CFD investigation of the shift of Nukiyama and Leidenfrost temperatures for high-pressure dense sprays hitting hot walls

In this paper the sensitivity of the Nukiyama and Leidenfrost effects to high velocity and high number density jets has been assessed. The effects of Nukiyama and Leidenfrost describe the non-linear and non-monotonous phase-change of liquid droplets upon a hot surface. According to fundamental experiments, these effects occur at two specific temperature values, i.e. the Nukiyama temperature and the Leidenfrost temperature. However, if dense jets are injected with high rate, the occurrence of these effects can be shifted at different temperatures values, mainly due to local thermos-kinetical effects produced by multiple hits of droplets at high frequency against the surface. In this work, this phenomenon has been investigated by means of three-dimensional computational fluid dynamics simulations of high-pressure fuel spray (300 bar - 700 bar) impinging a hot aluminium wall heated at different temperature values representative of the phase-change regimes. The surface temperature variation due to the spray impingement has been reproduced by means of the conjugate heat transfer approach, which relies on the coupling of the one-dimensional finite element method simulation of the surface. The methodology has been used to map the actual values of the Nukiyama and Leidenfrost temperatures and their shift compared to the reference experimental ones. The reasons behind the shift have been investigated by means of the three-dimensional results, then a correlation with the components of the average Weber number of the impinging droplets has been proposed.