Study On Flow Characteristics Of Dynamic Asymmetric Impinging Stream Reactor

Continuous improvement and optimization of reaction process is an important engineering task in human production and life.Impinging stream reactor is a novel technology for enhancing process.Short particle residence time leads to the incompleteness and insufficiency of reactions performed by impinging stream reactor,limiting the wide engineering development and application of impinging stream reactor.In this paper,a coaxial impinging stream reactor is taken as the research object.Compared with the symmetrical impinging stream reactor with equal mean inlet velocity,the flow and turbulence characteristics of dynamic asymmetric impinging stream reactor and asymmetric impinging stream reactor are numerically and experimentally studied.Discrete element method is used to build the particle motion model,and the effects of inlet velocity conditions on the motion characteristics and residence time of single and multiple particles are analyzed and studied.The main content and conclusions of this thesis are as follows:Three-dimensional numerical simulation is used to study the effects of the important parameters of inlet velocity conditions,type of dynamic inflow pattern and geometric parameters of reactor on fluid flow and turbulence characteristics.The geometric and mesh model of impinging stream reactor are built and solved by using the finite element software.CFD results show that impingement region of symmetric impinging stream reactor is located in the geometric center of the reactor,and fluid velocity distribution of the flow field is axial symmetry with respect to the impinging surface,and impingement region of asymmetric impinging stream reactor deviates toward the fluid with lower velocity and the fluid velocity distribution in the asymmetric impinging stream reactor is asymmetric.While,the flow regime is varied with time and impinging surface makes back and forth motion in the dynamic asymmetric impinging stream reactor.The maximum turbulent energy in the flow field always lies at the center of the symmetric,asymmetric and dynamic asymmetric impinging stream reactor.Increasing inlet fluid velocity at either side increases the fluid fluctuating velocity in the impingement region.The turbulent characteristics of the impingement region in the asymmetric impinging stream reactor are better than those in the symmetric impinging stream reactor.Under the dynamic inflow condition,a more intense turbulence will occur in the impingement region with weak mean inlet velocity.Compared with the symmetric and asymmetric impinging stream reactor,dynamic asymmetric impinging stream reactor has more intense turbulence characteristics.In order to verify CFD results,an experimental platform of dynamic asymmetric impinging stream reactor is established.The fluid flow characteristics,the velocity distribution in the main active region and the motion law of impinging surface are measured and studied by particle image velocimetry(PIV).Besides,CFD results and theoretical analysis abovementioned are verified.Discrete element method is used to build the motion model of single particle and multi-particle.Numerical studies and theoretical analysis show single particle motion characteristics and residence time are mainly determined by inlet velocity conditions,geometric parameters of impinging stream reactor and particle properties.Besides,the effects of step inflow conditions on the motion characteristics and mean residence time of single and multiple particles are investigated in the impinging stream reactors.A single particle with*<0.173s(*is particle relaxation time)is accelerated directly by the original fluid and leaves the main active area without oscillatory motion,and the single particle residence time values in three different impinging stream reactors are all the same;at Re_m<4465?4?L/D?8,a single particle exhibits at most three oscillatory motions in symmetric and asymmetric impinging stream reactor;however,a single particle with*?0.695s exhibits only three oscillatory motions in symmetric and asymmetric impinging stream reactor,but at least three oscillatory motions in dynamic asymmetric impinging stream reactor.Thus at*?0.173s,single particle residence time is greatest in the dynamic asymmetric impinging stream reactor,and is the shortest in the symmetric impinging stream reactor under identical simulation conditions.At the same time,this paper summarizes the inlet velocity conditions in which the single particle residence time reaches the maximum.That is,single particle residence time reaches a maximum value at the optimal period(T_b).Theoretical analyses and simulation results revealed that T_b is determined by the parameters*,Re_m and L/D.Besides,the function expression of optimal period is determined quantitatively.The main reason that multi-particle residence time is very short in the symmetric impinging stream,is no oscillatory motion,interparticle collision and rotation in the impingement region.The mean multi-particle residence time of the asymmetric impinging stream is larger than that of the symmetric impinging stream because a large number of the either-side particles undergo oscillatory motion.These new modes existing in dynamic asymmetric impinging stream reactor,where some particles are accelerated again by the original or reverse fluid flow,can cause some particles to perform oscillatory motion in the main active area.The mean multi-particle residence time of dynamic asymmetric impinging stream reactor is larger than that of the symmetric impinging stream reactors because of the three new particle motion modes which account for the majority of particle behavior.Moreover,when T=T_b,mean multi-particle residence time reaches a maximum value.