Evaluation of fuel spray ignition delay behavior using a two-stage Lagrangian model

In this study, a two-stage Lagrangian (TSL) model is used to investigate the ignition process of a fuel spray under engine-like conditions. The existing two-reactor TSL model developed for quasi-steady flames is improved to simulate the ignition process of a fuel spray at engine-like conditions, by inserting a well-validated entrainment model for the liquid fuel spray. The ignition delays of n-dodecane and n-heptane sprays predicted by the improved TSL model are validated with Engine Combustion Network Spray A and Spray H data over a wide range of ambient temperatures and oxygen concentrations. The results show that the two-reactor TSL model is capable of capturing the complex interaction between turbulent mixing and chemistry at the spray core and periphery regions during ignition. A 4-step ignition process at intermediate temperatures is observed. First, 1(st)-stage ignition appears at the fuel-lean spray periphery when the local residence time reaches a critical value. Then, the transport prompts the 1(st)-stage ignition in the core region. After a kinetic-dominated 2(nd)-stage ignition in the fuel-rich core region, the flame propagates into the whole equivalence ratio space. At ambient temperatures that are considered relatively high for internal combustion engines, the 1(st)-stage and 2(nd)-stage ignition in the fuel-rich core region is so short that the 1(st)-stage ignition does not occur at the spray periphery, resulting in a 3-step ignition process. The high equivalence ratio in core region restricts the local temperature rise, and the 2(nd)-stage ignition finally appears at the spray periphery with an increasing local residence time and the transport from the core region. The effectiveness of the two-reactor TSL model also demonstrates the dominant role of transport in the multiple-step ignition process of a high-pressure liquid fuel spray. The ignition process of high-pressure liquid fuel sprays is of great interest in engine combustion research, but the simulation of this process is computationally costly. In this study, a reduced order, two-stage Lagrangian (TSL) model, is extended to ignition process simulations. Based on the configuration of two perfectly stirred reactors with a well-developed 1-D fuel spray entrainment sub-model, the TSL model is able to accommodate detailed chemical kinetic mechanisms. Validated by ECN Spray A and Spray H data, this model provides good predictions of ignition delay for fuel sprays at engine-like conditions. Moreover, this model explains the roles of transport and chemistry at the spray periphery and core regions during the multiple-step ignition process. The model developed in this study helps the understanding of the multiple-step ignition process, and it can also be of great significance in chemical mechanism development and validation.