Stationary Gas Turbines Research Articles

Flame structure and emission signature in distributed combustion

Colorless distributed combustion (CDC) combustion technology offers significant advantages of ultra-low pollutants emission, stable operation and improved pattern factor for high intensity stationary gas turbines applications. Detailed knowledge on distributed combustion behavior is required to further deploy this technology. This paper reports the evolution of swirl flame shape, flame expansion and pollutants emission characteristics using propane, methane, and 20% and 40% hydrogen enriched methane fuels. The entrainment of hot reactive gases was simulated by diluting the inlet air stream with inert nitrogen or carbon dioxide. OH* chemiluminescence signatures, captured at different dilution levels, manifested gradual reduction of flame luminosity when CDC was approached. Flame boundaries derived using image threshold technique helped to visualize the broadened reaction zone under CDC conditions, which had much reduced chemiluminescence signal intensity. The rms to mean OH* signal variation at different dilution levels were analyzed to detect the initiation of CDC. The higher flame lift-off observed with CO2 dilution was due to higher heat capacity of CO2, resulting in greater flame speed reduction. Flame expansion, evaluated from the area encompassed within the flame boundary at different dilution levels showed a power-law behavior with both the diluents. Expansion of 4 to 5 times of initial flame volumes was observed under CDC using CO2 and N2 dilution, respectively. Significant reduction of NO and CO emission achieved under CDC was due to reduction of overall flame temperature, hotspot mitigation, and widened reaction zone occupying large flame volume, which makes it favorable for many practical combustor applications.

Investigation of Applicability of Transporting Water Mist for Cooling Turbine Blades

Abstract This paper presents a numerical study to investigate the feasibility of transporting water mist to the rotating blades of a high-pressure turbine. The idea of using mist film cooling to enhance conventional air cooling has been proven to be a feasible technique under laboratory conditions. However, there are challenges in implementing this scheme for real gas turbine systems. The first challenge is how to transport the mist to the rotating blades and the second challenge is delivering the mist to the injection holes and getting the particles to survive within the harsh gas turbine environment. Both a zero-dimensional mist evaporation analytical model and a 3D computational fluid dynamics (CFD) scheme are employed for analysis. In the CFD simulation, the Lagrangian–Eulerian method is used along with the discrete phase model (DPM) to track the evaporation process of each individual water droplet. For transporting the mist to the blades, the high-pressure water mist is injected into the stream of cooling air extracted from the compressor through two different passages. The first passage passes through the rotor cover-plate cavity before entering the blade base. The second passage passes through a diaphragm box on the base of the second vane, then tangentially through a cooling passage in the rotating shaft, and eventually to the blade base. The results show that it is feasible to transport the mist from the turbine casing to the blade through both passages, provided that droplets with sufficient particle diameter and mist loading are used. The shorter passage, through the nozzle diaphragm, alleviates a lot of challenges facing the passage through the blade cavity and seems to be more practical. A side benefit of transporting mist through the internal passages is the additional cooling of the preswirler and rotor cover plates. The results are encouraging for implementing the mist cooling technique under real gas turbine conditions.

A numerical approach to the heat transfer and thermal stress in a gas turbine stator blade made of HfB2

Conventional gas turbine blades often face corrosion and oxidation at high temperatures. For resolving these problems, another way in addition to the cooling ducts is selecting appropriate materials for the fragments manufacturing. The present work offers a solution to reduce the metallurgical issues at higher temperatures by using ultra-high temperature ceramics. Despite the rotor blades, stator ones are exposed to hot fluid without any rotation and consequence centrifugal forces. Because brittle materials such as ordinary ceramics are not applicable in the case of high tension forces, hafnium diboride is selected to examine its feasibility for the fabrication of gas turbine stator blades. Thermal stress and deformation of gas turbine stator blades are investigated numerically. Comsol Multiphysics software, utilizing Finite Element Method, was used to analyze the heat transfer and possibility of failure in the stator blades. HfB2 can be a resistant alternative for manufacturing the turbine blades, which does not fail against the applied compressive stresses according to both Von Mises and Coulomb-Mohr theories. In addition, a comparison study between HfB2 and two other diboride ceramics, ZrB2 and TiB2, is conducted.

Gas Turbine Condition Monitoring Using Acoustic Emission Signals

Acoustic emission (AE) signals are recognized as complementary measures for detecting incipient faults and condition monitoring in rotary machinery due to their containment of sources of potential fault energy. However, determining the potential sources of faults cannot be easily realized due to the non-stationarity of AE signals. Available techniques that are capable of evoking instantaneous characteristics of a particular AE signal cannot optimally perform in a sense that there is no guarantee that these characteristics (hereinafter referred to as the “features”) remain constant when another AE signal is obtained from the system, albeit operating under the same machine condition at a different time instant. This paper provides a theoretical framework for developing a highly reliable classification and detection methodology for gas turbine condition monitoring based on AE signals. Mathematical results obtained in this paper are evaluated and validated by using actual gas turbines that are operating in power generating plants, to demonstrate the practicality and simplicity of our methodologies. Emphasis is given to acoustic emissions of similar brand and sized gas turbine turbomachinery under different health conditions and/or aging characteristics.

Experimental Analysis and Phenomenological Model for Liquid Jet Breakup in Swirling Flow of Air

This paper presents detailed analysis of an experimental investigation of the impact of swirl number of subsonic cross-flowing air stream on liquid jet breakup at an airflow Mach number of 0.12, which is typical in gas turbine conditions. Experiments are performed for four different swirl numbers (0, 0.2, 0.42, and 0.73) using swirl vanes at air inlet having angles of 0 deg, 15 deg, 30 deg, and 45 deg, respectively. Liquid to air momentum flux ratios (q) have been varied from 1 to 25. High-speed images of the interaction of liquid and air streams are captured and processed to estimate the jet penetration height as well as the breakup location for various flow conditions. The results show unique behavior for each swirl number, which departs from the straight flow correlations available in the literature. Based on the results, an attempt has been made to understand the physics of the phenomena and come up with a simplified physical model for prediction of jet penetration. Furthermore, the high-speed images show a dominant influence of liquid column fluttering on fracture mechanism (column or shear breakup mechanism).

Ranking materials technologies by limiting characteristics of heat-resistant alloys and their longevity in the problems of import substitution

The structural strength properties determined by the true stress-deformation diagram and the fatigue characteristics of materials at combined action (tension and bending) under the conditions of temperature change are used to substantiate materials and technologies. The domestic alloy is suggested as an alternative to imported heat-resistant alloy IN-738LC used to manufacture the turbine rotor blades of stationary gas turbine plants (GTP). Expressions to determine the fatigue characteristics according to the criteria of structure stability are presented.

The development and performance of a perforated plate burner to produce vitiated flow with negligible swirl under engine-relevant gas turbine conditions.

The development and performance of a perforated plate burner (PPB) operating using premixed natural gas and air at engine-relevant inlet temperatures and combustor pressures with thermal powers up to 1 MW is discussed. A significant benefit of using burners with simplified flow fields, such as the PPB, for experimental studies in the laboratory is the potential for decoupling the complex fluid dynamics in typical combustors from the chemical kinetics. The primary motivation for developing this burner was to use it as a source of vitiated flow with negligible swirl for reacting jet in vitiated crossflow experiments. The design methodology for the PPB is described, including plate geometry selection and flashback mitigation features. The stable operation of the PPB within a high-pressure test rig was validated: successful ignition, effective use of red-lines for flashback mitigation, and long duration steady-state operation in both piloted and nonpiloted modes were all observed. Exhaust gas emissions measured using a Fourier-transform infrared (FTIR) spectrometer showed very good performance of the PPB in terms of the combustion efficiency (based on measured CO and UHC), and a stability diagram of the PPB was developed as a function of the equivalence ratio and the PPB hole velocity. FTIR measurements also showed very low levels of NOX in nonpiloted operation that were generally within 3 ppm (reported dry and referenced to 15% O2). The capability for steady-state operation, high combustion efficiency, and low levels of NOX makes this PPB an excellent burner candidate for combustion experiments in the laboratory.

Features of the Vortex Flow Structure around a One Fin Shroud

Modern gas turbines have high efficiency. A further increase in their economic efficiency can be achieved, on the one hand, by enhancing the accuracy of the parameter prediction at the design stage and, on the other hand, by the possibility of improving the design for different components of the flow path based on the results of calculating the complicated viscous spatial structure of the flow. One of the tools for enhancing the efficiency of gas turbines is the minimization of the rotor-tip leakage, the rate of which is reduced by shrouding the rotor blades. In particular, using numerical methods and software tools based on the former, one can perform thorough computational analysis of the vortex structure of the flow in the vicinity of the tip shroud and sufficiently accurately determine the rotor-tip leakage rates and other parameters of the stage. Such an approach allows a more accurate assessment of the leakage than semiempirical approaches used in practice. In particular, the assessment by the correlation dependence showed that the leakage rate for the tip shroud design in question with a tip clearance of 5 mm exceeded the leakage rate calculated using the method that considers the specific features of the tip shroud design and the vortex structure of the flow in the vicinity of it by 8.65%. As exemplified by the last-stage rotor blade of a stationary gas turbine, the possibility of controlling the leakage of the main flow through the tip clearance is demonstrated based on results of a numerical experiment.

Heat transfer, thermal stress and failure analyses in a TiB2 gas turbine stator blade

Abstract Gas turbine stator blades do not experience centrifugal force contrary to the rotor blades; but they are exposed to high–temperature combustion gases causing thermal stresses. In the present work, a series of numerical simulations were carried out to clarify the feasibility of TiB2 utilization as an appropriate material for gas turbine stator blades. The governing equations of heat transfer and solid mechanics were discretized by the finite element method and solved using Comsol Multiphysics software. The boundary conditions were applied, and temperature, displacement and maximum principle stress were obtained. The results showed that using ceramics such as TiB2 instead of conventional alloys can enhance the maximum displacement. Temperature distribution in the blade is more uniform than that of alloys, and consequently, the thermal stresses are reduced. The TiB2 can withstand the applied stresses according to the Coulomb–Mohr theory with a safety factor of 2.4.

A skeletal mechanism for prediction of ignition delay times and laminar premixed flame velocities of hydrogen-methane mixtures under gas turbine conditions

The aim of this study is to eliminate unimportant steps from a detailed chemical-kinetic mechanism in order to identify a skeletal kinetic mechanism that can predict with sufficient accuracy ignition delay times and laminar premixed-flame velocities for H2-CH4 mixtures under conditions of practical interest in gas-turbine applications, which pertain to high pressure, high reactant temperature, and primarily lean-to-stoichiometric mixture compositions (although somewhat rich conditions also are considered for completeness). The accuracy of selected detailed chemical-kinetic mechanisms that are suited to represent combustion of hydrogen-methane mixtures in air was evaluated through comparison of computed and measured ignition delay times and laminar flame velocities, and because of its relative simplicity and sufficient accuracy, the San Diego mechanism was selected for the needed chemical-kinetic reduction. Under the pressure and temperature conditions of the mixture composition addressed, thirty nine reversible elementary steps involving eighteen species were found to suffice to describe with acceptable accuracy both the ignition delay time and the laminar burning velocities. The skeletal mechanism is given here, along with discussion of its derivation and characteristics, as well as comparison of its predictions with those of the detailed mechanism and, where possible, with experiment.

Experimental Aerodynamic Investigations of the 100-MW Two-Shaft Gas Turbine Unit Exhaust Duct

The article describes the results from experimental investigations into the exhaust duct aerodynamics of a stationary two-shaft gas turbine unit (GTU) intended for operation in a combined-cycle plant equipped with a heat-recovery steam generator. The experiments were carried out at the Kirillov Turbine Construction Laboratory of Peter the Great Polytechnic University (SPbPU). The aim of investigations was to experimentally determine the 3D flow structure in the diffuser–exhaust hood flow path and its integral characteristics during operation in combination with the topping turbine stage. The pressure recovery factors and the energy loss coefficients in the diffuser and in the exhaust duct are determined. The circumferential and radial distributions of the 3D flow parameters in the diffuser inlet section are presented. Based on the experimental investigation results obtained for the exhaust duct operating jointly with the topping stage, the main energy loss sources in the duct are determined, and recommendations for improving its aerodynamics are given. A comparative analysis of the exhaust duct’s integral aerodynamic characteristics and of the flow structure parameters obtained during the tests with and without the topping stage is carried out. The results obtained from the comparative analysis of the data obtained in the experiments with the connected topping stage and with the isolated exhaust duct were used for working out recommendations on modeling the stream in the flow paths of two-shaft GTU exhaust ducts.

Numerical analyses of heat transfer and thermal stress in a ZrB2 gas turbine stator blade

Abstract Temperature of burned gases is a key parameter in output works of gas turbines. Nevertheless, high temperatures cause metallurgical problems in the turbine parts, specially the blades. Hence, ZrB2 as an ultra-high temperature ceramic (UHTC) with a melting point of 3246 °C , considerable high-temperature strength and relatively high thermal conductivity can be a proper candidate for manufacturing of gas turbine parts. In the present work, heat transfer and related thermal stresses of a gas turbine stator blade made of ZrB2 ceramic is investigated numerically by means of Comsol Multiphysics software and the obtained data are compared with M152 (UNS S64152) alloy. The results showed that ZrB2 has more uniform temperature distribution than M152, which is corresponded to its lower thermal gradient. Due to its lower thermal expansion coefficient, ZrB2 showed lower total displacement than M152 which is an important parameter in blade performance. Also, the Von Mises stress of ZrB2 showed higher value than M152.

Water vapor corrosion test using supersonic gas velocities

AbstractTesting of the corrosion resistance of environmental barrier coating (EBC) systems is necessary for developing reliable coatings. Unfortunately tests under realistic gas turbine conditions are difficult and expensive. The materials under investigation as well as parts of the test setup have to withstand high temperatures (≥1200°C), high pressure (up to 30 bar) as well as the corrosive atmosphere (H2O, O2, NOx). Therefore most lab scale test‐rigs focus on simplified test conditions. In this work water vapor corrosion testing of EBCs with a high velocity oxy fuel (HVOF) facility is introduced which combines high temperatures and high gas velocities. It leads to quite high recession rates in short periods of time, which are comparable to results from literature. It was found that high flow velocities can easily compensate low gas pressures. HVOF‐testing is a simple and fast way to measure the recession rate of an EBC‐system. As proof of concept the recession rates of an oxide/oxide CMC with and without EBC were measured.

A Novel Method for Gas Turbine Condition Monitoring Based on KPCA and Analysis of Statistics T2 and SPE

Gas turbines are widely used all over the world, in order to ensure the normal operation of gas turbines, it is necessary to monitor the condition of gas turbine and analyze the tested parameters to find the state information contained in parameters. There is a problem in gas turbine condition monitoring that how to locate the fault accurately if failure occurs. To solve the problem, this paper proposes a method to locate the fault of gas turbine components by evaluating the sensitivity of tested parameters to fault. Firstly, the tested parameters are decomposed by the kernel principal component analysis. Then construct the statistics of T2 and SPE in the principal elements space and residual space, respectively. Furthermore, the thresholds of the statistics must be calculated. The influence of tested parameters on faults is analyzed, and the degree of influence is quantified. The fault location can be realized according to the analysis results. The research results show that the proposed method can realize fault diagnosis and location accurately.

An overview on dry low NOx micromix combustor development for hydrogen-rich gas turbine applications

The paper presents a survey of the interactive optimization cycle at Aachen University of Applied Sciences, used for the development of a new low emission Micromix combustor module for application in hydrogen fueled industrial gas turbines. During the development process, experimental and numerical methods are applied to optimize a given baseline combustor with 0.3 mm nozzles with respect to combustion efficiency, combustion stability, higher thermal power output per nozzle and reduced manufacturing complexity.Within the described research cycle combustion and flow simulations are used in the context of parametric studies for generating optimized burner geometries and the phenomenological interpretation of the experimental results. Experimental tests, carried out on an atmospheric combustion chamber test stand provide the basis for validation of simulation results and proof of the predicted combustion characteristics under scaled down gas turbine conditions.In the presented studies, an integration-optimized Micromix combustor with a nozzle diameter of 0.84 mm is tested at atmospheric pressure over a range of gas turbine operating conditions with hydrogen fuel. The combustor module offers an increase in the thermal power output per nozzle by approx. 390% at a significant reduced number of injectors when compared to the baseline design. This greatly benefits manufacturing complexity and the robustness of the combustion process against fuel contamination by particles.During atmospheric testing, the optimized combustor module shows satisfactory operating behavior, combustion efficiency and pollutant emission level. Within the evaluated operating range, which correlates to gas turbine part-, full- and overload conditions, the investigated combustor module exceeds 99% combustion efficiency. The Micromix combustor achieves NOx emissions less than 2.5 ppm corrected to 15 Vol% O2 at the design point.Based on numerical analyses and experimental low pressure testing, a full-scale gas turbine combustion chamber is derived. High pressure testing in the auxiliary power unit Honeywell/Garrett GTCP 36–300 shows stable operation during acceleration of the engine, during IDLE and during load variations between IDLE and Main Engine Start (MES) mode. Throughout the investigated operating range, the combustion chamber generates low NOx emissions under full-scale gas turbine conditions.

A multi-objective optimization framework for robust axial compressor airfoil design

Airfoil design for stationary gas turbines is a challenging task involving both aerodynamic and structural aspects. The paper describes a multidisciplinary optimization process for axial compressor airfoils which is able to find optimal designs w.r.t. multiple objectives and constraints starting from a reference design and very few specifications of the new compressor. The process allows to simultaneously execute arbitrarily many instances of design evaluation processes independently from each other, which speeds it up, not just due to parallelization, but also because fast-running low-fidelity evaluation may take the design lead at an early design stage, whereas high-fidelity evaluation processes simultaneously contribute with more reliable results on the actual performance. For consistency of aerodynamic and structural analysis, an innovative method for direct loaded-to-unloaded design transformation is incorporated. Additionally, the process accounts for design robustness by utilizing production tolerances as an optimization objective. Therefore, a procedure is developed which allows to find the production tolerance which may be allowed without violating any constraints. An application example demonstrates that the proposed optimization process incorporating automatic detection of failure-critical eigenmode bands is able to shift them such that structurally reliable, robust, and simultaneously aerodynamically efficient designs are obtained.

Response of heat release to equivalence ratio variations in high Karlovitz premixed H2/air flames at 20 atm

This paper presents three-dimensional direct numerical simulations of lean premixed H2/air flames with equivalence ratios 0.4, 0.5 and 0.6, respectively. The initial Karlovitz number is around 2335 and the pressure is 20 atm, which is relevant to gas turbine conditions. The heat release in reaction zones under different equivalence ratios is examined statistically with the aim to extend our understanding of lean combustion under high-pressure conditions. With increasing equivalence ratio, the relative thickness of reaction zone (δf/δL) is increasing for both laminar and turbulent flames, but the extent of increase is reduced under high equivalence ratio. By examining the local structures of flame fronts, it is found that trenches and plateaus of local equivalence ratio are located on separate sides of the reaction zone edge. Due to the decreased Lewis number under high equivalence ratio, the trench ‘depth’ and plateau ‘height’ are reduced. For the flame under ultra-lean conditions, there are some spots with temperatures above adiabatic temperature. This is attributed to the high-fraction of radicals in these regions, which will promote heat release. Furthermore, the heat release rates of elementary reactions are investigated with the analysis of radical fractions and rate constants. When the mixture equivalence ratio varies, the local heat release is changed in different temperature windows due to the combined effects of radical fractions and reaction rate constants.

Reference natural gas flames at nominally autoignitive engine-relevant conditions

Laminar natural gas flames are investigated at engine-relevant thermochemical conditions where the ignition delay time τ is short due to very high ambient temperatures and pressures. At these conditions, it is not possible to measure or calculate well-defined values for the laminar flame speed sl, laminar flame thickness δl, and laminar flame time scale τl=δl/sl due to the explosive thermochemical state. Here, the corresponding reference values, sR, δR, and τR=δR/sR, that account for the effects of autoignition, are numerically estimated to investigate the enhancement of flame propagation, and the competition with autoignition that arises under nominally autoignitive conditions (characterised here by the number τ/τR). Large values of τ/τR indicate that autoignition is unimportant, values near or below unity indicate that flame propagation is not possible, and intermediate values indicate that a combination of both flame propagation and autoignition may be important, depending upon factors such as device geometry, turbulence, stratification, et cetera. The reference quantities are presented for a wide range of temperatures, equivalence ratios, pressures, and hydrogen concentrations, which includes conditions relevant to stationary gas turbine reheat burners and boosted spark ignition engines. It is demonstrated that the transition from flame propagation to autoignition is only dependent on residence time, when the results are non-dimensionalised by the reference values. The temporal evolution of the reference values are also reported for a modelled boosted SI engine. It is shown that the nominally autoignitive conditions enhance flame propagation, which may be an ameliorating factor for the onset of engine knock. The calculations are performed using a recently-developed, detailed 177 species mechanism for C0–C3 chemistry that is derived from theoretical chemistry and is suitable for a wide range of thermochemical conditions as it is not tuned or optimised for a particular operating condition.

Numerical Investigations of an Axial Exhaust Diffuser Coupling the Last Stage of a Generic Gas Turbine

It is well known that the last stage of a turbine and the subsequent diffuser should be viewed at and designed as a coupled system rather than as single standalone components. The turbine outlet flow imposes the inlet conditions to the diffuser, whereas the recovered dynamic pressure in the diffuser directly controls the turbine back pressure. With changing operating point, the turbine outflow can vary significantly. This results consequently in large variations of the diffuser performance. A major role in the coupled system of turbine and diffuser can be attributed to the tip leakage flow. While it is desirable to minimize the tip leakage with regard to the turbine, a higher leakage mass flow can often be beneficial for the diffuser performance. As there is currently a trend toward aggressive and hence shorter diffusers which are particularly prone to separation, the question arises where the optimum for this tradeoff problem lies. To investigate the performance in the coupled turbine/diffuser system, a generic last stage with shrouded rotor and axial exhaust diffuser has been designed. The components are representative for heavy duty stationary gas turbine applications. Results are presented for three different operating points representing part-load (PL), design-load (DL), and over-load (OL) condition. Three different seal gap widths are taken into account to control the leakage flow. The results indicate that an operating point-dependent optimum gap width can be found for the coupled system efficiency, whereas the maximum turbine performance is always achieved with a minimum gap width.

Thermodynamic calculation and experimental analysis of critical phase transformations in HVOF-sprayed NiCrAlY-coating alloys

Most of the current high temperature protective NiCrAlY-type coatings for stationary gas turbines are configured for base-load operation. During service at elevated temperatures, phase transformations occur which ultimately lead to a microstructural evolution and thus might have a substantial influence on service relevant materials properties. In consequence, these coating alloys need to be adapted for operation in high-cyclic and/or peak-loading conditions.The present investigation is focusing on the microstructural evolution of nine different NiCrAlY-coatings in the temperature range of up to 1150 °C. The study especially aims at determining the β-(NiAl) and γ′-(Ni3Al) phase transformation, which can have a substantial impact on the coefficient of thermal expansion, or the materials deformation behaviour respectively. For this, a combination of computational thermodynamics (Thermo-Calc 2015b, TCNi6 database) validated by dilatometry and X-ray diffraction was used. In addition, the obtained results were cross-checked using scanning electron microscopy and energy dispersive spectroscopy on water quenched, free-standing coating alloy specimens.In general, a good agreement between the thermodynamic equilibrium calculations, dilatometry and X-ray diffraction, as well as the microstructure analysis after water quenching was found with respect to the critical phase transformations of α-(Cr), β-(NiAl) and γ′-(Ni3Al) precipitates in the γ-(Ni) matrix. The great potential of computational thermodynamics combined with dilatometry and X-ray diffraction for the evaluation of critical phase transformations was proven for the different coating alloys. Using this methodology, novel and very promising NiCrAlY-coating compositions, without detrimental phase transformations around 1000 °C, were identified for the application on heavy-duty gas turbines hot gas path components. This manuscript presents a summary of the main results.