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2ndInternational Conference on Energy Systems and Technologies18 21 Feb. 2013, Cairo, Egypt

THE PERFORMANCE OF 0.6M IMPULSE TURBINE

OPERATING UNDER BI-DIRECTIONAL AIRFLOW

Rehil.O. Abdulhadi

Engineering Departments, Sebha University, Libya

This paper presents the performance analysis of 0.6m Impulse turbine operating under

unsteady bi-directional airflow conditions. In the study, two different turbines with 0.6 and0.7 hub to tip ratio (H/T) were tested. In addition, the effect of guide vanes shape 3D and 2Don the performance of 0.6 H/T ratio turbine was carried out. The experiments have beencarried out under irregular unsteady flow conditions based on Irish waves (site2). As a result,it was found that the maximum mean efficiency achieved by 0.6 H/T ratio was higher ascompared to 0.7 H/T ratio by a magnitude of 3 % under random conditions. In addition, themaximum mean efficiency achieved by 3D guide vanes was higher as compared to 2D guidevanes by a magnitude of 1.1 %. The hysteretic characteristic has been noticed from theexperimental results of 0.6 and 0.7 H/T ratio Impulse turbine. Furthermore, the steadyefficiency curve fails to present the rapid rise in efficiency during deceleration, which occursat low flow coefficient for unsteady bi-directional flow. In addition, a comparative analysis

between Impulse turbine and Wells turbine operating under similar condition was reported in

this paper.

Keywords: Fluid machinery; Impulse turbine; airfoil; Real sea condition; Wave powerconversion.

INTRODUCTION

For the last two decades, scientists have been investigating and defining differentmethod for power extraction from wave motion. The most successful and most extensivelystudied device for power extraction energy from ocean waves is the oscillating water columndevice (OWC). The OWC based wave energy power plants convert wave energy into low-

pressure pneumatic power in the form of bi-directional airflow. Self-rectifying air turbines areused to extract mechanical shift power. Two different turbines are currently in use around theworld for wave energy power generation, the Wells turbine, introduced by Dr A.A Wells [4]in1976 and Impulse turbine by some authors Kim [2]; Setoguchi [5] . Both these turbines arecurrently in operation in different power plants in Europe. The research around the world isfocused on improving the performance of both these turbines under different operatingconditions.

The present investigation deals with the Impulse turbine. The Impulse turbine wasinitially designed to operate with self-pitch controlled guide vanes, this type of Impulseturbine has disadvantage of maintenance of pivots on which the guide vanes are rotatedautomatically in a bi-directional air flow. In order to overcome this drawback, an Impulse

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turbine with fixed guide vanes has been proposed by Setoguchi [6]. There are many reportswhich describe the performance of Impulse turbine both at starting and running conditions[3], [5], [7], [8], [9], [10], [11], [12] and [13]. However, up to this point, all the experimentalanalysis was carried out under unidirectional flow conditions. These unidirectional flowconditions are not the flow conditions that the turbine will be operating under the real sea

conditions. In the real sea conditions the flow will be bi-directional and of a random nature.The objective of this paper is to present the performance of Impulse turbine operatingunder bi-directional unsteady flow condition. In the study, A comparative analysis of theImpulse turbine with different hub to tip ratio was carried out to confirm a better value of H/Tratio when the turbine operating under bi-directional unsteady flow conditions. In addition,the effect of guide vane shape 3D and 2D on the performance of 0.6 H/T ratio Impulse turbineoperating under unsteady bi-directional flow was carried out. Furthermore, the behaviour ofthe Impulse turbine operating under bi-directional airflow was investigated. Therefore, acomparison could be made between unidirectional and bi-directional unsteady flow. TheComparative analysis between the self-rectifying turbine Impulse turbine and Wells turbine is

part of this study.

EXPERIMENTAL SET-UP

A schematic layout of the experimental set-up of Wave Energy Research Team atUniversity of Limerick is shown in Fig 1. It consists of a 0.6m turbine test section, bi-directional valve, a plenum chamber with honeycomb section, a calibrated nozzle joining fanto plenum chamber, ductwork, centrifugal fan and two automated actuators (1), (2). The firstactuator (1) controls the flow rate while the second actuator (2) controls the direction of flow.Air is drawn in through the bi-directional valve either through side A or B depending on the

position of automated valve. More details drawing of this bi-directional valve are shown inFig 2. It then passes through the test section and into the plenum chamber. In the chamber theflow is conditioned through a calibrated nozzle and finally exhausting at the fan outlet. Theflow rate is controlled using the automated valve.

( )( )[ ]2//Pr 22 arRa zvblUvQCA +=

Ra Uv /=

Figure. 1. Schematic diagram of test rig

2

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In the study, the turbine test section had an internal diameter of 600mm and twofabricated rotor with a diameter of 598 mm, leaving tip clearance of 1mm. The hubs diameterselected as 358.8 mm and 418.6 mm, providing hub to tip ratio of 0.6 and 0.7 respectively.The chord length of 0.6 and 0.7 H/T ratio turbine were 100 mm and 106 mm respectively. Theguide vanes were mounted on the up-stream and down-stream hubs of the test rig.

A

BExit Path ofFlow

Figure 2. Bi-directional valve (Top removed for clarity)

2.1- Experimental Procedure

The random (Irish Sea Climate) wave inlet conditions to the turbine have been generated

by controlling the opening area of centrifugal fan outlet of test rig using an actuator valve.Initially, the actuator valve was calibrated to find the correlation between the open area of fanoutlet and pressure drop across the nozzle (Pn) located at inlet to the fan. The relationship

between the valve displacement and flow rate through the test section was arrived using thenozzle calibration curve. In order to generate a given regular or random wave, time history ofvalve displacement was calculated using the above mentioned method. The actuator valvecontroller was utilized to operate the valve according to the time history. For this purpose, acomputer program has been written Ryan, [1] and controller was interfaced with a computer.In order to create a bi-directional airflow, one controller controls the two actuators and a limitswitch. The limit switch is positioned such that it closes when the valve that controls airflowrate closes. When this flow control valve closes it passes the limit switch, causing the actuator

(2) to move in reverse direction, thus reversing the direction of airflow through the turbine.The experimental procedure involves fixing the rotor at a set speed and generating therequired flow pattern. Performance parameters were recorder using a high-speed logger that is

part of the Data Acquisition System Ryan, [1]. The overall performance of the turbine wasevaluated by the turbine angular velocity , torque generated T, flow rate Q and total pressuredrop across the rotor Pr. The results are expressed in the form of torque coefficient CT, input

power coefficient CA, efficiency and mean efficiency . The definitions are given below[5]:

Q

T

Pr= (1)

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( )( ) 2// 22 RrRa zrblUvTCT += (2)

( )( )[ ]2//Pr 22 arRa zvblUvQCA += (3)

Ra Uv /= (4)

= )Pr1

/()1

(00

dtQT

dtTT

TT

(5)

where,a : Axial flow velocity : DensityUR : Circumferential velocity atrRrR : Mid span radius

b : Blade heightlr : Chord length of impulse turbine rotor blade

z : Number of impulse turbine blades

RESULTS AND DISCUTION

1- Effect of Hub to Tip ratio

The effect of H/T ration on the mean efficiency of Impulse turbine operating under bi-directional irregular unsteady flow is shown in Figure (3). The curves of mean efficiency for0.6 and 0.7 H/T ratio are quite similar in trend, but the turbine with 0.6 H/T is giving highermean efficiency in the range of flow coefficient greater than 0.96. The peak efficiency of 0.6H/T was 41.9 % as compared to 38.97 % achieved for 0.7 H/T ratio turbine.

0

5

10

15

20

25

30

35

40

45

0 0.5 1 1.5 2 2.5

1/1/1/1/

meanefficiency(%)

0.6 H/T

0.7 H/T

Figure 3. Effect of H/T ratio on the mean efficiency of impulse turbine under site 2 conditions

In order to analysis the difference in behavior of Impulse turbine with 0.6 and 0.7 H/Tratio operating under unsteady bi-directional flow, instantaneous efficiency, torque coefficientand input coefficient have been plotted for a number of periods of site2 conditions at flow

coefficient of 1/ = 1.54. The non-dimensional term1/represents the flow coefficient

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for ordinary fluid machines. The parameter includes characteristic parameters of theirregular waves (Hsand Ts),turbine speed () and dimensions of the turbine and air chamber(rt and m). Figures (4a, b and c) show the variations of instantaneous efficiency, torquecoefficient and input coefficient at flow coefficient 1/=1.54. The turbine characteristics of0.7 H/T ratio under steady flow conditions have been plotted for comparison. The Figures

show the instantaneous efficiency, torque coefficient and input coefficient for 0.6 H/T and 0.7H/T follow two different paths during acceleration of inlet flow from zero to maximumvelocity and deceleration from maximum to zero velocity. Also it forms a counter clockwisehysteretic loop per half period of wave.

-150

-100

-50

0

50

100

0 0.5 1 1.5 2 2.5 3

InstEfficicncy

(%)

0.7-steady

0.7-site2

0.6-site2

(a)

-0.5

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2 2.5 3

CT

0.7-site2

0.6-site2

(b)

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0

0.5

1

1.5

2

2.5

3

3.5

0 0.5 1 1.5 2 2.5 3

CA

0.7-site2

0.6-site2

(c)

Figure 4. Variation 0.6 H/T and 0.7 H/T ratio of impulse turbine at value =1.54: (a) Efficiency,(b) Torque coefficient, (c) Input coefficient

Furthermore, during the acceleration the efficiency curve 0.7 H/T ratio obtained fromunsteady flow follow a similar trend to those obtained during steady flow conditions.However, the steady efficiency curve fails to present the rapid rise in efficiency duringdeceleration, which occurs at low flow coefficient for unsteady bi-directional flow. Inaddition, It can be observed from Figure (4a) that during the acceleration and deceleration ofthe inlet flow, the instantaneous efficiency of 0.6 H/T ratio is higher as compared to 0.7 H/Tratio in the range of flow coefficient greater than 0.71. In the lower flow coefficient range