The Effect of Ambient and Injection Pressure on Droplet Size of Ammonia Sprays in a Constant Volume Chamber

Ammonia, a zero carbon energy vector, is under consideration for decarbonising marine and energy storage applications due to its high mass-based energy density compared to many alternatives. In addition, there is widespread existing supply and transportation infrastructure due to ammonia’s use as a fertiliser. When injected in its liquid form, however, ammonia behaves quite differently to traditional fuels due to its high saturation pressure and enthalpy of vaporisation, amongst other things. This means that fundamental data on ammonia sprays need to be collected in order to understand ammonia spray behaviour and calibrate models of ammonia sprays needed for design in the virtual world. Previous work on ammonia sprays has mostly focused on spray morphology at a macroscopic level (such as liquid penetration length). However, there are fewer studies of ammonia sprays at a microscopic level. In this study, liquid ammonia was injected into a constant-volume chamber using a direct injector at two injection pressures (100 bar and 150 bar) and a range of ambient pressures from 3–13 bar. This range of ambient conditions spans regimes from flash-boiling to non-flash-boiling, thereby enabling systematic investigation of the transition between these regimes. A laser diffraction technique was used for measuring the droplet sizes of the spray at different locations (in a cylindrical volume with a diameter of 10 mm) within the spray plume at 10 kHz, and the nominal droplet sizes were quantified by the Sauter Mean Diameter (SMD). These SMD values provided, at a microscopic level, an insight of the atomisation of the spray as it left the nozzle and penetrated into an environment with different densities. It was found that the tested injector leads to a breakup dominant spray behaviour with liquid ammonia and hence the SMD values decrease as ambient pressure increases. In addition, the droplets are generally smaller at the outer edge of the spray plume compared to the inner part and both the injection pressure and injection duration have a strong effect on the droplet sizes.