Some New Experimental Observations of Combustion Characteristics near Lean Limit in a Tube under Microgravity

Experiments have been carried out with extremely lean, quiescent propane-air mixtures to examine the behavior of irregular flame propagation and to examine the lean limits of flame propagation in a tube under microgravity. The microgravity technique achieved in a freely falling chamber is employed because the realizations of symmetrical flame propagations in a tube are possible. Experimental conditions for the initial mixtures correspond to room temperature and 0.1 MPa and the fuel used is propane of 99.9% purity. The experimental results show that (1) though the lean limit of the flame propagation in a tube decreases with increasing diameter, it holds constant when the tube diameter is more than 50 mm, (2) with a large combustion tube, the lower flammability limit shifts to as low as φ = 0.15 of equivalence ratio under microgravity, (3) the irregular flame fronts of cellular nature are observed in a narrow range of 0.35 < φ < 0.55 under microgravity, (4) the lowest measured value of flame speed is of the order of 11.5 cm/s under microgravity which is less than half of that observed under normal gravity, and (5) the influence of radiation from the wall in a tube on flame speed appears negligible.