On Multi-Fluid models for spray-resolved LES of reacting jets

Abstract

Numerical simulation of sprays can potentially be used for assessing the performance and variability of engines. But today's simulations only provide average quantities, after calibration. Spray injection can be thought of as a multiscale problem with 4 levels (chamber, mixing layer, drop scale, and nozzle). Because of their complex interactions, increasing predictivity requires simulations to capture more of these scales. To assess the trade-offs in this regard, we perform a review of recent Diesel spray simulations. After highlighting the importance of the mixing layer in driving spray dynamics, we analyze various two-phase flow formalisms and show the potential benefits of a Eulerian spray formulation combined with Large Eddy Simulation (LES) to capture a variable-density mixing layer (VDML). We then present a framework and numerical methods to make this description operative and show how it performs on a realistic autoignition case called Spray A.

A semi-Lagrangian transport method for kinetic problems with application to dense-to-dilute polydisperse reacting spray flows

“…The conditions are hotter (T g = 1200 K) and correspond to a case where ignition takes place earlier and closer to the injector compared to the 900 K-case. The fuel vapor mass fraction, together with the level of turbulence and the temperature of the mixture, is considered to be a driver of autoignition [65,54,36]. The present results show the strong unevenness of the fuel vapor mass fraction and are promising for analyzing autoignition, its mechanisms, and its statistics, which is one of the long term goals of Diesel injection studies.…”

“…The authors agree that turbulent dispersion is important in sprays, but in this dense spray context no satisfactory model exists and the physics is not well known. The method has already been used for the study of momentum coupling in spray injection [35] and for preliminary studies on autoignition [36]. Here we analyze realistic spray results to prove the feasibility of the strategy for the simulation of dense sprays.…”

“…The strategy and numerics devised here focus on capturing near-nozzle scales and connecting dense spray dynamics to a dilute reacting spray. A review of spray injection and autoignition simulations [36] gives practical arguments to support this strategy. Choosing two moments is typically a good trade-off [39] for polydisperse problems with diverse drop dynamics.…”

“…We show numerical results of high fidelity simulations to clarify the potential of the method to act as a research tool on reacting spray physics and engine development. The strategy has been proposed and justified for engine sprays in [36].…”

“…The end of the plume has a large region where the temperature is above 800 K and the equivalence ratio ϕ is close to 2. At this temperature, high equivalence ratios are known to favor the so-called low-temperature reactions of the fuel so this rich region is a major candidate to investigate the path to autoignition [65,54,36]. The present strategy has therefore been able to finely describe the spray up to the point where it mixes as an ignitable turbulent flow.…”