Hot surface ignition of a wall-impinging fuel spray: Modeling and analysis using large-eddy simulation

Abstract

The accidental ignition of liquid fuels is an industrial safety concern due to the storage and transport of pressurized flammable liquids near components at elevated temperatures. In this work, a liquid n-dodecane fuel spray is considered that impinges on a hot surface, undergoing thermal ignition. Surface temperatures above the minimum hot surface ignition temperature (MHSIT) for n-dodecane ignition at atmospheric pressure are considered. At these temperatures, the interaction of the spray with the hot surface is governed by the Leidenfrost effect, resulting in inelastic reflection of impinging droplets. Large-eddy simulations are employed with finite-rate chemistry using a realistic 54-species chemical mechanism with low-temperature ignition chemistry. An Eulerian-Lagrangian approach is taken to the describe the spray dynamics. Ignition kernel formation and propagation are discussed in physical and composition spaces, and the flow field structure is compared to theory. At temperatures near the MHSIT, the secondary flow resulting from the spray impingement in the form of a toroidal vortex is shown to enhance scalar mixing. Low-temperature ignition within the vortex is seen to significantly precede high-temperature ignition near the wall and subsequent rapid flame propagation. At higher wall temperatures, the ignition delay is greatly reduced such that high temperature ignition occurs prior to the establishment of mixing structures, and hence the extent of flame propagation is diminished, with transition occurring rapidly to a steady-burning regime. This study provides fundamental understanding of the physical phenomena involved in the thermal ignition of impinging sprays in different temperature regimes toward the goal of improved industrial safety.