Transient Flame Development in a Constant-Volume Vessel Using a Split-Scheme Injection Strategy

Abstract

Multiple-injection strategies are characterized by a complex and transient interplay between high-and low-temperature reactions. Tracking low-temperature reaction products such as formaldehyde (CH2O) is particularly important to understand ignition phenomena and the so-called "combustion recession" that is observed in experiments. Experimentally, it is often difficult to discriminate between formaldehyde and other species such as poly-aromatic hydrocarbons, which is why a selective excitation approach is used in this work. Simultaneous high-speed imaging of the chemicallyexcited hydroxyl radical (OH*) is used to improve indication of flame location and second stage ignition. During experiments in a constant-volume vessel, two 0.5-ms injections of n-dodecane, separated by 0.5-ms dwell time, are injected into a 900-K ambient. The global flame development is characterized based on high-speed diagnostics, followed by an investigation into the spatial distribution of formaldehyde at four different times after start-of-injection (aSOI). Results show significant influence of the first injection on characteristics of the second. Ignition delay and lift-off location of the second injection are prominently reduced, while flame penetration is greatly enhanced by the wake of the first injection. Formaldehyde structure is observed during both end-of-injection transients, reaching as far upstream as 6 mm from the nozzle. Even after the second injection, the flame structure still appears to be influenced by the first, with a shorter lift-off length and compressed formaldehyde structure. Based on the selective excitation procedure, it becomes clear that the interpretation of laser-induced fluorescence (LIF) images obtained by 355-nm excitation alone is prone to ambiguity.

Heavy-Duty Diesel Engine Spray Combustion Processes: Experiments and Numerical Simulations

“…Additional filtering of the AHRR is applied after ignition, reducing the periodic pressure oscillations originating from the initial pre-mixed, auto-igniting fuel. A pressure based ignition delay is determined based on a 50-mbar increase of the vessel pressure after the corrections, which is slightly higher compared to a previous study [36]. Here, injection rates and the amount of injected mass are significantly higher due to the large orifice injector and long injection durations.…”

“…Fluorescence was recorded during a 100-ns gate time using an ICCD camera (PI-MAX 3, Unigen II photocathode) with a 50-mm objective (Nikkor AF-D f/1.4) and a band-pass filter centered around 380 nm (Semrock, 46 nm FWHM). By limiting the collection to the part of the spectrum below 440 nm, an attempt was made to limit the interference by poly-cyclic aromatic hydrocarbon (PAH) fluorescence and soot radiation excited by the same wavelength [35,36]. The experiments shown here were also performed shortly after auto-ignition of the fuel, before soot is formed.…”

“…The obtained pressure signal is corrected for the pressure decay before and after the fuel injection, as well as the local speed-of-sound in the vessel based on the location of the flame lift-off length, similar to ref. [36]. A low-order filtering procedure is used to smooth the pressure signal, prior to calculating the apparent heat release rate (AHRR) according to Heywood [38].…”

“…However, such a conclusion cannot be deduced based on this image. For proper distinction between formaldehyde and PAH in future experiments with large orifice injectors, an on-and off-resonant imaging strategy might be required to provide more conclusive results [35,36].…”

Comparative study on chemical kinetic schemes for dual-fuel combustion of n-dodecane/methane blends

“…Maes et al [67] investigated the evolution of the flame based on imaging of both the excited and ground state hydroxyl radicals (OH * /OH), together with CH 2 O visualization. Measurements of a split-injection strategy yield a more intermittent combustion during the premixed burn of the first injection, whereas the second fuel injection ignites faster [68,69].…”