FLAMMABILITY OF HYDROCARBON/CO2 MIXTURES: PART 2. PREDICTIVE MODELS FOR GAS JET IGNITION

The present work examines predictive models for the flammability of hydrocarbon and CO2 gas mixtures in jet releases. The extent of a flammable gas cloud from a jet release is often defined in terms of the location of the mean 50% Lower Flammable Limit (LFL) contour. Here, the use of 50% LFL rather than 100% LFL is to account for turbulent fluctuations above and below the mean gas concentration, which can produce pockets of flammable gas even where the mean mixture concentration is below the LFL. Rather than focus on mean gas concentrations, which provide only an indirect indictor of flammability, the present work examines the flammability of gas jets using the concept of the 'flammability factor'. This field variable provides essentially a prediction of the ignition probability, i.e. the likelihood of the gas mixture being ignitable at any particular location and time. Following the pioneering work undertaken at British Gas in the 1980's and using a combination of empirical relations taken from the literature, a simple predictive model is presented for the flammability factor in free, unobstructed, subsonic gas jets. The model is compared to previously published measurements of ignition probabilities and recent experiments conducted at the Health and Safety Laboratory (HSL). Good agreement between the model predictions and the previously published measurements is obtained. For the HSL experiments, the agreement between the model predictions and the measured ignition probability is reasonably good in the near-field of the jet but it deteriorates further downstream. The differences here are attributed to the effect of the wind in the experiments, which may have disturbed the downstream portion of jet, due to its relatively low momentum. The study demonstrates that even low wind speeds can have a significant influence on the dispersion behaviour of subsonic gas jets, and hence their ignition characteristics.