Alstom (Switzerland)

Study on development potential of

The author declares that there is no conflict of interest.

Steam turbine blades are highly subjected to water droplet erosion (WDE) caused by high energy impingement of liquid water droplets. However, most of the published research on this wear phenomenon is performed on laboratory test rigs, instead of addressing WDE of actual steam turbine blades. In this work, the progression of erosion on the surface of ex-service low pressure steam turbine blades was investigated using scanning electron microscopy. The erosion appearance and mechanisms are compared with laboratory test rig results that are carried out using a rotating disk rig according to ASTM G73 standard. Initial and advanced erosion stages could be observed on the steam turbine blades. Similar to the WDE rig coupons, initial pits and cracks were preceded by blade surface roughening through the formation of asperities and depressions. In addition, it was also observed that the twist angle of the turbine blade around its diagonal, is an important parameter that influences its WDE. Twist angle has an effect on: impact angle, erosion appearance, impact speed, and the affected area. Furthermore, according to the current experimental results, multi-ray rig erosion test results are considered the closest simulation to the actual ex-service blade in terms of damage appearance.

In heavy duty gas turbines, the loading boundary conditions of MCrAlY systems are differently weighted for different operation regimes as well as for each turbine component or even in individual part locations. For an overall optimized component protection it is therefore of interest to produce coatings with flexible and individually tailored properties. In this context, ALSTOM developed an Advanced Modular Coating Technology (AMCOTEC™), which is based on several powder constituents, each providing specific properties to the final coating, in combination with a new application method, allowing in-situ compositional changes. With this approach, coating properties, such as oxidation, corrosion, and cyclic lifetime, etc., can be modularly adjusted for individual component types and areas. For demonstration purpose, a MCrAlY coating with modular ductility increase was produced using the AMCOTEC™ methodology. The method was proven to be cost effective and a highly flexible solution, enabling fast compositional screening. A calculation method for final coating composition was defined and validated. The modular addition of ductility agent enabled increasing the coating ductility with up to factor 3 with only slight decrease of oxidation resistance. An optimum composition with respect to ductility is reached with addition of 20 wt.% of ductility agent.

The continuous monitoring of gas turbines in commercial power plant operation provides long-term engine data of field units. Evaluation of the engine performance is challenging as, apart from variations of operating points and environmental conditions, the state of the engine is subject to changes due to the ageing of engine components. The measurement devices applied to the unit influence the analysis by means of their accuracy, which may itself alter with time. Furthermore, the available measurements do usually not cover all necessary information for the evaluation of the engine performance. To overcome these issues, this paper describes a method to systematically evaluate long term operation data without the incorporation of engine design models since the latter do not cover performance changes when components are ageing. Key focus of the methodology thereby is to assess long-term emission performance in the most reliable manner. The analysis applies a data reconciliation method to long-term operating data in order to model the engine performance including non-measured variables and to account for measurement inaccuracies. This procedure relies on redundancies in the data set due to available measurements and the identification of suitable additional constituting equations that are independent of component ageing. The resulting over-determined set of equations allows for performing a data set optimization with respect to a minimal cumulated deviation to the measurement values, which represents the most probable, real state of the engine. The paper illustrates the development and application of the method to analyse the gas path of a commercial gas turbine in a combined cycle power plant with long-term operating data.

Natural gas is the primary fuel for stationary, powergeneration gas turbines, and it is necessary to understand its combustion characteristics under engine-relevant (turbulent) conditions. Since its composition varies depending on the fuel source, a natural gas surrogate (NG 18% C2+) and admixtures with H2 have been utilized recently by the authors to aid chemical kinetics modeling using ignition delay times and laminar flame speed experiments. The present study focused on measuring turbulent flame speeds (displacement speeds) of natural gas (NG2) and methane with H2 using a fan-stirred flame bomb. The apparatus is a closed, cylindrical chamber fitted with four radial impellers that generate a central spherical volume of homogeneous and isotropic turbulence with negligible mean flow. Schlieren imaging was used to visually track the growth of the spherically expanding turbulent kernels during the constant-pressure period. The turbulence levels were fixed at an average RMS intensity level of 1.5 m/s and at an integral length scale of 27 mm. Turbulent flame speeds (ST,0.1) of NG2 blends were measured over a wide range of equivalence ratios between 0.7 and 1.3. ST,0.1 for the natural gas surrogate closely matched with those of methane for near-stoichiometric mixtures. However, preferential-diffusion effects (fuel effects) were observed under turbulent conditions for off-stoichiometric cases. The effects of hydrogen addition on the turbulent flame speeds of NG2 (25/75 and 50/50 (by volume) blends of H2/NG2) were also investigated and were compared with the flame speeds reported in a recent paper by the authors (ASME GT2014-26742) on the effects of hydrogen addition to turbulent flame speeds of methane. The effect of the hydrogen addition was to increase the turbulent flame speed (by about a factor of two for 50% H2 addition), although this effect was much more pronounced for the lean and stoichiometric mixtures. Interestingly, the flame speeds (both laminar and turbulent) of the CH4 blends with H2 were slightly larger than those for the NG2 blend at equivalent conditions, or about 10–20% larger at 50% H2 addition. This behavior can be explained kinetically by the increased importance of the inhibiting reaction CH3 + H (+M) ↔ CH4 (+M), where ethane oxidation produces more CH3 radicals than methane at similar conditions.

The mechanical integrity design of front stages for heavy duty gas turbines is by definition a challenging task. Several design criteria need to be fulfilled, reaching from static stress requirements over sufficient cyclic lifetime to a defined dynamic behavior of the component. This design task becomes even more challenging in case the size of the engine increases. To satisfy the need for designing larger front stage blades, an automated and integrated design tool suite for Mechanical Integrity analysis tasks has been developed. This paper introduces the specific steps of the mechanical assessment in the early concept phase of new designs. The overall process relies on advanced numerical methods, namely Design of Experiments, numerical optimization and probabilistic techniques, which are combined into a single package. The delineated procedure represents an incremental approach, which contrasts to the integrated and partially multi-disciplinary optimization processes which are typically described in literature. The benefits of the incremental method can be identified with regard to computational time, enhancement of interfaces and feedback loops with neighboring disciplines. By applying the process outlined in this paper, a new generation of frontstage blades could be developed with further improved mechanical properties and reduced development efforts.

In the field of lightweight construction for transportation means, hybrid structures composed of high‐strength and low‐density materials exhibit a high potential for application. The composite extrusion process allows an easy embedding of metallic reinforcements into a multitude of light metal alloys. The current work shows that different metallic wire reinforced light alloys with a content of 11.1 vol% lead to a significant increase in lifetime of different aluminum and magnesium alloys under fully reversed stress controlled fatigue loading. Based on the knowledge of the quasi‐static behavior of the single components and the fatigue behavior of the matrix material, a new lifetime model is used to predict the lifetime for different unidirectionally reinforced material systems.

The fundamentals and thermodynamic analysis of High Temperature Air Combustion (HiTAC) technology is presented with focus on industrial furnaces as they are amongst the major energy users. The HiTAC is characterized by high temperature of combustion air having low oxygen concentration. This study provides a theoretical analysis of HiTAC a process from the thermodynamic point of view. The results demonstrate the possibilities of reducing thermodynamic irreversibility of combustion by considering an oxygen-deficient combustion process that utilizes both gas- and heat-recirculation. Furthermore, combustion with the use of oxygen (in place of air) is also analyzed. The results showed that a system which utilizes oxygen as an oxidizer results in higher 1st and 2nd law efficiencies as compared to the case with air as the oxidizer. This study is aimed at providing technical guidance to further improve efficiency of a combustion process which show very small temperature increases due to mild chemical reactions. The significant of these findings are now widely used in industrial furnaces with singular successes on energy savings, pollution reduction and reduced size of the equipment. The exergy analysis too can be used as a technical tool to improve efficiency in combustion processes.

A methodology is presented in this paper how to assess and to improve burner aerodynamic stability of a premixed swirl burner by means of CFD. Steady-state RANS has been widely used for complex industrial applications for decades because of its good reproducibility of mean flow and acceptable turn-around time. With affordability of high performance cluster it becomes feasible to extend steady-state RANS to unsteady-state LES for industrial applications. It is especially beneficial for swirl burners to assess burner stability directly because swirling flow is in fact an unsteady-state phenomenon, which RANS cannot fully capture. This paper shows an industrial practice for how to improve burner design using both RANS and LES. The former helps find potential problems in the flow field, e.g. flow separation. The latter quantifies their impact on flow stability/turbulent fluctuations, e.g. helical modes generated by coupling of flow disturbances and swirling flow. Improvement measures were worked out to supress such flow fluctuations and to enhance stability of burner aerodynamics. Another critical issue for burner design, reverse flow within the burner, was also discussed because it is a potential risk. When a flame instantaneously enters the burner backwards it can be stabilized in the recirculation zone and damage hardware. The risky region was eliminated by enhancing axial momentum of the air inflow. The strong turbulent fluctuations within the burners interfere with burner stability significantly and lead to a bad flashback resistance in macroscopic view [1]. In the second part of the paper, atmospheric tests verify the improvement of burner stability via improved flashback resistance and show an upgrading of aerodynamic behaviour via reduced burner pressure drop.