Study On Estimation Of Mass Transfer Coefficient Of Porous Material With Steady And Decoupling Experiment

Mass transfer coefficient is an important physical parameters in heat and mass transfer process of porous material.Measuring and calculating accurately mass transfer coefficient of porous material plays a significant role in the development and design,performance improvement,and optimization of new equipment.Also,it plays a significant role in improvement of processing quality and theoretical calculation precision of porous material.However,most of the methods used to measure and calculate water diffusion coefficient are based on transient method.There is a truncation error in the theoretical solution of Fick's second law,and the experimental parameters of the measurement are transient.The steady state cup method is used to measure and calculate the mass transfer coefficient by steady state method.This method only takes into account the relative humidity of the air on both sides of the sample and neglects the influence of the change of water content on the mass transfer coefficient in the sample.For the convective mass transfer coefficient,the parameters of theoretical model solving method cannot be measured,and the applicable condition of heat and mass analogy solution is narrow,and average dry base water content is used in the drying curve solution method.The literature on the measurement and calculation of mass transfer coefficient of porous materials at home and abroad is reviewed,aiming at the defects of transient method and steady state cup method,based on the Fick's first law,a new method for measuring and calculating the mass transfer coefficient of porous materials is proposed.The method has the following advantage:the theoretical foundation is reliable,and the moisture diffusing flux and moisture content of samples are stable during the experimental measurement.An experimental platform was built to measure and calculate the mass transfer coefficient of porous material by steady state-decoupling method.With clay and gypsum as the experimental object,the mass transfer coefficient of clay was researched in the thickness of 4cm,5cm,6cm,and natural convection and 5m/s of forced convection wind velocity.The mass transfer coefficients of gypsum at the thickness of 4cm,5cm,6cm and natural convection and forced convection speed of 5m/s,7.5 m/s and 10 m/s were calculated.It is found that the coefficient of moisture diffusivity of clay is between 4.27×10^-8m²/s and 4.49×10^-8m²/s,and the coefficient of convection mass transfer is between0.0031m/s and 0.0125m/s.Gypsum water diffusivity is between 8.25×10^-9m²/s and 8.56×10^-9m²/s,and the convective mass transfer coefficient is between 0.0087m/s and0.0427m/s.Secondly,combined with the possible errors of experiment,the factors that influence the mass transfer coefficient of clay and gypsum in steady state are analyzed.The results show that the thickness of the sample has no effect on the water diffusion coefficient and the convective mass transfer coefficient,and the convective mass transfer coefficient of the wind velocity affects the mass transfer coefficient,and the convective mass transfer coefficient will increase with the increase of the wind speed,but the wind speed has no effect on the water diffusion coefficient.