| Acting as a clean and abundant energy,solar energy plays an advantage in heat utilization fields,such as domestic and industrial heating.However,it has inherent defects such as unstable radiation intensity and intermittent supply at night,which restricts its further application,so it needs to be combined with a certain heat storage method.Microencapsulated phase change material(m PCM)slurry refers to dispersing m PCM particles into base fluids(such as water,ethanol,etc.)to form stable and uniform solid-liquid phase system.It has well specific heat capacity and fluidity.Different from traditional bulk phase change material,m PCM slurry can storage heat in the form of convective heat transfer.Meanwhile,owing to the poor lightabsorptive capacity of ordinary m PCM slurry,it can only be applied in the field of heat storage.In this paper,a light-absorptive graphene composite m PCM slurry was prepared,and its thermophysical properties were tested.Then,the heat collection experiments,numerical and experimental convective heat transfer work were carried out.The specific work is listed as follows:(1)The n-octadecane m PCM particle was prepared by in-situ polymerization,then its thermophysical properties,such as microscopic morphology and DSC curve were tested;The GNP composite m PCM slurry was prepared by two step method,and its thermophysical properties(such as density,viscosity and thermal conductivity)and optical properties were tested;The results show that the addition of graphene nanoparticles effectively improve the thermal conductivity.The binary particle system has higher optical absorption capacity than the single particle system.(2)The outdoor and indoor heat collection experiments were carried out respectively to study the influence of mass fraction of GNP and m PCM particles on the photo-thermal conversion performance.The results show that,under the effect of buoyancy,there exists obvious uneven temperature distribution in the solar collector,and the temperature of the upper fluid is always higher than that of the lower fluid;As for ordinary m PCM slurry,the maximum photo-thermal conversion efficiency is only 0.428,while after adding 0.05% graphene,it can be as high as 0.952;Although the photo-thermal conversion performance improves as the mass fraction of m PCM particles,the increase of latent heat refrains the rise of temperature,in this paper,when the mass fraction of m PCM particles is 10%,the heat storage rate can reach the highest.(3)The natural convective heat transfer characteristics of the m PCM slurry outside the helical coiled tube was simulated by CFD.The influence of particle concentrations and structures of the helical coiled tube on its unsteady state heat storage/transfer characteristics were studied.The influence of supercooling on its exergy efficiency in the heat release process was also discussed.Results show that the heat transfer coefficient of m PCM slurry is significantly higher than that of pure paraffin;Although the increase in particle concentrations reduces its heat transfer performance,the heat storage capacity of m PCM slurry with 30%particle concentrations increases by 54.4% compared with base fluid;The heat transfer and storage performances can be overall optimized when the pitch P is 0.15 m and the bending diameter D is 0.2 m.(4)The natural convective and forced convective heat transfer characteristics of the m PCM slurry outside the helical coiled tube was studied experimentally furtherly.The effects of different convection methods,stirring rates and inlet water flow rates on the actual heat storage/transfer characteristics were studied.The results show that the final completion of the m PCM slurry under the natural convective condition is only 43.70% during the time interval of0?3600s,while it is as high as 95.29% under the forced convective condition.The overall heat transfer coefficient K and external heat transfer coefficient ? are both sharply increased in the phase transition period and are improved with the stirring rates and inlet flow rates. |
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