Lift-off Heights of Turbulent H2/N2 Jet Flames in a Vitiated Co-flow

1. Lift-off Heights of Turbulent H2/N2 Jet Flames in a Vitiated Co-flow Zhijun WU, Sten H STÅRNER and Robert W BILGER The University of Sydney Dec 2003

2. Outline • 1 Background • 2 Experimental method • 3 Results and discussion • 4 Conclusions

3. 1 Background • Stabilizationmechanism of lifted turbulent jet flame • Auto-ignition in turbulent mixing flow • Dibble burner -Issues of auto-ignition in the stabilization mechanism of lifted flame -A rather simple, well-defined uniform boundary conditions -Attractive for modelling

4. 2 Experimental method • Burner -A replica of Dibble burner -Coflow diameter = 190 mm • Temperature measurement -Exposed type K thermocouple -Compensation for radiation loss • Liftoff height -Video image analysis • Flame noise level -Sound level meter

5. 3 Results and discussion • Control of H2/Air co-flow • Sensitivity of lifted jet flame • Typical cases • Hysteresis of lifted jet flame • Effects of co-flow velocity and noise levels

6. 3.1 Sensitivity of lifted jet flame • After 4 minutes of heating, the temperature of the co-flow is relatively stable and increases slowly with further heating time. • The temperature of co-flow rises linearly with increase of co-flow H2 and with decrease of co-flow air. A finely control for the co-flow H2 was performed using a bypass through a fine needle valve.

7. 3.2 Control of H2/Air co-flow • The lift-off height is very sensitive to the Tcoflow, especially at the case of bigger lift-off height. • The equivalence ratio Φ of coflow corresponding to the lift-off height changes little with the Tcoflow, and plays few effects on radicals produced in the coflow and hence flame stabilization.

8. 3.3 Typical Cases • In case A, the flame is bright and stable, and the fluctuation of lift-off height is small. • In case B, the flame looks weak and accompanied by a strong popping noise. The fluctuation of lift-off height is much bigger than case A. • Case A is for what appears to be a normally stabilized lifted jet flame, and case B for a lifted jet flame that appears to involve serial of auto-ignition phenomena indicated by a strong popping noise. * Thermocouple measurement in nozzle exit

9. Case A Case B 3.4 Hysteresis of lifted jet flame • Lift-off height increases with velocity of the jet. There is an obvious hysteresis region at low lift-off heights on varying the jet velocity from 30 m/s to 80 m/s. • Small variations of jet velocity have little effect on the lift-off height with case B being more sensitive than case A.

10. 3.5 Effects of co-flow velocity Case A • There is no apparent hysteresis region in the lift-off height. • The trend of lift-off height with the co-flow velocity here is definitely different from those in a cold co-flow (Brown et al, 1999), where the lift-off height increases monotonically with the co-flow velocity. Case B

11. 3.5 Effects of co-flow velocity (cont’d) Case A • The noise level of jet flame is obtained by subtracting the noise level of co-flow flame without the central jet flow from the noise level with the central jet flowing. • In case A, the variation of jet flame noise level shows a similar trend to the variation of lift-off height. However, a totally opposite trend is found in case B. • The noise results indicate that the stabilization mechanism of the jet flame in case B is different from that of case A. The reasons for this are of interest for the further investigation. Case B

12. 4 Conclusions • The effects of transients on the co-flow temperature can be controlled after at least 4 minutes heating up of stabilization plate. It is better to control the co-flow by the measured co-flow temperature with the co-flow H2 being adjusted within small limits to maintain the desired co-flow temperature. • The lifted jet flame is very sensitive to the co-flow temperature with the lift-off height decreasing strongly with increase of the co-flow temperature.

13. 4 Conclusions (cont’d) • There appear to be two different mechanisms of flame stabilization involved. Two typical cases have been selected for detailed study. For case A stabilization appears to be as in a normal lifted jet flame. For case B stabilization appears to involve a series of auto-ignition phenomena accompanied by a strong popping noise.

14. 4 Conclusions (cont’d) • The lift-off height increases monotonically with the velocity of the jet. For increasing velocity of the co-flow, the lift-off height increases at first and then decreases after a maximum. • The noise level of the jet flame shows a similar trend to the variation of lift-off height in case A, but is totally opposite in case B. The reason for this is not known, and further investigation is needed.

15. Acknowledgment • This work is supported by the Australian Research Council. Components of the burner were provided by Professor R W Dibble and this help is gratefully acknowledged. The assistance of Joshua Kent with some of the measurements is also gratefully acknowledged.

16. Thank you for your attention!