The Effects of Hydrogen on Steady-State and Transient Combustion Characteristics
Open Access
- Author:
- Strollo, John
- Graduate Program:
- Mechanical Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- March 16, 2020
- Committee Members:
- Jacqueline Antonia O'Connor, Thesis Advisor/Co-Advisor
Richard A Yetter, Committee Member
Karen Ann Thole, Program Head/Chair - Keywords:
- thermoacoustic instability
transient operation
hydrogen enrichment
thermo-acoustic instability
transient operation
hydrogen enrichment - Abstract:
- This thesis examines the effects of steady-state and transient hydrogen enrichment on thermo-acoustic instability in a model gas turbine combustor. Combustion instability, a feedback loop between flame heat release rate oscillations and combustor acoustics, is characterized in a swirl-stabilized flame operated at a range of hydrogen-natural gas fuel blends, heat rates, and mixing strategies. Measurements of combustor chamber pressure fluctuations and CH* chemiluminescence imaging are used to characterize instability at a range of operating conditions. Steady-state tests show that mixture heat rate, hydrogen content, and mixing strategy affect system stability. At a given heat rate, higher levels of hydrogen result in unstable combustion. As heat rate increases, instability occurs at generally lower concentrations of hydrogen in the fuel. While different mixing strategies yielded different stability map results, these two general trends remained. Utilization of technically premixed fuel was shown to alter the spectral density behavior of the combustor at various combustor states. This alteration of behavior was found in the form of multiple frequency peaks for various operating conditions. Steady-state flame imaging indicated general trends of increased mean and RMS CH* intensity with heat rate. For a given heat rate, all mixing strategies showed flame height reduction with increases in hydrogen content. All mixing strategies showed evidence of a nodal line (between the outer recirculation zone and upper flame region) in the RMS chemiluminescence images, where flame oscillation was at a minimum. While the fully premixed natural gas / fully premixed hydrogen and fully premixed natural gas / technically premixed hydrogen mixing strategies were generally axisymmetric, the images captured for the technically premixed natural gas / technically premixed hydrogen mixing strategy showed clear asymmetry in CH* distribution. Transient operation was tested for the fully premixed natural gas / technically premixed hydrogen mixing strategy in two directions – instability onset and decay – and two hydrogen-addition times – a short time of 1 millisecond and a longer time of 4 seconds. Results show that instability onset processes, through the transient addition of hydrogen, are highly repeatable regardless of the timescale of hydrogen addition. Certain instability decay processes are less repeatable, resulting in cases that do not fully transition from unstable to stable combustion despite similar changes in hydrogen fuel flow rate. Flame behavior before, during, and after the transient is characterized using high-speed CH* chemiluminescence imaging. Analysis of the high-speed images showed changes in flame stabilization and dynamics during the onset and decay processes. Finally, a steady-state stability map was generated for the multi-nozzle configuration of this combustor. Operating conditions for this configuration were set for the center nozzle of the combustor to match operating points for the single nozzle tests, yielding a natural gas-hydrogen flame at the center nozzle and natural gas flames at the outer nozzles. Results show that many operating points that were unstable for the single nozzle configuration were stable in the multi-nozzle configuration. Additionally, the only stability transition point that was discovered for the stability map coincided with the outer nozzles reaching the critical equivalence ratio of the combustor. These results indicate that the multi-nozzle configuration is less influenced by one nozzle’s operating point and more influenced by the operating points of the other four nozzles. The results of this study can have implications for systems that experience variations in fuel composition, particularly in light of growing interest in hydrogen as a renewable fuel.