Research On The Influence Mechanism Of Micro-mixing In Liquid Fast Reaction System Based On Lagrange PDF Method

Micro-mixing processes are widely present in many fields such as daily life,food processing,and industrial production.During the production process,especially in the rapid reaction system,micro-mixing has an important influence on the conversion of reaction materials and the production of target products.Aiming at the problem of the unclosed terms in the Reynolds stress transport equation,this paper uses the Lagrange PDF model method combining the Probability Density Function(PDF)proposed by Pope et al.with the turbulence model to close the term and solve the problem,and use CFD technology to explore the influence mechanism and law of micro mixing in the liquid fast reaction system.This paper derives the basic model formula based on the combination of the Probability Density Function method and the micro-mixed model.In view of the fact that the main simulation object of this subject is the fluid in the pipe,based on the classical transfer equation,the continuity equation and momentum equation in cylindrical coordinates are derived in this paper to fit the simulation calculation scenario.The traditional Reynolds stress transport equation has unclosed terms.In this paper,the probability density function method is used to couple with the turbulent transport equation to close the unknown term,and this paper derived the composition joint PDF transport equation,the velocity-composition joint PDF transport equation,Lagrange PDF transport equation Lagrange,etc.In this paper,the Lagrange PDF model method is validated by an experimental case.The verification system of acid-base neutralization and 2,2-dimethoxypropane hydrolysis competition reaction,and the micro impinging stream reactor are used to carry out simulation calculations in this paper.The simulated values are compared with the experimental value,to investigate the micro-mixing efficiency in the reaction system,and to verify applicability of the model method.The following conclusion can be draw.The fluid turbulence degree resulted from increasing the feed rate could enhance micro-mixing effect and accelerate the reaction rate.In the three sets of experiments those feeding for 4.8ms,6.5ms,and 9.5ms,separately,the simulation result of feeding for 9.5ms simulated reaction feed is in good agreement with the experimental value.In this work,the grid quality,wall densification function,and physical property parameter errors affect the accuracy of the simulation to a certain extent,and the improvement work can be started from these.With the Lagrange PDF method,this paper continues to carry out the simulation work based on the ideal reactor,and discusses the micro-mixing influence law of the system.This paper selected a double-pipe jet reactor as reactor,and the Beckmann rearrangement reaction of cyclohexanone oxime as the liquid-phase fast reaction system to discuss the mechanism and law of micro-mixing influence.The following conclusions can be draw from the work.In the continuous reaction,the feeding ratio’s change(the ratio of acid to oxime)can enhance the micro-mixing effect.Among the five experimental cases,the Micro-mixing effect is best when the ratio of acid to oxime is 1.5.Enhancement of the liquid fluid turbulence resulted from the feeding rate increasing can accelerate molecular diffusion process and improve the micro-mixing performance.Meanwhile,appropriately increasing the feed temperature can intensify the thermal movement of molecules,reduce the miceo-mixing time and enhance the micro-mixing effect.The applicable temperature adopted in this article is 105°C,which is also in line with the conditions adopted in the industry.