|M.Sc Student||Shilav Ramin|
|Subject||Apparatus and Experiments for Thermal|
Conductivity Study for Ablative Materials
|Department||Department of Aerospace Engineering||Supervisors||Professor Emeritus Alon Gany|
|Dr. Amiram Leitner|
Research on thermal protection systems is significant in the development of products for aerospace applications, such as internal insulation for solid rocket motors, nozzles, and external thermal protection. Ablative materials have been used as sacrificial materials protecting the underlying structural surfaces from the combination of high temperatures and high heat fluxes. Due to their unique chemical and physical characteristics, these materials can withstand the thermochemical ablation and mechanical erosion, resulting from the two-phase flows of hot combustion gases and alumina particles typical to rocket combustion chambers. Prediction of the thermal response of new ablative compositions requires prior work and experiments to determine the thermophysical and thermodynamic properties of the material, such as the temperature dependence of thermal conductivity and specific heat. The variation of thermal conductivity with temperature cannot be predicted analytically, and one needs to determine it experimentally. To date, there are no commercial thermal conductivity devices enabling ablative material testing at high temperatures. Since most reactions, which influence the thermal conductivity of ablative materials, occur up to a temperature of 1200K, it was decided to develop such an apparatus.
The present work presents the development of a thermal conductivity apparatus for research of ablative materials, following heat flow and mechanical finite element simulations. Derivation of a semi-empirical model for determining the instantaneous thermal conductivity of the ablative material has been implemented successfully. Verification testing of the apparatus was carried out in order to determine the accuracy and sensitivity of results to different factors. Tests with Teflon showed an uncertainty level of 4-8% (according to two standard deviations), conforming an earlier assessment. It was found that homogeneity of the material has great impact on the precision of the results.
Four compositions of EPDM-based insulations were tested using Teflon as a reference material.
In the present work, there is an attempt to calculate the thermal conductivity of the charred material via the assumption of conductive heat transfer through the solid residue and the pores. It was decided to use the Russell approximation for porous materials, and by utilizing the CEA thermochemical software for the volume fractions of the gas phase species, it was possible to determine the temperature dependent thermal conductivity of the charred material in the range of 2000-3500K. Very good agreement was found between the materials.
It was found that specific heat and thermal diffusivity of the ablative material could be determined as well, once the temperature dependent density is available experimentally. A comparative study showed good agreement between thermal diffusivity and heat capacity results obtained with the present apparatus and those obtained by a commercially available steady state Laser Flash apparatus. The Laser Flash apparatus yields a single value of thermal property at a specific temperature, during the test. This fact alone demonstrates one of the main advantages of the present apparatus.
The experiments revealed the influence of curing and fire retardant agents on the temperature dependent thermal conductivity of the ablative material.