Decoupling Simulation Study Of Phase Change Microcapsules Subjected To Flow Field In Centrifugal Pump

Phase change microcapsule suspension(MPCS)has both heat storage and heat transfer functions,and has broad application prospects.During the pumping cycle of heat storage and storage,part of the phase change microcapsules(MPCM)particles in the MPCS will be damaged under the cyclically changing complex external force,causing the system viscosity to increase and the heat storage and heat transfer capacity to decrease.This paper mainly conducts decoupling simulation research on the mechanical behavior of phase change microcapsules under the complex loading conditions during the pumping cycle for heat storage and release,and provides the necessary reference for the rational design of the pumping cycle system and the improvement of the mechanical properties of MPCM.The specific work completed is as follows:(1)Using No.58 paraffin as the core material and urea-formaldehyde resin as the wall material,multiple sets of MPCM samples were prepared by in-situ polymerization method,changing the core wall ratio,polymerization temperature,emulsification stirring speed,emulsifier dosage four experimental conditions,The nine groups of samples prepared based on the orthogonal experiment method were mainly tested and characterized by chemical composition,micro-morphology,thermal properties and particle size distribution.The best preparation process conditions obtained are:core wall ratio 1.5:1,polymerization temperature 70?,emulsification stirring speed 2000r/min,and the amount of emulsifier is 2%of the core material mass.The product obtained under this condition is a light yellow powder with an average particle size of 81.3?m,irregular particle shape,and a latent heat of phase change higher than 90 J/g.(2)The quasi-static compressive deformation behavior of urea-formaldehyde resin microspheres was preliminarily simulated and analyzed with ANSYS software.The purpose was to investigate the ability of MPCM wall materials to resist external pressure.The results show that the mechanical response of urea-formaldehyde resin solid and hollow microspheres under quasi-static compression conditions is related to their diameter;whether the hollow microspheres are subjected to concentrated load or uniform load,the thickness-to-diameter ratio significantly affects the simulation results.When the microspheres with radius and wall thickness of 40.65?m and 2.5?m are compressed by 0.001N concentrated force,the deformation will develop from the linear elastic stage to the strain hardening stage.When the urea-formaldehyde resin hollow microspheres with a radius of 40.65?m and wall thicknesses of 10?m,5?m and 2.5?m are subjected to a uniformly distributed load of 10⁴N/m²,they will not enter the yield stage and are always within the line of elastic deformation.(3)The solidification/melting model of FLUENT software is used to numerically solve the melting process of the core material of single MPCM particles.The purpose is to investigate the ability of the MPCM wall material to resist the internal pressure.Under the conditions of an initial temperature of 20°C and a surrounding fluid temperature of 70°C,the time required for the urea-formaldehyde resin-paraffin microcapsules with a radius of 40.5?m and a wall thickness of 10.125?m to completely melt is 8.3 ms.0?2ms is the heating stage,no phase change occurs;all phase transformation process is concentrated in 2?8.3ms,at this stage,the liquid phase ratio increases from 0 to 100%linearly.After the core material is completely melted,the internal pressure of the wall material is the largest,170m Pa,and will not cause the microcapsules to rupture.(4)Through the MPCS pumping cycle experiment and numerical simulation,the nature and magnitude of the main external forces on the MPCM particles in the complex flow field were studied.In the experiment,it was observed that the microcapsules adhered to the wall surface of the pipe,accumulated in the middle and rear of the two sides of the impeller blade,generated foam,and delaminated.The particle size distribution test results before and after the pumping cycle show that:MPCS with volume concentrations of 5%,10%and 20%,respectively,after 2 h of pumping cycle,the particle size distribution is remarkably shifted to the direction of large particle size compared to that before the pumping cycle and the particle size distribution range is narrowed.The simulation results of the internal flow field of the experimental pump show that:(1)The magnitude of the total pressure and static pressure is between 10⁴Pa and 10⁵Pa,that of the wall shear force on the MPCM particles is 10²Pa,and 10^-3N of maximum collision force.These flow fields will not directly cause the MPCM particles to rupture in a single cycle;(2)In a circulating flow,the MPCM particles alternating through the areas where static pressure is positive and negative at least twice,at least twice the collision with the solid wall surface,at least once through the high shear stress area;as the pumping cycle time increases,these three"alternating"loads will reach thousands and tens of thousands of times.Their cumulative effect may be a very important reason to induce microcapsule damage.(3)Among the three main simulation variables examined,the influence of the centrifugal pump speed and the average inlet flow rate on the above three flow field forces is significantly greater than that of the MPCM mass concentration.