Effect Of Ultrasound On Heat And Moisture Transfer Process In Fibrous Porous Media

The coupled heat and moisture transfer process in fibrous porous media is widely used in textile,military,medicine,biological,architectural and other field.At present,the study on the enhancement of heat and moisture transfer process in fibrous porous media focus on mechanical rotation and thermal drying.However,these enhancement methods have the disadvantages of low moisture removal efficiency,high damage to material and high energy consumption.Therefore,it is of great theoretical significance and practical value to study the heat and moisture transfer mechanism of convective drying process of fibrous porous media and search for more effective enhancement methods.In recent decades,ultrasound technology has been verified to be able to enhance the heat and mass transfer process in porous media by experiments and it shows important application prospect in food dehydration,sludge treatment and desiccant regeneration.However,the enhancement of ultrasound on heat and mass transfer process in fibrous porous media is rarely referred.Thus,this paper attempts to investigate the enhancement to the heat and mass transfer in fibrous porous media by ultrasound,the effect of ultrasound propagation on microstructure and heat and mass transfer process will be studied systematically.The main contents and conclusions are as follows:?1?The experimental and simulative studies on convective drying process of fibrous porous media are carried out:the drying curves under different drying media and physical parameters are measured by experiments;the velocity,temperature and humidity distribution in fibrous porous media during different drying periods are numerical simulated.This is convenient for comparative analysis for fibrous porous media dehydration process with and without ultrasound assisted.The results show that:The fibrous porous media drying process take a turn from constant rate period into falling rate period when the dry basis moisture content decreases to 0.5;the boundary layer theory gives an accurate description of the constant rate period which is controlled by external resistance,thus the moisture transfer rate can be significantly improved by changing the parameters of drying medium;in falling rate period,the dehydration process is determined by moisture diffusion process in porous media,the variation trend of moisture distribution is explained by the distribution of velocity and temperature at different height and time.?2?The basic physical quantities and mechanism of ultrasound are clarified and the propagation of ultrasound in fibrous porous media is studied.Based on the Helmholtz equation,the sound pressure distribution generated by ultrasound propagation in porous media is solved.Then,the propagation model of ultrasound in fibrous porous media is established and the effect of frequency and sound intensity on pressure and deformation distribution is investigated.The distribution trend is mainly affected by frequency and the magnitude is determined by sound intensity.Furthermore,the greater the sound intensity,the sound pressure is greater,and the deformation is more obvious.Besides,the porous media deformation due to ultrasound propagation is much smaller than its own size.?3?An experimental system for fibrous porous media convection drying with ultrasound assisted is designed and established,and then the drying curves under different ultrasound frequency and power are obtained,the effects of different drying media and physical parameters on ultrasound effectiveness are investigated.Besides,a mathematical model of drying process with ultrasound assisted for coupled heat and moisture transfer process in fibrous porous media is also established and numerical simulated.The results show that:in this drying process with ultrasound assisted,part of moisture is directly removed in the form of liquid,thus the whole process is falling rate dehydration;both absorption coefficient and attenuation increase with frequency so the enhancement of ultrasound on moisture transfer shows no obvious variation under different frequency;while the effectiveness of ultrasound enhancement is nonlinear enhanced with the increasing of ultrasound power;The more unfavorable the drying condition is,such as lower drying medium velocity and temperature,higher relative humidity,material thickness and initial moisture content,the effectiveness of ultrasound enhancement is more obvious;the response surface approach is adopted optimize the operating conditions;at the same time,along the direction of moisture transmission,the temperature,the liquid phase transfer rate and the rate of moisture content change increases,the moisture content decreases;at the same height,with the drying process proceeding,the temperature increases,the liquid phase transfer rate,the rate of moisture content change and the moisture content decreases gradually.?4?The microscopic pore structure of fibrous porous media after ultrasound pretreatment is observed,and then the microscopic images are analyzed statistically and described fractally,subsequently,the effects of pore structure changes on mass transfer characteristics are studied.The conclusions are concluded:with the increase of ultrasound power and pretreatment time,the average pore size,the maximum distribution size,the porosity and the surface fractal dimension of the fibrous porous media are nonlinearly increased;when the ultrasound power is 630W and the pretreatment time is 40min,the change of microstructure is nearly the most obvious;the moisture transfer rate is enhanced due to the change of microstructure,the dehydration time required to reduce the dry basis moisture content from 3 to 0.5 decreases from 690min to 504min after power pretreatment of 630W for 30min,and the effective moisture diffusion coefficient increases from 7.2265×10^-9m²/min to 9.04×10^-9m²/min.In summary,the current work systematically investigates the mechanism of convection drying with ultrasound assisted in fibrous porous media and the ultrasound propagation in porous media.The effect of ultrasound on microstructure and drying process of fibrous porous media is also explored.The corresponding research results provide a theoretical and technical support for the development of heat and moisture transfer enhancement in fibrous porous media with ultrasound assisted.