Prediction of the ignition characteristics of flammable jets using intermittency-based turbulence models and a prescribed pdf approach

A mathematical model capable of predicting the ignition hazards presented by turbulent releases of flammable materials is presented. The model is based on solutions of the fluid flow equations, with closure of this equation set achieved using either a k–ɛ–γ turbulence model or an intermittency-based second moment closure. Solutions are coupled to a prescribed, three-part probability density function (pdf) to allow the prediction of the bimodal scalar distributions observed in intermittent free shear flows which can have a significant influence on ignition characteristics. Integration of this pdf over the flammable range of the release material then leads to the probability of ignition at any point in the flow. Predictions of the complete model are compared with data obtained in a number of jets, with comparisons for velocity and concentration fields, intermittency, concentration pdf's and ignition probabilities demonstrating that both turbulence modelling approaches are capable of reliably predicting ignition probabilities in the jet flows examined. Overall, results derived from the second-order modelling approach are superior to k–ɛ–γ turbulence model predictions.