Study On Heat Transfer Characteristics Of Energy Storage Brick Filled With PCM And Its Optimization Design

Phase change material(PCM)is a new kind of material for energy saving,which can be used to solve the problem of energy mismatch in time and space in terms of its high energy storage density,stable chemical properties,wide selection range of phase transition temperature and convenient control.If the PCM could be applied to the building,the building energy consumption can be effectively reduced.However,low thermal conductivity and potential leakage of liquid are existing problems of PCM in the field of building energy conservation,which results in the expected control purpose unachievable in the application.In this paper,according to the internal and external present conditions,the main work is as follows:The paraffin was selected as PCM,and the carbon nanotubes(CNTs)was used to modify the paraffin to improve its thermal conductivity.The expanded perlite(EP)and paraffin were combined to form a shape-stabilized PCM,with white latex film were encapsulated to alleviate the leakage problem of PCM.And combining with experiment and numerical simulation to research the heat transfer characteristics of hollow brick filled with shape-stabilized PCM.At the same time,the finite element method is used to establish the hollow brick filled with shape-stabilized PCM corresponding to the experiment,and the heat transfer model is simulated under the given temperature boundary condition.Based on the established model,the cavity structure of testing brick is optimized and simulated,and a cavity structure type that can further improve the phase change storage efficiency of testing brick filled with PCM,eventually come to the conclusions:(1)Differential scanning calorimetry(DSC),thermal conductivity test and heat release rate test of CNTs/paraffin composite with different mass fraction and paraffin were carried out.The results show,the thermal conductivity of CNTs/paraffin composite was increased,while the phase transition temperature decreased.Moreover,the greater the mass fraction of CNTs,the greater the thermal conductivity of the paraffin composite,the lower the phase transition temperature,but less influence on the latent heat.(2)The best mixing ratio of EP and paraffin is about 1:4 for forming shape-stabilized PCM.Encapsulated with white latex can effectively prevent the leakage of PCM from solid to liquid transformation.(3)The bottom surface of the testing brick that the cavity filled with the shape-stabilized PCM sample was heated with constant temperature,the PCM can absorb heat and store it in the form of latent heat to reduce the temperature rise rate on the surface of the brick during the heating process.If the filled PCM is the composite,the time needed to store the heat released in the phase change is shortened and the temperature rise rate of the surface of the brick is further reduced.(4)When the cavity structure is fractal geometry and kept the same cavity area,the phase change energy storage efficiency of the brick filled with PCM is generally higher than that of the single cavity structure.If the cavity structure is fractal geometry,the distribution differences of cavity will change the phase change heat transfer characteristics of bricks.With the increase of the specific surface area of the hollow brick and the decrease of the distance between the cavity and the thermal boundary,the phase change energy storage efficiency of the phase change brick increases.In this paper,the research on the modification of the thermal conductivity of paraffin was carried out from the preparation of CNTs/paraffin composite,the test of thermal physical parameters and the test of the rate of heat and heat release.Secondly,the EP/paraffin and CNTs/EP/paraffin composite materials were made with EP as the carrier,and encapsulation was carried out to achieve the shape modification of paraffin,so as to alleviate the leakage of liquid paraffin.Thirdly,the EP/paraffin and CNTs/EP/paraffin composite materials were applied to the cavity of testing brick,and the experimental and simulation studies were carried out.The temperature distribution and the phase transformation rule of PCM in testing brick were obtained,thus revealing the passive temperature regulation mechanism of PCM when applied to building brick.Finally,through the fractal design of cavity structure of testing brick to further improve the phase change energy storage efficiency of testing brick filled with shape-stabilized PCM.The relevant research achievements provide some theoretical support and significant references for preparation of high performance building materials,forecasting building energy saving and guiding the design of building energy saving.