Dynamics of Lean Premixed Systems:

Measurements for Large Eddy Simulation

D. Galley 1,2

, A. Pubill Melsió 2, S. Ducruix

2, F. Lacas

2 and D. Veynante

2

Y. Sommerer 3 and T. Poinsot

3

1 SNECMA Moteurs, Département YKC, 77550 Moissy Cramayel, france

2 Laboratoire EM2C, CNRS UPR 288 - Ecole Centrale Paris, 92295 Chatenay-Malabry,France

3 CERFACS, CFD team, 42 Av. G. Coriolis, 31057 Toulouse CEDEX, France

ABSTRACTLean Premixed Prevaporized (LPP) injection systems have been designed to offer a minimum NOx

and soot emissions. The basic principle of LPP systems is to optimize combustion through an efficient

mixing of fuel and air. This can be achieved by vaporizing the initially liquid fuel and then mixing it

with the air before combustion using for example a swirling flow. It is well known that premixed

combustion can reduce pollutant emissions more than non-premixed combustion [1]. Moreover, a lean

mixture allows to control the flame temperature and then NOx production since it increases with

temperature. However, LPP systems are known to be very sensitive to couplings leading to many kind

of unstable behaviors. This work is a contribution to the understanding of the dynamical phenomena

occurring in a LPP combustor, using advanced laser diagnostics. This paper presents an experimental

and numerical study of a Laboratory-scale gas turbine combustion chamber designed and operated at

laboratoire EM2C. These results are compared with large eddy simulations (LES) performed at

CERFACS.

KEYWORDS

Gas Turbine, Lean Premixed Prevaporized (LPP) burner, Planar Laser Induced Fluorescence (PLIF),

Large Eddy Simulation (LES), Thickened Flame model, Flashback, Precessing Vortex Core (PVC).

EXPERIMENTAL FACILITY

The facility is a lean premixed burner operated at atmospheric pressure, using gaseous propane.

AirFlowmeter

FlowmeterFilter

Filter

Heater

Burner

Propane

Figure 1: Experimental Setup

The experimental setup is fed with dry compressed air and propane (see Figure 1). The flow rates are

monitored through two electronic mass flow meters. Air and propane are injected in the premixing

tube of the combustion chamber presented in Figure 2. Premixing tube and combustion chamber are

made in high quality quartz (fused silica) allowing visible and UV optical access. Main dimensions are

provided on Figure 2. Characteristic numbers of the combustion facility are summarized in Table 1.

The Reynolds number given in Table 1 is based on the bulk velocity and the diameter of the premixing

tube.

TABLE 1

CHARACTERISTIC NUMBERS OF THE COMBUSTION FACILITY

Max. Air mass flow rate 300 m3/h

Max. Propane mass flow rate 15 m3/h

Max. Reynolds Number From 40,000 up to 280,000

Rated thermal power 300 kW

Figure 2 : Swirler, Premixing tube, Combustion Chamber (dimensions in mm)

Mixing is enhanced using a radial-type swirl generator. Air and propane are introduced separately in

the premixing tube. The 6 mm diameter jet of propane is injected axially and sheared by the

surrounding swirling flow of air. Tangential air velocity in the premixing tube is produced using a

radial-type swirl generator (radial guide-vane cascades). Eighteen constant-section vanes, which

impart a helicoidal movement to the airflow, compose the swirl generator. The detailed geometry and

dimensions of the swirl generator are given in Figure 3. The mixture is ignited in the combustion

chamber using a spark plug.

Figure 3: Characteristics of the swirl generator (dimensions in mm)

COMBUSTION REGIMES

Depending on the swirl and Reynolds numbers, large-scale spatial fluctuations of the swirling flow are

coupled with a Central Toroidal Recirculation Zone (CTRZ) [2]. This recirculation zone plays an

important role in flame stabilization as it locally supplies the flame front with hot burned gases to

sustain combustion [3]. An example of swirl-stabilized flame is displayed in Figure 4 (a). Figure 4

shows OH* spontaneous emission of the flame, obtained with a ICCD camera using UV filters WG305

and UG5. In this case, the flame is stabilized in the combustion chamber by the central recirculation

zone created by the swirl (“compact flames”). In some situations, the flame propagates upstream in the

premixing tube as shown in Figure 4 (b). This phenomenon, called flashback, can lead to catastrophic

failure in real gas turbine, but is a stable regime of the facility (“flashback flames”). Since both swirl

stabilized flame and flashback can be safely investigated, this facility gives the opportunity to

understand the dynamics of partially premixed swirling flames and the phenomena leading to

flashback.

Flame regimes mainly depends on air and propane mass-flow rates, and are summarized in Figure 5.

For low and intermediate equivalence ratios (and intermediate air flows), the flame is stabilized in the

combustion chamber due to the CTRZ (“compact flames”). For higher values of the equivalence ratio,

the structure of the flame can be either flashback or compact depending on initial and transient

conditions (“hysteresis region”). The transition from flashback to compact flame takes place at

approximately the same equivalence ratio, = 0.68, whatever the air flow rate. Considering the

compact flame situation, decreasing equivalence ratio leads to a detached flame, stabilized downstream

in the combustion chamber. Then, for a lower equivalence ratio, the flame is spread all over the

combustion chamber. Further decrease of the equivalence ratio leads to blow off [4].

Figure 4 (a): Compact flame Figure 4 (b): Flashback flame

Figure 5: Burner Regimes

Laser Induced Fluorescence Imaging and Measurements

The phenomena occurring in LPP devices, such as flashback, are intrinsically unsteady. Diagnostics

used to understand these phenomena must take this into consideration. The key points of LPP behavior

are mixing efficiency and flame dynamics, which can be linked to acoustic couplings. Planar laser

induced fluorescence (PLIF) gives an instantaneous insight of these two aspects. The mixing is

quantified by seeding the propane flow with acetone vapor. PLIF of acetone as tracer, under restrictive

and well-controlled conditions, provides quantitative measurements of fuel mass fraction [5]. OH

radical displays the instantaneous position of flame front and burned gases. These two diagnostics

allows to study the flow dynamics either in the combustion chamber or in the premixing tube. The

imaging plane may be parallel or perpendicular to the symmetry axis of the experiment. In the last

case, as shown in figure 6, a cooled mirror is placed in the burned gases to transmit the fluorescence

signal to the camera. Longitudinal images have already been studied in [4]. In the present paper, we

focus on the transversal case. Both OH or acetone vapor PLIF can be carried out in this situation.

Quantitative results in the longitudinal situation can be found in [4].

Experimental setup

The whole experimental setup, including lasers and acetone seeding, are given in Figure 6.

Figure 6: Experimental setup and diagnostics

Results

The mixing process is first analyzed. Propane is seeded with acetone (10% in mass of acetone vapor in

propane) and a tranverse cut is made 5 mm downstream the exit plane of the premixing tube. Examples

of PLIF images of acetone is displayed Figure 7. These images show a very coherent structure: a

“comet plume” of fuel rotating in the same direction as the swirl movement created by the blades. This

offset structure seems to turn in the combustion chamber, feeding the flame front. As the laser

frequency is limited to 10 Hz, the images are not temporally connected. The direction of rotation is

deduced from the shape of the propane core, since the plume is at the rear part of the structure due to

the rotating movement. The rotation center is also slightly rotating (as can be deduced from the mean

field). Such coherent structure, known in the field of non reactive swirling flows, is called Precessing

Vortex Core (PVC) [2]. Figure 7 emphasizes the importance of unsteady structures. The average image

(top left) does not present any of them: from the mean point of view, the fuel concentration field is

isotropic in the radial direction. Nevertheless, instantaneous images show anisotropic structures that

control the flame behavior.

As a consequence, the OH instantaneous images exhibit a similar behavior. Indeed, even 2.5 cm

downstream the premixing tube, the reacting zone is not uniform. In each image (Figure 8), OH signal

presents a zone of weak signal, which also rotates from one image to another. This is because the

flame is stabilized on the PVC, the only region where fuel concentration exceeds the lean extinction

limit. This gives us information on the stabilization process of partially premixed swirled flame in this

kind of configuration. Due to the swirl movement of the airflow, a vortex is created in the premixing

tube and convected by the flow. This vortex presents a decreasing fuel concentration profile along its

radius. The inner core is fuel rich whereas the outer cell is lean [4]. Due to the swirl effects and the

sudden expansion in the combustion chamber, this vortex precesses in the combustion chamber. The

flame is then stabilized in a “precessing way”. This mode of stabilization has been confirmed using a

high-speed ICCD camera, recording spontaneous emission of the flame up to 10,000 images per

seconds. The precessing movement of the reactive zones has been confirmed, and a rotating frequency

has been estimated to 660 Hz.

The combination of these two diagnostics, OH and acetone PLIF, has permitted to explain the

stabilization mechanisms of swirled turbulent flame in this particular configuration. The mechanisms

controlling the flame stabilization are non-stationary. As a consequence, simulations of such burner

must be intrinsically unsteady. Reynolds Average Numerical Simulations (RANS) could only give

results corresponding to the mean propane concentration profile (Figure 7 top left) whereas the reality

is quite different as shown in Figure 7. Large Eddy Simulations (LES) resolves the structures of the

flow and thus is an adequate tool to simulate these phenomena.

Figure 7: Acetone LIF, transversal visualization of propane mass fraction 5 mm downstream thepremixing tube. First image: mean image obtained over 100 images.

Regime:120 m3/h of air and 3 m3/h of propane; Equivalence ratio: = 0.6, compact flame.

Figure 8: OH LIF, transversal visualization of reactive zones, 2.5 cm downstream the premixing tube.Regime: 60 m3/h of air and 1.5 m3/h of propane; Equivalence ratio: = 0.6, compact flame.

LARGE EDDY SIMULATIONS

The numerical solver for turbulent reacting flows

The calculations are carried out with the LES parallel solver AVBP developed by CERFACS [6]. The

full compressible Navier Stokes equations are solved on structured, unstructured or hybrid grids

allowing the simulation of reactive turbulent flows on complex geometries by using refined grid cells

only in the mixing and reactive regions of the flow. The numerical scheme provides third-order spatial

accuracy on hybrid meshes [7]. This point is important because high order numerical schemes are

particularly difficult to implement on hybrid meshes but required to perform precise LES. The time

integration is done by a third order accurate explicit multistage Runge-Kutta scheme. The Navier

Stokes characteristic boundary conditions (NSCBC) have been implemented [8] to ensure a physical

representation of the acoustic wave propagation. The objective of LES is to compute the large scale

motions of the turbulence while the effects of small scales are modeled. The WALE model [9] is

chosen to estimate subgrid scale stresses, whereas the flame-turbulence interaction is described by the

dynamic thickened flame model [10-12] which was found relevant to accurately predict partially

premixed flames.

The grid mesh used for this simulation is very fine in the mixing tube in order to resolve weakly

thickened flame. The thickening factor has been set to F = 5 (i.e. the thickness of the resolved flame

front is about five times the unstretched laminar flame thickness). This low value is required to allow

flashback since a too thick flame would not be able to penetrate in the mixing tube due to the

quenching distance. A non premixed flame is expected near the injector nozzle because mixing zones

between fuel and air are too small, while a well-premixed flame should occur in the combustion

chamber. The use of a small thickening factor increases the accuracy of the thickened flame model and

reduces the importance of the subgrid scale model. In such a case the model handles accurately both

mixing and perfectly premixed combustion, but also correctly reproduces pure diffusion flames. For

the present study, an hybrid grid combining hexahedral, prismatic and tetrahedral elements is used

with a total of about 600,000 cells (Figure 9). The walls are assumed to be adiabatic, and the gaseous

fuel injected is propane. A single step chemistry is used. The total physical time simulated for each

transition is about 0.05s corresponding to 3000 hours CPU time on a SGI O3800 R14000 500Mhz. The

computations are typically performed on 32 processors.

Figure 9: 3D view of the mesh

Numerical results

The simulations are carried out for the regimes explored experimentally [13]. Snapshots of a compact

and flashback regimes are given in figures 10 and 11. The burner dynamics are well reproduced. Both

compact and flashback flames can be simulated. Moreover, transitions between these regimes are also

well reproduced. Details are given in [13] and focus is put in the present paper on the mixing process.

Figure 12 compares propane mass fraction from simulations (left) and acetone LIF signal (right). The

coherent structure of mixing is well reproduced by the simulation. Moreover, Figure 10 reveals the

high dynamics of reactive zones (symbolized by the white temperature iso-surface) which is observed

experimentally in Figure 8 ([4]).

Figure 10: Instantaneous visualization of the compact flame. Iso-surface: temperature (T=1600K);vertical plane: axial velocity; black iso-line: zero axial velocity (U=0); gray iso-line: stoechiometric

mixture fraction. Regime:120 m3/h of air and 3 m3/h of propane; Equivalence ratio: = 0.6

Figure 11: Instantaneous visualization of the flashback flame. Iso-surface: temperature (T=1600K);vertical plane: axial velocity; black iso-line: zero axial velocity (U=0); gray iso-line: stoechiometric

mixture fraction. Regime: 21 m3/h of air and 0.75 m3/h of propane; Equivalence ratio: = 0.89

Figure 12: Numerical (left) and experimental visualizations of propane mass fraction, 5mmdownstream the combustion chamber. Regime:120 m3/h of air and 3 m3/h of propane; Equivalence

ratio: = 0.6, compact flame.

CONCLUSION

We have presented a numerical and experimental combined study. Advanced diagnostics have been

used to improve our understanding of the phenomena occurring in lean premixed prevaporized (LPP)

burners. Laser induced fluorescence of OH radical shows the high dynamics of the flame and its

chaotic behavior due to high turbulence levels. Laser induced fluorescence of acetone demonstrates the

presence of highly coherent structures in the mixing process (PVC). These structures are due to the

swirl movement imparted to the airflow. These unsteady phenomena, which explain the stabilization

process of swirled burners, are well reproduced by Large Eddy Simulations (LES).

Further calculations are presently carried out and close comparisons between experiments and

simulations will be presented.

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