The Research On Theheat-Transfer Mechanism And Thermal Insulation Properties Optimization Of The Cork

Cork is a natural thermal insulation material in the nature.The dissertation took the domestic cork (Quercus variabilis) as the research object, contrasted the Portuguese cork (Quercus suber), and studied the heat-transfer mechanism and the pyrolysis characteristics of the solid skeleton of the cork cells, and the heat-transfer characteristics changes of the solid skeleton of the cork cells after the heat treatment. Finally, the optimization research was implemented for the heat preservation and heat insulation performance of the cork products.The main research results are as follows:(1) Summarized and simplified the monomer structure of the cork cells, constructed the heat-transfer model of cork monomer cell, and achieved the equivalent coefficient formula of thermal conductivity for the solid-gas structural cork through using the series and parallel operational rule of the thermal resistance. The equivalent coefficient of thermal conductivity of the cork, which is relevant to the solid coefficient of thermal conductivity (λs) and the gas coefficient of the thermal conductivity (λa), is a function about the porosity (φ). The equivalent coefficient formula of thermal conductivity of the cork is:The the coefficient of thermal conductivity of the cork products can be predicted by the equivalent coefficient equationof thermal conductivity.(2) In the pyrolysis reaction process of the cork, the dissertation divided the pyrolysis processinto four phases. The first phase has an obvious endothermic peak, and the endothermic peak intensity of the domestic cork is higher than the Portuguese cork. The third phase is the main pyrolysisphase of the cork, and the weight loss rate of the phase reached up to77.14～81.59%. The entire pyrolysis process is the superposition of the pyrolysis process ofvarious composition such as the cork ester, the lignin and cellulose etc. The cork ester, the lignin, etc. have good thermal stability, so the temperature range of pyrolysis reaction is extended and the pyrolysis process of the cork is delayed.(3) Through calculating the activation energy, it is found that the corks from three production places have the larger activation energy, and the average activation energy of these corks respectively achieved172KJ·mol-1(Gansu),169KJ·mol-1(Shanxi) and164 KJ·mol-1(Portugal). Further, it indicated that the cork burning is hard to carry out, because it needed the higher temperatures and a lot of energy for the cork pyrolysis. According to the thermal decomposition rate equation, the corresponding pyrolysis kinetic equation of the cork was derived. The pyrolysis reaction mechanism of the cork can be better described by a first order reaction process.(4) Based on the research for the cork solid skeleton, it is found that theadsorption isotherm of the cork (Quercus variabilis) and the cork (Quercus suber) were the type III adsorption isotherm, the interaction between the adsorbent and the adsorbate is very weak, and there aren’t the absorbed water on the cell wall. On the cork cell walls (Quercus variabilis), there are more porosity and larger specific surface area, and the pore size is distributed mainly in the range of20nm. In the pores of Shaanxi cork, the mesoporous, which the pore diameter is under50nm, reached up to90.77%. The pore diameter of the mesoporous is less than the free path of gas molecules, so there is not the heat conduction of the gas inside the aperture of the cell wall, and the equivalent coefficient of thermal conductivity for Shanxi cork is very low.(5) After heat treatment, the chemical composition of cork cell wall changed:the asymmetric and symmetric stretching vibration of the C-H bond in the waxiness between2852cm-1-2920cm-1is abate. Thecharacteristic peak of carbonyl [)C=0) in the esters appeared between1700cm-1-1800cm-1. The vibration of the aromatic ring skeleton in lignin appeared at1506cm-1. The bending vibration of the CH2bond in hemicellulose appeared at1456cm-1. The characteristic absorption peak of β-D-glucose appeared at895cm-1, and the peak strength has weakened in the heat treatment.(6) After heat treatment, the specific surface area of Gansu cork cell walls showed an increase trend from2.438m2/g increases to2.907m2/g and decreased16.13%. The single point surface area and the total volume of BJH absorption pore showed a decrease trend. The pore diameter distribution at the cell walls concentrates to the mesoporous under20nm, and the distribution intensity of the mesoporous under20nm is higher, and the number of big pore more than50nm significantly reduced.(7) After heat treatment, the visual features appeared obvious changes:The brightness reduced, the chromatic aberration increased, and both showed an obvious increasing trend with the rise of temperature and the extension of time. After heat treatment, the chromaticity index a*of cork red-green axis and the chromaticity index b*of cork yellow-blue axis showed the regularity decrease trend at different degree. The influence of heat-treatment temperature on the brightness and chromatic aberration for the cork is greater than the influence of the heat treatment time.(8) The optimization research on thermal insulation properties of the cork products: Under the condition of the equal density, the cork products produced by the coarse particles have the minimum equivalent coefficient of thermal conductivity, and the best insulation performance. When cork particle size is smaller, the influence of the changes of the particle size on thermal insulation properties of the cork products is smaller. Under the condition of the same cork particle, the equivalent coefficient of thermal conductivity of cork products appeared an obvious increase trend with the increase of volume-weight. The heat treatment time and the volume-weight of cork products can obviously influence the coefficient of the thermal conductivity. According to the result of orthogonal experiment, and the dissertation found out the optimized solutions for:the cork particle size is the coarse particle, the volume-weight of the cork products is0.5g/cm3, and the heat treatment time is2hours. So, reducing the volume-weight of the cork products, and increasing the heat treatment time can decrease the equivalent coefficient of thermal conductivity, and efficiently improve the thermal insulating properties of cork products.