| The new generation of high performance aero-engine requires higher turbine gas inlet temperature.However,turbine blades face the risk of thermal fatigue due to exposure to extremely high temperature environment,especially the blade leading edge is directly impacted by the ultra-high temperature mainstream.Therefore,it is urgent to develop advanced cooling technologies to prolong the service life of turbine blades.Film cooling combined with internal cooling is widely employed to respond the severe challenges of local high temperature level in the leading edge region.Unfortunately,the leading edge with traditional cylindrical film-hole can cause the non-uniformity heat load at wall and lift-off of cooling air,which is not beneficial to improve thermal efficiency and propulsion output of advanced aero-engines.Meantime,the unsteady jet induces the rapid and nonlinear temporal evolution of temperature at wall,resulting in the unsteady fluctuation of the surface temperature and increasing the fatigue damage risk of blade leading edge.Therefore,the main objective of present work is to solve the practical engineering problems above.Firstly,the present work develops a timeresolved quantitative light sheet(QLS)system,to analyze the mechanisms of unsteady interaction between the mainstream and cooling air based on the simplified leadingedge model with traditional cylindrical film-holes.Secondly,the proper hole-geometry near stagnation line of a vane is evaluated by traditional time-resolved Infrared measurements and QLS,from a combined view of the steady film effectiveness and cooling unsteadiness level.Next,a reliable leading edge cooling structure combined with film holes and internal impingement cooling is proposed,through proper orthogonal decomposition(POD)analysis.Finally,the cooling unsteadiness levels in the stagnation zone of a double-wall cooling configuration with an actual vane geometry are studied.The leading-edge ablation area is identified accurately and an improved method is proposed.The main work and contents are listed as follows:(1)Owing to the lack of measurement technology to record the unsteady planar scalar transport behaviors,this work has improved and developed a time-resolved QLS system,and established the standard data conversion and correction process.The unsteady measurement and quantitative analysis of the film cooling are acquired.By comparing the previous research results,QLS can not only accurately capture timeaveraged film cooling performance,but also quantitatively record the transient mixing characteristics of hot and cooling air.At the same time,compared to the steady and unsteady surface cooling effectiveness obtained by the surface IR technology,the corresponding errors of the QLS results are less than 5.5%and 7.8%respectively,which confirms the reliability of the technology and solves the problem that IR measurement cannot record the transient mixing process between hot and cooling air.(2)Based on the fact that the unsteady characteristics of the leading edge cylindrical-hole film cooling are not clear,the effect of inclination angle of film-holes on the unsteady film coverage and the concentration-interactions of two rows is of concern.Transient tests reveal that the fluctuations of unsteady film coverage and jettrajectory are at random,and the ejection in stagnation line dominates much stronger unsteadiness relative to the downstream hole-row.The angle effect on film cooling unsteadiness in stagnation line is significant;however,in the downstream is nearly nonsensitive.Finally,from a view of unsteadiness of film cooling,some suggestions of leading-edge film cooling design can be proposed.(3)Aiming at the problem of strong cooling unsteady level caused by cylindricalhole cooling in the leading-edge stagnation zone,three advanced film-hole structures(including a fan-shaped-hole,the cylindrical-hole in a trench and an individual crater)are selected for the leading edge.Through employing the IR and QLS techniques,the unsteadiness of surface film effectiveness and in-plane jet-concentration are analyzed,and the proper hole-geometry near stagnation line of a vane is evaluated.Both techniques show that the fan-shaped and trenched holes can generate the lowmomentum ejections relative to the cylindrical-hole,and hence,improve the uniformity of film effectiveness over entire leading edge,and the maximum increments in areaaveraged effectiveness can be achieved above 10%.Under all cooling air flowrates,the trenched-hole can generate the lowest-level cooling unsteadiness,and thus,is recommended to be applied to the leading edge surface.However,the trenched-hole features a faster decay in film effectiveness,and the high-level cooling unsteadiness concentrates on the sidewalls of trench due to the internal flow impingement.The future task is to improve the trenched-hole shape or combine the fan-shaped holes to weaken its shortcomings.(4)In order to effectively improve the thermal protection ability of the leading edge,combining external film cooling and internal impingement cooling is required.Therefore,a new concept of combined cooling is proposed in the present work.Considering the combination of fan-shaped holes and trenched-hole in the leading edge,the effect of internal impingement cooling on unsteady film outflow is discussed.The measured concentration field is decomposed by the proper orthogonal decomposition(POD)method,and then the main unsteady structure and its spatial pattern evolution process are analyzed and extracted.Finally,the main contribution of flow structure to cooling unsteady is determined,and the most reliable combined cooling structure is proposed.(5)Due to the higher demand for advanced cooling technology,the double-wall cooling structure has a large potential to reduce the highest heat load level in turbine vane leading-edge region.Based on an actual vane geometry,the main goal of present experiments is to understand the effects of amount and location of jet-ejections on the time-averaged film effectiveness and cooling unsteadiness levels in the stagnation zone of a double-wall cooling configuration.The higher cooling unsteadiness levels near the hub and end-wall region has been successfully captured by QLS technology,and the intensive unsteadiness can cause a high risk of thermal damage of component.Generally,the preset work provides some new guidance for designs of highly efficient double-wall cooling scheme of leading edge. |
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