OH Radical Imaging in a DI Diesel Engine and the Structure of the Early Diffusion Flame

Abstract

Laser-sheet imaging studies have considerably advanced our understanding of diesel combustion; however, the location and nature of the flame zones within the combusting fuel jet have been largely unstudied. To address this issue, planar laser-induced fluorescence (PLIF) imaging of the OH radical has been applied to the reacting fuel jet of a direct-injection diesel engine of the "heavy-duty" size class, modified for optical access. An Nd:YAG-based laser system was used to pump the overlapping Q19 and Q28 lines of the (1,O) band of the A+X transition at 284.01 nm, while the fluorescent emission from both the (0,O) and (1,l) bands (308 to 320 nm) was imaged with an intensified video camera. This scheme allowed rejection of elastically scattered laser light, PAH fluorescence, and laser-induced incandescence.OH PLIF is shown to be an excellent diagnostic for diesel diffusion flames. The signal is strong, and it is confined to a narrow region about the flame front because the threebody recombination reactions that reduce high flame-front OH concentrations to equilibrium levels occur rapidly at diesel pressures. No signal was evident in the fuel-rich premixed flame regions where calculations and burner experiments indicate that OH concentrations will be below detectable limits. Temporal sequences of OH PLIF images are presented showing the onset and development of the early diffusion flame up to the time that soot obscures the images. These images show that the diffusion flame develops around the periphery of the downstream portion of the reacting fuel jet about half way through the premixed burn spike. Although affected by turbulence, the diffusion flame remains at the jet periphery for the rest of the imaged sequence. The images also show many details of the diffusion flame structure including its upstream extent. Finally, the location and nature of the diffusion flames are discussed with respect to previously reported soot and fuel distributions.

Experimental characterization of diesel ignition and lift-off length using a single-hole ECN injector

“…Dec and Coy [21]  When measuring the very weak emissions of CH* the sensitivity of the setup is fundamental, and even an intensified camera may not be able to catch the very beginning of cool flames [16]. Moreover, in the case CH* emissions are detected it is not straight forward to know that the setup is catching the emissions from the beginning.…”

A Phenomenological Model for Prediction Auto-Ignition and Soot Formation of Turbulent Diffusion Combustion in a High Pressure Common Rail Diesel Engine

“…Sandia National Laboratories reproduced the experiment in the optical engine using similar operating conditions. There are many advantages than DEC [19][20][21] conceptual model, particularly in the liquid spray penetration, fuel vapor permeability, flame structure and soot concentration on the spatial correlation. Tao et al knew, when the larger soot particles are in fuel-rich vortex downstream of the head region, there is a thin layer of a large concentration of soot particles in the very low quality flame of the external surface area.…”

Diesel Combustion: An Integrated View Combining Laser Diagnostics, Chemical Kinetics, And Empirical Validation

“…Over the past decade, a wide variety of these diagnostics have been applied to directinjection (DI) diesel combustion using a specially designed, optically accessible research engine (1)(2)(3)(4)(13)(14)(15)(16)(17)(18)(19)(24)(25)(26). These laser-imaging measurements have provided an abundance of new information about the details of diesel combustion and emissions formation.…”

“…Government laboratories in the U.S., Europe and several universities (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) have undertaken laser imaging studies to define the structure of diesel flames and their temporal evolution. Although these diagnostics were performed on a variety of engine and high-pressure vessel configurations, they show many similarities and begin to form a basis for the general description of a burning diesel fuel spray plume.…”