Thermal Stress in Semi-hot MHD Generation Duct Wall (Part 2)

Abstract

The semi-hot wall of MHD generation duct is constructed with many rectangular elements that are made of dense refractory ceramics.
In the steady state running of MHD generator, one side of the element is heated at constant temperature (T_w) by working fluid of MHD generator and the opposite side is cooled at constant temperature (T₀) by the element holder that is kept at constant temperature (T_b). Heat flux is flowed from the latter side of the element to the holder. The value of the heat flux is decided by temperature difference (T₀-T_b) and heat transfer coefficient between them (h). The thermal stress in the element is caused by temperature distribution and temperature-dependent physical properties of the element material.
The critical conditions of the element are determined in order to avoid the fracture caused by the steady thermal stress. The critical conditions are shown by the minimum values of T₀ or the maximum values of h, and decided by T_w and ratio of thickness to width (γ) of the element.
Results are as follows:
(1) The critical values of h became infinite whether γ>4 for magnesia ceramics or γ>1 for alumina ceramics when T_w=1150-1400K and T_b=300-310K. This indicated that the maximum values of tensile stress in the elements are always smaller than the strength of the element materials when T₀=T_b.
(2) A simplified method is developed in order to calculate the critical conditions when γ=0 (infinite plate). Results obtained by this method are almost the same as the critical conditions obtained by the method previously developed by the author when γ<0.5. This indicated that the maximum values of the tensile stress in a duct element where γ<0.5 is nearly the same as those in the infinite plate.
(3) The parameter Γ=(1-ν)Et/(Eα), (ν: Poisson's ratio, E_t: tensile strength, E: modulus of elasticiy, α: thermal expansion coefficient) is found to be a rough measure of the resistive strength against fracture caused by thermal stress under the steady state. The Γ values are about 19.5K for magnesia ceramics and about 46.7K for alumina ceramics which are kept at 1000K. This indicated that alumina ceramics is stronger than magnesia ceramics against fracture caused by steady thermal stress.
(4) The method described in the previous paper is only useful for T_w lower than the temperature (T_c) at which creap or plasticity of the element material occurs. The method described here, however, is available for T_w larger than T_c.