THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

Atomization17.2 Industrial Applications of Ultrasound 491Ultrasonic atomizers can produce small droplets of predictable size. For a givenliquid, droplet size depends on atomizer frequency and it gets smaller with increasingfrequency. Ultrasonic nebulizers are widely used for medical inhalation andoperate between 1 and 3 MHz to produce droplets between 1 and 5 μm. Increasedcombustion engine efficiency and reduction of pollution have been made possibleby ultrasound atomization of fuel, operating between 20 and 200 kHz.Emulsification and Flow EnhancementThe principal advantage of ultrasonic emulsification lies in the ability to mix someimmiscible liquids without additives (surfactants). Liquid flow through porousmedia can be increased by ultrasound and can find use in filtering and impregnation.Ultrasonic ViscometerThe ideal liquid should not support a shear stress, but the fact is that liquids dohave viscosity that gives rise to shear waves. A viscoelastic liquid combiningthe attributes of both fluid and solid behaviors (which produces shear stresses) isdescribed by− ∂ε∂y = 1 η p y + 1 G ṗy (17.1)where p y represents a variable shear stress; ṗ y is the time rate of the shear stress, εis the fluid particle displacement, η is the viscosity coefficient; and G is the shearcoefficient. The solution to Equation (17.1) is( ) ∂εp y =−∂y0(1 − e tη/G)(17.2)Equation (17.2) indicates that a periodically varying shear produces a relaxationprocess characterized by a time constant τ = η/G. The associated relaxation frequencyis given byf 0 = 12πτ = G(17.3)2πηFrom Equation (17.2) the attenuation of shear waves in a viscous liquid at a givenfrequency decreases with increasing viscosity. The damping of a vibrating shearwave transducer submerged in the liquid is a function of the coefficient of viscosityof the liquid. One way of measuring this quantity is to apply a pulse toa Y-cut crystal or to a torsionally vibrating rod immersed in the liquid so thatit vibrates freely with damped harmonic motion. When the amplitude of the vibrationdrops to a predetermined level, another pulse is generated. The rate ofpulse repetition increases with damping and hence decreases with increase in viscosity.The device is calibrated by using liquids having known coefficients ofviscosity.