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: 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: 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: 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: 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: 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.