Stress corrosion is a critical issue that leads to high costs in lost equipment and maintenance, affecting the operation and safety of aircraft platforms. Most aerospace structural components use the aluminum alloys 7xxx series, which contain Al, Cu, Zn, and Mg, due to the combined advantage of its high-strength and lightweight. However, such alloys, specifically AA7075-T4 and AA7075-T651, are susceptible to stress corrosion cracking (SCC) when exposed to both mechanical stresses and corrosive environments. SCC gives rise to a major technological challenge affecting aerospace systems as it leads to the degradation of mechanical properties. In addition, such corrosion presents an important yet complex modeling challenge due to the synergistic action of sustained tensile stresses and an aggressive environment. In light of this, we develop a finite element (FE) multiphysics model to investigate the interplay of mechanical loading and electrochemistry on the stress corrosion of aluminum alloys. The model includes a multiphysics coupling technique through which the kinetics of corrosion can be predicted in the presence of elastic and plastic deformation modes. The presented model provides useful information towards the kinetics of corrosion via tracking localized corrosion and stress distribution. Although the model is general, it has been made considering the characteristics of AA7xxx series, more specifically, taking AA7075.

For over forty years, coatings have been used as a means to prevent the build-up of foulants on the flow passages of centrifugal compressors. In general, this has been a “One Size Fits All” solution, with most OEMs and applicators offering nearly identical coating systems, despite the fact that there are distinct differences in the types of foulant that occurs in different services. To develop new coatings, there are two options, test “on the job” or test in a laboratory setting. This paper looks at various test methods to evaluate coatings by isolating the important aspects that a good fouling resistant coating should possess, namely fouling, erosion, and corrosion resistance. Each of these will be investigated separately in a battery of easy to perform tests. To the extent possible, the tests for each aspect attempt to replicate the complex conditions that occur in a centrifugal compressor that impact that specific aspect. The fouling and corrosion tests were performed with considerations for two different types of service. To illustrate the ability of the test to evaluate various coatings, the results from three different coatings will be discussed. These test methods will also be compared to other test methods used in literature.

Engine operating cost contributes a major share to an aircraft’s direct operating cost. Thus, the knowledge of the current and future state of their engines is a major concern to any airline operator. To be able to schedule shop visits, state-of-the-art diagnostic and prognostic tools including CFD methods are employed. These RANS-based turbulence and transition models are used to predict the overall efficiency and operational behavior of the engine components. Aerofoil surfaces undergo dynamic change during operation and surface roughness increases in complex non-homogeneous ways due to corrosion, erosion, and fouling processes; depending on the engine component and the environmental condition encountered. The influence of real fouling based roughness on the boundary layer transition is investigated experimentally and numerically within this study. For this purpose, the rotor midspan from the second HPC rotor of the CFM56 is used as the basis for experimental and numerical investigations. Realistic fouling based roughness is applied and investigated both in a cascade tunnel and a low speed compressor rig. The results shown here indicate that laminar boundary layers and their transition to turbulence must be included in the RANS model combination used. Furthermore, it is necessary to consider roughness effects in the respective turbulence and transition model. While the consideration of roughness for the turbulence models has already found wide acceptance, the results in this work motivate the additional extension of the transition model to include roughness effects.

U.S. Navy marine gas turbine engines serve as primarye and auxiliary power sources for several current classes of ships. Early observations noted in the 1960s and 1970s revealed severe corrosion attack on the first stage blade and vane components of a shipboard marine gas turbine engine that caused engine failure after only several hundred hours. In gas turbine development, there is always a drive and need to enhance the performance and life of engines. The virtues of using Ni-base superalloys in hot-section components has been well recognized and practiced as a means of substantial increase in turbine-inlet temperature, resulting in improvements in thermal efficiency, durability, and performance of engines. The USN shipboard environment (the marine environment) is high in salt laden air and water, coupled with air and fuel sulfur species that cause aggressive corrosion in gas turbine hot sections. Materials that can function in this environment are considered to be “Marinized”. Higher engine power density and pressure ratios for new engine designs will increase maximum blade, vane, and rotor metal temperatures from a mainly Low Temperature Hot Corrosion (LTHC) regime into both the High Temperature Hot Corrosion (HTHC) and Oxidation Corrosion regions. It is expected that future increased surface combatant loads and operational changes will require increased gas turbine operating temperatures and change the associated operating environment to one where Type I and Type II hot corrosion AND oxidation will be prevalent in newly anticipated operational profiles. The advanced gas turbine upgrade package will include better corrosion and oxidation resistant capability and/or higher temperature capable materials and their associated component overhaul methodologies. New materials need to be created and developed for use in more aggressive environments and higher temperature operations. The main cause of the shorter time between overhauls is the materials deterioration of the engine components associated with the hot section of the engine, e.g. turbine airfoils. The deterioration mechanisms are hot corrosion, with Type 1 hot corrosion mechanism becoming operative at the higher temperatures. The goal of this paper is to evaluate methods to enable running the engine at high power while getting back to the longer mean time between overhauls. The method to achieve the longer time is to evaluate and propose for implementation materials, which can withstand the higher temperatures and at the same time mitigate the operative corrosion mechanisms associated with marine environments.

The demand for flexible operation of stationary gas turbines, especially at part load, requires the simultaneous design for sufficient efficiency and life time. Both can be addressed by the secondary air system. The here applied concept modulates cooling air supply in off-design. Typically, a reduction of cooling air leads to higher efficiency but shorter turbine life time. This paper presents investigations on such concepts, aiming for trade-offs between fuel burn and turbine blade life. The considered life time mechanisms are creep, which is dominant in rotor blades, and oxidation. In addition, the effects on emissions from the combustion are outlined. The reference gas turbine is a literature-based, generic gas turbine in the 300 MWpower output segment. Regarding cooling air control, the focus is on the first two stages of the four-stage turbine. All simulations are performed by application of component zooming with an appropriate in-house tool: a previously introduced coupled model of the reference gas turbine that essentially connects gas turbine performance with a secondary air system network model. This coupled model is now extended with blade life evaluation and emission models. The results contain trade-offs for different operating points at base and part load. For example, the combined cooling air control of stage 1 rotor blade and stage 2 vane offers several benefits regarding fuel consumption: saving up to Δw fuel,rel = 0.5% in the heat recovery’s kink point operation at 60 % of base load of a combined cycle application. This saving is at the expense of creep life. However, some operating points could even operate at higher blade temperatures in order to improve life regarding hot corrosion. Furthermore, generic sensitivities of controlled secondary air supply to cooling layers and hot gas ingestion at rim seals are discussed. Overall, the presented trades mark promising potentials of modulated secondary air system concepts from a technical point of view.

Formation mechanisms for turbine roughness are manifold, including erosion, corrosion, deposition, and spallation or more recently additive manufacturing processes. Consequently, the resulting surfaces differ remarkably not only in roughness shape, height, and density, but also in element thermal conductivity. Because the roughness elements extend into the boundary layer, their temperature distribution has a direct influence on the thermal boundary layer and thus on the resulting convective heat transfer. In the current study, heat transfer distributions along a flat plate with more than 20 deterministic rough surface topographies that differ in element eccentricity, height and density are measured. For each surface roughness, measurements are conducted using two different element thermal conductivities (0.2 W/(mK) and 30 W/(mK)), two pressure distributions, four Reynolds numbers between 3 × 10 5 and 7.5 × 10 5 and various inlet turbulence intensities in the range of 1.5% to 8%. The pressure distributions resemble a typical suction and pressure side, respectively. Results show a heat transfer increase of up to 60% for the high thermal conductivity surfaces and up to 50% for the low conductivity ones. While heat transfer on the high conductivity surfaces is always higher than on the low conductivity ones, the difference becomes smaller with decreasing element density.

Most numerical engineering simulation is performed on pristine, as-designed representations of the components and systems in question. Although the rate of through-life performance degradation is hugely important when considering the total cost/benefit of a system, engineers have had profound difficulties in modelling the physical changes that components undergo in service due to the quasi-random and organic nature of the mechanisms such as wear, corrosion, icing and fouling. Typically, the creation of ‘worn’ models is based on a posteriori inspection or scanning of a failing or failed component. This paper presents a novel method for modifying geometry in response to scalar field variables directly accessed from the embedded physical models within physics-based simulation. It uses a distance field, managed as a Level-set, to drive time-dependent changes to the geometry surface, borrowing heavily from technology which has seen widespread use in the computer graphics industry to create and modify items in a natural organic way. A computational mesh can then be constructed around and within the modified geometry so that the simulation can be performed on the now ‘in-service’ version of the components. This greatly improves the predictive power of such simulations and provide a priori predictions of component performance in response to, for example, corrosive environments. The method is robust, can manage and create meshes for arbitrarily complex geometry, is insensitive to large scale topology changes such as hole blockage or passage burn-through, is highly suitable for automated simulation workflows and can represent both additive (fouling, icing) and reductive (erosion, corrosion, burn though) shape change.

Hot corrosion is defined as the accelerated oxidation/sulphidation in the presence of alkali metal molten salts. It is a form of chemical attack that causes good metal loss. Current lifing models in aircraft engines focus on creep, fatigue and oxidation while hot corrosion damage has been overlooked as being of secondary importance. However, the absence of hot corrosion lifing models for aircraft engines often leads to unexpected and unexplained hot corrosion findings by aircraft engine operators and Maintenance, Repair and Overhaul (MRO) providers during inspections. Although hot corrosion does not cause failure on its own, the interaction with other damage mechanisms can reduce component life significantly, consequently, there is a requirement for including hot corrosion in the damage prediction process of aircraft engines. In both theoretical and experimental studies in literature, deposition of molten salts is identified as one of the primary conditions for hot corrosion to occur and an increased amount of deposited liquid salts accelerates the attack. Currently, most hot corrosion studies are limited to experimental testing of superalloys which are pre-coated with a controlled layer of salts. Such experimental studies are disconnected from gas turbine operating conditions during service. The present paper analyses two deposition rate models applicable to gas turbine operating conditions using Design of Experiments. Design space exploration is presented by taking into account gas turbine operating parameters which vary during a flight as well as temperature ranges where hot corrosion can occur. Analysis of variance is presented for 6 input parameters using Box-Behnken 3-level factorial design. Results from the Analysis of Variance indicate that the deposition rate models are sensitive to pressure and salt concentration in the gas flow. Finally, the saturation point of sodium sulphate has been investigated within the operating range of gas turbine and it was found that it can vary significantly under different conditions.

In the development and assessment of critical gas turbine components, simulations have a crucial role. An accurate life prediction approach is needed to estimate lifespan of these components. Nickel base superalloys remain the material of choice for gas turbine blades in the energy industry. These blades are required to withstand both fatigue and creep at extreme temperatures during their usage time. Nickel-base superalloys present an excellent heat resistance at high temperatures. Presence of chromium in the chemical composition makes these alloys highly resistant to corrosion, which is critical for turbine blades. This study presents a flexible approach to combine creep and fatigue damages for a single crystal Nickel-base superalloy. Stress and strain states are used to compute life calculations, which makes this approach applicable for component level. The cumulative damage approach is utilized in this study, where dominant damage modes are capturing primary microstructural mechanism associated with failure. The total damage is divided into two distinctive modules: fatigue and creep. Flexibility is imparted to the model through its ability to emphasize the dominant damage mechanism which may vary among alloys. Fatigue module is governed by a modified version of Coffin-Manson and Basquin model, which captures the orientation dependence of the candidate material. Additionally, Robinson’s creep rupture model is applied to predict creep damage in this study. A novel crystal visco-plasticity (CVP) model is used to simulate deformation of the alloy under several different types of loading. This model has capability to illustrate the temperature-, rate-, orientation-, and history-dependence of the material. A user defined material (usermat) is created to be used in ANSYS APDL 19.0, where the CVP model is applied by User Programmable Feature (UPF). This deformation model is constructed of a flow rule and internal state variables, where the kinematic hardening phenomena is captured by back stress. Octahedral, cubic and cross slip systems are included to perform simulations in different orientations. An implicit integration process that uses Newton-Raphson iteration scheme is utilized to calculate the desired solutions. Several tensile, low-cycle fatigue (LCF) and creep experiments were conducted to inform modeling parameters for the life prediction and the CVP models.

Siemens Energy has a large fleet of aero-derivative gas turbines. The performance and durability of these power turbines largely depend on the capability of hot section components to resist high-temperature surface attacks and to maintain their mechanical properties. Hot corrosion attack occurs due to exposure of turbine components to sulfur-bearing fuels/air together with other corrosive compounds during turbine operation. This paper investigates the impact of low-temperature hot corrosion on the stress rupture of commonly used gas turbine disk alloys, including Inconel 718, Incoloy 901, and A-286. The results indicate that Inconel 718 and Incoloy 901 maintain their creep strength advantage over A-286 in a low-temperature hot corrosion inducing environment at 1100°F. All three materials exhibited an equivalent life reduction in the corrosive environments at 1100°F. Moreover, the results demonstrate that the stress-rupture life of materials in hot-corrosion environments depends on the combined and cumulative effects of corrosion-resistant and hardening elements.

There is a drive towards a broader range of fuels in industrial gas turbines, with higher levels of sulphur and potentially hydrogen. Due to these harsher environments, there is also a drive for corrosion resistant alloys and coatings. A number of key corrosion resistant superalloys, which are being employed to cope with these evolving conditions, exhibit primary creep. It is therefore imperative that fundamental material models, such as those for creep deformation, are developed to ensure they can accurately predict the material response to evolving operating conditions. The requirements for a creep model are complex. The model must be able to: predict forward creep deformation in regions dominated by primary loads (such as pressure); predict stress relaxation in regions dominated by secondary loads (such as differential thermal expansion); predict the effects of different creep hardening mechanisms. It is also clear that there is an interaction between fatigue and creep. With flexible operation, this interaction can be significant and should be catered for in lifing methods. A model that has the potential to account for the effect of plasticity on creep, and creep on plasticity is therefore desirable. In previous work the authors presented the concept for a backstress model to predict creep strain rates in superalloys. This model was fitted to a limited dataset at a single temperature. The approach was validated using simple creep-dwell tests at the same temperature. This paper expands on the previous work in several ways: 1) The creep model has been fitted over a wide range of temperatures. Including the effect of temperature in complex creep models presents a number of difficulties in model fitting and these are explored. 2) The model was fitted to constant load (forward creep) and constant strain (stress relaxation) tests since any creep model should be able to predict both forms of creep deformation. However, these are often considered separately due to the difficulty of fitting models to two different datasets. 3) The creep deformation model was validated on stress change tests to ensure the creep deformation response can cope with changes in response variables. 4) The approach was validated using creep-fatigue tests to show that the creep deformation model, in addition to our established fatigue models, can predict damage in materials under complex loading.

The evolution of industrial gas turbines towards increased efficiency and performance requires ever higher operating temperatures for the engines. In order to remain competitive in the market, OEM companies continuously need to develop maintenance programs and reconditioning technologies able to extend the life of these components as much as possible. During the operation, turbine blades and vanes are prone to deterioration due to the high temperature of the gas passing along the platform surface, causing hot corrosion and thermomechanical fatigue. In particular, this results in deterioration and crack formation at platform edges. If oxidized, cracked or otherwise deteriorated locations are found during service inspections, the component platform has to be rebuilt. The standard method for rebuilding the platform is to remove the deteriorated material volume and rebuild it by welding. In particular, the most common welding method for rotor blades and stator vanes restoration today is the tungsten inert gas welding (GTAW). However, tungsten inert gas welding has a large heat affected zone, which can later lead to major distortion, due the elevated thermal input, and loss of structural integrity of the reconditioned part. In particular, for single crystal material high thermal input can promote recrystallization issues and this phenomenon compromises the mechanical properties and considerably reduces the life of the component. The most recent reconditioning techniques developed by Ansaldo Energia involves the use of laser metal deposition technology to rebuild the damaged areas and restore the platform to the original shape. Prior to the automated process the area, on which the clad will be laid, requires precise preparation. This paper describes a customized, automatic process to restore the deteriorated blade or vane area by the laser metal deposition technique for equiaxial or single crystal material, minimizing the thermal input and resulting in high resistance rebuilding.

This paper presents the development of an artificial neural network (ANN) Gas Path Diagnostics (GPD) technique applied to pipeline compression system for fault detection and quantification. The work detailed the various degradation mechanisms and the effect of such degradations on the performance of natural gas compressors. The data used in demonstrating the ANN diagnostics is so derived using an advanced thermodynamic performance simulation model of integrated pipeline and compressor systems, which has embedded empirical compressor map data and pipeline resistance model. Implantation of faults within the model is in such a way to account for faults degradations caused by fouling, erosion and corrosion, of various degrees of severities, to obtain wide range of corresponding simulated “true” measurements. In order to account for uncertainties normally encountered in field measurements, Gaussian noise distribution was combined with simulated true measurements, which depends on the instrument’s tolerances. Furthermore, since judicious measurements selection are crucial in ensuring flawless GPD predictions, a sensitivity and correlation analysis of the available measurements revealed that discharge temperature, rotational speed and torque are the most effective measurements for the diagnostics with acceptable degrees of accuracies. The measurements observability technique is a novel approach in pipeline compressor diagnostics. Analytical case studies of the developed method show that, a selected ANN architecture can detect and quantify faults related to degradation in efficiency and flow capacities in the presence of instrument error, varied operational and environmental conditions.

The use of crude oil as a backup fuel in gas turbines remains an economical option for power plants in petroleum-rich countries like Saudi Arabia. However, operating gas turbines with crude oil can corrode their Hot-Gas-Path (HGP) components due to the formation of vanadium oxides and alkali metal vanadates, when preventative measures are not employed. While the pre-combustion treatment of crude oil fuel is effective at reducing the content of sodium (Na) and potassium (K) in the fuel and, therefore, their resulting combustion products, any pre-combustion removal of vanadium has remained unviable on a commercial scale. Typically, a best practice limit of 0.5 ppmw vanadium is imposed on liquid fuels when inhibitors are not employed, and the total alkali content is limited to 1.0 ppmw. To evaluate the effects of incremental exceedances of such limits on the structural materials and coatings of gas turbines, a high-pressure hot-corrosion test rig was built at King Abdullah University of Science and Technology (KAUST). Arabian Extra Light (AXL) crude oil with a vanadium content of up to 2 ppm was chosen as the test fuel. This burner rig was designed to accommodate high temperatures (∼1173 K/ 900 °C) and the combustion of unrefined crude oils at an elevated pressure of up to 15 bar. Highly reduced flow velocities (∼1 m/s) were employed to deconvolute erosional effects from corrosion. This experiment exposed the tested materials to combustion products under these conditions for extended periods (up to 500 hours). The development of the rig included several design iterations and changes to the configuration of various sub-systems, owing to some challenges in maintaining stable combustion at low flow rates, management of coke/ash formation from the burning of crude oil, and adequate control over the pressure and temperature during extended experimental campaigns. The present publication summarizes the design and commissioning of this high-pressure hot-corrosion rig facility, and the successful demonstration of several long duration (400+ hour) experimental runs with AXL crude oil, where the environment of the sample section with test coupons was maintained at 5.5 bar and 1173 K. The pressure and temperature inside the sample section of the rig were continuously monitored. The batch of material coupons was removed from the sample section after the runs and sent for corrosion analysis. Deposits resulting from the AXL combustion and fouling were collected from several sections of the rig for analytical analysis. The fuel wash procedure, the design of components in the burner rig, and best practices of operation were also documented.

The combustion of heavy fuel oil (HFO) in gas turbines (GT) generates some operational constraints and expenses which are too often accepted fatalistically. A first development has addressed the technical challenge caused by the deposition of ash on the hot parts of the turbines. Indeed, the combustion of HFO significantly increases the volume of ash as one must treat the fuel with a “vanadium inhibitor” that acts as an ash modifier preventing hot corrosion by vanadium. This fouling effect is the most serious drawback of HFO operation as it progressively shrinks the performances and reduces the availability of the machines. To tackle this issue, a genuine bimetallic vanadium inhibitor has been developed and field tested step by step between 2015 and 2018. The last step that took place at Yugadanavi in March-April 2018 has allowed validating a ready-to-use version of this bimetallic inhibitor product. Upon the completion of this program, the rate of power degradation during HFO operation has been halved and the GT availability significantly increased while the emission of particulates has been substantially reduced. As a further improvement effort, the teams have tested in the field the effect of changing the temperature of the HFO on its viscosity and monitored the impact induced on the quality of fuel atomization that underlies namely the level of particulate emissions. This second program devoted to the optimization of HFO heating, has enabled defining a rational minimum temperature of the fuel which establishes a fair compromise between atomization effectiveness and thermal energy consumption. This paper summarizes the main outcome of this multi-year collaborative test program.

Design of the combustor is of high priority in microturbine generators (MTG) due to the small and compact configuration of these type of generators and high range of the shaft revolution (normally over 100k rpm). Design process of the MTG components including the micro combustor and turbomachinery also require accurate description of the combustion phenomena, heat transfer, emission level and performance analysis of the system. Design of combustors for renewable fuels such as biogas has several complications including overcoming the lower heating value of the biogas (normally 1/3 of the natural gas), combustion instabilities and corrosion effects of burning these types of fuels. The main benefit of burning a carbon neutral fuel (e.g., biogas), however will be in reducing the carbon emission by avoiding fossil fuels and achieving the environmental targets (e.g., Paris Agreement). The tubular combustors are in the centre of attention in design and operations of the microturbines due to their low cost and the level of emission. This research work presents the design procedure and CFD modelling of a tubular combustor for a biogas burnt microturbine engine assembly. The biogas is generated from anaerobic digestions of agriculture waste and include a 57% and 43% mixture of methane and CO 2 respectively. All the combustor parts are designed with empirical and practical equations and dimensions are optimised by CFD simulations. Operation of the combustor is then analysed in terms of its gaseous emissions. Finally, the operation of the new combustor in a closed heat and power cycle was verified and compared with conventional combustor of the microturbine burning diesel fuel, and as a result all the benefits and considerations for the application of biogas in microturbine assembly are carefully remarked and discussed.

Optimizing safety and efficiencies is vital to turbomachinery design in the aerospace industry. However, aircraft engines subject to ingestion of airborne particle mixtures of unknown size and concentration have undergone unpredictable malfunction and power losses. Premature damage could be caused by increased erosion rates from mixtures with abrasive material. Similarly, corrosion rates could also increase for mixtures with corrosive material. Further, ingestions with melting points below the combustion exit temperature do melt, adhere to turbine blades, and thereby produce clogs and power losses. Additional blockage could be caused by accumulation of material of fluctuating volumes on blades and vanes. Such malfunction can be prevented by providing these engine systems with an on-board sensor capable of defining the particle size and size distribution and determining instantaneous and cumulative ingestion rates. This study demonstrates the methods used in the design of a fiber optic sensor to size the ingested material and measure its distribution and concentration directly in an engine’s flow path. A high-velocity, high-temperature contamination rig has been designed and built for testing the sensor and evaluating its functionality under engine conditions. Durability tests will be conducted to determine sensor lifespan and assess feasibility of incorporation in current turbomachinery for aircraft protection. Contaminant particle distribution and flow patterns in pipe cross section were studied through laser diffraction and light scattering, to enhance understanding of changes in sensor upon particle impingement.

This paper focuses on the development of a structural health monitoring system based on guided Lamb waves propagating over the structure and a network of surface acoustic sensors in communication at high frequencies. A time-of-flight (ToF), algorithm and a probabilistic diagnostic imaging and calibration method is developed to detect miniscule material losses or material adhesion as well as the defects like small scale holes and cracks in turbomachinery components like blades, rotors, plates and pipes. Using an advanced ToF algorithm, precise differences in timescales for arrival of symmetric / antisymmetric lamb wave packets are found for all possible combinations of actuator-sensor pairs. This leads to a deterministic mathematical construct for damage localization for various actuator-sensor pairs at focal points. In the probabilistic diagnostic imaging (PDI) method, field value is assigned based on fusion of wave signals rendered by various actuator-sensor paths to indicate the probability of the presence of a damage at a particular location on the structure. Correlation coefficients between healthy and damaged data for each of the actuator-sensor path is used to calculate the field value for each pixel on the structure. Damage calibration curve is developed by progressively increasing the damage and obtaining a magnitude of the probability density function of the severity of the damage. Proposed approach has been validated using experimental data for multiple damage cases on plates, internal surfaces of pipes and impeller blade to successfully detect submillimeter scale holes and cracks, material adhesion as well as rate of pipeline erosion and corrosion.

The changing energy landscape leads to a rising demand of more flexible power generation. A system for steam turbines warm-keeping provides the ability to shutdown conventional power plants during periods with a high share of renewable power. Simultaneously, these power plants are ready for grid stabilization on demand without an excessive consumption of lifetime during the start-up. One technical solution to keep a steam turbine warm is the use of hot air which is passed through the turbine. In addition, the air supply prevents corrosion during standstill and also enables the pre-warming after maintenance or long outages. This paper investigates the warm-keeping process of an intermediate pressure steam turbine (double shell configuration) through the use of dynamic numerical Finite-Elements (FE) simulations. As a representative test-case, warm-keeping calculations during a weekend shutdown (60h) are conducted to investigate the temperatures, their distribution and gradients within the rotor and the casing. For this purpose an improved numerical calculation model is developed. This detailed 3D FE model (including blades and vanes) uses heat transfer correlations conceived for warm-keeping with low air mass flows in gear mode operation. These analytical correlations take heat radiation, convection and contact heat transfer at the blade roots into account. The thermal boundary conditions at the outer walls of rotor and casing are determined by use of experimental natural cool-down data. The calculation model is finally compared and verified with this data set. The results offer valuable information about the thermal condition of the steam turbine for a subsequent start-up procedure. The warm-keeping operation with air is able to preserve hot start conditions for any time period. Most of the heat is transferred close to the steam inlet of the turbine, which is caused by similar flow directions of air and steam. Thus, temperatures in the last stages and in the casing stay well below material limits. This allows higher temperatures at the first blade groove of the turbine, which are highly loaded during a turbine startup and thus crucial to the lifetime.

High thermal load on the turbine blade tip surface leads to high temperature corrosion and severe structural damage. One common way is to deliver a part of coolant through bleed holes onto the tip portion for cooling purpose. In this study, numerical simulations are performed to investigate the effects of rotation on the internal tip heat transfer in a simplified rotating two-pass channel with a bleed hole, which is applicable to the internal cooling passage of typical gas turbine blade. The bleed hole is placed on the tip wall of a two-pass channel at different locations, i.e. the ratio of distance from the outlet-side wall to width of the tip wall is 0.07, 0.21, 0.5, 0.78, 0.93, respectively. A smooth channel without bleed hole is used as Baseline. The Reynolds number is fixed at 10,000. The Ro numbers are varied from 0 to 0.4. Results show that a three-dimensional vortex, which is induced by the Coriolis force, is found at the bend region. It transports the fluid from the trailing side to leading side, which is beneficial to enhance tip heat transfer. The middle-mounted hole shows a better heat transfer augmentation compared to other hole arrangement. The rotation have a notable effect on the heat transfer and flow structures. Compared to the smooth channel, the heat transfer augmentation is about 34%.