Simultaneous Schlieren–PLIF Studies for Ignition and Soot Luminosity Visualization With Close-Coupled High-Pressure Double Injections of n-Dodecane

Abstract

Studies are performed in a constant volume preburn type combustion vessel over a range of ambient temperatures (750 K, 800 K, and 900 K) at constant density (22.8 kg/m3) with 15% O2 by volume in the ambient at 1200 bar (n-dodecane) fuel injection pressure. The influence of the pilot (first) spray flame on the ignition and combustion characteristics of the main (second) injection is investigated while varying injection pressure, dwell time, and injection strategy. Simultaneous schlieren (with soot luminosity imaging) and 355 nm planar laser-induced fluorescence (PLIF) imaging for formaldehyde (CH2O) and polycyclic aromatic hydrocarbons (PAH) visualization was performed. At both 900 K and 800 K ambient, main injection exhibits a reduction in ignition delay (ID) by a factor of 2 over their respective pilots. For the ambient temperature condition of 750 K, reducing injection pressure from 1500 bar to 1200 bar causes a significant increase in ignition delay (by ∼0.8 ms), which was attributed to the influence of injection pressure on spray-mixing and early development of cool flame. Also, at 750 K ambient condition, multiple injection schedule having two 0.5 ms injections separated by a 0.5 ms dwell was found to have a shorter ignition delay than a single 0.5 ms injection. Studies carried at an 800 K ambient show that by increasing the dwell time, main interaction with pilot reactive intermediates can be controlled to avoid an early rich ignition of the main spray and to reduce soot precursors.

Optical study on characteristics of non-reacting and reacting diesel spray with different strategies of split injection

“…In general, the ignition delay (ID) of the second injection is reduced by a factor of two or more relative to that of the first injection, which is caused by the entrainment of high temperature combustion products and radical species remaining from first injection. The effects on ignition delay of dwell time between double injections have been also presented in 21,22 . However, more detailed analysis is still needed to be studied.…”

“…The effects on ID of dwell time between double injections have been also presented in Cung and colleagues. 20,21 However, more detailed analysis is still needed to be studied. The transient flame lift-off length (LOL) development of split injection has been recently measured by Maes et al 22 by means of the high-speed OH* chemiluminescence.…”

Flame–spray interaction and combustion features in split-injection spray flames under diesel engine-like conditions

“…In the experiments from Skeen et al [11], a double injection schedule (0.5 ms/0.5 dwell/0.5 ms) was employed at different temperatures. Moiz et al [8,16] performed double injections with shorter first and longer second injection durations (0.3ms /0.5 dwell/1.2 ms) to study soot production. Based on their works, double-injection strategies with short and long FIDs were studied at different dwell times.…”

“…The injected fuel mass or injection duration for the first injection is of great importance to improve fuel stratification [7], optimize the combustion phasing [8], and reduce soot emission [50,51]. Therefore, in order to optimize the combustion process, the ignition process for double injections with different FIDs should be further analysed in detail.…”

“…The proportion of fuel mass injected in the pilot and main injections is crucial in organizing the split-injection strategies. The fuel/air stratification [7] and the subsequent heat release process [8] will be influenced by the injection ratio. Previous study [9] found that the entrainment of oxygen was influenced by the dwell time.…”