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Parametric studies on gas turbine labyrinth seal for the secondary air flow optimization at static and rotating conditions
No full textVarious studies have been carried out related to the labyrinth seals and reported in the open literature using the different seal arrangements at the stator-rotor seal cavity region. In the present study, numerical analysis has been carried out for the static and rotational effects of labyrinth seals at various flow and geometrical, parametric conditions for the optimized leak flow using straight and steeped seal configurations. And, an experimental data has been generated for the straight through seals, and the numerical data of the same case is validated with the experimental data. The k-omega SST turbulence model is considered with 5% turbulence intensity for the CFD analysis. At a particular seal clearance, as the number of teeth increases the leakage flow is found to be decreased. The leak flow is found to be lower with the stepped labyrinth seals in comparison to the straight through seals. The leak flow amount is found to be lower at a rotational condition in comparison to the stationary condition. From the overall results, it is observed that the stepped seal with a lower clearance at a compressor bleed air temperature and rotational conditions have shown better performance with the lower leak air mass flow
Rotorcraft precision hover control in atmospheric turbulence via eigenstructure assignment
No full textThe rotorcraft Aeronautical Design Standard (ADS-33E-PRF) specifies a frequency domain criterion to define handling qualities rating for pitch-roll angular rate cross-axis coupling in the "bandwidth" frequency range. The specification is derived based on flight tests in forward flight conditions. For rotorcraft operating near "hover," cross-axis coupling in the pitch-rollyaw axes originates from both angular rate and low-frequency translational rate interaction. It is shown that this multiaxis cross-coupling including the entire rigid body mode spectrum is a dominant factor for handling qualities deterioration. Hover operations under turbulent atmospheric disturbances further degrade handling qualities, leading to aggravated pilot workload and cause safety of flight concerns. Control augmentation schemes, based on an eigenstructure assignment technique, are proposed to mitigate cross-axis rate coupling and improve disturbance rejection to enable precision "hover" control in turbulent atmospheric conditions
Relaxation effects in nonstoichiometric NBT-based ceramics
No full textEffects of cation nonstoichiomtry on crystal structure parameters, microstructure, and dielectric properties of ceramics (Na0.5+xBi0.5)TiO3 and (Na0.5-xBi0.5)TiO3 with Bi/Na<1 and of solid solutions [(Na0.5Bi0.5)1-xKx]TiO3 and (Na0.5Bi0.5)(Ti1-xMgx)O3 with x = 0 – 0.1 have been studied. Changes in the unit cell parameters and microstructure of the samples observed in ceramics prepared correlate well with both preparation conditions and radii of substituting cations. Ferroelectric phase transitions near ~ 400 K and ~ 600 K were confirmed ib the systems studied. Phase transitions near ~ 400 K demonstrate a pronounced relaxor behavior determined by the presence of polar regions in a nonpolar matrix. Besides, additional anomalies related to presence of relaxing dipoles formed by oxygen vacancies were observed on temperature dependences of dielectric permittivity at temperatures higher than 700 K. The results obtained confirmed that increase in nonstoichiometry lead to increase in ionic conductivity of the samples
Tantalum carbide based spectrally selective coatings for solar thermal absorber applications
No full textTantalum carbide (TaC) based tandem coatings for selective absorption of solar radiation were fabricated by magnetron sputtering technique. Tandem structure of Ti/TaC/Al2O3 coating was deposited on a stainless steel substrate in which Ti acts as back reflection layer, TaC acts as main absorber layer and Al2O3 acts as an antireflection layer. The composition and thicknesses of the individual component layers have been optimized by adjusting the sputtering process parameters. Reactively sputtered TaC coating deposited at 85 W of sputtering power and 8 sccm of C2H2 flow rate showed an absorptance of 0.845 and emissivity of 0.14. After depositing approximately 50 nm thick Al2O3 on Ti/TaC, the absorptance increased to 0.957 and emissivity increased to 0.15. The refractive indices and extinction coefficients of each layer were used to simulate the reflectance spectra of the deposited tandem stack using SCOUT simulation software. For individual layer thicknesses of Ti, TaC and Al2O3 of 15, 55 and 50 nm, respectively a good match was obtained with the experimental and simulated reflectance spectra. The layer thicknesses were consistent with the thickness data of the tandem stack obtained from cross-sectional field emission scanning electron microscope image. The thermal stability of the tandem stack was examined in vacuum at different temperatures for a short duration. The results showed that the tandem stack is stable up to 500 °C for 2 h
Influence of base cavity on base pressure at subsonic and supersonic Mach numbers
No full textAn experimental study has been conducted to understand the effectiveness of passive device over base pressure region by modifying the cavity lip radius of an axisymmetric afterbody. The study has been done at both subsonic and supersonic Mach numbers. The effectiveness of the base cavity lip geometry modifications is studied using unsteady base pressure measurements over the base region of the axisymmetric afterbody. Results show that the variation of the time-averaged pressure is approximately constant in the radial direction for both blunt base and rounded off base configurations at all Mach numbers. The normalized rms pressure fluctuation is approximately constant in the radial direction for both cases
Evolution of magnetoresistance behaviour at low temperatures in naturally oxidised specular spin valve systems
No full textThe temperature dependent magnetoresistive behaviour of field cooled naturally oxidised specular spin valve systems has been studied in the temperature range of 300–10 K. Inconsistent to the non-specular spin valve system, an anomalous behaviour was evolved with large exchange bias and higher coercivity, below 200 K. The structural investigations inferred the formation of magnetic oxides with higher density gradient in the pinned layer, and the observed anomalous behaviour at low temperatures was correlated with the antiferromagnetic ordering of these oxides in spin glass state. The uncompensated interfacial magnetism of the nano-oxide layer was further confirmed by comparing with low temperature magnetoresistive behaviour of non-magnetic oxide based specular spin valve systems
A novel route to synthesis polythiophene with great yield and high electrical conductivity without post doping process
No full textPolythiophene (PTh) was synthesized by a standard chemical oxidation procedure by using Ferric chloride (FeCl3) as an oxidizer without any post doping process. The synthesis procedure was optimized by varying different parameters like reaction temperature, monomer to oxidizer ratio, solvent system, reaction time and oxidizer addition time to realize the maximum electrical conductivity and without compromising on product yield. In the optimized synthesis procedure, highest yield of 80% with a maximum electrical conductivity of 9 Scm−1 was achieved. These results are first of its kind in the literature reported so far for the similar conditions. The enhancement of electrical conductivity was attributed to the increase in the degree of polymerization under the optimized reaction conditions and was substantiated with UV–Vis, Diffuse Reflectance Spectroscopy (DRS), FTIR, NMR solid state, SEM, XRD and DSC studies. This paper is the first of its kind to report the room temperature synthesis procedure of PTh with highest electrical conductivity and yield
Synthesis and properties of high velocity oxy-fuel sprayed FeCoCrNi2Al high entropy alloy coating
No full textThe present study aims to develop dense FeCoCrNi2Al high entropy alloy (HEA) coating of thickness ≅ 200 μm by high velocity oxy-fuel (HVOF) process. Studies on the microstructure and mechanical properties like microhardness, erosion resistance of the HVOF coated FeCoCrNi2Al HEA has not been reported by any investigator. An adherent coating has been deposited displaying a lamellar microstructure. The X-ray diffractogram of the coating revealed the formation of a major FCC and a minor BCC phase with small amount of oxide. The oxide phase was confirmed to be Al2O3 using Raman spectroscopy. The microhardness of the coating is found to be 600 ± 30 VHN with no crack formation upto a load of 1 kgf. The erosion resistance of the HEA coated steel was evaluated upto a temperature of 800 °C. The coating displays good erosion resistance and the depth of penetration of the erodent was well within the coating thickness with no delamination of the coating, thereby making it suitable for high temperature applications
The interaction of Indian monsoon depressions with northwesterly midlevel dry intrusions
No full textMonsoon depressions (MDs) bring substantial monsoon rainfall to northern and central India. These events usually form over the Bay of Bengal and travel across northern India toward Pakistan. Using European Centre for Medium-Range Weather Forecasts interim reanalysis, an MD-tracking algorithm, and an objective identification method, the authors find that about 40% of MDs interact with northerly intrusions of dry desert air masses as the MDs traverse the subcontinent. MD interactions with dry intrusions are often preceded by positive potential vorticity (PV) anomalies on the subtropical jet and low-level anticyclonic anomalies over the north Arabian Sea. Dry intrusions nearly halve the precipitation rate in the southwest quadrant of MDs, where MDs rain the most. However, dry intrusions increase the rainfall rate near the MD center. Similarly, ascent is reduced west of the MD center and enhanced at the MD center, especially in the upper troposphere. The reduced ascent west of MD centers is likely attributable to changes in vertical shear reducing differential cyclonic vorticity advection. Dry intrusions slightly reduce MDs’ propagation speed. For the mid- to upper-level vortex, this can be explained by anomalous westerlies reducing propagation by adiabatic advection. For the lower-tropospheric vortex, it is likely that reduced diabatic generation of PV plays a role in slowing propagation, along with reduced adiabatic advection
Corrosion and wear resistance properties of multilayered diamond‐like carbon nanocomposite coating
No full textMultilayered diamond‐like carbon (DLC) nanocomposite coating has been deposited on silicon and stainless steel substrates by combination of cathodic arc evaporation and magnetron sputtering. In order to make DLC coating adhered to metal substrate, a chromium interlayer has been deposited with constant bias voltage of −150 V applied to the substrate. Dense multilayered coating consists of metallic or nonmetallic and tetrahedral carbon (ta‐C) layers with total thickness of 1.44 μm. The coating has been studied for composition, morphology, surface nature, nanohardness, corrosion resistance, and tribological properties. The composition of the coating has been estimated by energy‐dispersive spectroscopy. Field‐emission scanning electron microscopy and atomic force microscopy have been used to study the surface morphology and topography. ID/IG ratio of ta‐C:N layer obtained from Raman spectroscopy is 1.2, indicating the disorder in the layer. X‐ray photoelectron spectroscopy studies of individual ta‐C:N, CrN, and Cr‐doped DLC layers confirm the presence of sp2C, sp3C, CrN, Cr2N, and carbidic carbon, and sp2C, sp3C, and Cr carbide. Nanohardness studies show the maximum penetration depth of 70 to 85 nm. Average nanohardness of the multilayered DLC coating is found to be 35 ± 2.8 GPa, and Young's modulus is 270 GPa. The coating demonstrates superior corrosion resistance with better passivation behavior in 3.5% NaCl solution, and corrosion potential is observed to move towards nobler (more positive) values. A low coefficient of friction (0.11) at different loads is observed from reciprocating wear studies. Wear volume is lower at all loads on the multilayered DLC nanocomposite coating compared to the substrate