In solid materials, alternative compressions and expansions
generated by the ultrasonic waves produce a similar effect to that
observed when a sponge is squeezed and released repeatedly (De
la Fuente et al., 2006). This ‘‘sponge effect’’ produces the release
of liquid from the inner part of the particle to the solid surface
and the entry of fluid from outside. The forces involved in this
mechanism can be higher than the surface tension which maintains
the water molecules inside the capillaries of the material, creating
microscopic channels (Muralidhara et al., 1985) and making
the interchanges of matter easier. Other effects to be considered
are the variation of viscosity, surface tension or the deformation/
degradation of the solid structure.
From a general point of view, all the effects produced by ultrasound
could influence mass and/or heat transfer phenomena. In
treatments with a solid immersed in a fluid, ultrasound could
accelerate the internal transport making the entry of fluids in the
solid matrix and/or their exit easier and also facilitating the exchanges
between the solid surface and the surrounding fluid. Then,
the use of ultrasound, when applied in an efficient way, could be
interesting in applications involving heat or mass transport,
decreasing both the external and internal resistance to transport.
In solid materials, alternative compressions and expansionsgenerated by the ultrasonic waves produce a similar effect to thatobserved when a sponge is squeezed and released repeatedly (Dela Fuente et al., 2006). This ‘‘sponge effect’’ produces the releaseof liquid from the inner part of the particle to the solid surfaceand the entry of fluid from outside. The forces involved in thismechanism can be higher than the surface tension which maintainsthe water molecules inside the capillaries of the material, creatingmicroscopic channels (Muralidhara et al., 1985) and makingthe interchanges of matter easier. Other effects to be consideredare the variation of viscosity, surface tension or the deformation/degradation of the solid structure.From a general point of view, all the effects produced by ultrasoundcould influence mass and/or heat transfer phenomena. Intreatments with a solid immersed in a fluid, ultrasound couldaccelerate the internal transport making the entry of fluids in thesolid matrix and/or their exit easier and also facilitating the exchangesbetween the solid surface and the surrounding fluid. Then,the use of ultrasound, when applied in an efficient way, could beinteresting in applications involving heat or mass transport,decreasing both the external and internal resistance to transport.
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