Parisa Sayad

Visualization of Different Flashback Mechanisms for H2/CH4 Mixtures in a Variable-Swirl Burner

Flame flashback from the combustion chamber to the premixing section is a major operability issue when using high H-2 content fuels in lean premixed combustors. Depending on the flow-field in the combustor, flashback can be triggered by different mechanisms. In this work, three flashback mechanisms of H-2/CH4 mixtures were visualized in an atmospheric variable-swirl burner using high speed OH* chemiluminescence imaging. The H-2 mole fraction of the tested fuel mixtures varied between 0.1 and 0.9. The flow-field in the combustor was varied by changing the swirl number from 0.0 to 0.66 and the total air mass-flow rate from 75 to 200 SLPM (standard liters per minute). The following three types of flashback mechanism were observed: Flashback caused by combustion induced vortex breakdown (CIVB) occurred at swirl numbers >= 0.53 for all of the tested fuel mixtures. Flashback in the boundary layer (BL) and flame propagation in the premixing tube caused by auto-ignition were observed at low swirl numbers and low total air mass-flow rates. The temporal and spatial propagation of the flame in the optical section of the premixing tube during flashback was studied and flashback speed for different mechanisms was estimated. The flame propagation speed during flashback was significantly different for the different mechanisms. (Less)

Experimental Investigation of the Effect of Steam Dilution on the Combustion of Methane for Humidified Micro Gas Turbine Applications

ABSTRACT Water introduction in the micro gas turbine (mGT) cycle is considered the optimal route for waste heat recovery and flexibility increase of such a small-scale combined heat and power (CHP) unit. However, humidification of the combustion air in a mGT affects combustion stability, efficiency, and exhaust gas emissions. This can lead to a non-stable, incomplete combustion, which will affect the global efficiency negatively. Additionally, CO emissions will increase. The non-stable, incomplete combustion might result in an engine shutdown due to a flameout. To study the impact of humidification on the combustion of methane in a humidified mGT, we performed combustion experiments in an atmospheric, variable-swirl, premixed combustion chamber. The results of these experiments are summarized in this article. The effect of the humidification of the combustion air was simulated by adding steam to the combustion air. The impact of the steam injection on methane combustion has been studied at variable swirl number and steam fraction. Experimental results showed a linearly increasing lean blowout (LBO) equivalence ratio for methane combustion with increasing steam fraction. In addition, CO emission levels started to rise at higher equivalence ratio for higher steam fractions compared to combustion under dry conditions. The CO emission levels at stable combustion were however still the same order of magnitude as for the dry combustion. The swirl number has little effect on the LBO limit. Final results indicated the possibility to maintain complete and stable combustion under humidified conditions with low CO emissions at higher equivalence ratio compared to the dry combustion.

Experimental Investigation of Methane Lean Blowout Limit; Effects of Dilution, Mass Flow Rate and Inlet Temperature

Lean blowout (LBO) is one of the major instability problems of premixed combustion. LBO equivalence ratio is a function of inlet temperature and pressure, mass flow or aerodynamic loading, and fuel composition. All these, except the last, vary during startup and with load. Developing gas turbine combustors capable of operating within wider range of fuel compositions requires extensive knowledge about instability limits of the combustor at different operating conditions. In this work an atmospheric variable swirl combustor was used to study the influence of inlet temperature, mass flow, swirl number and dilution on lean blowout of methane. The equivalence ratio at LBO was investigated for methane at 3 different inlet temperatures at various swirl numbers. The swirl number was varied by changing the ratio of axial and tangential flow through the combustor inlet, and was determined using Laser Doppler Anemometry. The experiments showed that increasing the swirl number reduced the lean blowout equivalence ratio for a given inlet temperature and that increasing the inlet temperature reduced the lean blowout equivalence ratio at a certain swirl number. In order to study the effect of inlet mass flow rate on lean stability limit, blowout experiments were conducted at 7 different mass flow rates. The measurements showed that the total mass flow has a nonmonotonic effect on the lean blowout limit. At total mass flow rates below 200 SLPM increasing the total mass flow extended lean stability limit whereas at mass flow rates higher than 300 SLPM the trend was reversed. The effect of fuel dilution on the LBO limit was also investigated by adding different fractions of N2 and CO2 to the fuel mixture. The results were compared with those for pure methane at the same swirl number. Dilution with either diluent reduced the LBO limit of methane. However at the concentrations lower than 50 % the effect of dilution on LBO equivalence ratio was relatively small and no significant difference was observed between N2 and CO 2 dilution.

Visualization of Different Flashback Mechanisms for H-2/CH4 Mixtures in a Variable-Swirl Burner

Flame flashback from the combustion chamber to the premixing section is a major operability issue when using high H-2 content fuels in lean premixed combustors. Depending on the flow-field in the combustor, flashback can be triggered by different mechanisms. In this work, three flashback mechanisms of H-2/CH4 mixtures were visualized in an atmospheric variable swirl burner using high speed OH chemiluminescence imaging. The H-2 mole fraction of the tested fuel mixtures varied between 0.1 and 0.9. The flow-field in the combustor was varied by changing the swirl number from 0.0 to 0.66 and the total air mass-flow rate from 75 to 200 SLPM (standard liters per minute). The following three types of flashback mechanism were observed: Flashback caused by combustion induced vortex breakdown occurred at swirl numbers >= 0.53 for all of the tested fuel mixtures. Flashback in the boundary layer and flashback due to autoignition were observed at low swirl numbers and low total air mass-flow rates. The temporal and spatial propagation of the flame in the optical section of the premixing tube during flashback was studied and flashback speed for different mechanisms was estimated. The flame propagation speed during flashback was significantly different for the different mechanisms. (Less)