Characterization And Simulation For Heat Transfer,Moisture And Air Permeating Behaviors Of Fiber Assemblies With Density Variation In-situ

The intrinsic reason affecting heat transfer,moisture diffusing and air permeating properties is the structure of the fiber assembly."Density" is thought as the main characteristic and the nature influence source of fiber assembly structure.However,quantitative investigation on density influence on heat transfer,moisture diffusing and air permeating behaviors of fiber assemblies is still the researching difficulty in the actual measurement and theory study,because there is no method to achieve the measurement in-situ.In addition to that,study on heat transfer,moisture diffusing and air permeating behaviors of fiber assemblies is usually carried out separately and very few people discuss the combined measurements of the three properties together.The measuring system" Fiber Assembly Complex Behavior Evaluation System(FACBES)" is developed in this paper to solve the combined measurement of fiber assemblies in-situ with density variation.On the basis of the "density variation" of fiber assemblies during the compression,heat transfer,moisture diffusing and air permeating of five types of fiber assemblies,including polyester,wool,cashmere,goose down and kapok fibers,are systematically studied and the factors,involving bulk density,fiber volume fraction,fiber arrangement and other factors,influencing the three properties,are also investigated and discussed.Meantime,the physical transport phenomena of fiber assemblies with different fiber types,packing densities and stacking modes are analyzed and interpreted.The following conclusions are withdrawn:(1)It is found that in the same low bulk densities,the ranking order of heat conductivities is:polyester,wool,cashmere,goose down and kapok,while in higher densities,heat conductivities of different fiber assemblies are becoming larger,with the exception of goose down who exhibits the unchanged heat conductivity,and the heat conductivity difference of the fibers are becoming smaller.This indicts that radiative heat transfer property exhibits obvious and unobvious state in lower and higher densities respectively,meantime,the heat conductivity of the single fibers is shown to be important in higher densities.It is also found that in the same low fiber volume percentage,the rank of heat conductivities of different fiber assemblies is:polyester,wool,cashmere and goose down,manifesting that heat conductivity is not only related to fiber volume percentage,but also influenced by other factors,such as tortuosity of air channels and the diameter,arrangement and distribution of pores in the fibe.At higher fiber volume percentage,wool and cashmere exhibits nearly the same heat conductivities and they also own higher values than the other two fibers,showing the lowest and unchanged values.Especially,goose down,showing unchanged heat conductivity at higher bulky densities,is due to its special structure which makes the heat flux channels are more tortuous.(2)The heat conductivity comparison of the wool fiber assemblies in three arrangements shows that the heat transfer property of fiber assemblies is mainly influenced by the number,arrangement and distribution of pores in the assemblies.(3)A comprehensive heat transfer model,involving both conductive and radiative heat transfer parts,is established.Series model,taking the contact heat resistance into consideration,is adopted in the conductive part and two-flux model,involving the absorption,emissivity and nonisotropic fiber scatting of fibers to radiation light waves,in the radiative part.The theoretical heat conductivity results derived by the model are in good accordance with the experimental results of the fiber assemblies except goose down assembly,which proves the accuracy of the model and the specific characteristic of goose down in heat transfer mechanism.Furthermore,Fourier Transform Infrared Spectroscopy(FTIR)is employed to measure the radiative heat conductivities of fiber assemblies with larger optical thickness,simultaneously,the testing results are compared with the theoretical results obtained by the simplified model and it is found that the two values are in good accordance with each other,indicating the accuracy of the model proposed here.However,the former value is larger the later one,demonstrating that the influence of density distribution and pore factors on heat transfer property of fiber assemblies,indicating that the model proposed here needs further improvement.(4)From the testing results of fabrics,it is found that the moisture diffusing property varies with time and the diffusing process can be divided into three stages:in the first stage,the moisture diffusing ability increases with time,attributing to the absorption or adsorption affect of fibers to water molecules;in the second stage,the moisture diffusing ability decreases with time,with the reason that the fiber assembly absorbs moisture and then releases heat,increasing the water vapor partial pressure,thus hindering the passing of moisture;in the third stage,the moisture diffusing ability increases with time,which can be explained with the reason that the assembly has accumulated a great mount of water molecules in the previous processes,and the water molecules will be released in this stage,leading to the improvement of moisture diffusing ability of fabrics.Different fabrics perform differently in the three stages,depending on porosity and moisture absorption of fibers.(5)From the study on the relationship between the moisture diffusing ability of the random arranged fiber assemblies and their density and fiber volume percentage,it can be found that in the same density,the ranking order of the moisture diffusing ability is as follows:?polyester?wool?cashmere?goose down?kapok.The ranking order is in accordance with that of the fiber volume percentage in the same assembly density,indicating that fiber volume percentage is an important factor influencing the moisture diffusing property of fiber assemblies.In the same fiber volume percentage,the moisture diffusing ranking order of the fiber assemblies is:?polyester?goose down?wool?kapok?cashmere,showing that fiber diameter and moisture absorption ability play an important role in affecting the property.(6)From the study on the moisture diffusing property of the fiber assemblies in three arrangements,it is found that the moisture diffusing ability is more correlated with the connectivity of the pores in the assembly and less influenced by other pore characteristics,such as pore diameter and distribution ect.(7)The study on the air permeating property of random arranged fiber assemblies manifests that the air permeating property of different fiber assemblies is very close in the low densities,while becoming very different in higher densities,depending on the fiber morphology and structure.In the same fiber volume percentage,the air permeating property of different fiber assemblies is significantly different,meaning that the property is not only related to fiber volume percentage,it is also related to other factors,such as pore size,shape,tortuosity and distribution ect,which is also indicated in the testing result of wool assemblies in three arrangements.(8)It is found from the analysis of the theoretical and experimental results that the air permeating ability of the 3-D random arranged fiber assemblies performs better that of the 2-D ones,indicating the obvious influence of the fiber arrangement.In the traditional theoretical study,the "average volume" method is usually adopted in simulating the three properties of fiber assemblies and microstructure influence on physical transfer properties is seldom considered,with is different from reality.Therefore,fractal method is proposed for studying the relationship between the microstructure and the heat transfer,moisture diffusing and air permeating properties of fiber assemblies.The fractal model built is confirmed to be more accurate than the existing theorectical models.Furthermore,a systematical analysis of factor influence on the three properties is conducted,which provides theoretical guidance for designing structural parameters of fiber assemblies to obtain the best heat insulating,moisture permeating or air permeating properties.From the fractal method,it is found that the porosity,i.e.bulk density,plays an important role in affecting the three physical properties.Heat transfer property of fiber assemblies exhibits increasing and decreasing trend successively with the increase of porosity,so there exists a certain porosity to provide the best heat insulating property,while moisture diffusing and air permeating properties increase significantly with the increase of porosity.Heat transfer and air permeating properties of fiber assemblies are proportional to fiber radius,however,moisture diffusing property is not influenced by this parameter.Heat transfer and moisture diffusing properties of fiber assemblies decreases with the increase of the fraction of perpendicular air channels,however,air permeating property is unaffected by this parameter.Moisture permeating and air permeating properties of fiber assemblies decreases with the increase of the tortuosity of air channels significantly,while heat transfer property is not influenced by this factor.In addition to that,air permeating property of fiber assemblies increases obviously with the increase of the pore fractal dimension and the maximum and minimum diameters of pores,what is more,air flow through the fiber assemblies depends on the maximum diameter of pores,however,heat transfer and moisture diffusing properties is less affected by the three parameters.