Premixed flame acceleration in straight and bend close pipe

There were many studies on premixed flame propagation in tubes, including
open tubes and enclosures. Yet, no sufficient data obtained for explosion properties in
medium scale piping system to assist engineers or practitioners in determining the
potential hazard posed due to explosion. In this work, an experimental study had been
carried out to investigate the explosion properties in a pipeline using two pipe
configurations, i.e. straight and 90 degree bend. A horizontal steel pipe, with 2 m long
(L) and 0.1 m diameter (D), giving L/D ratio of 20 was used in the range of equivalence
ratios (Ф) from 0.5 to 1.8. The 90 degree bend pipe had a bend radius of 0.1 m with
added a further 1 m to the length of the pipe (based on the centerline length of the
segment). Natural gas/pure oxygen mixture was prepared using partial pressure method
and a homogeneous composition was achieved by circulating the mixture using a solid
ball which was placed in the mixing cell. It was shown that stoichiometric mixtures gave
the highest flame speed measurement, both on straight and bend pipes. Stoichiometric
concentration (Ф = 1.0) gave significant maximum overpressure of 5.5 bars for bend
pipe, compared to 2.0 bars on straight pipe explosion test; approximately 3 times higher.
This was due to bending part that acted like obstacles. This mechanism could induce and
created more turbulence, initiated the combustion of unburned pocket at the corner
region, causing high mass burning rate and hence, increased the flame speed. It was also
shown that the flame speed was enhanced by factor of 3 for explosion in bend pipe
compared to straight pipe. It can be concluded that the bend can create greater
turbulence effect compared to straight pipe configuration and applying appropriate
safety devices before the area of the bends is recommended as one of the effective
methods to prevent the explosion from happen.