| The rapid development of modern economy has accelerated the pace of technological civilization,but also led to many environmental pollution problems.Phase change material(PCM)is a new type of energy storage material with high energy density and low temperature change,but at the same time there are some problems,such as poor thermal conductivity,easy leakage of phase change.Researchers have found that the high thermal conductivity of carbon-containing materials can be used to improve the thermal conductivity of composite PCM,and carbon nanotubes(CNTs)are widely used in heat storage of PCMs because of their high thermal conductivity,strong electrical conductivity and high chemical stability.In this paper,three different particle sizes of 8-12 nm,10-20 nm and 20-40 nm of industrial grade CNTs powders with four CNTs contents of 1wt%,3wt%,5wt%and 7wt%were incorporated into paraffin to make twelve samples.The samples were characterized thermally using DSC,SEM,and Hot Disk thermal property parameter testing instruments;a charge and discharge performance test bench was set up to test the charge and discharge temperature of different kinds of samples,so as to screen out the most suitable CNTs composite PCM under different application scenarios and needs;finally,COMSOL numerical simulation software was then used to establish a model of the thermal storage box,the empirical formula of thermal conductivity with the best applicability was selected and substituted to calculate the thermal conductivity to analyze the mechanism of its internal temperature field,velocity field and density field.The main findings are summarized as follows:(1)In terms of thermal properties,hydrocarbon dispersants in CNTs can disperse spherical condensates,thus increasing heat transfer paths and improving heat transfer coefficients;with the increase of the diameter and content of CNTS,the phase transition temperature and latent heat decrease;the thermal conductivity of the composites increases with the increase of the mass fraction of CNTs and the decrease of the diameter.With the increase of the mass fraction of CNTs,the specific heat capacity decreases,and the average specific heat capacity of the three CNTs diameter decrease significantly from0.35MJ/m ~3?Kto 0.27MJ/m ~3?Kwhen the CNTs filler increases from 3wt%to 7wt%;the thermal diffusivity showed an increasing trend,from about 2.36mm~2?s-1to about3.44mm~2?s-1.(2)In terms of the charge and discharge of the composite PCM,the temperature variation law with time was analyzed,and the results of the study showed that the composite PCM(CNT-A-7)with CNTs mass fraction of 7wt%and diameter of 8-12nm has the best charge effect in the heat storage box in the horizontal direction at the position away from the heat source,composite PCM(CNT-B-7)with a CNTs mass fraction of 7 wt%and a diameter of 10-20 nm has the best charge effect in the thermal storage box in the vertical direction;the composite PCM(CNT-A-7)with a CNTs mass fraction of 7 wt%and a diameter of 8-12 nm has the best charge effect in the horizontal and vertical directions in the thermal storage chamber at a location close to the heat source;the composite PCM(CNT-A-7)with a CNTs mass fraction of 7 wt%and a diameter of 8-12 nm showed the best discharge effect in the thermal accumulator.(3)Based on the analysis of the thermal conductivity mechanism of CNTs,three empirical formulas for thermal conductivity of CNTs composites were selected to calculate the thermal conductivity for simulations,which were fitted with the experimental data of heat storage of PCMs.The XUE equation is closer to the charge experimental curve,especially in the phase change interval phase,so its simulation results for calculating the thermal conductivity are used to analyze the simulated physical field changes.(4)The thermal storage device model was established,and the thermal storage simulation can be derived by the composite PCM with the best thermal conductivity.Temperature field:high-temperature liquid composite PCM concentrated in the top movement,the bottom mixing area appears vortex,composite materials can accelerate the reduction of the lower mixing area by increasing the thermal conductivity to increase the temperature uniformity;velocity field:both sides of the wall to the upper region of the liquid zone and the bottom of the mixing zone vortex,the overall flow velocity changes in the mixing zone compared to the liquid zone is sensitive to the natural convection effect,the central velocity of the liquid zone due to the close proximity of the heat source than the central velocity of the mixed flow,the composite material can increase the velocity uniformity by reducing the time of the existence of vortices in the mixing zone;density field:there is a difference in the density of solid and liquid in the mixing zone,resulting in uneven temperature distribution in the mixing zone,a small part of the heat will be consumed in the phase change in the mixing zone,increasing the melting time,composite materials can be increased by making the bottom liquid PCM area expansion rate,suppressing natural convection and increasing the density uniformity. |
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