Combustion and Emission Performance in a Can Annular Combustor

Transcription

1 Combusion and Emission Performance in a Can Annular Combusor Gang Pan, Hongao Zheng Absrac In order o design a dual-fuel combusor for he Chemically Recuperaed Gas Turbine (CRGT), numerical research on he performance of a can annular combusor is presened in his paper. The objecive is o undersand he impac of differen acual operaing condiions on combusion and emission performance. The load of he combusor is varied from 30% o 100%. The mahemaical models used for fuel combusion consis of he realizable k-ε model for urbulen flow, discree phase model (DPM) for racking he oil drople pah lines, probabiliy densiy funcion (PDF) model for non-premixed oil combusion and polluion model for nirogen oxides (NO X ) emission. The obained resuls using hese models show good agreemen wih oher experimenal resuls. The effecs of differen condiions on he flow field, flame shape, gas emperaure and NO X emission are presened in his paper. The resuls obained in his sudy show he change in gas urbine can annular combusor performance wih differen acual operaing condiions. Index Terms Numerical simulaion, differen condiions, flow field, NO X emission G I. INTRODUCTION AS urbines ha operae in simple cycles have low efficiencies because he urbine exhaus gases come ou very ho and his energy is los o he amosphere [1]. In order o recuperae of he exhaus energy from gas urbines, he CRGT sysem has been presened. Concerning CRGT sysems, here are some repors on he hermal efficiency and feasibiliy of he sysems [1]-[4]. Bu few lieraures exis on he combusor design which is a major problem in developing a CRGT sysem. The combusor is he mos heavily hermal load par in he gas urbine, in which boh igniion and combusion keep going on, and normally whose service life is very shor [5]. The performance of he CRGT, paricularly he operaing efficiency and reliabiliy would be grea affeced by he combusor. In order o shoren he design period and increase he design efficiency, a mehod of modifying an original oil fuel combusor ino a dual-fuel one is adoped in his paper. In order o design a dual-fuel combusor wih good performance, i is necessary o sudy he performance of he original oil fuel combusor under differen operaing condiions. Wih he developmen of compuer echnology, he mehod Gang Pan is wih he College of Power and Energy Engineering, Harbin Engineering Universiy, Harbin , Heilongjiang, China. ( markpg0808@gmail.com). Hongao Zheng is wih he College of Power and Energy Engineering, Harbin Engineering Universiy, Harbin , Heilongjiang, China. ( zhenghongao9000@163.com). of numerical simulaion is more and more employed o sudy he urbulen flow in differen fields [5]-[7]. So far here are very pleny of available researchers have employed CFD o invesigae he flow, combusion and hea ransfer in a combusor. For example, Li e al. [5] made use of he CFD mehod o analyze combusion and cooling performance in an aero-engine combusor. Wang e al. [8] sudied he srucure of a hydrogen/air premixed flame in a micro combusor under one condiion by he direc numerical simulaion mehod. Gobbao e al. [9] performed a CFD simulaion on a hydrogen fuelled single can gas urbine combusor. Ghenai [10] carried ou a numerical invesigaion on he combusion of syngas fuel mixure in gas urbine can combusor. The effecs of syngas fuel composiion and lower heaing value on he flame shape, gas emperaure, and nirogen oxides per uni of energy generaion were presened. Orbay e al. [11] employed he LES model o sudy he swirling isohermal and reacive flows of premixed naural gas/air mixure in a laboraory combusion chamber. Wan e al. [12] invesigaed he effecs of he inle velociy and equivalence raio on he blow-off limi, combusion efficiency, exhaus gas emperaure and ouer wall emperaure profile numerically. Zhang e al. [13] sudied he NO X emission in a model commercial aircraf engine combusor by he CFD approach. Hu e al. [14] proposed a new mehodology named FIA (fuel ieraive approximaion) for LBO limi predicions of combusors, and hen he CFD was employed o predic he LBO limis for an aero-engine combusor. However, all above-menioned sudies on he performance of combusor wih differen geomeric srucures or inle parameers under consan load, bu a limied number of people have sudied he performance of a can annular combusor under differen acual operaing condiions. In his paper, in order o modify he original oil fuel combusor ino a dual-fuel one, he numerical simulaion is adoped o sudy he performance of he oil combusor under differen acual operaing condiions in deail. And he velociy field, emperaure field, pressure loss, NO X emission and so on are obained and analyzed. II. GOVERNING EQUATIONS The mahemaical equaions describing he oil combusion are based on he equaions of conservaion of mass, momenum, and energy ogeher wih oher supplemenary equaions for he urbulence and combusion. The realizable k-ε urbulence model is used in his sudy. The equaions for he urbulen kineic energy k and he dissipaion rae of he urbulen kineic energy ε are solved. For non-premixed combusion modeling, he PDF model is used. The equaions

2 are as follows (ignore radiaion and graviaion) [15]: Mass Conservaion Equaion: ( V) Sm (1) Where he S m is he mass added o he coninuous phase from he dispersed second phase. Momenum Conservaion Equaion: ( VV ) p F (2) Where and F are he sress ensor, force beween dispersed second phase and coninuous phase, respecively. Energy Conservaion Equaion: VE N s k T h J V pv S q Where eff i i h D i1 (3) k eff, J i, S h and q D are he effecive conduciviy, diffusion flux of species i, hea of chemical reacion, hea exchanger beween dispersed second phase and coninuous phase, respecively. Paricles Moion Equaion: dv d p F V V F (4) D p x Where F x is an addiional acceleraion (force/uni paricle mass) erm. PDF Combusion Model: V f f S (5) m ' 2 ( Vf ) ( f ) C ( f ) C f k Where f and ' 2 2 ' 2 g d (6) '2 f are he mixure fracion and mixure fracion variance, respecively. The consan values of, C and g Cd are 0.85, 2.86 and 2.0, respecively. Realizable k-ε model: Vk ( ) k G G Y S k k b k k V ( ) 2 C S C C C G S k k b Where he k =1.0, =1.2, C1 =1.44, C 2 =1.9. III. GEOMETRY, BOUNDARY CONDITIONS, MESH, AND NUMERICAL METHOD (7) (8) The gas urbine can annular combusor is designed o burn he fuel efficienly. The basic geomery of he gas urbine can annular combusor is shown in Fig. 1(a). The size of he combusor is 0.391m in he Z direcion, 0.245m in he Y direcion, and 0.743m in he X direcion. A qualiy mesh was generaed for he can annular combusor (see Fig. 1(b)). (a) Geomery model (b) Meshes for combusor Fig. 1. Geomery and meshes of he gas urbine can annular combusor The finie volume mehod and he firs-order upwind mehod were used o solve he governing equaions. The convergence crieria were se o 10-4 for he mass, momenum, urbulen kineic energy, dissipaion rae of he urbulen kineic energy, and he mixure fracion. For he energy and he polluion equaions, he convergence crieria were se o The boundary condiions of he air and oil are shown in Table I. TABLE I CONDITIONS IN COMBUSTOR Condiions A B C D E Combusor load (%) Air mass flow rae (kg/s) Air Temperaure (K) Air pressure (MPa) Oil mass flow rae (kg/s) Oil Temperaure (K) IV. MODEL VALIDATION The experimenal daa of [16] was used o validae he numerical model. In he experimenal and numerical cases, he fuel of aviaion kerosene and C 12 H 23 were adoped, respecively. The boundary condiions of he experimenal and numerical cases are as follows: air mass flow rae, 0.24kg/s, he emperaure is 500K, he pressure is 0.13 MPa; oil mass flow rae, kg/s, he emperaure is 300K. The emperaure and CO 2 mass fracion radial profiles a he combusor oule are compared o he resuls of [16] in

3 Fig.2, respecively. The values of emperaure and CO 2 mass fracion in he presen resuls and he experimenal resuls of [16] are compleely similar. The iny differences beween he experimenal and numerical resuls may be due o he alernaive fuel adoped in he numerical case and he measuremen error in he experimenal case. Based on he comparisons, he obained profiles have an acceped behavior and accuracy; hus he soluion can appropriaely sudy he performance of combusor. (a) Temperaure from he swirler o he combusor oule can be mainly divided ino hree zones, namely primary zone, middle zone and mixing zone. In he primary zone, he air leaves he swirler o he combusor liner a high swirling velociy. Then, he air mixes wih fuel injeced from he nozzle. Afer ha, he swirling mixure of he air and fuel flows forward and enraps he air in he cener area of he combusor liner, and a he same ime, he downsream air refills he region. As a resul, a CRZ (Cenral Recirculaion Zone) wih a couner-roaing vorex pair forms in he head of he combusor liner, which ensures igniion and sable combusion in he combusor liner. Bu due o he non-symmery of he shell, he flow raes of air flowing ino he combusor liner from he upper and lower surfaces are differen, so he vorexes are asymmeric. I can also be seen from Fig.4, he gas in CRZ jus flows back o he swirler exi which can avoid he phenomenon of flashback occurring. In he downsream area of he primary zone, srong fresh air injecs ino he combusor liner from he primary holes, as a resul, he primary zone is cu off by he fresh air. Alhough a small amoun of he gas is inhaled ino he primary zone, mos gas flows downsream and mixes wih he cold air from he diluion holes. This makes he flow in he combusor more uniform. (b) CO 2 mass fracion Fig. 2. Temperaure and CO 2 mass fracion radial profiles a combusor oule (a) Meridian plane V. RESULTS A. Fuel Drople Spray and Aomizaion When fuel is inroduced ino he combusor from he nozzle, i is broken up ino iny fuel droples firsly and hen burned wih air. The movemen of he fuel droples was raced under 100% load, as shown in Fig. 3. The longes drople residence ime is 2.53ms, which means ha fuel is aomized and vaporized quickly and burned immediaely afer evaporaion. Fig. 3. Fuel droples pah line B. Velociy Field Due o lile difference on he sreamlines of velociy field under he loads from 30% o 100%, only he velociy field in he combusor under full load is presened in his paper, as shown in Fig. 4. I can be seen from he figure ha he flow (b) Horizonal secion Fig. 4. Sreamlines of velociy field in combusor under 100% load C. The Air Flow Disribuion The air flow disribuion is a basic problem in combusor design and developmen. I affecs he combusor igniion, combusion efficiency, flame sabiliy, oal pressure loss, wall cooling, oule emperaure disribuion and so on. In order o modify he original oil fuel combusor ino a dual-fuel one, he air flow disribuion in he origin combusor was sudied. Fig. 5 shows he proporions of swirler and primary holes air disribuion. I can be seen from he figure, he proporion of he air disribuion under differen condiions is almos he same: swirler air-15%, primary air -29.5%. So i can be concluded ha he air disribuion is mainly deermined by he combusor srucure, no he operaing condiions.

4 (a) 30% Load Fig. 5. Air flow disribuion under differen condiions D. Pressure Loss Fig. 6 shows he numerical and experimenal daa of (pressure recovery facor) under differen condiions, which can reflec he pressure loss direcly. As shown in he figure, he numerical daa and experimenal daa show a good agreemen and he slighly increases as he combusor load increases. When he combusor load is 30%, he of he experimen and simulaion are 94.52% and 93.18%, respecively. And when he combusor load is 100%, he of he experimen and simulaion are 94.93% and 94.13%, respecively. (b) 50% Load (c) 70% Load (d) 80% Load Fig. 6. Pressure recovery facor under differen condiions E. Temperaure Field The conours of he prediced gas emperaure for he combusion of C 12 H 23 in combusor are shown in Fig. 7. The peak gas emperaure is locaed in he primary zone. However, due o he diluion of burned gas wih he air, he gas emperaure is lower in middle and mixing zones han ha in primary zone. I can be seen from Fig.7, he high emperaure zone becomes bigger as he load increases. I is because more fuel is burned in he combusor under he high load. Due o he unsymmerical flow field in he combusor, he emperaure disribuion in he combusor is also asymmeric. As can be seen from he Fig.7, he gas emperaure near he lower surface of he combusor liner is higher han ha near he upper surface in he primary zone. However, he gas emperaure near he lower surface of he combusor liner is lower han ha near he upper surface in he middle zone and mixing zone. The maximum emperaure difference beween upper and lower surfaces appears in he middle zone, which is abou 400K. In order o proec he combusor liner, a symmerical flow field is required. Fig. 7. Temperaure field (e) 100% Load In order o sudy he high emperaure zone in deail, he conours of T=1800K under differen condiions are shown in Fig. 8. As can be seen from Fig. 8, here are wo high emperaure zones in he combusor. One is he CRZ, which increases as he load increases along he cenral axis of combusor liner. The oher locaes behind he primary holes, which is no exisence under 30% load bu becomes bigges under full load. (a) 30% Load (c) 70% Load (b) 50% Load (d) 80% Load

5 kmol m -3 s -1, respecively. (e) 100% Load Fig. 8. Conours of T=1800K under differen condiions The emperaure disribuion along he cenral axis of combusor liner is shown in Fig. 9. As seen in he figure, due o more fuel burned under he high load, he oule emperaure increases as he load increases. In he region from he fuel nozzle o x=0.15m, due o he aomizaion and evaporaion processes of fuel, he emperaure a he cenral axis of combusor liner has lile change as he load increases. However, in he region from x=0.15m o combusor oule, he emperaure a he cenral axis of combusor liner increases as he load increases. I can also be seen from Fig. 9, due o he air from he primary holes akes more hea o reach he equilibrium emperaure, he emperaure a he cenral axis decreases sharply a x=0.2 ~ 0.22m. A x=0.35m, he emperaure a he cenral axis decreases under all condiions which illusraes ha he air from diluion holes can blend wih he burned gas effecively. And he mixing of diluion air and burned gas is useful o improve he uniformiy of combusor oule emperaure disribuion. Fig. 9. Temperaure disribuion along he cenral axis Fig. 10. Rae of hermal and promp NO along he cenral axis (100% load) Fig. 11. Rae of NO along he cenral axis Fig. 12 shows he NO X emission a combusor oule. Due o he formaion rae of NO increases as he load increases, he NO X emission a combusor oule increases. The lowes NO X emission is 111.6ppm a 30% load and he highes is ppm a 100% load. The NO X emission of he combusor does no mee he requiremens of he regulaions. I can be concluded from Figs. 7, 11 and 12 ha he higher emperaure in he primary zone can lead o higher formaion rae of NO, hen more NO X emission a he combusor oule. So in order o modify he origin combusor ino a dual-fuel one wih low NO X emission, some effecive measures should be adoped o conrol he emperaure in he primary zone of his combusor. F. NO X Emission In his paper, in order o modify an oil combusor ino a dual-fuel combusor and conrol he NO X emission a he same ime, he NO X emission performance of he origin combusor was sudied here. The hermal NO X and promp NO X were considered in his paper. Fig. 10 shows he formaion raes of hermal and promp NO along he cenral axis of combusor liner under 100% load. As can be seen from he figure, he maximum formaion rae of promp NO is far less han ha of he hermal NO, which are and kmol m -3 s -1, respecively. I illusraes ha he hermal NO X emission is he mainly NO X emission in his combusor. Fig. 11 shows he formaion raes of NO along he cenral axis of combusor liner under differen condiions. The formaion rae of NO including he formaion raes of hermal NO and promp NO. I can be seen from he figure, he maximum formaion rae of NO under each condiion is locaed in he primary zone and he maximum formaion rae of NO increase as he load increases. This is mainly due o he emperaure increases as he load increases. I can also be seen from Fig. 11, he maximum formaion rae of NO under 30% load is far less han ha under 100% load, which are and Fig. 12. NO X emission a combusor oule under differen condiions VI. CONCLUSION In order o design a dual-fuel combusor for he CRGT, hree-dimensional CFD analysis of C 12 H 23 combusion in gas urbine can annular combusor is presened in his sudy. The effecs of differen condiions on he flow field, flame shape, gas emperaure and NO X emission were deermined in his sudy. The obained resuls were in good agreemen wih oher experimenal resuls. The resuls derived in his paper are lised briefly as follows: 1) The combusor burns fuel effecively under differen condiions. A CRZ wih wo unsymmerical vorexes is

6 creaed in combusor, which could improve he flame sabiliy under differen condiions. 2) Due o he unsymmerical flow field, he emperaure disribuion in combusor is asymmerical also. The gas emperaures near he upper and lower surfaces of combusor liner are differen, and he maximum emperaure difference is abou 400K in he middle zone, which is no conducive o proec he liner. So a symmerical flow field in he combusor may exend he life of he combusor liner. 3) The NO X emission a he combusor oule monoonous increases as he load increases. The values of he NO X emission are ppm and 111.6ppm under 100% load and 30% load, respecively. In order o reduce he NO X emission, an effecive way should be adoped o lower he emperaure in he primary zone. In summary, in order o modify he oil combusor ino a dual-fuel one successfully, he high NO X emission problem of he oil combusor mus be solved. disseraion, Nanjing Universiy of Aeronauics and Asronauics, Nanjing, REFERENCES [1] L. G. Alves and S. A. Nebra, Thermoeconomic Evaluaion of a Basic Opimized Chemically Recuperaed Gas Turbine Cycle, Inernaional Journal of Thermo-dynamics, vol. 6, no. 1, pp.13 22, [2] R. Carapellucci and A. Milazzo, Thermodynamic opimizaion of a rehea chemically recuperaed gas urbine, Energy Conversion and Managemen, vol. 46, no , pp , [3] C. Luo, N. Zhang, N. Lior and H. Lin, Proposal and analysis of a dual-purpose sysem inegraing a chemically recuperaed gas urbine cycle wih hermal seawaer desalinaion, Energy, vol. 36, no. 6, pp , [4] W. Han, H. Jin, N. Zhang and X. Zhang, Cascade uilizaion of chemical energy of naural gas in an improved CRGT cycle, Energy, vol. 32, no. 4, pp , [5] L. Li, X. F. Peng and T. Liu, Combusion and cooling performance in an aero-engine annular combusor, Applied Thermal Engineering, vol. 26, pp , [6] J. C. Chen and W. Chen, How flow becomes urbulen, IAENG Inernaional Journal of Applied Mahemaics, vol. 42, no. 2, pp , [7] L. A. Danao, J. Edwards, O. Eboibi and R. Howell, A Numerical Invesigaion ino he Effecs of Flucuaing Wind on he Performance of a Small Scale Verical Axis Wind Turbine, Engineering Leers, vol. 21, no. 3, pp , [8] H. Wang, K. Luo, S. Lu and J. Fan, Direc numerical simulaion and analysis of a hydrogen/air swirling premixed flame in a micro combusor, Inernaional Journal of Hydrogen Energy, vol. 36, no. 21, pp , [9] P. Gobbao, M. Masi, A. Toffolo and A. Lazzareo, Numerical simulaion of a hydrogen fuelled gas urbine combusor, Inernaional Journal of Hydrogen Energy, vol. 36, no. 13, pp , [10] C. Ghenai, Combusion of Syngas Fuel in Gas Turbine Can Combusor, Advances in Mechanical Engineering, vol. 2010, pp.1 13, [11] R. C. Orbay, K. J. Nogenmyr, J. Klingmann and X. S. Bai, Swirling urbulen flows in a combusion chamber wih and wihou hea release, Fuel, vol. 104, pp , [12] J. Wan, A. Fan, K. Marua, H. Yao and W. Liu, Experimenal and numerical invesigaion on combusion characerisics of premixed hydrogen/air flame in a micro-combusor wih a bluff body, Inernaional Journal of Hydrogen Energy, vol. 37, no. 24, pp , [13] M. Zhang, Z. Fu, Y. Lin and J. Li, CFD Sudy of NO X Emissions in a Model Commercial Aircraf Engine Combusor, Chinese Journal of Aeronauics, vol. 25, pp , [14] B. Hu, Y. Huang, F. Wang and F. Xie, CFD predicions of LBO limis for aero-engine combusors using fuel ieraive approximaion, Chinese Journal of Aeronauics, vol. 26. no. 1, pp , [15] ANSYS Fluen 12.0 Theory Guide, ANSYS, Inc., January [16] X. X. Dang, Experimenal invesigaion and numerical simulaion of a gas urbine annular combusor wih dual-sage swirler, Ph.D.