Schlieren Methodology for the Analysis of Transient Diesel Flame Evolution

Abstract

Schlieren/shadowgraphy has been adopted in the combustion research as a standard technique for tip penetration analysis of sprays under diesel-like engine conditions. When dealing with schlieren images of reacting sprays, the combustion process and the subsequent light emission from the soot within the flame have revealed both limitations as well as considerations that deserve further investigation. Seeking for answers to such concerns, the current work reports an experimental study with this imaging technique where, besides spatial filtering at the Fourier plane, both short exposure time and chromatic filtering were performed to improve the resulting schlieren image, as well as the reliability of the subsequent tip penetration measurement. The proposed methodology has reduced uncertainties caused by artificial pixel saturation (blooming). Additionally, an algorithm to calculate Lift-off length from schlieren images has been developed and compared to the more conventional OH* imaging technique.By means of the improved setup, the analysis of transient diesel flame has been performed throughout the ignition and early combustion phases. When compared to a non-reacting vaporizing spray, the temporal evolution of the reacting case departs from the inert one at the start of combustion, when expansion in both axial (i.e. spray tip increase) and radial direction are observed. After that, the tip penetration progressively decreases on a clear transient period where the flow tries to re-organize itself in response to the induced reaction zone. Eventually, the spray undergoes an acceleration period where the reacting spray tip progresses above the nonreacting one. A parametric study has been conducted for three injection pressure levels to investigate the influence of this parameter upon transient flame evolution.

An experimental analysis on the evolution of the transient tip penetration in reacting Diesel sprays

“…In previous works authors have investigated about the uncertainties and limitations on the particular schlieren optical set-up [25] as well on the final digital analysis and measurements [23]. In that sense, particularities on the collection section are briefly recalled here in order to empathize the fact of having a "controlled" schlieren system.…”

“…The same arrangement as in previous works [23,25] has been used, namely a "focused single pass" schlieren setup, as sketched in Figure 1. This optical arrangement can be divided in two different sections, which are referred to as the illumination and collection sections.…”

“…This fact is highly desirable for image digital analysis avoiding artificial pixel saturation, i.e. blooming phenomena [26] on Diesel-like flames (further details on digital analysis can be found in [23]). …”

“…A processing routine to calculate the lift-off length in a flame schlieren image (LOL s ) has been developed. This routine comprises the following steps (further details can be found in [23]). …”

“…In agreement with preceding works and also with literature reports, schlieren imaging of the reactive spray evolution will be performed. Careful considerations presented in [23] have been accounted for when setting up the system optics to enable reliable schlieren measurements. Tests have been performed in a Constant-Pressure Flow (CPF) vessel with a much larger visualization system than in [21].…”