A Research Of Enhancing Boiling Heat Transfer And Mass Transfer During Osmotic Dehydration With Acoustic Cavitation

Acoustic cavitation refers to the formation of tiny bubbles under certain pressure within a liquid, their turbulence and surge in response to change of acoustic pressure, or their subsequent growth, shrinkage and collapse when ultrasound travels across the liquid. The process of acoustic cavitation is to focus the acoustic field energy and release rapidly. In recent decade, the acoustic cavitation and its relative effects attract more and more concerns among the international experts because of its theoretical value and great applied potentiality. The technology of enhancing heat and mass transfer with acoustic cavitation is not only the third generation active intensification of heat and mass transfer technology, to which close attention was paid extensively just in recent years, but also the new application in the field of heat and mass transfer .In view of acoustic cavitation phenomenon induced by ultrasound traveling in a medium, cavitating threshold value and dynamic behavior of cavitating bubble was analyzed. High temperature, high pressure and strong press pulse during acoustic cavitation are regarded as the root of enhancing heat and mass transfer with acoustic cavitation.A series of experiments were carried out systematically to make clear the effects of acoustic cavitation on natural convective heat transfer. The temperature of the liquid and test section surface and heat transfer coefficients, with and without acoustic cavitation, were measured and analyzed in order to study the effect of the parameters of acoustic cavitation on natural convective heat transfer. The physical mechanism and behavior of enhancing natural convection heat transfer using acoustic cavitation were analyzed and discussed in the meantime. The results show that a distinctive augmentative effect on natural convection is observed with acoustic cavitation, and cavitating intensity and orientation are important factors affecting natural convection enhanced by acoustic cavitation.The acoustic shock wave caused by ultrasonic cavitation induces macroscopic turbulence of the solution and changs fluid filed around horizontal circular tube. At the same time, both the thickness of the boundary layer and thermal resistance which exists between a liquid and a solid are decreased because acoustic streaming impinged directly on the wall. Therefore natural convection heat transfer was intensified.Saturated nucleate boiling heat transfer of horizontal copper tube in infinite space was taken as an example, experiments were carried out to study the macroscopical physical behavior during nucleate boiling heat transfer with acoustic cavitation. The research found that the maximum augmentation ratio occurs in the starting region of nucleate boiling heat transfer intensified with acoustic cavitation, and augmentation ratio becomes smaller along with the increase of heat flux. The augmentation ratio increases with cavitating intensity when heat flux wasn't changed. On the basis of experimental study, starting from bubble dynamics of homogeneous boiling, the effects of acoustic cavitation on boiling nuclei and bubble formation and growth and departure were analyzed from two aspects, force and thermal equilibrium. The number of boiling nuclei in the wall increases, the diameter of the bubble initially growing decreases, the speed of growth is quickened, the frequency of departure increases because of the effect of acoustic cavitation.An experiment was carried out to study the enhancement of mass transfer during osmotic dehydration with acoustic cavitation. The trends of water losses and solute gain rates of materials were discussed along with solute concentration, cavitating intensity, thickness of materials, stage and time during osmotic dehydration. The influence of different materials on mass transfer during osmotic dehydration was also investigated. Meanwhile the physical mechanism and behavior of enhancing mass transfer during osmotic dehydration with acoustic cavitation were clarified. The results show that osmotic dehydration is distinctively augmented with acoustic cavitation. The water losses and solute gain rates of materials all increase, because macroscopic turbulence of the solution andshock to boundary layer of acoustic streaming increase with the increase of cavitating intensity. The local pressure fluctuation induced by cavitation accelerates degassing of material's tissue, increases the available mass transfer surface. All of these accelerated the mass transfer process during osmotic dehydration.The mathematical model of moisture and solute diffusion during osmotic dehydration accelerated with acoustic cavitation was established. The coefficients of moisture and solute diffusion were calculated. These values are similar to the apparent diffusivities proposed by different authors for similar system. The numerical simulation was used to calculate moisture content and dry matter gain rates of materials at different time points. The calculated results agree well with experimental data. The mathematical model represents well the rule of mass transfer during osmotic dehydration intensified with acoustic cavitation.