Investigation On The Effective Thermal Conductivity Of Carbonized Fabric-Type Ablative Materials

As a phenolic resin-based fabric-type ablative material,high silica/phenolic and carbon/phenolic composites are widely used in aerospace field.Their mechanical properties and thermal properties have been comprehensively studied.But the thermal properties of the carbonized ablator were rarely reported.Considering the carbonized ablator is an important part of the ablative material after ablation,the thermal properties of the carbonized ablator are key parameters in the studies of the heat transfer performance in the process of application.Thus,The thermal properties of the carbonized ablator have been measured,and the corresponding theoretical prediction models have been established in this work.The carbonization experiment was designed based on the thermogravimetric analysis(TGA)results of the ablative material,and the carbonized samples of high silica/phenolic,carbon/phenolic and the phenolic resin were obtained.The results of composition and structural characterizations showed that the carbonized ablator and the carbonized sample are basically the same in composition and structure.Thus,the carbonized samples obtained through carbonization experiments can be studied as the carbonized ablator.The thermal diffusivities of the carbonized samples were measured by a flash method.Combined with the measured values of the density and the specific heat,the effective thermal conductivities of carbonized samples were calculated from 100 to 970 ?.In addition,an analysis model of the effective thermal conductivity of the carbonized ablator consisting of fiber yarns and carbonized phenolic resin was established based on the result of structure analysis.The measured effective thermal conductivity of the carbonized phenolic resin was used to inverse the transverse effective thermal conductivity of the fiber yarns.A simulation model of fiber yarns was adopted to verify the reliability of inversed values.At the same time,the deviation analysis on the inversed values was conducted in combination with the results of empirical model.The analysis result showed that the inversed values are reliable.When the inversed values were adopted in different empirical models,it was found that the Clayton model and the Pilling model are suitable for predicting the effective thermal conductivities of the carbonized high silica/phenolic and carbonized carbon/phenolic,respectively.An experimental study of the carbonized three-phase ablator has been carried out using a high porosity lightweight resin composite as the object.The specific heat from room temperature to 1350 ? and the thermal diffusivity from room temperature to 1400 ? of the high porosity lightweight resin composite were measured by experiments,and the effective thermal conductivity was calculated combined with the measured values of the density.The result showed that the density and the effective thermal conductivity of the high porosity lightweight resin composite are lower than that of two-phase composites apparently.The effect of the ply scheme of the woven fabric was analyzed by simulation,including the dislocation and the angle of layers.The results show that the effective thermal conductivity of the carbonized ablator increases initially then decreases from the start of dislocation to 1/2-unit dislocation,and the value reaches the maximum at 1/4-dislocation.The effective thermal conductivity of the carbonized ablator increases with the angle of layers.In addition,based on the curves of the effective thermal conductivity of the carbonized ablator with the ratio of the effective thermal conductivity of the fiber yarns kt/ka,a dimensionless value X was defined,which can be used to distinguish the effect of the anisotropic conductivity of the fiber yarns on the effective thermal conductivity of the carbonized ablator with ply scheme.It is found that the dislocation among layers does not affect the relationship between the effective thermal conductivity of the carbonized ablator and the ratio of transverse and axial thermal conductivity of the fiber yarns,while the angle of layers did.