Temperature Rise in the Multistage Axial Flow Compressor During Rotating Stall and Surge

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1 THE AMERCAN SOCETY OF MECHANCAL ENGNEERS 345 E. 47 St., Ne York, N.Y GT-323 The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published in an ASME Journal. Papers are available from ASME for fifteen months after the meeting. Printed in USA. Copyright 1988 by ASME Temperature Rise in the Multistage Axial Flo Compressor During Rotating Stall and Surge KATERNA NACOVSKA CKD Kompresory 19 2 Praha 9 Czechoslovakia ABSTRACT An experimental investigation of rotating stall and surge as carried out on a four stage axial flo compressor. Results of flo and blade temperature measurements in the compressor are presented. nternal temperature levels during rotating stall and surge are considerably higher than those obtained during un stalled compressor operation. n the pure rotating stall regime, the temperature is almost identical in all compressor stages and depends only on rotor speed and mass flo rate. During surge, the highest temperature is found at the tip diameter prior to the first stage rotor. The absolute level depends on rotor speed, mass flo rate (i. e. throttle position) and on the number of compressor stages. A model of the temperature changes in the multistage compressor during the surge cycle has been derived from the experiments. NOMENCLATURE c annulus average axial velocity ax n rotational speed n stall cell relative speed of propagation P static pressure P total pressure c 6P static pressure rise across the compressor: 6 P = P - P. Q inlet volume flo Pate 1 s stall cell blockage T temperature of flo T temperature of compressor blade L 6 T temperature rise across the compressor: 6 T = T - T e i u rotor velocity at midspan W hot ire anemometer velocity density Q subscripts: i e SL LSL r E NTRODUCTON axial c velocity parameter ti'\ _ ax _ Q '+' - --U- - const. u pressure rise coefficient ljj = at compressor inlet at compressor exit uniform flo stability limit point (on unstalled part of compressor character is tic) lo stability limit point (on stalled part of compressor characteristic) transient equilibrium point p e u When the mass flo trough an axial compressor is reduced from the design point, the steady axisymmetric flo becomes unstable. The phenomenon resulting from this instability can be either surge or pure rotating stall. Which kind of instability ill occur depends on the compression system (i. e. on th compressor and its circuit) and on the rotational speed. Rotating stall or surge arising in an axial flo compressor markedly affect compressor performance and life expectancy. Unstable compressor operation often leads to high internal temperatures and serious blade stresses. On the basis of theoretical and experimental ork reported in references (Day et al., 1978) and (Greitzer, 1976), an extesive experimental investigation using a four-stage axial flo compressor as carried out (Feierfeil et al, 198). Rotating stall and surge as experienced during hich special attention as given to flo temperatures ithin the compressor, blade temperatures, blade Presented at the Gas Turbine and Aeroengine Congress Amsterdam, The Netherlands-June 6-9, 1988 Donloaded From: on 1/18/218 Terms of Use:

2 stresses, rotor blade clearances and rotor vibrations. This paper describes the result of flo and blade metal temperature measurements and finally the model is shon to provide an adequate qualitative description of the temperature changes that occur in the flo path of the multistage compressor during the surge cycle. EXPERMENTAL FACLTY The experimental program as carried out on a four-stage axial flo compressor facility (Fig. 1). The compressor as driven by a 25 kw electric motor and the speed could be continuously varied up to the maximum of 81 rpm. Mass flo rate at maximum speed of 81 rpm as 5 kg/h. --- ill AC B - : i,.----' T; Pi P, l, GEAR MOTOR sed circuit the compressor inlet pressure could be reduced up tu 1 Pa. By reducing the inlet pressure it as possible to decrease the blade stresses so that the surge investigation as not limited by inadmissibly high stress. Both steady-state and high frequency response instrumentations ere used for mass flo and pressure rise measurement. The measurement planes,,, V,V and V in the compressor duct are indicated in Fig. 1. The planes of rotor relative vibration (A and B) and rotor blade clearances (C) are also indicated. The measuring stations in the flo path of the compressor are shon in Fig. 2. For rotating stall diagnosis, to bot ire anemometers and to high response total pressure transducers ere installed at measuring stations and 3 (Fig. 2). The blade metal temperatures of to stator blades in every blade ro including the inlt guide vanes ere measured by standard thermocouples. The flo temperature measurements ere taken beteen all blade ros using Ni - NiCr thermocouple probes of special design (Fig. 3) ith l,.><j- 'iz[ Fig. 1 Compressor test facility The blading consisted of four stages of the same type ith a constant 55 mm outside diameter as can be seen in Fig. 2. Hub-tip ratio as. 67 in the first stage and increa- Fig. 2 MEASURNG STATONS 1R 2R 3R 3 To T1R 11 12R 12 T 3R T 3 Wo P ea L BLAOC TEMPERAT URE FLOW TEMPERATURE W HOT WRE ANEMOMETER VELOCTY P c TOT AL PRESSURE 2 Measuring stations in four stage compressor sed gradually up to. 81 in the last stage. Design rotor velocity at midspan as 185 m/s and the maximum rotor velocity corresponding to the rotational speed of 81 rpm as m/s. Design flo coefficient of the stages a. 83 and temperature rise of a stage as 2 C. The profiles of rotor and stator blades ere NACA 65 series ith a circular arc camber line. The blading as designed in terms of the free vortex la. The compressor as tested both in an open and in a closed circuit. They both are typical high volume industry compression system configurations and are shon in Fig. 1. n the clo- 14 Fig. 3 Thermocouple probe of special design and its typical transient response. Mach number M =. 29 thin ires of.1 mm diameter laser elded. The number of the probes at measuring stations and their spanise location ere varied during investigation. A time constant of the thermocouple as.2 to. 4 s and as both calculated and experimentally determined for various flo speeds and temperature differences. A typical thermocouple transient response is also shon in Fig. 3. EXPERMENTAL RESULTS The four stage compressor steady-state characteristic shon in Fig. 4 is presented in terms of a pressure rise coefficient here P is static pressure rise across the compressor, as a function of axial flo velocity parameter (nondimensional mass flo parameter) Donloaded From: on 1/18/218 Terms of Use: 2

3 Fig cp = = u " n =225 RPM Q const. u 3 STABLTY LMT LNE 35 {', D 7 = 81 L,,.- ", UNSTALLED PART OF SL C"'ARACTERSTC ; -,, \,,,,,,v,,, 225 RPM 24 ;RT OF CHARACTERSTC. PURE ROTA]rlNG STALL REGME),4,6,8 Four stage compressor steady-state characteristic This ay of expressing the compressor characteristic has been taken over from (Day et al., 1978) and Greitzer, (1976) and is suitable for incompressible flo hose characteristic \j) = f ( cj)) does not depend on the rotor speed. The tested four stage compressor at rotational speeds above 3 rpm shos the effect of compressibility and the unstalled branch of the characteristic shifts toards higher values of and <D The unstalled part of the characteristic measured at 225 rpm and 81 rpm can be seen in Fig. 4. For the investigations reported here, the points laying at the stall limit line (i. e. at the uniform flo stability limit line) are only important in unstalled flo region and are shon in Fig. 4 for various rotatinal speeds. The temperature rise across the compressor at the stability limit points is presented in Fig. 5. Fig f-. U.J <lz _J f:::1 a: _J n:: 2 5- ri ;i m Q<J: tli t f-. f-. 2 <i: ROTATONAL SPEED [RPM] Variation of temperature rise across the four stage compressor at the stability limit line for various rotational speeds f the stability limit line is surpassed by throttling then at <t> < <PsL the flo becomes unstable. The result of this instability is either surge or pure rotating stall. When the test compressor as operating in the open circuit the surge appeared at the speeds higher than 25 rpm. At loer speeds it orked in the pure rotating stall regime at the point of the lo stability limit (LSL). Then, by further throttling other points of the stalled branch of the characteristic could be meassured. 25 rpm is the surge/rotating stall boundary speed for the compression system under investigation (i. e. for the test four stage compressor and its open circuit as seen in Fig. 1). When the test four stage compressor as operating in the closed circuit (Fig. 1) the surge/rotating stall boundary speed as higher, approximately 43 rpm. n the pure rotating stall region measurements of the stalled part of characteristic could be undertaken ithin a ider range of speeds, as shon in Fig. 4. The mode of surge arising in the compression system, and its properties (i. e. frequency and transient response) depend on the compressor, its circuit, rotational speed, and the position of the equilibrium point that is determined by throttle setting. Deep surge as the primary mode of surge observed over the hole range of surging operation of both compression systems. Claseic surge appeared occasionally. Fig. 6 REVERSE FLOW ROTATNG STALL CLASSC SURGE " TMN steady-state equilibrium point steady-state axial velocity parameter determined by throttle setting transient axial velocity parameter Schematic diagram of transient compression system behavior 3 Donloaded From: on 1/18/218 Terms of Use:

4 The classic and deep surge ere defined in (Greitzer, 1976) and schematically are shon in Fig. 6. During classic surge the compressor operates in rotating stall over part of the cycle and unstalled over the rest. Deep surge cycles consist of the folloing parts: rotating stall, reverse flo, rotating stall and unstalled flo. Pressure variation during classic surge can be seen in Fig. 7 and during deep surge in Fig RPM <P =,41 peratures and small clearances. The aim of the research ork described in this paper as to find out hat ere the temperatures in the flo path of multistage compressor during surge.,8,-.4:, J L t -*=--=== r- 35 RPM 3RPM ' 225RPM 1 * Fig. 7 Pressure and anemometer velocity variation during classic surge 5 RPM SURGE CYCLE PEROD REVERSE - ;:-_:_Lo l' NSTALLED FLOW, - 1\ ROTATNG STALL t =.4,15..., - Fig. 9 s 1, oi The rotating stall alays constitutes a part of the surge cycle. For this reason the temperatures during pure rotating stall ere examined first. The pure rotating stall observed at the points of the stalled part of compressor characteristic at different axial velocity parameters (.5 to.39) and different speeds (225 to 4 rpm) as total single-celled stall. The speed of propagation as at 4 percent of rotor speed approximately constant for all speeds and axial velocity parameters as seen in Fig. 9. The stall cell increased in size ith decreasing axial velocity parameter (Fig. 1) and the stall cell blon.5- l.1 - i j l, l J,2.3 t,4 Variation of stall cell relative speed ith axial velocity parameter,- -- l-- --; -35 RPM 3 RPM RPM' OM-,2L L,1,2 _ l,.3,4 Fig. 1 Variation of stall cell blockage ith axial velocity parameter Fig. 8 TME Pressure and temperature variation during deep surge The range of the surge investigation is usually limited by high blade stresses, high temperatures and lo rotor blade clearances. n the open circuit surge measurements could only be performed up to 54 rpm due to blade stresses. n the closed circuit ith loer inlet pressure surge investigations ere made up to 81 rpm but at speeds of 6 to 81 rpm measurements could only be undertaken near the lo stability limit point due to high tem- ckage as approximatelly the same at both the section and 3. No measurements ere made to determine local flo direction and magnitude inside the stall cell. Temperature rise in compressor during pure rotating stall Rapid changes in flo temperature are experienced during compressor rotating stall. Special thermocouples (Fig. 3) located beteen blade ros of the four stage test compressor ere used to indicate these flo temperature fluctuations. The thermocouple signal during a typical rotating stall is presented in Fig. 11. The other trace is total pressure variation. The flo temperature fluctuation frequency 4 Donloaded From: on 1/18/218 Terms of Use:

5 : :: > < : Cl. '- : :: VJ VJ : o. J ;: 4 RPM <p:.17 5 s [ c_f<tng _ TA,LL LL:TN5JLrn - CELL FLOW ZONE - --' ' --- TME p SQ_ nt. Flo temperatures at stations, 2 and 3 are presented in Fig. 13 as a function of axial velocity parameter for a speed of 225 rpm. The flo temperatures shon in Fig ' --- T" _ a rno l , STATON _J a + 3 NLE T DSCHARGE Fig. 11 Thermocouple signal during rotating stall n = 225 RPM '' =, 17.1 Fig :;) & To 8:' 6. /..... T d} A (PEAK. 7e:; / VALUE) f- 4 V -v.j. ' ---- TMN 3 1R LL 2Q,1,2 TME Variation of flo temperature upstream and donstream the first stage rotor during pure rotating stall is identical to rotating stall frequency. Flo temperature variations upstream and donstream of the first stage rotor are shon in Fig. 12. The flo temperature increases in the stall cell due to the dissipation of mechanical energy supplied by the rotor and decreases in the zone of unstalled flo. The measured difference beteen the maximum and minimum value of temperature pulsations (T - T. shon in Fig. 12) at a measuring. st Wfl on i Wi q nfluenced by heat exchange beteen the flo and the metal of blades, rotor and casing and the time constant of the th _ r mocouple. fhis measured difference is considerably loer than the actual difference beteen the stalled and unstalled flo temperatures. Flo temperature fluctuations beteen blade ros in pure rotating stall regime ere obtained at various speeds and axial velocity parameters. Steady flo temperatures ere reached after to minutes at the operating poi-,3 Fig. 13 Variation of peak value of flo temperature ith axial velocity parameter at various stations of compressor in rotating stall represent the peak measure vaues of the pulsating temperatures (T in Fig. 12) during _ rotating stall. The vapition of compressor exit temperature is also shon in Fig. 13. nlet temperature remained constant. The spanise flo temperature measurements ere made prior to the first stage rotor. The highest temperature as found near the tip diameter but the temperature variation ith radius as small. This result, though, may not be generally valid and depends probably on the compressor stage geometry. The comparison of the tip and hub flo temperatures ia presented in Fig. 14. The blade metal temperature measurements in rotating stall regime ere taken after the compressor had reached thermal equilibrium (approximately 5 minutes). The blade temperatures obtained ere constant in time corresponding to a mean value of flo temperature pulsations. The results are shon in Fig. 15. The figure illustrates the variation of blade temperature ith axial velocity parameter at 225 rpm that is compared ith flo temperature upstream of the first stage rotor. The variation of the highest blade temperature found in the compressor ith speed at various velocity parameters is given in Fig. 16. t shos that if the compression system (compressor circuit) alloed the compressor to ork in pure rotating stall at a higher speed (for instance at the maximum of 81 rpm), the temperatures in the compressor ould rise con- Donloaded From: on 1/18/218 Terms of Use: 5

6 siderably. u._ 12 r 1 '(JO -i- oo \ 6 4 '. - n RPM 1 l o To TP e To t 1 HUB , -ej 2- Ct'. =i Ct'. o_ 1 2 f-- <t _J m Fig. 16 Tl1-1 =.5 =.1 P 'o, cplsl ROTATONAL SPEED [RPM] Variation of the highest blade metal temperature ith rotational speed at various axial velocity parameters in rotating stall regime Fig. 14 Fig L,1 - L _J.2.3,4 Variation of peak value of flo temperature ith axial velocity parameter near hub and near tip diameter at entry to the first stage in rotating stall ;_; 1 ' n < 225 RPM T - TL1.. Tu Tu rl4?.. t -.. t - t 4 - J.1.2 OJ.4 Variation of blade metal temperature ith axial velocity parameter in rotating stall The flo and blade temperature measurements in the rotating stall regime indicate that: (1) the flo temperatures beteen blade ros of a compressor are approximately identical except for the temperature upstream of the last stage that is loer (Fig. 11). The discharge temperature of compressor during rotating stall operation is higher than the discharge temperature during unstalled operation at the stability limit point. Hoever the discharge temperature is loer than flo temperatures beteen blade ros (Fig. (2) (,3) (4) 13). spanise variation of flo temperature is small (Fig. 14), the highest blade temperatures are found in the first or second stage stator, but, the difference beteen blade temperatures of all the four stages is relatively small (Fig. 15), the flo and blade temperatures in rotating stall regime are a function of rotational speed, i. e. of stall cell frequency and of axial velocity parameter, i. e. of stall cell blockage (Fig. 16) Temperature rise in compressor during surge The foregoing paragraph has indicated that the temperature in the pure rotating stall regime is almost identical in all compressor stages. ts absolute value depends on the value depends on the value of the axial velocity parameter. The same could be applied to the rotating stall that occures during a part of the surge cycle. Classic surge cycles consist of rotating stall part and unstalled flo part (Fig. 6). The changes of flo temperature during classic surge cycle are indicated in Fig. 17. Fig. 17 shos the signal of the thermocouples located at stations lr, 1, 2R, 2 and JR during classic surge. The peak values of temperature pulsations are not very different at these stations. Donloaded From: on 1/18/218 Terms of Use: 6

7 3 RPM n = 6 RPM, 'f' =,35 HUB Fig. 17 :::) Vl Vl Q_ :::) Q_ 2: f-- i Ji.QJs TME Thermocouple signals during classic surge During deep surge (Fig. 6), the reverse flo in the part of the surge cycle bas a decisive effect on the temperature levels in the flo path of the compressor. During the reverse flo a temperature rise occurs in each stage begining ith the last stage due to the ork input of the rotor ros. The stage temperature rise is probably a function of the negative transient axial velocity parameter, rotor speed and stage geometry. n = 6 RPM ( =,35 TP Fig i:r 1 2: f-- 5 LL T o \,2,4 \ ',,6,8 TME [s] Variation of flo temperature near hub diameter at the stations 1 and lr during deep surge cycle ture in front of the first rotor ro measured near the hub diameter at the same operating point as in the Fig. 18. The peak value of temperature pulsations near the bub is considerably loer compared to the value measured near the outside diameter. n 6 RPM (a) :::) Q_ 2: t-- :; J LL 3' 2 ; o v SHORT TME SURGE RANGE OF TEMPERATURES DURNG D LONG TME SURGE Fig. 18 Qr J......_t'--'-....2,4,6 QB TME [s] Variation of flo temperature near tip diameter at various stations of the compressor during deep surge cycle The time variations of flo temperatures during deep surge at various measuring stations ithin the test compressor are shon in Fig. 18. The temperatures ere measured near the outside diameter. The difference in the peak values of temperature pulsations prior to the first and the last stage of the four stage compressor is relatively large. Fig. 19 indicates the variation of the flo tempera- (b) Fig. 2 :::) 15 j 15 Q_ 1'. f-- ' <{ J co 1,1,2 5o,1,2 <P Peak value of flo and blade metal temperature variation ith axial velocity parameter during deep surge,3,3 <P,4 Donloaded From: on 1/18/218 Terms of Use: 7

8 The variation of flo and blade temperatures ith axial velocity parameter for the steady-state equilibrium points at 6 rpm ispresented in Fig. 2. The flo temperatures in Fig. 2a are the peak values of the temperature pulsations during surge. The highest flo temperature as found prior to the first stage rotor. ts absolute value depends on the axial velocity parameter of the steady-state point. The highest measured blade temperature as located at the first stage stator and its variation ith the axial velocity parameter is shon in Fig. 2b. Most of surge investigations ere made during short-time surge, i. e. during surge that lasted from three-to-five surge cycles. The blade metal temperatures in Fig. 2b ere indicated during this short-time surge. The temperature measurements ere also realized during long-time surge hen the number of surge cycles at the steady-state equilibrium point of l' cj) =.15 and 6 rpm as very high. n = 5 RPM f '' _(_ U a:: :J a:: 121 l+---h-+-r--- t-- D Fig i +- L= _[ j TME [ s] Variation of blade metal temperatures ith time during long-time surge Fig. 21 shos the blade metal temperature variation ith time during the surge that lasted about six minutes. With surge frequency of 1.7 Hz at this steady-state point it represents about 6 surge cycles. The flo temperatures changed only a little during the long-time surge, the range being as shon in Fig. 2a. The variation of flo temperature ith rotational speed prior to the first rotor ro at the lo stability limit equilibrium point is given in Fig. 22. This flo temperature T L1 - er :::> 1-- <l'. er 2 a._ :L 1-- <l'. J m - 3: '3 u_ Fig. 22 iool l A = <jl LSL 2 To - -- i - T /j. :; f /f j/ T u ' ROTATONAL SPEED [RPM] -1 J Variation of flo temperature ith rotational speed at entry to the first stage rotor and variation of blade metal temperature of the first stage stator at the LSL (steady-state point) during surge rise due to deep surge can be compared to flo temperature values reached during unstalled compressor operation that ere shon in Fig. 5. n addition, the variation of blade metal temperature of first stage stator ith speed during short-time surge can also be seen in Fig. 22. The flo and blade temperature measurements during surge indicate that: ( 1) (2) ( 3) the highest flo temperature that occurs during deep surge is found in front of the first stage rotor near the tip diameter (Fig. 18, 19 and 2a), its absolute value is a function of rotational speed, the value of axial velocity parametr of the steady-state equilibrium point and of the number of compressor stages (Figs. 2a and 22), the highest blade temperature as found in the first stage and its value depends on the flo temperature and the number of surge cycles (Fig. 21). MODEL OF TEMPERATURE CHANGES DURNG SURGE The above described extensive experiments of the surge in the multistage axial-flo copressor have been analysed in detail. From the large number of measured surge cycles a model has been suggested. t provides an adequate qualitative description of the temperature changes in the flo path of the multistage compressor during the surge cycle. A schematic diagram of these temperature changes Donloaded From: on 1/18/218 Terms of Use: 8

9 according to the model is shon in Fig. 23. (a) CLASSC SURGE (b) DEEP SURGE ROTATNG STALL SURGE CYCLE rcitatnlled 1RER1 lunstalled STALL FLOW 1FLOW i FLOW --T,;;;;, L._+_ - at lo speed, high temperatures may occur at lo mass flo rate near throttle shut off (Fig. 16). (3) During surge, dangerous compressor temperatures may occur particularly in the deep surge regime. During deep surge the highest temperature as observed prior to the rotor ro of the first stage at high speed or/and near throttle shut off depending upon the number of compressor stages. Long-time surge involes very high blade metal temperatures that may cause compressor failures. (4) The temperature changes during the surge cycle can be described by a simple model (Fig. 23). = ;;L Fig. 23 i = V_L TME- TME- Schematic diagram of the model of temperature changes during classic and deep surge cycle During classic surge cycle (Fig. 23a) the temperature variation in the flo path of the compressor is caused by rotating stall, namely by stall cell blockage variation. n rotating stall the flo temperatures begin tr rise and then sink again in accordance ith the changing transient velocity parameter value. The peak values of temperature pulsations during classic surge are almost the same in all compressor stages. During the unstalled part of the cycle the temperatures approach to the values corresponding to stage unstalled conditions. During deep surge cycle (Fig. 23b) the temperatures increase in rotating stall ith decreasing transient axial velocity parameter. Then, during the reverse flo there is a considerable difference beteen the first, middle and last stage of the compressor. Obviously, the higher the number of stages, the greater the increase of the temperature in the reverse flo. Then the rotating stall part of the surge cycle follos and the temperatures decrease due to increasing transient axial velocity parameter. During unstalled flo the temperatures approache to the values of the unstalled ork of compressor stages. Day,. J., Greitzer, E. M. and Cumpsty, N. A., 1978, "Prediction of Compressor Performance in Rotating Stall, " ASME Journal of Engineering for Poer, Vol. 1, pp Feierfeil, J., Blaha, P., Nacovska, K., Goldsmid,., Andera, V. et al., 198, "The Effect of Unstable Operation of Multistage Axial Flo Compressor on its Main Operation Characteristics," (in Czeck).KO Kompresory Report. Greitzer, E. M., 197 6, "Surge and Rotating Stall in Axial Compressors," Part, Part, ASME Journal of Engineering for Poer, Vol. 98, PP REFERENCES CONCLUSONS (1) Multistage axial compressor operation in both pure rotating stall and surge regime results in high internal temperature levels that may be dangerous to compressor operation. (2) f the compression system has a high rotating stall/surge boundary speed, then high temperatures may arise at high rotor speed in the pure rotating stall regime. Hoever, even Donloaded From: on 1/18/218 Terms of Use: 9