Experimental Measurements and Numerical Modelling of Conductive and Radiative Heat Transfer in Polytetrafluoroethylene

摘要

Polytetrafluoroethylene (PTFE, or Teflon) components are used in propulsion and power systems due to their resistance in high temperature environments. Heat transfer in PTFE samples is investigated, both experimentally and numerically, to characterize the thermal protection behavior of PTFE samples under conductive and radiative heat sources. PTFE is known to undergo a sharp, reversible transition at 600 K, changing from the white, virgin substance to a transparent, amorphous material. As the temperature of PTFE is raised above 620- 670 K, pyrolysis occurs and the polymer decomposes into gaseous products. Experiments performed using a CO2 laser show that the internal absorption of radiation causes a continuos increase of both the internal temperature and the thickness of the gel layer. These experimental data are then used to validate a model of the thermal response and degradation process of PTFE samples. To model the ablation processes, under the same operating conditions of the experimental approach, a transient one- dimensional model, including variable thermal properties and radiation absorption is written. Energy, mass conservation equation and the decomposition rate kinetic equations are discretized by means of a finite difference technique. The comparison between experimental and numerical results is then discussed. Conclusions stress the importance of a coordinated approach to investigate the design and the sizing of PTFE thermal protection systems under ablating and non-ablating conditions.