The Study On Numerical Model Of Heat And Mass Transfer In Wood During Convection Drying

People have long looked forward to establishing drying models through the research of drying theory to obtain reliable and instant information in wood during drying,instead of the laborious and time-consuming experiments.But today,this is still a luxury,except a few cases.For most of the drying operation,it is only through test data to guide the production practice for the inexperience case.In order to achieve the above goals,this paper mainly studied the mathematical simulation of wood drying process from the establishment,solving and verification.Based on heat transfer,thermal model of moisture content detection and preheating mathematical model were developed.Based on heat and mass transfer,model including moisture field were developed.A numerical inverse analysis method for the effective parameters of wood was introduced.Based on above,a comprehensive mathematical model including fluid field was developed.At last,numerical solution and verification of all models were made.The aims are to improve and promote the application for mathematical model in the field of wood drying and to make it can be a practical and effective method to assist in wood drying feature detection.It provides the necessary information for the improvement of drying process and reliable implementation.Based on the thermal properties of wood,moisture content was determined indirectly by calculating the difference of heating rate in different moisture content conditions by means of an external hot line providing a stable heat source.The obtained results were verified combined with the uncertainty analysis,and satisfying results were obtained.Overall,1)measuring error ranged from-4.30?3.72%and standard uncertainty ranged from 1.29?6.08%with moisture content from 0.0%to 23.5%.This illustrated that hot line technology for measuring moisture content in lumber is feasible and the results were stable and with high reliability.2)The main factors that affect the test results can be considered as the differences of wood properties,the distribution of temperature and moisture content and detection precision.The 2D mathematical model based on the heat transfer law was developed to simulate the temperature profile in larch boxed-heart square timber during preheating process.The numerical and experimental results were compared and the reasonable preheating time was obtained.The influence of moisture content on preheating time and temperature distribution was discussed.The results showed:1)the simulated values of preheating time were close to measured values with maximum error within 9.5%,which illustrated that the model for forecasting preheating time was feasible.2)Moisture content had no appreciable effect on preheating time,but the profile of moisture content had appreciable effect on precision above the fiber saturation point.3)Only taking uniform heating into consideration,the recommended preheating time can be set to 7h for the squared lumber with cross section of 120mm×120mm.In practice,in order to improve the simulation accuracy of the model,the fluctuations of moisture content must also be considered.Based on the mechanism of heat and mass transfer,a one-dimensional mathematical model that accounts for the moving evaporation interface was developed to simulate coupled heat and mass transfer during the convective drying of wood.Mass and energy controlling equation and corresponding boundary conditions covering the entire range of moisture content were given.The proposed model was validated against experimental data for the average moisture content and central temperature of wood,revealing good agreement between experiment and simulation.The simulation results for the density of vapor and the volume rate of evaporation indicated that moisture migration was mainly in the form of gas under lower moisture content conditions.The evaporation interface moved at approximately constant speed and the evaporation rate of the interface decreased with time.Although the simulation results for the density of vapor and the volume(interface)rate of evaporation were not verified,reasonable simulation results can assist in analyzing the mechanism of wood drying and optimizing the process.The necessary parameters in solution of model were introduced.The inverse determination of thermal conductivity in wood based on finite difference and genetic algorithm was emphatically introduced,respectively.And,the calculation results were verified experimentally.The two factors regression equation of moisture content and temperature was obtained,satisfactorily.The 3D mathematical model of heat and mass was developed,which considers environmental drying medium together with wood entities.The details of the control equations and boundary conditions was given and solved by multi-physics coupling finite element software(Comsol Multiphysics).The results showed that ambient fluid property around wood was not uniform during drying especially at the initial period of drying,which illustrated that drying model must take into consideration of the environmental flow field properties to describe wood drying process and improve the accuracy of model.The comparison between simulation and the measured values showed that simulation accuracy for core of temperature and average of moisture was acceptable.The profile of moisture content and temperature in wood should be validated and optimized.The model parameters,i.e.,temperature,humidity and velocity should be adjusted to validate the stability of the models and the models meet with the actual production should be established.