Modelling of liquid ammonia spray based on flash boiling break-up driven by thermal growth of multiple bubbles

Ammonia is a sustainable zero-carbon fuel with promising application prospects. Compared with traditional fossil fuels, it has a lower boiling point and is prone to intense phase changes, leading to unique flash boiling phenomenon in low-pressure engine spray scenarios. Therefore, it is necessary to re-evaluate the reliability of the breakup model with trans critical state under gradient variation of ambient pressure range. This study proposed a flash boiling breakup model driven by thermal growth of multi-bubble based on Rayleigh-Plesset equation and nucleation theory. Droplet number and radius within every parcel is re-distributed after breakup. The droplet velocity is then corrected based on energy conservation. The new breakup model is coupled to the liquid ammonia spray simulations. Compared with experimental results, the trans critical spray characteristic is well captured in low-pressure injection scenarios. It is indicated that the bubble growth driven by flash boiling leads to an increasing bubble nucleation number as ambient pressure gradient decreases, which in turn, leads to frequent breakup of droplets, causing the spray tip penetration to decrease and the spray cone angle to increase, and simultaneously reducing the SMD. The development of this model provides a foundation for studying the combustion characteristics of ammonia engines.