Due to the high temperature of the flue gas flowing at high velocity and pressure, the wall cooling is extremely important for the liner of a gas turbine engine combustor. The liner material is heat resistant steel with relatively low heat conductivity. To accommodate outside wall forced air cooling, the liner is designed to be thin, which unfortunately facilitates the possibility of high amplitude wall vibrations (and failure due to fatigue) in case of pressure fluctuations in the combustor. The latter may occur due to a possible occurrence of a feedback loop between the aerodynamics, the combustion, the acoustics and the structural vibrations. The structural vibrations act as a source of acoustic emitting the acoustic waves to the confined fluid. This leads to amplification in the acoustic filed and hence the magnitude of instability in the system. The aim of this paper is to explore the mechanism of fluid-structure interaction on the LIMOUSINE setup which leads to limit cycle of pressure oscillations (LCO). Computational fluid dynamics (CFD) analysis using a RANS approach is performed to obtain the thermal and mechanical loading of the combustor liner and finite element model (FEM) renders the temperature, stress distribution, and deformation in the liner. Results are compared to other numerical approaches like zero-way interaction and conjugated heat transfer model (CHT). To recognize the advantage/disadvantage of each method validation is made with the available measured data for the pressure and vibration signals, showing that the thermoacoustic instabilities are well predicted using the CHT and two-way coupled approaches. While the zero-way interaction model prediction gives the largest discrepancy from experimental results.
Supercritical water gasification is an efficient thermochemical conversion process to convert wet biomass into a high grade fuel. However, char formation during the conversion might influence the process efficiency and operational stability. Providing insight into char formation behavior during the gasification process will facilitate the utilization of this technology. Computational fluid dynamic modelling is used to study the supercritical water gasification of glucose in a tubular helical reactor, which includes an injector tube to mix glucose feed with pre-heated water to provide fast heating. Validation against experimental data confirms that the developed model performs well. An average discrepancy of 4% is obtained for the total feed conversion whereas the gas yield at high temperature is computed with 15% difference. The char yield trend is also captured well. Sensitivity analysis reveals that the presence of low temperature zone at the injection point plays a significant role in char formation.
In the past 25 years high temperature air combustion (HiTAC) technology has been proved and utilized in industry as a promising way to increase thermal efficiency, create a relatively uniform temperature distribution, and reduce emissions of harmful pollutants such as NOXand CO. However, due to the complexity of fuel-oil combustion, to date HiTAC is mainly applied to gaseous fuel or coal, and little is known about spray combustion under HiTAC condition. In the present study, we numerically investigate the Delft Spray-in-Hot-Coflow (DSHC) using ethanol in high temperature diluted combustion air, and extend it to more co-flow conditions. We employ different temperatures and oxygen concentrations of the co-flow in order to dilute the oxidizer/fuel before it reacts with the fuel/ oxidizer. The pressure-swirl atomizer model with an Eulerian-Lagrangian approach was implemented for the spray modeling. Collision, coalescence, secondary breakup and evaporation of the drops were taken into account. The steady laminar flamelet model for the combustion of ethanol, the Discrete Ordinate model for radiation and the k-ε model for the turbulence with enhanced wall treatment were validated by the simulation of the NIST flame under conventional conditions and then used in the current study.
The results indicate that the decreased peak temperature in many HiTAC applications with high temperature combustion air is mainly due to the reduced oxygen concentration by entraining flue gas.
In the present study, a low oxygen concentration slows the evaporation process of droplets. It results in an enlarged combustion zone, a lowered peak temperature and minor NOXemission. However, decreasing the oxygen concentration may lead to problems of cracking, soot formation and flame extinction, especially for heavy oils. The optimization needs to be carried out based on the analysis of a specific fuel in order to create a HiTAC-like condition.
Based on the results of the current study, the 1500 K and 6%vol oxygen concentration case is considered as a HiTAC condition.
Combustion of pyrolysis oil has attracted many attention in recent years as a renewable and environmental friendly fuel. However, pyrolysis oil as an multi-component fuel has some differences compared to conventional fossil fuels. One of the main differences is the formation of solid char in the droplet during
evaporation. The goal of this work is to study the effect of the solid char on the combustion characteristics of multi-component fuel. An Euler-Lagrange model of three phase gas/liquid/solid combustion is developed to study the detailed information about every phenomena in the process such as: heat, mass and momentum transfer between droplet and gas phase, droplet evaporation, homogeneous and heterogeneous reactions. The results indicate that the presence of the solid char and consequently its combustion elongates significantly the combustion region in a typical spray injection chamber/burner. Moreover, the gas phase reaches higher temperatures as a result of char combustion that creates more heat by heterogeneous oxidation as a kind of afterburner
Gasification in supercritical water is a very promising technology to process wet biomass into a valuable gas. Providing insight of the process behavior is therefore very important. In this research a computational fluid dynamic model is developed to investigate glycerol gasification in supercritical water, which takes place in a cylindrical reactor with a tee junction. The performance of the developed model is validated against experiment, and it shows that the model is able to describe the process very well. The experimental validation shows that the model slightly overestimates the outlet temperature on average by 6% and underestimates the carbon gasification efficiency on average by 16%. The flow behavior in the supercritical water gasification process is successfully described and a sensitivity analysis is conducted. It is revealed that the flow pattern of the process is heavily influenced by gravitational forces which significantly influences mixing and heat transfer.
In order to develop an alternative spray drying technology, a high drying rate in a smaller volume must be achieved. In this paper, results of CFD study are presented, carried out to investigate the possibility of spray drying in a novel design vortex chamber. The model is validated against experimental data, that makes a good agreement with an average error of 7% with only air and 24% with water spray. Results of temperature fields and droplet impact positions are discussed. The computations demonstrate that vortex chamber spray dryer can be an attractive solution for drying technology.
In order to achieve efficient combustion of liquid fuel a proper atomization of the fuel is needed. In case of many biomass fuels the atomization process is obstructed and hindered by high viscosity of the fuel. Preheating to reduce the viscosity in many cases is not possible because of fuel polymerization processes and secondary cracking reactions which finally result in fuel flow restriction. In this work, a novel flow blurring atomizer is presented and discussed in view to atomization and combustion of regular and highly viscous fuels. A detailed results regarding droplets SMD and distributions are presented followed by the combustion experiments in 50kWe full scale gas turbine. The outcome of the research shows that flow blurring atomizer is not sensitive for changes in the fuel viscosity and can be efficiently used for combustion applications.
The worldwide concern regarding global warming has increased the interest of using biomass as a renewable and CO2 neutral source of energy. Pyrolysis oil (PO), as one of the most important product of biomass conversion, has the potential to be used as a fuel oil substitute in many applications for heat and electricity generation. However, pyrolysis oil properties and its behavior during combustion are considerably different from conventional fossil fuels. From a chemical point of view, PO contains large number of oxygenated compounds derived from the decomposition of biomass during thermal treatment. It has also considerable amount of water originated from both moisture content and the decomposition reactions. Water is homogeneously dissolved in the oil and cannot be eliminated with drying processes without losing volatile hydrocarbon compounds . From the physical point of view, bio-oils are characterized by high viscosity and surface tension, low heating value and, due to multicomponent composition, a very wide boiling range . Moreover, they are thermally unstable and when heated, undergo polymerization processes, leading to the formation of carbonaceous solid material (char) in the fuel supply line, at the nozzle tip and in the combustion chamber . Van Rossum et al.  found that pyrolysis oil evaporation is always coupled with the formation of char. Literature survey indicates that combustion behavior of pyrolysis oil is still unknown process. More investigations is required to understand PO spray formation, evaporation and combustion. Especially, the impact of char formation on the combustion characteristics, which has been not yet explored, needs detailed assessment. Knowledge and data about the specifics of the processes and phenomena which interact during combustion of PO will support efficiency increase and design of new generation of burners operating on this bio-fuel.
The objective of this work is to investigate pyrolysis oil combustion, taking into account mutual interactions between gaseous, liquid and solid fields. A numerical model that takes into account liquid fuel evaporation and gaseous and char combustion has been developed in OpenFOAM. The char is considered to be present in the fuel droplets and its oxidation is modeled after complete evaporation of liquid.
Due to high energy density, storability and transportability pyrolysis oil has an advantage over the originating solid biomass. However, the combustion behaviour of the oxygenated pyrolysis oil is not comparable to fossil oil due to significant differences in physical and chemical properties. This different behaviour have an impact on the combustion efficiency, emissions and life-time of the burner.
For the investigations a 50 kWe gas turbine working in low temperature combustion regime (idle and low power operation mode) was used. Pyrolysis oil was blended with diesel fuel by utilizing alcohols (ethanol or butanol) or surfactants (Zephrym PD2206 and Atlox 4912) as binding agents. Stable blends with pyrolysis oil content up to 45 wt% for alcohols and 20 wt% for surfactants were obtained.
The recorded NO emissions were at level of few ppm, i.e. in the range of gas analyser error. Depends on the investigated conditions, blend composition and preheating temperature (effect of viscosity) the CO emissions were in the range of 550-700 ppm and generally agreed with the results of diesel fuel tests. The turbine was inspected after finalizing runs with surfactants as binding media, showing no signs of deterioration nor contamination on its components (in total few hours of operation).
It has been concluded that combusting of pyrolysis oil blends with diesel distillate is an interesting option for biomass co-firing and can give an important contribution to power generation sector.
Abstract: In this paper transient fluid-structure thermal analyses of the Limousine test rig have been conducted while the combustor was exposed to saturated amplitude limit cycle combustion oscillations (LCO). The heat transfer between hot combustion gases and the liner wall cooled by convection will affect thermo-acoustic instabilities, and therefore the relevance of prediction of the transient heat transfer rate in gas turbine combustors is explored. The commercial CFD code ANSYS CFX is used to analyze the problem. Fluid and solid regions are resolved simultaneously in a monolithical approach with application of a finite volume approach. Since the spatial scales of the solid temperature profiles are different in case of steady state and transient oscillatory heat transfer, special care has to be taken in the meshing strategy. It is shown that for the transient oscillatory heat transfer in to the solid in LCO operation, the mesh distribution and size of the grid in the solid part of the domain will play a very important role in determining the magnitude for the heat flow in the solid and the gas pressure fluctuations, and the grid resolution needs to be adapted to the thermal penetration depth. Moreover, compared to the calculations of only the fluid domain with adiabatic/isothermal boundary wall conditions, the results demonstrated that application of the Conjugated Heat Transfer (CHT) model leads to significant accuracy improvements in the prediction of the characteristics of the combustion instability.
Abstract: The concept of using pyrolysis oil (PO) derived from biomass via a fast pyrolysis route for power and heat generation encounters problems due to an incompatibility between properties (physical and chemical) of bio-oil and gas turbines designed for fossil fuels. An extensive research has been performed on the production and improvement of pyrolysis oil but only few investigations were carried out on its utilization. The latter have shown a major difference in behavior of pyrolysis oil compared to fossil fuels during combustion processes. In this work, pyrolysis oil is co-fired with diesel in a 50 kWe gas turbine operating in idle mode. Stable mixtures with up to 20 wt.% of pyrolysis oil and diesel fuel were produced with utilization of a surfactant agent. To prevent feeding line deterioration due to acidic character of pyrolysis oil, a stainless steel nozzle was employed. Furthermore, the fuel emulsion was preheated up to maximum temperature of 80 oC in order to reduce the effect of high viscosity on the atomization process. Diesel distillate #2 was used as a reference fuel for a comparison of gas turbine performance and emissions with various PO content in the blends. During the combustion investigations, the amount of pyrolysis oil was gradually increased with simultaneous decrease of preheating temperature. In all investigated cases, the gas turbine was running stable at its maximum rotational speed (RPM). The CO level resulting from the study with different blends was generally slightly higher in relation to the diesel distillate fuel. NO emissions were in the range of few ppm and almost no detectable with common gas analyzing equipment. After a few hours of continuous operation, there were no signs of deterioration or contaminations inside the combustor. The study shows that pyrolysis oil gradually can be introduced in the market of fossil fuels and benefit to green power generation.
Abstract: The turbulent reacting flow over a backward facing step shares some essential characteristics of premixed combustion occurring in a typical gas turbine combustor, while it is a simpler configuration to observe and model. For this reason and to explore the characteristics of the turbulent flow, in this study the combustion and flow dynamics in a backward facing step as a most elementary part of a combustor is studied numerically in atmospheric conditions. Two different configurations representing two laboratory devices are considered. As a first necessary step, the accuracy of predicted results is validated through the detailed comparison of numerical predictions and experimental measurements for a non-reacting flow. First, based on these non-reacting calculations, the turbulent model is selected and then the reacting simulations are done using a standard combustion model (available in CFX). Calculations are well supported with experimental data available from literature. Among the investigated turbulence models (k − ω, SST and SAS–SST), SAS–SST model showed the best agreement with the experimental data. The chosen turbulence model was used for the calculation of well documented case of turbulent flow over a backward facing step with the heated wall, showing satisfactory results compared to experimental data. For modeling of the reacting flow, the BVM combustion model was used. The predicted results using this model showed accurate results with an error about 2% on prediction of reattachment length
Abstract: A methanol spray flame in a combustion chamber of the NIST was simulated using an Eulerian–Lagrangian RANS model. Experimental data and previous numerical investigations by other researchers on this flame were analyzed to develop methods for more comprehensive model validation. The inlet boundary conditions of the spray were generated using semi-empirical models representing atomization, collision, coalescence, and secondary breakup. Experimental information on the trajectory of the spray was used to optimize the parameters of the pressure-swirl atomizer model. The standard k-ϵ turbulence model was used with enhanced wall treatment. A detailed reaction mechanism of gaseous combustion of methanol was used in the frame of the steady laminar flamelet model. The radiative transfer equations were solved using the discrete ordinates method. In general, the predicted mean velocity components of the gaseous flow and the droplets, the droplet number density, and the Sauter mean diameter (SMD) of the droplets at various heights in the present study show good agreement with the experimental data. Special attention is paid to the relative merits of the employed method to set inlet boundary conditions compared to the alternative method of using a measured droplet size and velocity distribution
Abstract: The atomization of biodiesel, vegetable oil and glycerin has been studied in an atmospheric spray rig by using digital imaging (PDIA). Images of the spray were captured in the near field, just 18 mm downstream of the atomizer, and processed to automatically determine the size of both ligaments and droplets. The effect of the spray structure in this region is of major interest for the combustion of biofuels in gas turbines. The sprays were produced by a pressure-swirl atomizer that originates from the multifuel micro gas turbine (MMGT) setup. Various injection conditions have been tested to investigate the influence of viscosity on the spray characteristics and to assess the overall performance of the atomizer. The spray measurements have been compared to combustion experiments with biodiesel and vegetable oil in the micro gas turbine at similar injection conditions. The results show that the primary breakup process rapidly deteriorates when the viscosity is increased. A higher viscosity increases the breakup length, which becomes visible at the measurement location in the form of ligaments. This effect leads to an unacceptable spray quality once the viscosity slightly exceeds the typical range for conventional gas turbine fuels. The SMD in the investigated spray region was not significantly affected by viscosity, but mainly influenced by injection pressure. The data furthermore indicate an increase in SMD with surface tension. It was found that the penetration depth of ligaments can have major impact on the combustion process, and that the droplet size is not always the critical factor responsible for efficient combustion. The measured delay in primary breakup at increased viscosity shows that pressure-swirl atomization is unsuitable for the application of pure pyrolysis oil in an unmodified gas turbine engine
Abstract: The relation between spray quality and combustion performance in a micro gas turbine has been studied by burning a viscous biofuel at different fuel injection conditions. Emissions from the combustion of a viscous mixture of straight vegetable oils have been compared to reference measurements with diesel No. 2.
The effect of fuel viscosity on pollutant emissions is determined by adjusting the injection temperature. The measurements confirm that a reduction in fuel viscosity improves the spray quality, resulting in faster droplet evaporation and more complete combustion. CO emission levels were observed to decrease linearly with viscosity in the tested range. For the pressure-swirl nozzle used in the tests, the upper viscosity limit is found to be 9 cP. Above this value, droplet evaporation seems to be incomplete as the exhaust gas contains a considerable amount of unburned fuel.
Additionally, the influence of increased injection pressure and combustor temperature is evaluated by varying the load. Adding more load resulted in improved combustion when burning diesel. In case of vegetable oil, however, this trend is less consistent as the decrease in CO emissions is not observed over the full load range.
The outcome of this study gives directions for the application of pyrolysis oil in gas turbines, a more advanced biofuel with high viscosity
Abstract: The objective of this study is to investigate the sensitivity and accuracy of the reaction flow-field prediction for the LIMOUSINE combustor with regard to choices in computational mesh and turbulent combustion model. The LIMOUSINE combustor is a partially premixed, bluff body-stabilized natural gas combustor designed to operate at 40–80 kW and atmospheric pressure and used to study combustion instabilities. The transient simulation of a turbulent combusting flow with the purpose to study thermoacoustic instabilities is a very time-consuming process. For that reason, the meshing approach leading to accurate numerical prediction, known sensitivity, and minimized amount of mesh elements is important. Since the numerical dissipation (and dispersion) is highly dependent on, and affected by, the geometrical mesh quality, it is of high importance to control the mesh distribution and element size across the computational domain. Typically, the structural mesh topology allows using much fewer grid elements compared to the unstructured grid; however, an unstructured mesh is favorable for flows in complex geometries. To explore computational stability and accuracy, the numerical dissipation of the cold flow with mixing of fuel and air is studied first in the absence of the combustion process. Thereafter, the studies are extended to combustible flows using standard available ansys-cfx combustion models. To validate the predicted variable fields of the combustor’s transient reactive flows, the numerical results for dynamic pressure and temperature variations, resolved under structured and unstructured mesh conditions, are compared with experimental data. The obtained results show minor dependence on the used mesh in the velocity and pressure profiles of the investigated grids under nonreacting conditions. More significant differences are observed in the mixing behavior of air and fuel flows. Here, the numerical dissipation of the (unstructured) tetrahedral mesh topology is higher than in the case of the (structured) hexahedral mesh. For that reason, the combusting flow, resolved with the use of the hexahedral mesh, presents better agreement with experimental data and demands less computational effort. Finally, in the paper, the performance of the combustion model for reacting flow is presented and the main issues of the applied combustion modeling are reviewed.
Abstract: Prediction of mutual interaction between flow, combustion, acoustic, and vibration phenomena occurring in a combustion chamber is crucial for the reliable operation of any combustion device. In this paper, this is studied with application to the combustion chamber of a gas turbine. Very dangerous for the integrity of a gas turbine structure can be the coupling between unsteady heat release by the flame, acoustic wave propagation, and liner vibrations. This can lead to a closed-loop feedback system resulting in mechanical failure of the combustor liner due to fatigue and fatal damage to the turbine. Experimental and numerical investigations of the process are performed on a pressurized laboratory-scale combustor. To take into account interaction between reacting flow, acoustics, and vibrations of a liner, the computational fluid dynamics (CFD) and computational structural dynamics (CSD) calculations are combined into one calculation process using a partitioning technique. Computed pressure fluctuations inside the combustion chamber and associated liner vibrations are validated with experiments performed at the state-of-the-art pressurized combustion setup. Three liner structures with different thicknesses are studied. The numerical results agree well with the experimental data. The research shows that the combustion instabilities can be amplified by vibrating walls. The modeling approach discussed in this paper allows to decrease the risk of the gas turbine failure by prediction, for given operating conditions, of the hazardous frequency at which the thermoacoustic instabilities appear.
Abstract: Combustion tests with bioethanol and diesel as a reference have been performed in OPRA’s 2 MWe class OP16 gas turbine combustor. The main purposes of this work are to investigate the combustion quality of ethanol with respect to diesel and to validate the developed CFD model for ethanol spray combustion. The experimental investigation has been conducted in a modified OP16 gas turbine combustor, which is a reverse-flow tubular combustor of the diffusion type. Bioethanol and diesel burning experiments have been performed at atmospheric pressure with a thermal input ranging from 29 to 59 kW. Exhaust gas temperature and emissions (CO, CO2, O2, NOx) were measured at various fuel flow rates while keeping the air flow rate and air temperature constant. In addition, the temperature profile of the combustor liner has been determined by applying thermochromic paint. CFD simulations have been performed with ethanol for five different operating conditions using ANSYS FLUENT. The simulations are based on a 3D RANS code. Fuel droplets representing the fuel spray are tracked throughout the domain while they interact with the gas phase. A liner temperature measurement has been used to account for heat transfer through the flame tube wall. Detailed combustion chemistry is included by using the steady laminar flamelet model. Comparison between diesel and bioethanol burning tests show similar CO emissions, but NOx concentrations are lower for bioethanol. The CFD results for CO2 and O2 are in good agreement, proving the overall integrity of the model. NOx concentrations were found to be in fair agreement, but the model failed to predict CO levels in the exhaust gas. Simulations of the fuel spray suggest that some liner wetting might have occurred. However, this finding could not be clearly confirmed by the test data.
Abstract: The paper presents a numerical study of the mechanisms driving thermoacoustic instabilities in a lean partially premixed combustor in conditions representative of gas turbine combustion systems. Various combustion models and modeling approaches able to predict the onset of thermoacoustic instabilities are examined and applied to the experimental test rig in order to assess their validity. The influence of the imposed acoustic and thermal boundary conditions on characterization of the coupling between heat release rate fluctuations and the acoustic field is investigated. Predicted data is used to improve the understanding of mutual interactions between pressure fluctuations and unsteady heat release in the unstable combustors which play an essential role in characterizing limit-cycle behavior. The mean convective time delay between heat release and the perturbation in the flow is used to determine the stability condition of the combustor. The study shows that heat transfer is important parameter regulating pressure oscillations.
Abstract: Thermo-acoustic instability can be caused by the feedback mechanism between unsteady
heat release, acoustic oscillations and flow perturbations. In a gas turbine combustor limit cycles of pressure oscillations at elevated temperatures generated by the unstable combustion process enhance the structural vibration levels of the combustor. In this paper, the behavior of turbulent partially premixed flames in a laboratory-scale lean partially premixed combustor (called as LIMOUSINE combustor) operating on natural gas- methane fuel mixtures is studied by using CFD methods. Depending on the operating conditions, the flame shows a stable or an unstable behavior. In order to predict the frequency and magnitude of the thermo-acoustic instability, and also to capture the reacting flow physics within the combustor, the influence of operating conditions on combustion characteristics is examined by using unsteady three-dimensional RANS solution of the conservation equations. To understand the effects of operating conditions on the observed stability characteristics, the time averaged velocity fields were measured with Particle Image Velocimetry (PIV) for the thermoacoustically stable and unstable operating conditions of the combustor. The comparison of the CFD calculations with the mean velocity fields shows good agreement. The results of the present study demonstrate the relationship between the flame structure, the mean velocity filed and pressure fluctuations under different operating conditions.
Abstract: An accurate prediction of the flow and the thermal boundary layer is required to properly simulate gas to wall heat transfer in a turbulent flow. This is studied with a view to application to gas turbine combustors. A typical gas turbine combustion chamber flow presents similarities with the well-studied case of turbulent flow over a backward facing step, especially in the near-wall regions where the heat transfer phenomena take place. However, the combustion flow in a gas turbine engine is often of a dynamic nature and enclosed by a vibrating liner. Therefore apart from steady state situations, cases with an oscillatory inlet flow and vibrating walls are investigated. Results of steady state and transient calculations for the flow field, friction coefficient, and heat transfer coefficient, with the use of various turbulence models, are compared with literature data. It has been observed that the variations in the excitation frequency of the inlet flow and wall vibrations have an influence on the instantaneous heat transfer coefficient profile. However, significant effect on the time mean value and position of the heat transfer peak is only visible for the inlet velocity profile fluctuations with frequency approximately equal to the turbulence bursting frequency.
Abstract: The multi-domain problem, the limit cycle behaviour of unstable oscillations in the LIMOUSINE model combustor has been investigated by numerical and experimental studies. A strong interaction between the aerodynamics-combustion-acoustic oscillations has been observed during the operation. In this regime, the unsteady heat release by the flame is the acoustic source inducing pressure waves and subsequently the acoustic field acts as a pressure load on the structure. The vibration of the liner walls generates a displacement of the flue gas near the wall inside the combustor which generates an acoustic field proportional to the liner wall acceleration. The two-way interaction between the oscillating pressure load in the fluid and the motion of the structure under the limit cycle oscillation can bring up elevated vibration levels, which accelerates the degradation of liner material at high temperatures. Therefore, fatigue and/or creep lead the failure mechanism. In this paper the time dependent pressures on the liner and corresponding structural velocity amplitudes are calculated by using ANSYS workbench V13.1 software, in which pressure and displacement values have been exchanged between CFD and structural domains transiently creating two-way fluid-structure coupling. The flow of information is sustained between the fluid dynamics and structural dynamics. A validation check has been performed between the numerical pressure and liner velocity results and experimental results. The excitation frequency of the structure in the combustor has been assessed by numerical, analytical and experimental modal analysis in order to distinct the acoustic and structural contribution.
Abstract: In this paper, lean premixed combustion on natural gas is studied in experimental and numerical way. Experiments are done at the state-of-the-art 500 kW thermal power combustion setup. The test rig resembles combustion chamber of gas turbine and can be pressurised up to 5 bar absolute pressure. The experimental study are applied for validation of numerical computations. For numerical calculations a hybrid approach combining CFD and FEM methods is used. Mutual interaction between acoustic wave propagation inside the combustion chamber and structural vibrations is studied applying
acousto-elastic model. During the CFD computations, pressure fluctuations created by the flame in the combustion chamber, are calculated first. The results of the CFD are exported then to the FEM code, where interaction between acoustic waves and wall vibrations is resolved. To reduce the effect of numerical dispersion and dissipation of acoustic waves in the CFD code, only the pressure recorded near the flame region is transferred. To simulate acoustic waves next to the vibrating liner, the investigated model is equipped with acoustic elements designed to recognize a structure on one side and a fluid on the other side of the element. The frequencies at which thermo-acoustic instabilities may appear at given operational conditions are predicted. Furthermore, a modal analysis to mark the hazardous structural, acoustic and coupled modes and eigenfrequencies is performed. Computational results are validated against experimental data. Results are in good agreement.
Abstract: In this paper, a one dimensional acoustic network model is presented which can be used as a design tool to predict the limit cycle pressure oscillations in a gas turbine combustor. Analytically represented models are combined with measured flame transfer functions and well defined boundary conditions. Additionally acoustic damping due to turbulence, acoustic reflection at contractions, modification of the acoustic speed of sound due to a mean flow and effect of temperature gradient that play a role in the acoustic modeling of combustion systems have been included in this network model. The model is applied on a high-pressure laboratory combustor. Finally, the measured and predicted dynamic behaviour in the combustor is compared. The results indicate the network modelling approach is a promising design tool for gas turbine combustion applications.
Abstract: The lean premixed combustion suffers from a high sensitivity to thermo-acoustic instabilities which may occur in a combustion chamber of a gas turbine. The high level of acoustic excitation is hazardous to the combustion chamber walls (liner). The situation is even worse when mutual interaction between thermo-acoustic instabilities and liner vibration is present; then both processes may enhance each other. This behaviour reduces the life time of the gas turbine significantly. Therefore, the possibilities of thermo-acoustic instabilities to appear and their interaction with vibrating walls must be
predicted in advance to avoid combustion system destruction. This multi-phenomena interaction is presented and studied in this thesis. The experimental and numerical techniques are employed to investigate the interaction between coupled fields. The experimental part of the study is done on the laboratory scale combustion test rig, which mimics the combustion conditions as encountered in the full scale gas turbine. Experiments are performed at operating conditions, which differ with respect to power and absolute pressure, using two different liner configurations. The obtained results are used for validation of the numerical models. In the fluid-structure interaction analysis (FSI), the thermo-acoustic instabilities are correlated with walls vibration using partitioning approach. Here, two numerical solvers applying CFD (Ansys-CFX) and FEM (Ansys-Multiphysics) are employed to calculate phenomena occurring in the fluid and structural domain, respectively. These solvers exchange information about mechanical loads and structural displacement every time step through the interface connection created
between them. Both one-way and two-way data transfer is studied. For the acousto-elastic analysis (AE) a hybrid approach is used. First the combustible flow is calculated by CFD and latter a pressure data from the near-flame region is transferred to FEM code as the input conditions. This solution allows solving
acoustics inside the combustion chamber more precise than the FSI model, but in costs of only one-way interaction between pressure waves and flame. Additionally, a modal analysis of acoustic, structural and coupled modes is performed. The results of the numerical investigations have shown a good agreement with experimental data. Both models were able to predict correctly the frequencies of thermo-acoustic instabilities and liner vibration.
Abstract: In order to fulfil requirements regarding emission of harmful gases to atmosphere, the gas turbine technologies had to develop into clean techniques for energy generation. Lean premixed combustion of natural gas is one of them. Since during this process exceed of air is used, the total combustion temperature is relatively low. In consequence fewer pollutants are produced. The major drawback of this process is high sensitivity on the thermo-acoustic instabilities. Inside the combustion chamber interaction between several phenomena takes place. Three of them, i.e. combustion, acoustics and the combustion chamber walls vibration coupled together into closed feedback loop might finally lead to the gas turbine failure. The destruction process has an origin in flame intrinsic instabilities. When those are promoted by coupling the heat release fluctuations with acoustic field perturbations, the unsteady self-excited oscillations of the pressure field inside the combustion chamber grows up in the amplitude and exert significant forces on the chamber walls called liner. The liner is a critical component since has to operate reliably at extremely high temperatures. This has a significant negative influence on the liner performance and its material properties. Additional pressure forces acting on the walls surface due to unstable combustion reduce significantly the life time of the liner and gas turbine itself.
In this paper the thermo-acoustic instabilities are investigated in combination with liner vibrations. The investigations are done at the combustion test rig which may operate with maximum power of 500kW and absolute pressure equal to 5bar. In order to observe influence of the wall configuration on the overall instabilities two liners constructions i.e. stiff and flexible one are taken into account. Both liners are investigated at various pressure levels. Finally, relation between perturbations upstream of the burner and system response in form of flame transfer function is obtained.
Abstract: Gas turbine combustors have at industrial scale a thermal power released by combustion of 1 to 400 MW. As the flames in these combustors are very turbulent, the combustion generates high levels of thermo acoustic noise. Of crucial importance for the operation of the engine is not the noise emitted, but its structural integrity. This may be at hazard when the combustor liner starts to vibrate in a mode linked to the thermo acoustic noise. This is even more likely when the combustion noise changes to an unstable closed loop feed back system. Another dangerous situation may arise when there is a two way interaction between the combustion oscillations and the liner vibration. For these reasons the understanding of transient combustion and its coupling with wall vibration in a typical gas turbine combustion chamber is of prime interest. This phenomenon is investigated in the project FLUISTCOM in both experimental and numerical work.
In the project a liner was designed with a thin, flexible section with a significant amplitude response on changes in the pressure field caused by the combustion oscillations. Numerical calculations of eigenfrequencies and eigenmodes were performed, followed by transient numerical calculations of the transient combusting flow within the combustion chamber with the use of CFX-ANSYS. The flame investigated was a 1.5 bar, 150 kW premixed natural gas flame.
Solutions for the pressure field obtained during numerical computations of the combustor flow were collected and implemented in the structural code (Ansys) as surface loads on the liner side. Results show the one way response of the liner structure as a result of the transient pressure generated by the combustion of the gas flow.
The paper will present the predicted results on the combustion field, the accompanying oscillating pressure field, and the induced structural vibration of the combustor liner as predicted by the finite element structural code.
Abstract: The resulting limit cycle amplitude and frequency spectrum of a flame placed in a combustor of rectangular cross section is investigated. The partially premixed flame is stabilized on a bluff body placed in the upstream half of the combustor. The bluff body is an equilateral triangular wedge with one of the edges pointing in upstream direction. Acoustically there is an open downstream end and theer are variable acoustic conditions at the upstream end.
In order to assess the properties of the flame in this combustor, steady state flame simulations have been performed of the flame in the enclosure. These provided the fields of the mixing of gases, temperature and the velocity.
A test rig was manufactured for this burner at the University of Twente. In a first set of experiments, gas temperature, pressure field and flame chemiluminescence in the combustor were measured as a function of power and acoustic inlet condition. It was observed that the combustor exhibited strong natural pressure oscillations. The measured pressure, temperature and chemiluminescence data are compared to the CFD simulations and to numerical calculations of the acoustics presented in a companion paper by M.Heckl.
Abstract: Introduction of lean premixed combustion to gas turbine technology reduced the emission of harmful exhaust gas species, but due to the high sensitivity of lean flames to acoustic perturbations, the average life time of gas turbine engines was decreased significantly. Very dangerous to the integrity of the gas turbine structure is the mutual interaction between combustion, acoustics and wall vibration. This phenomenon can lead to a closed loop feedback system, with as a result fatigue failure of the combustor liner and fatal damage to the gas turbine rotor.
In this paper the use of numerical tools for CFD and CSD analysis is described to predict the hazardous frequencies at which the instabilities can occur. The two way interaction of the combustible compressible flow and structural walls is investigated with the application of the partitioning fluid-structure interaction approach. In this technique the fluid and structural model are considered as individual but coupled dynamic systems. Information of conditions at the fluid-structure interface is exchanged at given time steps through the interface connection created between the numerical domains. Therefore, the partitioned approach can take the full advantage of existing, well developed and tested codes for both, fluid and structure analysis. Next to the fluid-structure interaction analysis, acousto-elastic and modal models are applied to get insight into the acoustic and vibration pattern during the instability process. The calculations use elements devoted to the solution of the acoustic and structural fields. This approach has the advantage of high resolution of the acoustics, but takes into account only one way combustion dynamics (taken from the CFD results). All numerical solutions are compared to experimental results obtained on a laboratory test rig. The data is evaluated for both, pressure and velocity fields.
Abstract: The computation of fluid–structure interaction (FSI) problems requires solving simultaneously the coupled fluid and structure equations. A partitioned approach using a volume spline solution procedure is applied for the coupling of fluid dynamics and structural dynamics codes. For comparative study, two commercial packages for combustion and structural analysis are used. Results of numerical investigations of FSI between unsteady flow and vibrating liner in a combustion chamber are presented and show good agreement with experimental data.
Abstract: Steady state and transient heat transfer is a very important aspect of any combustion process. To properly simulate gas to wall heat transfer in a turbulent flow, an accurate prediction of the flow and the thermal boundary layer is required. A typical gas turbine combustion chamber flow presents similarities with the academic backward facing step case, especially in the near wall regions where the heat transfer phenomena take place. For this reason, due to its simple geometry and the availability of well documented experiments, the backward facing step with wall heat transfer represents an interesting validation case. Results of steady-state and transient calculations with the use of various turbulence models are compared here with available experimental data.
Abstract: The turbulent flame in the lean combustion regime in a gas turbine combustor generates significant thermo-acoustic noise. The thermo-acoustic noise induces liner vibrations that may lead to fatigue damage of the combustion system. This phenomenon is investigated in the project FLUISTCOM using both an experimental and a numerical approach. The correlation between acoustic pressure oscillations on one side and liner vibrations on the other side is a prime interest.
In order to have better insight in the processes present in the combustion chamber, a combustion test rig was designed and manufactured at the University of Twente. One of the most important parts of the test rig is a liner with a flexible section and optical access to measure the vibration pattern and amplitude. This paper describes a flame investigated at 1.5 bar, 125 kW with premixed natural gas and air. The experimental measurements of the vibrations are done with the use of a Laser Doppler Vibrometer. CFX-Ansys was used for the transient numerical calculations of the transient combustion flow within the combustion chamber. Simultaneously, the pressure results from the near-wall region were collected and sent as initial conditions to a structural code (Ansys). Results show the one way response of the liner structure as a result of the transient pressure generated by the combustion of the gas flow.
The paper will present the numerically predicted results on the combustion field, the accompanying oscillating pressure field, and the induced structural vibration of the combustor liner. These results will be compared with the available experimental data.
Abstract: This paper presents numerical results of the fluid-structure investigation in a generic gas turbine combustion chamber. Results are obtained with the use of CFX-10 and ANSYS-10 commercial codes. The influence of the pressure changes inside the combustion chamber on the vibration pattern of the liner walls and vice versa is investigated.
Abstract: Numerical investigations of fluid structure interaction between unsteady flow and vibrating liner in a combustion chamber are undertaken. The computational study consist of two approaches. Firstly, a partioned procedure consists of coupling the LES code AVBP for combustion modelling with the FEM code CaluliX for structural dynamic analysis. The CFD code CFX together with the FEM Ansys package are then used.
Results of unsteady fluid structure interaction applied to combustion system are presented and compare well with experimental results.