Comparison Between PIV Measurements and CFD Simulation on a Model of GT Annular Burner D. Giordano, S. Giammartini, M. Rufoloni, G. Calchetti, F. Manfredi, E. Giacomazzi ENEA - C. R. Casaccia Sec. ENE-IMP Rome, Italy XXV Event of the Italian Section of the Combustion Institute 3-6 June 2 Rome, Italy
Comparison between PIV Measurements and CFD Simulation on a Model of GT Annular Burner D. Giordano, S. Giammartini, M. Rufoloni, G. Calchetti, F. Manfredi, E. Giacomazzi ENEA - Casaccia - Dipartimento di Energia Via Anguillarese n.31, 6 Casaccia (Roma), Italy ABSTRACT A model of industrial burner for air breathing engines has been investigated experimentally and numerically. The combustor is formed by an external circular duct (obtained with the overimposition of a diffuser on a circular pipe) and a stabiliser (consisting of an inverted frustum of cone) fixed axially. Burner shape has been designed to generate an abrupt increase of vorticity along the inner wall of the outletting annular jet. A ring like vortex steady evolves downstream the bluff body as a consequence of the strong velocity gradient of the outletting mixture (methane-air or LPG-air). Exhaust gasses, coming from annular front flame, evolve in a central counter rotating zone (generated by the annular vortex) and form an hot bubble useful for flame stabilization. A comparison between experimental PIV measurements and numerical calculations (RANS 2D, 3D and LES) allowed us to point out either flame base properties either numerical performances, with a description of GT burner behaviour (from a fluid mechanic, thermal an chemical points of view). Key words: CFD simulation, PIV measurements, GT burner INTRODUCTION Recirculation zone properties, downstream a conical bluff body burner (applied on gas turbines combustion chambers), have been investigated with the join venture of experimental measurements (PIV and LDA velocities measurements, thermocouples measurements) and numerical evaluations like: Reynolds Average Navier Stoke Simulation (RANS) and 3D Large Eddy Simulation (LES). Ring like vortex (steady evolving downstream an axial bluff body stabiliser) fixes the premixed flame at the burner exit by a maximum Reynolds number of Re=1 and equivalence ratio (φ=fuel flow/air flow) changeable in the field φ=.57-. Experiences showed in the following have been made on methane-air premixed flame with constant regime. This working condition corresponds to low pollutant emissions 1 (low NOx and limited CO). To put in evidence different numerical performances we compared the same mean experimental field with RANS and LES results to point out advantages and disadvantages. EXPERIMENTAL SET-UP PIV measurements have been acquired by means of the following set up: CCD camera 484 x 768 pixels; double pulsed Nd:Yag lasers (energy = 3 mj, wave length = 532 nm); air feeding systems (with Al2O3 air seeder); Dantec PIV cross correlation and synchronisation unit. Flame evolves inside a cylindrical test chamber (diameter =,4 m, height = 1,5 m), with a conical upper hood and a perforated base (porosity = 4%) and
outcomes from a conical bluff body burner placed axially 1 mm over the greed above mentioned. Burner quarl is formed by an external 23.4 mm steel pipe and an inner stabiliser formed by a 15 mm frustum of cone 1. Air and fuel flow in downward the burner, in a premixing cylindrical volume, with a cross flow configuration. Fuel comes from the bottom while four air jets comes from the vertical walls, crossing the fuel axially. EXPERIMENTAL AND NUMERICAL RESULTS Experimental measurements have been done with the same Re= 8 and equivalence ratio φ=,6 (corresponding to low NOx emission 1, limited CO production 1 and a steady flame evolution). Velocity field downstream the conical stabiliser shows the evolution of a counter rotating ring vortex (fig.2), where exhaust gasses evolve heating the axial hot bubble. This burning regime was chosen for low NO x and CO emissions 1,2,3 and steady flame evolution 4,5. To obtain a good numerical representation of flow evolution we measured the following boundary conditions: a. turbulence level of the outletting mixture at the burner exit is u /Um=13% (measured by means of 2D LDA technique); b. mean velocity of the air incoming from the base of the combustion chamber is equal to,5 m/s. To verify flame stability and properties we performed PIV acquisition and two dymensional (choosen for the symmetry of the burner shape) RANS simulations (useful for the rapidity of the result) on the same working regime. Numerical simulation was made without air entrainment from the base of the combustion chamber. The result of was a numerical unsteady flame that burned out fastly (due to oxygen deficiency). As a matter of fact we considered the air entrainment and obtained a steady numerical configuration for flame behaviour. To validate numerical results we closed the base of the combustion chamber and verified the unsteady behaviour of the flame. This confirmed the usefulness of air entrainment for flame feeding and stabilisation (in figure1 there is a scheme of the base of the combustion chamber). The first comparison between PIV and numerical data showed a great difference. This result was due to the angularity of the outletting mixture, ought to an error of angularity for burner pipe installation. As a consequence, several LDA measurements have been performed on the burner exit to control and optimize the symmetry of air-methane mixture outlet. Burner Combustion Chamber Air entrainment Fig. 1 Scheme of the base of the combustion chamber and entrainment The final comparison between two and three dimensional RANS evaluation and the experimental mean trend (reported in fig.2 only for the 2D case for the sake of brevity) allowed us to point out the following results for the simulation : 1. Understimation of the recirculation zone extension; 2. Fast reduction of momentum downstream the ring like vortex, while the real trend shows a annular jet maximum velocity quite constant. Contrary of the real case this seems to be due to high turbulent viscosity that doesn t decrease with temperature.
V(m/s) PIV RESULT RANS RESULT V(m/s).8.6 6 5 4 3 1 - -1 y(mm) x(mm) 6 5 4 3 1 5 1 15 y(mm).8.6 METHANE AIR INLET Fig. 2 Comparison between mean velocity field (calcultated on the base of 6 PIV samples) and 2D RANS simulation (Re=8, φ=,6) V(m/s) PIV RESULT LES RESULT Vm (m/s).8.6 6 5 4 3 1 - -1 y (mm) 6 4 1 y (mm) 3..8.6 Fig. 3 Comparison between mean velocity field (calcultated on the base of 6 PIV samples) and 2D LES simulation (Re=8, φ=,6)
LES simulation was made in two and three dymension. We succed to acquire the flame with a 2 D PIV for the symmetry of the phenomenum and the absence of swirl movements of outletting mixture. The comparison between 2D simulation and experimental samples shows a good agreement, even if we doesn t consider 3D vortex stretching tipically considered with LES calculations (fig.3). This semplification allowed us to obtain the same result of the 3D case but with a lower computational time. Temperature field obtained with numerical simulation gave a good qualitative rappresentation of temperature variation useful to predict pollutant emissions. CONCLUDING REMARKS Annular bluff body burner has been investigated by means of two dymensional PIV technique. Ring vortex rotates downstream the conical stabiliser, forced by burner shape. This recirculation zone creates an hot central volume crossed by exhaust gas. The provisional RANS simulation pointed out the relevancy of air entertainment (from the base of the combustion chamber) for flame oxygen feeding and stabilisation. An improved ventilation system was designed for hook duct to increase air intake downward the combustio chamber. The comparison between RANS and LES simulation with PIV mean data allowed us to point out high turbulent viscosity of the first code in hot region with an apparent velocity reduction, causing an under estimation of the recirculation zone extension. Greater efficiency of LESS simulation allowed us to have a quite good reproduction of the whole field. Two dimensional application of LES code showed good performances due to the swirl absence. Squishing movements generate a rotation in plane that can be measured with 2D PIV and probably, makeing forgettable vortex stretching, Numerical result for temperature field gave an over estimation of local values especially for hotter regions. The comparison between thermocouples measurements confirmed us the good qualitative representation of numerical result, useful to predict hottest regions (useful for NOx emission control and preview) and to detect ignition zone (useful for flame stabilisation and control). ACKNOWLEDGMENT We gratefully acknowledge Prof. G. Gori, Prof. C. Coppa and Prof. G. Bella who allowed us to perform numerical simulations at the University of Rome Tor Vergata and for insightful discussion about numerical results. We acknowledge Mrs R. Gallodoro for technical and scientific support. REFERENCES 1. Giordano D., Giammartini S., Manfredi F.: Sixth International Conference on Technologies and Combustion for a Clean Environment, Oporto, Portogallo, July, p.1487 (1) 2. Giordano D., Giammartini S., Giacomazzi E., Manfredi F.: Combustion and the Environment XXIV Event of The Italian Section of the Combustion Institute, S. Margherita, Italy, September, p. V.7, (1) 3. Lefebvre A. H., Ibrahim A. R. A. F. and Benson N. C.: Combustion and Flames,. 1, (1966). 4. Lefebvre A. H.: Gas turbine combustion, Mc Graw-Hill, New York (1983) 5. Chen R. H., Driscoll J.: Proceedings of the Combustion Institute, 22:531 (1988):