Thermal Autoignition of Aqueous Urea Ammonium Nitrate as a Function of Equivalence Ratio, Water Content, and Nitrogen Pressure

Aqueous urea ammonium nitrate has been previously suggested as a hydrogen-carrying monofuel. The addition of helium and water to the fuel is known to increase its autoignition temperature. Nevertheless, both the reason for this behavior and the reaction pathways that lead to the ignition of the fuel are not completely clear. In this work, the effect of fuel-rich conditions (1.1≤φ≤2.5) on the thermal autoignition of this fuel was investigated. The pre-ignition reaction network was explored using a kinetic gas-phase model. Furthermore, the effects of helium, water, and fuel-rich conditions on these reactions were studied using simulations. The results indicated the autoignition temperature rises with the increase of the urea content of the fuel mixture. Moreover, this behavior was suggested to stem partially from the inhibiting effect of low nitric acid levels on amidogen generation. Additionally, helium and water were identified to act as diluents that increase the pressure and the flux through three-body reactions. This work provides a valuable in-depth look into the reactions that lead to the ignition of aqueous urea ammonium nitrate and their dependence on the process parameters.