As a hydrogen energy carrier and carbon-free fuel, ammonia (NH3) is being growingly considered as an alternative fuel for combustion systems in an effort to reduce carbon emissions. Unlike conventional fossil fuels, its relatively low flame speed hinders its widespread use in practice. Utilizing the 1D freely propagating flame model in Chemkin-Pro 2019, this work numerically evaluates the impact of blending ammonia with dimethyl ether (DME) on the laminar burning velocity of NH3/air flames. For this, the reaction mechanism is first validated by comparing predictions to experimental data available in the literature. The addition of DME is found to significantly enhance the laminar burning velocity (SL) of NH3 flames, which can be comparable to that of hydrocarbon fuels. This enhancement is a result of the combined thermal and chemical effects. Reaction pathway and sensitivity analyses reveal that adding DME enables the major reaction steps and the sensitivities to be altered, thus positively affecting the overall reaction rate. Furthermore, the SL of NH3/air flames is weakly dependent on the inlet thermodynamic pressure, but becomes stronger with more DME blended. The explanation behind this phenomenon is that the relative importance of the termination versus branching reactions is more significant in binary flames. The overall reaction order characterizing the importance of the chain branching and the terminating reactions shows a decreasing trend with increasing inlet pressure. In general, the present work provides a design guideline on the selection of fuel flexibility and sheds light on the fundamental NH3-DME-fueled flame enhancement mechanisms. 

Enhancing and assessing ammonia-air combustion performance by blending with dimethyl ether

In ramjet propulsion systems, syelf-excited combustion instability is highly undesirable. It may lead to violent structural vibration and even mission failure. For this, self-excited combustion instabilities in such SFRJ are investigated numerically by 2D unsteady Reynolds-Average Navier-Stokes (URANS) model and varying the inlet mass flow rate ṁair. When combustion instability occurs, large-amplitude temperature, velocity, and reacting species wavy motions are observed along the axial flow direction. To shed light on the flow features and the energy distribution among the unstable eigenmodes, proper orthogonal (POD) and dynamic mode decomposition (DMD) analyses of nonlinear combustion instabilities in a solid-fuel Ramjet (SFRJ) combustor are conducted. The POD studies reveal that the fluctuation energy of the first six modes contributes to 99% of the total fluctuation energy of all modes. It means that the main features of the SFRJ combustors could be captured by using the first six modes. Unlike POD studies, DMD analysis captures the frequency information and spatial structures in the entire flow field. Examining the DMD growth rate reveals that all dominant DMD eigenmodes are marginally stable (growth rate = 0) or unstable (positive growth rate). And the frequency of each dominant mode could be enhanced by 3–10 Hz as theṁair is increased by 0.2 kg/s. The mode energy decreases with the increase of the mode order, with the decrease up to around 90%. The DMD analysis on velocity field indicated that the flow separation could be observed near the backward facing step. And the DMD mode structure of the temperature field can significantly affect the mode structures of Mach number and YCO2 field. In general, the present work provides detailed analyses of the SFRJ flow field in the presence of self-excited combustion instability by using POD and DMD approaches. 

Kan Xie

Papers

Numerical investigation on regression rate and thrust regulation behaviors of a combined solid rocket motor with aluminum-based fuel