Numerical Simulation Of Ultrasound-Assisted Extraction Based On Multi-physical Field Coupling

Ultrasound-assisted extraction(UAE)is considered to have advantages of high extraction yield and short extraction time,However,it is difficult to apply this technology to industrial production on a large scale.The main reasons include:(1)There is a lack of effective evaluation criteria for the physical fields involved in UAE process.(2)Standing wave effect leads to non-uniform distribution of acoustic field,which makes cavitation effect difficult to occur.(3)The mechanism of enhanced fluid mass transfer by ultrasound is still unclear.Based on the above situation,the multi-physical field coupling model in UAE process was constructed,and the visualization of the physical field distribution of ultrasonic equipment was realized.The evaluation indexes of acoustic field characteristics in UAE process were established,and the influence of different acoustic parameters,physical parameters and transducer array arrangement on the acoustic field characteristics were studied,which further elucidating the mechanism of ultrasonic.Taking soybean protein extraction as an example,the effect of ultrasonic frequency on protein extraction rate and microstructure were studied in order to verify the effect of ultrasonic wave and the reliability of numerical simulation.The conclusions of the research are as follows:(1)The physical fields and its interaction relationships in UAE process were analyzed.Based on the mechanism of acoustic wave generation and acoustic-fluid interaction,the governing equations of acoustic-solid coupling and fluid-solid coupling were derived,and the multi-physical field coupling mathematical model in UAE process was established.(2)Based on the mathematical model,the UAE process was simulated by adding appropriate boundary conditions.The simulation results of acoustic field showed that: There are several alternating periods of positive and negative pressure in the process of ultrasonic propagation,which cause the vibration of medium molecule centered on the equilibrium position.With the increase of power density,the absolute sound pressure increases first and then tends to slow down,and the degree of non-uniformity of acoustic field increases continuously.With the change of ultrasonic frequency,the absolute sound pressure and relative standard deviation change according to the same trend,indicating that the acoustic field uniformity becomes worse if the absolute sound pressure is improved.Compared with the system flow rate,dynamic viscosity,transducer spacing and array arrangement have greater influence on the acoustic field characteristics.The cylinde-shaped ultrasound-assisted extraction equipment can improve the uniformity of acoustic field by frequency modulation,which can promote the occurrence of cavitation effect.The results of model calculation showed that the extraction effect of this equipment at and near the resonance frequency of 28 kHz,35 kHz,45 kHz is better.The simulation results of fluid field showed that the eddy current effect induced by ultrasound can enhance fluid disturbance,change fluid field distribution and promote mass transfer mixing.(3)Taking soybean protein extraction as an example,the optimum process of protein extraction was determined by single-factor experiment and orthogonal experiment.The optimum process of protein extraction was as follows: power density 120 W/L,extraction time 25 min,solid-to-liquid ratio 1:20 g/mL,pH 11.0,extraction temperature 45℃,grinding fineness 80 mesh.The effect of UAE method under the optimum conditions was significantly better than those of traditional alkali extraction method(p<0.05).The extraction rate decreases with the increase of ultrasonic frequency,and the extraction effect of this equipment at and near the resonance frequency is better.The UV spectrum,the FT-IR spectrum,the fluorescence spectrum and the circular dichroism spectrum of soybean protein treated with different frequencies showed that the influence of low frequency ultrasound on protein structure is more obvious than that of high frequency ultrasound.The resonance frequency ultrasound will destroy the structure of protein and cause its extreme denaturation.The experimental results are consistent with numerical simulation,which verifies the effect of ultrasonic wave and the reliability of numerical simulation.