1982 Volume 90 Issue 1044 Pages 447-458
Pressureless-sintered or reaction-bonded silicon carbide and silicon nitride (S-SC, S-SN, RB-SC, RB-SN, respectively) were selected as the candidate materials for advanced gas-turbine combustors. Data of thermal and mechanical properties of these materials representing the present state of the manufacturing technology were evaluated. Characteristic tensile strengths corresponding to the spontaneous fracture of the materials were deduced from the available bending strength data on the basis of Weibull statistics. A simplified model of combustor tube was selected, which corresponds to the primary combustion zone without film-cooling. Computations of the steady-state temperatures and thermal stresses of the combustor wall were carried out by FEM, and the failure probabilities associated with the spontaneous fracture by thermal stress were calculated. The dependences of the results on the selection of material, wall thickness, flame temperature and Weibull modulus of the strength of the material were studied. The selection of material gave large effects on the maximum temperatures and the temperature gradients, and their trend was as RB-SN<S-SN<RB-SC≅S-SC. The change of thermal stress with the selection of material was as S-SN≅S-SC≅RB-SC<RB-SN at flame temperatures below 1600°C. The increase in flame temperature resulted in the increase in thermal stress, which was steepest in the case of S-SN because of the largest temperature dependence of the properties of that material. The failure probabilities of the ceramic combustor tubes associated with the thermal stress increased with flame temperature. Especially those of S-SN and RB-SC showed highly steep increase due to the steep decrease of the strengths of these materials with temperature. The failure probabilities were as RB-SN<RB-SC≅S-SC≥S-SN at flame temperatures>1600°C, but were as S-SN<RB-SC<RB-SN<S-SC at flame temperatures≅1800°C. The Weibull modulus associated with the strength of material had an extremely large effect on the failure probabilities of the combustor tube. Reasonably high reliabilities are not expected from materials with Weibull moduli>10.
