Centrifugal-force effects on combustion

The effects of centrifugal force on the combustion of propane-air and hydrogen-air mixtureswere evaluated in a combustion centrifuge. Tests using propane-air mixtures over a range of equivalence ratios, and hydrogen-air mixtures at an equivalence ratio of 0.6, showed that the flame-propagation rate was relatively independent of centrifugal force in acceleration fields of less than 200 times that of the earth (200 g), but increased approximately in proportion to the square root of the acceleration field at values above 200 g. Conversely, for a stoichiometric hydrogen-air mixture, the flame-propagation rates were independent of centrifugal force. A hypothesis, based on a “bubble transport” flamespreading mechanism, is proposed to explain the conflicting experimental results. Buoyant forces acting on a flame “bubble” move it in a direction opposite to that of the applied centrifugal force at a velocity corresponding to the condition at which the buoyant and drag forces on the bubble are equal. At large values of centrifugal force, the bubble velocity exceeds the turbulent flamespeed and controls the flamespreading rate; at lower values, the bubble velocity is less than the turbulent flamespeed and does not affect flame propagation. In this investigation, bubble velocities to 65 fps have been measured; these exceed conventional propane-air turbulent flamespeeds nearly fourfold. In many tests, an upper limit of the imposed acceleration field was reached, above which combustion could not be sustained. This limit, which was preceded by an abrupt decrease in flamespeed with increasing acceleration, appears to be a function of laminar flamespeed, and can be expressed approimately by g=4500Su, where Su is the laminar flamespeed for the mixture.