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.

已知液氮甲烷的燃烧速率随压力的变化会出现斜率折断，仅在有限的压力区域内遵循圣罗伯特定律。本文介绍了实验和建模结果，目的是更好地了解这种行为。一种新的实验设备用于肉眼观察从3到101 MPa的燃烧过程，从而可以通过高速摄影测量各种直径的管中的燃烧速率。进行一系列建模计算会增加复杂性，首先使用理想状态方程，然后使用真实状态方程研究气相火焰传播。使用相同的动力学模型和理想的气体状态方程，将这些结果与先前发布的液体回归速率预测进行了比较。理论表明，在高压下，自由传播的气相火焰速度应与液体回归速率一致。结合起来，这些结果使您可以进一步了解坡度折断和复杂燃烧行为的机理。在燃烧速率中，根据压力确定了三种状态。存在着压力指数为1.16的低压状态，其中燃烧发生时在液体和气体之间存在明显的界面。在大约18 MPa时，燃烧速率突然增加，这与硝基甲烷和近地表物种的混合物的临界点相关，从而产生2-10的压力指数。在此压力以上，表面张力的损失会引起表面波和大规模的流体动力不稳定性。随着压力的增加，燃烧速率随着压力的增加而继续增加，但压力指数逐渐降低至约0.75。在最高压力下，大规模的不稳定性消失，火焰以湍流的细胞前沿传播，其速度取决于管的直径。