• R KUMAR

      Articles written in Sadhana

    • Foreword

      R A Mashelkar R Kumar

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    • Breakage and coalescence of drops in turbulent stirred dispersions

      D K R Nambiar R Kumar K S Gandhi

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      The various existing models for predicting the maximum stable drop diameterdmax in turbulent stirred dispersions have been reviewed. Variations in the basic framework dictated by additional complexities such as the presence of drag reducing agents in the continuous phase, or viscoelasticity of the dispersed phase have been outlined. Drop breakage in the presence of surfactants in the continuous phase has also been analysed. Finally, the various approaches to obtaining expressions for the breakage and coalescence frequencies, needed to solve the population balance equation for the number density function of the dispersed phase droplets, have been discussed.

    • Sonochemical reaction engineering

      K S Gandhi R Kumar

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      Ultrasound has been widely used by chemists to enhance yields as well as rates of homogeneous as well as heterogeneous chemical reactions. The effect of ultrasound on the course of chemical reactions is mediated through cavitation bubbles it generates. High temperatures and pressures are attained inside the cavitating bubbles when they collapse. The extreme conditions so generated lead to the formation of reactive intermediates, e.g., free radiacls, inside the bubbles, which cause chemical reactions to occur when they enter the surrounding liquid. This is the mechanism through which ultrasound influences the path of homogeneous reactions. The cavitation bubbles collapse asymmetrically in the vicinity of solids, e.g., catalyst particles. Asymmetric collapse lead to formation of high speed microjets. The microjets can enhance transport rates, the increase surface area through pitting as well as particle fragmentation through collisions. Both can alter the rates of heterogeneous reaction rates. It however appears that these effects do not exhaust the scope of the influence of ultrasound on heterogeneous reactions. Modelling and quantitative prediction of the effect of ultrasound on chemical reactions is however at a stage of infancy as the phenomena are complex. Only a few examples of modelling exist in literature. Apart from this, reactor design and scaleup pose significant problems. Thus sonochemical reaction engineering offers large scope for research and development efforts.

    • Influence of cutting parameters on cutting forces and surface roughness in dry turning of Al using PCD and different coated tools

      R KUMAR S K PATTNAIK J K MINZ S PADHI S K SARANGI

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      Aluminium (Al) is the suitable material for aerospace and automotive industries due its light weight, corrosion resistance, weldability, non-magnetic and mechanical properties. But, machining of Al and its alloy and finding the suitable tool is really a big challenge because of its formation of BUE (Built-up Edge) andBUL (Built-up Layer). This paper presents the influence of cutting parameters (speed, feed and depth of cut) and its effect on the cutting force and the surface finish. Five different advanced cutting tool inserts (SPUN WC, SPGN WC, PCD, WC + TiN and WC + Ti(C, N) TiN + Al₂O₃) at different cutting speed (Vc) ranging between 300 m/min and 700 m/min and feed rate (f) of 0.045, 0.06, 0.09 and 0.125 mm/rev at a depth of cut of 0.2 mm (constant throughout the experiment) were taken for the experiment. Tool inserts were characterized by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) analysis. The cutting forces were measured using Kistler force dynamometer. Amongst all tools, PCD provided a better result in all aspects butsurprisingly WC tool provided a better surface finish with lesser tool wear. For all cutting conditions, high speed (670 m/min) and low feed rate (0.045 mm/rev) were recommended.

    • Experimental and FE analysis for the buckling behavior of hat-stiffened panels under edge compressive loading

      S KUMAR R KUMAR S MANDAL

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      In this paper, experimental studies on two laminated composite hat-stiffened panels with equally spaced stiffeners have been carried out with application of axial compression load on the panel for the determination of the pre-buckling and post buckling behavior. A non-linear buckling analysis on the hat-stiffenedpanel has also been performed under compression load with application of finite element tool ABAQUS. From strain analysis at different locations, the local buckling of skin has been observed before the buckling of the panel, and a visual damage has been found near the skin-stiffener and debonding skin-stiffener of the panel during failure of the hat-stiffened panel. The compression load-axial displacement curve of the experiments has correlated well with simulated finite element model result for determination of the buckling behavior of thepanel up to the failure load. The out of plane displacement pattern shows that the compressive failure initiated at the edges of the panel and gone to skin-stiffener bonding, and finally failure of the panel occurred due to debonding between skin-stiffener.

    • Optimization of the growth of GaN epitaxial layers in an indigenously developed MOVPE system

      ABHISHEK CHATTERJEE V K AGNIHOTRI R KUMAR S PORWAL A KHAKHA G JAYAPRAKASH TAPAS GANGULI T K SHARMA

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      Growth of GaN epitaxial layers is optimized in an indigenously developed nitride metal organic vapour phase epitaxy (MOVPE) system. The motivation of present work is to develop a simple MOVPE reactor which is though affordable but can deliver GaN epilayers of desired quality. The design and fabrication methodology of nitride MOVPE system is briefly discussed. MOVPE growth of GaN epilayers is carried out through a two-step growth process where influence of the growth parameters of low temperature buffer layer on the crystalline properties of high temperature GaN epilayer is discussed in detail. Optimum values of the growthtemperature, annealing duration and thickness of GaN buffer layer are achieved. High resolution X-ray diffraction measurements (HRXRD) confirmed that the crystalline quality of GaN epitaxial layers is reasonable. A good surface morphology of the optimized sample is strongly corroborated by the results obtained from photoluminescence (PL) and HRXRD techniques. Usefulness of a systematic Williamson-Hall analysis in the optimization of growth of GaN epitaxial layers is demonstrated where a high value of lateral coherence length ofGaN buffer layer is found to be a key parameter. Such an optimization process leads to the good crystalline quality of GaN epitaxial layers with low dislocation density. It is found that the high temperature GaN epilayers grown on the optimized GaN buffer layer are compressively stressed which is also revealed by the PL measurements

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