Mixture fraction measurements during the flame transmission at equip ment of the type of protection "flameproof enclosure"

The test for non-transmission of an internal ignition as required by IEC 60079-1 is crucial for flameproof enclosures. Internal ignition may cause an external ignition by (typically turbulent) mixing of hot exhaust gases with a surrounding combustible atmosphere. Understanding this process in detail is a key for a systematic development of flameproof enclosures. We investigate the mixing processes of hot turbulent flow with unburned fuel/air mixtures using laser-diagnostic methods. The focus of this work lies on the quantitative measurement of mixture fraction, defined as the fractional part of mass that originates from the hot jet. A novel strategy based on LIF of nitric oxide radical (NO) and computed correlations between the NO LIF-signal and mixture fraction is used to measure planar, instantaneous maps of mixture fraction. Using numerical simulations of the physical and chemical processes leading to ignition, and also using detailed simulations of NO-fluorescence spectroscopy, correlations between NO-LIF signal and mixture fraction are assessed. A comparison of the axial and lateral mixture fraction profiles deduced from the NO-LIF images shows higher gradients in the lateral direction than in the axial directions based on higher strain rates at the boundaries of the hot free jet leading to faster mixing in these areas. This explains the high probability of re-ignition along the axial direction rather than at the lateral direction as seen from earlier OH-LIF studies.