Spray, Combustion and Soot of Water-in-Fuel (n-Dodecane) Emulsions in a Constant Volume Combustion Chamber; Part II: Effects of Low Temperature Conditions and Oxygen Concentrations

Abstract

The application of water-in-fuel (n-dodecane) emulsions (WFE) with a fixed water mass fraction of 10 wt% has been investigated in an optically accessible constant volume combustion chamber (CVCC) at varying low ambient temperatures conditions (Tgas=760–800K/ρgas=22.8kg/m3) such as found in direct-injection (DI) Diesel engine operations with (extreme) “Miller”-timing. Furthermore, oxygen concentrations have been decreased to 18 vol% and 15 vol% in order to emulate exhausRd in DI engines.As in Part I two injection strategies have been employed: first, a constant injection pressure case as for neat fuel and prolonged injection duration (strategy (A)), second, the injection pressure has been increased to keep a constant energy input flow, therefore, with constant injection duration (strategy (B)).Spray parameters and combustion characteristics have been assessed by Schlieren imaging, Mie-scattering, diffused back-illumination extinction imaging (DBIEI) and OH∗-chemiluminescence, respectively. Furthermore, the spray flame’s soot formation has been investigated through the detection of soot incandescence signals.For the WFE liquid lengths (LL) penetrate farther downstream the respective LOL at all ambient conditions, whereas, utilizing neat n-dodecane LLs remain upstream the LOL. The interaction between LL and the high-temperature reaction zone dominates the soot behaviour of WFE resulting in decreased dwell times between ignition and first soot occurrence. Decreasing gas temperatures at atmospheric oxygen concentrations generally decrease dwell times, even though, ignition delays and LOLs increase by the utilization of neat n-dodecane.Soot incandescence signal amplitudes decrease strongly employing WFE (150–270 times lower maximum signal values) compared to neat n-dodecane injections. During mixing-controlled combustion phase apparent heat-release rate (aHRR) amplitudes increase for WFE with injection strategy (B) compared to the neat fuel sprays. At the gas temperature of 760 K non-sooting lifted flames have been detected for both fuels. Calculated fuel-to-air ratios Φ evaluated at the respective LOL agree well with soot occurrences for both fuels. Utilizing WFE, the additional latent heat introduced by the water increases LOLs and to a further extent the liquid fuel core which is found within the spray flame and therefore, a fuel rich combustion zone is suggested to surround the liquid-phase WFE. Hence, soot formation persists for calculated fuel-to-air ratios Φ<2 at the LOL, although, detected soot incandescence signals were on very low amplitude levels injecting the WFE.