October 2000 Newsletter - Naval Postgraduate School

FEATURED PROJECT
JP-10/AEROSOL FOR PULSE DETONATION APPLICATIONS, continued from page 7
Due to the increased difficulty of detonating a fuel/air
mixture, a small (pilot) combustor which operated on JP-10
and oxygen was used to generate a very strong detonation
wave and provide the initiating shock wave for the JP-10/air
mixture. The geometry was previously determined by LT
Dave Forster, USN, to provide the most rapid and reliable
generation of a detonation wave in a JP-10/oxygen aerosol.
The strong shock wave generated by this pilot combustor
results in the direct transition of the detonation wave into the
fuel/air mixture as long as the mixture is within a known
detonability limit.
The atomizers used in this work produced sprays with low
Sauter Mean Diameters (SMD), typically 7mm to 10mm over
a wide range of flow rates, thereby allowing the spray SMD to
be approximately constant through a wide range of fuel/air
ratios. SMD values of the aerosol exiting the inlet arms were
measured for conditions ranging from a temperature of 285K
to 500K and air flow rates of 0.07 kg/s to 0.30 kg/s. By
combining the measured particle size and concentration and
knowing the initial amount of fuel sprayed, the amount of
fuel vaporized at the point of injection into the main combustor
was estimated. The results for an air flow rate of 0.20 kg/s
and overall equivalence ratio of 1.5 for the JP-10/air mixture
are shown below in Figure 4 for varying engine inlet temperatures.
Detonations were only observed for inlet temperature
above approximately 345K. The corresponding aerosols
detonated therefore had SMD values below approximately 3
microns and an estimated fuel vapor above 70%. Test runs
which demonstrated a higher success rate at producing
detonations were for higher inlet air temperatures and
subsequently smaller droplets and fully vaporized fuel. A
sample trace for one cycle of a JP-10/air detonation is shown
in Figure 5 for an engine inlet temperature of 425K. The fuel
was completely vaporized before initiation for this case. The
pressure-time traces reveal the extremely high detonation
wave velocities observed and the result head-end pressure
which produces the thrust for these systems.
The successful detonations observed possessed detonation
wave speeds generally within 2% of the expected Chapman-
Jouget values. This result appears to indicate that although a
portion of the fuel may have been in a liquid state, enough
fuel vapor was present to initiate a reaction and provide rapid
heating, vaporization, and reaction rates behind the shock
wave to result in near Chapman-Jouget velocities.
The JP10-fueled pulse detonation engine has been operated
at frequencies up to 5Hz and is in the process of being
retrofitted with new fuel injectors to investigate operation at
up to 100Hz. Performance parameters will be measured as a
function of cycle frequency, simulated flight velocity, partial
--continued on page 42
Figure 4 (left). Sauter Mean Diameter and Percent Fuel
Vaporization as Function of Temperature.
Figure 5 (above). Pressure-Time Trace for JP10/Air Detonation.
NPS Research page 41
October 2000