National Aerospace Laboratories

About: National Aerospace Laboratories is a facility organization based out in Bengaluru, India. It is known for research contribution in the topics: Coating & Corrosion. The organization has 1838 authors who have published 2349 publications receiving 36888 citations.

Minimization and identification of conducted emission bearing current in variable speed induction motor drives using PWM inverter

Abstract: The recent increase in the use of speed control of ac induction motor for variable speed drive using pulse width modulation (PWM) inverter is due to the advent of modern power electronic devices and introduction of microprocessors. There are many advantages of using ac induction motor for speed control applications in process and aerospace industries, but due to fast switching of the modern power electronic devices, the parasitic coupling produces undesirable effects. The undesirable effects include radiated and conducted electromagnetic interference (EMI) which adversely affect nearby computers, electronic/electrical instruments and give rise to the flow of bearing current in the induction motor. Due to the flow of bearing current in the induction motor, electrical discharge machining takes place in the inner race of the bearing which reduces the life of the bearing. In high power converters and inverters, the conducted and radiated emissions become a major concern. In this paper, identification of bearing current due to conducted emission, the measurement of bearing current in a modified induction motor and to minimize the bearing current are discussed. The standard current probe, the standard line impedance stabilization network (LISN)), the electronics interface circuits are used to measure high frequency common mode current, bearing current and to minimize the conducted noise from the system. The LISN will prevent the EMI noise entering the system from the supply source by conductive methods, at the same time prevents the EMI generated if any due to PWM, fast switching in the system, will not be allowed to enter the supply line. For comparing the results with Federal Communications Commission (FCC) and Special Committee on Radio Interference (CISPR) standards, the graphs are plotted with frequency Vs, line voltage in dBµ V, common mode voltage in dBµ V and the bearing current in dBµ A with out and with minimizing circuits.

Modeling the effective elastic properties of nanocomposites with circular straight CNT fibers reinforced in the epoxy matrix

Abstract: In the present study, the consistent effective elastic properties of straight, circular carbon nanotube epoxy composites are derived using the micromechanics theory. The CNT composites are known to provide high stiffness and elastic properties when the shape of the fibers is cylindrical and straight. Accordingly, in the present work, the effective elastic moduli of composite are newly obtained for straight, circular CNTs aligned in the specified direction as well as distributed randomly in the matrix. In this direction, novel analytical expressions are proposed for four cases of fiber property. First, aligned, and straight CNTs are considered with transverse isotropy in fiber coordinates, and the composite properties are also transversely isotropic in global coordinates. The short comings in the earlier developments are effectively addressed by deriving the consistent form of the strain tensor and the stiffness tensor of the CNT nanocomposite. Subsequently, effective relations for composites reinforced with aligned, straight CNTs but fibers isotropic in local coordinates are newly developed under hydrostatic loading. The effect of the unsymmetric Eshelby tensor for cylindrical fibers on the overall properties of the nanocomposite is included by deriving the strain concentration tensors. Next, the random distribution of CNT fibers in the matrix is studied with fibers being transversely isotropic as well as isotropic when CNT nanocomposites are subjected to uniform loading. The corresponding relations for the effective elastic properties are newly derived. The modeling technique is validated with results reported, and the variations in the effective properties for different CNT volume fractions are presented.

An Investigation on the Wear and Corrosion Behavior of HVOF-Sprayed WC-12Co-Al 2 O 3 Cermet Coating

Abstract: The aim of the present study is to develop thermally sprayable WC-12Co and WC-12Co-xAl2O3 (x = 10 and 15 wt.%) cermet coatings on steel substrate (SS 304) by high-velocity oxy fuel (HVOF) method. Influence of Al2O3 addition on the wear and corrosion behavior of WC-12Co coating has been studied. The microstructure and chemical composition of the coatings were analyzed using field emission scanning electron microscope (FESEM), and phase identification was carried out using x-ray diffraction (XRD) studies. The morphology of the coating appears as coarse granular structure. The XRD studies revealed the presence of hexagonal WC phase along with η-Co6W6C phase. It has been observed from the microhardness measurements, that the values gradually increase from 950 to 1300 HK with the addition of Al2O3 from 0 to 15 wt.%. The wear rate of WC-12Co-15Al2O3 (3.19 × 10−6 mm3/Nm) and WC-12Co-10Al2O3 (5.26 × 10−6 mm3/Nm) coatings was seen to be one order of magnitude lower than that of WC-12Co (2.9 × 10−5 mm3/Nm) coating. The polarization studies revealed that WC-12Co-15Al2O3 cermet coating showed superior corrosion protection than that of WC-12Co-10Al2O3 and WC-12Co coatings. This has been attributed to the gradual decrease in the porosity levels with an increase in Al2O3 content which is supported by morphology studies. The microhardness and wear behavior of WC-12Co-Al2O3 coatings are equivalent to those of hard chrome suggesting the possibility of its replacement.

Corrosion protection behaviour of silica–titania hybrid coatings embedded with silica nanoparticles

Abstract: Hybrid sol-gel coatings prepared using organically modified silanes and corrosion inhibitors are used for the surface protection of metals and alloys. This paper focuses on the corrosion behaviour and compatibility of silica nanoparticles as nanocontainers in silica–titania hybrid system. The surface protection of the alloy is improved by using silica nanoparticles as nanocontainers for cerium (III) inhibitor. The hybrid coating system protected AA2024-T3 alloy effectively against corrosion when immersed in NaCl solution for prolonged hours. The barrier property of the sol-gel film was found to be reinforced due to the presence of cerium nitrate inhibitor in the silica nanoparticles.

Pressure fluctuation measurements in the NAL 0.2-m supersonic wind tunnel

Abstract: The pressure fluctuations were measured along the NAL 0.2m supersonic wind tunnel circuit with a microphone. Its attenuation is about 20dB between the compressor and the settling chamber. There is no cause of generating noise strongly up to the settling chamber exit in this tunnel circuit. The settling chamber pressure fluctuations were measured by the two ways of a microphone and a small pressure transducer. The two results obtained are almost the same mutually. The ratio of the rms value of the fluctuations to the total pressure of 75kPa remains under 0.1% at all tested Mach numbers from 1.5 to 2.5. It is possible to utilize an axial flow compressor for the quiet wind tunnel. Mach number and flow angle distributions along central 0.12 m long vertical line passing through the test section center were measured with a static pressure tube and a yaw meter. The change of largest 0.05 in Mach number and the change of largest about 1.0 degree in flow angle are observed. The comparison between the measured and numerical results at Mach number 2.0 suggests that the boundary layer on the nozzle plate maintains a laminar flow. The measured rms values of the pitot pressure fluctuations at the test section center show the values more than 0.1% of the Pitot pressure. The rms value becomes larger as the Mach number increases. The increment of the rms value suggests that some phenomenon of increasing the fluctuations like the eddy Mach wave radiation has originated during the flow passing the nozzle. Because the power spectrum density curve remains its shape between the settling chamber and the test section if the phenomenon increasing the pressure fluctuation does not exist, the rms value of the pressure fluctuation at the settling chamber must be less than 0.1% of the total pressure unlike 0.2% in order to make the rms value less than 0.1% at the test section.