Laminar burning velocities of methane-hydrogen-air mixtures

In a future sustainable society, hydrogen is likely to play an important role as an energy carrier. In an EET-project called Greening of Gas (VG2) the transition path from pure natural gas towards the use of mixtures containing more and more hydrogen is investigated. The research carried out at the TU/e is focused on the safety of burner devices. A crucial parameter for the safety of burner devices is the laminar burning velocity. In this thesis the laminar burning velocity of methane-hydrogen mixtures is experimentally determined and compared to numerical data using several combustion reactionmechanisms. An asymptotic theory for stoichiometric methane hydrogen flames is presented. This theory is validated with the experimental and numerical data. To measure the laminar burning velocity accurately the heat flux burner is used, which is developed previously at TU/e. Based on the earlier works of van Maaren and Bosschaart the heat flux method is further analysed in this thesis. This analysis results in a better understanding of several aspects of themethod. For example it is shown that the influence of the heating jacket is negligible when using a temperature difference of at least 30 K between the unburnt gas temperature and the temperature of the heating jacket should be maintained. Furthermore, it is not likely that the burner surface influences the heat flux experiments in the presented measurement range. However when higher unburnt gas temperatures will be used this influence should be regarded. In the present research, three sets of laminar adiabatic burning velocities have been measured and presented using 95% confidence error intervals. The first set consists of hydrogen-oxygen-nitrogen mixtures at various fuel equivalence ratios and several nitrogen dilutions. The second set of measurements deals with methane-hydrogen-air mixtures at various fuel equivalence ratios and hydrogen contents up to 40%. The last set of measurements show a glimpse towards gas turbine situations. Here the unburnt gas temperature is increased from298 K up to 420 K for methane-hydrogen-airmixtures. The laminar burning velocity measurement data of hydrogen-oxygen-nitrogen mixtures, show significant differences with experimental results of other authors. This discrepancy is probably related to the non-linear stretch correction performed by them. The differences between the combustion reaction mechanisms and the heat flux data show significant differences in the performance of the methane based combustion reaction mechanisms in the case of hydrogen-oxygen-nitrogenmixtures. Especially the commonly used GRI-mechanism deviates from the experimental data. Remarkably the performance of the methane based SKG03 mechanism is comparable or even better compared to hydrogen based mechanisms for fuel lean flames to slightly rich hydrogen-oxygennitrogen flames. Generally, the hydrogen based kinetic mechanisms perform quite well for the investigated parameter range; especially the Konnov mechanism. When comparing the measurements of the laminar burning velocities at ambient conditions as well as increased unburnt gas temperatures of methane-hydrogen-air mixtures with numerical combustion mechanisms it is shown that both the SKG03 mechanism and the GRI-mechanism perform very well. Experimental data of the laminar burning velocities of methane-air flames show that the measurements of Bosschaart give comparable results with the present measurements. Regrettably experimental data of methanehydrogen- air flames is scarce; the data of Halter et al. show comparable results. In order to get more insight in the basic properties describing methane-hydrogen-air flames, the asymptotic theory of Peters and Williams for stoichiometric methane-air flames is adapted to stoichiometricmethane-hydrogen-air flames. This theory is validated both with experiments performed using the heat flux burner and numerical simulations using CHEM1D. With this theory for stoichiometric flames the laminar burning velocity as a function of the hydrogen content can be predicted qualitatively even for higher pressures and temperatures. The resulting equations show that the driving force for the increase in burning velocity of a methane-hydrogen flame is the increase in temperature difference between the inner layer temperature and the adiabatic flame temperature

Asymptotic analysis of methane-hydrogen-air mixtures

In this paper an asymptotic analysis of de Goey et al.concerning premixed stoichiometric methane-hydrogen-air flames is analyzed in depth. The analysis is performed with up to 50 mole percent of hydrogen in the fuel, at gas inlet temperatures ranging from 300 K to 650 K and pressures from 1 to 15 atm, which are about gasturbine conditions. The results of this analysis are compared with experimental and numerical data and give accurate predictions concerning the variation of the nvestigatedparameters with increasing hydrogen content. Especially the flame thickness and laminar burning velocity give good results

Asymptotic analysis of methane-hydrogen-air mixtures

In this paper an asymptotic analysis of de Goey et al.concerning premixed stoichiometric methane-hydrogen-air flames is analyzed in depth. The analysis is performed with up to 50 mole percent of hydrogen in the fuel, at gas inlet temperatures ranging from 300 K to 650 K and pressures from 1 to 15 atm, which are about gasturbine conditions. The results of this analysis are compared with experimental and numerical data and give accurate predictions concerning the variation of the nvestigatedparameters with increasing hydrogen content. Especially the flame thickness and laminar burning velocity give good results

Analysis of the flame thickness of turbulent flamelets in the thin reaction zones regime

The thickness of the instantaneous flamelets in a turbulent flame brush on a weak-swirl burner burning in the thin reaction zones regime was analyzed experimentally, theoretically, and numerically. The experimental flame thickness was measured correlating two simultaneous Rayleigh images and one OH-image from two closely spaced cross sections in the flame. The low temperature edge of the flame was thickened by turbulent eddies but these structures could not penetrate far enough into the flame front to distort the inner layer for the moderate Karlovitz numbers used. The flame front based on the temperature gradient at the inner layer became thinner for lean flames and thicker for rich methane-air flames.</p

Analysis of the flame thickness of turbulent flamelets in the thin reaction zones regime

The thickness of the instantaneous flamelets in a turbulent flame brush on a weak-swirl burner burning in the thin reaction zones regime has been analysed experimentally, theoretically, and numerically. The experimental flame thickness has been measured correlating two simultaneous Rayleigh images and one OH-image from two closely spaced cross sections in the flame. It appears that the low temperature edge of the flame is thickened by turbulent eddies but that these structures cannot penetrate far enough into the flame front to distort the inner layer for the moderate Karlovitz numbers used. The flame front based on the temperature gradient at the inner layer becomes thinner for lean flames and thicker for rich methane–air flames. This has been explained theoretically and numerically by studying the influence of flame stretch and preferential diffusion on the flame thickness. It appears that the flame front thickness at the inner layer (and mass burning rate) is not influenced by turbulent mixing processes, and it seems that eddies of the size of the inner layer have to be used to change this picture. Experiments closer to the boundary of the broken reaction zones regime have to confirm this in the future

Analysis of the flame thickness of turbulent flamelets in the thin reaction zones regime

The thickness of the instantaneous flamelets in a turbulent flame brush on a weak-swirl burner burning in the thin reaction zones regime was analyzed experimentally, theoretically, and numerically. The experimental flame thickness was measured correlating two simultaneous Rayleigh images and one OH-image from two closely spaced cross sections in the flame. The low temperature edge of the flame was thickened by turbulent eddies but these structures could not penetrate far enough into the flame front to distort the inner layer for the moderate Karlovitz numbers used. The flame front based on the temperature gradient at the inner layer became thinner for lean flames and thicker for rich methane-air flames

Effects of temperature and composition on the laminar burning velocity of CH4/H2/O2/N2 flames

This work summarises available measurements of laminar burning velocities in CH4 + H2 + O2 + N2 flames at atmospheric pressure performed using a heat flux method. Hydrogen content in the fuel was varied from 0% to 40%, amount of oxygen in the oxidiser was varied from 20.9% down to 16%, and initial temperature of the mixtures was varied from 298 to 418 K. These mixtures could be formed when enrichment by hydrogen is combined with flue gas recirculation. An empirical correlation for the laminar burning velocity covering a complete range of these measurements is derived and compared with experiments and other correlations from the literature. © 2009 Elsevier Ltd. All rights reserved