S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Mechanical Aerospace Engineering (기계항공공학부) Theses (Ph.D. / Sc.D._기계항공공학부)
High Harmonic Combustion Instability Characteristics of H2/CH4 Fuel in a Partially Premixed Combustor
부분예혼합 연소기에서 H2/CH4 연료조성 변화에 따른 고주파 공진 불안정 특성
- 공과대학 기계항공공학부
- Issue Date
- 서울대학교 대학원
- Partially Premixed Combustor; Combustion Instability; Fuel Flexibility; High Harmonic Instability; Instability Mode Shifting; Multi-Mode; Flame Transfer Function; OH-PLIF; PIV; Instability Prediction
- 학위논문 (박사)-- 서울대학교 대학원 : 기계항공공학부, 2017. 2. 윤영빈.
- Advancing the current technology of power-generation sources is essential because of increasing concerns related to emission regulations and the specific necessity of reducing emissions of CO2. The main source of power generation is slowly changing from fossil fuels to renewable energy sources. However, renewable energy technology is not adequate at this time to replace the total amount of power generated by fossil fuels. Therefore, interest is increasing in the combined-cycle gas turbine, which is both efficient and eco-friendly. The gas turbine, the core technology of the combined cycle, has better emission characteristics compared to those of the coal-fired plant, and can be easily applied to new fuels including Syngas, Synthetic Natural Gas (SNG), and gas extracted from shale. The newer fuels contain a higher H2 composition, which has been shown to be correlated with problems related to flashback. A different approach to minimize the disadvantage of higher H2 composition is essential. Therefore, partially premixed or diffusion-type combustion systems were introduced to minimize hazards based on combustion characteristics (including flashback). Various fuel compositions must be usable and detailed research must inform fuel composition, with the minimization of combustion instability being one of the key issues for these new types of turbines.
This paper details a set of experiments related to combustion instability and how it is affected by the fuel composition in the partially premixed combustor. The reason of high harmonic instability mode shifting and the categorization of the occurred multi-mode instability were investigated. In addition, the flame transfer function was also measured to form a foundation of research that could predict the high harmonic combustion instability.
The first step of these experiments was to examine the H2/CH4/CO composition variation in 45 test cases conducted to delineate the characteristics of the combustor. Combustion instabilities primarily occurred in fuel including H2, CH4 composed fuel composition and the mode analysis of each instability cases were conducted. Combustion instabilities in the partially premixed combustor were found in the 3rd, 4th, and 6th harmonic modes of the longitudinal fundamental mode. The fuel composition was reduced as H2/CH4 to conduct intensive research into instability characteristics. Combustor length and load variation tests were conducted to this end
the instabilities that occurred were confirmed as occurring in the longitudinal mode during the combustor length variation tests. In addition, high harmonic instability and instability-related mode shifting were found.
One of the specific objectives was to build understanding of the reasons for high harmonic combustion instability. The combustion instability mode varied from the 1st mode to the 6th mode with variations in the fuel composition. The other parameters (such as air flow rate and equivalence ratio) were varied to find the reason for instability mode shifting. From the parametric study, the structural variation of the flame was suspected to be the main parameter for instability mode shifting, and related to the convection time in time-lag analysis. Therefore, the simplified convection times of all cases were calculated based on OH planar laser–induced fluorescence (OH-PLIF) flame images, leading to finding the inverse relationships between convection time and the instability mode/frequency shifting. The convection-time reductions directly related to the decrease in total time delay is the main reason for shifts in the instability mode/frequency, which were analyzed based on the simplified Rayleigh criterion. In conclusion, the reduction of the combustion system’s convection time is the main reason for instability mode shifting.
During the instability mode shifting analysis, the complex multi-mode occurrence were found in some experimental cases. The measured instability cases were categorized based on the time and frequency domain in 6 cases. The continuous wavelet transform was applied in these representative cases to determine the frequency characteristics during the time variations. In detail, the multi-mode instability can be divided into two cases. In one case, the various frequencies co-existed during the entire period of instability
in the other case, one of the dominant frequencies was found, but its duration was very short (i.e., its intermittency characteristics were found). The removal of multi-mode instability cases in the convection time–instability frequency graph, which were found during mode-shifting analysis, shows the distinct convection time range of each mode. Convection-time overlapping indicated that the condition satisfies several instability modes at the same time. In conclusion, the reason for multi-mode instability is the co-satisfaction of multiple potential regions of instability.
Finally, the flame transfer function, which can be directly used in the instability prediction, was measured at different selected instability conditions. The flame transfer function gain decreasing tendency and flame transfer function phase decreasing gradient reduction was found during the increase of H2 composition. The increase of H2 composition directly related with the increase of volume flow rate of the fuel and it can reduce the inlet perturbation delivery time. In addition, the importance of flame structure and convection time was found based on the airflow rate and equivalence ratio variation tests. The measured flame transfer function gain was compared with that reported in previous research to determine the reasons for the trend. Last, the measured flame transfer function was used to predict the combustion instability phenomenon. The prediction results successfully follow the characteristics of instability mode shifting. These results allowed confirmation of accurate prediction of instability based on an accurate flame transfer function.
The high harmonic instability characteristics in a partially premixed combustor during various fuel compositions were understandable based on the experimental approach. In addition, accurate predictions of the possibility of instability were also made based on the measurements yielded by the flame-transfer function. Hopefully, these results can be used as the basic parameter for the development of a fuel-flexible gas turbine that uses premixed fuel with a high H2 composition.