femlab conference stockholm 2005 university of catania department of industrial and mechanical...

FEMLAB Conference Stockholm 20FEMLAB Conference Stockholm 200505

UNIVERSITY OF CATANIA Department of Industrial and Mechanical

Engineering

Authors: M. ALECCI, G. CAMMARATA, G. PETRONE

ANALYSIS AND MODELLING OF ANALYSIS AND MODELLING OF A LOW NOx SWIRL BURNERA LOW NOx SWIRL BURNER


PROBLEM FACED :

CFDCOMPUTATIONAL FLUID DYNAMIC

ADVANTAGES:

•Reduction of planning time and costs.

•Availability to study systems for which the experimentation is difficult and expensive.

•Availability to study systems in 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.


OBJECTIVES OF THE STUDY:

FEM modelling of the “cold” fluid-dynamics of a swirl burner.

Evaluation and analysis of the velocityand pressure fields.

Comparison of the obtained results with those coming from literature.


SWIRL EFFECT:““SSwirlwirl” is defined as the spiral rotational motion imparted to a fluid ” is defined as the spiral rotational motion imparted to a fluid

upstream of an orifice. This spiral develops in a direction parallel upstream of an orifice. This spiral develops in a direction parallel to the injection one.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 decrease in flame temperature.3) Flame stabilization. 4) High performance combustion forseveral carboneous materials.

NOx REDUCTION


THE SWIRL BURNER: The modelled burner is used in several industrial applications:


The anterior side is characterized by the following devices:

Holes for the fuel injectionDuct for the flame revelation probe Axial swirler


MODELLING STEPS:

Construction of the geometrical model

Femlab module choice and physics settings.

Meshing the model

Plotting e post-processing of theresults.

Problem solving


GEOMETRICAL MODEL

The swirler has been realized by a CAD software, due to its complex shape,

and further imported into the Femlab drawing grid.


EQUATIONS AND MODULE CHOICE:

FLOW HYPOTHESES :

INCOMPRESSIBLE(Ma<0.3)

TURBULENT(Re>2000)

NEWTONIAN FLUID(homogeneous gases mixture)

T Fu u p u

0u

i Tij

j k

uu k k

x

21 1/ /i T

ijj

uu c k c k

x

Momentum balance

Mass balance (continuity)Turbulent Kinetic energy (K)

equationDissipative turbulent (e) energy equation

K-e Turbulence module


PHYSICS SETTINGS:

•Density: 1 kg/m3

•Kinematic viscosity: 1 E-5 m2/s

•Volume forces neglected

Inlet flow with axial velocity: u=20 m/s.

No slip conditions: U=0.

Pressure: p=3 bar

SUBDOMAINSETTINGS:

BOUNDARY CONDITIONS:


COMPUTATIONAL GRID AND USED SOLVER

Used solver:

DIRECT (UMFPACK), NON LINEAR

Finer mesh close to the swirler zone


PLOTTING E POST-PROCESSING OF THE RESULTSCross sections: velocity field

It is possible to observe how in the first duct the fluid accelerates whenit goes through the swirler.


Longitudinal section:

When the fluid enters the reactor, it expands with the classical cone course, up to velocity of 1-2 m/s.


Streamlines of the fluid:

Spiral motion inside the “core”, typical of“swirling jets”.


“SWIRL NUMBER” AND LITERATURE RESULTS

3

2

12tan

3 1

h

x h

R RGS

G R R R

“Swirl number”:

S<0.6S<0.6 Weak Weak swirlswirl

0.6<S<1 0.6<S<1 Medium swirlMedium swirl

S>1 S>1 Strong Strong SwirlSwirl

LDV(Laser Doppler

Velocimetry)

Swirl number of the analyzed system: S=0.77


Radial distribution of the axial velocity close to the burner’s outlet:

The negative values correspond to the RFZ developmentaccording to the literature results.


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)


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.


CONCLUSIONS AND FURTHER DEVELOPMENTS:

1. 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.

2. 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.

3. In a further development the combustion reaction will be introduced into the model, analyzing how it may influence the velocity and pressure fields.

4. The thermal characterization of heat exchanges will complete the entire model.


ACNOWLEDGEMENTS:This work has been developed at the This work has been developed at the

DepartmentDepartment

of Industrial and Mechanical of Industrial and Mechanical Engineering of theEngineering of the

University of Catania with the University of Catania with the precious collaboration ofprecious collaboration of

ITEA S.p.A, SOFINTER Group www.iteaspa.com

AUTHORS’ REFERENCES:[email protected] [email protected]@diim.unict.itWork phone: +39 095 7382452

femlab conference stockholm 2005 university of catania department of industrial and mechanical...

Documents

femlab conference stockholm

axial swirler slide

femlab drawing grid

nox reduction slide

e turbulence module

low nox swirl burner

swirl effect

axial velocity