THRUST AUGMENTATION It is often possible to increase the thrust of the pulse jet very appreciably
without increasing the flow rate of fuel. By transferring some energy from the flow of the hot combustion
products to other propulsive flows, this augmentation of thrust may be accomplished in several different
ways.The process of augmentation involves by definition a double flow or even a multiple flow system.
Thrust thrust augmentation ratio as defined is the ratio between the thrust of such a multiple-flow system
and the thrust which would be produced by primary component alone under the same conditions.The forms
of thrust augmentation are as follows.. .
AUGMENTATION BY BACKFLOW Thrust augmentation is accomplished by modifications to the primary cycle,
which in turn produce a further increase of thrust. Backflow is believed to produce a beneficial modification
of the heating mode. In low flight speeds the standard pulse jet is a double flow jet engine because
of the occurrence of backflow. Backflow air does not undergo the same thermal cycle as air which flows
through the combustion chamber. The only energy received by the backflow areas is transferred to it
from the hot combustion products in a substantial pressure exchange process which takes place in the
tailpipe. In the pressure exchange process the velocities on the two sides of the interface separating
the two flows become equal neglecting the waves which are generated under certain circumstances.The wave
discontinues at the duct exit.The backflow wave there loses its entire free stream kinetic energy without
pressure recovery as it is brought to rest within the engine.
BACK FLOW TYPE At low speeds it is possible to increase the backflow mass ratio by flaring out the
end of the tailpipe. Tailpipe flares have been found to be sufficient to increase a steady thrust by
20 percent or more. As a flight speed increases these tailpipe flares become less and less effective
until their presence may actually result in a net loss of thrust. Introduction of cold air into the tailpipe
through side ports makes it possible to make even at relatively high speeds the conditions similar to
that which is produced by backflow at the lower speeds. Improper configuration of these inlets may have
a detrimental effect on performance at high speeds. Effect of redistributing the energy released in combustion
or a large mass of gas by pressure exchange is not equivalent to that of releasing the same amount of
heat in the same amount of gas through a larger engine. Back low has a beneficial effect on heating.Though
in that the lower temperature of the gas column in the tailpipe increases the inertial confinement of
the burning region. This effect is most appreciable in the static operation conditions or very low flight
speed.
