Mechanical Engineering

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Shiv Nadar University

Performance of hydropower plant is severely affected by the presence of sand particles in river water. Degree of degradation significantly depends on the level of operating parameters (velocity, impingement angle, concentration, particle size and shape), which is further related to erosion mechanism. In this investigation, the effect of some of these operating parameters on erosion mechanism of generally used hydroturbine steel, CA6NM (13Cr4Ni), is reported. Morphology and variation in the martensite and austenite phases of the eroded surfaces were investigated using SEM and XRD. It was observed that velocity and impingement angle affect the erosion mechanism of CA6NM steel. Erosion mechanism was also significantly affected by the radial distance from the impact zone. Primary mechanism responsible for the removal of material at normal impingement angle was the formation and removal of platelets. At acute impingement angle, ploughing was observed to be one of the prime mechanisms responsible for the loss of the material. Other than these two well-known erosion mechanisms, the presence of another two erosion mechanisms was also observed. Models have been proposed for these unfamiliar erosion mechanisms. Interaction amongst different operating parameters was studied using line and contour plots. It was observed that the interaction between velocity and concentration was most significant. Using the experimental results, a statistical model based on regression approach was developed. Validity of this statistical model was checked using the experimental results from the literature and present study. © 2013 Springer Science+Business Media New York.

(SoE) School of Engineering

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Tribology Letters

Slurry erosion mechanism of hydroturbine steel: Effect of operating parameters

Materials with improved tribological properties have become the pre-requisite of advanced engineering design. Surface metal matrix composites (SMMCs) exhibit a unified combination of good tribological properties and high toughness of the interior bulk metal when compared with monolithic materials. Friction stir processing, a microstructure modification technique, has emerged as one of the processes used for fabrication of SMMCs. Commercial cast or wrought type Mg-Al- Zn AZ-series alloys, such as AZ91, AZ61 or AZ31 with 3-9 wt.% Al and 1 wt.% Zn, have been widely used in automobiles or electronic appliances. Due to low percentage of alloying elements/second phase elements in these alloys, the grain refinement and hence the mechanical properties such as hardness cannot be appreciably enhanced. In this study, FSP was used to fabricate AZ31 based nanocomposite using TiC as reinforcement. FSP was performed at a fixed volume fraction of TiC under varying cooling conditions. Tribological performance of the fabricated composite was investigated using pin-on-disc wear test arrangement. The analysis of the wear surfaces as well as wear debris analysis was done using SEM equipped with EDS. The fabricated composite was found to exhibit superior wear resistance. © (2012) Trans Tech Publications, Switzerland.

Advanced Materials Research

Improving the tribological properties of mg based AZ31 alloy using friction stir processing

Hydrotubines do face regular exposure to cavitating environment, causing damage to guide vanes, impeller, nozzle, buckets and spear. Quite a large number of surface modification techniques have been employed to counter such a deleterious effect. Among these large numbers of techniques, thermal spray coatings has been mostly focused and studied. In the present work one such technique has been opted for study. Selection of coating technique and composition was made keeping their economical viability in mind. Hardness and toughness have been identified as significant factors effecting cavitation erosion of materials. In-depth analysis of erosion mechanism of coated and un-coated samples was undertaken using SEM/EDS, micro-hardness tester and XRD. Erosion mechanism of steel was mainly plastic deformation, whereas in case of coating, (i) lack of adhesion between splats (ii) lower toughness of alumina (iii) loosely bonded un-melted alumina particles, were dominant factor controlling the erosion mechanism of coating. © (2012) Trans Tech Publications, Switzerland.

Evaluation and development of economically viable coatings for erosion protection of hydroturbines

In this study, AZ91 based hybrid nanocomposite was fabricated using mixture of TiC and alumina by friction stir processing (FSP). FSP of AZ91 was conducted with as well as without particle addition and under different cooling conditions. Distribution of nano TiC and Al2O3 particles in the Mg matrix was studied using SEM. Grain size analysis of the as-cast and FSP AZ91/(TiC +Al2O3) composite was done using optical microscopy. The performance of the fabricated nanocomposite was investigated using microhardness, bulk hardness and scratch tests. Microhardness testing was done at 50 gf load and bulk hardness at 5 Kgf load. Scratch test was performed at 2 and 5 N normal load values on a universal tribometer using Rockwell indentor. AZ91/(TiC+Al2O3) composite exhibited improved mechanical properties.

Transactions of the Indian Institute of Metals

Some investigations on friction stir processed zone of AZ91 alloy

Ocean Engineering

Erratum: A 3D fully coupled analysis of nonlinear sloshing and ship motion (Ocean Engineering (2012) 39 (1-13) DOI: 10.1016/j.oceaneng.2011.09.015)

This study investigates the coupling effects of six degrees of freedom in ship motion with fluid oscillation inside a three-dimensional rectangular container using a novel time domain simulation scheme. During the time marching, the tank-sloshing algorithm is coupled with the vessel-motion algorithm so that the influence of tank sloshing on vessel motions and vice versa can be assessed. Several factors influencing the dynamic behavior of tankliquid system due to moving ship are also investigated. These factors include container parameters, environmental settings such as the significant wave height, current velocity as well as the direction of wind, wave and flow current acting on the ship. The nonlinear sloshing is studied using a finite element model whereas nonlinear ship motion is simulated using a hybrid marine control system. Computed roll response is compared with the existing results, showing fair agreement. Although the two hull forms and the sea states are not identical, the numerical result shows the same trend of the roll motion when the anti-rolling tanks are considered. Thus, the numerical approach presented in this paper is expected to be very useful and realistic in evaluating the coupling effects of nonlinear sloshing and 6-DOF ship motion. © 2011 Elsevier Ltd All rights reserved.

A 3D fully coupled analysis of nonlinear sloshing and ship motion

Erosion in industrial appliances causes huge loss of valuable resources especially when solid particles are present in the working medium. A number of erosion models that are available in the literature claimed to predict the erosion response. In this work, five erosion models have been evaluated using computational fluid dynamics (CFD) methodology. Three most commonly used experimental test rigs (solid particle and unsubmerged slurry jet type and pot type) have been simulated and comparisons with the experimental results have been performed. The study helps in understanding the effect of fluid medium and flow pattern on the erosion process. The knowledge acquired shows that phenomenological based erosion models give better results considering the wide range of operating parameters. The in-depth study of the flow field shows the possibility that erosion mechanism, being parameter dependent, could change with location on the same specimen. © 2012 The Minerals, Metals, & Materials Society. All rights reserved.

CFD Modeling and Simulation in Materials Processing

Numerical Simulation of Erosion Using Computational Fluid Dynamics

WC-Co-Cr coatings were deposited on some hydroturbine 13Cr4Ni and 16Cr5Ni steels by the detonation-gun spray process. An in-depth characterization of the as-sprayed coating was done using X-ray diffraction (XRD) and scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDS) techniques. Microhardness and porosity measurements were also made. The coating was found to have a typical splat-like morphology with some indications of unmelted carbide particles. The XRD results showed the presence of WC as the primary phase along with W2C and Co6W6C as secondary phases. Furthermore, the slurry erosion behavior of the coatings was investigated to ascertain the usefulness of the coatings to reduce the slurry erosion of the steels. The effect of four operating factors viz. the velocity, impact angle, concentration, and particle size on the slurry erosion of coated and bare steels has been studied using a high-speed jet-type test rig. The sand used as an erodent was collected from a power plant to replicate the actual turbine conditions. It has been observed that the given cermet coating can enhance the erosion resistance of the steel. Velocity was found to be the most significant factor affecting the erosion behavior of the coating, whereas it was the erodent particle size in the case of uncoated steel. As evidenced from the SEM images, the platelet mechanism of erosion seemed to be the prominent one, causing the removal of material from the surface of the steel, whereas for the coating, the formation and interlinking of cracks resulted in the removal of material. © The Mineral, Metals & Materials Society and ASM International 2012.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Parametric study of slurry-erosion of hydroturbine steels with and without detonation gun spray coatings using taguchi technique

Erosion in industrial appliances causes huge loss of valuable resources especially when solid particles are present in the working medium. A number of erosion models that are available in the literature claimed to predict the erosion response. In this work, five erosion models have been evaluated using computational fluid dynamics (CFD) methodology. Three most commonly used experimental test rigs (solid particle and unsubmerged slurry jet type and pot type) have been simulated and comparisons with the experimental results have been performed. The study helps in understanding the effect of fluid medium and flow pattern on the erosion process. The knowledge acquired shows that phenomenological based erosion models give better results considering the wide range of operating parameters. The in-depth study of the flow field shows the possibility that erosion mechanism, being parameter dependent, could change with location on the same specimen.

TMS Annual Meeting

Numerical simulation of erosion using computational fluid dynamics

Cavitation erosion (CE) of various components in hydroturbines is a critical issue. Several attempts have been made in the past to counter CE through various surface modification techniques. In this study, a novel attempt has been made to modify the surface properties of hydroturbine steel with the aid of friction stir processing (FSP). Metallurgical characterization of the processed steel was undertaken with the help of XRD, SEM/EDS, EBSD and optical microscopy. For mechanical characterization micro-hardness and cavitation erosion studies were conducted. EBSD results confirmed the presence of grains in nano range. Processed steel showed gain in hardness of the order of 160 % in comparison to unprocessed steel. XRD results show a significant variation in the type and quantity of various phases in pre-and post processed steel. It was observed that FSPed steel showed a significant improvement in the erosion resistance in contrast to the unprocessed steel.