R Nagarajan

Cavitation erosion is predominant in pipelines for liquid transportation, causing damage to pipe wall, impeller and their accessories. The present study is focused on development of cavitation -wear resistant nano-ceramic particle-reinforced polymer matrix material; and on study of its feasibility to be used as lining material in hydraulic transportation. The polymer/nano composite is fabricated using power ultrasound in all three process steps: synthesis of nano-dimensional particles of white fused alumina (WFA) from micron size particles, optimized blending and finally reinforcement into poly methyl methacrylate (PMMA) matrix. The effect of ultrasonic parameters on nanocomposite/ virgin polymers (like polyethylene and polypropylene) is studied by measuring mass loss of the materials and suspension turbidity during exposure time. At low frequency (20-60 kHz), cavitation intensity is predominant; this effect is utilized for fabricating sub-micron particles, and for performing accelerated cavitation erosion tests. At high frequency, acoustic streaming is predominant; this effect is utilized for blending and reinforcing of the nano ceramic particles into polymer matrix. The size and quantity of the particles generated by cavitation erosion was analyzed by Laser Particle Size Analyzer (20 nm-1400 micron range). The nano-composite coupons were analyzed before and after the ultrasonic erosion test using SEM. It is concluded that lowfrequency sonication is a viable option for cavitaton erosion testing of ceramic/polymer composites.

The present work investigates utilization of ultrasound in reagent-based coal de-ashing and de-sulfurization. The coal under study was received from Girald mine, Rajasthan, India. Three different ultrasonic frequencies (25kHz, Dual (58/192kHz) and 430kHz) and three reagents (HCl, HNO3 and H2O2) were used. The study employed a Taguchi fractional-factorial L27 DOE. Experimental data were used to derive an empirical model for the prediction of total sulfur removal. The model incorporates cavitational intensity, reagent concentration, sonication time, coal particle size and coal concentration as key parameters. Effects of above factors on reagent-based ultrasonic coal-desulfurization are presented here. An optimum set of process parameters are identified and validated. Larger-scale trial with high-ash and high-sulfur coals is strongly recommended. © 2011 Elsevier B.V.

Ultrasonic cleaning, employing frequencies in the range between 40 kHz to 200 kHz, is widely used in many industries requiring precision cleanliness in the micrometer to submicron particle size range-e.g., semiconductor wafer fabrication, hard disk drive manufacturing, integrated circuit assembly, etc. One overriding concern with the use of ultrasonic cleaning for delicate components and assemblies has been the specter of cavitation erosion-surface material loss and other functional degradation due to impact of shock waves generated by collapsing bubbles and bubble-clusters in an oscillating acoustic field. The simultaneous processes of surface cleaning and of surface erosion in the presence of a highfrequency ultrasonic field (>/= 58 kHz) are described here mathematically, and the equations are coupled in such a way as to allow conceptual optimization of parametric settings to maximize the cleaning efficiency, even while minimizing the level of erosion damage. This theoretical analysis is presented for various ultrasonic field conditions (frequency, intensity, etc.), fluid medium properties (viscosity, density) and various surface conditions (hardness, smoothness, etc.). The contribution of "acoustic streaming" to surface cleaning is incorporated in the model, and is shown to have minimal influence on the optimum cluster collapse pressure, but to have a significant effect on the net cleaning efficiency for the surface.

Ultrasonic cleaning, employing frequencies in the range of 40-200 kHz, is widely used in many industries requiring precision cleanliness in the micrometer to submicron particle size range-e.g., semiconductor wafer fabrication, hard disk drive manufacturing, and integrated circuit assembly. One overriding concern with the use of ultrasonic cleaning for delicate components and assemblies has been the specter of cavitation erosion-surface material loss and other functional degradation due to the impact of shock waves generated by collapsing bubbles and bubble clusters in an oscillating acoustic field. The simultaneous processes of surface cleaning and surface erosion in the presence of a high-frequency ultrasonic field (≥ 58 kHz) are described here mathematically, and the equations are coupled to allow conceptual optimization of parametric settings to maximize cleaning efficiency while minimizing the level of erosion damage. This theoretical analysis is presented for various ultrasonic field conditions (frequency, intensity, etc.), fluid medium properties (viscosity, density), and surface conditions (hardness, smoothness, etc.). The contribution of acoustic streaming to surface cleaning is incorporated in the model, and is shown to have minimal influence on the optimum cluster collapse pressure, but to have a significant effect on the net cleaning efficiency for the surface.

Cavitation erosion is predominant in pipelines for liquid transportation, causing damage to pipe wall, impeller and their accessories. The present study is focused on development of cavitation -wear resistant nano-ceramic particle-reinforced polymer matrix material; and on study of its feasibility to be used as lining material in hydraulic transportation. The polymer/nano composite is fabricated using power ultrasound in all three process steps: synthesis of nano-dimensional particles of white fused alumina (WFA) from micron size particles, optimized blending and finally reinforcement into poly methyl methacrylate (PMMA) matrix. The effect of ultrasonic parameters on nano-composite/ virgin polymers (like polyethylene and polypropylene) is studied by measuring mass loss of the materials and suspension turbidity during exposure time. At low frequency (20-60 kHz), cavitation intensity is predominant; this effect is utilized for fabricating sub-micron particles, and for performing accelerated cavitation erosion tests. At high frequency, acoustic streaming is predominant; this effect is utilized for blending and reinforcing of the nano ceramic particles into polymer matrix. The size and quantity of the particles generated by cavitation erosion was analyzed by Laser Particle Size Analyzer (20 nm-1400 micron range). The nano-composite coupons were analyzed before and after the ultrasonic erosion test using SEM. It is concluded that low-frequency sonication is a viable option for cavitaton erosion testing of ceramic/polymer composites.

This paper investigates the accelerated mixing of hot and cold liquid layers in storage tanks of different physical dimensions by the application of high-frequency, high-intensity pulsed ultrasound. In pulsed operation, the ultrasonic field is switched on for a few seconds and then switched off. This cycle is repeated several times. Pulsed mixing of hot and cold water due to ultrasonics was measured in this study. Acoustic streaming and cavitation phenomena associated with the ultrasonic field can induce enhanced mixing in the storage containers leading to de-stratification of liquid. The experimental results indicate that dual frequency operation, which combines one high-frequency mode with one low-frequency mode, is optimal in enhancing mixing compared to other frequencies. Mixing efficiency increases with cavitation intensity and the introduction of acoustic streaming augments it further. The experimental result indicates that as the height of the cylinder increases, the mixing time also increases. The ultrasonic mixing times obtained for different frequencies indicate that as the frequency increases, the time required to reach the steady state temperature also increases, due to decrease in cavitation intensity. © 2008 The Institution of Chemical Engineers.

Deposition of fly ash particles onto heat-transfer surfaces is often one of the reasons for unscheduled shut-downs of coal-fired boilers. Fouling deposits encountered in convective sections of a boiler are characterized by arrival of ash particles in solidified (solid) state. Fouling is most frequently caused by condensation and chemical reaction of alkali vapors with the deposited ash particles creating a wet surface conducive to collect impacting ash particles. Hence, the amount of alkali elements present in coals, which, in turn, is available in the flue gas as condensable vapors, determines the formation and growth of fouling deposits. In this context, removal of alkali elements becomes vital when inferior coals having high-ash content are utilized for power generation. With the concept of reducing alkali elements present in a coal entering the combustor, whereby the fouling deposits can either be minimized or be weakened due to absence of alkali gluing effect, the ultrasonic leaching of alkali elements from coals is investigated in this study. Ultrasonic water-washing and chemical-washing, in comparison with agitation, are studied in order to estimate the intensification of the alkali removal process by sonication.

Enhancement of heat transfer from a heat source to a flowing fluid within a tube is a challenging problem with many practical applications. In this paper, experimental investigation of a low-frequency (20-33. kHz), high-intensity (500-1000. W) ultrasonic field as a potential heat-transfer " process intensifier" is undertaken. Heat-transfer enhancement data collected in a miniaturized furnace tube over a range of flow conditions and ultrasonic process parameters indicate that sonication provides significant augmentation only under near-static (e.g., stagnant) and low-Reynolds number flow conditions. With increasing flow velocity, cavitational and acoustic-streaming fields associated with ultrasound are rapidly diminished in importance, hence playing no role in bulk fluid heat transfer (unless input power levels or frequencies are suitably increased). However, the relevance to some locations, such as those under porous deposits in water-wall tubes of boilers near the goose-neck portion, can spur further study to exploit the impact of ultrasonic heat-transfer enhancement. The critical parameter that determines the efficacy of ultrasonic enhancement of heat transfer appears to be the ratio of the characteristic ultrasonic field velocity (sum of cavitational and acoustic streaming velocities) to the prevailing flow velocity. © 2012 Elsevier B.V.

Stratification is one of the main causes for vaporization of cryogens and increase of tank pressure during cryogenic storage. This leads subsequent problems such as cavitation in cryo-pumps, reduced length of storage time. Hence, it is vital to prevent stratification to improve the cost efficiency of storage systems. If stratified layers exist inside the tank, they have to be removed by suitable methods without venting the vapor. Sonication is one such method capable of keeping fluid layers mixed. In the present work, a mechanistic model for ultrasonic destratification is proposed and validated with destratification experiments done in water. Then, the same model is used to predict the destratification characteristics of cryogenic liquids such as liquid nitrogen (LN2), liquid hydrogen (LH2) and liquid ammonia (LNH3). The destratification parameters are analysed for different frequencies of ultrasound and storage pressures by considering continuous and pulsed modes of ultrasonic operation. From the results, it is determined that use of high frequency ultrasound (low-power/continuous; high-power/pulsing) or low frequency ultrasound (continuous operation with moderate power) can both be effective in removing stratification. © 2013 Elsevier B.V. All rights reserved.