William L. Roberts

Thermally induced secondary atomization (TISA) is a complex phenomenon that accelerates phase change in the combustion chamber. It occurs if multi-component fuels, having a wide boiling range, are exposed to high temperatures. Several airlines are recently experimenting with bio- and fossil fuels blends. However, the characteristics of droplet TISA are primarily unknown because of the challenges associated with experimental activities like suspended or falling droplets. In this scenario, numerical models become essential to study TISA. That is why a new multi-component, multi-phase volume of fluid computational fluid dynamics solver was developed to simulate droplets TISA. The solver takes advantage of the OpenFOAM framework and uses the isoAdvector methodology. The bio- and fossil fuels were represented by n-heptane and n-hexadecane, respectively, to simplify the problem. Evaporation was implemented by assuming that the mixture could only boil at that temperature. Surface tension and other relevant mixture properties were considered as a function of species concentration and temperature to replicate all phenomena comprehensively. An analysis of bubble expansion based on the Rayleigh–Plesset equation preceded the breakup tests. The test cases consisted of a droplet suspended in microgravity having a bubble initialized at the interface. The bubble eventually expanded, and the bubble cap collapsed, leading to the micro-explosion. A parametric study of breakup cases under different pressures and at a fixed temperature of 1200 K was performed. The atomization mechanism was tested at 1, 3, 10, and 20 bar and compared. It was observed that while high pressure slows down the process, it finally leads to a higher surface area. This behavior was confirmed by testing two different bubble sizes. Together with the atomization intensity, also the morphology of the particles changed. At atmospheric pressure, the maximum surface area was reached when the droplet had a disk-like shape, while at higher pressures, it evolved in an elongated shape.