Lady Davis Building 384
Thermodynamic Simulation of Continues Closed Loop Heated Micro-Turbine Test Facility
סימולציה טרמודינמית של מתקן לניסויי מיקרו טורבינות במעגל סגור עם יכולת חומום של הזרימה.
Thesis Type: M.Sc. Project
Faculty: Aerospace Engineering, Technion, Haifa
Adviser: Dr. Beni Cukurel
For a small gas turbine, in comparison to their larger counter parts, a reduced amount of fluid is supplied to an almost unchanged thermodynamic cycle; hence, the aero-thermo and chemo dynamics remain mostly the same . However, the work exchange between the compressor or the turbine and the fluid is proportional to the square of the peripheral rotor speed, therefore, since rotational speed scales inversely with diameter, miniaturization of turbomachinery components results in rotational speeds of the order 105 rpm . Nevertheless, a large decrease in Reynolds number is unavoidable . Thus, this not only alters the aero-thermal distribution on the blades and vanes , but also results in higher viscous losses . Therefore, the component’s, and hence the cycle’s, efficiencies are reduced .
In conclusion, above the inherent design complexity associated with all gas turbine engines, the physics associated with mini and micro gas turbines are further complicated by dimension-specific challenges. The small scales have detrimental implications on the efficiency of the components due to the increased viscous friction losses, augmented relative clearances, and constrained turbine inlet temperatures. Therefore, to obtain positive cycle efficiencies, the same design guidelines cannot be applied to large and miniaturized engines, and the key technological barrier towards broad spectrum implementation of micro gas turbines is lack of knowledge in aerodynamically coupled heat transfer and thermal management issues [5-7] .
Developmental project is to be structured around a versatile closed-loop pressurized high speed turbine facility, which will provide unique research capabilities to the global research environment. In the current academic world, a continuously running high pressure micro gas turbine facility, which is capable of matching all of the engine similarity conditions (Mach and Reynolds number, pressure ratio / loading, as well as operation representative blade to gas temperature ratio) does not exist, however this is most desirable. To this end, a wind tunnel is to be constructed, which will incorporate an interchangeable test section to provide hot (~600K) transonic conditions for fixed blade cascade and rotating high/low pressure turbine stages of micro-engines. In light of micro turbine operating conditions, the final capabilities are: maximum turbine diameter up to 200 mm, closed loop turbine pressure ratio up to 6:1, maximum mass flow rate of 1 kg/sec, transonic Mach distribution on the blades, rotational rate of 120,000 rpm and flow to metal temperature ratio of up to 2:1.
The unique facility is being developed in a manner which allows component testing of small scale high pressure turbines, as found in micro gas turbine generators, Figures 1. The main components comprising the closed test facility include a compressor, small pressure equalization tank to damp out transients, pressure drop valves, electric heater, test turbine, large dump tank and cooler, which reduces the inlet temperature to the compressor. The facility operates through the drive of a 500hp (370 kW) compressor which creates pressure ratios up to 9:1, with a maximum flow rate of 1 kg/sec. Considering the aerodynamic similarity parameters of the test turbine stage, the pressure ratio is set by combination of compressor operating point and the valve pressure loss mechanisms and the mass flow rate is predominantly defined by the compressor rotational speed. The initial mass introduced into the isolated system defines the Reynolds number, which is coarsely adjusted prior to the startup, and fine-tuned during operation. The closed loop nature of the facility allows independent aerodynamic testing of the micro gas turbine in engine representative conditions, independent from the other operational parameters including the engine speed line. Since small gas turbines are used with several low pressure turbine stages (energy generator), it is critical to decouple the nominal pressure level from the pressure ratio across the turbine. The only remaining aerodynamic parameter is the control of the turbine rotational
Figure 1: Conceptual Schematic of Continuous, Closed Loop, and Heated High Pressure Turbines Facility
speed, which can be established by connecting the turbine shaft to a loading mechanism.
The project’s goal is to thermodynamically model the operational principles in an interdependent multi-component system, which is controlled by independent variables (compressor speed, turbine speed, heater power and total mass enclosed in the system) and to predict the non-dimensional turbine parameters (mcor, Ncor, Re, Tflow/Tblade) associated with the experimental facility.
Procedure Steps and Working Method
1) Aerodynamic and Thermal Similarity Analysis of Micro Turbine Component
Via: Buckingham-Pi (non-dimensional) analysis of turbine stage in scaled conditions
2) Steady State and Transient Performance Analysis of Interdependent Components in a Closed Loop System
Via: Thermodynamic characterization of each component with moving control volume analysis
3) Simulation Computing Turbine Dimensional State at Specified Operation of Auxiliary Components
Via: Solving the simulation of the system iteratively to a converged state of dimensional turbine operation
4) Simulation Predicting Required Auxiliary Component Operational Points for a Desired Turbine State
Via: Creating a dense matrix of solutions states and interpolating among the set of independent parameters for a desired non-dimensional turbine state
Novelty of Modeling Effort
An investigation of a much simplified mathematical model for a cascade type wind tunnel (not a turbine testing facility) has been conducted by University of Notre Dame. The results of the effort are published in AIAA Ground Testing Conference (http://arc.aiaa.org/doi/abs/10.2514/6.2013-2491), and currently under peer-review for AIAA Journal of Thermophysics and Heat Transfer. The proposed work describes a similar investigation, applicable to the more challenging environment of closed loop micro-turbine testing.
Facility’s Relation to Previous Work
The present research proposal is concerned with conjugate micro turbine testing, where both time scales (aerodynamic and thermal) must be respected. This requires the capability of a continuously running facility, which can independently set Reynolds, Mach, and blade to flow temperature ratio. Globally, there are only a few turbine facilities operating at high speed. The short-duration rigs are a cost-effective way to generate engine similar conditions for a very small temporal frame (less than 500ms) and, it is possible to extrapolate the findings to steady aerodynamic operating conditions. Historically, three different types of transient tunnels (blow-down , shock-tube driven , and isentropic piston compression [10, 11]) have been developed. They are located at Wright Patterson Air Force Base, Ohio State University and University of Oxford, as well as von Karman Institute, respectively. The operation principles differ by their air supply and type of power absorption system. Cons of such short duration facilities include difficulties in: performing optical measurements due to the reduced test times characterizing aero-thermal performance mapping due to transient conditions conducting coupled heat transfer studies, due to large thermal time scales. Addressing these issues, there is limited number of high speed continuously running turbine research rigs, which include open  and closed loop   systems located at Graz University, and DLR Gottingen, as well as, ETH Zurich. Although the design philosophy of these closed loop test rigs, especially NG-Turb, is overlapping with the proposed high pressure micro turbine facility, the dimensional scales and the target market are significantly different. They are commissioned to satisfy the requirements of large physical scale aviation derivative gas turbines with higher mass flow rates at reduced rotational speeds. As the physics associated with micro gas turbines are complicated by dimension-specific challenges, the same design guidelines cannot be applied to large and miniaturized engines, and dedicated research laboratories are necessary to provide the scientific advancements in dimension specific challenges associated with micro-gas turbines.
Air breathing propulsion, fluid mechanics and heat transfer, measurement techniques and instrumentation, system analysis, turbomachinery
1. Peirs, J., et al., Micropower generation with microgasturbines: a challenge. Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, 2007. 221(4): p. 489-500.
2. Gong, Y., et al. Aerothermodynamics of micro-turbomachinery. in ASME Turbo Expo 2004: Power for Land, Sea, and Air. 2004. American Society of Mechanical Engineers.
3. Verstraete, T., Z. Alsalihi, and R. Van den Braembussche, Numerical study of the heat transfer in micro gas turbines. Journal of turbomachinery, 2007. 129(4): p. 835-841.
4. Walsh, P.P. and P. Fletcher, Gas turbine performance. 2004: Wiley. com.
5. Epstein, A.H., Millimeter-scale, micro-electro-mechanical systems gas turbine engines. Journal of Engineering for Gas Turbines and Power, 2004. 126(2): p. 205-226.
6. Fernandez-Pello, A.C., Micropower generation using combustion: issues and approaches. Proceedings of the Combustion Institute, 2002. 29(1): p. 883-899.
7. Epstein, A., et al. Micro Heat Engines, Gas Turbines, And Rocket Engines – The MIT Microengine Project. in 28th AIAA Fluid Dynamics Conference, 4th AIAA Shear Flow Control Conference, Snowmass Village, CO, AIAA Paper. 1997.
8. Anthony, R.J., et al., Modifications and upgrades to the AFRL Turbine Research Facility. ASME, Paper GT2012-70084, 2012.
9. Haldeman, C., et al., The USAF Advanced Turbine Aerothermal Research Rig(ATARR). AGARD CP-319, 1992.
10. Jones, T., D. Schultz, and A. Hendley, On the flow in an isentropic light piston tunnel. 1973: HM Stationery Office.
11. Arts, T., J. Duboue, and G. Rollin, Aerothermal performance measurements and analysis of a two-dimensional high turning rotor blade. Journal of turbomachinery, 1998. 120(CONF-970604–).
12. Erhard, J. and A. Gehrer, Design and construction of a transonic test turbine facility. ASME Paper 2000-GT, 2000. 480.
13. Kost, F. and P.-A. Giess, Experimental Turbine Research at DLR Goettingen. Journal of the Gas Turbine Society of Japan, 2004. 32(6): p. 485-493.
14. Behr, T., A. Kalfas, and R. Abhari, Unsteady flow physics and performance of a one-and-1/2-stage unshrouded high work turbine. Journal of turbomachinery, 2007. 129(2): p. 348.