DUCTED PULSEJET
AND
PRESSURE EXCHANGE
|
DUCTED PULSE JET
Shrouding can be used to improve the performance of the pulse jet in important
ways. The first being the utilization of ram precompression at high flight speeds. Thrust augmentation
results from the fact that shrouding brings an additional flow into the system. Shrouding only the tailpipe
section is preferable to shrouding the entire duct. This is because shrouding the rear section eliminates
most of the secondary flow loss in the forward portion of the engine. Putting the entire duct inside
a shrouding helps to accomplish ram precompression. A duct impulse jet comprises three flows, the direct
flow, the backflow in the primary engine, and the flow in the annular space surrounding it. Backflow
is almost negligible in this case therefore engines with tailpipe flares are not necessarily well adapted
to shroud use. The use of shrouding can be self-defeating. Considerations for use of shrouding include
its weight, and drag characteristics. If the engine is to be used in low speed applications a tailpipe
flare will generally be as efficient as a shrouding system.
|
PRESSURE EXCHANGE
Pressure exchange is commonly defined as a process in which a flow or region of
the flow is compressed by piston like action of another flow expanding between the same pressure limits.
Pressure exchange plays an important role in the operation of the pulsejet. Pressure waves are utilized
to transfer energy from the combustion products to the fresh charge for the purpose of precompression
and also some forms of thrust augmentation. Entropy increments of pressure exchanges isthe some of all
increments produced on any given particle by moving shock waves. Within the engine the pressure increment
can only be calculated on the basis of the detailed study of the pressure exchange process in each individual
engine.
|
|
