Flame acceleration in a tube with variable cross-section

Abstract
An experimental study is presented on the effect of tube cross-section variations on flame acceleration. A geometry where flames propagated from a tube with large diameter to another tube with smaller diameter was studied. Experiments were performed in an explosion tube combined of two parts, which were equal in length and different in diameters (174 and 520 mm id). Tests were made with lean hydrogen-air mixtures. The mixtures were chosen to be near the critical composition of 10.5% H ₂ in air for fast flames in tubes with constant cross-section. Critical mixture compositions for development of fast flames were determined in the tests. It was found that compared to cases with a constant tube diameter, the critical mixture composition for strong flame acceleration was shifted towards less energetic mixtures in our case. It was shown that this shift is due to an effective compression and heating of the reactants ahead of the flame in the geometry studied. This effect appeared to be in a good qualitative and quantitative agreement with the critical values of mixture expansion ratio for strong flame acceleration determined in our earlier studies for hydrogen mixtures.