Characterization of an Aerosol Shock Tube Facility for Heterogeneous Combustion Studies

Combustion is responsible for providing energy for many applications, especially in propulsion and rocket propellants. Shock tubes provide a controlled, repeatable means of studying combustion characteristics; although, most of these studies require the fuel in a mixture to exist in pure gas-phase. This makes it challenging to test low-vapor-pressure fuels that tend to remain in condensed form. Low-vapor-pressure fuels are commonly used in many combustion applications, making combustion studies of these fuels important.

A method to study low-vapor-pressure fuels using a shock tube approach is to inject the fuel into the shock tube as tiny, uniformly-sized aerosol droplets. The sub-micron-sized aerosol droplets remain uniformly suspended in the shock tube prior to running the experiment. An incident shock wave vaporizes the liquid fuel droplets, then the reflected shock wave initiates ignition of the mixture. This study presents the characterization of an aerosol fuel injection method to the shock tube to study the combustion of low-vapor-pressure fuels. An aerosol generator was used to produce repeatable, uniformly-sized fuel droplets, and flow controllers were used to control and measure oxygen and argon dilution gas injected into the shock tube. A technique was developed to ensure consistent and repeatable aerosol fuel production rates over which calibration curves were found. This study presents the ignition delay times for C7H16 (? = 1.0) at a pressure of 2.0 atm for temperatures from 1220 - 1427 K, C7H8 (? = 1.0) at 1.9 atm over a temperature range of 1406 ? 1791 K, and C12H26 (? = 0.3) at 3.0 atm for the temperature range of 1293 ? 1455 K. The ignition delay times for heptane and toluene were compared to the literature values at the same conditions and were found to be in good agreement. Laser extinction (visible laser at 632nm) was used to verify the presence of aerosol fuel droplets inside the shock tube for dodecane, but showed the heptane aerosol vaporized upon injection into the shock tube. Initial laser absorption (3.39 ?m) measurements were also taken. This aerosol technique was found to successfully evaluate combustion effects of low-vapor-pressure fuels; however, was limited by the range of possible fuel concentrations. Further work needs to be performed on the verification of aerosol spatial uniformity and obtaining higher fuel concentrations.