 6(c) and 8(c), it is seen thathigher efficiencies are obtained when the flow is from atmo-sphere.

3.2. Turbine characteristics under sinusoidalflow conditions

Actually, air turbine for wave power conversion is operatedunder unsteady flow conditions since the airflow is generatedby OWC. Therefore, it is inadequate to compare the perfor-mance of the proposed radial turbine with that of the con-ventional radial turbine under steady flow conditions. Here,investigations are made for the turbine characteristics underreciprocating flow conditions in order to clarify the useful-ness of the proposed radial turbine and the effect of pitch-controlled guide vanes on the turbine performance.

When the turbine is in the running condition, the pa-rameters of turbine performance such as To, Q, Δp, and ω


6

4

2

0

−2

CT

0 0.5 1 1.5 2 2.5

φ

θdo = 30◦θdo = 40◦θdo = 50◦

θdo = 60◦θdo = 70◦

θdo = 80◦θdo = 90◦

(a)

12

10

8

6

4

2

0

−2

CA

0 0.5 1 1.5 2 2.5

φ

θdo = 30◦θdo = 40◦θdo = 50◦

θdo = 60◦θdo = 70◦

θdo = 80◦θdo = 90◦

(b)

0.4

0.3

0.2

0.1

0

η

0 0.5 1 1.5 2 2.5

φ

θdo = 30◦θdo = 40◦θdo = 50◦

θdo = 60◦θdo = 70◦θdo = 80◦

θdo = 90◦Fixed GV

(c)

Figure 8: Effect of diffuser setting angle of outer guide vane on tur-bine characteristics (exhalation, θni = 15◦). (a) Torque coefficient.(b) Input coefficient. (c) Efficiency.

0.36

0.34

0.32

0.3

0.28

ηp

20 40 60 80 100

θdo (deg)

Figure 9: Effect of diffuser setting angle of outer guide vane on peakefficiency (exhalation, θni = 15◦).

0.5

0.4

0.3

0.2

0.1

0

η

0 0.5 1 1.5 2 2.5

Φ

θdi = 30◦, θdo = 70◦θdi = 40◦, θdo = 70◦

θdi = 50◦, θdo = 70◦Fixed GV

(a)

0.45

0.425

0.4

0.375

0.35

ηp

20 30 40 50 60

θdi (deg)

(b)

Figure 10: Effect of diffuser setting angle of inner guide vaneon turbine characteristics under sinusoidal flow conditions (θni =θno = 15◦). (a) Mean efficiency. (b) Peak efficiency.

vary periodically in a sinusoidal oscillating flow. In this case,turbine performance should be represented by mean value.The running characteristics are evaluated in the relationshipbetween the mean efficiency η and the flow coefficient Φ


0.5

0.4

0.3

0.2

0.1

0

η

0 0.5 1 1.5 2 2.5

Φ

θdi = 40◦, θdo = 50◦θdi = 40◦, θdo = 60◦θdi = 40◦, θdo = 70◦

θdi = 40◦, θdo = 80◦θdi = 40◦, θdo = 90◦Fixed GV

(a)

0.42

0.41

0.4

0.39

0.38

ηp

40 50 60 70 80 90 100

θdo (deg)

(b)

Figure 11: Effect of diffuser setting angle of outer guide vaneon turbine characteristics under sinusoidal flow conditions (θni =θno = 15◦). (a) Mean efficiency. (b) Peak efficiency.

defined as

η =(1/T)

∫ T

0Toω dt

(1/T)∫ T

0ΔpQdt

,

Φ = VRUR

,

(4)

where η is evaluated over one wave period (see Figure 4) withthe rotor rotating at constant speed and VR is the amplitudeof the radial velocity of sinusoidal airflow.

Figures 10 and 11 show the effect of setting angles of in-ner and outer guide vanes on the mean efficiency under si-nusoidal oscillating flow conditions. As is evident from Fig-ures 10(b) and 11(b), the highest efficiency was obtained incase of the combination of θdi = 40◦ and θdo = 70◦. Thecause of the above result may be understood by consider-ing the turbine characteristics under steady flow conditions

(Figures 6 and 8). Furthermore, the efficiency of the new ra-dial turbine is higher than that of the conventional radial tur-bine by approximately 0.11 as shown in Figures 10(a) and11(a).

Therefore, it has been concluded from the above resultsthat the performance of the radial turbine can be improvedconsiderably by using pitch-controlled guide vanes.

4. CONCLUSIONS

In order to develop a high-performance radial turbinefor wave power conversion, a radial turbine with pitch-controlled guide vanes has been proposed and investigatedexperimentally by model testing. The experiment has beencarried out under steady and sinusoidal flow conditions inthe study. Then, the results have been compared with thoseof a radial turbine with fixed guide vanes.

As a result, the performances of the proposed radial tur-bine under steady and sinusoidal flow conditions have beenclarified. Furthermore, it seems that the proposed radial tur-bine is much superior to the conventional radial turbine, thatis, the radial turbine with fixed guide vanes.

ACKNOWLEDGMENT

The first author wishes to thank the Shimane Industrial Pro-motion Foundation (SIPF) for the financial support whichmade this investigation possible.

REFERENCES

[1] K. Kaneko, T. Setoguchi, and S. Raghunathan, “Self-rectifyingturbine for wave energy conversion,” in Proceedings of 1st In-ternational Offshore Polar Engineering Conference (ISOPE ’91),vol. 1, pp. 385–392, Edinburgh, Scotland, UK, August 1991.

[2] K. Kaneko, T. Setoguchi, and S. Raghunathan, “Self-rectifyingturbines for wave energy conversion,” International Journal ofOffshore and Polar Engineering, vol. 2, no. 3, pp. 238–240, 1992.

[3] M. E. McCormick, J. G. Rehak, and B. D. Williams, “An exper-imental study of a bidirectional radial turbine for pneumaticwave energy conversion,” in Proceedings of Mastering the OceansThrough Technology (OCEANS ’92), vol. 2, pp. 866–870, New-port, RI, USA, October 1992.

[4] M. E. McCormick and B. Cochran, “A performance study ofa bi-directional radial turbine,” in Proceedings of 1st EuropeanWave Energy Conference, pp. 443–448, Edinburgh, Scotland,UK, July 1993.

[5] T. N. Veziroglu, Ed., Alternative Energy Sources VI, Vol. 3,Wind/Ocean/Nuclear/Hydrogen, Hemisphere, Washington, DC,USA, 1985.

[6] T. Setoguchi, S. Santhakumar, M. Takao, T. H. Kim, and K.Kaneko, “A performance study of a radial turbine for wave en-ergy conversion,” Journal of Power and Energy, vol. 216, no. A1,pp. 15–22, 2002.

[7] M. Takao, K. Itakura, T. Setoguchi, T. H. Kim, K. Kaneko, and A.Thakker, “Performance of a radial turbine for wave power con-version,” in Proceedings of the 12th International Offshore andPolar Engineering Conference (ISOPE ’02), vol. 1, pp. 562–567,Kitakyushu, Japan, May 2002.


[8] T. Setoguchi, K. Kaneko, H. Taniyama, H. Maeda, and M. In-oue, “Impulse turbine with self-pitch-controlled guide vanesfor wave power conversion,” International Journal of OffshorePolar Engineering, vol. 6, no. 1, pp. 76–80, 1996.

[9] T. Setoguchi, M. Takao, Y. Kinoue, K. Kaneko, S. Santhakumar,and M. Inoue, “Study on an impulse turbine for wave energyconversion,” International Journal of Offshore Polar Engineering,vol. 10, no. 2, pp. 145–152, 2000.

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Experimental Study of a Radial Turbine Using Pitch-Controlled Guide Vanes … · 2019. 8. 1. ·...

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