Investigation of self-excited combustion instabilities in two different combustion systems

The objective of this paper is to characterize dynamic pressure traces measured at self-excited combustion instabilities occurring in two combustion systems of different hardware. One system is a model lean premixed gas turbine combustor and the other a fullscale bipropellant liquid rocket thrust chamber. It is commonly observed in both systems that low frequency waves at around 300Hz are first excited at the onset of combustion instabilities and after a short duration, the instability mode becomes coupled to the resonant acoustic modes of the combustion chamber, the first longitudinal mode for the lean premixed combustor and the first tangential mode for the rocket thrust chamber. Low frequency waves seem to get excited at first since flame shows the higher heat release response on the lower frequency perturbations with the smaller phase differences between heat release and pressure fluctuations. Nonlinear time series analysis of pressure traces reveals that even stable combustion might have chaotic behavior with the positive maximum Lyapunov exponent. Also, pressure fluctuations under combustion instabilities reach a limit cycle or quasi-periodic oscillations at the very similar run conditions, which manifest that a self-excited high frequency instability has strong nonlinear characteristics.     

An experiment analysis of MILD combustion with liquid fuel spray in a combustion vessel

Mild combustion is characterized by its distinguished features, such as suppressed pollutant emission, homogeneous temperature distribution, reduced noise, and thermal stress. Recently, many studies have revealed the potential of MILD combustion in various power systems but most studies have been focused on gas phase fuel MILD combustion. Therefore, further study on MILD combustion using liquid fuel is needed for the application to a liquid-fueled gas turbine especially. In this work, we studied experimentally on the formation of liquid fuel MILD combustion under the condition of high dilution by burnt gas generated from a first premixed flame in two stages combustor which consists of the first premixed burner and secondary combustor. In particular, the effects of burnt gas velocity and oxygen level of burnt gas on the formation of liquid fuel MILD combustion were investigated. The results show that as the burnt gas velocity through the nozzle becomes higher, the color of flames was changed from yellow to pale blue and flames became very short. The OH radical measured by ICCD camera was uniformly distributed on the pale blue flame surface and its intensity was very low compared to conventional liquid diffusion flame. As burnt gas velocity is increased, local high-temperature region appeared to be diminished and the flame temperature became spatially uniform. And CO emission was sampled around 1 ppm and NOx emission was measured around 10 ppm under the overall equivalence ratio of 0.8 to 0.98 for 40 mm or less diameter of velocity control nozzle. This low NOx emission seems to be attributed to maintaining the average temperature in secondary combustor below the threshold temperature of thermal NOx formation. In view of the uniform temperature distribution, low OH radical intensity and low NOx emission data in the secondary combustor, formation of stable MILD combustion using kerosene liquid fuel could be verified at high burnt gas velocity.

     

Experimental study on combustion instability and attenuation characteristics in the lab-scale gas turbine combustor with a sponge-like porous medium

The present study has experimentally investigated the combustion instability and its attenuation characteristics in the lab-scale swirlstabilized premixed combustor with a sponge-like porous medium. Unlike the conventional premixed burners, this model combustor has the unique features including a porous dump plane and an acoustic cavity, which was devised for attenuating the combustion instability. When replacing the dump plane with a non-porous medium, the burner becomes the conventional design. In order to evaluate the damping effects of the porous medium on the unstable flame dynamics, various acoustic and photonic measurements and flame visualization were made. Special emphasis is given to the effects of the acoustic cavity length on the stabilization characteristics. Results showed that the model combustor with the porous dump plane and the acoustic cavity exhibited dramatic attenuation of the pressure oscillation intensity by up to about 40 %. The attenuation was increased with increasing the acoustic cavity length. It was also found that the attenuation is effective not only for the fundamental resonance but also for its higher harmonics. Based on the experimental results, detailed discussions are made for the combustion instability and its attenuation characteristics in the model gas turbine combustor with porous and nonporous media.

     

Analysis of the combustion oscillation in a silo-type gas turbine combustor and its suppression

The characteristics of combustion oscillation of a silo-type 79.5 MW gas turbine combustor in commercial operation and its suppression have been investigated. The oscillation of the lean premixed gas turbine combustor resulting from the combustion instability occurred at near full load operation. An FFT analysis of the combustion dynamics showed that the dominant frequency of the oscillation would be that of the 1st longitudinal acoustic resonance mode of the combustor. To suppress the combustion oscillation, a passive control technique for reducing the combustion instability was employed; that is, the fuel to the combustor was redistributed by adjusting the operational schedule of one of six fuel control valves, which would lead the increase of the local operational equivalence ratio near the central recirculation zone of the combustor. By doing so, the oscillation was successfully reduced to the permissible level while the amount of NO_x emission met proper regulatory level set by the local government.     

A simulation for prediction of nitrogen oxide emissions in lean premixed combustor

Chemical reactor networks (CRN) models were developed for lean premixed gas turbine combustor to predict the NOx emissions. In this study, CRN models are constructed based on the computational fluid dynamics (CFD) for both non-pilot and pilot flame cases. Predictions of NOx emissions in combustor with the developed models were made by using CHEMKIN code and full GRI 3.0 chemical kinetic mechanism in the CRN. The predicted results agree reasonably well with the experimental data obtained from a simplified test combustor for the GE7FA gas turbine. The effects of overall equivalence ratio, swirl angle and pilot fuel ratio on the NOx emissions were investigated.     

某重型燃气轮机燃烧室燃烧流动的数值模拟

对某重型燃气轮机燃烧室燃烧天然气进行数值模拟,在模拟过程中采用了雷诺应力模型、EBU-Arrheniue湍流燃烧模型和六通量辐射模型来描述其燃烧流动过程,运用FLUENT软件求解了三维流场和温场分布.计算结果能够很好地反映该重型燃气轮机燃烧室燃烧流动特点,对预测燃烧室内的燃烧流动有一定参考价值.     

Feasibility study of ultra-low NOx Gas turbine combustor using the RML combustion concept

A new combustion concept, the so called RML, was investigated to validate its application as a gas turbine combustor for combustor outlet temperatures over 1973 K. The feasibility study of the RML combustor was conducted with zero dimensional combustion calculations. The emission characteristics of RQL, LEAN, EGR and RML combustors were compared. The calculation results showed that the RQL combustor has lower NOx emissions than the LEAN at high outlet temperature. NOx emissions of the RML combustor at equivalence ratio of the rich chamber of 2.0 can be reduced by 30 % compared with the EGR combustor, and lower than the RQL combustor at a combustor outlet temperature over 1973 K. However, the CO emissions of the RML combustor were higher than those of the LEAN and EGR combustors. Also, the possibility of applying the RML combustor to gas turbines was discussed considering residence time, equivalence ratio of the rich chamber and recirculation rate. Although further research to design and realize the proposed RML combustor is needed, this study verified that the RML concept can be successfully used in a gas turbine combustor.

     

Linear stability analysis of acoustically driven pressure oscillations in a lean premixed gas turbine combustor

The dynamic response of a turbulent premixed flame to acoustic velocity perturbations was experimentally determined in a swirl-stabilized lean-premixed gas turbine combustor. CH* chemiluminescence intensity and the twomicrophone method were used to measure heat release rates and inlet velocity fluctuations, respectively. Using the n-τ formulation, gain and phase of flame transfer functions were incorporated into an analytic thermoacoustic model to predict instability frequencies and modal structures. Self-excited instability measurements were performed to verify eigenfrequencies predicted by the thermoacoustic model. Instability frequency predicted by the model is supported by experimental results. Results show that the self-excited instability frequency of ～ 220 Hz results from the fact that the flames amplify flow perturbations with f = 150 ～ 250 Hz. The other instability frequency of ～ 350 Hz occurs because the whole combustion system has an eigenfrequency corresponding to the ¼-wave eigenmode of the mixing section.     

Idealized gas turbine combustor for performance research and validation of large eddy simulations

This paper details the design of a premixed, swirl-stabilized combustor that was designed and built for the express purpose of obtaining validation-quality data for the development of large eddy simulations (LES) of gas turbine combustors. The combustor features nonambiguous boundary conditions, a geometrically simple design that retains the essential fluid dynamics and thermochemical processes that occur in actual gas turbine combustors, and unrestrictive access for laser and optical diagnostic measurements. After discussing the design detail, a preliminary investigation of the performance and operating envelope of the combustor is presented. With the combustor operating on premixed methane/air, both the equivalence ratio and the inlet velocity were systematically varied and the flame structure was recorded via digital photography. Interesting lean flame blowout and resonance characteristics were observed. In addition, the combustor exhibited a large region of stable, acoustically clean combustion that is suitable for preliminary validation of LES models.     

基于CFD的氢气／空气当量比对微尺度燃烧室燃烧特性影响的分析

以美国麻省理工学院（MIT）研制的硅基六晶片微燃烧室为研究对象,提出利用二维CFD（计算流体动力学）数值模拟的方法,研究在保持微尺度燃烧室进口氢气／空气流量不变的情况下,改变氢气／空气当量比对燃烧室燃烧特性的影响。整个模拟计算主要包括氢气／空气的流动路径、微燃烧室的内部区域以及整个燃烧室的墙壁面;同时在计算过程中我们考虑了氢气／空气的流体动力学特性、传热学特性和详细的基元反应机理。结果表明,利用二维CFD数值模拟的方法研究微尺度燃烧室燃烧特性可行,与国外实际测量结果较为相似,为今后微型燃气轮机燃烧室的研制及改进提供了一定的参考依据。     

Biparametric assessment of the combustion stability in an industrial gas turbine combustor

Here we propose a generalized procedure for a two-parameter assessment of the Combustion stability (CS) of industrial gas turbines. In evaluating the CS, this procedure employs two parameters of measured dynamic pressure data: the Root-mean-square (RMS) pressure as the primary parameter and the damping ratio as a secondary parameter. The former tells the time-averaged level of the dynamic pressure, and, the latter, the degree of acoustic energy loss. A data point pairing the two parameters, which are evaluated at a specific instance of a combustion process, identifies Instantaneous combustion stability (ICS) by its location on a 2-D domain of both parameters. Collective representation of the ICS points on the domain produces a CS map of the combustion process. The locus of the ICS point on the map represents the temporal variation of CS during the combustion process. The biparametric assessment procedure divides the CS map into three regimes (i.e., stable, transitional and unstable) by utilizing two threshold values for the RMS pressure and one for the damping ratio. The feasibility of the proposed procedure was tested with the dynamic pressure data from a model gas turbine combustor burning synthetic natural gas. Then the technique was applied experimental data obtained from a laboratory-scale lean premixed combustor to identify the three regimes of the combustion process of a reported case. We found that the procedure is able to provide gas turbine operators with valuable information on CS during a combustion process, especially on the transitional regime.

     

Application of dfb diode laser sensor to reacting flow (II) —liquid-gas 2-phase reacting flow—

Diode laser sensor is conducted to measure the gas temperature in the liquid-gas 2-phase counter flow flame. C₁₀H₂₂ and city gas were used as liquid fuel and gas fuel, respectively. Two vibrational overtones of H₂O were selected and measurements were carried out in the spray flame region stabilized the above gaseous premixed flame. The path-averaged temperature measurement using diode laser absorption method succeeded in the liquid fuel combustion environment regardless of droplets of wide range diameter. The path-averaged temperature measured in the post flame of liquid-gas 2-phase counter flow flame showed qualitative reliable results. The successful demonstration of time series temperature measurement in the liquid-gas 2-phase counter flow flame gave us motivation of trying to establish the effective control system in practical combustion system. These results demonstrated the ability of real-time feedback from combustor inside using the non-intrusive measurement as well as the possibility of application to practical combustion system. Failure case due to influence of spray flame was also discussed.     

Model and field testing of a heavy-duty gas turbine combustor

The results of stand and field testing of a combustion chamber for a heavy-duty 150 MW gas turbine are discussed. The model represented one of 14 identical segments of a tubular multican combustor constructed 1∶1 scale. The model experiments were executed at a lower pressure than that in a real gas turbine. Combustion efficiency, pressure loss factor, pattern factor, liner wall temperature, flame radiation, fluctuating pressure and NOx emission were measured at partial and full loads for both model and on-site testing. The comparison of these items in the stand and field test results led to has the development of a method of calculation and the improvement of gas turbine combustors.     

Analysis of the combustion instability of a model gas turbine combustor by the transfer matrix method

Combustion instability is a major issue in design of gas turbine combustors for efficient operation with low emissions. A transfer matrix-based approach is developed in this work for the stability analysis of gas turbine combustors. By viewing the combustor cavity as a one-dimensional acoustic system with a side branch, the heat source located inside the cavity can be described as the input to the system. The combustion process is modeled as a closed-loop feedback system, which enables utilization of well-established classic control theories for the stability analysis. Due to the inherent advantage of the transfer matrix method and control system representation, modeling and analysis of the system becomes a straightforward task even for a combustor of the complex geometry. The approach is applied to the stability analysis of a simple combustion system to demonstrate its validity and effectiveness. This paper was recommended for publication in revised form by Associate Editor Ohchae Kwon Dong Jin Cha received his B.S. and M.S. degrees from Hanyang University in Seoul, Korea, in 1981 and 1983, respectively. He then received his Ph.D. in ME from the University of Illinois at Chicago in 1992, and worked at the US DOE NETL for the next three years as a National Research Council (NRC) Associate. Dr. Cha is currently a Professor at the Department of Building Services Engineering at Hanbat National University in Daejeon, Korea. His research interests include combustion instability of gas turbine for power generation and fluid flows in building services engineering. Jay H. Kim received his BSME from Seoul National University in 1977, MSME from KAIST in 1979 and Ph.D. in ME from Purdue University in 1988. He has joined the Mechanical Engineering faculty of the University of Cincinnati in 1990, and is currently a Professor. Before joining the University of Cincinnati, he worked in industry for six years in Korea and US. His research interests have been in broad areas of acoustics, vibrations and applied mechanics with recent focuses on human/bioacoustics and vibration, gas pulsations and elastic stability. Yong-Jin Joo received his BSME and MSME from Sung Kyun Kwan University in Seoul, Korea, in 1990 and 1992, respectively. Mr. Joo is currently a Project Leader for IGCC Operation Technologies at KEPRI (Korea Electric Power Research Institute) which is the central R&D center of KEPCO (Korea Electric Power Corporation). His research interests include the development of operation and maintenance simulator for power plants including IGCC.     

Ignition and combustion characteristics of the gas turbine slinger combustor

This paper describes the ignition and combustion characteristics of a gas turbine slinger combustor with rotating fuel injection system. An ignition test was performed under various airflow, temperature and pressure conditions with fuel nozzle rotational speed. From the test, there are two major factors influencing the ignition limits: the rotational speed of the fuel nozzle, and the mass flow parameter. Better ignition capability could be attained through increasing the rotational speed and air mass flow. From the spray visualization and drop size measurement, it was verified that there is a strong correlation between ignition performance and drop size distribution. Also, we performed a combustion test to determine the effects of rotational speed by measuring gas temperature and emission. The combustion efficiency was smoothly enhanced from 99% to 99.6% with increasing rotational speed. The measured pattern factor was 15% and profile factor was 3%.     

Experimental investigations on combustion characteristics of a swirl-stabilized premixed burner

Experimental investigations have been conducted to understand the combustion characteristics of a swirl-stabilized double-cone pre-mixed burner used for industrial gas turbines for power generation. NOx and CO emissions, extinction limit, combustion noise, pressure loss, and wall temperature distributions were measured for various operating conditions. Results show that NOx emissions are decreased with increasing air/fuel ratio or decreasing air load unless the air load is too small. CO emissions are also decreased with increasing air/fuel ratio, leading to a positive correlation between NOx and CO emissions. Flame extinction limit is reduced with increasing air flow rate as the flow residence time is reduced. Combustion noise has its peak amplitude at the frequencies of 150 or 300 Hz, which are considered to be the resonance frequencies of the longitudinal mode of the combustor. The noise level at the peak frequency is maximized when the flame is considered to be located near the burner exit. Pressure loss is decreased with the A/F ratio as the flame moves downstream out of the burner.

     

燃气轮机燃烧室冲击射流冷却火焰筒壁的结构参数研究（英文）

The flame temperature in the combustor of a gas turbine is usually as high as 2000 K, while the maximum temperature that can be endured by metal materials is less than 1200 K at present. Therefore, various protective and cooling measures are needed to ensure the operation life of the liner wall which wraps the flame. The lean premixed combustor can meet the increasingly stringent emission requirements, but it requires more air for premixed combustion and then less air for cooling and dilution. In order to obtain a better impingement jet cooling structure, this paper studied the impingement jet cooling structure with vertical circular holes of equal diameter under single outlet condition. The structural variables studied include the jet hole diameter D, the impinging distance Z, the jet hole length(jet plate thickness) t, and the jet-to-jet spacing X is ignored. Among them, X/D(the ratio of the jet-to-jet spacing to the jet diameter) is inversely correlated with the mass flow rate. Within the constant X/D being equal to 10, the influence of D, Z and t on the average heat transfer coefficient h of the target surface under same mass flow was determined by means of conjugate numerical heat transfer analysis and orthogonal test. The results show that Z has significant influence on h, D has moderate influence on h, and t has negligible influence on h. Further, by means of regression orthogonal test, the influence trend of parameters Z and D on h at X/D=10 was studied. The optimal values of Z and D within the research scope were found.     

燃气轮机流动大涡模拟的进展与挑战（英文）

Gas turbine flows are complex and very difficult to be predicted accurately not only due to that they are inherently unsteady but also because the presence of many complex flow phenomena such as transition,separation,substantial secondary flow,combustion and so on.Those complex flow phenomena cannot be captured accurately by the traditional Reynolds-Averaged Navier-Stokes(RANS)and Unsteady RANS(URANS)methods although they have been the main numerical tools for computing gas turbine flows in the past decades due to their computational efficiency and reasonable accuracy.Therefore,the desire for greater accuracy has led to the development and application of high fidelity numerical simulation tools for gas turbine flows.Two such tools available are Direct Numerical Simulation(DNS)which captures directly all details of turbulent flow in space and time,and Large Eddy Simulation(LES)which computes large scale motions of turbulent flow directly in space and time while the small scale motions are modelled.DNS is computationally very expensive and even with the available most powerful supercomputers today or in the foreseeable future it is still prohibitive to apply DNS for gas turbine flows.LES is the most promising simulation tool which has already reasonably widely used for gas turbine flows.This paper will very briefly review first the applications of LES in turbomachinery flows and then focus on two gas turbine combustor related flow cases,summarizing the current status of LES applications in gas turbines and pointing out the challenges that we are facing.     

Conjugated heat transfer and temperature distributions in a gas turbine combustion liner under base-load operation

Prediction of temperature distributions on hot components is important in development of a gas turbine combustion liner. The present study investigated conjugated heat transfer to obtain temperature distributions in a combustion liner with six combustion nozzles. 3D-numerical simulations using FVM commercial codes, Fluent and CFX were performed to calculate combustion and heat transfer distributions. The temperature distributions in the combustor liner were calculated by conjugation of conduction and convection (heat transfer coefficients) obtained by combustion and cooling flow analysis. The wall temperature was the highest on the attachment points of the combustion gas from combustion nozzles, but the temperature gradient was high at the after shell section with low wall temperature.     

Hot-fire injector test for determination of combustion stability boundaries using model chamber

This study realizes the conceptual method to predict combustion instability in actual full-scale combustion chamber of rocket engines by experimental tests with model (sub-scale) chamber. The model chamber was designed based on the methodologies proposed in the previous work regarding geometrical dimensions and operating conditions, and hot-fire test procedures were followed to obtain stability boundaries. From the experimental tests, two instability regions are presented by the parameters of combustion-chamber pressure and mixture (oxidizer/fuel) ratio, which are customary for combustor designers. It is found that instability characteristics in the chamber with the adopted jet injectors can be explained by the correlation between the characteristic burning or mixing time and the characteristic acoustic time. In each instability region, dynamic behaviors of flames are investigated to verify the hydrodynamically-derived characteristic lengths of the jet injectors. Large-amplitude pressure oscillation observed in upper instability region is found to be generated by lifted-off flames.