ENSURING ADEQUATE COOLANT PURITY FOR ADVANCED GAS TURBINES Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1995/78811/V004T10A014/2406576/v004t10a014-95-gt-281.pdf by guest on 30 September 2021 D.E. Woodmansee, A.K. Tolpadi, T.H. Hwang Corporate Research and Development General Electric Company Schenectady, NY 12345 A.O. Maddaus Industrial and Power Systems General Electric Company Schenectady, NY 12345 ABSTRACT water was placed on a wheel rim and was metered through The role of particulate contaminants in advanced gas turbine bucket cooling channels to emerge at the bucket tip (Homer, et coolants is discussed, especially in light of the extremely high G­ al., 1980) in a partially vaporized state. The fluids proposed for field regions they will experience in service. Predictions of sedi­ coolants include air, steam, and inert gases. mentation in both laminar and highly turbulent accelerating Major concerns in cooling any parts subjected to high G­ flows using a computational fluid dynamics code are made for a fields are the sedimentation of coolant particulates in these range of particulate sizes to show that particles over 0.5 µm are critical regions, causing primary problems of erosion, plugging, of concern. Possible techniques for limiting access of these and insulation, as well as possible secondary problems of particulates to the gas turbines themselves are presented. sediment release, re-entrainment, and redeposition. The G-fields Overall, contaminant deposition appears controllable, limiting act as enhanced particulate settlers and can lead to a buildup of required cleaning of coolant channels to regularly scheduled deposits in the cooling channels of rotating parts. Open-cycle­ inspections. cooled gas turbine bucket cooling passages are usually provided with bleed holes sufficiently large that accumulated particulates INTRODUCTION in the channels can be blown out of the buckets. Minimum Particulates in any coolant system can cause a number of allowable bleed hole sizes are speci fled on these exit holes concerns, for example, containment erosion, deposition in (usually 30 to 40 mils) so that the exiting particulates do not critical regions causing flow restrictions and flow redistribution, gradually block and ultimately plug the exit aperture. Such bleed and insulation in regions where high h~at transfer is desired. holes were also specified for bucket cooling systems in which a Secondary concerns also include undesirable concentrations of small fraction of the coolant would be "blown-down" at each contaminant chemical components, leading to corrosion (or bucket tip, preventing a longterm buildup of particulates, in the nuclear activation in some systems) and the release and re­ same way that steam boiler solids are often controlled by entrainment of sediments to aggravate the primary concerns. blowdown. However, for coolant gases where the loss of the This paper deals specifically with coolants in gas turbine engines. blow-down coolant is a major replacement cost item, or where The cooling of hot gas path parts in advanced gas turbine the coolant pressure is high, then too much coolant would be lost (Brayton) cycles has commonly been accomplished by extract­ through a minimum-sized, non-blinding hole, and the use of ing compressor discharge air and directing it to both static and blow down would be prohibited in practice. rotating parts immersed in the hot combustor discharge gas. Rotating part cooling, in particular, has been accomplished in EROSION an open cycle in which the air convectively cools the bucket Particle erosion is a well-known problem in steam turbine de­ interior or a trailing edge and is discharged into the working sign as well as in aircraft gas turbines operating in dusty environ­ fluid, or it film cools external airfoil structures as it is discharged ments. Steam turbine nozzle and blade erosion is due to particu­ through arrays of small holes providing a cool gas barrier. How­ lates that travel into the boiler from the steam generation ever, air has not been the only coolant attempted. An open­ system and its delivery piping. Studies of the erosion by cycle water-cooled turbine concept has been developed in which particulates from a supercritical boiler plant have shown that Presented at the International Gas Turbine and Aeroengine Congress and Exposition Houston, Texas - June 5-8, 1995 1.00E+OO 1.00E-01 - ~ e• 1.00E-02 • E i-• DI C ~mputed from T&C ~ns ~ i-- c:i 1.00E-03 - Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1995/78811/V004T10A014/2406576/v004t10a014-95-gt-281.pdf by guest on 30 September 2021 E -·- e ~ c ~ --38 I licron Asn -~ a ii 1.00E-04 ~ 01 304 ~s a w ~ 1.00E-05 ~ Umll --- - Upper - I l Of J GT 1.00E-06 .. - 100 1000 Gas Velocity (fl/sec) FIGURE 1. Erosion of Aluminum Alloy by 200 Micron Quartz Particles particulates over 100 µm are found sufficiently often that they which are the primary concern in gas turbine coolants, can have can create erosion problems for the steam turbine far out of very high insulating values in thin films. In many cases, such in­ proportion to their number frequency (Duncan, et al., 1992). SJ lating deposits may provide the limiting criteria for particle The constitutive relations proposed to describe erosion behavior deposition as compared to the undesirable flow redistribution are several, but a more detailed study related to fan erosion by that would be caused by flows through metering orifices as de­ fly ash found that erosion is proportional to the third or fourth scribed above. power of the relative velocity, and to the square or the cube of In addition to sedimentation in a G-field, mechanisms for the particle size (i.e., its mass) (Yamaguchi, et al., 1982.). particulate deposition include impaction, electrostatic attrac­ Data for the erosion of aluminum by silica particles is shown tion, and electrophoresis. The last two are not considered as im­ in Figure 1 (Tabakoff, 1982). Aluminum is much more erodable portant to this problem for the following reasons. than the high alloy materials needed for hot gas path parts in gas Electrostatic attraction can attract the particulates to the turbines. Nevertheless, if it is conservatively assumed that the walls of the duct in which they are traveling. Sometimes this oc­ largest particulates allowed into the gas turbine coolant would be curs because of the generation of a streaming potential along an 2 µm, the aluminum erosion rate shown would be a negligibly insulating duct carrying a nonconductive fluid, such as in hydro­ low 0.02 mils for 20,000 hours of exposure. Therefore, erosion of carbon fuels flowing through rubber hoses. This is believed to be the cooling passages for a coolant pre-cleaned to 2 µm or less unlikely here since the coolant ducts have conductive metal should show no measurable effect of the types of erosion seen in walls. other power generation equipment. Thermophoresis is the motion of a small particle in a strong thermal gradient. The particle moves down the temperature DEPOSITION gradient because of the more intense molecular motion of the The deposition of particulates from the coolant can poten­ gas on the hotter side. In the case of cooling hot surfaces as in tially lead to plugging of flow channels, insulation of internal the gas turbine example, this mechanism would actually reduce heat transfer surfaces, and stiffening of any brush type rotary gas the rate of approach to the hot surface, therefore it is not an seals. A voiding the plugging or flow redistribution is not simply operative mechanism. an issue of having the individual coolant particles pass through a Gradient coagulation results when a particle-containing gas given critical aperture. Particulates are well known to adhere to stream is sheared, creating large velocity gradients such as near surfaces and each other by static charges, forming larger a wall. Particles in adjoining streamlines run into each other agglomerates that may be re-entrained. because of the velocity gradient, in addition to whatever random The formation of deposits on high heat flux surfaces can also particle motions occur because of Brownian motion. This create insulating films that interfere with the coolant picking up contact in shear fields is a major reason that dust will tend to heat from the coolant channel walls. Inorganic oxide materials, build up at the edge of flow orifices in contaminated flows. 2 1000.00 .. -- - - - 100.00 u.. ,_ Ill s_ -:::::::::; ........ 10.00 u~ 0 v~ 'i - Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1995/78811/V004T10A014/2406576/v004t10a014-95-gt-281.pdf by guest on 30 September 2021 > J" OI 1.00 ~ . , • 82 Tip ·' - 15600 Gs C/J ./ .I '"" • --a--- Bolt Circle - 9070 /k"/ 0.10 Gs E ,r - 0.01 0.01 0.10 1.00 10.00 100.00 Particia Diameter (microns) FIGURE 2. Particle Settling Velocities on a Gas Turbine Rotor THEORY AND RESULTS Particle Trajectories from CFO Model Calculations. A more accurate estimate, however, can be developed for Sedimentation specific geometries using 3D computational fluid dynamic The well-established Stokes Law theory (Perry, 1984) for the (CFD) codes that predict the gas velocities acting on the settling of particles in liquids and gases is applicable here. For particles. The code used in this work was a fully elliptic, 3D, very small particles, the Reynolds Number for settling tends to body-fitted code based on pressure correction techniques be very low, well under 0.1 where the discharge coefficient is (Tolpadi and Braaten, 1992, and Tolpadi, 1994). In this proportional only to the inverse of the Reynolds Number. approach, a 3D gas flow solution is generated in the channel However, the acceleration term for the gas turbine rotor and geometry of interest.