Measurements of Cellular Structure in Spray Detonation

Abstract

The cellular structure of heterogeneous detonations in a low vapor pressure fuel (decane) droplet mixture with oxygen and nitrogen was studied in the present investigation. The aerosol was generated by an ultrasonic nebulizer and the fuel concentration of the mixture was regulated by monitoring the volume flow rate of oxygen and nitrogen through the nebulizer. The vertical detonation tube is 64 mm in diameter and 3 m long and ignition was by a powerful spark (120 joules stored energy) or by a high explosive detonator. Velocity was measured with ionization probes, pressure by a PCB piezoelectric transducer and cell size by a smoked metallic foil inserted either into the top end or at the center of the detonation tube. The initial pressure of all the experiments was 1 atmosphere. In order to compare the time scales associated with the physical processes of droplet breakup, heat transfer, evaporation and mixing, experiments were also carried out in the tube heated to 100°C and 185°C, using electrical heating tape to ensure a homogeneous gas-phase mixture of decaneoxygen-nitrogen. Comparison of the cell size for the same mixture in the cold and the heated tube permits one to separate the time scales associated with the physical processes from the chemical kinetic rate processes. The results from the heated tube for the homogeneous vapor phase decane detonations are similar to those for the common gaseous fuels in the alkane group (i.e., ethane, propane, butane). Corresponding results for the heterogeneous case (cold tube) of aerosol decane detonation indicate that the cell size is larger by a factor of about two for the present case of 5 \im particle size. The measurements of cellular structure obtained experimentally have been compared to the computed results determined using the 2ND chemical kinetic detonation model. The detonation cell size was chosen as being 60 times the calculated induction length.