Khanh Cung

Ignition, lift-off, and soot formation studies in n-dodecane split injection spray-flames:

A close-coupled double injection strategy with two 0.5-ms injections separated by a 0.5-ms dwell is implemented. Studies are performed in a constant volume pre-burn type combustion vessel over two ambient temperatures (900 and 800 K) at constant density (22.8 kg/m3) with 15% O2 by volume in the ambient. The aim of this work is to investigate the establishment and dependence of ignition delay and flame stabilization on the ambient temperature conditions especially for the main injection, and thereby investigating eventual soot production. Simultaneous schlieren and planar laser -induced fluorescence experiments as well as three-dimensional Reynolds-averaged numerical simulation computational fluid dynamic modeling with chemical kinetics in every computational fluid dynamic cell were performed. It was observed experimentally that at 900 K, the second injection is injected in a high-temperature combustion recessed ambient of the first injection whereas at 800 K it is injected in a low temperature, possibly reactive species environment. It was found from Reynolds-averaged numerical simulation modeling that combustion recession at 900 K in the present case entails rich presence of hydroxyl radical species and also the ambient of 800 K is source of reactive radicals like peroxides, leading to acceleration of main ignition. Flame stabilization of the second injection occurs closer to the injector due to short ignition delays with flame being sustained in the fuel–air premixing zone. Flame stabilization of the second injection was found to follow a premixed flame propagation mechanism. Investigation in mixture fraction and temperature space of pilot-main spray combustion revealed that the lower lift-off of main results in lower air-entrainment which causes richer ignition of main resulting in quicker and higher soot formation. The effect of the second injection in enhancing the oxidation of soot from the first injection by inducing enhanced mixing was also revealed.

Turbulent Spray Combustion

Understanding turbulence is one of the most difficult topics in science and engineering. This is because turbulent spray combustion involves many areas of physics and chemistry which accompany a variety of mathematical challenges. Defining the various length and timescales existing in turbulent flow provides a better way to understand and characterize this chaotic phenomenon. However, the degree of complexity increases when there is a strong interaction between turbulence flow and chemistry. Here, characteristic times of chemical reaction in a molecular level (chemical) and fluid-mechanic level (physical) determine which of these are more dominant. This interaction remains as one of the most important and challenging aspects of turbulent reacting spray. In the present chapter, we begin with a general discussion on turbulence. The following section covers description of key features involved in a spray combustion scenario. Concepts involving higher fidelity in description of turbulent combustion are covered by discussion of interaction of turbulence and combustion. In most actual spray combustion applications, the combustion is dominantly non-premixed. There is a minor aspect of premixed combustion too which are discussed in this chapter. New advanced combustion modes such as partially premixed combustion (PPC) and multiple injections, topics with growing interests, are introduced and discussed later. Finally, numerically simulating these aspects is a key area of combustion research. It is of utmost important to optimize the combustion system using computer-based simulations to avoid higher cost for experimentally parametric study. Reynolds-averaged Navier–Stokes (RANS) models are mostly used in commercial sector for computationally tractable simulation time. Large-eddy simulation (LES) offers a higher fidelity approach. With the advent of higher computational resources, LES approaches are becoming more popular for obtaining solutions of turbulent combustion. Aspects of both RANS and LES relevant to spray combustion scenarios are discussed. Although usually requiring very high computational power, direct numerical simulation (DNS) can provide an actual representative of many chemical and physical aspects of spray combustion such as evaporation and auto-ignition, which are discussed at the end of this chapter.