Investigation On Rheological Property Of Aqueous Suspension Of Microencapsulated Phase Change Materials With Controllable Size

The use of phase change material microcapsules is one of the most effective methods for storing thermal energy.Due to their high storage capacity in the phase change process,the same medium can be used to transmit or store energy,which can reduce the heat transfer loss in the heat transfer process and reduce the pumping power.However,the paraffin phase change microcapsules prepared by traditional methods have the problems of uneven particle size,irregular shape and appearance,and poor dispersion and suspension stability,which is not conducive to the use of it in related research on rheological properties.In this thesis,phase change material microcapsules with n-heptadecane as the core material and HDDA as the shell layer are used as the research object.The microfluidic method is used to prepare paraffin phase change microcapsules with uniform particle size,regular appearance and extremely high monodispersity.For the capsule,by controlling the density of the capsule to be consistent with the density of the base liquid,a phase-change microcapsule suspension with stable dispersion is obtained.To study the effect of the particle size and concentration of the phase change microcapsule suspension on its viscosity under different dispersion systems under the same test conditions,Numerical study of the value of the constant A in the Vand viscosity formula through experiments,and the change of the viscosity of the phase change microcapsule suspension under different coating ratios and temperatures were studied.The specific work is as follows:(1)Using dual chips,prepare n-heptadecane/HDDA phase change microcapsules through microfluidic technology.The density of the capsules is controlled to be equal to the density of the base liquid,and a phase-change microcapsule suspension with stable dispersion is obtained.Double glass chips are prepared,and phase change microcapsules with uniform distribution are prepared by the method of coaxial flow microfluidic chips.Characterize the morphology,structure,thermal stability and thermal properties of phase change microcapsules by SEM,FTIR,TG and DSC,and prepare n-heptadecane with controllable particle size,regular morphology,stable structure and high monodispersity.The injection pump is used to precisely control the flow rates of the dispersed phase,the intermediate phase and the continuous phase to control the shell-to-core ratio of the capsule,making the density of the phase change material microcapsules controllable.Seven phase change paraffin wax capsules with different particle sizes but the same density are prepared by this method,and a phase change microencapsule suspension with the same density of the paraffin phase change capsule and the base liquid is prepared.Seven partial phase-change microencapsule suspensions with different particle sizes were taken as samples,and their dispersion stability was observed at regular intervals.The suspension still had good dispersion stability after one month.(2)To study the effect of particle size and concentration on the rheological properties of paraffin phase change microencapsule suspension under monodisperse system.In this study,the fixture plate in the Malvern rheometer was used,and the gap value used in the test was 1.2mm.The seven different particle diameters obtained by the test configuration,and the viscosity of four different mass fractions of paraffin phase change microcapsule suspensions under each group of particle diameters.The experimental results show that when the shear rate of the rheometer is between 100s^-1and 200s^-1,the mass fraction of the PCM capsule suspension in this interval ranges from 5%to 20%,which all behave as Newtonian fluid characteristics.When the concentration of paraffin phase change microcapsules is constant,the larger the particle size,the greater the viscosity;when the particle diameter of paraffin phase change microcapsules is constant,the greater the concentration,the greater the viscosity.At the same time,the value of the constant A is obtained by fitting the Vand viscosity formula through experimental numerical values.(3)Study the effect of particle size and concentration on the rheological properties of paraffin phase change microcapsule suspension in polydisperse system.Two polydisperse paraffin phase change microcapsule systems with different dispersion coefficients under two mass fractions were established.By comparing the viscosity of different dispersion systems when the average particle size is the same,the influence of the polydispersity of the phase change microcapsule particle size on the viscosity of the capsule suspension is determined.At the same time,the change of the constant A in the Vand viscosity formula is studied.The experimental results show that compared with the monodisperse system,in the polydisperse system,with the increase of the polydispersity coefficient PDI,the viscosity of the capsule in the polydisperse system increases.(4)Study the effect of temperature and the coating rate of the capsule on the rheological properties of the paraffin phase change microcapsule suspension.Five phase change microcapsules with the same outer diameter and different inner diameters were prepared,and the influence of the density of the phase change microcapsules on the viscosity of the phase change microcapsule suspension was studied;And studied the viscosity change of the capsule under the condition of changing temperature.The experimental results show that the viscosity of the microcapsule suspension increases with the increase of the density;the viscosity of the capsule suspension decreases with the increase of temperature.