Analysis Of Heat Transfer Of Multi-phase Fibrous Materials Under Unsteady-state Conditions

Multi-phase fibrous materials are not only widely utilized in the apparel field owing to their well thermal insulation, ventilation and cooling properties, but also gradually used in some industrial engineering areas because the outstanding properties of some novel high performance fiber are recognized in these areas. In some kinds of extreme high/low conditions, such as high-temperature fire place, transient thermal stimulation and some frozen cases, their heat transfers are commonly under the unsteady-state conditions whose internal heat transfer modes always become complex and changeable along with their procedures. Therefore, theoretical and quantitative analysis of the heat transfer in fibrous materials is a better approach to investigate and design some new structures of multi-phase fibrous materials. In our project, based on the theories of porous media heat transfer and local thermal equilibrium, three research methods, i.e. experimental measurements, numerical simulations and analytical solutions, are applied to elucidate some special issues, i.e. the heat transfer of fibrous materials, measurements of effective thermophysical properties and the applications of these measured properties unpon some unsteady-state cases.The general works and conclusions of our research project are listed as:(1) A new two-phase heat transfer model of fibrous materials is built according to the local thermal equilibrium theory and non-Fourier heat transfer, the temperature profile of each phase is calculated by the theoretical method, then a criterion is proposed to assess the local thermal equilibrium status of fibrous materials. Hereafter, we can find that in most conditions, fibrous materials are approximately at local thermal equilibrium status and we also illustrate that the grade of heat source, boundary conditions and thermophysical properties of samples can influence the local thermal equilibrium conditions.(2) A new unsteady-state apparatus used to measure sample thermophysical properties is set up following the essential principle of fibrous materials measurement and its impact factors, i.e. imperfect heat source, boundary heat loss and heat source power are discussed, the validation of this apparatus is also verified. This apparatus can yield the "pure" parameters, thermal conductivity, thermal diffusion and volumetric heat capacity.(3) Choosing the stacked composites as the object, the heat transfer equations of multi-layer samples are solved by the Green function method. These equations are used to predict the effects of stacking sequence of composites on the sample thermal responses, here two-layer and sandwich are chosen as the samples. Afterwards, an experimental apparatus set up in our lab is utilized to verify the accuracy of our prediction model. The results show that the stacking sequence of two-layer samples plays no effect on their thermal response similar with the sandwich samples with the same filling layers, however, the stacking sequences play an important role in the thermal response of the sandwich with different filling layers.(4) Thermal insulation of multi-layer fibrous materials is conducted in this part, the stacking sequence is also considered as an important factor of sample thermal insulation. Numerical simulation and experimental verification are both used to illustrate this issue considering the internal natural convection and boundary heat loss. From the above results, we find that the layer contacting the heat source is the most important layer to determine the thermal insulation of the whole system and its volumetric heat capacity is the key property to regulate the system thermal insulation.(5) A reasonable heat transfer model between two-layer fabric and human skin is built to describe the thermal stimulation of skin upon contacting the cooling fabric. We find that the thermal stimulation depends on the fabric thickness in both transient and steady-state stages, and a thicker fabric thus with lower values of the proposed parameters will generate a higher thermal stimulus faster during both stages. During the transient period, the inner layer interfacing with skin is the primarily important layer to affect the heat flux parameters and the marginal influence from the outer layer can be ignored. Once entering the steady-state however, the thermal diffusivity ceases to have influence and the thermal conductivities of both fabric layers are combined in affecting the skin thermal stimulation with roughly equal significance. In addition, different stacking sequences can generate different thermal stimulations, and the inner layer is more dominant than the outer layer during the dynamic stage. However the stacking sequence no longer matters once in the steady-state stage. This model can be used as the theoretical foundation to instruct the materials of winter quilt and underwear selection and design.Base on the investigation in our project, we can use the criterion proposed in this project to quantitatively analyze the local thermal equilibrium status of multi-phase fibrous materials and make a clear recognition of micro-heat exchange between internal phases. Three basic thermophysical properties can be measured accurately by our apparatus. Then several unsteady-state cases are investigated by numerical analysis and experimental measurement to demonstrate the important position of thermophysical properties to describe fibrous materials heat transfer. In a word, this project can provide the better and more concrete methods to conduct the physical properties of fibrous materials, exploit new product and predict the function of multi-layer materials etc.