Abstract
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 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.
Highlights
Wet compression is the process whereby vaporization is achieved via volumetric compression heating of the gas-phase of a droplet laden aerosol
A range of volatilities and other physical properties typical of diesel are covered with these surrogate fuels, a few of which are listed in Table 1. n-Eicosane is utilized in this study to represent some of the most involatile petro- or bio-diesel components where typical T90 distillation points are near 600K for petro- and near 670K for bio-diesel [51]
Argon has been demonstrated to have superior heat retention characteristics relative to lighter monatomic diluent gases including helium and this minimizes heat loss to the reaction chamber walls during the ignition delay period [53]. These characteristics are due to argon’s lower heat diffusivity, κ. This behavior is unfavorable during the wet compression process because it impedes the transport of heat to and mass from the fuel droplets in the aerosol resulting in longer evaporation times and greater thermal and/or compositional stratification within the gas phase
Summary
Wet compression is the process whereby vaporization is achieved via volumetric compression heating of the gas-phase of a droplet laden aerosol. In shock tubes where test temperatures generally range from 900 to 2000K the gas-phase compression event is achieved via a rapidly traveling shock wave (Δtcomp ~ 30μs; Tcomp ~ 600-700K); the passing of the initial compression wave increases the pressure and temperature of the surrounding bath gases but it fragments the initial droplets and results in high convective velocities near the droplet surface. These features result in very fast vaporization (e.g., Δtevap ~ 100μs [13]). Evaporation must be achieved at lower temperatures than in STs (e.g., by ~500K to precede low temperature chemical reactivity)
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