Role of low-temperature chemistry in detonation of n-heptane/oxygen/diluent mixtures

The ZND structure of mixtures of n-heptane, oxygen and diluent has been investigated using a reduced reaction scheme that includes the low-temperature chemistry (LTC) pathways. It is shown that for high CO2 contents (XCO2 > 0.82) such that the reaction temperature is relatively low, the structure is affected by LTC and exhibits two distinct stages of energy release caused by low- and high-temperature chemistry, respectively. Based on the ZND structures, the dynamic parameters such as cell size, direct initiation energy and critical tube diameter of detonation within the LTC affected regime have been evaluated using various semi-empirical models. Such detonation structures exhibiting two distinct length scales lead to two distinct values for each of these dynamic parameters. For the evolution of the induction length and cell size, although the total length scales do not show negative response with increasing temperature, the length scale of the first-stage ignition demonstrates negative response when the post-shock temperature decreases within the LTC controlled regime. For the evolution of direct initiation energy, the models based on critical curvature and critical decay rate show negative temperature response, but the model based on cell size predicts continuous increase of critical initiation energy with increasing dilution. For the critical tube diameter, the model based on the critical decay rate approach also exhibits the negative temperature response.