Comparison of Transient Modeling Techniques for a Micro Turbine Engine

A large number of papers have been published on transient modeling of large industrial and military gas turbines. Few, however, have examined micro turbines. The decrease in size affects the relative rates of change of shaft speed, gas dynamics and heat soak. This paper compares the modeled transient effects of a micro turbojet engine comprised of a single stage of radial compression and a single stage of axial expansion, with a diameter of 12cm. The model was validated with experimental data. Several forms of the model were produced starting with the shaft and fuel transients. Conservation of mass, and then energy, was subsequently added for the compressor, combustor and turbine, and a large inlet plenum that was part of the experimental apparatus. Heat soak to the engine body was incorporated into both the shaft and energy models. Heat soak was considered in the compressor, combustor and turbine. Since the engine diameter appears in the differential equations to different powers, the relative rates of change vary with diameter. The rate of change of shaft speed is very strongly influenced. The responses of the different transient effects are compared. The relative solution times are also discussed, since the relative size of the required time steps changes when compared to a large engine.

Lifetime Prediction of Components Including Initiation Phase

The lifetime distribution of a component subjected to fatigue loading is calculated using a micro-mechanics model for crack initiation and a fracture mechanics model for crack growth. These models are implemented in a computer code which uses the local stress field obtained in a Finite Element analysis as input data. Elemental failure probabilities are defined which allow to identify critical regions and are independent of mesh refinement. An example is given to illustrate the capabilities of the code. Special emphasis is put on the effect of the initiation phase on the lifetime distribution.

Additional Engine Testing of an Advanced Alloy for Microturbine Primary Surface Recuperators

HAYNES ® alloy HR-120 ® is being evaluated as a replacement for type 347 stainless steel for use in Microturbine Primary Surface Recuperators. The material has been characterized after being subjected to both steady-state and cyclic engine exposure in a Capstone C60 MicroTurbine™ operating at 100°F above the normal operating temperature. Oxide scale growth and elemental depletion has been analyzed and documented after 1,800 and 2,500 hours of exposure. A preliminary estimate of the remaining usable oxidation life has been made using a simplified parabolic model. Engine test results indicate that HR-120 has improved oxidation resistance and elemental stability.

Analytical Derivatives Technology for Parametric Shape Design and Analysis in Structural Applications

The ability to perform and evaluate the effect of shape changes on the stress, modal and thermal response of components is an important ingredient in the ‘design’ of aircraft engine components. The classical design of experiments (DOE) based approach that is motivated from statistics (for physical experiments) is one of the possible approaches for the evaluation of the component response with respect to design parameters [1]. Since the underlying physical model used for the component response is deterministic and understood through a computer simulation model, one needs to re-think the use of the classical DOE techniques for this class of problems. In this paper, we explore an alternate sensitivity analysis based technique where a deterministic parametric response is constructed using exact derivatives of the complex finite-element (FE) based computer models to design parameters. The method is based on a discrete sensitivity analysis formulation using semi-automatic differentiation [2,3] to compute the Taylor series or its Pade equivalent for finite element based responses. Shape design or optimization in the context of finite element modeling is challenging because the evaluation of the response for different shape requires the need for a meshing consistent with the new geometry. This paper examines the differences in the nature and performance (accuracy and efficiency) of the analytical derivatives approach against other existing approaches with validation on several benchmark structural applications. The use of analytical derivatives for parametric analysis is demonstrated to have accuracy benefits on certain classes of shape applications.

Forced Response Variation of Aerodynamically and Structurally Mistuned Turbo-Machinery Rotors

The paper presents the results of a study looking into changes in the forced response levels of bladed disc assemblies subject to both structural and aerodynamic mistuning. A whole annulus FE model, representative of a civil aero-engine fan with 26 blades was used in the calculations. The forced response of all blades of 1000 random mistuned patterns was calculated. The aerodynamic parameters, frequency shifts and damping, were calculated using a three-dimensional Reynolds-averaged Navier-Stokes aero-elasticity code. They were randomly varied for each mistuning pattern, with the assumption that the system would remain stable, i.e. flutter would not occur due to aerodynamic mistuning. The results show the variation of the forced response with different types of mistuning, with structural mistuning only, with aerodynamic mistuning only and with both structural and aerodynamic mistuning.

Numerical and Experimental Analysis of the Thermo-Mechanical Load on Turbine Wheels of Turbochargers

CFD, FEA, and experimental testing have been combined in order to investigate the lifetime limiting design deficiencies of a turbine wheel in a turbo charger. Thermocouples have been applied to the same radial turbine wheel to provide boundary conditions and validation data for the simulations. The tests have been performed on a turbocharger gas-stand. Based on two steady state CHT-calculations for two distinctly different operating points the heating process of the wheel has been simulated in a transient temperature calculation. Since the resulting temperature gradients induce thermal stresses, the temperature distribution serves as a boundary condition for the subsequent structural analysis. To obtain realistic stress distributions, centrifugal forces also need to be accounted for. In this way, the influence of the thermal stress on the overall stress can be evaluated.

Technology Insertion in the U.S. Navy

With a new perspective on how to conduct business through acquisition reform, the Navy faces infrastructure challenges that are not necessarily in sync with acquisition principles. Historically, the U.S. Navy (Navy hereinafter) has always spearheaded the means of developing design for form, fit, and function of Navy machinery. This leadership role has its roots on the unique requirements that the U.S. Navy has to fulfill its mission. Unfortunately, this process does not always prove to be cost effective since its implementation normally carries heavy restrictions, unique applications, and little competition. This is commonplace for most technology insertion efforts into Navy Ships.

Experimental Results From Controlled Blade Tip/Shroud Rubs at Engine Speed

Experimental results obtained for an Inconel compressor blade rubbing a steel casing at engine speed are described. Load cell, strain gauge and accelerometer measurements are discussed and then applied to analyze the metal-on-metal interaction resulting from sudden incursions of varying severity, defined by incursion depths ranging from 13 μm to 762 μm (0.0005-in to 0.030-in). The results presented describe the transient dynamics of rotor and casing vibro-impact response at engine operational speed similar to those experienced in flight. Force components at the blade tip in axial and circumferential directions for a rub of moderate incursion depth (140 μm) are compared to those for a severe rub (406 μm). Similar general trends of variation during the metal-to-metal contact are observed. However, in the nearly three-fold higher incursion the maximum incurred circumferential load increases significantly, while the maximum incurred axial load increases much less, demonstrating the non-linear nature of the rub phenomena. Concurrently, the stress magnification on the rubbing blade at root mid-chord, at tip leading edge, and at tip trailing edge is discussed. The results point to the possibility of failure occurring first at the airfoil trailing edge. Such a failure was in fact observed in the most severe rub obtained to date in the laboratory, consistent with field observations. Computational models to analyze the non-linear dynamic response of a rotating beam with periodic pulse loading at the free-end are currently under development and are noted.

A Design to Improve the Effective Damping Characteristics of Hole-Pattern-Stator Annular Gas Seals

Predictions are presented for hole pattern seals for which the hole depth is varied axially, showing that various depth patterns can significantly improve damping performance in terms of both increasing the seal’s effective damping and reducing its cross-over frequency — the frequency at which effective damping changes from positive to negative. Test results are presented for the seal with the best variable hole depth (VHD) pattern showing an increase in peak damping by a factor of 1.6 over a constant hole depth (CHD) design, versus predictions of 2.7. The cross over frequency is reduced by approximately 40%. The VHD seal has a significant negative static stiffness that probably arises from a friction-factor jump phenomena, not flow path divergence. The VHD seal leaks less than a comparable CHD design.

Stability Increase of Aerodynamically Unstable Rotors Using Intentional Mistuning

A new simple asymptotic mistuning model (AMM), which constitutes an extension of the well known Fundamental Mistuning Model for groups of modes belonging to a modal family exhibiting a large variation of the tuned vibration characteristics, is used to analyze the effect of mistuning on the stability properties of aerodynamically unstable rotors. The model assumes that both, the aerodynamics and the structural dynamics of the assembly are linear, and retains the first order terms of a fully consistent asymptotic expansion of the tuned system where the small parameter is the blade mistuning. The simplicity of the model allows the optimization of the blade mistuning pattern to achieve maximum rotor stability. The results of the application of this technique to realistic welded-in-pair and interlock low-pressure-turbine rotors are also presented.