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THE TEST RIG:
The ramjet intake was mounted just behind the output of a 160 mph
(measured) leaf blower. Both the blower and ram jet were mounted on a
board which was placed on rollers. The board was attached to a spring
loaded weight gauge which was also connected to ground to measure the
force of thrust generated. The fuel was supplied by a flexible fuel
line from a BBQ propane tank. The tank was placed off to the side so it
would not be part of the test fixture or interfere with the thrust
results.
THE FLAME HOLDER AND START UP:
On the first run, my concern was to test the effectiveness of the flame
holder. I had no idea how it would work. I started the leaf blower and
set it to the lowest velocity. I then used a blow torch to blast a hot
flame in the exhaust port of the ram jet and up toward the flame holder.
At this point I slowly open the fuel valve of the propane tank. I
observed small wispy traces of blue flames exiting the exhaust port but
the flame had not yet taken hold. I increased the fuel flow rate
slightly and the concentration of wispy flames increased then suddenly
BOOM and it roared into
life. It was a bit startling and made me jump. Flame had taken hold
behind the flame holder!
It worked surprisingly well at all velocities and fuel flow rates. I was
able to make an educated guess on the stoichiometry by observing the
flame color within the combustion chamber. Orange indicates fuel rich
and blue indicates a balanced or lean mixture. After playing around
with the fuel flow rate at a low wind velocity I wanted to see the
actual combustion and mixing process.
COMBUSTION OBSERVATION:
The blower was set to low velocity and the fuel flow was set low as
well. I walked about 15' behind and to the side of the exhaust port
and looked into the combustion chamber. The chamber is 6" diameter and
the flame holder is about 3" diameter. The flame was attached directly
behind the flame holder. It was very turbulent and had a dark to light
blue color. The flame never came in contact with the exhaust port or
combustion chamber walls. In fact, cool air completely enveloped the
flame. The combustion chamber remained cool being completely isolated
from the flame.
I moved closer to the engine to take notice of the exhaust plume. From
several feet away I felt the blast of the exhaust plume. It was quite
warm but not hot. It also had the odor of cleanly burned fuel. It
didn't have that sooty smell. Apparently the engine was at a very lean
setting.
Without adjusting the fuel flow rate I turned up the velocity of the
blower. I wanted to test the limits of the flame holder at extremely
lean settings. I turned up the blower to medium and the engine roared
even louder. With the blower at high setting the flame was still
holding but the exhaust plume was notably cooler. In fact, the plume
was as warm as a hair dryer. At the highest setting the flame became
unstable was not able to hold for too long.
With this first round of tests the fuel flow was kept very, very low. I
noticed the engine produced little or no thrust but the volume and
velocity of exhaust gas was greatly enhanced by the heat of combustion.
TURNING UP THE HEAT:
On the second round of tests I increased the fuel flow rate
considerably. With the blower at low the exhaust plume was a bright
orange with flames extending 2-3 feet beyond the exhaust port. When the
blower was turned up to medium the flames moved closer to the exhaust
port and became more blue. I moved far behind the exhaust port and
slightly to the side. I observed the flame holder doing its job with a
turbulent, orange, blue flame attached behind it. The flames did not
come in contact with the chamber walls but now they were much closer and
the chamber walls were too hot to be touched.
Keeping the fuel flow rate constant I turned the blower to its highest
setting. The flames moved completely inside the chamber and were
completely blue. The combustion process was very turbulent and getting
quite loud. The roar was so loud I didn't want to sustain it at this
level for too long (neighbors sake).
MORE HEAT:
The next set of tests used the maximum gas phase output of the BBQ
tank. With the blower set at idle the exhaust plume consisted of a
large red orange flame that extended 6' beyond the exhaust port. When
the blower was set to medium the flame became shorter and brighter. The
volume of exhaust gas was tremendous. When the blower was set to
maximum (160 mph) the flame became blue and was almost completely inside
the combustor. It was very, very loud. I moved far
behind the exhaust port and noticed the flame inside was much larger and
in complete contact with the convergent section of the exhaust port and
part of the combustion chamber. The combustion chamber was becoming
much hotter and began to radiate heat. The exhaust plume was very loud,
radiant and powerful. In fact, the plume induced much of the
surrounding air and dust on the concrete to be drawn in and blown back.
The combustion process was adding tremendous force to the air flowing
from the leaf blower. From 25' away it produced what could be described
as a strong desert wind on a hot, smoggy day.
VARYING FUEL FLOW RATE VARIES THRUST
The next tests used liquid phase propane. I first started up the ramjet
using gas phase propane and the blower was set at low. I gradually
increased both the blower output and (gas phase) fuel flow rate to
maximum. The test rig was vibrating and the noise was too much. When
the blower was as high as it could possibly go and a strong flame was
held inside the combustion chamber it was ready for liquid phase fuel.
With everything ready to go I turned the BBQ tank upside down and it was
Rock and Roll.
There was a huge blast of blue flame from the exhaust port and the test
rig lurched forward. It was producing thrust! Despite all the special
effects it only produced 7 lbs of *net force. The sound was a mix
between a deep rumble and a brissant roar. I estimate it was roughly
115 db of pink noise @ 4'. The sound was too much to go for more than
30 seconds. The sound was vibrating the entire work area. But while
the test lasted I throttled the fuel flow rate up and down. Sure
enough, the spring gauge indicated force ranging between 1 and 7 pounds
as I varied the fuel flow rate. Thrust follows fuel flow rate.
When I felt the neighbors had enough noise and vibration I slammed the
fuel flow off and the thrust immediately terminated. Then I turned off
the leaf blower and when everything was finally quiet I noticed my ears
were ringing.
The entire range of tests consumed about a gallon of propane. Even at
this low a velocity the ramjet really eats fuel. I calculated the
flight vehicle will consume 45 grams of diethyl ether per second at a
flight speed of 600 MPH. That's essentially a fuel dump.
In any case this is much better than the most efficient chemical
rocket. The delivered ISP should be 450 to 500.
There will be no further tests until I can get the funds for a much
larger blower and bring it to the MTA.
-Anthony Colette
rokitman@sprintmail.com
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