Effect of Outer Fin Axial Gap on the Sealing Effectiveness and Fluid Dynamics of Radial Rim Seal

The modern gas turbine is widely applied in the aviation propulsion and power generation. The rim seal is usually designed at the periphery of the wheel-space and prevented the hot gas ingestion in modern gas turbines. The high sealing effectiveness of rim seal can improve the aerodynamic performance of gas turbines and avoid of the disc overheating. Effect of outer fin axial gap of radial rim seal on the sealing effectiveness and fluid dynamics was numerically investigated in this work. The sealing effectiveness and fluid dynamics of radial rim seal with three different outer fin axial gaps was conducted at different coolant flow rates using three-dimensional Reynolds-Averaged Navier-Stokes (RANS) and SST turbulent model solutions. The accuracy of the presented numerical approach for the prediction of the sealing performance of the turbine rim seal was demonstrated. The obtained results show that the sealing effectiveness of radial rim seal increases with increase of coolant flow rate at the fixed axial outer fin gap. The sealing effectiveness increases with decrease of the axial outer fin gap at the fixed coolant flow rate. Furthermore, at the fixed coolant flow rate, the hot gas ingestion increases with the increase of the axial outer fin gap. This flow behavior intensifies the interaction between the hot gas and coolant flow at the clearance of radial rim seal. The preswirl coefficient in the wheel-space cavity is also illustrated to analyze the flow dynamics of radial rim seal at different axial outer fin gaps.

CFD Leakage Predictions of Unworn and Worn Labyrinth Seals With and Without Tooth Axial Offset

The objective of this study is to develop a Computational Fluid Dynamics (CFD) based methodology for analyzing and predicting leakage of worn or rub-intended labyrinth seals during operation. The simulations include intended tooth axial offset and numerical modeling of the flow field. The purpose is to predict total leakage through the seal when an axial tooth offset is provided after the intended/unintended rub. Results indicate that as expected, the leakage for the in-line worn land case (i.e. tooth under rub) is higher compared to unworn. Furthermore, the intended rotor/teeth forward axial offset/shift with respect to the rubbed land reduces the seal leakage. The overall leakage of a rubbed seal with axial tooth offset is observed to be considerably reduced, and it can become even less than a small clearance seal designed not to rub. The reduced leakage during steady state is due to a targeted smaller running gap because of tooth offset under the intended/worn land groove shape, higher blockages, higher turbulence and flow deflection as compared to worn seal model without axial tooth offset.

Experimental and Computational Investigation of Rayleigh-Bénard Flow in the Rotating Cavities of a Core Compressor

This paper presents new experimental measurements, at conditions representative of an aero engine, of heat transfer from the inner peripheral surface (shroud) of a rotating cavity. The results are taken from the University of Sussex Multiple Cavity Rig, which is designed to be similar to a gas turbine high pressure compressor internal air system. The shroud Nusselt numbers are shown to be dependent on the shroud Grashof number and insensitive to throughflow axial Reynolds number. The magnitude of the shroud Nusselt numbers are consistent with accepted correlations for turbulent free convection from a horizontal plate, yet show a trend (gradient of Nusselt to Grashof numbers) that is similar to laminar free convection. A supporting high-resolution 3D unsteady RANS simulation was conducted to investigate the cavity flow structure with particular attention paid to the near shroud region. This demonstrated flow structures that are consistent with published work but also show the existence of a type of Rayleigh-Bénard flow that manifests as a series of streaks that propagate along the periphery of the cavity. These structures can be found in the literature albeit in different circumstances. Whilst these streaks have been shown in the simulation their existence cannot be ratified without experimental confirmation.

A New Heat Transfer Correlation for Supercritical RP-3 Flowing in Vertical Tubes

A new empirical correlation for upward flowing supercritical aviation kerosene RP-3 in the vertical tubes is proposed. In order to obtain the database, numerical simulation with a four-component surrogate model on RP-3 and LS low Reynolds turbulence model in vertical circular tube has been performed. Tubes of diameter 2mm to 10mm are studied and operating conditions cover pressure from 3MPa to 6MPa. Heat flux is 500KW/m2, mass flow rate is 700kg/(m2·s). The numerical results on wall temperature distribution under various conditions are compared with experimental data and a good agreement is achieved. The existing correlations are summarized and classified into three categories. Three representative correlations of each category are selected out to evaluate the applicability in heat transfer of supercritical RP-3. The result shows that correlations concluded from water and carbon-dioxide do not perform well in predicting heat transfer of hydrocarbon fuel. The mean absolute deviation of them is up to 20% and predict about 80% of the entire database within 30% error bands. So a new correlation which is applicable to different working conditions for supercritical RP-3 is put forward. Gnielinski type has been adapted as the basis of the new correlation for its higher accuracy. In consideration of major influence factors of supercritical heat transfer, correction terms of density and buoyancy effect are added in. The new correlation has a MAD of 9.26%, predicting 90.6% of the entire database within ±15% error bands. The comparisons validate the applicability of the new correlation.

Numerical Characterization of Flow and Heat Transfer in Pre-Swirl Systems

This paper deals with a numerical study aimed at the validation of a computational procedure for the aerothermal characterization of pre-swirl systems employed in axial gas turbines. The numerical campaign focused on an experimental facility which models the flow field inside a direct-flow pre-swirl system. Steady and unsteady simulation techniques were adopted in conjunction with both a standard two-equations RANS/URANS modelling and more advanced approaches such as the Scale-Adaptive-Simulation principle, the SBES and LES. The comparisons between CFD and experiments were done in terms of swirl number development, static and total pressure distributions, receiving holes discharge coefficient and heat transfer on the rotor disc surface. Several operating conditions were accounted for, spanning 0.78·106<Reφ<1.21·106 and 0.123<λt<0.376. Overall the steady-state CFD predictions are in good agreement with the experimental evidences even though it is not able to confidently mimic the experimental swirl and pressure behaviour in some regions. Although the use of unsteady sliding mesh and direct turbulence modelling, would in principle increase the insight in the physical phenomenon, from a design perspective the tradeoff between accuracy and computational costs is not always favourable.

Investigation of Strip Seal Leakage With Special Focus on Seal Groove Design and Relative Displacement of Sealing Surfaces

Tight sealing lines are vital in large gas turbines (GT) to achieve high performance and efficiency. Leakage including rim purge air can sum up to 30% of the total cooling and leakage air consumption of a gas turbine. Leakage through static strip seals contributes about 1/3 to all leakage air. Considering the seal design as on drawings, sealing quality is generally influenced by the seal type, sealing groove curvature and the sealing groove roughness. In addition the sealing quality depends strongly on the geometric deviation of the groove compared to ideal design. This is caused by manufacturing deviations or relative movements of the grooves during operation of the parts containing the sealing. In the article at hand, different seal designs and pertinent sealing quality is discussed. More in detail, it is discussed the geometric relation of seal, groove and misalignment to predict the seal position relative to its groove confinements. The risk of seal clamping can be judged and adaptation of seal or groove geometry can be derived. The effect of leakage increase due to misalignment is investigated by a test matrix varying seal length and curvature radius of groove as well as radial misalignment.

Evaluation of Numerical Methods to Predict Temperature Distributions of an Experimentally Investigated Convection-Cooled Gas-Turbine Blade

This paper presents two different numerical methods to predict the thermal load of a convection-cooled gas-turbine blade under realistic operating temperature conditions. The subject of the investigation is a gas-turbine rotor blade equipped with an academic convection-cooling system and investigated at a cascade test-rig. It consists of three cooling channels, which are connected outside the blade, so allowing cooling air temperature measurements. Both methods use FE models to obtain the temperature distribution of the solid blade. The difference between these methods lies in the generation of the heat transfer coefficients along the cooling channel walls which serve as a boundary condition for the FE model. One method, referred to as the FEM1D method, uses empirical one-dimensional correlations known from the available literature. The other method, the FEM2D method, uses three-dimensional CFD simulations to obtain two-dimensional heat transfer coefficient distributions. The numerical results are compared to each other as well as to experimental data, so that the benefits and limitations of each method can be shown and validated. Overall, this paper provides an evaluation of the different methods which are used to predict temperature distributions in convection-cooled gas-turbines with regard to accuracy, numerical cost and the limitations of each method. The temperature profiles obtained in all methods generally show good agreement with the experiments. However, the more detailed methods produce more accurate results by causing higher numerical costs.

Preliminary CFD Simulations of Lubrication and Heat Transfer in a Gearbox

When designing a gearbox it is important to consider the heat rise generated inside the gearbox due to the gear meshing action of gear teeth. Providing efficient lubrication helps keep the gearbox at lower temperatures and reduce friction, which in return leads to a longer lifespan. Given the difficulty in obtaining experimental data within the gearbox, the authors investigate and present the setup and methods using Computational Fluid Dynamics (CFD) modelling of the process. The main purpose of this work is to implement and demonstrate numerical techniques that are needed in order to perform CFD simulations on this subject. There are currently no widely used techniques known to the authors that would allow to carry out parametric CFD study of gearbox lubrication and cooling. There are only limited empirical models that are used to find a best design. When developed, CFD methods may allow to do parametric studies and therefore significantly improve the quality of the gearbox design. In order to capture the fluid behaviour in a continuously changing topology around rotating gears, dynamic mesh technique with remeshing and smoothing is used. Dynamic mesh is a complex and expensive technique on its own; and becomes even more so when have to be implemented along with the two-phase flow and conjugated heat transfer. For that reason the development and implementation of this method requires an incremental approach with very gradual increase of difficulty and separation of the large task into small ones, which essentially what has been done in this work. Furthermore, investigation of how to reduce the cost of the simulation is an important part so that the method can then be used more widely. Two types of lubrication are considered: partial dipping into oil (rotational submersion) and jet spraying. Rotational speeds of up to 8,000rpm are studied. Temperature of the gears and the surrounding fluids are initially defined as uniform. Additional heat sources are created in the solid cells of the gears where the teeth come into contact, also using a UDF. 2.5D dynamic remeshing is used for models with spur gears, whereas full 3D remeshing is used with helical gears. Simulations are performed using the Volume of Fluid method and the standard k-omega turbulence model. Simulations are run with varying degrees of complexity (low- and high-fidelity). Some results of basic preliminary simulations are compared with available results from the literature, demonstrating a good agreement. Validation of the results demonstrate the ability of the presented methods to accurately predict the gear losses and the fluid flow in a gearbox. More complex simulations are run in order to observe and analyse both the fluid flow and the heat distribution in the gearbox. Main attention is given to the temperatures of the housing and the meshing teeth. Since all simulations with meshing gears require a small gap between the gears (i.e. with no direct contact of the gears), three different gap sizes are investigated. For these simulations a comparison of the oil flow is provided. This comparison is used to justify which model can be used most efficiently without significant loss of accuracy when modelling the temperature distribution at the housing. Current work is an essential first step towards the detailed study that is currently of great interest of both research and industry. Future work is necessary to fully justify the methods, however the current work is essential and will hopefully provide an inspiration and encouraging of the topic advancement.

Unsteady Pressure Characteristics in the Mainstream/Disc Cavity of a Turbine-Stage

The interaction between the mainstream and disc cavity purge flows in a turbine stage is an unsteady 360° phenomenon. Most of the current rotating rigs have used steady pressure transducers to measure the mainstream annulus pressure distributions as well as the pressure distribution in the disc cavity. Unsteady static pressure measurements in these regions using fast-response transducers have also been reported but to a much lesser degree, mainly at ASU, OSU, VKI, and ETH. To gain better insight into the prevailing unsteady flow phenomena, and to assess the difference between steady and time-averaged unsteady pressure data, new unsteady static pressure measurements were recently carried out at three locations in an ASU-Honeywell turbine stage, namely, in the main gas path on the outer shroud near vane trailing edge as well as on the vane platform lip, and on the stator surface rim seal. They are reported in this paper along with the comparative results of the corresponding URANS CFD simulation reported in an earlier publication. Experiments were carried out at five different purge air flow conditions for each of the two mainstream air flow rate and rotor speed combinations. The current unsteady measurements indicate that the rim cavity pressure frequency is governed by the blade passage frequency. The unsteadiness amplitude increases with purge flow in the main gas path, but decreases with increase in purge flow for the rim cavity where the sensitivity to change in purge flow is smaller at the lower mainstream flow rate. The difference in the ambient-corrected time-averaged static pressures between those evaluated from the current unsteady measurements and the previously published steady measurements are found to be within the measurement uncertainties.

Experimental and Analytical Assessment of Cavity Modes in a Gas Turbine Wheelspace

Fast response pressure data acquired in a high-speed 1.5-stage turbine Hot Gas Ingestion Rig shows the existence of pressure oscillation modes in the rim-seal-wheelspace cavity of a high pressure gas turbine stage with purge flow. The experimental results and observations are complemented by computational assessments of pressure oscillation modes associated with the flow in canonical cavity configurations. The cavity modes identified include shallow cavity modes and Helmholtz resonance. The response of the cavity modes to variation in design and operating parameters are assessed. These parameters include cavity aspect ratio, purge flow ratio, and flow direction defined by the ratio of primary tangential to axial velocity. Scaling the cavity modal response based on computational results and available experimental data in terms of the appropriate reduced frequencies appears to indicate the potential presence of a deep cavity mode as well. While the role of cavity modes on hot gas ingestion cannot be clarified based on the current set of data, the unsteady pressure field associated with turbine rim cavity modal response can be expected to drive ingress/egress.