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Stretch Effects on Hydrogen/Methane/Air Laminar Flame Propagation and Extinction

Final Report Summary - STRELA (Stretch effects on hydrogen/methane/air laminar flame propagation and extinction)

The addition of hydrogen to natural gas, the main constituent of which is methane, allows for leaner operation of premixed combustion at lower temperatures, which leads to reduced pollutants emission and to higher efficiencies of engines and turbines. Hydrogen-containing fuels exhibit specific behavior caused by the high diffusivity of hydrogen: so called preferential diffusion effects.

The main objective of the project STRELA was to establish mechanisms through which these effects influence propagation and extinction of fuel-lean methane-hydrogen-air (hythane-air) flames, for the fuel gas concentrations down to the lean flammability limits (LFL).

To achieve this goal, propagation of ultra-lean flames in hythane-air mixtures, with varied hydrogen content in the fuel gas, in half-opened tubes of different diameters, ranging from 6 mm to 50 mm, have been studied experimentally.

Detailed gas-dynamic structure of studied flames was explored using the particle image velocimetry method, and flames temperature fields were measured by the thin filament pyrometry method, a 2-D version of which has been developed in the framework of the project. 2-D distributions of OH radical (an indicator of the local burning rate) was measured using the planar lased induced fluorescence method. Values of the LFL and speeds of the flame propagation have been measured.

Researchers of Combustion Technology Group (CTG), Dr. A. Sepman and Prof. N.J. Dam, participated in experiments, so that extensive mutual exchange of knowledge and experience between the CTG scientists and incoming researcher was thereby provided. In parallel, computer simulations of studied flames were carried out by the CTG scientist Prof. J. van Oijen, using the 2-D LamFla2D code developed at TU/e.

For all hydrogen-containing mixtures, the determined LFLs were much wider than minimum fuel concentrations at which adiabatic planar flame could propagate, which was explained by strong preferential diffusion effects. Unexpectedly, a variety of qualitatively different regimes has been observed for ultra-lean flames as experimental parameters were varied. Besides usual long 'open bubble'-shaped flames, which a commonly observed in methane-air and propane-air mixtures in similar conditions, flames shaped as a very short spherical segment (flame cups) and small enclosed spherical flames (flame balls) have been observed. Only for regular-type long flames the expected anomalous tendency of the extending of the LFL with decreasing tube diameter has been observed. For the flame-cap and flame-ball regime, the LFL depended very weakly on the tube diameter for fixed contents of hydrogen in the fuel gas. It was established that, in the flame ball regime, reactants are delivered to the flame and heat is removed from the flame almost solely by diffusion, despite the fact the reactants are initially premixed.

Earlier, this regime was observed only in microgravity conditions, and the discovery of normal gravity flame balls was the major finding of the project. Moreover, for the first time continuous transition between two currently known fundamental regimes of the flame enhancement by the preferential diffusion: flame ball regime and so-called flame stretch-induced preferential diffusion; has been observed for the first time in experiments and simulations.

An important outcome of the project was the design of a prototype burner which allows stabilising ultra lean sublimit flames, including normal gravity flame balls. This burner potentially allows more detailed, then in the current project, study of fundamental preferential diffusion effects, in particular the transition to the flame-ball regime. Such studies would be of a significant potential value for the better understanding and correct modelling of lean combustion of hydrogen-containing fuels in practical devices.

Two research proposals have been prepared and submitted to Dutch national science foundation programs: STW (Technology Foundation) and FOM (Foundation for Fundamental Research on Matter). In the case if at least one of the proposals is granted, the incoming researcher will be invited to participate in carrying out a new project at CTG, Eindhoven University of Technology.

The results of the project can be used by for establishing safety standards for the production, storage, delivery and distribution of hythane-air blends. Industrial companies developing novel lean combustion devices operating on such mixtures can find the obtained knowledge useful for the evaluation of lean limits of operation for such combustion devices and of corresponding combustion temperatures, and for prediction/identification of the regimes of combustion of ultra-lean hythane-air mixtures.

Contact details:
TU/e, Mechanical Engineering, Combustion Technology
Den Dolech 2, 5600 MB Eindhoven.
Email: (Prof. Philip de Goey)