Fuel and diluent property effects during wet compression of a fuel aerosol under RCM conditions
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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 (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 dieselrepresentative 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 d0 = 3 and 8μm are used based on results of previous studies where the overall compression time is set to 15.3ms with the maximum volumetric compression ratio 13.4. An overall equivalence ratio of ϕ = 1.0 is used. It is shown that under these conditions, involatile fuels up to ~ n-hexadecane appear to be candidates for aerosol RCM experiments. However, the use of small droplets (d0 < 5μm) 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 (d0 < 1μm) and low pressures (e.g., P0 < 0.5bar) are employed along with longer compression times. In addition, the boiling curve (i.e., saturation pressure) and Lf are found to be the dominant fuel properties while the density-weighted mass diffusivity, ρgDg, 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 (Leg) contributes to either greater compositional (Leg>1) or thermal (Leg<1) stratification. For large hydrocarbons and oxygenated hydrocarbons that are representative of diesel fuels Leg ~ 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.