Fuel, Vol.93, No.1, 454-467, 2012
Fuel and diluent property effects during wet compression of a fuel aerosol under RCM conditions
Wet compression of fuel aerosols has been proposed as a means of creating gas-phase mixtures of involatile diesel-representative fuels and oxidizer + diluent gases for rapid compression machine (RCM) experiments. The intent of this study is to investigate the effects of fuel and diluent gas properties on the wet compression process, specifically to: (a) explore a range of fuels which could have applicability in aerosol RCM experiments, and illustrate important limitations due to fuel properties, and (b) fundamentally understand how fuel and diluent gas properties affect the wet compression process and assess which ones are most important. Insight gained from this work can be utilized to aid the design and successful operation of aerosol RCMs. A spherically-symmetric, single-droplet wet compression model is used where n-heptane, n-dodecane, 2,2,4,4,6,8,8-heptamethylnonane (isocetane), n-hexadecane (cetane) and n-eicosane are investigated as the diesel-representative fuels, while comparisons are made to water droplets. Nitrogen, neon and argon are selected as the gas-phase diluents while the oxidizer is considered to be oxygen at atmospheric concentrations. Initial droplet diameters of d(0) = 3 and 8 mu m are used based on results of previous studies where the overall compression time is set to 15.3 ms with the maximum volumetric compression ratio 13.4. An overall equivalence ratio of phi = 1.0 is used. It is shown that under these conditions, involatile fuels up to similar to n-hexadecane appear to be candidates for aerosol RCM experiments. However, the use of small droplets (d(0) < 5 mu m) will be necessary in order to ensure complete vaporization and adequate gas-phase mixing in advance of low temperature chemical reactivity. Fuels with higher boiling points might not be useable unless extremely small droplets (d(0) < 1 mu m) and low pressures (e. g., P-0 < 0.5 bar) are employed along with longer compression times. In addition, the boiling curve (i.e., saturation pressure) and L-f are found to be the dominant fuel properties while the density-weighted mass diffusivity, rho D-g(g), which controls the rate of gas phase mass diffusion, and thus compositional stratification, generally plays a secondary role. The heat capacity and molar mass are the dominant diluent properties that affect the near-droplet and 'far-field' conditions. The gas-phase mixture Lewis number (Le(g)) contributes to either greater compositional (Le(g) > 1) or thermal (Le(g) < 1) stratification. For large hydrocarbons and oxygenated hydrocarbons that are representative of diesel fuels Le(g) similar to 3-5, and therefore compositional stratification could be significant; this characteristic has the potential to complicate interpretation of ignition/oxidation data acquired from these machines. (C) 2011 Elsevier Ltd. All rights reserved.