Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02834478 2013-11-26
A Method of Operating a Scramjet Engine
TECHNICAL FIELD OF INVENTION
The present invention relates to engines that utilize air moving at supersonic
speeds for
combustion and expansion. Engines that use combustion at supersonic speed of
air in a
combustor are known as scramjets. More specifically, the present invention
relates to the
engines for supersonic and hypersonic aircrafts.
BACKGROUND OF THE INVENTION AND PRIOR ART
A ramjet engine uses the momentum of the airstream to compress air within an
inlet, without the
use of moving parts such as rotating compressors or fans. The airstream is
slowed down to
subsonic speeds for burning a fuel in the combustor. A scramjet engine is
similar to a ramjet
engine except that the flow is not slowed to subsonic speeds and in the
combustor remains at
supersonic speeds. A dual mode scramjet engine is an engine that can operate
in both
ramjet and scramjet modes.
In the prior art various methods of operating a scramjet engine are described
as shown in the
following patent applications and patents granted in Canada.
Patent application CA2034466, George A. Coffinberry, discloses a method of
operating a
scramjet engine. The method includes the steps of providing supersonic
compressed airflow to
a combustor and supplying fuel to the combustor for generating a fuel/air
mixture having a
predetermined temperature less than a temperature required for spontaneous
ignition of the
fuel/air mixture. The method also includes igniting the fuel/air mixture and
generating
combustion gases.
Unites States patent US 5255513 A, John C. Blanton, Paul H. Kutschenreuter,
Jr., discloses a
method of operating a scramjet engine that includes injecting fuel into the
inlet-combustor for
mixing fuel with supersonic airflow for generating supersonic combustion gases
in the inlet-
combustor. In the preferred embodiment of the invention, the fuel is injected
to create a fluid
boundary defining a subsonic fuel zone and a supersonic fluid zone. The fluid
boundary is
variable and eliminates start and unstart problems requiring variable inlet
geometry in a
conventional scramjet engine.
It is known that at relatively low supersonic speeds the thrust is limited due
to low ram pressure.
At hypersonic speeds the thrust is limited due to less efficient use of energy
of combustion
process due to high temperature of the exhaust gases and relatively slow
recombination of
ionized gases. Another issue that is not effectively addressed is very high
temperature in a
combustor.
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CA 02834478 2013-11-26
SUMMARY OF THE INVENTION
An objective of the invention is to provide a method of operating of the
scramjet engine (or a
dual mode engine that incorporates operation in a scramjet mode) having higher
thrust and
specific impulse at supersonic and hypersonic speeds. Sequence of combustible
and non-
combustible zones is generated by modulating the fuel injection rate with
sequential ignition and
energy transfer between combustible and non-combustible zones. Invention is
based on a
pressure-gain combustion being implemented without valves and can be used at
supersonic and
hypersonic speeds.
The proposed method of operating a scramjet engine includes providing
supersonic (or
hypersonic) airstream from the inlet of the engine to a combustor, injection
of fuel in the
airstream, mixing the injected fuel with the air, igniting and burning the
injected fuel generating
heat. The hot products of combustion are discharged through the nozzles
producing thrust. Fuel
is being injected with periodically varying flow rate forming a continuous
series of zones in the
airstream with subsequently higher and lower concentration of fuel in the
fuel/air mixture. At the
maximum fuel flow rate the injection of fuel produces stoichiometric (or
substantially
stoichiometric) mixture ratio in the airstream, thus forming a gaseous
combustion zone (CZ) that
moves with the airstream along the axis of the combustor towards the nozzle.
At the minimum
(or zero) fuel flow rate the injection of fuel produces gaseous non-combustion
zones (NCZ) in
the airstream being devoid or substantially devoid of fuel.
Heat being produced by burning the fuel increases temperature and pressure in
the moving CZ.
The CZ expands into the adjacent NCZs at the speed of sound while moving
towards the nozzle.
Pressure in the NCZ increases until pressures in the adjacent CZ and NCZ are
equalized. Full
equalization of pressure is not necessary for implementation of this
invention. Gases from CZs
and NCZs are discharged through the nozzle generating thrust. Due to energy
transfer between
CZs and NCZs, the average velocity of gases being discharged through the
nozzle reduces but
overall thrust increases due to increased mass flow.
Thrust and specific impulse will increase due to pressure gain, associated
higher discharge speed,
and increased mass flow being discharged through the nozzle while having the
same fuel flow
rate as compared with the known methods of operation of the scramjet engine.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG.1 illustrates a method of operating a scramjet engine at the moment of
beginning of
formation of a new CZ and combustion and expansion of previously formed CZs.
FIG.2 through FIG.6 illustrate subsequent phases of combustion and expansion
of CZs and
compression of NCZs.
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FIG.7 illustrates a method of operating a scramjet engine by forming a
continuous combustion
zone, specifically at the moment of beginning of formation of a new CZ and
combastion and
expansion of previously formed CZs.
= FIG.7 through FIG.12 illustrate subsequent phases of combustiont and
expansion of CZs and
compression of NCZs for the method of operating a scramjet engine comprising a
process of
forming a continuous combustion zone.
FIG. 13 shows distribution of pressure along the axis of a scramjet engine at
the moment of the
combustion and expansion process shown on FIG. 4.
FIG. 14 shows the change of pressure in the individual CZ and NCZ during
movement from the
inlet 40 to the nozzle 50 along the axis of a scramjet engine.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with a first embodiment of the invention, the proposed method of
operating a
scramjet engine includes providing supersonic (or hypersonic) airstream from
the inlet of the
engine to a combustor, injection of fuel in the airstream downstream of the
inlet, mixing the
injected fuel with the air, igniting by the ignition means, and burning the
injected fuel generating
heat. The hot products of combustion are discharged through the nozzles
producing thrust. Fuel
is being injected with periodically varying flow rate within predetermined
minimum and
maximum values and forming a continuous series of zones in the airstream with
subsequently
higher and lower concentration of fuel in the fuel/air mixture. At the maximum
fuel flow rate the
injection of fuel produces stoichiometric (or substantially stoichiometric)
mixture ratio in the
airstream, thus forming a gaseous combustion zone (CZ) that moves with the
airstream along the
axis of the combustor towards the nozzle. At the minimum (or zero) fuel flow
rate the injection
of fuel produces gaseous non-combustion zones (NCZ) in the airstream being
devoid or
substantially devoid of fuel. The NCZ moves with the airstream along the axis
of the combustor
towards the nozzle. Subsequently, the fuel flow rate increases to a maximum
predetermined
value forming a next CZ followed by the next NCZ, and the process continues.
The CZs and
NCZs have common boundaries allowing mass and energy transfer between the CZs
and NCZs.
Heat being produced by burning the fuel increases temperature and pressure in
the moving CZ.
Combustion takes place as a detonation or a deflagration process. Due to
increasing temperature,
pressure of the combustion products in the CZ starts increasing and the CZ
expands into the
adjacent NCZs at the speed of sound for deflagration combustion while moving
towards the
nozzle. Pressure in the NCZ increases until pressures in the adjacent CZ and
NCZ are equalized.
Gases from CZs and NCZs are discharged through the nozzle generating thrust.
The speed of the
airstream and the CZ in the combustor is supersonic (moving from the inlet to
the nozzle). The
pressure boundary of the CZ propagates towards the inlet of the engine at the
speed of sound
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=
against the flow of the supersonic airstream. Therefore, the higher pressure
generated in the CZ
never reaches the inlet of the engine.
The process of operation of the engine according to the present invention is
represented by two
= processes: combustion in the CZs with temperature and pressure increase,
partial expansion in
the combustor and further expansion in the nozzle, and compression of the NCZ
with associated
pressure increase in the combustor and expansion in the nozzle. If total mass
flow of the
airstream is divided equally between the CZs and NCZs, and assumed
instantaneous pressure
increase ratio in the CZs (for hydrogen fuel) is equal to 6, the overall
pressure gain ratio will be
equal to 3 when the pressure is equalized between the CZs and NCZs. Thrust and
specific
impulse will increase due to pressure gain, associated higher discharge speed,
and increased by
100% mass flow being discharged through the nozzle while having the same fuel
flow rate as
compared with the known methods of operation of the scramjet engine. If total
mass flow of the
airstream is divided between the CZs and NCZs in a ratio of 1:2, and assumed
instantaneous
pressure increase ratio in the CZs (for hydrogen fuel) is equal to 6, the
overall pressure gain ratio
will be equal to 2 when the pressure is equalized between the CZs and NCZs.
Thrust and specific
impulse will increase due to pressure gain, associated higher discharge speed,
and increased by
200% mass flow being discharged through the nozzle while having the same fuel
flow rate as
compared with the known methods of operation of the scramjet engine.
FIG.1 through FIG. 6 illustrate sequence of forming the CZs and NCZs by
injection of a fuel by
a fuel injection means (a group of fuel injection nozzles 60) and their
movement from the inlet
30 to the nozzle 50. FIG.1 shows initiation of forming of the CZ 1 downstream
of the inlet 30.
The CZ 2 and CZ 3 are shown at different stages of combustion (the moment of
ignition is not
shown) and expansion in the combustor 40, and the NCZ 10, NCZ 11, and NCZ 12
at different
stages of compression. The CZ 4 is shown at the stage of expansion in the
nozzle 50. FIG. 2 and
FIG.3 also show as the CZs and NCZs move along the engine axis in the
combustor 40 towards
the nozzle 50. FIG. 3 shows formation of a new NCZ 13 immediately downstream
of the inlet
30. FIG.4, FIG.5, and FIG.6 sequentially show combustion and expansion of CZ
1, CZ 2, and CZ
3 and compression of NCZ 10, NCZ 11, and NCZ 12 with subsequent expansion in
the nozzle 50
of CZ 4 (FIG. 4), compressed NCZ 12 (FIG. 4 and FIG. 5), and CZ 3 (FIG. 5 and
FIG. 6). Each
of the newly formed CZs is ignited during an expansion phase of the
combustible zone formed
immediately prior to said newly formed CZ allowing better pressure stability
in the combustor 40
and better overall efficiency of the scramjet engine.
For better stability of the combustion process (flame stability) at least one
continuous CZ is
formed along the axis of the combustor filling part of the cross-sectional
area of the combustor
by continuous injection of fuel through one nozzle (or a group of the nozzles)
in the range of
10% to 20% of the total fuel flow rate.
FIG. 7 through FIG. 12 illustrate a method of operation of a scramjet engine
comprising
formation of a continuous CZ along the axis of the combustor 40 that occupies
part of the cross-
sectional area of the combustor 40. FIG. 7 through FIG. 12 also illustrate
sequence of forming
the CZs and NCZs and their movement from the inlet 30 to the nozzle 50. FIG.7
shows initiation
of forming of the CZ 1 downstream of the inlet 30. The CZ 2 and CZ 3 are shown
at different
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stages of combustion and expansion in the combustor 40, and the NCZ 10, NCZ
11, and NCZ 12
at different stages of compression. The CZ 4 is shown at the stage of
expansion in the nozzle 50.
FIG. 8 and FIG.9 also show as the CZs and NCZs move along the engine axis in
the combustor
40 towards the nozzle 50. FIG. 9 shows formation of a new NCZ 13 immediately
downstream of
the inlet 30. FIG. 10, FIG. 11, and FIG. 12 sequentially show combustion and
expansion of CZ 1,
CZ 2, and CZ 3 and compression of NCZ 10, NCZ 11, and NCZ 12 with subsequent
expansion
in the nozzle 50 of CZ 4 (FIG. 10), NCZ 12 (FIG.10 and FIG. 11), and CZ 3
(FIG. 11 and FIG.
12).
Depending on the speed of the aircraft, a ratio of the masses (mass flows) of
CZs and NCZs, and
the speed of airstream in the combustor, the overall improvement of the thrust
and specific
impulse varies between 30% and 110% for hydrogen or or hydrocarbons used as a
fuel. These
numbers indicate that the scramjet engine operated in accordance with this
invention will have a
specific impulse values similar to the specific impulse values of a turbojet
engine.
The engine thrust can be controlled by changing the average fuel injection
flow rate and
frequency of modulation of the fuel injection flow rate, thus changing the
ratio of mass flows of
CZs and NCZs and average pressure in the combustor 40 before the nozzle 50.
Additional
advantage of the present invention is lower average temperature in the
combustor. Partial
expansion of CZs in the combustor reduces potential for efficiency losses due
to incomplete
recombination of free radicals in the nozzle.
Distribution of pressure (not to scale) in the scramj et engine is shown on
FIG. 13, at the moment
of the CZs expansion shown on FIG.4. FIG. 14 shows the change of pressure in
the individual
CZ and NCZ during movement from the inlet 40 to the nozzle 50 along the axis
of a scramjet
engine.
Area between the inlet 30 and a combustion zone in the combustor 40 can also
perform functions
of an isolator (not shown on the drawings). Different design of the inlet can
be used, including a
diffuser, a Buseman type inlet, a conical inlet, an inlet using a magnetic
field for deceleration of
the airstream, or other types known in the art. The thermodynamic cycle used
in this method of
operation of the scramjet engine differs from the Brayton cycle because the
heat addition process
is not isobaric.
Supersonic airstream is defined as having speed greater than the speed of
sound (above Mach 1)
that includes hypersonic speeds (above Mach 5). Speed of airstream in the
combustor is
supersonic in the scramjet mode of operation.
The invention improves the performance of a scramjet engine and provides a
higher thrust and
greater specific impulse. Several embodiments of the present invention have
been described and
further details are understood by any person knowledgeable in this art and
area of knowledge. It
is understood that various modifications may be made without departing from
the principles and
scope of the present invention.
