Study On Temperature Control Mechanism Of Cementitious Composites Integrated With Spherical Phase Change Particles

With the rapid development of global economy,the increasing consumption of fossil energy has seriously increased the global carbon emissions,resulting in a serious threat to the ecological environment.How to effectively control carbon dioxide emissions and slow down the global warming process have received extensive attention of the most countries.One of effective ways to achieve low carbon goals is to explore efficient storage and conversion technology of sustainable renewable energy to reduce energy consumption.As a typical heat storage technology,phase change materials(PCM)can absorb or release large amounts of heat storage in the process of phase change,during which their own temperature basically keep constant.Therefore,it is promissing in the field of green buildings to integrate phase change materials with building materials,i.e.cement,to construct composited phase change materials(CPCMs)so that the interior temperature of buildings can be controlled passively,resulting in an effectively reduction of building energy consumption in the process of refrigeration and heating.However,PCMs have such disadvantages as liquid phase leakage and the erosion of building materials in the process of phase change.More seriously,the low thermal conductivity of PCMs leads to a time-consuming phase change process,which seriously affects the efficiency of heat storage and release,and is not beneficial to the efficient temperature control inside buildings.To do this,this thesis aims to study the performance of cement-based phase change composite,which involves interdisciplinary activities of heat transfer,mechanics,computational mathematics,materials science and so on.The study begins from simple phase change particle encapsulation to complex phase change based composite materials,to finally establish the systematic method of design,fabrication and characterization to guide the application of cement-based phase change composite materials to serve the development of low-carbon buildings.The main works of the thesis include:1.Establishing a heat transfer model of spherical phase change particle(SPCP)involving natural convection.As the basic units,the SPCP’s properties determine the thermal performance of SPCP-based composites.Thus,the melting experiment of spherical paraffin particles was first conducted to observe the melting process of paraffin wax as well as the shape evolution of phase change interface,and further varidate the corresponding computational model.Then,based on this numerical model,the mechanism of natural convection of liquid phase in accelerating the phase change process was discussed in details,and the influences of spherical size and wall heating temperature on the natural convection are analyzed.Finally,based on the natural convection effect,an equavilent thermal conductivity formula in terms of variables of particle radius and heating temperature are proposed by converting the natural convection effect in the spherical particle into the thermal conductivity of paraffin wax,resulting in an extremely simplified phase change simulation including heat conduction only.This formulation will be used for the efficient analysis of composites containing a large amount of SPCPs.2.Determining the effective thermal conductivity of CPCMs.The thermal conductivity represents the heat conduction capacity of material,so CPCM’s thermal conductivity is firstly investigated.First,the SPCP-based CPCMs with different particle volume contents are fabricated and their thermal conductivity is measured by using the transient heat source method.Simultaneously,a microstructure-guided computational model is established by considering the random distributions of SPCPs and the geometric periodicity requirement and its effectiveness is validated by comparing to the experiment.Subsequently,a generalized self-consistent theoretical formula on the effective thermal conductivity of three-phase composites is derived and the influence of microstructure parameters on the effective thermal conductivity of composites are indicated numerically and theoretically to understand the variations of the thermal conductivity of composites.3.Investigating the energy storage performance of the CPCMs.Because the thermal conductivity of composite can’t fully determine its energy storage capacity,an investigation of energy storage capacity of composite is necessary.First,a test platform for heat storage performance of cement-based phase change composite is built to apply constant-temperature heating on the specific surface of the specimen and the temperature distribution of the composite is measured.Secondly,a dynamic heat transfer model of the composite materials is developed based on the formulated equavilent thermal conductivity of phase change materials to obtain the heat transfer routine and temperature distribution inside the specimen.The effectiveness of the computational model is verified by comparing with the experimental results.Then,the influence of microstructure parameters such as phase change particle content,on the heat transfer characteristics,heat storage capacity and temperature control effect of the CPCMs is investigated.Especially the temperatures change on the opposite surface of the block to the heating surface is analyzed to assess the temperatureadjustable capacity of the composite.4.Revealing the heat transfer performance of CPCM under radiation heating condition.The radiation heating is more general than the constant temperature heating in the practice,so the thermal response of CPCM under radiation heating condition is investigated here.To do this,a test platform of CPCM board is prepared to meet the requirement of condition of radiation heating by setting a heat lamp over the specimen.The temperature and heat flux data are collected by using the heat flux sensors.Simultaneously,the corresponding calculation model of composite board is established and verified through the compassion beteeen the numerical and experimental results.Then,the heat transfer characteristics and energy storage mechanism of the composite board under radiation heating condition is studied,and the temperature control capability of the composite board is depicted by analyzing the variation of temperature on the opposite surface.