Conjugate flow and heat transfer analysis between segmented thermal barrier coatings and cooling film

The segmented thermal barrier coatings (TBCs) with vertical cracks have similar thermal insulation properties to layered coatings prepared by atmosphere plasma spraying (APS), and have similar mechanical properties to columnar coatings prepared by electron beam physical deposition (EB-PVD), which are very suitable for gas turbines with frequent start-up and variable load. Vertical cracks in the segmented APS-TBCs change the surface morphologies and internal microstructures, so it is necessary to study the internal heat transfer performance of the coating and the conjugate heat transfer characteristics between the TBCs and the cooling film. In this work, the numerical reconstruction models of TBCs with different crack lengths and openings were obtained based on the optimized quartet structure generation set (QSGS) method. By using the double distribution function lattice Boltzmann method (DDF-LBM), the temperature distribution of the coating, the conjugate heat transfer between TBCs and the cooling film and the flow characteristics of the cooling film were studied. The results showed that the vertical cracks with diverse sizes have significant effects on the coating-film cooling system, on which the crack opening has a greater effect. The temperature non-uniformity coefficient shows bimodal distribution in TBCs and the highest temperature is located at the crack center on the coating surface, which can easily lead to local hot spots near crack tips and further result in an acceleration of coating structure's fatigue failure. Meanwhile, the presence of cracks strengthens the interfacial convection heat transfer characteristics. The convection heat transfer coefficient change rate goes up 10% with the crack opening widens. Correspondingly, the greater deformation of the cooling film streamline and value of the vertical velocity are generated under the pre-existing larger size cracks. The results can provide us a powerful guide for the design of advanced coating structures and a strong foundation for a better understanding of the failure behavior of segmented TBCs.