2-2008

PROBLEMY EKSPLOATACJI

239

Paweł WIRKOWSKI Polish Naval Academy, Gdynia

GAS TURBINE ENGINE WORK PARAMETERS DURING INCORRECT SETTING OF AXIAL COMPRESSOR VARIABLE STATOR VANES

Key words Gas turbine engine, axial compressor, variable stator vanes. Summary The paper deals with the problem concerning the influence of changes in the variable stator vanes axial compressor settings of a gas turbine engine on work parameters of compressor and engine. Incorrect operation of the change setting system of variable vanes could make the work of compressor and engine unstable. This paper presents a theoretical analysis of the situation described above and presents the results of research done on a real engine. Next, the results of mathematical modelling of the changes in gas turbine engine parameters during change of angle setting of axial compressor variable stator vanes are presented. Introduction When in compressor construction is assembled, the system of setting the change of variable stator vanes is made optimal in cooperation with engine units during the permanent improvement of compressor characteristic. Perturbations in the operation of this system could cause changes in the work of compressor and engine similar to changes caused by changes of rotational speed or polluted inter-blade ducts of the compressor. Fig. 1 presents the general characteristics of

240

PROBLEMY EKSPLOATACJI

2-2008

a gas turbine axial compressor where the change of variable stator vanes’ angle settings is taken into consideration. 0,8

αKW = + 10o

πS

αKW = 0o

0,7

αKW = - 10o

0,6

0,5

n2 = idem 0,4

0,3

n1 = idem 0,2 0,2

0,3

Wielom. αKW = (Serie7) - 10o

0,4

0,5

m& m o Wielom. αKW = 0(Serie8)

0,6

0,7

0,8

Wielom. (Serie9) α =+ 10o KW

Fig. 1. General characteristic of axial compressor equipped in variable stator vanes: n1, n2 – rotational velocity of compressor rotor, αKW – variable stator vanes angle setting; πS – relative compression of compressor, m& – relative air mass flow

The compressor stage unitary work on the radius is defined based on the equitation of angular momentum and has the following form: (1) lst = ωr(c2u – c1u) = u∆cu = u∆wu where: ω – angular velocity, u – tangential velocity, r – rotor radius, c1u, c2u – circumferential components of air stream absolute velocity on inlet and outlet rotor blades on radius r, ∆cu, ∆wu – air stream whirl in rotor. Work is constant on the whole depth of the rotor blade. The sum of work is the unitary work of stage. The involved change of the variable stator vanes’ angle setting, when kept at a constant level of rotational velocity (constant u), causes a change of the air stream inlet angle in rotor vane β 1 (Fig. 2) [2, 5]. It causes a change in the axial component of the air stream’s absolute velocity on

2-2008

PROBLEMY EKSPLOATACJI

241

inlet c1a, which is equivalent to the change of air mass flow m& and a change in the air stream whirl ∆wu in the rotor. It influences efficiency and work of stage. a)

b)

c) k

α1

a1’

w1” w1

a1’’

c1a”

w1’ c1a’ u

c1a

β1 w

Fig. 2. The essence of control of the compressor’s axial stage by changing the setting of the angle of the stator vanes’ ring at changeable air flow velocity: a) decreased axial velocity, b) calculation axial velocity, c) increased axial velocity, k – variable stator vanes ring, w – rotor vanes ring [1]

The purpose of investigations made on a real engine was to determine the influence of an incorrect operation of the axial compressor inlet guide variable stator vanes’ control system of a gas turbine engine on compressor and engine work parameters. The compressor characteristics are indicated in the relationship between & and compression ratio π∗S, compressor efficiency ηS and air flow mass m compressor rotational velocity n. This makes it possible to determine the best conditions of mating the compressor and another engine units. The characteristics are used to select the optimal conditions of airflow regulation and to make an assessment of operational factors on compressor parameters. Therefore, the compressor should be controlled in the operational range of rotational velocity, so that the mating of the compressor and engine mating has a standard of stable work. The main rule of compressor control during a change of their rotational velocity or flow intensity is to keep up the stream inlet angles i value near zero. One of the most popular ways of axial compressor control is by changing their flow duct geometry by application of inlet guide stator vanes or variable stator vanes of several first compressor stages [1]. This solution makes it possible to change the air stream inlet angle on the rotor blades of compressor stages by changing the stator vanes setting angles during a change of compressor rotational velocity. Fig. 2 illustrates, as an

242

PROBLEMY EKSPLOATACJI

2-2008

example, one stage of compression and a rule for the regulation of variable stator vanes [1]. For average values of the operational range of the compressor rotor speed, as shown in Fig. 2b, the speed values and directions are indicated with subscript 1. This situation is the intermediate angle setting of the stator vanes. The air stream inlet angle on the rotor blades does not cause a disturbance of stream flow by the inter-blades ducts. For lower values of compressor rotor speed and, in consequence lower values of absolute axial component velocity c1a’, it is necessary to reduce the stream outlet angle of the variable stator vanes α1 (Fig. 2a). The angle reduction range should allow the keeping of the same value of the stream inlet angle on the rotor blades. A similar situation takes place during the work of the compressor with a higher rotational speed. For higher rotational speeds, the absolute axial component speed c1a’’ increases. In this situation, for keeping the work of compressor stable and, in consequence, constant value of stream inlet angle on rotor blades, it is necessary to increase the stream outlet angle of the variable stator vanes (Fig. 2c). Application in gas turbine engine construction of a control system of the flow ducts geometry has a bearing on the running of unstable processes [3].

1. Object of researches The object of researches is type DR 77 marine gas turbine engine, which is part of power transmission system of a war ship [4]. It is three-shaft engine with a can-ring-type combustion chamber and reversible power turbine. Fig. 3 illustrates a block diagram of a DR gas turbine engine with marked control sections of the flow duct and measuring parameters. In compressor construction, the configuration of this engine uses inlet guide stator vanes, which make it possible to change the setting angle incidences (change of compressor flow duct geometry) dependent on engine load. This process is operated by a control system in which the working medium is compressed air received from the last stage of the high-pressure compressor [5–7]. Fig. 4 presents a block diagram of the flow control signal of the system of variable stator vanes. Compressed air from the last stage of the high-pressure compressor is supplied to the working space of the control actuator by the cleaning and cooling block. Compressed air exerts pressure on control actuator elements. It causes movement of the control piston, which is connected with a moving ring. This ring moves on the circumference of compressor body. The ring is connected to the stator vanes by levers. When the ring is moving, the stator vanes accomplish rotational motion changing the air stream outlet angle α1 (Fig. 2).

2-2008

PROBLEMY EKSPLOATACJI

243

Fig. 3. Block diagram of DR type gas turbine engine: LPC - low pressure compressor, HPC - high pessure compressor, CO - combustor, HPT - high pressure turbine, LPT - low pressure turbine, PT – power turbine

CO

HPC

LPC

CLEANING AND COOLING BLOCK

BLEED CONTROL ACTUATOR

MOVING RING ŁK ŁK ŁK ŁK 1 2 … z

Fig. 4. Block diagram of the change of stator vanes’ setting mechanism; CO – combustor, HPC – high-pressure compressor, LPC – low-pressure compressor, ŁK – variable stator vane

In the cleaning and cooling block are holes. During the research air stream was bled away by holes and less air was supplied to actuator. It caused the change of the setting angle αKW of the variable stator vanes. In consequence, flow duct geometry was changed. Experiment was carried out on an engine load of 0,5Pnom, taking into consideration atmospheric conditions. For this load setting, the angle of the variable vanes has a of value αKW =- 4o. During change in the engine load, from idle to full load, the setting angle αKW of the variable vanes changes in the range from -18o to + 18o. During the experiment, a few parameters of engine work was measured and registered for three different setting angle αKW of variable vanes:

244

PROBLEMY EKSPLOATACJI

2-2008

A–– αKW = - 4o, B–– αKW = - 11o, C–– αKW = - 18o. Table 1 presents the measured and registered parameters of engine work. Table 1. Parameters of engine DR work measured during researches Parameter nLPC nHPC nPT p1 p21 p2 pp T1 T42

Measurement range -1

0÷20000 [min ] 0÷22000 [min-1] 0÷10000 [min-1] -0,04÷0 [MPa] 0÷0,6 [MPa] 0÷1,6 [MPa] 0÷10,0 [MPa] -203÷453 [K] 273÷1273 [K]

Parameter name Low pressure rotor speed High pressure rotor speed Power turbine rotor speed Sub-atmospheric pressure on compressor inlet Air pressure on low pressure compressor outlet Air pressure on high pressure compressor outlet Fuel pressure before injectors Air temperature on compressor inlet Exhaust gases temperature on inlet power turbine

2. Results of research Figure 5 presents the results of the experiment. Presented are those parameters which are the most sensitive to the change of vanes’ setting angle. Changing the setting of the vanes from position A to position C caused an increase in airflow resistance by the stator vanes. In consequence, subatmospheric pressure on compressor inlet p1 decreases (Fig. 5c). It causes a pressure decrease in the next parts of the compressor and engine flow duct (Fig. 5de). In this way, reduced air density flowing by compressor, for stable quantity of stream flow supplied to combustion, causes an increase in the rotor speed of compressors. The most visible is the increase of the rotor of the low-pressure compressor (Fig. 5a), caused by the direct influence on this compressor incorrectly setting the variable stator vanes. The range of change of this parameter is above 2% the value of rotational speed for an undisturbed angle setting of the vanes. The gas-dynamic connection between the low-pressure compressor and high-pressure compressor absorbs the disturbances of the work of the lowpressure compressor, which are transferred to the high-pressure compressor. Therefore, range of change of the speed of the high-pressure compressor rotor (Fig. 5b) is lower than low-pressure compressor. In this experiment, it is below 1% and it is in the measuring error of sensor range. The change of sub-atmospheric pressure is above 5% of the undisturbed value of this parameter. Changes of low- and high-pressure compressor outlet pressure are adequately above 1,3% and above 2,4% of the undisturbed value of angle setting αKW = - 4o.

2-2008

PROBLEMY EKSPLOATACJI

245

Changes of the pressure and air mass flow intensity values accompanied disturbances in the work of compressor, during constant fuel mass flow intensity in the combustion chamber, which caused an enrichment of fuel mixture. As a result, the temperature of the outlet gases of the combustion chamber increases (Fig. 5f). The experiment confirmed the tendency of changes of the values of gas temperatures, even though the range of those changes is in measuring error of sensor range. Based on the results of the experiment, the mathematical equations modelling changes of particular engine work parameters as a function as a of variable inlet guide stator vanes setting angles αKW were determined as follows: n SNC = 0,7449αKW 2 + 2,602αKW + 9234,5 [obr/min]

(2)

n SWC = 0,0204αKW 2 – 1,1224αKW + 12598 [obr/min]

(3)

p1 = -10-7αKW 2 – 10-7αKW + 0,00077 [MPa]

(4)

p21 = 0,00029αKW + 0,29814 [MPa]

(5)

p2 = 0,00143αKW + 0,81771 [MPa]

(6)

T42 = 0,0204αKW 2 + 0,1633αKW + 799,48 [K]

(7)

Figure 6 presents the results of the mathematical modelling of those same engine work parameters as in Fig. 5. Modelling was carried out as steady engine load, which is equivalent to unchanging fuel mass flow. In this case, the range of change of variable inlet guide stator vanes’ setting angle αKW was widened from -18o to +18o. Research in the range αKW from -4o to +18o was not possible on a real engine, due to the technical restrictions on the engine. A change of variable inlet guide stator vanes’ setting angle αKW from -4o to o +18 caused an increase in the stream outlet angle of variable stator guide vanes α1 (Fig. 2). It decreases the drag on the airflow on the low-pressure compressor inlet that caused a decrease of the sub-atmospheric pressure (Fig. 6c). During constant engine load (constant fuel mass flow), the absolute axial component velocity c1a increases. It exerts an influence on the increase of the air mass flow m& . Simultaneously, the absolute axial component velocity c1a increase caused a decrease of the air stream whirl in rotor ∆wu. The effect of above is a reduction of the compressor stage unitary work – equation (1). In consequence of the low pressure, the compressor rotor speed increases (Fig. 6a). In connection with a decrease of the sub-atmospheric pressure, it caused an increase in air pressure on the low-pressure outlet compressor (Fig. 6d). In spite of a slight decrease of

246

PROBLEMY EKSPLOATACJI

2-2008

low pressure compressor rotor speed high pressure compressor rotor speed a) 12650 of the variable inlet guide stator vanes Fig.9500 5. Change of engine work parameters as a b) function _ [obr/min]

9400

B

9300

12600

SNC

[obr/min]

_

o o o setting C angle: A –– αKW = - 4 , B –– αKW = - 11 , C –– αKW = - 18

n

SWC

A

n

9200

9100

12550

-20 -18 -16 -14 -12 -10 -8

-6

-4

-2

angle setting of controllable vanes α KW [o]

0

subatmospheric pressure on compressor inlet

c)

-20 -18 -16 -14 -12 -10 -8

-4

-2

0

air pressure on low pressure outlet compressor

d)

0,3

_

0,0008

-6

angle setting of controllable vanes α KW [o]

0,298

0,00076

[MPa]

0,00074

p 21

p 1 [MPa]

0,00078

0,296 0,294 0,292

0,00072

0,0007

0,29 -20 -18 -16 -14 -12 -10 -8 -6 -4 angle setting of controllable vanes αKW [o]

e)

-2

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 angle setting of controllable vanes αKW [o ]

0

air pressure on high pressure outlet compressor

0,85

f)f)

0

exhaust gas temperature on power turbine inlet

805

T42 [K] _

p2 [MPa]

_

803

0,8

801 799 797

0,75 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 angle setting of controllable vanes α KW [o]

0

795 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2

0

angle setting of controllable vanes α KW [o]

2-2008

PROBLEMY EKSPLOATACJI

low pressure compressor rotor speed

_

_ [obr/min]

b)12650

[obr/min]

a) 9600 9500 9400

247

high pressure compressor rotor speed

12600

n

SWC

SNC

9300

n

9200 9100

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20

12550

angle setting of controllable vanes α KW [ ] o

c) 0,0008

subatmospheric pressure on compressor inlet

angle setting of controllable vanes α KW [o]

d)0,304

air pressure on low pressure outlet compressor

0,302

p 1 [MPa] _

p21 [MPa] _

0,00078

0,00076

0,3

0,298 0,296

0,00074

0,294 0,00072 0,292 0,0007

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20

0,29 angle setting of controllable vanes αKW [o]

e)0,85

air pressure on high pressure outlet compressor

angle setting of controllable vanes αKW [o]

exhaust gas temperature on power turbine inlet

f) 809 807

T42 [K] _

p2 [MPa] _

805

0,8

803 801 799 797

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 angle setting of controllable vanes αKW [o]

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20

795 0,75

angle setting of controllable vanes αKW [o]

Fig. 6. Change of engine work parameters as a function of the setting angle of variable inlet guide stator vanes during mathematical simulation

248

PROBLEMY EKSPLOATACJI

2-2008

the high-pressure compressor rotor speed (Fig. 6b), the increase of air pressure on the low-pressure outlet compressor involves an increases of air pressure on the high-pressure outlet compressor (Fig. 6e). This slight decrease of the speed of the high-pressure compressor rotor caused an increase in the drag on gas flow in the next stage of the gas turbine engine for combustion. The effect of the above is a slight increase in the exhaust gas temperature on the power turbine inlet. There is a noticeable fact that the parameters connected with the lowpressure rotor react more to change in the variable inlet guide stator vanes’ setting angle αKW than in the parameters connected with the high-pressure rotor.

Conclusions Based on theoretical consideration and experimental research, we can draw a conclusion that the incorrect operation of the control system of inlet guide variable stator vanes or the first stage stators vanes gas turbine engine compressor exerts a negative influence on compressor work and engine performances. Multi-shaft construction of a gas turbine engine reduces the effects of incorrectly setting the variable vanes. Therefore, compressors of three-shaft gas turbine engines do not require variable stator vanes for as many stages as the compressor of two-shaft engines with the same achievements. Preliminary research confirms the necessity for making an inspection of the correct operation of the system control of variable stator vanes. This makes possible to eliminate this factor from group of factors concerning the technical state of engines, which are identified during diagnostic inspections.

References 1. Dżygadło Z. et al.: Rotor unit sof gas turbine engines. Transport and Telecommunication Publishing House (WKiŁ), Warszawa, 1982 (in Polish). 2. Marschal D.J., Muir D.E., Saravanamuttoo H.I.H.: Health monitoring of variable geometry gas turbines for the Canadian navy. The American Society of Mechanical Engineers, 345 E, 47 St., New York, N.Y.10017. 3. Korczewski Z.: Identification of gas-dynamic processes in compressor unit of marine gas turbine engine for diagnostics purposes. Polish Naval Academy. Ass. Prof. qualifying dissertation, Gdynia, 1998 (in Polish). 4. Charchalis A.: Diagnostics of marine gas turbine engines. Polish Naval Academy, Gdynia, 1991 (in Polish). 5. Korczewski Z.: Wirkowski, P.: Modeling gasodynamic processes within turbine engines’ compressors eqipped with variable geometry of flow duct.

2-2008

PROBLEMY EKSPLOATACJI

249

IV International Scientifically-Technical Conference “Explo-Diesel & Gas Turbine ‘05”, Gdańsk-Międzyzdroje-Kopenhaga, Gdańsk University of Technology, Gdańsk, 2005, 227–236. 6. Wirkowski P.: Modeling the characteristics of axial compressor of variable flow passage geometry, working in the gas turbine engin system. Polish Maritime Researsch, Gdańsk University of Technology, Gdańsk, 2007, 3, 27–32. 7. Wirkowski P.: Gas turbine engine work parametrs in conditions of changeable geometry of axial compressor flow duct. 11th Intern. Conf. „Computer systems aided science, industry and transport” – TRANSCOMP, Zakopane, 2007. Published by Radom University of Technology, Radom 2007, 383–388. Reviewer: Marek ORKISZ

Parametry pracy silnika turbinowego w warunkach nieprawidłowego ustawienia regulowanych łopatek kierownicy sprężarki osiowej Słowa kluczowe Silnik turbinowy, sprężarka osiowa, regulowane łopatki kierownicy. Streszczenie W artykule przedstawiono wpływ zmian ustawienia regulowanych łopatek kierownicy sprężarki osiowej silnika turbinowego na parametry pracy sprężarki oraz całego silnika. Nieprawidłowe funkcjonowanie systemu zmiany ustawienia regulowanych łopatek może powodować niestabilną pracę sprężarki przenoszoną na konstrukcję silnika. Sytuacja taka jest niepożądana ze względu na obciążenia mechaniczne, które w skrajnych przypadkach mogą doprowadzić do uszkodzenia silnika. W artykule zaprezentowana została analiza teoretyczna powyższego zjawiska oraz przedstawiono wyniki badań przeprowadzonych na rzeczywistym obiekcie. Następnie na bazie przeprowadzonych badań określone zostały równania matematyczne opisujące zależności pomiędzy wartościami rozpatrywanych parametrów pracy silnika a kątem ustawienia regulowanych łopatek. Równania te posłużyły do określenia zmian wartości parametrów dla zakresu zmian kąta regulowanych łopatek nieosiągalnego podczas badań na obiekcie rzeczywistym.

250

PROBLEMY EKSPLOATACJI

2-2008