Sergey V. Komarov

Enhancement of gas phase heat transfer by acoustic field application

This study discusses a possibility for enhancement of heat transfer between solids and ambient gas by application of powerful acoustic fields. Experiments are carried out by using preheated Pt wires (length 0.1-0.15 m, diameter 50 and 100 micro m) positioned at the velocity antinode of a standing wave (frequency range 216-1031 Hz) or in the path of a travelling wave (frequency range 6.9-17.2 kHz). A number of experiments were conducted under conditions of gas flowing across the wire surface. Effects of sound frequency, sound strength, gas flow velocity and wire preheating temperature on the Nusselt number are examined with and without sound application. The gas phase heat transfer rate is enhanced with acoustic field strength. Higher temperatures result in a vigorous radiation from the wire surface and attenuate the effect of sound. The larger the gas flow velocity, the smaller is the effect of sound wave on heat transfer enhancement.

Suppression of slag foaming by a sound wave

The aim of this work was to study the effects of sound frequency, sound intensity and viscosity of slag on the slag foaming rate and the steady-state foam height. Experiments were carried out using two slags (BaO-B2O3) melted at a temperature of 1223 or 1273 K, as well as water-glycerin solutions at room temperature. Low frequency sound waves (< 1.3 kHz) are found to be more effective in the slag foaming suppression than high frequency waves (1.3-12 kHz). The steady-state foam height decreases abruptly when the sound pressure reaches a threshold value that depends on sound frequency and liquid viscosity. The results can be explained in terms of enhancing the rates of liquid drainage and film rupture induced by sound.

Nanosized zero-valent iron as Fenton-like reagent for ultrasonic-assisted leaching of zinc from blast furnace sludge

Ultrasonic-assisted sulphuric acid leaching combined with a Fenton-like process, utilizing nanoscale zero-valent iron (nZVI), was investigated to enhance the leaching of zinc from the blast furnace sludge (BFS). The leaching of iron (Fe) and zinc (Zn) from the sludge was investigated using Milli-Q water/BFS ratio of 10 and varying the concentration of hydrogen peroxide, sulphuric acid, the temperature, the input energy for ultrasound irradiation, and the presence or absence of nZVI as a Fenton reagent. The results showed that with 1g/l addition of nZVI and 0.05M of hydrogen peroxide, the kinetic rate of Zn leaching increased with a maximum dissolution degree of 80.2%, after 5min treatment. In the absence of nZVI, the maximum dissolution degree of Zn was 99.2%, after 15min treatment with 0.1M of hydrogen peroxide. The rate of Zn leaching at several concentrations of hydrogen peroxide is accelerated in the presence of nZVI although a reduction in efficiency was observed. The loss of Fe was no more than 3%. On the basis of these results, the possible route for BFS recycling has been proposed (BFS slurry mixed with sulphuric acid and hydrogen peroxide is recirculated under ultrasonic irradiation then separated).

Cavitation and acoustic streaming generated by different sonotrode tips

Aiming at improving the efficiency of cavitation treatment, this work investigates characteristics of acoustic streaming and cavitation generated in water by dumbbell-shaped sonotrodes with plane, truncated and conical tips. The main emphasis was placed on elucidating the effects of tip shape and vibration amplitude ranged from 40 to 60 μm. The PIV technique and Weissler reaction were used to measure flow pattern and velocity of acoustic streaming, and cavitation efficiency, respectively. To provide a theoretical explanation to the experimental results, a self-developed mathematical model was used to simulate the acoustic streaming and predict the size of cavitation zone numerically. Both the experimental and numerical results revealed that the sonotrode tip shape affects the acoustic streaming significantly, altering the flow magnitude and direction from fast and downward under the plane and truncated tips to relatively slow and upward near the conical tip. Besides, the conical tip provides a more efficient cavitation treatment in comparison with the plane and truncated tips. The simulation results showed that widening of cavitation zone and altering of acoustic streaming velocity and direction near the sonotrode tip are responsible for the enhancement of cavitation treatment efficiency.

Effect of Sound Waves on Decarburization Rate of Fe-C Melt

Sound waves have the ability to propagate through a gas phase and, thus, to supply the acoustic energy from a sound generator to materials being processed. This offers an attractive tool, for example, for controlling the rates of interfacial reactions in steelmaking processes. This study investigates the kinetics of decarburization in molten Fe-C alloys, the surface of which was exposed to sound waves and Ar-O2 gas blown onto the melt surface. The main emphasis is placed on clarifying effects of sound frequency, sound pressure, and gas flow rate. A series of water model experiments and numerical simulations are also performed to explain the results of high-temperature experiments and to elucidate the mechanism of sound wave application. This is explained by two phenomena that occur simultaneously: (1) turbulization of Ar-O2 gas flow by sound wave above the melt surface and (2) motion and agitation of the melt surface when exposed to sound wave. It is found that sound waves can both accelerate and inhibit the decarburization rate depending on the Ar-O2 gas flow rate and the presence of oxide film on the melt surface. The effect of sound waves is clearly observed only at higher sound pressures on resonance frequencies, which are defined by geometrical features of the experimental setup. The resonance phenomenon makes it difficult to separate the effect of sound frequency from that of sound pressure under the present experimental conditions.

Generation of OH Radical by Ultrasonic Irradiation in Batch and Circulatory Reactor

Ultrasonic technology has been widely investigated in the past as one of the advance oxidation processes to treat wastewater, in this process acoustic cavitation causes generation of OH radical, which play a vital role in improving the treatment efficiency. In this study, OH radical formation rate was measured in batch and circulatory reactor by using Weissler reaction at various ultrasound output power. It is found that the generation rate in batch reactor is higher than that in circulatory reactor at the same output power. The generation rate tended to be slower when output power exceeds 137W. The optimum condition for circulatory reactor was found to be 137W output and 4L/min flow rate. Results of aluminum foil erosion test revealed a strong dependence of cavitation zone length on the ultrasound output power. This is assumed to be one of the reasons why the generation rate of HO radicals becomes slower at higher output power in circulatory reactor.

Ultrasonic Assisted Reduction of Hot-Tearing During High-Speed DC Casting of 6000 Series Aluminum Alloys

This work presents results of preliminary investigations concerning the effect of ultrasonic vibrations on the solidification structure and hot-tearing susceptibility of 6000 series aluminum alloys in high-speed direct chill casting processes. A pilot DC caster was used to produce billets of 82–97 mm in diameter. Ultrasonic vibrations were introduced directly into the mold through a high-amplitude ceramic sonotrode, the tip of which was positioned at different distances from the melt crystallization front. The cast billets were then investigated for the microstructure and hot tearing susceptibility. It is shown that the ultrasonic treatment leads to a significant reduction in hot tearing susceptibility, and at the same time to a rise in mechanical properties of the alloys. The results suggest that at least two ultrasonic effects contribute to these improvements. The first one is cavitation which results in forming more refined and uniform microstructure of alloys. The second one is acoustic streaming which is responsible for macro agitation of melt in the sump. This causes the liquid-solid system to approach an equilibrium state that results in increasing the fraction of eutectic phase solidified at the grain boundaries of α-Al phase.

Investigation of Phosphorus-Containing Compounds in Aluminium Alloys with Emphasis on the Formation Mechanism

It is well known that the presence of phosphorus in hypoeutectic Al-Si alloys causes coarsening of Al-Si eutectic even when the phosphorus concentration is as low as a few ppm. The reason is that phosphorus reacts with aluminum to form AlP, the particles of which serve as potential nuclei for silicon due to a very similar lattice parameter with Si. The purpose of this study is to investigate the possibilities for reducing the nucleation potency of AlP by changing the alloy chemistry. The experiments include melting and solidification of a number of aluminum alloys under different conditions. After that, the samples are examined using SEM and TEM microscopy to elucidate the presence of phosphorus-containing compounds and to explain their formation mechanism. The experimentally obtained data are discussed on a systematic basis of thermodynamic calculations and compared with the corresponding data from the relevant literature. Particularly, the results reveal that such elements as Mg, Ca and Sr are potentially capable of neutralizing the nucleation activity of AlP although their effect is dependent on the presence of other chemical elements in the aluminium alloy.