Simulation Study On Heat And Moisture Conduction Of Weft Knitted Fabric

In recent years,people's requirements for the comfort of apparel fabrics have become higher and higher,which is becoming the first condition that consumers need to consider during the purchasing process.The thermal and wet comfort is an important performance of fabric comfort,because the comfort perception of human skin mainly comes from temperature and humidity,and the fabric is between human skin and external environment,so it will directly affect the comfort feeling of human body.For the study of the thermal and wet comfort of fabrics,most of the previous people focused on the discussion of influencing factors and the establishment of related numerical models.The process was time-consuming,consumable,and larger accidental errors,and the internal transfer law could not be observed and understood at the same time.Regarding the finite element simulation of fabric performance,previous researches are mostly about woven fabrics,because the three-dimensional model of woven fabrics is simple and easy to create.Most of the research is about the stress analysis of fabrics,and a few of the simulations on fabric permeability.The simulation research about thermal conductivity and moisture conductivity of fabrics is still in a difficult exploration stage.This thesis presents a method for quickly creating the three-dimensional model of knitted fabrics by a hexagonal grid structure and NURBS spline curves.This method could create the three-dimensional knitted fabrics model that close to the actual fabric and can be imported for finite element simulation analysis.According to the thermal and moisture performance test conditions of the fabric,the finite element simulation in the virtual state is carried out to obtain the heat and moisture transfer status in the fabric.And the feasibility of the finite element simulation analysis of the thermal conductivity and moisture conductivity of the fabric system is verified.Firstly,the hexagon mesh structure is obtained by selecting weft knitted fabric interweaving points as special value points.According to the geometric relationship and the trigonometric function,the position coordinates of each type value point are obtained.The yarn path of loop can be obtained by inserting a non-uniform and rational B-spline curve with eight special value points.The cross section is drawn according to the yarn fineness,then the fabric yarn system is drawn scan along the path,and finally the knitted fabric model is obtained by repeated arrangement of the array in the transverse and longitudinal directions.This method not only has less selection of shape value points,simple calculation process,but also achieve very close appearance effect with actual fabric.And the error between the transverse,longitudinal and thickness dimensions of the fabric model and the actual measured value is within 2.5%.In general,because the fabric size is small and the error is within the acceptable range,this method has certain practicability for establishing the geometric model of the knitted fabric.In order to carry out the finite element simulation of the thermal conductivity of the fabric,the created fabric model was cut into the size of the unit loop and imported into the finite element analysis software.Because the model is too large,which may cause problems with meshing,and the simulation calculation process is complicated and longer.The simulation conditions and environment settings can be based on the test environment of the fabric thermal comfort experiment,and the model attribute parameters can be obtained from the corresponding fabric attributes.In the software STAR-CCM~+,a series of steps such as boundary condition setting,surface mesh repairing,volume meshing,physical model selection and setting,and definition solver are operated to process the analysis.According to the derivation formula,the thermal resistance value is obtained and compared with the actual measurement value.The simulation results of pure cotton knitted fabric and the actual error are within 4%.Comparing other fiber materials knitted fabrics,it is found that the thermal conductivity simulation analysis of single material knitted fabrics is closer to the actual test results.The simulation results of CVC and T/R blended fabrics have larger errors,ranging from 16%to 24%.In addition,in all simulation calculations,the performance of the fabric to prevent heat transfer is slightly less than the actual situation,In order to carry out the finite element simulation of the moisture transmission of the fabric,the polyester weft knitted fabric models with yarn counts of 28.1tex(21~s),22.4tex(26~s),and 18.5tex(32~s)were created by the same modeling method described above.And the element model was imported into the finite element analysis software.According to the moisture permeability test of the fabric,the simulated experimental environment and conditions are determined.The relationship between the pressure and speed when the air flow through the fabric is established through the air permeability experiment of the fabric,in order to explore the viscous drag coefficient and the inertial drag coefficient in the fabric system.In order to obtain the porosity of the yarn in the fabric system,the method of immersion experiment and geometric model volume ratio is used.These parameters are brought into the simulation process,and the calculation and analysis are carried out through the steps of boundary condition setting,surface mesh repair,volume meshing,selection and setting of the physical model,and solver definition,and the moisture permeability of three kinds of polyester weft knitted fabrics are calculated by physical functions.Compared with the measured value under actual conditions,the result has an error between 10%?15%,within an acceptable range,and the simulation result is generally larger than the actual measurement result.The finite element simulation study of the thermal conductivity and moisture conductivity of the fabric not only explores the feasibility scheme and parameter settings,but also can observe the heat and moisture transfer in the fabric through the cloud diagram.When the heat is transferred in the knitted fabric system,the yarn surface is distributed in an arc shape from the upper surface to the lower surface,and the lag phenomenon occurs when the solid yarn is transferred at the interface with the air fluid.When moisture is transmitted in the knitted fabric system,it preferentially passes through the middle gap,the fluid velocity gradually increases from top to bottom,and the intermediate velocity is greater than the surroundings.Through observation,we can further understand the internal transfer law about the thermal conductivity and moisture conductivity of knitted fabrics,and provide a certain reference for studying the thermal and moisture comfort of fabrics.