PILOTED FLAME DATABASE

INTRODUCTION:

The piloted jet burner produces a simple streaming flow (parabolic) and uses a heat source from a set of premixed flames to stabilise the main jet to the burner's exit plane. The burner consists of an axisymmectric jet centred in an annulus in which a number of premixed flames are stabilised on a flame holder. The burner is centred in an unconfined coflowing stream of air. The unburnt pilot gases are a premixed mixture which, when burned to completion produce combustion products which have the same C/H and O/H ratio as that of a stoichiometric mixture of the main fuel and air. If the fuel mixture used in the pilot is different from the jet fuel, then the adiabatic temperature of the pilot flame is likely to be different from that of the main fuel mixture. This is believed not to be a major problem and is accounted for in the computation. The boundary conditions are very simple and well specified. If the initial conditions are not measured directly then laminar flow can be assumed at the jet exit plane for the pilot gases and fully developed turbulent pipe flow can be assumed for the central fuel jet.

Hot gases from the pilot stabilise the jet flame to the burner, regardless of the fuel flowrate in the main jet. This forces flames extinction to occur, at high enough jet velocities, further downstream in a region where turbulent mixing time scales become of the same order of magnitude as the chemical time scales. This makes the burner extremely useful for studying the effects of the interaction between turbulence and chemistry in flames without the added complications of soot formation and thermal radiation. Flame re-ignition may occur further downstream of the extinction zone where turbulent mixing rates are less intense. Piloted flames of hydrocarbon fuels, while generally blue and visibly soot free in the upstream regions, may become yellow and loaded with soot further downstream. The length of the blue flame zone depends on the fuel and on how close the flame is to blowoff.

The stability characteristics of piloted flames of various fuel mixtures have been reported elsewhere [3,8]. The parameters which control the flame stability are the fuel jet velocity, and the stoichiometry of the pilot as well as its flowrate. The mixture fraction in the pilot stream, is generally equal to the stoichiometric mixture fraction of the main fuel except in some cases where it is kept slightly rich to prevent overheating of the burner tip. The simplicity of the flow, and the existence of a fully turbulent region of the flame where the chemical kinetic effects are significant makes this burner an ideal test case for the testing and development of computer models. The existence of the pilot stream can be easily accounted for and the pilot flame gases have no noticeable influence on the flame composition in the region where extinction occurs.

EXPERIMENTAL SETUP:

INITIAL & BOUNDARY CONDITIONS:

The initial conditions (at the jet exit plane) and boundary conditions (in the coflowing stream) are described here. The mean mixture fraction, the mean velocities and the variances and covariances for the velocities are specified. The mean mixture fraction is taken to be one in the jet stream and zero in the air stream. Mixture fraction in the pilot stream is generally taken as stoichiometric unless otherwise specified. The measured initial conditions for the mean axial velocity, u and its rms fluctuations u' normalised with the mean velocity are tabulated.

Initial Conditions for:
Pilot-Stabilised Flames file: icpilot.dat

Modellers may chose to use the actual measured initial conditions as specified in the tables. This will ensure that the jet initial velocity and momentum are accounted for. Initial radial and circumferential velocities are taken as zero, v = w  = 0. It is also assumed that the initial profiles of v' and w' are identical to those of u'. The initial covariances have not been measured but may be taken as:

where C_cv  is a constant equal to 0.5. All other covariances are zero.

An alternative to using the tabulated initial conditions is to use power law fits for the initial velocity profiles both in the jet (approximating fully developed turbulent pipe flow) and the coflow. Initial profiles for u' are specified as piece-wise linear. Sharp changes in velocity may be intentionally avoided for computational purposes. Although these specified velocities may deviate slightly from the measured ones, the momentum both in the jet and the coflow streams are within 2% of the momentum at the specified conditions. The net momentum deficit in the coflow stream due to the boundary layer is accounted for in the computations.

The coflow air boundaries should be set wide enough to include the full boundary layer on the coflow side and to ensure that the boundary specifications do not influence the jet. The pressure gradient at the coflow boundaries is taken as zero. Constant pressure could also be applied at the exit plane. The pilot gas stream is strictly not adiabatic as some heat is dissipated to the burner. This is generally small but could be accounted for in the computations. It is good practice to extend the grid upstream of the nozzle exit to ensure that fully developed pipe flow conditions prevail at the nozzle exit plane.

ACCURACY CONSIDERATIONS:

A detailed description of the sources of error on the Raman and Rayleigh measurements is given in Readme-fst.txt. Typical signal to noise ratios (S/N) are given in the following files:

S/N ratios for data collected 1992 and before.  stn92.dat

FLOW FIELD DATA:

Mean and rms fluctuations of axial and radial velocities

SPONTANEOUS RAMEN/RAYLEIGH/LIF DATA:

Temperature and species mass fractions

Conditional and Radial Means & RMS Data:

REFERENCES:

1. Masri, A.R. and Bilger, R.W., `Turbulent Diffusion Flames of Hydrocarbon Fuels Stabilised on a Bluff Body', Twentieth Symposium (International) on Combustion. The Combustion Institute, Pittsburgh, 1985, pp. 319-326.

2. Dibble, R.W., Masri, A.R. and Bilger, R.W., `The Spontaneous Raman Scattering Technique Applied to Nonpremixed Flames of Methane', Combust. Flame 67:189-206 (1987).

3. Masri, A.R. and Bilger, R.W., `Turbulent Nonpremixed Flames of Hydrocarbon Fuels Near Extinction: Mean Structure from Probe Measurements', Twenty-first Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, pp. 1511-5120.

4. Masri, A.R., Dibble, R.W. and Bilger, R.W., `Turbulent Nonpremixed Flames of Methane Near Extinction: Mean Structure from Raman Measurements', Combust. Flame 71:245-266 (1988).

5. Masri, A.R., Bilger, R.W. and Dibble, R.W., `Turbulent Nonpremixed Flames of Methane Near Extinction: Probability Density Functions', Combust. Flame 73:261-285 (1988).

6. Masri, A.R., Bilger, R.W. and Dibble, R.W., `Conditional Probability Density Functions Measured in Turbulent Nonpremixed Flames of Methane Near Extinction', Combust. Flame 74:267-284 (1988).

7. Masri, A.R., Bilger, R.W. and Dibble, R.W., `The Local Structure of Turbulent Nonpremixed Flames Near Extinction', Combust. Flame 81:260-276 (1990).

8. Masri, A.R., Dibble, R.W. and Barlow, R.S., `The Structure of Turbulent Nonpremixed Flames of Methanol over a Range of Mixing Rates', Combust. Flame 89:167-185 (1992).

9. Masri, A.R., Dibble, R.W. and Barlow, R.S., `Chemical Kinetic Effects in Nonpremixed Flames of $H_{2}/CO_{2}$ Fuel', Combust. Flame 91:285-309 (1992).

10. Masri, A.R., Dibble, R.W. and Barlow, R.S., `Raman-Rayleigh Measurements in Bluff Body Stabilised Flames of Hydrocarbon Fuels', Twenty-fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 317-324.

11. Masri, A.R., Dally, B.B., Barlow, R.S. and Carter, C.D., `The Structure of The Recirculation Zone of a Bluff-Body Combustor', Twenty-fifth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1994, pp.1301-1308.

12. Masri, A.R., Dibble, R.W., and Barlow, R.S., `The Structure of Turbulent Nonpremixed Flames Revealed by Raman-Rayleigh-LIF Measurements', Prog. Energy Combust. Sci., 22:307-362 (1997).

13. Dally, B.B, Masri, A.R., Barlow, R.S., Fiechtner, G.J., and Fletcher, D.F., `Measurements of NO in Turbulent Nonpremixed Flames Stabilised on a Bluff-Body', Twenty-sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1996, Vol. 2, pp.2191-2197.

14. Dally, B.B, Masri, A.R., Barlow, R.S., Fiechtner, G.J., and Fletcher, D.F., 'Instantaneous and Mean Compositional Structure of Bluff-Body Stabilised Nonpremixed Flames', Combust. Flame 114:119-148 (1998).

15. Dally, B.B, D.F. Fletcher, and Masri, A.R., 'Flow and Mixing Fields of Turbulent Bluff-Body Jets and Flames',  Combustion Theory and Modeling 2:193-219 (1998).

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