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Abstract: This work presents an experimental investigation to study the characteristics of combustion using a premixed methane–air mixture within a non-homogeneous porous inert medium (PIM) under high pressure and temperature. In order to obtain a stable flame under these operating conditions within PIM, a novel flame stabilization technique in porous inert media (PIM) combustion under high pressure and temperature has been developed and evaluated. The proposed technique avoids the draw backs of the hitherto developed techniques by properly matching the flow and flame speeds and, consequently, ensuring a stable combustion, for a wide range of operating pressure and temperature. The success of this technique permits the extension of PIM combustion to new applications such as gas turbines. The validity of this new technique has been assessed experimentally in detail by analyzing combustion inside a prototype burner. The effects of various operating conditions, such as initial preheating temperature and elevated pressure, have been examined for an output power range between 5 and 40kW. The experiments covered a broad spectrum of operating conditions ranging from a mixture inlet temperature of 20°C and pressure ratio of 1 up to a temperature of 400°C and a pressure ratio of 9. Evaluation of the results revealed excellent flame stability with respect to both flashback and blow-out limits throughout all the operating conditions studied, including relative air ratios far beyond the normal lean limit. While the blow-out stability showed no significant dependence on pressure, it was strongly determined by the preheating mixture inlet temperature. A remarkable broadening of the stability range from 0.6 to 1.0 on preheating to 400°C was observed. This reveals the potential of pre-heat temperature to improve the dynamic modularity of the burner. [ABSTRACT FROM AUTHOR]

This paper presents both experimental and modeling investigations of a catalytic wall fuel processor consisting of coupled methane reforming and methane combustion sections. The reacting systems are both catalytic and the latter generates the heat required for the occurrence of the former. The catalytic wall reactor was examined for light-off behavior and for steady-state product distribution. On one hand, the analysis of the reaction products distribution after catalyst ignition indicated that in both combustion and reforming sections catalysts undergo to a relatively long transient (about 40 min) before reaching steady state conditions. On the other hand, a much longer reactor thermal transient was observed and the two transient behaviors appear independent of each other. Analysis of the reactor operating under real conditions (nonadiabatic) showed that a 3D model is needed to accurately predict the reactor performance because a 2D model, although much more convenient, cannot allow for the whole heat loss thereby yielding unreliable results. [ABSTRACT FROM AUTHOR]

Abstract: The effect of temperature on the adiabatic laminar burning velocities of CH4 +air and H2 +air flames was analyzed. Available measurements were interpreted using correlation S L = S L0 (T/T 0) α . Particular attention was paid to the variation of the power exponent α with equivalence ratio at fixed (atmospheric) pressure. Experimental data and proposed empirical expressions for α as a function of equivalence ratio were summarized. They were compared with predictions of detailed kinetic models in methane+air and hydrogen+air flames. Unexpected non-monotonic behavior of α was found in rich methane+air flames. Modeling results are further examined using sensitivity analysis to elucidate the reason of particular dependences of the power exponent α on equivalence ratio. [Copyright &y& Elsevier]

A study has been made of the distinctive features of self-ignition and combustion of fuel-air mixtures containing methane, hydrogen, and ethylene in adiabatic compression. [ABSTRACT FROM AUTHOR]

Supersonic film cooling with and without induced shock waves was investigated numerically. The computations used a freestream Mach number of 3.13 and tangential-slot injection-cooling-stream Mach numbers of 1.3 and 1.8. Parameters investigated included shock wave intensity and coolant properties (nitrogen, methane, and helium). The results indicate that the adiabatic effectiveness is reduced by the impinging oblique shock wave because the shock wave reduces the local Mach number in the coolant layer, which increases the adiabatic-wall temperature. In addition, the oblique shock wave enhances the mixing of the cooling stream with the freestream, especially for stronger shock waves or lighter-gas coolants, which also increases the adiabatic-wall temperature. For the same injection-cooling-stream Mach number, the effects of the oblique shock wave are stronger in helium; however, helium had a higher adiabatic effectiveness than methane and nitrogen, even though the helium mass flow rate was lower than that of either methane or nitrogen. The adiabatic effectiveness is reduced much more by the 10 deg shock waves than by either the 7 or 4 deg shock waves. [ABSTRACT FROM AUTHOR]

Abstract: Experimental studies of adiabatic flat and cellular premixed flames of (CH4 +H2)+(O2 +CO2) are presented. The hydrogen content in the fuel was varied from 0% to 35% and the oxygen content in the oxidizer was 31.55%. These mixtures could be formed when oxy-fuel combustion technology is combined with hydrogen enrichment. Non-stretched flames were stabilized at atmospheric pressure on a perforated plate burner. A heat flux method was used to determine propagation speeds under conditions when the net heat loss of the flame is zero. Adiabatic burning velocities of methane+hydrogen+carbon dioxide+oxygen mixtures were found in satisfactory agreement with the detailed kinetic modeling employing the Konnov mechanism. Under specific experimental conditions the flames become cellular; this leads to significant modification of the flame propagation speed. The onset of cellularity was observed throughout the stoichiometric range of the mixtures studied. Visual and photographic observations of the flames were performed to quantify their cellular structure. The results obtained in the present work in (CH4 +H2)+O2 +CO2 mixtures are in good accordance with the previous observations for different fuels, CH4, C2H6 and C3H8. The enrichment by hydrogen leads to: the increase of the laminar burning velocities; the increase of the number of cells observed; the decrease of the mean cell diameter. The flame acceleration due to cellularity was not affected by the hydrogen enrichment. [Copyright &y& Elsevier]

It is widely known that detailed kinetic mechanisms with identical reaction steps but with very different rate parameters may provide similar simulation results in combustion calculations. This phenomenon is related to the similarity of sensitivity functions, which arises if low-dimensional manifolds in the space of variables, and autocatalytic processes are present. We demonstrated the similarity of sensitivity functions for adiabatic explosions and burner-stabilized laminar flames of stoichiometric methane–air mixtures. The cause of similarities was investigated by calculating the dimension of the corresponding manifolds, and the pseudo-homogeneous property of the sensitivity ordinary differential equation (ODE). The methane explosion model showed global similarity, which means that different parameter sets could provide the same simulation results. This was demonstrated by numerical experiments, in which two significantly different parameter sets resulted in identical concentration profiles for all species. This phenomenon is important from a practical point of view in the fields of ‘validation’ of complex reaction mechanisms and parameter estimation of chemical kinetic systems. [ABSTRACT FROM AUTHOR]