EXPERIMENTAL SOME NEW OBSERVATIONS OF COMBUSTION CHARACTERISTICS NEAR LEAN LIMIT IN A TUBE UNDER MICROGRAVITY

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Experiments have been carried out with extremely lean, quiescent propane-air mixtures to examine the behavior of irregular flame propagation and to examine how 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 condition for the initial mixtures corresponds 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 its 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 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.
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