Browsing by Subject "Actuator"
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Item Analysis of oscillating flow cooled SMA actuator(Texas A&M University, 2005-11-01) Pachalla Seshadri, RajagopalShape Memory Alloys (SMA) are a group of metallic alloys that have the ability to return to some previously defined shape or size when subjected to an appropriate thermal cycling procedure. In recent years there has been a lot of research on the development of small, light and, yet, powerful actuators for use in areas like robotics, prosthetics, biomimetics, shape control and grippers. Many of the miniaturized conventional actuators do not have sufficient power output to be useful and SMAs can be used advantageously here. The widespread use of SMAs in actuators is limited by their low bandwidth. Use of SMAs in two-way actuators requires that they undergo thermal cycling (heating and cooling). While SMAs can be heated quickly by resistive heating, conventional convection cooling mechanisms are much slower as the exothermic austenitic to martensitic phase transformation is accompanied by the release of significant amount of latent heat. While a number of cooling mechanisms have been studied in SMA actuator literature, most of the cooling mechanisms involve unidirectional forced convection. This may not be the most effective method. Oscillating flow in a channel can sometimes enhance heat transfer over a unidirectional flow. One possible explanation for this heat transfer enhancement is that the oscillatory flow creates a very thin Stokes viscous boundary-layer and hence a large time-dependent transverse temperature gradient at the heated wall. Therefore heat transfer takes place at a large temperature difference, thereby enhancing the heat transfer. In this work, the heat transfer from an SMA actuator under an oscillating channel is investigated and is compared to steady, unidirectional flow heat transfer. Oscillating flow is simulated using a finite volume based method. The resulting velocity field is made use of in solving the heat transfer problem using a finite difference scheme. A parametric study is undertaken to identify the optimal flow conditions required to produce the maximum output for a given geometry of the SMA actuator. The latent heat of transformation of the SMA is accounted for by means of a temperature dependent specific heat.Item Simulations of atmospheric pressure plasma discharges(2013-08) Breden, Douglas Paul; Raja, Laxminarayan L.This document presents a study of the numerical simulation of non-equilibrium plasma discharges in air mixtures in the atmospheric pressure regime. Such plasma is formed by applying a very high electric field over a very short time duration (nano-microsecond) which preferentially heats the electrons to very high temperatures (10 electron Volts or more) while preventing thermalization of the gas. Preferentially heating the electrons to very high temperatures allows the discharge to efficiently and rapidly ionize and dissociate the gas mixture without losing too much energy to thermalization or vibrational excitation. Consequently, two useful characteristics of these discharges are low gas temperatures and rapid electron chemistry. This study focuses on two applications of interest: ignition of fuel-air mixtures and plasma enhanced medicine. For ignition, there are two situations that arise where it is difficult for traditional spark ignition systems to operate. The first is at the supersonic flow regime where the residence time of the flow in the engine is low. The second is high pressure ignition of lean fuel-air mixtures. For plasma medicine and surface treatment, non-equilibrium plasma is an effective means of delivering reactive radical species to the surface while limiting damage due to thermal heating. The problems of interest are characterized by the formation of weakly ionized plasma in the presence of flow fields such as supersonic boundary layers or low speed jets. To simulate the coupled plasma-fluid flow physics of these discharges, two numerical tools are utilized. The first is a two-temperature, multiple species, self-consistent plasma solver with finite rate chemistry which is used to simulate the plasma as it forms in a neutral background gas. The second tool is a multiple-species compressible flow solver which calculates the flow field properties of the background gas mixture.Item Thermomechanical Cyclic Response of TiNiPd High-Temperature Shape Memory Alloys(2012-10-19) Atli, KadriTiNiPd high-temperature shape memory alloys (HTSMAs) have attracted considerable attention as potential solid-state actuators capable of operating at temperatures up to 500 ?C, exhibiting excellent corrosion resistance, adequate ductility levels and significant strain recovery under both constrained and unconstrained thermomechanical conditions. During operation, these actuators may be subjected to multiple cycles and from an application point of view, the functional stability, i.e. conservation of original actuator dimensions and transformation temperatures during repeated employment, is of considerable importance. This study addresses the issue of functional stability in a model HTSMA, Ti50.5Ni24.5Pd25, for its use as a compact solid-state actuator. Since the primary reason for functional instability is the creation of lattice defects (dislocations, vacancies, etc.) during repeated transformation cycles, several methods were successfully undertaken to improve the functional stability through inhibiting the generation of these defects. Solid-solution strengthening through Sc microalloying and thermomechanical treatments via severe plastic deformation were the two approaches used to strengthen the HTSMA against defect generation. Thermal cycling the HTSMA under stress was the third method to voluntarily introduce defects into the microstructure such that further defect generation during application would be impeded. Overall, severe plastic deformation was found to be more efficient than other strengthening methods in improving the functional stability of TiNiPd HTSMA, yet it brought about disadvantages such as reduction in transformation strain and transformation temperatures. While functional instability is due to the creation of lattice defects, the generation of these defects is mainly controlled by the crystallographic incompatibility between martensitically transforming phases and the strength levels for plastic deformation. It was shown that TiNiPd HTSMAs, which exhibited martensitic transformation from a cubic (B2) to orthorhombic (B19) symmetry, illustrated better compatibility and thus better functional stability levels compared to TiNi SMAs, which had a B2 to monoclinic (B19?) transition. Although crystallographic incompatibility seems to be the governing factor for the functional stability of the TiNiPd HTSMA, the strength differential between the onset of plastic deformation and local constraint due to the martensitic transformation was also found to be an influential factor determining the overall stable behavior. Functional stability was also investigated for the two-way shape memory effect (TWSME) in TiNiPd HTSMAs. Better strength and compatibility levels compared to TiNi SMAs were also reflected in the TWSME characteristics in the form of enhanced stability under stress-free thermal cycling. The stability during constrained thermal cycling was not as good and TWSME degraded rapidly while doing work against an opposing stress. Nevertheless, work output levels were much higher as compared to those obtained from conventional TiNi and Cu-based SMAs.