Alessandro Schönborn

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 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.

ESS Target Helium Experiments at Lund University

In ESS, a 5 MW proton beam will hit a helium cooled tungsten target to generate neutrons by spallation. For the purpose of investigating various aspects of cooling with helium, an experimental system has been constructed at Lund University, Department of Energy Sciences. A helium flow of 3 g/s is circulated in a closed loop with pre-heater, cooler, filters and a test vessel. The vessel has windows for measurements and is designed for 10 bar and above 400°C. In the test vessel, a tube forms a helium jet of up to 150 m/s onto a coin shaped material sample. One purpose of the system is to investigate erosion of sample surfaces. Drilling and attaching equipment to the surface had to be avoided, so the setup has been benchmarked and the comparison was used to tune a CFD model. Then the model is used to assess the conditions in the experiments. The setup, as well as the tests and calculations, are described and it is concluded that the temperature and velocity estimates are sufficient for the experiments to be performed. (Less)

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)

Experimental Evaluation of a Novel High Frequency Ignition System Using a Flow-Reactor Set-up

Using diluted methane/air mixtures in internal combustion engines has a potential of reducing emissions and increasing efficiency. However, the ignition systems used today show difficulties igniting lean mixtures. For this purpose a new high frequency (HF) ignition system using pulse generators and a resonance circuit to achieve a controlled number of sparks during a controlled period of time has been developed. A first prototype of this high frequency system has been tested in a flow-reactor and compared to a conventional ignition system. Results show that the high frequency system improves the flame development under lean conditions compared to the conventional system. Higher frequencies have higher capability of igniting lean mixtures than lower frequencies. Lower spark frequencies were found to travel faster across the electrodes than high frequencies and also compared to the conventional system. High pressure and high flow rates affected the lean limit of all ignition strategies, but especially high spark frequencies had difficulties igniting the charge under high pressures, due to the resonant frequency changing with pressure. The high frequency system was also limited in the amount of available voltage. However, this will be improved with further development of the ignition system. (Less)