Abstract
The burning rate of liquid nitromethane as a function of pressure is known to exhibit slope breaks, following Saint-Robert's law only in limited regions of pressure. The present paper presents experimental and modeling results with the objective to better understand this behavior. A new experimental facility is used to visually observe the combustion process from 3 to 101 MPa allowing for measurement of burning rates in tubes of various diameters with high-speed cinematography. A series of modeling calculations are performed increasing in complexity first studying vapor phase flame propagation with the ideal gas equation of state and then a real gas equation of state. These results are compared with previously published predictions of the liquid regression rate using the same kinetic model and ideal gas equation of state. Theory indicates that, at high pressures, the freely propagating vapor-phase flame speed should agree with liquid regression rates. Combined, these results enable further insight into the mechanisms for the slope breaks and complex burning behavior. Three regimes were identified in the burning rate as a function of pressure. A low-pressure regime with pressure exponent of 1.16 exists where burning occurs with a distinct interface between liquid and gas. At approximately 18 MPa, an abrupt increase in burning rate occurs that is associated with the critical point of the mixture of nitromethane and near-surface species producing pressure exponents ranging from 2–10. Above this pressure, the loss of surface tension induces surface waves and large-scale hydrodynamic instabilities. With increasing pressure, burning rates continue to increase with pressure but with a gradual decrease in pressure exponent towards a value of ∼0.75. At the highest pressure the large-scale instabilities disappear, and the flame propagates with a turbulent cellular front, the speed of which depends upon the tube diameter.
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