The vaporization characteristics of a single fuel droplet subjected to rapid gas-phase compression (i.e., wet compression) are computationally investigated using two spherically-symmetric models: quasi-steady (QS) and fully transient (TS). Features of the wet compression process under rapid compression machine (RCM) conditions are discussed with these compared to simulations where the far-field conditions are essentially invariant. It is observed that wet compression can significantly increase the rate of evaporation primarily due to the increase in droplet temperature and corresponding saturation pressure (fugacity); an increase in the density-weighted mass diffusivity is also beneficial in reducing the droplet consumption times. The QS model predicts substantially longer rates of evaporation relative to the TS model due to transient behavior associated with the initial evaporative cooling process, and the gas-phase compression heating process. Increases in the rate of volumetric compression can lead to more rapid droplet consumption, however there is a corresponding increase in spatial stratification in the gas- and liquid-phases which may not be advantageous for RCM applications. An ‘operating map’ has been developed based on parametric simulations of an n-dodecane droplet evaporating into nitrogen.
Wet compression is the process whereby droplet evaporation is achieved through compression heating of the gas-phase of a droplet laden aerosol. This phenomenon has received increasing attention in recent years with applications to internal combustion (IC) and gas turbine (GT) engines, as well as laboratory devices such as aerosol shock tubes (STs) and rapid compression machines (RCMs). In advanced IC engines non-conventional combustion strategies are being investigated in order to reduce soot, NOx and unburned hydrocarbon (UHC) emissions while maintaining high energy conversion efficiencies [1], [2], [3], [4] and [5]. In these, sometimes referred to as low temperature combustion (LTC) schemes, the fuel can be introduced very early in the compression stroke, in some cases well in advance of maximum piston compression. Under such conditions the in-cylinder gases are relatively cool, meaning they may be at or below the fuel’s boiling point (e.g., Tb ∼ 650 K). During the piston’s compression stroke the liquid droplets are vaporized due to the gas-phase volumetric compression, and the fuel vapor is subsequently mixed with the gas-phase oxidizer. Droplet coalescence and wall wetting during compression, especially important for highly involatile fuels or fuel components, can lead to extended evaporation times and the formation of unwanted emissions [6]. Wet compression is also important in IC engines that utilize “wet” ethanol, which is a minimally-processed ethanol-based fuel with high water content [7]. In these engines much of the ethanol readily evaporates during the induction stroke however, the water may not completely vaporize until well into the compression stroke.
In GT engines wet compression of water aerosols has been used to achieve “continuous cooling” in the compressor component of the engine [8], [9], [10], [11], [12] and [13]. In engines employing this process, water droplets with diameters on the order of ∼15 μm are injected into the intake stream via fogging systems with high droplet output. The pressure-driven injectors used in these systems achieve very high relative droplet–air velocities and rapid mixing of the evolving water vapor with the air. This can result in significantly increased power densities along with cost and performance advantages relative to conventional inter-cooling units. However, it is only effective when the humidity ratio of the intake air is low and the residence time in the compressor is adequate to achieve complete evaporation.
In shock tubes (STs) and rapid compression machines (RCMs) wet compression has been proposed as a means of preparing test gases of high molecular weight (MW), involatile liquid fuels relevant to the transportation industry. Traditional charge preparation techniques use external mixing protocols based on partial pressure methodologies [14] and [15]. Diesel-representative fuels for example, have very low vapor pressures at standard conditions (e.g., <1 Torr) which make this option difficult or impossible. Heating the mixing tanks and equipment can lead to better fuel vaporization but this can result in seal degradation issues in RCMs, as well as concern for pre-test reactivity during the preparation process. Aerosols of suspended fuel droplets (Sauter mean diameter, Dsm ∼ 8–18 μm) have been used to deliver liquid fuels to the machines where subsequent compression of the surrounding gas phase leads to evaporation of the liquid fuel droplets [16] and [17]. In shock tubes where test temperatures range from 900 to 2000 K the gas-phase compression event is achieved via a rapidly traveling shock wave (compression achieved in ∼30 μs to 600–700 K); the passing of the initial wave not only 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 (which enhances vaporization). The subsequent reflected wave compresses the evaporated mixture to the test conditions. In RCMs where test temperatures are generally between 600 and 1100 K the compression event is much slower (e.g., ∼15–60 ms) and the bulk gas motion is often suppressed in order to minimize unwanted heat loss during the test period [17]. Droplet evaporation is much slower and is mainly diffusion-limited. Evaporation must also be achieved at lower temperatures, i.e., before the test temperature is reached.
