АННОТАЦИЯ T. MYKHAILENKO, D. NEMCHENKO, DOUAISSIA OMAR HADJ AISSA, I. PETUKHOV APPROACHES TO THE SIMULATION OF THERMAL HYDRAULIC PROCESSES IN THE OIL SYSTEM ELEMENTS OF GAS TURBINE ENGINE ABSTRACT Design of the oil-system for gas-turbine engines requires the calculation of the exact fuel consumption and pressure losses in the pipelines, the heat exchange between the oil and the lubrication system elements, critical modes of the flow in the pipelines and local resistances. A development of the model of oil system that will enable the measurement of the flow rate and other parameters at any place inside the oil system, the numerical investigation of the influence of structural improvements taking into account these parameters is a rather promising outlook. A specific feature of the oil system of gas turbine engine is that the oil and air mixture is passing instead of single-phase liquid (oil) actually in each element of it, which affects the behavior of thermohydraulic processes in these elements. This scientific paper gives consideration to the peculiarities of the two-phase flow of oil-gas mixture, heat-mass exchange processes that occur in the oil cavity of the rotor rack of gas turbine engine and prevalent approaches to their analysis. Consideration was also given to specific features of

The complex interrelation of thermal and hydraulic processes in a gas turbine engine bearing chamber requires modelling methods based on the multiphase flow mechanics and Computational Fluid Dynamics (CFD) to predict the fluid distribution and heat transfer phenomena. This paper presents a study of different approaches to CFD modelling of multiphase oil-air flow in the bearing chamber. The Volume of Fluid and Eulerian multiphase models, Steady and Transient solvers, “Realizable k-ε” and “k-ω SST” turbulence models were analysed. The Eulerian Wall Film Model implemented in ANSYS Fluent was applied to model an oil film formation on the bearing chamber walls. The CFD results were compared with available experimental data to formulate practical recommendations for precise modelling of processes in the bearing chamber.

The heat transfer coefficient (HTC) is one of the key parameters that should be known at the stage of the bearing chamber design. This ensures safe temperature conditions for the lubrication oil and reliable operation of the gas turbine engine. The temperature gradient method is commonly used in experimental practice to determinate the HTC. The accuracy of the HTC determination is sensitive to changing of the bearing chamber operating conditions and should be analyzed at the stage of experimental studies planning. This paper presents a study on the accuracy of HTC determination when the external cooling of the bearing chamber is used to obtain the temperature difference sufficient for measurement. Three ways to reduce the relative error of the HTC determination in the bearing chamber were analyzed: i) decreasing the temperature measurement error; ii) decreasing the temperature of external cooling medium; iii) increasing the external heat transfer coefficient and contribution of wall thermal resistance optimization. For different operating conditions of the bearing chamber, the temperature of the outer wall that ensures the specified accuracy of the experimental HTC and the required parameters of the cooling medium were determined and recommended for practical implementation.

The gas-liquid flow in the bearing chamber (BC) of the gas turbine engine is realized due to the interaction of the sealing air and oil supplied for lubrication and cooling of friction units. The complex nature of the flow movement is determined not only by the BC geometry but also by the presence of rotating elements and the manner of oil supply and flow removal. The most important result for the engineering practice of BC flow modelling is the determination of the heat transfer coefficient to the inner wall. The variety of influencing factors causes difficulties even at the stage of an integral mathematical model of the process formation, which makes it possible to determine this coefficient. As a result, significantly different approaches are used – from three-dimensional CFD modelling of a heterogeneous flow up to the use of a criterion equation that formally considers the effect of geometric and regime parameters on a heat transfer. The first approach requires significant computational resources, and certain difficulties arise in setting the initial and boundary conditions, especially in terms of droplet parameters. Homogeneous models somewhat simplify the problem, including when formulating the boundary conditions. However, all the effects of the influence of the droplet diameter are levelled. In both cases, the computation time is long, and the results of CFD simulations require selective experimental verification. Therefore, it is problematic to use this approach solely for engineering problems, as well as when generalizing experimental data in several regime parameters. When using the criterion equation, the calculation procedures are as simple as possible. However, dimensionless complexes do not ensue from the basic equations of heterogeneous media mechanics and do not consider the effects of interfacial interaction during heat transfer in the BC. Therefore, the possibility of the application of the proposed correlations for other BC geometries and oil and air supplying conditions also needs to be experimentally confirmed. This makes it problematic to use such correlations at the design stage of the BC and the oil system as a whole. The preferred approach for engineering practice is when the thermohydraulic processes in the BC are described on the basis of the proven equations of heterogeneous media mechanics with the transition to a two-dimensional problem by averaging the phase parameters along the axis. This averaging is justified by the fact that the main heat carrier from the core to the inner wall of the BC is the radial flow of droplets. In view of the low volume fraction of droplets, the Lagrange approach is used to simulate a two-phase flow in the BC core. The droplet parameters along the trajectory are calculated considering the interfacial interaction with the air. In this regard, the air velocity field is determined by considering the geometry of the BC, the flow through the seals and the shaft speed. Here, it is possible to consider not only the droplet polydispersity but also the effects of primary droplet reflection, the formation, and movement of secondary droplets during the formation of a near-wall oil film, the thermal resistance of which directly affects the value of the internal heat transfer coefficient.

Обґрунтовано вибір структури математичної моделі теплогідравлічних процесів в масляних порожнинах опор ротора ГТД. Сформована тривимірна CFD-модель для розрахунку багатофазних течій з використанням інформації про потокорозподіл і теплообмін, що представлені в науковій літературі. Розглянуті підходи і окремі моделі, що використовуються для даних цілей. Отримані рішення узгоджуються з результатами експерименту на модельній опорі і загальноприйнятими уявленнями про процеси в пристроях цьому класу. Приведені розподіл масла в камері, лінії течії фаз, поля температури і швидкості, а також вектори швидкості для різних CFD-моделей (VOF, Euler, Inhomogeneous) і типів вирішувача (стаціонарний і нестаціонарний). На основі аналізу отриманих результатів встановлено, що модель Euler з використанням нестаціонарного вирішувача дає найменшу розбіжність з експериментальними значеннями коефіцієнта тепловіддачі. В усіх випадках при врахуванні сили тяжіння має місце асиметричний розподіл масляної плівки. В результаті змінюється термічний опір пограничного шару і, отже, коефіцієнт тепловіддачі по окружності камери підшипника. Це в значній мірі визначає тепловий потік через стінку камери. Запропонований метод моделювання робочого процесу в масляній порожнині опори базується на математичному описанні гетерогенного монодисперсного масляно-повітряного потоку з умовою інверсії структури двофазного потоку в пристінній області з краплинної у бульбашкову. Це дозволяє більш точно розраховувати температурний стан елементів опори ротора ГТД і системи забезпечення працездатності підшипника шляхом коректного визначення коефіцієнта тепловіддачі з боку масляно-повітряної суміші. Розроблена модель дає можливість чисельно досліджувати дієздатність відомих і отримати нові кореляційні залежності для середнього значення коефіцієнта тепловіддачі в масляній порожнині опори ротора, використовуваного при інженерних розрахунках. Також дозволяє чисельно досліджувати вплив геометрії, частоти обертання ротора і витрат фаз на тепловіддачу в масляній порожниниКлючовi слова: чисельне моделювання, багатофазні потоки, масляна порожнина, масло-повітряна суміш, коефіцієнт тепловіддачі UDC 629.7.036.3