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

3. 3 1 Background Stabilization mechanism 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. 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 -S ound level meter

5. 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. 6 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. 3.1 Sensitivity of lifted jet flame

7. 7 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. 3.2 Control of H2/Air co-flow

8. 8 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. 3.3 Typical Cases CaseAB Central JetQ H2 (slm)25 Q N2 (slm)75 T jet (K)317.5* V jet (m/s)110 Re jet 25,760 Co-flowQ H2 (slm)190.1185.2 Q air (slm)1720 T coflow (K)10441013 V coflow (m/s)4.0 Re coflow 18,96519,200 Lift-offH Liftoff (mm)~25~100 Flame SoundPopping noise NoYes * Thermocouple measurement in nozzle exit

9. 9 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. 3.4 Hysteresis of lifted jet flame Case A Case B

10. 10 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. 3.5 Effects of co-flow velocity Case A Case B

11. 11 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. 3.5 Effects of co-flow velocity (cont’d) Case A Case B

12. 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. 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. 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. 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. 16 Thank you for your attention!