Numerical Simulation Of Self-oscillating Cavitation Of Flow Field Characteristics In Chitosan Solution

Oligochitosan have been widely used in many fields because of their unique functional properties.However,the preparation of oligochitosan by chitosan has many disadvantages,such as high cost,degradation process is cumbersome and so on.Therefore,it is of great significance to look for an economical,efficient,environmental-friendly and easy-to-use method for degrading chitosan to broaden its use and improve its application value.Based on the previous work which the degradation of chitosan had been studied by the hydraulic cavitation process such as orifice plate and Venturi tube,the influence of different structural parameters and operating parameters on the flow field characteristics of Helmholtz self-oscillating cavitation device were simulated and analyzed in this paper.And a structure suitable for the degradation of chitosan solution by the self-oscillating cavitation was established.The main contents and results of this paper are as follows:Through theoretical investigation,the modulation mechanism of self-oscillating cavitation jet was analyzed,the judgment method of cavitation inception and the main factors affecting cavitation inception are discussed,and the basic theory of cavitation dynamics is made clear through the derivation of Rayleigh-Plesset equation and the discussion of cavitation expansion and collapse process.In the self-oscillating cavitation device,the key structure to determine the performance of the self-oscillating cavitation jet is the oscillation cavity.According to the basic structure of Helmholtz self-oscillating cavitation device,the numerical simulation of the inner flow field in Helmholtz self-oscillating cavitation device was carried out by using the multiphase flow Mixture model,Schnerr and Sauer cavitation model and LES turbulence model based on CFD software,the mechanism of the self-oscillating cavitation jet is revealed and the pressurization characteristic of the self-oscillating cavitation jet is expounded.Meanwhile the effects of the main structural parameters,such as the length of the oscillating cavity,the outlet diameter of the oscillating cavity and the impact angle of the wall in the oscillating cavity which affect the fluid oscillation characteristics wre simulated,and the influence of the structural parameters of the self-oscillating cavitation device on the jet performance were analyzed.And the optimum structure parameters of the self-oscillating cavitation device are obtained by comparing the results.At the same time,the simulation of the internal flow field characteristics of the hydraulic cavitation device under different structural types were carried out,and the unique results of the self-oscillating cavitation structure are obtained which are different from the general cavitation structure.Based on the simulation study of the generation mechanism of self-oscillating cavitation jet and the analysis of the influence of structural parameters on the jet performance,a structure suitable for the degradation of chitosan solution by self-oscillating cavitation was established.The effects of different structural parameters,operating parameters,the concentration of chitosan solution and the temperature of chitosan solution on the flow field characteristics were analyzed by numerical simulation.At the same time,through the study of orthogonal experiment,the optimum combination of structural parameters of self-oscillating cavitation is obtained,the optimum structural combination is that the oscillatory cavity length is Lc=9 mm,the diameter of the outlet section of the oscillator cavity is d₂=2.6 mm,the impact angle at the outlet of the oscillatory cavity is?₁=120°.The results will provide a theoretical reference for the enhancement of chitosan degradation by self-oscillating cavitation in the future.