| The principle of phase change materials?PCMs?used in building envelope was relieve the contradiction of the energy consumed in buildings at the time and space level,which can store or release the latent heat energy in a passive manner.Incorporating PCM into the building envelope can provide a high heat capacity to achieve the maximum employment of natural resources though the building envelope and reduce the thermal amplitude and improve the indoor thermal environment.However,the organic PCMs exhibits low thermal conductivity and the application methods of this technique were unressonnable.These problems lead to latent heat were not perform timely and the PCMs effifiency rate was low.The unique thermal performance of PCMs are difficult to achieve effectively and atable and control the indoor thermal environment for a long time.Based on architectural thermal engineering,this study proposed an optimized PCM panel which appiled in lightweight wall.The theoretical analysis,experimental research and numerical simulation were promoted at micro-level of PCM and macro-level of PCM panel,respectively.This study aims at the method of thermal conductivity enhancement,the heat transfer enhancement for the internal structure in PCM panel and dynamic thermal characteristic of different composite wall structures,the main research work and results are as follows:?1?The carbon nanotubes was selected as the microscopic heat-conducting medium for industrial grade paraffin.The preparation method of the composite PCM combined physical dispersion with chemical dispersion.The best ratio of CNTs and hydrocarbon dispersant for the CNTs?TNHDIS?in the composite PCM was determined though visual observations and ultraviolet-visible?UV-Vis?spectrophotometer.The measurement of thermal conductivity of the composite PCM increased by 18.9%and 21.1%compared with pure paraffin at liquid state and solid state,respectively.The average latent heat value of the composite PCM is 122 kJ·kg-1,which could satisfy the requirements for building application completely.The infrared thermograph images of both samples determined the heat transfer rate of the composite PCM was more rapid and homogeneous than that of the paraffin,thereby reducing the adverse effect of natural convection during the heat storage process.?2?The numerical model of PCM panel was established through the finite element software COMSOL Multiphysics coupled with multiple physical fields.The necessity of natural convection of liquid state in the heat transfer simulation of PCM panel was discussed at heat storage process.The optimized geometric parameters for the fin in the PCM panel were determined by the complete melting time of PCM,the temperature difference between the heating surfaces and the melting uniformity of the PCM panel,which design used for reference of the fins in heat transfer equipment.The simulation results show that the optimized geometric parameters for the fin in the PCM panel were determined as:fin spacing=5mm,fin length=8mm,and fin thickness=0.2 mm.Thus,longer and thinner fins enhanced the heat transfer efficiency of the PCM panel.?3?The typical hottest summer day in Shenzhen for the natural ventilation and air conditioning room as the boundary conditions?the western wall?.Firstly,the appropriate values for the phase change temperature of the PCM panel was determined through the integral mean value temperature of aerated concrete wall under the different boundary conditions.The appropriate thickness of the PCM panel was determined through the latent heat utilization of the PCM and heat accumulation capacity of the wall's interior surface.Secondly,the dynamic thermal characteristic of different composite wall structures were compared and analyzed with non-optimized PCM panel and optimized PCM panel.The accumulated heat gain of the indoor,hourly heat transfer quantity for the composite wall and the accumulated value of heat transfer quantity were determined by the interior surface temperature and heat flux,which can evaluate thermal performance of the compsite walls.Then the optimized structure of the composite wall with PCM panels were obtained at different boundary conditions.Simulation results showed that the composite wall with optimized PCM panel has the best thermal performance in the natural ventilation room.The interior surface temperature of the wall with optimized PCM panel was lower 0.4?and the accumulated heat gain of the indoor for 24 h was reduced by7.8%than that of the non-optimized PCM panel under the same amount of PCM.In addition,the total heat storage quantity of the composite wall with optimized PCM panel in daylight and at night were higher 8.8 kJ·m-2 and 15.2 kJ·m-2 than that that of the non-optimized PCM panel.The optimized PCM panel had better heat storage performance,which could effectively achieve the balance between heat storage and heat release of building walls in summer. |
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