Spray, combustion and soot of water-in-fuel (n-dodecane) emulsions investigated in a constant volume combustion chamber part 1: Influence of low water content

Abstract

This experimental study focuses on the characteristics of water-in-fuel-emulsions (WFE) with low water mass fractions (⩽10wt%) on injection, spray combustion and soot in regard to direct-injection (DI) diesel fuel engines. Investigations have been performed in an optically accessible constant volume combustion chamber (CVCC) at engine relevant conditions (Tgas=860K/ρgas=22.8kg/m3) with n-dodecane as a reference diesel fuels surrogate fuel. Optical diagnostics has been applied to characterize the influences related to the water content in the fuel on injection such as spray momentum flux and hence, air entrainment as well as on spray combustion and soot formation/oxidation.In order to further enhance the fundamental understanding of the fuel mixing and chemical effects of WFE, two injection strategies have been employed: first, a constant injection pressure as for neat fuel and thus a prolonged injection duration was employed. Second, a constant energy input flow has been obtained by increasing injection pressure dependent on water content while keeping the injection duration constant. The injection is described by injection rate/momentum analysis. The spray parameters (penetration length, cone angle) have been assessed by Schlieren imaging and the combustion characteristics (ignition delay, flame lift-off length) were acquired by OH∗-chemiluminescence. To investigate the (temporal and spatial) soot behaviour two-color pyrometry has been applied and is compared with diffused back-illumination extinction imaging (DBIEI).n-Dodecane and WFE show nearly equal momentum flux and spray penetrations for constant injection pressure, i.e. the first injection strategy. Compared to neat fuel, ignition delay is gradually increased with addition of water. Measured quasi-steady lift-off lengths (LOL) increased and evaluated fuel-to-air ratios Φ at axial LOL decreased, however, for both WFEs differences of Φ are within their respective standard deviation. Increasing water contents reduce axial and radial soot extension as well as the optical soot densities (KL factor). Moreover, the injection strategy of constant injected energy input flow further decreased soot area and optical soot densities (KL). Decreasing fuel-to-air ratios Φ on the spray centerline at the axial soot inception location and increasing fuel flow residence times from LOL to axial start of soot formation with increasing water mass fractions indicate a direct influence of the water content in WFE on soot formation/oxidation processes.