Thus d evelopment r esearch and technology is needed to suppress the thermo- a coustic instability Model- based design and control, as well as to shor

Transcription

1 2016 International Conference ISBN: on Material Science and Civil Engineering (MSCE 2016) D evelopment Segmented Dynamic Combustion M odel to I nvestigate Instability G as Turbine Jiwoong Jeong 1 1 1, Jisoo Park, Jaeyoung Han and 1Mechanical Eng ine 1, Sangseok Yu ering, Chungnam national university, Daejeon, Korea T his p aper presents a dynamic combustion model with 10 segmented well- stirred reactors which thermo- acoustic instability gas turbine is able to be evolved Because combustion instability in gas turbine r esults in s erious damage to the combustor system, it i s inevitable to understand t he physics instability In this study, segmented dynamic combusti on model and thermo- acoustic instability model has been developed in Matl ab/simulink platform The dynamic segmented model is developed to understand the consumption f uel and oxidizer and the volume segmentation is discretized by finite difference method T he thermo- acoustic instability model is based on the wave equation to describe the pressure fluctuation Model is validated with reference experiment which shows that frequency pressure appears 430Hz, 630Hz under the low-speed and high- speed range respectively Also fluctuations speed and temperature corresponding to the two input condition in the chamber were presented As a result, reliable combustor simulation model has been d eveloped to analyze combustion characteristics under the operating condition K eywords: G as Turbine, Combustion, Instability, Segment, Fluctuation 1 I ntroduction A stationary ga s turbine is a power plant that is converting chemical energy fuel t o mechani cal work As emission regulation is stringent, the stationary gas turbine employs Low NOx technology which makes worse in stability combustion i nside the burner Combustor a gas turbine is divided into a diffusion flame and a premixed flame method Diffusion flame sharply r aises NOx that is due to local h igh t emperature There are a number methods such as catalytic reduction and d ilution air to reduce emissions nitrogen oxides Nevertheless, the combustor is a trend that turns into premixed way t o meet even the strin gent r egulations Most the gas turbines are being used turbulent lean premixed combustion mode to meet t he regulatory standards However, premixed combustion technology is difficult to d evelop because the sensitivity the driv ing characteristics Also, thermo-acoustic instability occurs due to the structure and chemical reaction the combustor Thermo- acoustic instability is caused by t he acoustic oscillation in the c ombustion chamber by the interaction the heating mechanism and the pressure f luctuations S uch combustion instability is a problem because it can seriously damage the entire system, as well as the gas turbine combustor durability must be a ddressed in the development the combustor, the core technology [ 2, 7] 734

2 Thus d evelopment r esearch and technology is needed to suppress the thermo- a coustic instability Model- based design and control, as well as to shorten the d uration development technique can be made a systematic understanding t he existing technology Also it may create a variety strategies, while minimizing physical damage which may occur during the controller design and o peration Through the development a model-b ased controller, it is possible to encapsulate acoustic instability combusti on under allowable margin The purpose this study is to develop a d ynamic m odel segmented c ombustion in t he one dimensional chamber u nder M ATLAB / SIMULINK environment that w ill be a predicted model for model- based controller design T he model i s then applied to understand acoustic instability low NOx gas turbine over various o perating conditions Figu re 1 Feedback mechanism combustion instability 2 M odeling approach 2 1 Thermo-a coustic I nstability Mechanism W hen the gas turbine is operated, the acoustic instability is observed The acoustic instability is known as feedback thermal instability to acoustic instability The thermal instability is also working to fluctuate equivalence ratio which is coupled with thermal instability Thermo- acoustic instability is generated by the interaction o f three components, as shown in Fig 1 W here a nd are the combustor pressure oscillations, t h periodic heat addition process, and i a coustic energy loss process, respectively 735

3 Combustor is unstable when the net rate energy addition to the acoustic field e xceeds the net rate damping provided by inherent dissipative processes [ 2, 11] 2 2 Thermo-a coustic instability modeling Thermo- a coustic instability is a phenomenon that is amplified by the pressure and t he heat release perturbation as defined by the In this study, the pressure and heat release instability is modeled in three different regimes which are Ⅰ, Ⅱ, Ⅲ, a s shown in Fig 3 The model regimes are nozzle regime, principal combustion regime, and post oxidation regime Each regime is modeled as well stirred reactor and each regime is then segmented 3 nodes in regime I, 4 nodes in r egime II, and 3 nodes in regime III so that the longitudinal aspects can be i nvestigated more specifically F low and pressure fluctuations are observed in Regime Ⅰ and Ⅲ Since the fluctuation flow and pressure are closely coupled with dynamics pressure and turbulence, model is decoupled to mean and fluctuated variables A nd the perturbation governing equations thus configured through Regime Ⅰ, Ⅱ a nd Ⅲ s o that interactions momentum and energy conservation can grasp a c haracteristic instability 736

4 F igure 2 Schematic d efinition computational regions in the combustor G overning equations In this study, combustor is assumed as well- stirred reactor and the region is then segmented to investigate axial variations pressure and velocity The governing equations a well-s tirred reactor are given by M ass Conservation: M omentum C onservation: ( 3) E nergy Conservation: 737

5 ( 4) ( 5) Region Ⅰ disturbance equation can be solved by the induced wave equation a ssociated with acoustic Induced wave equation is as follows W ave Equation: T o obtain the instability pressure value through the wave equation, and can obtain the instability density and velocity values from the continuity equation and t he momentum equation 3 R esults and Discussion ( 6) 3 1 P ressure F requency Characteristics the I nlet Velocity Thermo-a coustic instability model wa s v alidated with experimental r esults L NGT combustor [2] T he instability characteristics LNGT combustor is shown i n Figure 3 The experimental results were compared with the modeling results If you have given your entrance speed to 16m/s thermo-a coustic instability results for characteristics is shown in Figure 6 At this time, pressure instability due to the wave equation at the nozzle area is measured by the FFT result 430Hz, it is propagated to a well-s tirred reactor It was confirmed that a constant oscillation occurs repeatedly in the burner model In addition, the propagated frequency from the inlet through the analysis the vibration was presente d 4 C onclusion 1 ) A thermo- a coustic instability mechanism with segmented geometry model is d eveloped In particular, thermo- acoustic instability indicated characteristic according to inlet velocity In order to express the thermo- acoustic instability expressed as pressure, speed, density and average sum fluctuations was c onstructed the governing equations It was derived by integrating the 738

6 p ressure perturbation equations nonlinear wave equations form F igu re 3 Fourier transform pressure perturbation at low velocity(16m/s) Figu re 4 FFT pressure perturbation at inlet duct under low velocity( 16 m/s) 2 ) Compared to thermo- acoustic instability characteristics under the same conditions as the reduced combustion test specifications Georgia Tech a ppeared similar to the pressure instability frequency T hus th e developed model may represent the combustor dynamic characteristics according to the change in the equivalent ratio Because combustor is represented by a finite difference it can be seen by sections characteristics Through the developed combustor model you can see the combustor operation c harac teristics according to the change the control input operation 739

7 A cknowledgement This research was funded by the Energy Development Project ( A) o f the Korea Institute Energy Technology Evaluation and Planning R eferences 1 J WS Rayleigh, T he Theory Sound 1945, V olume 2 2 T imothy C Lieuwen, Investigation combustion instability mechnisms in p remixed gas turbine, Georgia institute technology, B T Zinn, ME Lores, Application the galerkin method in the solution n onlinear axial combustioninstbility problems in liquid rockets Combustion S cience and Technology, 1972, vol4, pp J G Le e and D A Stantavicca, Experimental Diagnostics for the study c ombustion instability in lean premixed combustors Journal propulsion and p ower, 2003, Vol 19, No 5, pp S Park, A M Annaswamy, and AF Ghoniem, Heat release dynamics m odeling kinetically controlled burning Combustion and Flame, 2002, v ol 128, pp S Turns, I ntroduction to combustion New York: McGraw-H ill, C Li euwen, Vigor Yang, Combustion instabilities in gas turbine engines USA: AIAA, M arble, FE, and Candel, S M, Acoustic Disturbance from gas non-u niformities convected through a nozzle, Journal Sound and Vibration, 1977, Vol 55, No 2, pp Pankiewitz, C, and Sattelmayer, T, Time domain simulation combustion instabilities in annular combustors, Journal Engineering for Gas Turbine a nd Power, 2003, Vol 125, No 3, pp A A Peracchio and W Proscia, Nonlinear heat release/acoustic model for thermoacoustic instability in lean premixed combustors, ASME Gas Turbine a nd Aerospace Congress, D Kim, Introduction to flame transfer function in lean premixed gas turbine c ombustor, T ransaction th e Korean society mechanical engineers B, 2011, Vol B, No 9, pp