The necessity for a combustion system to work with premixed flames and its capability to cope with rapid load variations avoiding the occurrence of thermo-acoustic instabilities, has led to investigate the complex dynamic phenomena that occur during combustion. Thanks to numerical simulations it is possible to examine these complex mechanisms getting useful information to optimize the combustion system. The aim of this work is to describe a numerical procedure developed in Ansaldo Energia for the investigation of combustion dynamics occurring in Ansaldo Energia gas turbines. In this paper, firstly the experimental apparatus of a full scale atmospheric test rig equipped with Ansaldo Energia burner is described. Secondly, the flame behavior is studied by means of a Large Eddy Simulation (LES). Once the LES has reached a statistically stationary state, a forcing is added to the system to compute the Flame Transfer Function (FTF), in terms of amplitude n and delay time τ, related to initial phases of humming. Thirdly, the forced flame simulations are used as the input of an Helmholtz solver to analyze the acoustic behavior of the system, which is then compared to experimental data. Finally, to evaluate the feasibility of a less computationally intensive approach, a RANS simulation of the same configuration is described and the results are transferred to FEM (Finite Element Method) Helmholtz solver: a comparison between the LES approach and the RANS approach is carried out with reference to the experimental data.
This paper concerns the study of self–sustained combustion instabilities that occur in a test rig characterized by a single longitudinal combustion chamber equipped with a full scale industrial burner and a longitudinal plenum. The length of both plenum and combustion chamber can be continuously varied. During tests, at a fixed value of the length of the combustion chamber, a sensibility of the amplitude of pressure oscillations to the length of the plenum has been registered, while the frequency remained constant. To investigate this behavior, a linear stability analysis has been performed evaluating the influence of the length of the plenum on the frequency and growth rate of the registered unstable mode. The analysis has been performed by means of a finite element method (FEM) code with a three–dimensional distribution of the n-τ Flame Transfer Function (FTF) computed by means of computational fluid dynamics (CFD) simulations. According to the Rayleigh criterion, the distribution of the local Rayleigh index has been computed in order to evaluate the acoustic energy production, while the scattering matrix of the entire system has been used to evaluate the acoustic energy losses. Numerical results show that the reduction of the plenum length induces an increase of acoustic energy losses while the energy production remains almost constant. This result is in agreement with the reduction of the pressure oscillations amplitude observed during tests.
VeLoNOx™ series (Very Low NOx) burners have been developed by Ansaldo Energia to satisfy the most restrictive emissions limits required by various countries. For these combustion systems the characterization of the field of stable operating conditions is a key design element. In particular, flame transition is of paramount importance for the stability of these combustion systems. Usually, this phenomenon occurs during load ramps and is characterized by a change of the flame shape, occurring in a well-defined load range. Proper flame shape control is essential to achieve more stable operating conditions up to base load. At transition, experiments in a full scale pressure test-rig clearly show both a modification of the flame shape, a change of the measured air side burner effective area, and a reduction of the combustion chamber wall temperatures. LES simulations were conducted using the same boundary conditions as RANS simulations discussed in a previous work [1]. In this paper, we show that both RANS and LES were able to correctly predict flame transition and a RANS-LES-Experiment comparison is performed.
VeLoNOx™ (Very Low NOx) burners have been developed by Ansaldo Energia to satisfy the most restrictive emission limits of many countries. The stability field range is a key element for the design of such combustion systems; ever-deeper knowledge is therefore essential. The impact on stability of flame transition is of paramount relevance. Usually, flame transition occurs during load ramp: two different flame shapes take place in a well-defined load range. When loading up to base load condition, stability requires proper choice of flame shape. Observation of flame transition in a full-scale pressure test-rig implies a) unambiguous observation of an alteration in the flame shape b) measurement of both the burner area discharge coefficient “Alfa” (on the air side) and of a temperature drop on the walls of the combustion chamber. The aim of the present work was to verify that RANS simulations with particular setting of the physical models can provide us with realistic description of the global flame behaviour in the full-scale, pressurised test rig even during the crucial flame transition at partial base load. This aim was achieved. CFD RANS has led to two different solutions with different shape, just like in the experiment. The main global parameters of the combustion system undergo similar changes and with similar magnitude in CFD results and in the experiment. In particular, the burner discharge coefficient “Alfa” increases with flame transition from “open” to “closed” flame, while the combustion chamber wall temperature decreases with flame transition from “open” to “closed” flame. It means that this CFD RANS simulation provides us with a reliable flow and temperature fields which can be used as meaningful input data for the more detailed analysis of both thermal load at combustion chamber wall and CO and NOx emission level, thermo-acoustic behaviour of the combustion system by means of more specific tools. Such information could be useful when designing more efficient, less polluting combustion systems.
Recently, because of environmental regulations, gas turbine manufacturers are restricted to produce machines that work in the lean combustion regime. Gas turbines operating in this regime are prone to combustion-driven acoustic oscillations referred as combustion instabilities. These oscillations could have such high amplitude that they can damage gas turbine hardware. In this study, the three-step approach for combustion instabilities prediction is applied to an industrial test rig. As the first step, the flame transfer function (FTF) of the burner is obtained performing unsteady computational fluid dynamics (CFD) simulations. As the second step, the obtained FTF is approximated with an analytical time-lag-distributed model. The third step is the time-domain simulations using a network model. The obtained results are compared against the experimental data. The obtained results show a good agreement with the experimental ones and the developed approach is able to predict thermoacoustic instabilities in gas turbines combustion chambers.
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