Jong Ho Uhm

Effects of Hydrogen Enrichment on Confined Methane Flame Behavior

Many land based power generation units presently operate on natural gas, whose major constituent is methane, and many of them would need to tackle the challenges due to a fuel switch towards synthesis gas in the near future. Operating conditions and stability of a pre-mixed gas turbine combustor is quite sensitive to the changes in the fuel composition. Behavior of a premixed confined hydrogen enriched methane flame is studied with regard to thermo-acoustic instability induced flame flashback, emissions, flammability limits and acoustics over a wide range of operating conditions. However, most emphasis is put on lean combustion, which is an industry standard method used to lower pollutant emissions by reducing adiabatic flame temperatures. Hydrogen addition extends the flammability limits and enables lower nitric oxide emissions levels to be achieved at leaner equivalence ratios. On the other hand, increased root-mean-square pressure fluctuation levels, and higher susceptibility to flashback is observed with increasing hydrogen volume fraction inside the fuel mixture. This phenomenon is mostly attributed to much higher burning speeds of hydrogen in contrast to pure methane. A semi-analytical model has been utilized to capture the flame holding and thermo-acoustically induced flame flashback dynamics for a pre-mixed gas turbine combustor. A simple linearized acoustic model, derived from the basic conservation laws, and a front-tracking algorithm based on the Markstein’s G-equation are coupled together in order to track the flame initiation front, which in turn yields in an understanding of dynamic flame holding characteristics. A limit cycle behavior in the flame front movement is observed during simulations due to a non-linearity in the feedback term that relates acoustic velocity to heat release. Sets of experiments including flashback speed measurements have been performed at varying fuel composition. Phase locked CH radical imaging measurements have also been performed in order to track the flame initiation front in time with respect to the dominant instability cycle. Computer simulations are performed to study flashback and combustor acoustics together numerically and it is observed that these are in good qualitative agreement with the experiments.Copyright

Hydrogen Enriched Confined Methane Flame Behavior and Flashback Modeling

Most gas fueled power generation units presently operate on natural gas and many of them would need to tackle the challenges due to a fuel switch towards syngas in the near future. Operating regime of a gas turbine combustor is sensitive to the changes in the fuel composition. Behavior of a premixed enclosed methane-hydrogen flame is studied with regard to thermo-acoustic instability induced flame flashback, emissions, flammability limits and acoustics over a wide range of operating conditions. Hydrogen addition extends the lean flammability limits and enables lower NOx emissions levels to be achieved. However increased RMS pressure fluctuation levels and higher susceptibility to flashback is observed with increasing hydrogen volume fraction. This is associated with higher burning speeds of hydrogen in comparison to methane. Furthermore flashback in the experimental facility is triggered by thermo-acoustic oscillations. Therefore, an analytical model has been developed to capture the flame holding and thermo-acoustically induced flashback dynamics for a pre-mixed gas turbine combustor. A simple linearized acoustic model is derived from the basic conservation laws, and a front-tracking algorithm based on the Marksteins G-equation is coupled to combustor acoustics it in order to track the flame-front which yields in an understanding of dynamic flame holding and flashback behavior. Due to the non-linear nature of the coupling between acoustic velocity and heat release a limit cycle behavior in the flame front movement is observed during simulations. Sets of experiments including phase locked CH radical imaging have been performed in order to time resolve the flame initiation front behavior. Numerical simulations are performed to study flashback and combustor acoustics together and it is found that these are in good qualitative agreement with the experiments.

Dynamics, NOx and Flashback Characteristics of Confined Pre-Mixed Hydrogen Enriched Methane Flames

The operating regime of a gas turbine combustor is highly sensitive to fuel composition changes. In particular, the addition of hydrogen, a major constituent of syngas, has a major effect on flame behavior due to the higher burning rates associated with hydrogen. A laboratory scale pre-mixed test rig is constructed in order to study such effects. The fuel composition is incremented with increasing hydrogen starting from 100% methane. It is observed that increased RMS pressure levels and higher susceptibility to flashback occurs with increasing hydrogen volume fraction. Furthermore, hydrogen enrichment can cause an abrupt change in the dominant acoustic mode. Phase locked hydroxyl PLIF measurements have been performed with respect to the dominant acoustic instability limit cycle. These measurements are complemented with real time heat release, emissions and flashback measurements. Particular emphasis is put on time resolving the thermoacoustic instability induced flashback cycle of the wedgeshaped flame front and the temporal events associated with flashback.Copyright

CONTROL OF HEAT RELEASE AND NO EMISSIONS IN A COMBUSTOR THROUGH MODULATION OF TRANSVERSE AIR-JETS

Abstract The effect of modulated air jets, introduced through the combustor shell, on the temperature distribution and nitric oxide emissions is investigated. Temperature and emissions measurements have been made at a number of forcing frequencies in the range of 100–850 Hz, blowing ratios in the range of 4–10 and equivalence ratios between 0.6 and 1.0. Open-loop flame response to forcing has also been acquired by recording pressure spectra. Results show that substantial reductions in nitric oxide emissions index (15–30%) can be obtained over a wide range of flow conditions with side-air jet forcing. In addition, forcing also alters the time averaged temperature field, with higher mean temperatures close to the dump plane, due to enhanced fuel-air mixing. The higher temperatures and volumetric heat release obtained with forcing can enable more compact combustor designs. The lower emissions are potentially linked to greater unsteadiness with forcing.

Open Loop Control of Combustion Instability With a High-Momentum Air-Jet

A new strategy for open-loop control of combustion oscillations using a high-momentum air-jet modulated at low frequencies is presented in this paper. The oscillations in the swirl-stabilized spray combustor of interest are dominated by an acoustic mode (235 Hz) with a low frequency (13 Hz) bulkmode (of the upstream cavity) oscillation superimposed. The most effective strategy for control is shown to be achieved through the use of a new concept which utilizes a high-momentum air-jet injected directly into the region of flame dynamics. It is shown that with a low frequency modulation (15 Hz) of the high momentum air-jet, the pressure oscillations can be reduced significantly (by a factor of nearly 10). Square wave modulation is shown to be considerably more effective than sine-wave modulation. These results are extremely promising since high bandwidth actuation is not required for effective control.Copyright

Amplitude-Limiting Feedback Control of Combustion Instability With a High-Momentum Air Jet

A new strategy that integrates low-frequency modulation of a high-momentum air-jet with amplitude feedback is presented for control of combustion oscillations in a swirl-stabilized spray combustor. The oscillations in the combustor of interest are dominated by an acoustic mode (235 Hz) with a low frequency (13 Hz) bulk-mode (of the upstream cavity) superimposed. An effective strategy for control is shown to be achieved through the use of a concept which utilizes low bandwidth modulation of a high-momentum air-jet that penetrates into the regions of positive Rayleigh index. It is shown that with a low frequency modulation (5 Hz) of the high momentum air-jet, the pressure oscillations can be reduced significantly (by a factor of nearly 6). Further improvement in control is achieved with an amplitude-limiting feedback strategy, in which, the valve opening and closing of the control air-jet is driven by the pressure amplitude relative to a specified threshold. The goal of the controller is to maintain pressure oscillations below the pre-set threshold level. With this strategy, the valve frequency and duty cycle are automatically adjusted based on the amplitude of the pressure signal. It is observed that modulation frequencies are typically in the range of 5–30 Hz (although higher frequencies, as high as 130 Hz, are needed occasionally). Duty cycles less than 50% are required for effective control. The amplitude-limiting feedback controller is shown to combine the benefits of low-bandwidth actuation, low-duty cycles, and greater reductions in pressure oscillations.© 2004 ASME