1. EXPERIMENTAL EXAMINATION OF THE THERMOACOUSTIC INSTABILITY OF A LOW SWIRL FLAME WITH PLANAR LASER INDUCED FLUORESCENCE OF OH Jianan Zhang, Kelsey Kaufman, and Albert Ratner Department of Mechanical and Industrial Engineering The University of Iowa June 2014

2. Outline  Motivation  Combustion Heat Release Measurement Methods  Experimental System  Results and Discussion  Acknowledgements

3. Outline

4. Motivation  In the real system, the flame is highly turbulent and the flame structure is complicated.  Proper measurement of heat release of combustion is critical for understanding the feature of a burner used in the real energy system.

5. Motivation  The low swirl burner is a newly designed burner that operates under lean regime to ensure low NOx production.  In the real lean combustion system, thermoacoustic instability is a common combustion instability form.  Thermoacoustic instability results from coupling between combustion heat release and pressure oscillation in the system.

6. Outline

7. Chemiluminescence Measurement Chemiluminescence spectra of OH*(508 nm), CH*(431 nm), and C 2 *(513 nm) in laminar flame (J. Kojima. 2013) CH+O 2  OH*+CO Chemiluminescence of OH*(508 nm) of a swirling flame  Chemiluminescence results can show the heat release information globally.  It is easy to record with photomultiplier or ICCD camera equipped with filter.  Line of sight method, which has low space resolution and lose the detail information about the flame structure

8. Planar Laser Induced Fluorescence of OH  PLIF can get cross-section view of the flame, good spatial resolution  Two-dimensional result  OH exists in both of the flame front and burnt gas OH spectrum (Hult, 2005) PLIF setup OH PLIF

9. Combination of OH and CH 2 O PLIF CH 2 O+OH  HCO + H 2 O  Can get local heat release rate accurately  Measurement system is more complicated  Two Dimensional result Images of OH PLIF and CH2O PLIF (B.O. Ayoola, 2006)

10. Outline

11. Experimental System  Low swirl burner: flow divergence for stabilization provided by Dr. Robert Cheng of LBNL 1 inch burner and swirler with low swirl number (0.5) Flame shape Velocity vectors and shear stresses (background contours) obtained in R= 2.54 cm LSB. Adapted from Chen [1] [1] Cheng, Robert K. "Low swirl combustion." DOE Gas Turbine Handbook (2006).

12. A: Acoustic Chamber; B: Air compressor; C: Fuel cylinder; D: Premixer; E: Burner; F: Quartz window; G: ICCD camera; H: Signal generator; I: Power amplifier; J: Signal transducer; K: Signal transducer; L: computer; P: Pressure sensor; S: Speaker; T: Thermocouple Experimental System

13.  Laser system Final laser beam with power of 27 mJ/Pulse

14. Outline

15. Result (Analytical Method) Rayleigh Index map Rayleigh Index based on Rayleigh Criteria is used in current research. Instability is damped Instability is encouraged

16. Results(Rayleigh Index Map) Rayleigh Index map of 5 m/s flame with perturbation frequency of 125 Hz  Results with OH-PLIF clearly show the coupling structure and location.  Coupling strength increases with the forcing level and then becomes stable.  Along the flame edge, the coupling trend is also clearly illustrated. Response of Rayleigh Index Distribution to Perturbation Amplitude

17. Results(Weighted R RMS ) Original Rayleigh Index Map  The general trend can be divided into two regions, which include a linear and nonlinear region.  The maximum uncertainty is 10.89% Global RMS Rayleigh Index Rayleigh Index considering oscillation level

18. Acknowledgement  Lawrence Berkeley National Laboratory for providing the low swirl burner  The University of Iowa for financial support and facilities

19. QUESTIONS?