Gas Turbine Engine Test Cell Modeling

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York, N.Y GT-244 The Society shall not be responsible for statements or opinions advanced in papers or in do - cussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. c Discussion Is pnnted only it the paper is published in an ASME Journal. Papers are available ]^L from ASME for fifteen months alter the meeting. Pnnted in USA. Copyright 1990 by ASME Gas Turbine Engine Test Cell Modeling D. SALINAS Naval Postgraduate School Monterey, CA (ASME Member) E. E. COOPER Naval Civil Engineering Laboratory Port Hueneme, CA (ASME Member) A numerical simulation of the aerothermal characteristics of a gas turbine engine test cell is presented. The three-dimensional system is modeled using the PHOENICS computational fluid dynamics code. Results predict the velocity field, temperatures, pressures, kinetic energy of turbulence, and dissipation rates of turbulent kinetic energy. Numerical results from two versions, a cartesian coordinate model and a body fitted coordinate model, are compared to experimental data. The comparison shows good quantitative and very good qualitative agreement, suggesting that numerical modeling would be useful in the preliminary design of gas turbine test facilities. Introduction To assure acceptable performance under all conditions, new and overhauled jet engines must pass a rigorous certification test prior to installation in U.S. Navy aircraft. The jet engines undergo the certification tests in test cell facilities at various locations around the world. Test cells are fully enclosed, sound absorbent "hangers" capable of dissipating the energy of the engine exhaust flow while the engine is put through a controlled test series, and its performance is monitored. The requirement for enclosed test cells with noise abatement arose during the 1950s with the introduction of jet engines. Over the years, the U.S. Navy has had several test cell designs. As engine exhaust temperatures and mass flow rates increased, older test cell designs became obsolete. At the present time, the T-10 test cell, illustrated schematically in Figure 1, is considered the "standard" Navy test cell. It is capable of testing all carrier-based turbofan and turbojet engines in the Navy inventory. Proper operation of a test cell requires that: (1) noise levels are reduced sufficiently for personnel to work in the vicinity of the test cells without ear protection; (2) distortion-free airflow is provided to the engine inlet; and (3) in some locations, gases exiting the test facility meet air quality standards. The principles governing the operation of a test cell are quite simple. Ambient air entering through the intakes is entrained by the jet engine exhaust. This provides a transfer of energy and momentum from the hot, high velocity jet to the cool, slow entrained air. Further transfer of momentum and energy takes place as the air moves through the augmenter tube into the exhaust stack for emission to the environment. Although the operating principles of test cells are simple, the physics of the flow is extremely complex. Complications inherent to test cells include turbulence; non-concentric flow; subsonic, transonic, and supersonic velocities; extreme temperature and pressure gradients; and complex three-dimensional surfaces. Historically, test cells have been designed without benefit of detailed analyses of the aerodynamic and thermodynamic characteristics of the gas flows through the cells. Designers relied upon their experience, on scale model test data for which scaling laws (particularly of turbulent mixing parameters) were vague, and on test data and empirical aerothermal relationships acquired from older designs. Recently, computational fluid dynamics (CFD) computer programs such as the PHOENICS code have become available. These codes can be used to mathematically determine the complex aerothermal characteristics of test cells and, therefore, hold promise as a design tool. However, experience is required for the efficient use of such programs, and there must be confidence in the results. One important aspect of the U.S. Navy test 'Presented at the Gas Turbine and Aeroengine Congress and Exposition June 11-14, 1990 Brussels ; Belgium

2 cell R&D program is to determine the feasibility of using CFD codes to predict the heat transfer and fluid flow characteristics of jet engine test facilities. Jet Engine Test Cells A typical test cell facility is comprised of five sections: a test bay, primary and secondary air intakes, an augmenter tube, and an exhaust stack. See Figure 1. The engine is mounted on a platform in the test bay. Combustion air for the jet engine enters through the primary air intake. Some of the primary intake air flows around the engine to mix with the hot engine exhaust. A secondary intake provides additional cool ambient air for mixing with the engine exhaust. The augmenter tube enhances mixing of the hot jet exhaust and the cooler ambient air it is dragging along. In addition, absorption and dissipation of acoustic energy takes place in the augmenter tube and the exhaust stack. The exhaust stack turns the horizontal flow through the tube into a vertical flow for discharge into the environment. Gas turbine engine test cell components and their functions are described in more detail in Reference 1. Modeling of the Test Cell Facility The test facility at Cubi Point in the Republic of the Philippines is simulated here as an example. The PHOENICS code is used to model the facility. PHOENICS is a general purpose finite volume program for mathematically modeling fluid flow, heat and mass transfer phenomena (Reference 2). Two versions of the test cell model are developed, a cartesian coordinate (CC) model and a body fitted coordinate(bfc) model. With a cartesian coordinate model, all geometric boundaries are necessarily parallel to one of the three perpendicular planes formed by the three orthogonal axes of the coordinate system, e.g., the x, y, z axes. Boundaries of the problem are "modified" to make them rectangular. Figure 2 shows the PHOENICS grid used for the Cubi Point cartesian coordinate model. With a BFC system, the grid can be body fitted to the actual shapes of the structure's components as shown in Figure 3. Both versions are three-dimensional. However, in order to keep the problem within manageable size, a vertical plane of symmetry at the midplane of the structure is imposed, and thus only one half of the structure was analyzed. Figure 4 illustrates this half structure. This restriction requires that the center plane of the engine coincides with the midplane of the structure. To further simplify the grid, primary intake turning vanes are neglected; the front of the test bay is assumed to be open. PHOENICS' assumptions include the use of the ideal gas law for the determination of density, the log law for wall friction, and the k-epsilon turbulence model (Reference 2). The jet engine is modeled as a solid cylindrical body with a specified mass flow sink at the intake end, and, an equal mass flow source at the exhaust end, and a specified exhaust temperature. One important goal of the present work is to compare the results obtained from the cartesian model with the results from the body fitted coordinate model to assess the need for BFC. In addition to the engine, body fitted coordinates are used for the cylindrical augmentor tube, the inclined ramp at the base of the exhaust stack, and for the complex curved surface walls in the transition region. See Figures 3 and 4. Because iteration convergence of a body fitted coordinate simulation requires a nearly orthogonal grid, the generation of a BFC grid is arduous and time consuming. Computer storage requirements and run times are also about an order of magnitude greater when BFC are used. Establishing the y-z plane as the symmetry plane of the structure, the half model is comprised of 12 segments in the x direction, then 27 segments in the y direction and 40 segments in the z direction for a total grid mesh divided into 12,960 three dimensional segments. Seven response variables per cell (pressure, temperature, 3 velocity components, kinetic energy of turbulence and dissipative rate of turbulent kinetic energy) result in a total of unknowns and equations. Typical Results Figures 5, 6 and 7 illustrate the type of information that can be acquired through the use of aerothermal models. Figure 5 shows predicted gas velocities throughout the symmetry plane of the test cell. Engine nozzle vectors have been deleted to keep from cluttering the plot. Of particular interest in this figure is the mixing of the jet and secondary air along the augmentor tube. By the time the jet reaches the end of the tube, mixing is complete; the velocity profile resembles the classical profile for turbulent flow through a pipe. Such a condition of fully developed flow is optimum for minimizing both thermal and noise discharge to the surroundings. Less desireable and perhaps a problem area are the large velocity gradients that develop against the ramp forming the back of the exhaust stack. Figure 6 is an expanded view of the test bay and secondary intake. Again the nozzle velocity vectors have been deleted. The flow in the test bay is evenly distributed, approaching the engine straight on and with velocities less than the 15 m/sec design maximum. Mixing of the jet and secondary flow appears to be smooth with only small velocity gradients and no indication of hot exhaust recirculating back around to the engine inlet. A finer grid is used for this important region to increase the accuracy of the simulation. Thermodynamic properties of the system can be examined in a similar manner. Figure 7 is an example, plotting isotherms through the engine exhaust and down the augmentor tube. The rapid cooling of the jet is immediately apparent. Transfer of thermal energy to the secondary flow is occurring more quickly than the transfer of momentum. Compare Figure 7 with Figure 5. (Radiation from the jet nozzle and convection to the test cell walls have not yet been considered but both are pertinent 2

3 phenomena and will be added to the models.) The hot core of the jet never reaches the augmentor tube walls. An aspect of test cell behavior of great concern is the quality of the gas being discharged from the exhaust stack into the environment. Both simulation models are in good agreement with discharge temperatures ranging from 400 degrees Kelvin to 620 degrees Kelvin, at velocities from minus 15 m/sec. to plus 80 m/sec. The negative velocity is associated with a recirculation region in the exhaust stack. The models also predict recirculation zones in the secondary inlet, and in the augmentor tube. Accuracy of the Models Figures 8 through 11 show comparisons between the simulation models and experimental results (Reference 1). Both versions of the Cubi Point jet engine test cell model showed fair to very good quantitative agreement, and very good to excellent qualitative agreement with one another and to the available experimental results. Although it was expected that the body fitted coordinate model would show better agreement with experimental results, for the most part the cartesian coordinate model was in fact in better agreement. The additional refinement associated with the body fitted model was negated by the selection of the k-epsilon turbulence model which does not work well with grids which contain non-orthogonal cells. This problem is expected to be resolved in future updated versions of the code. It should also be pointed out that time and machine limitations did not permit analyses with more refined grids. The grids used in these analyses were "minimum" grids in that they were grids with the smallest number of cells that led to converged solutions. Thus the results obtained in this work are not likely to be "grid independent" solutions. Nevertheless, in the spirit of this work, which is to show the potential for using CFD codes in conjunction with experiments for test cell design, the following comparisons are provided. Figure 8 shows that both models capture the qualitative nature of the flow temperature along the augmentor tube, with the cartesian model comparing very well with the experimental results, and the body fitted model providing only a fair comparison. Figure 9 is a plot of flow temperature across the horizontal diameter at two locations (9 meters and 18 meters) down the tube. Here the results of the cartesian model show good agreement with the experimental results, while the results of the body fitted model show only fair quantitative agreement. Figure 10 shows flow velocities as a function of distance along the vertical diameter of the augmentor tube at a location 18 meters down the tube. Again the cartesian model provides good agreement, while the body fitted model provides only a fair agreement. Figure 11 shows static pressures (in inches water) along the axis of the augmentor tube. The model predicted results are qualitatively good and quantitatively fair. Conclusions The results of this work show that under certain conditions, stated below, CFD codes may be an effective tool in the design of test cell facilities. Some of the factors that determine the effectiveness of CFD modeling are as follows. In order to obtain accurate predictions, the user of a CFD code needs to know not only what modeling features of the system are required, but also their relative importance in the overall synthesis of models. That is, what heat transfer modes are relevant? How does radiation transfer compare with convection and conduction transfer? What are the appropriate turbulence models? What are the "correct" boundary conditions for the model? Exclusion of phenomena, or the selection of inappropriate models will result in a poor model. The PHOENICS CFD code selected for this work provides a very extensive model option menu for user selection. The results of this investigation suggest that the k-epsilon model did not work well with the BFC model. There is the question of whether some other turbulence model would have yielded better results. If sufficient computer resources are available, it becomes possible to try alternate models and then select those which gives results which compare best with experimental results. The accuracy of CFD models depends upon the number of grid cells used in modeling of the system. The more grid cells one uses, the better the results, but also the more iterations needed to obtain a converged solution. To obtain more accurate results, the large and complicated three dimensional Point Cubi test cell structure with its complex aerothermal behavior requires a more refined grid model than was possible for the desk top VAX-2000 computer system used in this work. A bigger and faster computer would have provided better quantitative predictions than was obtained with the VAX-2000 computer. Finally, there is the question of the value of CFD modeling of test cell facilities. Without numerical models, the complexity of the aerothermodynamics of test cell facilities makes prediction of changes in behavior due to system modification extremely difficult if at all possible. Although smaller scaled physical replica models can be built and used, there is the problem of scaling up the experimental results to predict the behavior of the corresponding full size structure. On the other hand, the modification of a numerical model to reflect anticipated changes in dimensions is not at all difficult. Moreover once a converged solution for a model is obtained, changes in dimensions, to study their effect on system performance, can be easily accommodated into the model and convergence is achieved without too much additional computational effort. In this way, numerical models using CFD codes can effectively be used by engineers in the process of test cell design. It is important that model predictions be checked by experimental data. 3

4 References 1. Kodres, C.A., Cooper, E.E., and Stone, P.L., "Experimental Examination of the Aerothermal P erformance of the T-10 Test Cell at NAS, Cub! Point", Naval Engineering Laboratory Technical Note N- 1788, September Rosten, H.I. and Spalding, D.B., "The PHOENICS Beginner's Guide, CHAM TR/100, Revision 4, Wimbledon, London, England, October

5 PRIMARY INTAKE TEST BAY SECONDARY INTA E AUGMENTER TUBE EXHAUST STACK ACOUSTIC BAf FEES ACOUSTIC BAFFLES TURNING MNEB ^vl ENGINE E THROTTLE ACOUSTIC EXHAUST PLATE LINING RAMP Figure 1. Components of the standard Navy test cell, Nas, Cubi Point. (b) Side view Figure 2. Grid pattern for cartesian coordinate model of T-10 test cell. 5

6 Figure 3. Perspective view of the Cubi Point test cell CFD grid using body fitted coordinates. Figure 4. Cubi Point test cell BFC grid emphasizing vertical symmetry plane. 6

7 Vector scale >400 rn/sec J : it au, luau. - :..:asal..: : : : t I ii a a ----> z Figure 5. Velocity field in the vertical symmetry plane of the NAS Cub! Point test facility. to j.4 Vector scale ). 10 m\sec ', f,, f i ). Figure 6. Velocity field (m/sec) in the test bay and secondary intake. 7

8 Figure 7. Isotherms in the symmetry plane of the NAS Cubi Point gas turbine engine test cell. 40C U 35C 0) N c U 30C 25C 0 0N 16 20C Cl) II) 15C a) O. E a> IOC 0 U_ 5C C U 4 b 1L 1b LU 24 Lb Distance along augmenter tube, meters Figure 8. Comparison of analytical model results and experimental data for flow temperatures at the acoustic liner plates. 8

9 Imo`` a) (Ii E 300 Li. 200 W Experimental BFC Analysis 9m down tube (( 9m down tube data from NAS BFC Analysis 18m down tube 18m down tube Cubi Point, T-10 + CC Analysis 9m down tube E stack ramp test cell 0 p CC Analysis---18m down tube I 0 0 r-url 2 4 oidfuueuu Position across horizontal diameter of augmenter tube, meters I F- Figure 9. Comparison of analytical models and experimental data for flow temperatures in the augmenter tube (3) ~^j a 110 E 100 ai j Ii Comparison for 18m down tube Experimental data 0-- Throttle plate removed from NAS Cubi Point, + Throttle plate installed T-10 test cell BFC Analysis Throttle plate installed CC Analysis Throttle plate installed 0 toonoin) 2 4 trop Distance along vertical diameter of augmeter tube, meters Figure 10. Comparison of analytical models and experimental data for flow velocities in the augmenter tube. 9

10 O 2- U) 1 v -1 0) -2- (5 3 0) 4. 5 N U) \. J s l Experimental 0 Port side data from WAS Cubi Point, 0 + Top Bottom T-10 test cell A Test Bay BFC Analysis CC Analysis Distance along augmenter tube, meters Figure 11. Comparison of analytical model and experimental data for pressure variation in the secondary intake and along the augmenter tube. 10