Xingqian Mao

The phase transition dynamics of ammonia droplets under high-pressure conditions, representative of advanced engine combustors, exhibit a remarkable complexity driven by drastic variations in thermophysical properties and the progressive diffusion of the vapor–liquid interface. This phenomenon renders classical evaporation models inadequate for accurately predicting the critical fuel–gas mixing process. To overcome this limitation, we have developed a high-pressure transient phase transition model that could capture the complete lifecycle of a single ammonia droplet in a nitrogen environment, from injection to full mixing. Results show that the process is not simple evaporation but instead evolves through three continuous stages governed by distinct physical mechanisms, i.e., a classical two-phase stage, a previously overlooked transitional two-phase stage, and a supercritical single-phase mixing stage. The latter two stages prove to be particularly critical. During the transitional stage, a rapid interface thickening and a sharp reduction in surface tension not only challenge the validity of the classical phase-equilibrium assumption but also lead to a dramatic collapse in the droplet's stability against external perturbations. Upon entering the supercritical stage, the efficient conversion from a liquid-like to a gas-like fluid is driven by a pseudo-boiling phenomenon, resulting in mixing rates far exceeding those dictated by conventional evaporation. These findings provide a deeper physical understanding of high-pressure fuel phase transitions and establish a critical theoretical basis for developing high-fidelity computational fluid dynamic sub-models to enable precise prediction and control of ammonia spray combustion.

Hydrogen nanobubbles dispersed in liquid ammonia represent a promising strategy for ammonia-hydrogen synergy, offering a simplified approach to hydrogen storage while enhancing the ammonia combustion efficiency for transportation propulsion. However, due to limited experimental data and simulations, the fundamental mechanisms governing this system are not yet fully understood. In this study, molecular dynamics (MD) simulations were employed to investigate the dynamic behaviors and evolution of hydrogen nanobubbles within liquid ammonia. The thermophysical properties of hydrogen nanobubbles were determined, and their stability mechanisms were identified across a range of supersaturation levels. Results show that stable hydrogen nanobubbles form only within a moderate supersaturation range. Low supersaturation fails to induce nucleation due to insufficient bulk free energy, whereas excessively high supersaturation leads to gas-liquid phase separation. The hydrogen nanobubbles exhibit high internal pressure and density, but their pressure-volume-temperature relationship deviates from predictions based on macroscopic experiments or conventional equations of state. The incorporation of hydrogen nanobubbles significantly reduces the viscosity of liquid ammonia but increases thermal conductivity and diffusion coefficients, with these effects well modeled by quadratic interpolation. Moreover, gas supersaturation and surface tension play crucial roles in maintaining the mechanical equilibrium of the stable hydrogen nanobubbles. As supersaturation increases, hydrogen's diffusion coefficient decreases, and its transport behavior increasingly reflects random thermal motion. Finally, by combining MD simulations with classical bubble theory, the maximum achievable hydrogen-to-ammonia ratio is estimated at 9.07%. This study offers valuable insights and foundational data for ammonia-hydrogen fuel preparation for combustion.