Авіаційно-космічна техніка та технологія (Aug 2024)
Determination of the thermophysical characteristics of composite materials at high temperatures
Abstract
This article discusses a methodology for determining the thermophysical characteristics of composite materials at high temperatures. The primary focus is on improving methods for determining the thermal conductivity of materials used in the aviation and space industries. The proposed methodology accounts for material loss from the surface during heating, which increases the accuracy of thermal conductivity calculations. The thermal conductivity was determined by solving the inverse heat conduction problem using a computer model of heat transfer in a flat sample in a one-dimensional formulation. The model was built using the COMSOL Multiphysics® software, which is designed for finite element calculations for various multiphysics problems. The model consists of the following components: 1) initial conditions (ambient temperature); 2) boundary conditions; 3) material properties (density, temperature dependencies of specific heat capacity and thermal conductivity). The boundary conditions are the experimental time dependencies of the temperature of the heated surface and the heat flux from the back (cold) surface due to radiative cooling. The inverse heat conduction problem in the model was solved by iteratively calculating the temperature of the sample’s back surface while simultaneously adjusting the sought temperature dependence of the thermal conductivity coefficient. The iteration procedure terminated when the calculated back surface temperature matched the experimental one. The model includes a Prescribed Normal Velocity node to account for the effect of linear wear, which specifies the time-dependent wear rate. An industrial propane-oxygen welding torch serves as the source of one-sided heating of samples, allowing the study of materials at temperatures above 2000 °C. Temperature measurements on the opposite surfaces of the samples were performed using pyrometers with PC registration. The unique feature of the proposed methodology is the simultaneous determination of temperature fields in the samples, thermoerosive wear, and the emissivity coefficient at selected temperature values based on experimental results. For each subsequent sample temperature, the values of the listed model parameters are calculated based on previously determined values. The developed methodology was used to determine the thermophysical characteristics of the carbon-carbon composite material CCCM-88, which is a layered composite made of twill-weave carbon fabric with a density of 1.57 g/cm² and a porosity of 20%. The thermal conductivity coefficient of CCCM-88 in the temperature range up to 2000 °C varies from 3 to 8.2 W/m/K, and the linear wear rate at 2000 °C reaches 55 μm/s. The obtained values allow the experimental curve of the back surface temperature to be described with an error of approximately 2.2% and an absolute deviation of 35° at the end of the test when the hot surface temperature is 2000 °C.
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