UNIVERSITY OF CATANIA Department of Industrial and Mechanical Engineering

1. UNIVERSITY OF CATANIA Department of Industrial and Mechanical Engineering Authors: M. ALECCI, G. CAMMARATA, G. PETRONE ANALYSIS AND MODELLING OF A LOW NOx SWIRL BURNER FEMLAB Conference Stockholm 2005

2. PROBLEM FACED: CFD COMPUTATIONAL FLUID DYNAMIC • ADVANTAGES: • Reduction of planning time and costs. • Availability to study systems for which the experimentation is difficult and expensive. • Availability to study systemsin conditions of extreme safety . • DISADVANTAGES: • Discretized models present inevitable PDE approximation . • In the linear systems solution iterative methods are used. These allow to obtain only solutions close to the exact ones. FEMLAB Conference Stockholm 2005

3. OBJECTIVES OF THE STUDY: FEM modelling of the “cold” fluid-dynamics of a swirl burner. Evaluation and analysis of the velocity and pressure fields. Comparison of the obtained results with those coming from literature. FEMLAB Conference Stockholm 2005

4. SWIRL EFFECT: “Swirl” is defined as the spiral rotational motion imparted to a fluid upstream of an orifice. This spiral develops in a direction parallel to the injection one. Then, a tangential velocity component and high pressure gradients (axial and radial) develop. The low pressure zone inside the spiral core is characterized by toroidal vortexes: (Precessing Vortex Core phenomenon PVC) This results (for strong degree of swirl) in the setting up of a Reverse Flow Zone (RFZ) where the fluid is recirculated towards the burner’s outlet. 1) Good mixing of reactants. 2) A decreaseinflame temperature. 3) Flame stabilization. 4) High performance combustion for several carboneous materials. NOx REDUCTION FEMLAB Conference Stockholm 2005

5. THE SWIRL BURNER: The modelled burner is used in several industrial applications: FEMLAB Conference Stockholm 2005

6. The anterior side is characterized by the following devices: Duct for the flame revelation probe Axial swirler Holes for the fuel injection FEMLAB Conference Stockholm 2005

7. MODELLING STEPS: Construction of the geometrical model Femlab module choice and physics settings. Problem solving Meshing the model Plotting e post-processing of the results. FEMLAB Conference Stockholm 2005

8. GEOMETRICAL MODEL The swirler has been realized by a CAD software, due to its complex shape, and further imported into the Femlab drawing grid. FEMLAB Conference Stockholm 2005

9. EQUATIONS AND MODULE CHOICE: FLOW HYPOTHESES : INCOMPRESSIBLE (Ma<0.3) TURBULENT (Re>2000) NEWTONIAN FLUID (homogeneous gases mixture) K-e Turbulence module Momentum balance Mass balance (continuity) Turbulent Kinetic energy (K) equation Dissipative turbulent (e) energy equation FEMLAB Conference Stockholm 2005

10. PHYSICS SETTINGS: • Density: 1 kg/m3 • Kinematic viscosity: 1 E-5 m2/s • Volume forces neglected SUBDOMAIN SETTINGS: Pressure: p=3 bar BOUNDARY CONDITIONS: Inlet flow with axial velocity: u=20 m/s. No slip conditions: U=0. FEMLAB Conference Stockholm 2005

11. COMPUTATIONAL GRID AND USED SOLVER Finer mesh close to the swirler zone DIRECT (UMFPACK), NON LINEAR Used solver: FEMLAB Conference Stockholm 2005

12. PLOTTING E POST-PROCESSING OF THE RESULTS Cross sections: velocity field It is possible to observe how in the first duct the fluid accelerates when it goes through the swirler. FEMLAB Conference Stockholm 2005

13. Longitudinal section: When the fluid enters the reactor, it expands with the classical cone course, up to velocity of 1-2 m/s. FEMLAB Conference Stockholm 2005

14. Streamlines of the fluid: Spiral motion inside the “core”, typical of “swirling jets”. FEMLAB Conference Stockholm 2005

15. “SWIRL NUMBER” AND LITERATURE RESULTS S<0.6Weak swirl 0.6<S<1 Medium swirl S>1 Strong Swirl “Swirl number”: LDV (Laser Doppler Velocimetry) Swirl number of the analyzed system: S=0.77 FEMLAB Conference Stockholm 2005

16. Radial distribution of the axial velocity close to the burner’s outlet: The negative values correspond to the RFZ development according to the literature results. FEMLAB Conference Stockholm 2005

17. Iso-surfaces of axial velocity: The bulb, located in the central core, corresponds to negative values of axial velocity. That means the fluid is recirculated towards the burner outlet section. (RFZ development) FEMLAB Conference Stockholm 2005

18. Radial distribution of the axial velocity close to the burner’s outlet and 10 cm and 20 cm from it: RFZ results stronger close to the burner’s outlet and it decreases as soon as the fluid reaches a certain distance from it. FEMLAB Conference Stockholm 2005

19. CONCLUSIONS AND FURTHER DEVELOPMENTS: • A three-dimensional simulation of a low NOx “swirl burner” is reported in this study. The analysis has been focused on the swirl device by the evaluation of the velocity and pressure fields of the jet entering the combustion reactor. • The model reflects, with good approximation, the real behaviour of the system, and finds a good correspondence with literature. Thus, it may be used to simulate different operative conditions (such as other fluids or other inlet velocities), avoiding expensive experimentation. • In a further development the combustion reaction will be introduced into the model, analyzing how it may influence the velocity and pressure fields. • The thermal characterization of heat exchanges will complete the entire model. FEMLAB Conference Stockholm 2005

20. ACNOWLEDGEMENTS: This work has been developed at the Department of Industrial and Mechanical Engineering of the University of Catania with the precious collaboration of ITEA S.p.A, SOFINTER Group www.iteaspa.com AUTHORS’ REFERENCES: m_alecci@yahoo.it gcamma@diim.unict.it gpetrone@diim.unict.it Work phone: +39 095 7382452 FEMLAB Conference Stockholm 2005