2. Thesis and Dissertations

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    Experimental Investigations on Single/Two Phase Carbon Dioxide Based Natural Circulation Loops
    (National Institute of Technology Karnataka, Surathkal, 2021) R, Thippeswamy L.; Yadav, Ajay Kumar.
    The natural circulation loop (NCL) is a highly reliable and noise-free heat transfer device due to the absence of moving components. Working fluid used in the natural circulation loop plays an important role in enhancing the heat transfer capability of the loop. This experimental study investigates the thermo-hydraulic performance of subcritical and supercritical CO2 based natural circulation loop (NCL). In the subcritical region, different phases of CO2 such as liquid, vapour and two phase are considered for the study. Operating pressures and temperatures are varied in such a way that the loop fluid should remain in the specified state (subcooled liquid, two phase, superheated vapor, supercritical). Water and methanol are used as external fluids in cold and hot heat exchangers for temperatures above and below 0 °C respectively, depending on the operating temperature. For loop fluids, the performance of CO2 is compared with water for above zero and with brine solution for the subzero case. Further, the impact of loop operating pressure (35-90 bar) on the performance of the system is also studied. Results are obtained for hot heat exchanger inlet temperature ranging from 5 to 70 °C and cold heat exchanger inlet temperature from -18 to 32 °C. It was observed that the maximum heat transfer rates in the case of subcritical vapor, subcritical liquid, twophase and supercritical CO2 based systems are 400%, 500%, 900%, and 800% higher than the water/brine-based system respectively. However, the instability associated with a regular change in fluid flow direction due to an imbalance between friction and buoyant forces is a major disadvantage of NCL. Tilting the entire loop by a certain angle is one of the method to reduce the instability of NCL, with an inherent penalty in heat transfer and pressure drop. Pressure drop and heat transfer performance of CO2 based NCL with end heat exchangers, operating under subcritical (vapour, liquid and two phase) and supercritical conditions for various tilt angles (0°, 30°, 45°) in XY and YZ planes, have been investigated. Results are obtained for different operating pressures (35-90 bar) and temperatures (-18 to 70 °C). Results show that the effect of tilting on heat transfer rate, pressure drop and temperature distribution in the case of supercritical, subcritical liquid, subcritical vapour and two phase CO2 based system shows a small variation on the performance of the loop. Hence, loop tilting could be a one of the solution to reduce the instability problem associated with NCL without compromising the performance of the loop.
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    Dynamic Behaviour of “Hybrid Composite Shaft Rotor-Bearing System” of an Aero Gas Turbine Engine
    (National Institute of Technology Karnataka, Surathkal, 2021) Gonsalves, Thimothy Harold.; Mohan Kumar, G C.; Ramesh, M R.
    In the present study, the hybrid composite shaft's dynamic behaviour is evaluated in detail with the primary intention of using it in the high-speed rotor-bearing system of an aero gas turbine engine. In this research work, the top-down research approach is adopted wherein the most appropriate rotor-bearing system for the composite shaft application was first assessed. A preliminary rotor dynamic study was carried out on the three prospective rotor-bearing systems to verify the possibility of using the composite shaft. The preliminary rotor dynamic study indicated that the power turbine rotor-bearing system of a front driving turboshaft engine powering the rotorcraft as the most suitable application of composite shaft. To avoid the gas turbine engine's high temperatures, the composite shaft was proposed to be within the compressor section while the existing steel alloy was retained in the hot section. The hybrid composite shaft made of composite material sandwiched between the steel tubes was envisaged to avoid the direct exposure of composite material to the gas turbine's harsh environment and for the easier interface with the subsequent metallic assembly. A parametric study was carried out to select the right combination of material, thickness, and stacking sequences of the composite laminate layers. The mechanical characterization was carried out to estimate the material strength using a universal testing machine. The damping estimation was carried out using the free vibration and dynamic mechanical analysis techniques. Based on the parametric study and characterization tests, the carbon-epoxy laminate of 10 layers with the stacking sequence of [90, 45, -45, 06, 90] sandwiched by steel was found to be the best material configuration for the hybrid power turbine shaft. Finally, the rotor dynamic analysis of the power turbine shaft was carried out for a couple of variations in the thickness of steel tubes and length of the hybrid shaft to obtain the best configuration in comparison to the existing steel shaft. The results obtained in this work have successfully demonstrated the utility of the hybrid composite shaft in line with the objectives laid down for the thesis work.
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    Parametric Investigation and Off-Design Simulation of Low Temperature Organic Rankine Cycle for Residential Applications
    (National Institute of Technology Karnataka, Surathkal, 2021) Upadhyaya, Suhas.; Gumtapure, Veershetty.
    The energy demand across the world is increasing rapidly as a result of massive urbanization and industrialization. This is coupled with scarcity of traditional energy sources and severe environmental issues such as global warming and climate change. In order to counter this problem, there is a sense of urgency to explore alternative sources of energy. In this regard, harnessing the renewable energies and waste heat recovery are considered as potential solutions that can effectively address these issues. The organic Rankine cycle is proved to be reliable technology that can efficiently convert these low to medium-grade heat sources into useful power. The ORC power block consists of a pump which is used for pumping the working fluid to the desired pressure. This pressurized fluid is then passed on to the evaporator where heat addition takes place. The pressurized vapor passes through the expander, where the actual expansion of the working fluid takes place and the pressure drops. Finally, the vapor condenses in the condenser to complete the cycle. In ORC systems, the enthalpy drop across the expanders is less. Net work output per unit mass of the working fluid is small in ORC plants. In order to achieve higher power output (greater than 10 kWe), mass flow rate of working fluids has to increase. This will increase the size and cost of the ORC system. Therefore, it is reasonable to adopt small capacity ORC systems (1-5 kWe). The present study focussed in detail the overall system performance in terms of thermal and exergy efficiencies. Moreover, major components constituting the ORC system such as evaporator and expander are assessed. A thermodynamic model for ORC system was developed based on laws of mass and energy conservation. Using this model, ORC thermal and exergy efficiencies were evaluated for four different working fluids; R245fa, R123, Isobutane and R134a. Sensitivity analysis was performed using key thermodynamic parameters including expander inlet temperature, expander inlet pressure, condensation temperature and pinch point temperature difference (PPTD) to study its effect on net work output, mass flow rate, thermal and exergy efficiencies. Optimization of the system was also performed using genetic algorithm. The system was optimized to maximize cycle exergy efficiency. Parametric analysis was carried out to investigate the impact of evaporator pressure, condensation pressure, superheat, dead state temperature and PPTD on the system performance. Thermo-hydraulic model of plate heat ii exchanger evaporator was used to study the effect of evaporator pressure, PPTD and superheat on evaporator area. The effect of expansion ratio, shaft speed and expander inlet temperature on mass flow rate, work output and efficiency of open-drive scroll expander was studied using validated semi-empirical model. Finally, cost analysis and exergoeconomic optimization of 1 kWe driven solar ORC system was performed to compare the cost of solar ORC with solar PV in India and to determine the minimum cost of electricity respectively. Optimization results showed that the highest thermal efficiency (7.1%) and exergy efficiency (45.53%) at lowest expander inlet pressure (3.66 bar) was attained with R123. This was followed by R245fa with thermal efficiency of 7.04% and exergy efficiency of 44.98% at expander inlet pressure of 6.07 bar. R245fa was preferred for this study as it is a zero ODP fluid and also has a lower specific volume compared to R123. Sensitivity analysis showed that, expander inlet pressure showed the highest degree of sensitiveness for all working fluids. Detailed exergy analysis of ORC components was performed to identify the location and to assess the magnitude of exergy losses occurring within the ORC system. Exergy analysis of 1 kWe ORC system showed that, evaporator accounted for the maximum exergy loss. 41% of the total exergy loss occurred in the evaporator. It was also observed that evaporator pressure had significant effect on both energy and exergy efficiencies of the ORC. Significant reduction in evaporator area (75.87%) and cost (63.59%) was observed when evaporator pressure was increased from 4 to 10 bar. Heat exchanger area decreased by 89.65% and evaporator cost was reduced by 74.86% when PPTD was increased from 2 to 14 ºC. It was also observed from the model that the cost increases whereas the pressure drop decreases with increase in plate width and plate spacing. The trade off point for plate width was at 0.0065 m, where the evaporator cost was found to be 1166 USD (Rs 87,450) and frictional pressure drop was 2.03 kPa. In case of plate spacing, it was at 0.003 m, where the evaporator cost was 1210 USD (Rs 90,750) and frictional pressure drop was 1.27 kPa. Parametric investigation of scroll expander showed that the scroll expander should be operated in a range close to its adapted expansion ratio to achieve maximum efficiency. It was also revealed that increasing expander inlet temperature led to increase in thermal energy dissipation. This leads to the deterioration in efficiency of the expander. The economic analysis showed that the capital cost of small scale ORC systems is very high (Rs 7,42,500) compared to equivalent solar PV system (Rs 85,000) in India. Exergoeconomic optimization showed that minimum electricity cost of 3.9 Rs/kWh could be attained at maximum evaporator pressure of 13.9 bar.
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    Heat Transfer Studies of Flame Jet Impinging Over Wedge
    (National Institute of Technology Karnataka, Surathkal, 2021) Parida, Ritesh Kumar.; M, Vasudeva.
    A transient inverse heat conduction problem concerning jet impingement heat transfer has been solved analytically in this work. Experimentally obtained transient temperature history at the non-impinging face, assumed to be the exposed surface in real practice, is the only input data. Towards developing and validate the experimental setup, a study on the effect of pressure on the volumetric flow rate of compressible gas flowing through a rotameter is undertaken. Both air rotameter (range 40 500 milliliters/ minute at STP) and methane rotameter (range 400 5000 milliliters/ minute at STP) are calibrated using a standard Soap Bubble Flow Meter (SBFM). The experimental observations towards change in the volumetric flow rate at STP with a change in gas pressure are in agreement with theoretical understanding. The predicted methane-air mixture flow rates are further verified using the blow-off flame stability concept, thus validating the experimental set up. This study aims to estimate two unknown parameters - heat transfer coefficient and adiabatic wall temperature - at the impinging face simultaneously. The Green's Function Approach to accommodate both the transient convective boundary conditions and radiation heat loss is used to derive the forward model, which is purely an analytical method. Levenberg Marquardt Algorithm, a fundamental approach to optimisation is used as a solution procedure to the inverse problem. An in-house computer code using MATLAB (version R2014a) is used for analysis. The method is applied for a case of a methane-air flame impinging on one face of a flat 3mm thick stainless steel plate. It keeps Reynolds number of the flame 1000, and dimensionless burner tip to impinging plate distance equals to 4 while maintaining the equivalence ratio one. Inclusion of both radiation and convection losses in the Green's function solution for the forward problem, enhances the accuracy in the forward model, thereby increasing the possibility of estimating the parameters with better accuracy. The results are found to be in good agreement with the literature. This methodology is independent of external fluid flow and heating conditions; and can be applied even to high-temperature applications. Heat transfer characteristics of impinging flame jet over a wedge-shaped structure similar to a deflector plate of a missile launch-pad are studied using the same analytical technique. The transient temperature of the non-impinging surface of the 4-mm-thick v test object made of stainless-steel is measured experimentally. Multiple experimental cases are considered in this work by varying methane-air gas mixture Reynolds number (800-1500), non-dimensional nozzle tip to test object distance (2-6), and wedge-angle (90o and 120o). The observations concerning heat transfer characteristics are discussed in detail. Uncertainty of estimation is evaluated using the Monte Carlo technique.
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    Synthesis and Characterization of Magnetorheological (MR) Fluid for Different Engineering Applications
    (National Institute of Technology Karnataka, Surathkal, 2021) Acharya, Subash.; Kumar, Hemantha.
    Magnetorheological fluid (MRF) are suspensions of iron particles in a carrier oil. They are controllable smart fluid whose rheological properties change under the application of magnetic field. The design of Magnetorheological (MR) device and the composition of MR fluid used in it have a significant effect on its performance. In this study, MRF composition suitable for MR damper, MR brake and MR beam were determined based on optimization. Initially, the key ingredient of MRF, that is, iron particles of different average sizes, were characterized to determine their morphology, particle size distribution and magnetic properties. The morphology of iron particles were observed using Field Emission Scanning Electron Microscope. The particle size distribution was measured using particle size analyzer. The magnetic properties of different iron particles were measured using vibrating sample magnetometer. In the first part of this study, optimal dimensions of MR damper and composition of MRF suitable for MR damper were determined. A shear mode monotube MR damper was designed by using optimization technique. A damper was manufactured in accordance with the optimized size and was filled with commercially available commercial MR fluid, MRF 132DG (Lord Corporation) to determine its damping characteristics using damper testing machine. Experimentally determined values were validated with computational ones. Further, six MR fluid samples (MRFs) were prepared composed of combination of three different particle mass fractions and two sizes of iron particles. Rheological tests were conducted on these samples to determine the flow curves at off-state and on-state magnetic field conditions and they were compared with those of commercial MR fluid, MRF 132DG (Lord Corporation). In addition, the sedimentation stability of prepared fluid were examined. These MRFs were filled in the MR damper and their damper characteristics were determined. The area bounded by the force-displacement graphs was used to calculate the energy dissipated which was then used to calculate equivalent damping coefficient. Finally, using multi-objective genetic algorithm (MOGA) optimization, based on maximization of on-state damping coefficient and minimization of off-state damping coefficient, the optimal mass fraction and particle size was determined. iv In the next part of the study, optimal dimensions of MR brake and composition of MRF suitable for MR brake were determined. At first, optimum dimensions of MR brake were computed considering the properties of commercially available MRF132DG fluid using MOGA optimization. Maximization of field induced braking torque and minimization of off-state torque were chosen as the objectives. This was performed in MATLAB software coupled with magnetostatic analyses in ANSYS APDL software. The braking torque of designed and fabricated MR brake utilizing commercial MR fluid, MRF 132DG (Lord Corporation) was experimentally determined and validated with computational ones. Selection of optimal composition of MRF was done considering In-house MR fluid samples composed of different combinations of particle mass fractions, mean particle diameters and base oil viscosities. A design of experiments technique was employed and braking torque corresponding to the synthesized MRFs at different speeds and current supplied along with the variation of shaft speed during braking process were measured. Based on the experimental results, MOGA optimization technique was used to determine optimal MR fluid composition with the objectives of maximizing field induced braking torque and minimizing off-state torque. Further, the effect of particle size and mass fraction of iron powder in the MRF on the vibration behaviour of MRF sandwich beams were studied. Six MRFs composed of combination of two particle sizes and three mass fractions of carbonyl iron powder were prepared and their viscoelastic properties were measured. The MRFs were used to fabricate different MRF core aluminium sandwich beams. Additionally, a sandwich beam with commercially available commercial MR fluid, MRF 132DG (Lord Corporation) as core was fabricated. The modal parameters of the cantilever MRF sandwich beams were determined at different magnetic fields. Further, sinusoidal sweep excitation tests were performed on these beams at different magnetic fields to investigate their vibration suppression behaviour. Finally, optimal particle size and mass fraction of iron powder suitable for sandwich beam were determined based on maximization of damping ratio and minimization of mass of MRF.
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    Design, Fabrication and Characterization of an Optimal Magnetorheological (Mr) Damper for Prosthetic Knee Application
    (National Institute of Technology Karnataka, Surathkal, 2021) Saini, Tak Radhe Shyam.; Kumar, Hemantha.; Sujatha, C.
    A transfemoral amputation is the removal of lower limb above the knee joint in the thigh, through the femur bone. The design of any device which can make a transfemoral amputee’s gait similar to a normal gait is a challenging problem. An even more challenging task is to make the prosthetic leg adapt to variable walking speeds, terrains and intents. Magnetorheological (MR) fluids with their magnetic field controllable rheological properties, along with their fast response have been applied to design prosthetic knee devices capable of assisting a transfemoral amputee in replicating a near-normal gait. In this study, various design configurations based on MR fluids are explored for their suitability in prosthetic knee domain and an optimal design is selected among them. The device should be capable enough of producing the required knee braking torque sufficient for normal human walking and a low off-state resistance with a least possible mass, which forms a design optimization problem. The optimal design of any device configuration based on MR fluid involves coupling a mechanical design and an electromagnetic design. The mechanical design is based on the application of fluid constitutive models on the problem geometry. Equivalent magnetic methods (EMM) and finite element magnetostatics (FEM) are the two methods used in the electromagnetic design process. The former results in producing significant errors in magnetic field variables, whereas the latter requires a large computational time and effort. In this study, a combined magnetostatics approach is proposed which can address the various shortcomings associated with the available optimization methodologies. The proposed algorithm is compared with frequently used optimization methodologies based on FEM, EMM as well as neural network based data-driven methods. A statistical comparison of hypervolume indicator revealed that the proposed methodology produces similar design points compared to optimization based on FEM method and also substantially reduces the computational time. Although there exists only one commercially available design based on MR fluid, many alternative knee design configurations have been studied by various researchers. However, not all the previously studied design configurations are optimally iii designed specific to prosthetic knee applications. In this study, four design configurations based on MR fluids are selected based on intuition and also from models based on literature. The design configurations namely waveform arc boundary MR brake, multi-pole MR brake, twin rod MR damper and rotary vane MR damper are considered for a preliminary design process. The commercially available multi-plate MR brake has been extensively studied in the literature and thus is avoided from optimal design, although it is used in comparative studies at a later stage in this work. Among the chosen design configurations, twin rod MR damper and rotary vane MR damper are selected based on the criteria of producing normal human knee braking torque adequately. The multi-pole MR brake is found to produce a braking torque of 14 Nm, which is insufficient for normal human walking and thus is rejected for further testing. Although the waveform arc boundary MR brake is capable of producing the required braking torque, the design has limitations similar to that of commercially available multi-plate MR brakes and thus is also rejected for further testing. A prototype of the other two dampers is fabricated with random dimensions so as to obtain a few insights into the working nature of the device. Later, optimal design of selected design configurations is performed using the developed optimization methodology. The twin rod MR damper is characterized on a linear dynamic testing machine using harmonic excitations of varying amplitudes, frequencies and currents. This device configuration is capable of producing a damping force of 1020 N (equivalent to 40.8 Nm at 40 mm force moment arm) at a current of 1 A and also has a mass of 0.71 kg. An equivalent test setup to characterize the rotary vane MR damper is developed. This device configuration is found to produce a damping torque of 33 Nm at a current of 1A and has a mass of 1.1 kg. Based on the experimental findings and a comparison of the dampers with the available commercial model and models based on literature, the twin rod MR damper is selected as the optimal design configuration for prosthetic knee application. Finally, the twin rod MR damper is mathematically modelled using Bouc-Wen model with the model parameters evaluated by minimizing the error norms for time, displacement and velocity between the experimental and the model-generated results using a multi-objective genetic algorithm optimization. In the process, two different iv experimental data sets are used, one for mathematical modeling and the other for assessing the accuracy of the fit model. Also, an inverse model based on the forward damper model is proposed and validated later. This model predicts the current directly and avoids the necessity of solving any quadratic equation, which is otherwise required in the case of inverse models based on the modified Bouc-Wen model. The dynamic model of a single axis two segmental prosthetic knee is coupled with the forward Bouc-Wen model, the inverse model and a proportional derivative (PD) plus controlled torque (CT) controller to realize a complete semi-active prosthetic knee model. The parameters of PD plus CT controller are tuned to minimize the error between the desired and the controller-estimated torques. A closed loop control study is performed for the swing phase of the gait cycle. The results from the dynamic analysis predict that the damper is suitable for reproducing knee angle trajectories similar to those of normal gait and thus can be applied for prosthetic knee applications. Further, it was observed that the shank reaches full knee extension at the end of the swing phase with terminal velocity small enough to be handled by an extension stop.
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    Theoretical and Experimental Investigation of Intelligent Non-Linear Controls for Magnetorheological Elastomer based Vibration Systems
    (National Institute of Technology Karnataka, Surathkal, 2021) Kumar, Susheel.; Murigendrappa, S M.; Gangadharan, K V.
    Magnetorheological elastomer (MRE) based semi-active isolators and absorbers are prominently used to reduce undesirable vibration for a wide range of operating frequencies. The real-time implementation of these systems requires the controllers to vary the electric current of the electromagnet. Previous researchers have focused on the model-based and fuzzy controllers to control input current. However, due to the viscoelastic behavior of MRE, it exhibits nonlinearities and time-varying properties. This behavior makes the real-time implementation of the MRE-based vibration isolator and absorber less effective with existing controllers. The present work concentrates on the theoretical and experimental investigation of the intelligent adaptive nonlinear controls on the MRE-based vibration isolation and adaptive tuned vibration absorber (ATVA). For the implementation of the controllers, a thorough knowledge of the field-dependent properties of MRE is studied using an in-house custom-made dynamic characterization setup. The dynamic characterization of the MRE is carried out for variable input frequency, displacement and magnetic field. Further, the Bouc-Wen model is employed to comprehend constitutive the relationship between the individual parameters. The characterized MRE is used in the MRE vibration isolator. The properties of the MRE vibration isolator are extracted from shift frequency data. Furthermore, the performance of the MRE vibration isolation is investigated for the model-based PID and LQR controllers. MRE exhibits nonlinearity and time-varying properties that limit the application of linear controllers. To overcome the limitations of the linear controllers, the nonlinear and intelligent controls based on neural networks and fuzzy systems are designed. The designed boundary sliding mode control (BSMC) and neural network-based adaptive observer neural network fuzzy sliding mode control (NNAONFSMC) are implemented on the MRE vibration isolation system. The Lyapunov theorem assesses the asymptotical stability of the designed observer and controls. The controllers' effect is compared without and with parameter uncertainties of MRE vibration isolation at the single frequency excitation. Further, The NNAONFSMC has been analyzed for variable excitation frequency, and the maximum percentage reduction of the measured ii acceleration is 34%. From these outcomes, it is evident that the NNAONFSMC is more effective with time-varying parameter uncertainties of MRE than the BSMC at single and variable frequency excitation. Furthermore, the performance of model-free adaptive fuzzy sliding mode control for the magnetorheological elastomer-based adaptive tuned vibration absorber (MRE ATVA) has been investigated. MRE ATVA is fabricated with anisotropic MRE. Sliding mode and adaptive fuzzy sliding mode controls have been developed. The boundary layer is applied to the sliding surface to reduce the chattering effect in the sliding mode control. In the adaptive fuzzy sliding mode control, two fuzzy systems approximate the equivalent control and switching control. The Lyapunov theorem evaluates the asymptotical stability of the developed adaptive control based on fuzzy systems. The performance is compared for both the controls subjected to single-frequency excitation. Further, the adaptive fuzzy sliding mode control has been investigated for variable frequency excitation. The maximum reduction of transmissibility of primary mass is 38.14%. Based on the results, the model-free adaptive fuzzy sliding mode control is more effective in tuning the natural frequency of MRE ATVA by 0.5 s with parameter uncertainties and under variable frequency excitation compared to the boundary layer sliding mode control.
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    Aerodynamic Performance of Low Aspect Ratio Turbine Blade in the Presence of Purge Flow
    (National Institute of Technology Karnataka, Surathkal, 2021) Babu, Sushanlal.; S, Anish.
    In aero engines, purge flow is directly fed from the compressor which bypasses the combustion chamber and introduced into the disk space between blade rows to prevent the hot ingress. Higher quantity of purge gas fed through the disk space can provide additional thermal protection to passage endwall and blade surfaces. Moreover interaction of the purge air with the mainstream flow can alter the flow characteristics of turbine blade passage. The objective of the present investigation is to understand the secondary vortices and its aerodynamic behavior within a low aspect ratio turbine blade passage in the presence of purge flow. An attempt is made to understand the influence of velocity ratios and purge ejection angles on these secondary vortices. The objective is broadened by investigating the unsteadiness generated by upstream wakes over the secondary vortex formations in th presence of purge flow. Further the thesis aims to judge the feasibility of implementing endwall contouring to curb the additional losses generated by the purge flow. To accomplish these objectives, a combination of experimental measurements and computational simulations are executed on a common blade geometry. The most reliable commercial software ANSYS CFX which solves three dimensional Reynolds Averaged Navier Stokes Equations together with Shear Stress Transport (SST) turbulence model has been used to carry out computational simulations. Along with steady state analysis, in order to reveal the time dependent nature of the flow variables, transient analysis has been conducted for certain selected computational domains. The numerical results are validated with experimental measurements obtained at the blade exit region using five hole probe and Scanivalve. The experimental analysis is conducted for the base case without purge (BC) and base case with purge (BCp) configurations having flat endwalls. vi In the present analysis, it is observed that with an increase in the velocity ratio, the mass averaged total pressure losses also increases. In an effort to reduce the losses, purge ejection angle is reduced to 350 from 900 with a step size of 150. Significant loss reduction and improved endwall protection are observed at lower ejection angles. Numerical investigation of upstream disturbances/wakes explore the interaction effects of two additional vortices, viz. the cylinder vortex (Vc) and the purge vortex (Vp). Steady state analysis shows an increase in the underturning at blade exit due to the squeezing of the pressure side leg of horseshoe vortex (PSL) towards the pressure surface by the cylinder vortices (Vp). The unsteady analysis reveals the formation of filament shaped wake structures which breaks into smaller vortical structures at the blade leading edge for stagnation wake configuration (STW). On the contrary, in midpassage wake configuration (MW), the obstruction created by the purge flow causes the upper portion of cylinder vortices bend forward, creating a shearing action along the spanwise direction. Investigation of contoured endwall geometries shows that, endwall curvature either accelerate or decelerate the flow thereby a control over the endwall static pressure can be obtained. Out of three contoured endwalls investigated, the stagnation zones generated at the contour valleys has resulted in the additional loss generation for the first two profiles. Reduced valley depth and optimum hump height of the third configuration has effectively redistributed the endwall static pressure. Moreover an increase in the static pressure distribution at the endwall near to pressure surface has eliminated the pressure side bubble formation. Computational results of URANS (Unsteady Reynolds Averaged Navier Stokes) simulations are obtained for analyzing transient behaviour of pressure side bubble, with more emphasis on its migration on pressure surface and across the blade passage.
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    Influence of Heat-Treatment on Structure and Properties of Nickel Titanium Alloy
    (National Institute of Technology Karnataka, Surathkal, 2021) Mukunda, Sriram.; S, Narendranath.; Herbert, Mervin A.
    In this present investigation, the low temperature annealing heat-treatment was carried out at four different temperatures between 300oC and 450oC. The observation of the microstructure has been carried out using transmission electron microscopy as per ASTM F86. An EDAX analysis has been carried out as per ASTM F1375 - 92(2012) to ascertain the chemical composition. An x-ray diffraction has been carried out as per ASTM F2024 - 10(2016) to ascertain the phases present in the alloy. The DSC has been carried out as per ASTM D3418 on the alloy to analyze the transformation temperatures to confirm the superelastic nature of the material. The material has been subjected to mechanical testing by performing the tensile test as per ASTM E8 and Vickers hardness test as per ASTM E92 – 17. The tribological characteristics of the material has been analyzed as per ASTM G-132a by conducting abrasive wear test at room temperature. The superelasticity test has been performed as per ASTM F2516-18 at room temperature by varying the magnitude of strain. An electrochemical corrosion test has been conducted as per ASTM F-2129 on NiTi alloy with a prepared solution to study the corrosion resistance of the same. The salient results of the systematic investigation carried out within the scope of the investigation indicate that the chemical composition of the constituents present in the alloy assessed by EDAX analysis indicate that the Ni-Ti alloy used in this investigation is a 50:50 Ti-Ni alloy which is well within the tolerance limit for 50:50 TiNi alloy as per ASTM F1375 - 92(2012). The alloy is seen to be slightly on the Titanium-rich side. The TEM of the as-received NiTi alloy indicates the presence of martensite which appears as a needle-like region and also a shaded region indicating presence of dislocation network. Whereas the 50% Nickel - 50% Titanium alloy subjected to optimum low temperature annealing heat-treatment at 350oC for one-hour duration indicates the presence of martensite, dislocation network and formation of NiTi alloy grains. The extent of dislocation network has relatively reduced and the grain size of NiTi alloy has relatively increased. vi The XRD of as-received NiTi alloy indicates that the presence of martensitic and austenitic phase. Whereas, the X-ray diffractogram of the 50% Nickel – 50% Titanium alloy subjected to optimum low temperature heat-treatment at 350oC for one-hour duration indicates that the presence of martensitic phase, austenitic phase and an additional Rhombohedral phase. The DSC thermogram of the as-received NiTi alloy indicates that there are no significant peaks seen in the heating as well as the cooling curves meaning that there are no distinct phase transformations of either Austenite-Martensite or Martensite- Austenite present in the material. Whereas, the DSC thermogram of 50% Nickel - 50% Titanium alloy subjected to optimum low temperature heat-treatment at 350oC for onehour duration indicates that the material shows a single-stage transformation from austenite-martensite phase in the cooling cycle and a two-stage transformation from martensite-rhombohedral phase and rhombohedral-austenite phase in the heating cycle. The tensile test carried out for the as-received material in this investigation indicates that the material is super-elastic by nature. The comparison of the ultimate tensile strength of as-received 50% Nickel - 50% Titanium alloy and the alloy sample subjected to an optimum low temperature annealing heat-treatment of 350oC for a duration of 1 hour indicates that there is an improvement in ultimate tensile strength of 350oC heat-treated sample by 44.40% as compared to that of as-received 50% Nickel - 50% Titanium alloy. The Vickers Pyramid Number of as-received material is 421 VPN. The comparison of hardness of 50% Nickel - 50% Titanium alloy subjected to optimum low temperature annealing heat-treatment at 350oC for one-hour duration with the hardness of as-received material indicates that the hardness has increased by 14.5% as compared to hardness of as-received 50% Nickel - 50% Titanium alloy. The abrasive wear test indicates that when the load is increased, the wear mass loss rate is relatively at a higher rate upto 15N and with further increase in the load, the mass loss rate is relatively at a slower rate. The comparison of the abrasive wear between the as-received 50% Nickel - 50% Titanium alloy and 50% Nickel - 50% Titanium alloy subjected to optimum low temperature annealing treatment at 350oC for one-hour duration indicates that the mass loss of low temperature annealed 50% Nickel - 50% Titanium alloy is lesser by 37.17% to 47.58% than the mass loss of as -received 50% vii Nickel - 50% Titanium alloy. This trend of reduction in the mass loss or in other words the improvement in wear resistance has been found true at all the axial loads investigated within the scope of this investigation. The variation of strain for different levels of stress during loading and after release of load at different pre-determined strain values indicate that the material even after loading upto stress level of 700 MPa does not break but returns back to the original stress value after release of the load indicating that the unloading curve had followed a hysteresis path compared to loading curve by returning back to the same point which means that the material is exhibiting superelastic behavior. The extent of improvement in Superelasticity of 50% Nickel - 50% Titanium alloy subjected to optimum low temperature annealing heat-treatment of 450oC for one-hour duration is in the range of 54.5% to 95.2 % as compared to superelasticity of as-received material. The electrochemical corrosion study carried out for as-received NiTi alloy indicates that the electrochemical corrosion rate for the material was found to be 0.0613 mm/year. The corrosion rate of 50% Nickel - 50% Titanium alloy subjected to low temperature annealing heat-treatment at different temperatures is less than the corrosion rate of asreceived 50% Nickel - 50% Titanium alloy. The extent of improvement in the corrosion resistance of 50% Nickel - 50% Titanium alloy subjected to optimum low temperature annealing heat-treatment at 350oC for one-hour duration is 35.72% as compared to corrosion resistance of as-received 50% Nickel - 50% Titanium alloy.
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    Experimental Investigation and Modelling of Magneto Rheological Elastomer for Torsional Vibration Isolation
    (National Institute of Technology Karnataka, Surathkal, 2021) K, Praveen Shenoy.; Gangadharan, K V.
    Torsional vibration isolation is an effective method to mitigate unwanted disturbances arising from dynamic loading conditions. Typically this is achieved with conventional passive isolators such as centrifugal pendulum absorbers, torsion springs, sprocket dampers, fluid cased vibration dampers and others. A drawback of the existing passive isolators is the inability to tune themselves to varying operating conditions. With smart materials as suitable substitutes, the conventional passive systems have attained attributes of semi-active and active control isolators. Of the various available smart materials, the Magnetorheological Elastomers (MRE) offers a field-dependent property variation for variable operating parameters. Though the MRE has been effectively studied to isolate the structures in the linear/translatory systems, its capabilities as an effective torsiocal isolator are yet to be understood fully. In lieu of the same, in the present study, Magnetorheological Elastomers' attributes as an effective torsional vibration isolator have been explored. To comprehend the isolation capabilities of Magnetorheological Elastomers, a thorough understanding of the influencing parameters is necessary. Hence, the initial part of the current study focuses on the dynamic property characterization of the Magnetorheological Elastomers under torsional loading conditions. The dynamic properties of Magnetorheological Elastomers are predominantly affected by variation in the input displacements, applied magnetic fields and input frequency. Though the characteristics have been extensively studied under lateral shear, the property variations under torsional shear have not been explored. The present study develops a novel method to study the influence of angular displacement, applied magnetic fields and input frequency on the dynamic properties of Magnetorheological Elastomers under torsional loading conditions. The experimental setup is developed according to the ISO 10846-2 standard to evaluate the dynamic torsional stiffness and loss factor variations. Viscoelastic properties represented in-terms of complex torsional stiffness and loss factor are estimated from the Lissajous curves within the linear viscoelastic (LVE) limit. Experiments are conducted for varying input angular displacements (in the Linear Viscoelastic limit) and input frequency (in the range of 10Hz to 30Hz). The frequency range corresponds to the torsional frequency range of shafts rotating in the lower speed vii range under 2000 rpm. Magnetic field sensitive characteristics are evaluated under the field produced by a custom-made electromagnet in the range of 0T to 0.3T. The volume fraction of the CIP is set at 27% for the RTV based isotropic MRE. The results reveal a strong influence of field-dependent variations on the complex stiffness compared to the input frequency. Variations observed in the loss factor suggest a dominance of the imaginary part of the complex stiffness on the energy dissipation. The reduced field-induced enhancements in the complex stiffness are interpreted from the Magneto-static and structural based numerical simulations using ANSYS 19.1. The angular displacement dependent variations highlight the effectiveness of the developed method in capturing the rheological properties under torsion. Changes in the dynamic torsional stiffness suggest the dominant behaviour of the input angular displacement. The bound rubber theory is used to interpret the displacement-dependent variations on the torsional stiffness. It is also observed that the MRE's damping capacity depends on the angular displacement and the dissipation capacity of the elastomer is evaluated in terms of loss factor. Results indicate a significant contribution of the interfacial damping over the intrinsic and magneto-mechanical hysteresis damping. To formulate the actual implementation of the MRE as a semi-active isolator, it is required to model the complex behaviour of the MRE through its stiffness and damping. Though much research has been carried out in understanding MR fluids' behavior, the same cammt be said of its elastomer counterparts. The constitutive relationship between the operating parameters is derived using a viscoelastic parametric modelling technique based on the Kelvin-Voight model. Results highlight the derived model's effectiveness in predicting the experimentally obtained viscoelastic behaviour of the Magnetorheological Elastomers regarding the stiffness and the energy dissipation capacity. To evaluate the Magnetorheological Elastomer isolator's isolation capabilities, a novel, custom-made SDoF torsional isolation system is developed. Field-dependent reduction in the transmissibility ratio highlights the semi-active vibration capabilities of the isolator and a maximum reduction of 42% is observed in the transmitted amplitudes. Further, a shift in the natural frequency is detected due to the field-induced viii variations in the isolator's torsional stiffness. The isolation capabilities are calculated for different input angular displacements, inertia and magnetic fields and the effect of the individual parameters is studied. The torsional stiffness and the damping factor are ascertained individually for the individual parameters. Also, a model-based PID control strategy is adopted to assess the semi-active vibration capabilities of the Magnetorheological Elastomer isolator.