Studies On Mechanisms Of Film Cooling Coupling Thermal Barrier Coatings Under Turbine Cascade Environment

Design of a scheme of active cooling coupling with passive insulation has become an important method to effectively protect the hot components in future aero-engines from melting.To achieve the goals of the less consumption of cooling air,the higher whole engine efficiency and the higher component durability,it is necessary to design properly the film cooling and spraying efficiently thermal barrier coating(TBC),when the hot gas temperature has exceeded the melting point of the superalloy material.However,the thermal protection of the coupled scheme was worse than that caused by the superposition of solo film cooling and TBC.The main causes included the nonuniform heat fluxes at blade vane due to the cascade flow and the unsteady film outflows.Additionally,the large thermal gradient near film-holes can induce the thermal failure of local TBCs.Such temperature gradient often is generated by the spatial-temporal evaluation of flow interaction between hot gas and cooling air jet,which has been widely accepted to be unsteady.The main objective of present work was to solve the key problems above.The experimental facilities with the typical cascade flow environment were built up,constructed by the combinations of different wall curvatures and streamwise pressure gradients Firstly,the proper spraying parameter and the optimum thickness and geometry of TBC at different regions of blade were acquired,through applying the conjugate heat transfer measurements and simulations.Secondly,the spatial-temporal evaluations of film effectiveness of advanced cooling structures at different regions of blade were obtained by the highfrequent infrared thermal imaging technique.The mechanisms of thermal failure of the TBCs near film-holes were discussed by the numerical simulation.The suggestions of improving the durability of TBC were proposed.The main connects and conclusions of this thesis were listed as follows.(1)By matching the Biot number on the hot side of the actual turbine material,a coupled heat transfer model of TBC/film cooling is established.and the relationship between the surface roughness of TBC and the overall cooling effectiveness of the system is discussed under different wall curvatures,mainstream pressure gradients and cooling air flow rates.The results of the coupled heat transfer experiment show that the overall cooling effectiveness of the metal can change by 15%when the surface roughness of the TBC is increased by 70 times from smooth,which is enough to prove the importance of strict requirements for the TBC preparation process.The design thresholds of TBC surface characteristic parameters are extracted by sorting out the experimental data.For the local area with small curvature of the blade surface,we strive to achieve smooth TBC surface.Only the suction surface can be prepared by appropriately relaxing the roughness of TBC while meeting the requirements of material stress level.(2)In view of the problems that the overall thermal protection capability of the existing thermal insulation/cooling system is not ideal and the utilization rate of cold air is not high,coupled heat transfer experiments and fluid-solid weak coupling numerical prediction are carried out in different areas of the blade.Research on the best matching among TBC design parameters(thickness and coating structure),film cooling structure parameters(jet direction)and cooling air consumption.The results confirm that the TBC design method with partially exposed substrate can significantly reduce the risk of coolant being lifted away from the forward hole,and can improve the comprehensive thermal protection performance by up to 50%.Reversing the direction of the jet can increase the overall cooling effectiveness of the metal by a maximum of 45.3%,and the trench of backward jet can improve the ability to prevent hot air backflow and dust accumulation.When the mechanical properties of the material allow,better protection of the metal can be achieved by coating thicker TBC,and the metal surface cooling effectiveness can be increased by up to 11%when the insulation material thickens by 30%.However,in terms of the TBC’s own operating life,the partially exposed design of the substrate is more suitable for the use of thick coatings.The thick coating of film hole area has the negative effect of heating the outer surface.(3)With the help of a high-frequency infrared thermal imager and proper correction of the influence of the thermal conductivity of the model,the local highprecision quantitative evaluation of the spatiotemporal evolution characteristics of the film effectiveness and the qualitative evaluation of some regions are realized.Combined with high-precision numerical simulation,the variation and influence mechanism of the average effectiveness and the unsteady characteristics of the average effectiveness on the local area of the airfoil is discussed for the design of the partial exposure of the substrate.The selected advanced cooling structure includes:compound angled round hole(including forward to backward jet)and fan-shaped hole(forward jet).The valuable references include:Partial exposure of the base material can improve the film effectiveness by 20%-52%for the round holes with 0-180° compound angle.However,the partial exposure of the base material design can only achieve high reliability of near-hole TBC for the film hole with 60°,90°,120°and 135° compound angles;Breaking through the research point of view of flat plate,the partial exposure of the base material design can achieve a maximum increase of 27%in film effectiveness for fan-shaped holes under the constraint of curved surface.The significant timing pulsation of the concave film leads to the rejection of the exposure of the base material design;The extraction of the spatiotemporal evolution characteristics of the near-hole makes fan-shaped hole with compound angle not suitable for the pressure surface.Comparing to the simple angle outflow,the film effectiveness can be increased about 50%by the fan-shaped hole with 30° compound angle in other areas.