Neelesh Nandkumar Sarawate

Magnetomechanical characterization and unified energy model for the quasistatic behavior of ferromagnetic shape memory Ni–Mn–Ga

This paper presents an overview of the characterization and modeling of single crystal ferromagnetic shape memory Ni–Mn–Ga. A continuum thermodynamics model is presented which describes the magnetomechanical characterization of single crystal Ni–Mn–Ga for the following behavior: (i) sensing effect; (ii) actuation effect; (iii) blocked force (stress generation). The thermodynamic potentials, namely the magnetic Gibbs energy and the Gibbs energy, are obtained from the Helmholtz energy in order to arrive at the set of required independent and dependent variables; the potentials include magnetic energy consisting of Zeeman, magnetostatic and anisotropy components, and mechanical energy consisting of elastic and twinning components. Mechanical dissipation and the microstructure of Ni–Mn–Ga are incorporated in the continuum model through the internal state variables volume fraction, domain fraction, and magnetization rotation angle. The constitutive response of the material is obtained by restricting the process through the second law of thermodynamics. The model requires only seven parameters identified from two simple experiments. Several interesting characteristics of Ni–Mn–Ga are examined in concert with the magnetomechanical characterization.

Dynamic sensing behavior of ferromagnetic shape memory Ni?Mn?Ga

This paper is focused on the characterization and modeling of a commercial Ni?Mn?Ga alloy for use as a dynamic deformation sensor. The flux density is experimentally determined as a function of cyclic strain loading at frequencies from 0.2 to 160?Hz. With increasing frequency, the stress versus strain response remains almost unchanged whereas the flux density versus strain response shows increasing hysteresis. This behavior indicates that twin-variant reorientation occurs in concert with the mechanical loading, whereas the rotation of magnetization vectors occurs with a delay as the loading frequency increases. The increasing hysteresis in magnetization must be considered when utilizing the material in dynamic sensing applications. A modeling strategy is developed which incorporates magnetic diffusion and a linear constitutive equation. The model is used to describe the hysteretic dependence of magnetic flux density on strain at dynamic frequencies.

Characterization of Metallic W-Seals for Inner to Outer Shroud Sealing in Industrial Gas Turbines

Sealing and clearance control are two of the most cost effective methods to reach desired goals of efficiency, power output, operational life and emission levels in turbomachinery. Metallic seals such as W seals are widely used in gas turbines to seal axial gaps between adjacent static components such as shrouds and nozzles. Often the seals are characterized in a laboratory controlled environment, and the test results are used in modeling the secondary flows. However in real operating conditions, the static components can shift relative to each other creating misalignments that result in non-uniform sealing surfaces. One such application includes sealing between the stage 1 outer and inner shrouds. The inner shrouds are often stacked with axial misalignments relative to the neighboring shrouds due to manufacturing and assembly tolerances. Characterizing the effect of shroud misalignments on the W seal leakage is reported in this article. A comprehensive test matrix is conducted to characterize W seal leakage for four different magnitudes of shroud offsets, three types of seals having varying stiffness, and two compression levels. It is observed that the W seal leakage is fairly insensitive to the compression levels and the type of seal at zero misalignments. The seal leakage increases substantially with misalignments up to four times than for a perfectly aligned condition. The seal behavior also changes with increasing offsets. The seal exhibits typical properties of a positively loaded member for small misalignments, however, the behavior resembles a loose seal for larger misalignments. For a positively loaded seal, the effective clearance of the seal increases with pressure differential, whereas in case of a loose seal, the effective clearance decreases with increasing pressure differential. The effect of misalignments must be considered when modeling the seal in the engine flow models using a weighted average of the effective clearance.Copyright

Development of Laminated Seals for Stator-Stator Sealing in Gas Turbines

This paper describes the development of laminated seals for stator-stator sealing in gas turbines. Cloth seals were introduced as stator-stator seals in the 1990’s and resulted in a significant improvement in sealing performance over the rigid seals then in service. Laminated seals presented here are proposed as an improvement to the existing cloth seals. They demonstrate improved leakage performance over cloth seals in aligned and offset conditions. Several versions of laminated seals were developed and tested before arriving at a seal geometry that satisfied the leakage, manufacturability and assembly requirements. These seals therefore provide an alternative to cloth seals that leak less and are durable, cost-effective and robust in assembly.Copyright

Stiffness and Leakage Characterization of Ceramic-Metallic Rope Seals

Ceramic-metallic rope seals are suitable for use in high-temperature and low-cycle displacement applications. This paper discusses performance aspects that need to be considered during seal selection such as the maximum load generated at assembly and during operation, seal resiliency and leakage performance for a given application. Extensive load-displacement and leakage tests are conducted on several candidate seals. Selected rope seals are of hybrid seal type consisting of ceramic fiber core along with an outer sheath of fine metallic wires. Test parameters include rope manufacturers, rope diameters, and combination of core/sheath materials. Testing is performed on as-received ropes as well as ropes that are heat-treated at temperatures over 1000°F. The load-displacement behavior is non-linear, hysteretic and varies with the number of loading cycles. The force generated by the rope at a given displacement decreases with increasing number of loading cycles for a given displacement. The ropes do not recover to their original size after the first cycle of compression, and the residual height decreases with increasing cycles. The seal behavior tends to saturate after about 5 loading cycles. The rope selection for a particular application needs to consider the saturating residual deflection and maximum force generated. Leakage testing was also performed for rope seals placed in a rectangular race-track type geometry. The seals demonstrate good leakage resistance under desired pressure conditions. The ropes with smaller diameters show the best leakage performance, but exhibit low spring-back.Copyright