Evolution of Spanwise-Hole Blade Cooling in Industrial Combustion Turbines

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York, N.Y GT-397 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 1992 by ASME Evolution of Spanwise-Hole Blade Cooling in Industrial Combustion Turbines G. L. WHIDDEN W. E. NORTH Power Generation Business Unit Westinghouse Electric Corporation Orlando, Florida The evolution of the spanwise-hole blade cooling technique is described from its employment in the original blade cooling design of the late 1960's to its current applications in modern industrial combustion turbines. Descriptions of the design, along with analytical and manufacturing advances which have enabled this technique to remain competitive at higher firing temperatures with the relatively complex designs are included. Verification of the spanwise cooling hole designs by means of pyrometer testing is also presented. NOMENCLATURE friction factor dimensionless smooth smooth hole friction factor dimensionless ( D hole diameter m F, inlet effect factor dimensionless F ffriction effect factor dimensionless F rfactor for rotational effects dimensionless F wwall temperature factor dimensionless L hole length m Nu DNusselt number dimensionless Pr Prandtl number dimensionless Re preynolds number dimensionless Tcbulk coolant temperature K Twwall temperature K INTRODUCTION Spanwise cooling holes have been successfully used to cool rotating turbine blades in aero and industrial combustion turbines for many years. As the drive to improve power and performance by increasing firing temperatures continued, the thermal demands on current cooling schemes grew. To meet this need, spanwise-hole cooling has evolved over the years and represents an effective and efficient means of cooling rotating blades in modern combustion turbines. This paper reports on the evolutionary development of Westinghouse Electric Corporation's use of spanwisehole cooling schemes in large industrial combustion.ci { r:s1 W.'S, ant,' kelrayarg.f SC AP,r,!?...Ydg, SO di APS.J.1 :1 4 1VSIT ' test which was used to evaluate the latest spanwise-hole cooling designs. THEORETICAL BACKGROUND The Dittus-Boelter equation provides the basic formulation for analysis of forced convection through a circular hole. Additional factors are applied to account for friction, buoyancy, Coriolis, inlet, and wall temperature effects as shown in the equation below: Nu D =.02 Re D.8 Pr.33 F, F r F, F weq. (1) The effect of surface roughness is accounted for by the factor F f. This factor results from simple surface effects or from the existence of transverse ribs on the hole surface. A rough surface increases heat transfer by increasing turbulent eddy effects. Kays and Crawford (1980) recommend the following correlation. where: Ff = f }smooth r Eq. (2) n =.68 Pr 215 Eq. (3) Transverse ribs in a channel significantly affect heat transfer. Stankiewicz and Kirkham (1991 reported that turbulators in round holes can increase the heat transfer in certain cases by as much as a factor of three. The heat transfer effect due to transverse ribs is quantified by Webb et al. (1971). The net effect due to rotation is represented by the factor F r. In a rotating blade where the force fields are on the order of 10,000 times that of gravity, buoyancy forces caused by variations in density through the boundary layer become very significant. Rotation also causes Coriolis forces which give rise to secondary flows in the flow path. These secondary flows may enhance heat transfer on the trailing side of the hole, (with respect to rotation) and decrease the heat transfer on the leading side of the hole. An extensive list of references for the effect of rotation on Presented at the International Gas Turbine and Aeroengine Congress and Exposition Cologne, Germany June 1-4, 1992

2 heat transfer in round holes is provided by Morris and Salemi (1991). The inlet effect F; also must be accounted for since the thermal boundary layer is not fully developed until some distance downstream from the inlet. This factor generally takes the form the combined rotor and blade designs were verified, but the tests also indicated that future higher temperature rotor cooling systems could be better served by the use of cooled cooling air without preswirling. F i = 1 + A(D/L) B Eq. (4) where A and B are selected to simulate the effect of the particular inlet condition being considered. The wall temperature factor Fw accounts for the effect due to the ratio of the bulk coolant temperature to the wall temperature. This factor is obtained from F = ( T o / T w ) c Eq. (5) where a typical value of for the exponent c is appropriate. EVOLUTION OF SPANWISE-HOLE COOLING Background The rotor cooling schemes employed by Westinghouse have historically entailed supplying the rotor with air which has been extracted from the compressor discharge, externally cooled, and then filtered. Cooling the air enabled the use of relatively inexpensive ferritic rotor materials. Filtering the cooling air permitted the design of relatively small cooling passages minimizing the possibility of small particles obstructing the coolant flow. As firing temperatures increased to a level that required blade cooling, the spanwise cooling hole technique was adopted because of its relative simplicity and ease of manufacture. As cycle temperatures and pressure ratios continued to rise, the cooling air cooler permitted the continued use of ferritic rotor materials. The cooler also eliminated the need to preswirl the rotor cooling air, thus permitting relatively high cooling air pressures within the rotor. This also allowed for higher cold side heat transfer. The result of this is the ability to attain the higher firing temperatures typical of modern combustion turbines, while retaining the relatively simple spanwise-hole blade cooling technique. A question that needed addressing for each new engine design was whether the cooling air cooler was a liability to either the thermodynamic cycle or to the cost of production of the turbine power plant. In every case to date the result has shown retaining the cooler to be favorable. The thermodynamic penalty of rejected heat by the cooler was countered by the resulting lower cooling airflow made possible by a reduced cooling air temperature. In some combined cycle applications this rejected heat is returned to the cycle. The added cost of the cooling air cooler and external piping is balanced by the lower cost of the ferritic rotor materials and, in more recent engines, by the reduced cost of the simpler blade designs. W251B Spanwise-hole blade cooling was first introduced by Westinghouse into the W251B combustion turbine. The W251B was a 31 MW geared unit that began commercial operation in 1970, with a turbine rotor inlet temperature of 986 C (1806 F). The first stage blade (Figure 1) was cast with cored spanwise cooling holes. In this case, the rotor cooling circuit was conservatively designed to include both a cooling air cooler and preswirled cooling air. The performances of both the blade cooling design and rotor cooling circuit were thoroughly tested with the in-house W251B test engine at the Westinghouse South Philadelphia factory. The high degree of conservatism in Fig. 1 W251B stage 1 turbine blade W501B The second generation of Westinghouse spanwisehole blades appeared in the W501 B. The W501B was a 60 Hz 80 MW direct drive unit that entered commercial operation in 1973 with a turbine rotor inlet temperature of 993 C (1819 F). In this case, the blade was forged and the holes were added by an electrochemical machining process (Figure 2). The W501B rotor cooling circuit took on the basic configuration used by all subsequent Westinghouse designs (Figure 3); i.e., the preswirl feature was omitted. Fig. 2 W501B stage 1 turbine blade 2

3 FILTER COOLER ROTOR COOLING AIR empi THERMOCOUPLE LOCATION 1- gill PH I 3.14 : j imilmose ROTOR ENTRY HOLES Fig. 3 W501 rotor cooling circuit Since the full pressure of the rotor cooling air supply circuit was not required, the flow entering the rotor was throttled at the rotor-entry holes. The blade cooling performance was tested and verified as part of a prototype engine test. Data acquisition included rotating blade thermocouple readings obtained through a slip ring. W501D The W501D at 95 MW was introduced in 1975, with a turbine rotor inlet temperature of 1096 C (2005 F). The rotor geometry was almost identical to that of the W501 B, with the exception that the cooling air inlet holes in the rotor were opened up to provide the full pressure required to satisfy the greater stage 1 blade cooling demands. A retrofit blade was recently developed to provide upgrade capability for the earlier W501 AA turbines, and to also provide retrofit for the W501 B's and D's. The new blade, shown in Figure 4, is a more efficient design by virtue of its 22 small cooling holes instead of the 14 larger holes contained in the original blade. Fig. 4 W501 AA, B, D retrofit stage 1 blade cooling design W501D5 The W501D5 entered commercial service in 1981, with an introductory rotor inlet temperature of 1085 C (1985 F), which was later increased to 1132 C (2070 F) at 107 MW. The stage 1 blade was a completely new design from that of the previous engines, but retained basically the same cooling scheme as that of previous W501-series turbines. The blade cooling scheme has been subsequently redesigned to reduce the leading edge temperature and to improve the life of the NiCoCrAIY corrosion-resistant coating. In this new design, the leading edge of the blade was cut back approximately 1.52 mm. A new leading edge hole was also used, containing transverse ribs in the outer 75% portion of the blade height to increase turbulence and enhance heat transfer. The new blade was specifically designed to control the leading edge wall thickness. Variations in wall thickness of the original blade caused large blade-to-blade variations in leading edge temperature. The result of this new leading edge configuration was a substantial reduction in stagnation-point surface temperature. The new leading edge hole also used considerably more flow than did the original hole. In order to keep the total cooling flow requirement for the blade equal to that of the original design, the remainder of the blade had to be cooled more efficiently. This was achieved by replacing the 16 camber line holes with 28 smaller holes, which were repositioned around the periphery of the blade, except at the trailing edge where they were placed along the camber line. The cooling schemes for both the original and new designs are shown in Figure 5. Positioning the cooling holes close to the surface reduced the conduction path, which then lowered the required temperature drop across the wall. This peripherally cooled blade had a higher cooling effectiveness while maintaining the same level of coolant flow. Two-dimensional finite element models for the camber line and peripheral blades were used to analyze the designs. Isotherms comparing the camber-line cooling and the peripheral cooling schemes are shown in Figure 6. The isotherms for the camber-line cooling scheme show how 3

4 the relatively long conduction path between the cooling holes and the blade surface impedes the heat transfer. mentioned earlier. The cooling scheme for the stage 1 blade consists of spanwise holes located along the camber line. These holes are cast into the blade and contain turbulating ribs which are added by an electrochemical machining process. W501 D5 ORIGINAL BLADE W501 D5 "PERIPHERAL" BLADE 701DA The turbine rotor inlet temperature for the 701DA is 1177 C (2150 F), and may represent the practical limits for using spanwise-hole cooling technology in a first stage turbine blade. The peripheral cooling design of the W501D5 provided the foundation for the cooling scheme for the 701DA stage one blade. Twenty-five spanwise cooling holes were positioned around the periphery of the blade. All of these holes, except for the trailing edge hole, contained transverse ribs to increase the turbulence in the flow and increase the heat transfer coefficient. However, the cooling requirement at the leading edge was beyond the capability of a turbulated spanwisehole. The solution was to replace the leading edge hole with a single-pass cored cavity utilizing a showerhead leading edge. The need for a leading edge cavity dictated that the blade be cast instead of forged, as was the W501D5 stage one blade. A cross section of this blade is shown in Figure 7. Isotherms from a two-dimensional finite element model are also displayed in Figure 7. Fig. 5 W501D5 stage 1 blade cooling hole configurations W501 D5 ORIGINAL BLADE W501D5 'PERIPHERAL" BLADE Fig DA stage 1 blade cooling scheme and isotherms Fig. 6 W501D5 stage 1 blade isotherms CW251B12 The CW251B12 was introduced at 47 MW with a base load firing temperature of 1149 C (2100 F) and represents the latest configuration of the W251B The leading edge cavity contains transverse ribs on the suction and pressure sides. The showerhead holes are oriented at a 30-degree angle in the spanwise direction. The reason for the steep angle of the showerhead holes was to maximize the hole surface area, and to minimize the lift-off of the cooling air from the leading edge. A small film cooling effect is realized from the showerhead, but the bulk of the cooling is done by the flow of air through the holes. Also shown in Figure 7 are gill holes on the suction side of the airfoil. These gill holes are located only on the outer portion of the blade near the tip. They serve to locally supplement cooling in the cavity which has 4

5 diminished from bleed-off of the flow by the showerhead. The aft cavity serves no cooling purpose but is there for mechanical design considerations. The cooling design for this blade will be verified by a pyrometer installed in the first engine. PYROMETER TEST A pyrometer installation developed by Dow Chemical, U.S.A. and described by Kirby et al. (1986) was used to test the new cooling designs in a Dow W501D5. Three types of blades were placed in the engine for the test. These test blades consisted of standard blades, new peripheral blades, and some developmental blades. The developmental blades were identical to the peripheral blades, except that all but the trailing edge hole contained transverse ribs to turbulate the flow. The minor diameter of the fully turbulated holes was the same as the diameter of the standard peripheral holes. The flow through the turbulated holes was substantially reduced compared to the non-turbulated holes due to increased friction caused by the ribs. The pyrometer sight tube passed through the combustor casing and then through the transition piece, viewing the first stage blade between two first stage vanes. The position of the pyrometer sight tube was as shown in Figure 8. The pyrometer was capable of seeing the leading edge of the blade between the 60% and 80% height, and the pressure surface up to 40 mm back from the leading edge, as shown in Figure 9. That region of the blade was considered to be the most significant, since that was where previous tests had shown the maximum surface temperature was located. The plot in Figure 10 represents the temperatures of the first stage blades through one revolution of the rotor. Three distinct temperature patterns are evident, corresponding to the three types of blades used in the test. The first pattern of temperatures represents the Fig. 8 W501D5 pyrometer installation Fig. 9 W501D5 pyrometer viewing area ORIGINAL BLADES ORIGINAL BLADES PERIPHERAL BLADES Fig. 10 Pyrometer data from one revolution of the rotor at 60% blade height 5 FULLY TURBULATED PERIPHERAL BLADES

6 standard blades with the camber-line cooling holes. The temperature difference between the leading edge and the remainder of the blade is very evident, as shown by the sharp peaks. The second pattern of temperature profiles represents the peripheral blades, and is improved compared to the first. The leading edge temperatures have been substantially reduced, while the temperatures over the rest of the blade have been reduced by a lesser degree. The third type of temperature pattern represents the fully turbulated peripherally cooled blades. The leading edge temperatures are comparable to the peripheral blade, since both blades have the same leading edge cooling scheme. However, the body of the fully turbulated blade is significantly cooler. The leading edge temperatures shown in Figure 10 are artificially low, due to the lower effective emissivity found at the leading edge. The pyrometer assumes an average value of effective emissivity for the entire viewed surface. This value works well for the pressure surface, but is too high for the leading edge. Figure 11 shows an expanded view of the pyrometer trace for each of the three blade types that has been corrected for leading edge emissivity differences. This figure demonstrates the success of the peripheral blade cooling design in reducing the surface temperature without a corresponding increase in coolant flow. SUMMARY The evolution of spanwise-holes for blade cooling has been presented. Results of a pyrometer test used to verify the cooling designs were also discussed. It was shown that spanwise-holes have evolved to meet the challenges of modern combustion turbines. ACKNOWLEDGEMENTS The Authors wish to acknowledge Dow Chemical U.S.A. and Mr. Richard E. Zachary for their help and expertise in infrared pyrometry and for providing the test data presented in this paper. REFERENCES p cc 7 < CC 50 LLJ 0. UJ ORIGINAL BLADE PERIPHERAL BLADE FULLY TURBULATED PERIPHERAL BLADE LEADING EDGE PRESSURE SURFACE SURFACE DISTANCE Fig. 11 Pyrometer data at 60% blade height for three individual blades corrected for leading edge emmisivity. 1. Kays, W. M., and Crawford, M. E., Convective Heat and Mass Transfer, Second Edition, McGraw-Hill, New York, Kirby, P. J., Zachary, R. E., and Ruiz, F., "Infrared Thermometry for Control and Monitoring of Industrial Gas Turbines," ASME 86-GT-267. Morris, W. D., and Salemi, R., "An Attempt to Experimentally Uncouple the Effect of Coriolis and Buoyancy Forces on Heat Transfer in Smooth Circular Tubes which Rotate in the Orthogonal Mode," ASME 91- GT-17. Stankiewicz, D. J., and Kirkham, T. R., "The Application of Convective Cooling Enhancement of Span- Wise Cooling Holes in a Typical First Stage Turbine Blade," ASME 91-GT-12. Webb, R., Eckert, E., and Goldstein, R., 1971, "Heat Transfer and Friction in Tubes with Repeated-Rib Roughness," International Journal of Heat Mass Transfer, Pergamon Press, Vol. 14, No. 4, pp