【作者】 姚然；

【导师】 王建华； 徐华昭；

【作者基本信息】 中国科学技术大学 ， 动力工程及工程热物理， 2021， 博士

【摘要】 随着航空发动机技术的持续发展,透平进口温度和压力不断提升,远远超过了现有材料的最高耐受温度。为了保障包括端壁和前缘在内的透平高温部件的正常运行,亟需发展先进高效的冷却技术。目前,常用的冷却技术可以分为外部冷却和内部冷却,前者包括气膜冷却和层板冷却等,后者包括肋化通道冷却、冲击冷却和涡流冷却等。近年来,对这些冷却方法的传热机理、影响因素和结构优化的研究大量涌现,受到了研究者的广泛关注。尽管如此,一些冷却技术从理论研究到实际应用还有一定距离,仍然有一些问题亟需解决。例如:现有的机理实验研究大多采用简化的透平冷却叶片几何模型代替真实的复杂模型,或采用简化的边界条件代替复杂的真实边界条件。显然,这类简化的做法会对冷却效果的预测造成一定的偏差。此外,如何在减少冷气消耗的同时获得较高的冷却效率依然始终是透平冷却叶片设计者面临的一大难题。为此,本文采用通过实验验证的流固耦合数值方法,对不同冷却结构下,真实透平叶片端壁和前缘的冷却特性进行深入研究,主要工作如下:(1)讨论了环形透平端壁简化为平板对气膜冷却效率的影响。通过真实环形端壁和简化平板端壁上气膜冷却效率的比较,发现在不同工况下,端壁平板简化对绝热和综合冷却效率的影响是截然不同的。在高主流雷诺数的真实工况下,真实环形端壁会导致综合冷却效率出现“马太效应”,即:在气膜覆盖较好的区域产生高冷却效率,在气膜覆盖较差的区域产生低冷却效率。因此,在环形端壁上的温度梯度更大,也就是说,使用平板端壁简化可能会高估端壁的使用寿命。(2)分析了利用传热传质相似性测量端壁气膜效率的可靠性和准确性。本文先对湍流状态下传热和传质过程的控制方程进行系数比较,再分别使用空气和二氧化碳作为冷却工质,用数值计算方法分别比较透平端壁的传热和传质气膜效率。研究发现,以传质实验代替传热实验的可靠性和当地湍流特征有关。只有当湍流输运占主导地位,且湍流传热和传质扩散系数十分接近时,用传质实验预测传热特性的准确度较高。对真实透平端壁来说,在上游层流区域产生的误差较大,中游湍流区域误差较低,而下游区域传质气膜效率低于真实的传热气膜效率。(3)针对叶片前缘很难冷却的问题,提出了一种新型的多级涡流冷却概念,并通过数值模拟研究了该结构的流动和传热特性。结果表明,与传统的单级涡流冷却结构相比,这一新型的多级结构能够在不增加冷气消耗的前提下,获得更高、更均匀的传热速率。虽然使用该结构付出了总压损失升高的代价,但气热性能是总体提高的。(4)研究了叶片前缘双层壁冷却结构的传热和流动特性。通过在不同总压比下的数值模拟,发现了高温燃气倒灌进入叶片内部的不利现象,并分析了该现象形成的原因,据此提出了一系列改进措施,有效地提升了目标壁面的冷却效率。(5)在考虑主流入口热斑的真实条件下,研究了热斑运动和扩散规律,分析了叶片前缘气膜冷却和双层壁冷却结构的冷却特性,比较了两种结构在热斑条件下的冷却效果。结果表明,热斑会导致叶片前缘产生局部的高温区域,该区域的特性同热斑的强度、位置有关。在主流温度非均匀的条件下,双层壁冷却结构相比气膜冷却结构能够获得更高的冷却效率,但是在扰流柱上存在较高的温度梯度和较大的热应力。更多还原

【Abstract】 With the continuous development of aero-engine technology,modern gas turbines are usually operating at the high temperatures exceeding the melting point of turbine blade materials.Therefore,it is important and necessary to develop effective cooling methods,especially for the turbine blade endwall and leading edge,because they are directly exposed to the extreme environment with heavy heat load and aerodynamic force.The present cooling strategies can be divided into external cooling and internal cooling.The external cooling strategy mainly includes film cooling and laminated cooling,while the internal cooling strategy includes ribbed-channel cooling,impingement cooling and vortex cooling.In recent years,there have been a large number of studies concerned on the heat transfer mechanism,influence factors and structural optimization of the cooling strategies of blade endwall and leading edge.However,there is still a certain gap between the current theoretical turbine cooling investigations and practical applications.For example,in most investigations,the simplified geometrical models or boundary conditions were usually used to substitute the real complicated turbine structures or running conditions,respectively,which could cause non-ignorable errors on the predictions of cooling effectiveness.In addition,it is still a major problem to get a high cooling performance without increasing the coolant consumption.In this dissertation,validated numerical calculations by conjugate heat transfer algorithm are used to obtain the heat transfer and flow characteristics of the realistic cooling structures in blade endwall and leading edge.The main work is summarized as follows:(1)The influence of flat endwall simplification on film cooling effectiveness is discussed.Through the comparison of film cooling effectiveness between a real annular turbine endwall and simplified flat endwall,it can be concluded that the influences of flat endwall simplification on film cooling effectiveness are different under different conditions.Under the real running conditions with high mainstream Reynolds number,the "Matthew effect" is exhibited on the annular endwall,i.e.,better overall cooling performance appears in good film-covered region,but worse overall cooling performance appears in poor film-covered region.Therefore,the temperature gradient in annular endwall is much higher.With other words,using flat endwall simplification may cause a high risk of endwall lifespan.(2)The feasibility and accuracy of the mass transfer analogy experiments in turbine endwall film cooling are analyzed.Through theoretical analyses,the turbulent heat and mass diffusive coefficients are compared.Then the heat and mass transfer simulations on the film effectiveness of turbine endwall are conducted,using air and CO2 as coolant,respectively.It can be concluded that the feasibility and reliability of the analogy experiments are closely related to the local turbulent characteristics.Only when the values of turbulent heat and mass diffusive coefficients are close,and the turbulent transportation dominates in the whole field of heat or mass transfer,the experimental results obtained by the analogy are reliable.For a real turbine endwall,the difference of film effectiveness between the heat and mass simulations is large in the upstream laminar region,but small in the mid-passage with turbulence transportation.In the downstream of the endwall,the film effectiveness by mass transfer data is lower than that by real heat transfer.(3)A novel conception of multistage vortex cooling configuration is introduced,aiming to improve the cooling performances of the leading edges without increasing the coolant consumption.The numerical simulations are carried out to study the flow and heat transfer characteristics of the novel cooling configuration,and the results show that compared to the traditional single stage model,the multistage cooling strategy can provide a better cooling performance,a more uniform heat transfer rate.Although the total pressure loss becomes higher,the thermal performance of the multistage model is better.(4)The flow and heat transfer characteristics in the double wall cooling structure of blade leading edge are investigated.Three different total pressure ratio of coolant to mainstream are included in the numerical simulations,and from the results,a unfavorable phenomenon is observed,i.e.,high-temperature-mainstream reversely flows back to the film holes.In addition,the reasons of this phenomenon are revealed,a series of improvements are proposed,and the cooling effectiveness on the target wall is successfully improved(5)Under the real conditions of non-uniform mainstream inlet temperature(hot streak),the performances of film cooling and double wall cooling in the blade leading edge are numerically investigated.The results show that,a local high temperature region can be observed in the leading edge surface caused by hot streak.Compared to the film cooling,double wall cooling structure shows a better cooling ability and a lower thermal stress on the vane surface,but the problem of the extremely high thermal stress within the pin-fins should be carefully considered.更多还原