Measurements of flammability and flame acceleration limits at low pressures

Results of an experimental study on limits for strong flame acceleration (FA) as a function of initial pressure are presented. The range of initial conditions studied covers the range of conditions typical for VVPSS ex-vessel LOCA scenarios. The experimental data give a basis for extension of FA #sigma#-criterion to the range of sub-atmospheric pressures typical for ITER accident scenarios. (orig.)Report on EFDA Subtask TW2-TSS-SEA5.4Available from TIB Hannover: ZA 5141(6811) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Effect of obstacle geometry on behaviour of turbulent flames. Auswirkung der Hindernisgeometrie auf das Verhalten von turbulenten Flammen

Results of experimental studies of turbulent flame propagation in obstructed tubes (channels) with different blockage ratios are presented. Explosion channel 80x80 mm and cylindrical tubes 174, 350, 520 mm i.d. were used in the tests. Blockage ratios were 0.09, 0.1, 0.3, 0.6, and 0.9. Explosion processes were studied in hydrogen-air and stoichiometric hydrogen-oxygen mixtures diluted with N_2, Ar, He, and CO_2. All tests were made with normal initial temperature and pressure. Flame propagation in explosion channels resulted in one of four characteristic combustion regimes: relatively slow flames with global quenching, slow/unstable flames, choked flames, and quasi-detonations. The difference between slow regimes and fast regimes (choked flames and detonations) is shown to be the most pronounced for all mixtures, scales and geometrical configurations tested. At sufficiently large scale, possibility of development of fast combustion regimes is shown to depend only on mixture composition. Mixture properties, thus, are shown to provide a potential for effective flame acceleration. This potential can be realized, if geometry of confinement and scale provide favorable conditions for flame acceleration. Correlations of potential for flame acceleration with mixture properties are discussed. For mixtures and initial thermodynamic conditions used in the tests, mixture expansion ratio is shown to be the most significant parameter in these correlations. Fast combustion regimes are found to be only possible in mixtures with #sigma#>#sigma#*= 3.75#+-# 0.25. This critical value can be a function of mixture type and initial conditions. Effects of mixture composition and blockage ratio on DDT conditions are also discussed. The requirement of small enough mixture detonation cell size #lambda# compared to characteristic size of a tube (channel) is confirmed as the necessary condition for onset of detonations. Critical conditions for onset of detonations in mixtures with regular cellular structure (Ar and He dilution) are shown to be different from those for irregular mixturesSIGLEAvailable from TIB Hannover: ZA 5141(6328) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman