tailieunhanh - Advanced Topics in Mass Transfer Part 3

Tham khảo tài liệu 'advanced topics in mass transfer part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Application of Airborne Sound Waves for Mass Transfer Enhancement 69 It is generally recognized that there three types of acoustic streamings. They differ each other by the spatial scale on which they can spread. The acoustic streaming of the first type is generated in an unbounded body of fluid. In this case the streaming is a steady flow directed away from the sonic generator in the direction of wave oscillations. As its scale can be larger compared to the sound wavelength it is called large-scale streaming. It is generally agreed that the large-scale streaming originates from the sound energy absorption briefly considered above. Under some conditions the velocity of this type of streaming can be as high as several m s Lighthill 1978 . The acoustic streamings of the second and third types are generated in the presence of solid obstacles walls particulates etc. placed in an acoustic field. In this case the attenuation occurs because of frictional dissipation between an oscillating gas volumes and solid surface within the resulting boundary layer. The acoustic streaming of the second type is generated outside the boundaly layer. The scale of such an outer streaming is much smaller than that of the first type and is equal approximately to the wavelength. The acoustic streaming of the third type is called inner small-scale streaming because it is induced within the bounday layer the dimension of which is much smaller than wavelength. The effective thickness of the boundaly layer is about 5 times larger than that of acoustic boundaly layer which is given by the following expression s v ro 05 17 where V is the kinematic viscosity of fluid ro is the angular frequency of sound ro 2nf Zarembo 1971 . 3. Combustion and environmental control applications. Mechanisms of mass transfer enhancements As pointed in the above section application of acoustic oscillations can lead to the occurrence of several phenomena responsible for improvements in the gas-phase mass transfer .