Direct Numerical Simulation on the Unsteady Flow Field of Low-Pressure Turbine Cascades With and Without Upstream Wake at Low Reynolds Number

The spectral/hp element method is a high fidelity method that has good numerical dispersion-diffusion characteristics and is flexible and applicable to quasi-three-dimensional aerodynamic problems with complex geometric configurations in the streamline direction and the pitch direction. This paper uses this method to directly solve the incompressible Navier-Stokes equations, and analyzes the aerodynamic performance of the T106A low-pressure turbine cascade at low Reynolds number. Two different conditions, i.e. uniform inlet flow and cylinder’s wake flow, are adopted and their basic characteristics of flow separation and transition are quantitatively analyzed and compared, by observing the distribution of cascade wall surface pressure and friction coefficient, the distribution of wake profile pressure loss and the evolution characteristics of boundary layer flow structure. The numerical results show that the spectral/hp element method can accurately predict the flow separation and transition performance of low-pressure turbine cascades, which implies that it can be used as a high-fidelity simulation and calculation tool for the optimal design of this type of cascade. It is also found that cylinder’s wake can effectively inhibit boundary layer separation of the T106A LPT blade and improve aerodynamic performance and efficiency of it.

Improving the methodology for calculation of energy losses in axial turbine cascades

Представлена оценка неопределенности прогнозирования потерь энергии в решетках профилей осевых турбин. В сравнении с экспериментальными данными рассмотрены эмпирическая модель ЦИАМ и метод CFD анализа в рамках RANS модели. Геометрические и режимные параметры решеток профилей варьируются в широком диапазоне. Результаты CFD расчета отличаются существенно в зависимости от модели турбулентности. Наименьшая неопределенность получена для модели рейнольдсовых напряжений RSM. Определено выборочное стандартное относительное отклонение для анализируемой базы данных. Применительно к CFD расчету данное отклонение составило 18,6%, применительно к эмпирической модели ЦИАМ 46,4%. Разработана эмпирическая модель коррекции потерь полученных по результатам CFD анализа с моделью турбулентности RSM. Корректирующая функция включает в себя геометрические и режимные параметры решеток и особенности течения в межлопаточном канале (всего 14 параметров). Использование разработанного подхода позволило снизить неопределённость прогнозирования потерь в 2 раза. В результате работы выборочное стандартное относительное отклонение предсказания потерь для рассматриваемой базы решеток профилей составило 9,3%. Estimation of the uncertainty in predicting profile losses using various models was performed. In comparison with the experimental data, empirical model of CIAM and method of CFD analysis are considered. RANS models are used. The geometric and operating parameters of the analyzed turbine cascades vary over a wide range. Turbulence models strongly influence loss prediction uncertainty. The smallest uncertainty was obtained using the RSM turbulence model. The sample standard deviation for the considered turbine cascades base was determined. The deviation for CFD analysis is 18.6%. For the empirical model of CIAM the deviation is 46.4%. The new empirical model has been created to correct the results of calculating losses according to the RANS model using the RSM turbulence model. The corrective function takes into account the influence of the geometric and operating parameters of the turbine cascades and the features of the airfoil flow (14 parameters in total). The developed approach allows reducing the uncertainty in the estimation of losses according to the RANS model by 2 times. As a result, the sample standard deviation in the prediction of losses is 9.3% for the considered turbine cascades base.

Critical and Choking Mach Numbers for Organic Vapor Flows Through Turbine Cascades

Results are presented of a theoretical and experimental study dealing with critical and choking Mach numbers of organic vapor flows through turbine cascades. A correlation was derived for predicting choking Mach numbers for organic vapor flows using an asymptotic series expansion valid for isentropic exponents close to unity. The theoretical prediction was tested employing a linear turbine cascade and a circular cylinder in a closed-loop organic vapor wind tunnel. The cascade was based on a classical transonic turbine airfoil for which perfect gas literature data were available. The cascade was manufactured by Selective Laser Melting (SLM), and a comparable low surface roughness level was established by subsequent surface finishing. Because the return of the closed-loop wind tunnel was equipped with an independent mass flow sensor and the test facility enabled stable long-term operation behavior, it was possible to obtain the choking Mach number with high accuracy. It was observed that non-perfect gas dynamics affect the critical Mach number locally, but the observed choking behavior of the turbine cascade was in good agreement with the asymptotic result for the considered dilute gas flow regime.

A Discrete Adjoint Method for Two-Phase Condensing Flows Applied to the Shape Optimization of Turbine Cascades

This paper presents a fully turbulent two-phase discrete adjoint method for metastable condensing flows targeted to turbomachinery applications. The method is based on a duality preserving algorithm and implemented in the open-source CFD tool SU2. The optimization framework is applied to the shape optimization of two canonical steam turbine cascades, commonly referred to as White cascade and Dykas cascade. The optimization were carried out by minimizing either the liquid volume fraction downstream of the cascade or the total entropy generation due viscous effects and heat transfer. In the first case, the amount of condensate turned out to be reduced by as much as 24%, but without reduction of the generated entropy, while the opposite resulted in the second case. The outcomes demonstrate the capability and computational efficiency of adjoint-based automated design for the shape optimization of turbomachinery operating with phase change flow.

Loss classification and review of secondary flow models in gas turbine cascades

Much attention is paid to increasing the efficiency of turbofan engines by increasing the efficiency of the main modules. The aerodynamic efficiency of a turbine depends on the level of total pressure and kinetic energy losses, which are determined by the scale of secondary flows in the channels of the turbine cascades. There are many studies and articles on the topic of secondary flows, in which vortex structures are often given incorrect names. The problem lies in the absence of a unified model of secondary flows and mismatch in the names of the components of secondary flows in adaptation of model descriptions from English to Russian. The purpose of this review article is to consider the existing classifications of losses and the most famous models of secondary flows in turbine cascades, including the Wang model, the Goldstein and Spores model, the Sharma and Butler model, etc. The considered sources of information made it possible to single out the most complete classification of losses, compare with each other the components of secondary flows of various models, describe the mechanism of their occurrence and give the most complete nomenclature of secondary flows in turbine cascades.