Study On Heat And Moisture Transfer Process Of Diatomite-based Humidity Control Material And Its Application In Buildings

Indoor humidity environment directly affects the comfort of the occupants, the storage of goods, the durability of the building envelope and energy consumption. It involves many fields, such as medicine, chemical engineering, food, historic preservation, instrument maintenance, et al. Humidity Control Material (HCM) is a kind of porous building material which can regulate and control indoor air relative humidity (RH) passively. There are a variety of forms of HCM, like plates, coating, decoration, et al. Considering the geometric complexity of HCM and the nonlinear characteristic of humidity control process, the mechanism of heat and moisture migration in HCM, especially the effect of pore structure on heat and moisture migration in HCM are not fully revealed. Meanwhile, humidity control effects when HCM is applied to the inner side of a wall in buildings still lack deep and sufficient research. Therefore, researches on humidity control performance optimization of Diatomite Based Humidity Control Material (DBHCM), theoretical analysis/numerical simulation/experimental verification of humidity control mechanism, as well as analogue simulation when DBHCM was applied to the inner side of a wall in buildings were all carried out in the paper and obtained the following conclusions:(1) Scanning Electron Microscope (SEM) and Energy Dispersive x-ray Spectroscopy (EDS) were adopted to characterize the microscope pore morphology and elements distribution analysis of diatomite before and after calcinations, respectively. The calcination of diatomite can effectively improve its pore volume and the formation of tiny pores, thereby the humidity control performance of diatomite also improved. Meanwhile, an experimental platform was established to measure the humidity control performance of diatomite treated with different calcination times and calcination temperatures. The results show that the macro humidity control performance difference of diatomite comes from its micro structure difference and the variation of its elementary composition. No matter the calcination temperature is above 800℃ or the calcination time is above 8 h will result in the destruction of pore structure of diatomite, which will also worsen the humidity control performance of diatomite. Better humidity control performance of diatomite demands more tiny and dense pores and thin fractures. Appropriate calcination can remove the impurities of diatomite which will enhance the interaction between diatomite and water vapor in air. When the calcination temperature (500℃) is constant, the humidity control performance of diatomite with 3 h calcination is the best, thereafter, the humidity control performance of diatomite with 5h/8h calcination decreases. When the calcination time (3 h) is changeless, the humidity control performance of diatomite calcinated at over high temperature (800℃) or over low temperature (200℃) cannot be optimized, there is an optimum calcination temperature 500℃.(2) A kind of intellective DBHCM blocks were prepared. When preparing DBHCM, several raw materials were mixed at a certain ratio. Natural mineral diatomite was used as the main moisture modulating ingredient. It was important that the calcinations of diatomite should be performed at certain temperature for a specific time to increase the porosity and pores connectivity of the DBHCM. Cement was applied for structural support. Gypsum was served as cement retarder to control the pore formation rate of DBHCM. Pulverized fuel ash was used as a supplement to reduce the cement consumption which could provide a way to utilize the industrial waste and lower the cost of DBHCM. Poplar cellulosine was used as an assistant humidity control material. Mildew preventive and antimicrobial substances were added to DBHCM for preservation. The surface pore structure and the pore size distribution of DBHCM were visually characterized by micrographs and statistical analysis methods, respectively. The contact angle of DBHCM was measured using an optical contact angle measuring device to evaluate the surface free energy which laid the experimental foundation for analyzing the impact of surface pore structure, pore size distribution and surface free energy on humidity control performance of DBHCM.(3) A 1-D couple heat and moisture migration mathematical model for DBHCM under convective boundary condition was established. The model took into consideration the water vapor diffusion in pores of DBHCM under capillary effect, the liquid water/water vapor diffusion and their phase change in pores of DBHCM under water vapor pressure. The coupled heat and moisture migration process at different levels of porosity, ambient temperature and ambient relative humidity were simulated. Meanwhile, the adsorption/ desorption experiments were carried out learning from some adsorption/desorption criterions of relevant materials at home and abroad. The results show that the humidity control performance has pore structure sensitivity, the adsorption/desorption curves vary nonlinearly with the variation of porosity. With the decrease of porosity, the amounts of adsorption and desorption increase. Ambient temperature does not significantly influence the coupled heat and moisture migration process, the effect of ambient temperature (the maximum deviation is 20℃) on the adsorption/desorption of DBHCM is about 10% and on the temperature distribution of DBHCM themselves is less than 1%. Better humidity control performance demands smaller pore diameter and larger quantity of small pores.(4) Due to complex porous structure of DBHCM, isotropic, anisotropic porous media with the same porosity, different pore size distributions and fractal dimensions were reconstructed by random growth method. Suitability of box counting dimension and fractal spectrum dimension studying mass diffusion in pores of porous media was analyzed. Pore structure and pore size distribution of porous media were characterized by micrographs and statistical analysis methods, it was aim to theoretically study the impact of microscopic pore structure on water vapor diffusion process in porous media. The simulation results show that the water vapor diffusion property in pores of porous media largely depends on the pores connectivity, better pores connectivity leads to better water vapor diffusion property in pores of porous media. Pore size distribution and box counting dimension have some limitations when reflecting moisture dynamic transport properties in pores of porous media. However, fractal spectral dimension can effectively analyze and reflect pores connectivity and moisture dynamic transport properties of porous media from the microscopic perspective. The pores connectivity and water vapor diffusion performance in pores of porous media get better with the increase of fractal spectral dimension of porous media. Fractal spectral dimension of parallel anisotropic porous media-2 (water vapor diffusion direction is parallel to the direction of larger growth rate of anisotropic porous media) is more than that of perpendicular anisotropic porous media-1 (water vapor diffusion direction is perpendicular to the direction of larger growth rate of anisotropic porous media). Fractal spectral dimension of isotropic porous media is between parallel anisotropic porous media and perpendicular anisotropic porous media. Other macroscopic parameters such as equilibrium diffusion coefficient of water vapor, water vapor concentration variation at right boundary when equilibrium, the time when water vapor diffusion process reaches stable also can characterize the pores connectivity and water vapor diffusion properties in pores of porous media.(5) Indoor air temperature, RH and moisture content in a enclosed space with/without DBHCM and enclosed/natural ventilation space with DBHCM were simulated by using the effective moisture penetration depth model under typical climate in winter in Nanjing and indoor periodic moisture load boundary conditions. The results show that DBHCM can effectively weaken the influence of outdoor climate fluctuation and indoor periodic moisture load change on indoor air temperature and RH. DBHCM also can control indoor air humidity parameters in a narrow range which can meet people’s comfort requirement. The thickness of DBHCM should be more than or equal to the optimum thickness (0.02 m). When the thickness of DBHCM is 0.02 m, natural ventilation space has more ways to gain heat than enclosed space, thus the temperature in natural ventilation space is higher than that in enclosed space. The RH and moisture content in natural ventilation space are closer to the RH and moisture content of outdoors than that in enclosed space. Indoor air RH can be controlled in about 45% and 50% in the enclosed space and natural ventilation space with DBHCM, respectively, which can meet people’s comfort requirement.In summary, the above researches systematically obtained the humidity control mechanism of DBHCM from aspects of preparation of DBHCM, pore structure characterization of DBHCM, adsorption/desorption properties measurement, establishment of heat and moisture migration model of DBHCM, analogue simulation when DBHCM was applied to the inner side of a wall in buildings, et al. The relevant research achievements provide some theoretical support and significant references for preparation of high-performance HCM, assessing performance of building material, predicting energy-saving level of architecture and guiding the architectural design, which enrich the current research methods of HCM.