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

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FIGURE 1. Erosion of Aluminum Alloy by 200 Micron Quartz Particles

particulates over 100 µm are found sufficiently which are the often that they primary concern in gas turbine coolants, can have can create erosion problems for the steam very high insulating turbine far out of values in thin films. In many cases, such in­ proportion to their number frequency SJ lating deposits (Duncan, et al., 1992). may provide the limiting criteria for particle The constitutive relations proposed deposition to describe erosion behavior as compared to the undesirable flow redistribution are several, but a more detailed study that related to fan erosion by would be caused by flows through metering orifices fly ash found as de­ that erosion is proportional to the third or fourth scribed above. power of the relative velocity, and to the square In addition or the cube of to sedimentation in a G-field, mechanisms for the particle size (i.e., its mass) (Yamaguchi, particulate deposition et al., 1982.). include impaction, electrostatic attrac­ Data for the erosion of aluminum by silica tion, and electrophoresis. particles is shown 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 Electrostatic gas path parts in gas attraction can attract the particulates to the turbines. Nevertheless, if it is conservatively walls of the assumed that the duct in which they are traveling. Sometimes this oc­ largest particulates allowed into the gas turbine curs because of coolant would be the generation of a streaming potential along an 2 µm, the aluminum erosion rate shown insulating duct would be a negligibly carrying a nonconductive fluid, such as in hydro­ low 0.02 mils for 20,000 hours of exposure. Therefore, carbon fuels flowing erosion of through rubber hoses. This is believed to be the cooling passages for a coolant unlikely pre-cleaned to 2 µm or less here since the coolant ducts have conductive should show no metal 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 The deposition of of the particulates from the coolant can poten­ gas on the hotter side. In the case tially of cooling hot surfaces as in lead to plugging of flow channels, insulation the gas turbine of internal 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, seals. A voiding therefore it is not an 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 given critical aperture. gas Particulates are well known to adhere to stream is sheared, creating large surfaces velocity gradients such as near and each other by static charges, forming a wall. Particles larger in adjoining streamlines run into each other agglomerates that may be re-entrained. because of the velocity gradient, in addition to whatever The formation of deposits random on high heat flux surfaces can also particle motions occur because of create insulating Brownian motion. This films that interfere with the coolant picking contact in shear up fields is a major reason that dust will tend to heat from the coolant channel walls. Inorganic build oxide materials, up at the edge of flow orifices in contaminated flows.

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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. Onto these solutions are "injected" bucket application is a large multiple of the normal gravity field individual particles of specific diameter at specific positions at used to calculate particle settling in a normal, quiescent fluid. In the flow entry point. The particle motion is assumed to be the case of the gas turbine, these G-forces can exceed 15000 dictated by local velocities, but the flows are assumed to be times sea level gravity because of the large diameters of unaffected by the presence of the particles. In the current machines running at from 3000 to 3600 rpm, and smaller application, the particle concentration is so low that this is a diameters but rotational speeds to 5000 Rpm for smaller gas reasonable assumption. The trajectories of the particles are then turbines. Figure 2 displays the settling velocities of particles followed stochastically to determine the number of collisions based on the Stokes Law friction coefficients for spherical iron made with the wall, and, in the case of a return bend at a large oxide particles in a gaseous coolant. The high-G environment radius, the number of the injected particles that ultimately make provides very high settling velocities; a 1-µm particle would it through the turn and back into the returning coolant flow settle at 10 ft/s in this environment. From such calculations one where the G-fields are lower. For this study, rebound can estimate the time it would take a particle to move across a characteristics of the particles have been taken from the laminar flow field and contact an outer boundary wall. Figure 3 literature (Tabakoff). shows the distance a particle in the flow along the inside wall Figure 4 shows a flow solution for a simple return bend and would flow before hitting the outside wall. If it is assumed that Figure 5 shows a similar solution for a more realistic turn the particle would attach to the outer wall under these demonstrating the capability of the CFD code. Examples of the conditions, then this provides a conservative estimate of what trajectories of particles of increasing diameter in a simple tum particle sizes would be completely removed from the flow. In are shown in the three images in Figure 6. Based on averaging this case, a 250 ft/s flow along a 5-in.-long bucket tip would the results from a number of particle injections, Figure 7 remove all particles equal to, or larger than, 0.4 µm. This type of demonstrates the increase in the number of collision as the par­ calculation can be employed to estimate how clean a coolant ticle size increases, and the introduction of elastic propertiesof stream must be, given a known particle size distribution. rebound reduces the number of collisions in half for 1-µm

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FIGURE 3. Particle Removal in a 0.25-in. High Axial Flow Channel

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FIGURE 4. Flow Solution for a Square Return Bend

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FIGURE 6. Trajectory Traces of Particles studies of coolant systems will clearly necessitate that pre-filtra­ tion or sedimentation systems should be provided to ensure that particles. However, all particles injected into the flow stream at cooling channels would neither plug nor build up insulating lay­ different entry positions do not behave the same. Figure 8 ers quickly. Moreover, while these large particulates may repre­ demonstrates how the wall collisions of 1-µm particles can vary sent a very small fraction of particle number and even total par­ by over two orders of magnitude depending on where the ticle massflow, it is clear that their presence can not be tolerated particles enter the channel. The large, 10- µm particles continue since they would be one-third the size of the smallest apertures to bounce in the channel because they are driven to the outer in high intensity stator cooling regions and create potential wall, while the 0.5- µm particles bounce over three orders of plugging-as well as insulating-problems at the bucket tips. magnitude less often. The fraction of particles making it all the way from the entrance to the exit of the return bend is shown in MEANS OF REMOVING PARTICULATES FROM Figure 9. While the stochastic nature of the prediction results in COOLANTS a curve that is not smooth, the results clearly show that There are a number of techniques for removing particulates approximately 90% of the 0.5-µm particles will pass through the from fluids, including filtration, magnetic separation, channel, while less than half of the 3-µm particles pass and are electrostatic attraction, and sedimentation. Absolute filtration trapped in the channel boundary layer, never to exit with the would be desirable because no large particulates can be allowed flow. in the cooling passages, but the pressure drop through such systems compromises overall performance. Hot gas filtration DISCUSSION through ceramic filter bags, sintered ceramics, and sintered The results of the sedimentation calculations by the simple metals has been done. For example, steam and air used in food Stokes Law approximation for settling across a laminar flow gap or pharmaceutical applications have been filtered through generally agreed with the more accurate 30-CFD calculations sintered metal filters to ensure cleanliness, where the volumes which provide statistics based on a stochastic particle tracking are usually so small that a large associated pressure drop is not a procedure. Both indicate that the majority of particles under significant product cost. about 0.5 µm should pass through the tip turns of bucket coolant While the other options cannot be entirely ruled out for channels. For a monotonic particle dispersion in this size range, possible use, they do have disadvantages. Work done on high the particle entrapment will depend upon where the particles gradient magnetic separation showed that large fields and are in the flow field cross-section when they enter the turn. This distributed collection media (steel wool) could effectively position sensitivity appears to be partially a result of how the remove ferromagnetic materials. While metal corrosion Coriolis forces might drive particles toward a bounding wall and products are indeed often ferromagnetic, there is a concern that lead to deceleration and ultimate entrapment at the tip. other materials like Si02 would not be captured in such filters. However, it is also clear that larger particles will progressively Also, the filtration is not absolute and any momentary failure of become a concern for deposition. Large particles found in earlier the magnet would release contaminant and would necessitate an

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FIGURE 8. Average Number of Wall Collisions with Entry FIGURE 9. Percentage of Particles Passing Through the Position Channel

transients such as shutdown and startup. Consideration should emergency shutdown. Similarly, electrostatic precipitation be given to providing internal traps ahead of critical cooling re­ micrht pick up many contaminants, but the operation of charged gions and small apertures that would hold any re-entrained ag­ media at high pressures has been found to be difficult in practice. glomerates. Clearly the design engineer needs to select from a series of options, each one of which has disadvantages. CONCLUSIONS MEANS OF REMOVING DEPOSITS FROM The analytical predictions on particle settling provide a SURFACES useful tool for determining how the complex internal cooling A system for cleaning internal gas turbine cooling passages flows can affect particulate sedimentation. The agreement would be desirable, although implementation may require between two calculational approaches indicates that oxide considerable effort. Nevertheless, several options are available particulates under 0.5 µm are substantially conducted through for such deposit removal. They include on-line deposit removal the outer turn. A number of design considerations for a cooling system are by pulsing coolant flows or erodants and off-line cleaning outlined. Among these are absolute particulate removal at the procedures that do not require engine disassembly, including entrance to the coolant system, prediction of locations where chemical cleaning. However, the gas turbine coolant passages coolant flows and G-fields might deposit breakthrough are very small, and the existence of mechanical joints offers particulates, and provision for trapping re-entrained opportunities for chemicals to diffuse into the joints and to be deposits within the stator- and rotor- cooling circuits. Application of this extremely difficult to rinse out. Chemical cleaning would knowledge will enable the design of cooling systems that meet probably be a last resort and only be used upon turbine disassembly. performance requirements for the interval between regular inspections. A final caution must be discussed in connection with inter­ nal, gas turbine cooling systems. Even if the system operates suc­ ACKNOWLEDGMENTS cessfully, relative to a coolant particulate loading specification The authors would like to thank the Department of Energy to limit the buildup of deposits in both stator and rotor passages, for partial support of the study. They also appreciate the helpful subsequent release of previously deposited material is a system discussions with associates A. Whitehead and L. Tomlinson of hazard and must be handled in the system design. Deposit re­ GE Industrial & Power Systems. lease can allow particle agglomerate re-entrainment into the flowing coolant, which would potentially happen on system

6 REFERENCES Horner, M.W., Cohn, A., Smith, D.P., and Caruvana, A., Perry,JH (Ed), 1984, Chemical Engineers Handbook, 6th 1980, "Near Term Application -of Water Cooling," ASME Edition, Wiley & Sons, New York, pp5-64. 80GT159 Gas Turbine Conference New Orleans, LA. Tolpadi, AK, and Braaten, M.E., 1992, "Study of Branched Duncan, D., Vohr, J.H., Shalvoy, R.S., 1982, "Field Measure­ Turboprop Inlet Ducts Using a Multiple Block Grid Calculation ment of Solid Particle Erosion in Utility Steam Turbines," EPRI Procedure," ASME J. Fluids Engineering, Vol. 114, No.3, pp379- TR-100215, Research Project 1885-2, Final Report, January. 385. Yamaguchi, N., Taguchi, S., and Shida, H., 1982, "Fly-Ash Tolpadi, AK, 1994, "Calculation of Heat Transfer in a Radi­ Behavior in the Suction Box of Axial Induced Draft Fans for ally Rotating Coolant Passage," AIAA Paper 94-0261, accepted Coal-Fired Boilers", in Tabakoff, et al (Ed), Particulate Laden for publication by Numerical Heat Transfer (in press). Downloaded from http://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1995/78811/V004T10A014/2406576/v004t10a014-95-gt-281.pdf by guest on 30 September 2021 Flows in Turbomachinery, ASME, New York, p. 23. Tabakoff, W., "Measurements of Particle Rebound Charac­ Tabakoff, W., 1982, "Performance Deterioration on Turbo­ teristics on Materials Used in Gas Turbines," AIAA J. Propul­ machinery with Presence of Solid Particles," in Tabakoff, et al sion and Power, Vol. 7, pp.805-813. (Ed), Particulate Laden Flows in Turbomachinery, ASME. New Yo!X,p. 3.

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