Similarity of high-pressure direct-injection liquid ammonia spray for different-sized engines

Carbon-free ammonia has been regarded as a promising alternative fuel for the shipping industry, and the high-pressure direct-injection (HPDI) liquid ammonia spray combustion mode has attracted significant attention in recent years. Scaled model experiment based on similarity theory has a significant role in the intensive development of a series of engines with different bore sizes, but there is no information available on scaled model experiments of ammonia engine. To facilitate the intensive development of ammonia engines with different bore sizes and meet the urgent GHG reduction targets, the present study conducts a pioneering theoretical and numerical study on scaled model experiments for ammonia engine, primarily focusing on the HPDI mode and the similarity of liquid ammonia spray from different-sized nozzle holes. In the theoretical section, the single value conditions and two similarity laws for scaling liquid ammonia spray are summarized, and the similarity of fuel injection rate, spray breakup, spray tip penetration evolution and maximum liquid length is theoretically analyzed. Then, an optical combustion chamber coupled with high-speed photography techniques are used to obtain liquid ammonia spray data across various engine-like conditions. After validating the 3D-CFD simulations against the experiment data, the similarity of liquid ammonia spray is numerically analyzed under different similarity ratios and ambient conditions. Generally, the spray vapor- and liquid-phase penetration as well as spray morphology and ammonia mass fraction distributions can be well scaled under the non-flashing region, indicating the effectiveness of the single value conditions and similarity laws. Moreover, the speed law leads to a slightly longer maximum liquid length, while the pressure law exhibits great prediction. This aligns with the theoretical analysis that the evaporation process of liquid ammonia spray is controlled by fuel and air mixing process, suggesting that the local interphase transport at the droplet surface is not the rate-controlling factor.