The influence of flow parameters on minimum ignition energy and quenching distance

Abstract

Experiments have been carried out on the effects of pressure, velocity, mixture strength, turbulence intensity and turbulence scale on minimum ignition energy and quenching distance. Tests were conducted at room temperature in a specially designed closed-circuit tunnel in which a fan was used to drive propane/air mixtures at subatmospheric pressures through a 9 cm square working section at velocities up to 50 m/sec. Performated located at the upstream end of the woring section provided near-isotropic turbulence in the ignition zone ranging from 1 to 22 percent in intensity, with values of turbulence scale up to 0.8 cm. Ignition was effected using capacitance sparks whose energy and duration could be varied independently. The results of these tests showed that rectangular, arc-type sparks of 60 μsec duration gave lower than previously reported values of ignition energy for both stagnant and flowing mixtures. It was found that both quenching distance and minimum ignition energy increased with (a) increase in velocity, (b) reduction in pressure, (c) departures from stoichiometric fuel/air ratio, and (d) increase in turbulence intensity. Increase in turbulence scale either raised or lowered ignition enegy, depending on the level of tubulence intensity. Equations based on an idealized model of the ignition process satisfactorily predicted all the experimental data on minimum ignition energy.