Device for generating a hot air flow

The present invention relates to a device generating a hot air flow, including an air source generating an air flow at its outlet (17), a heating system (13) generating heat energy and heat-exchange elements (27) capable of storing heat energy generated by said heating system (13) and of yielding this heat energy to said air flow. According to the invention, said heating system (13) and said air source are mounted fixed, and said heat-exchange elements (27) are arranged on a mobile support (3) capable of continuously renewing the heat-exchange elements (27) arranged in said air flow and previously heated by said heating system (13).

The present invention relates to a device for generating a hot air flow. 
Although the present invention can be used in numerous installations 
requiring a hot air flow, such as, for example, installations for testing 
the stability of materials or for aerodynamic tests or for validating wall 
cooling systems, it will be described more particularly hereafter in the 
context of a test rig for developing ramjets. 
It is known that, for such development, in particular of ramjets intended 
to operate at hypersonic speeds, it is necessary to feed the ramjets 
mounted on the test rig with air which is both at high temperature and as 
pure as possible. In addition, such an air feed must be carried out 
continuously over a period of time which is generally very long, so as to 
simulate a flight or to verify the thermal stability of the ramjet in 
thermal steady state. 
In order to generate hot air, static heaters are known which include a 
heating device, for example a gas burner or an electrical device, and 
intended to heat a material having a high heat capacity, such as, for 
example, alumina balls or metallic tubes. When said material is heated, 
pure air is passed into the heater, which makes it possible to obtain air 
which is both at elevated temperature and is furthermore pure, since 
passage through said heater does not change the composition of the air 
used. 
However, since the heat capacity of the material used is obviously limited, 
it is not possible in this way to generate hot air over a very long period 
of time, and one of the essential characteristics for the operation of 
test rigs of the type previously described, namely continuous air feed, is 
therefore not satisfied. 
Furthermore, active heaters are known which make it possible to heat air 
continuously by combustion using, for example, hydrogen or kerosene, as 
well as the oxygen in the air to be heated, the consumed oxygen being 
replenished after heating. The hot air thus obtained is composed of air 
and of products generated during this combustion. This air is therefore 
not pure, which raises drawbacks for combustion studies, in particular in 
cases where kinetics play an important role, since it is then very 
difficult to reproduce on the ground functioning conditions similar to 
those existing in flight. 
The object of the present invention is to overcome these drawbacks. It 
relates to a device for generating hot air which can provide 
uncontaminated air at high temperature, and do this without time 
limitation. 
To this end, according to the invention the device generating a hot air 
flow, including an air source generating an air flow at its outlet, a 
heating system generating heat energy at its outlet and heat-exchange 
elements capable of storing heat energy generated by said heating system 
and of yielding this heat energy to said air flow, is noteworthy in that 
said heating system and said air source are respectively mounted fixed so 
that said air flow generated by said air source cannot be subjected to the 
action of said heating system, and in that said heat-exchange elements are 
arranged on a mobile support capable of passing said heat-exchange 
elements from a first position in which they are in front of the outlet of 
said heating system to a second position in which they are in front of the 
outlet of said air source, so as continuously to renew the heat-exchange 
elements arranged in said air flow and previously heated by said heating 
system. 
Thus, since the heat-exchange elements which transport the heat stored in 
the heating system and are used for heating said air flow are renewed 
continuously, the hot air flow is generated without interruption and 
without time limitation, that is to say for as long as is desired. 
In addition, since the air flow is not subjected to the action of the 
heating system, the hot air flow generated is not contaminated by said 
heating system, which makes it possible to obtain hot air having the same 
purity as the air generated at the outlet of the air source. 
Advantageously, said heating system includes at least one burner fed with a 
combustible fluid, said burner being mounted so that the gases generated 
by combustion of said combustible fluid do not mix with said heated air 
flow. 
Furthermore, said burner advantageously uses, for combustion of said 
combustible fluid, air coming from said air source and not used for 
generating the hot air flow. 
In a preferred embodiment of the invention, said mobile support is rotary, 
which makes it possible to simplify the device according to the invention. 
In addition, preferably, said mobile support has the form of a cylindrical 
container, at least partially filled with said heat-exchange elements. 
Advantageously, said cylindrical container is provided in the upper wall 
with a first annular opening, concentric with the axis of the cylindrical 
container and, in the lower wall, with a second annular opening, also 
concentric with the axis of the cylindrical container, said second opening 
being provided with a metallic mesh, and the outlet of the heating system 
and the outlet of the air source being arranged below said second opening, 
diametrically opposite relative to the axis of the cylindrical container. 
Furthermore, the device according to the invention advantageously includes 
an enclosure which is mounted fixed, said container being mobile in said 
enclosure and the outlets of said air source and of said heating system 
being arranged in said enclosure. 
In addition, advantageously, said device includes isolation means arranged 
between the container and the enclosure and intended to prevent 
circulation of air between said container and said enclosure, which makes 
it possible to prevent unheated air coming directly from the air source, 
or possibly contaminated air coming from the heating system from mixing 
with the hot air flow generated. 
In addition, so as to allow adequate removal of the various air flows, said 
enclosure may be provided in its upper wall: 
with a first opening, arranged in front of said outlet of the air source, 
to which first opening a nozzle may be connected; and 
with a second opening, in front of said outlet of the heating system, to 
which second opening an exhaust pipe may be connected. 
Thus, the hot air flow generated can be removed via the nozzle then 
transmitted to an application device, such as, for example, a combustion 
chamber of a ramjet mounted on a test rig, and the possibly contaminated 
air flow coming from the heating system can be removed via the exhaust 
pipe, the two air flows thus being incapable of mixing with each other. 
In order to make it possible to control the distribution of the air flow 
through said nozzle and said exhaust pipe, said exhaust pipe is 
advantageously provided with a controllable internal valve capable of 
altering the opening intended for exhaust. 
In addition, advantageously, a passage is made between said container and 
said enclosure, in order to allow circulation air from the outlet of the 
air source to the heating system.

The device 1 according to the invention and represented in FIGS. 1 to 3 is 
intended to generate a hot air flow. 
Said device 1 includes an external enclosure 2 mounted fixed and a mobile 
system produced in the form of a cylindrical container 3 mounted so as to 
move inside said enclosure 2. 
To this end, said cylindrical container 3 is solidly attached to a shaft 4 
of axis X--X, mounted so as to rotate in the upper wall 5 and the lower 
wall of 6 of the enclosure 2 and capable of being driven in rotation, as 
indicated by an arrow E, by a drive device of known type and not 
represented. Said container 3 has symmetry of revolution about said axis 
X--X and is provided in its upper wall 7 with an annular opening 8 
concentric with the axis X--X and, in its lower wall 9, with an annular 
opening 10 which faces the opening 8. Said annular opening 10 is covered 
with a metallic mesh 12. 
It will be noted that, in the context of the present invention, it is 
possible to produce the entire lower wall 9 in the form of a metallic 
mesh. 
Under said annular opening 10 are arranged: 
on the one hand, a heating system 13 generating heat energy at its outlet 
16, said heating system 13 including burners 14 fed, in a manner not 
represented, with a combustible fluid, for example propane, which are 
fixed on the internal face 15 of the lower wall 6 of the enclosure 2 and 
distributed in a circular arc at the center of the annular opening 10, as 
represented in FIG. 2, the burners 14 being indicated schematically by 
points in this FIG. 2; and 
on the other hand, opposite said heating system 13 with respect to the axis 
X--X, the outlet 17 of an air source, not represented and capable of 
generating two air flows A and B, said outlet 17 passing through an 
opening 18 made in the lower wall 6 of the enclosure 2 and being solidly 
attached to said enclosure 2. 
One A of said airflows generated by the air source can flow through a 
passage 19 formed between the lower walls 6 and 9 of said enclosure 2 and 
of said container 3, in order to feed said burners 14 with air during 
combustion of the combustible fluid, while the other air flow B is capable 
of flowing through said container 3, in which it can be heated as will be 
seen hereafter, in the direction of the annular opening 8 made in the 
upper wall 7 of said container 3. 
The upper wall 5 of the enclosure 2 is, for its part, provided: 
on the one hand, with a circular opening 20 made in front of the outlet 17 
of the air source, to which opening 20 a nozzle 21, which is provided with 
a mixer system 22 capable of mixing the air and arranged close to said 
opening 20, inside said nozzle 21, is connected; and 
on the other hand, an opening 23 of oblong shape, made in front of the 
outlet 16 of the heating system 13 and formed in a circular arc in a 
fashion corresponding to the arrangement of the burners 14, as represented 
in FIG. 3. An exhaust pipe 25, represented in dots and dashes in FIG. 1 
and provided with a controllable internal valve 26 capable of altering the 
free cross section of said exhaust pipe 25, is connected to said opening 
23 via a nozzle element 24. 
Furthermore, said container 3 is filled with heat-exchange elements 27, for 
example balls or tubes, only represented in the bottom of the container 3 
in FIG. 1. In addition, for reasons of clarity of the drawing, said 
heat-exchange elements 27 are represented spread-out in this FIG. 1. Of 
course, said container 3 may be entirely filled with such heat-exchange 
elements 27, in very compact fashion, filling being defined as a function 
of the results to be obtained, as will be seen hereafter. Said 
heat-exchange elements 27 preferably have a high ratio between their 
external surface area and their volume and are preferably made of a 
material having a high specific heat capacity, such as, for example, 
zirconium dioxide. 
In order to generate a hot air flow, the device 1 according to the 
invention operates as follows. 
For this purpose, the following operations are first of all carried out: 
the shaft 4 is driven in rotation at low and constant speed and thus 
rotates said container 3 about the axis X--X; 
the air source 17 which generates the two aforementioned air flows A and B 
is turned on; and 
the burners 14 of the heating system 13, which are fed with combustible 
fuel, in a manner not represented, as well as with air by means of the air 
flow A, are turned on. 
Said burners 14 heat the heat-exchange elements 27 located above them. The 
combustion gases coming from said burners 14 then flow out of the 
container 3 and the enclosure 2 via the openings 8 and 23, as indicated by 
arrows C, and are removed via the exhaust pipe 25. 
The heat-exchange elements 27 thus store energy above the outlet 16 of the 
heating system 13. They yield this stored energy, after rotation by one 
half turn of the container 3, bringing said heat-exchange elements 27 into 
said air flow B, to said air flow B. Said air flow B is thus heated. In 
addition, the distribution of heat in this heated air flow B is made 
homogeneous by passage of said air flow B through said mixer system 22, so 
as to obtain a homogeneously heated air flow at the outlet of said nozzle 
21. 
Thus, by virtue of the invention, said hot air flow B has the following 
characteristics: 
its temperature can be determined precisely as a function, in particular, 
of the intensity of the heating, as well as of the characteristics and of 
the quantity of the heat-exchange elements 27 used; 
it is uncontaminated, the products generated by combustion in the burners 
14, and capable of contaminating said air flow B, being removed via the 
exhaust pipe 25; and 
the hot air is supplied continuously and homogeneously because of the 
constant speed of rotation of the container 3, which makes it possible 
continuously to renew the heat-exchange elements 27 arranged in the air 
flow B and intended to heat said air flow B. 
It will be noted that the air generated by the air source can furthermore 
flow directly via the opening 23, as indicated by arrows D, the strength 
of this flow depending in particular on the aerodynamic resistance of the 
heat-exchange elements 27 placed in the container 3. The distribution of 
the flow out of the container 3, respectively through the exhaust pipe 25 
and the nozzle 21, depends in particular on the size of their respective 
cross sections. This distribution can thereby be controlled by altering 
the cross section of the exhaust pipe 25 by means of the controllable 
valve 26. 
Furthermore, isolation is established between the container 3 and the 
enclosure 2, for example with the aid of labyrinth seals 28 arranged 
between their vertical walls and a mobile wall, not represented, arranged 
between their upper walls, which makes it possible to prevent circulation 
of air between the enclosure 2 and the container 3, so as to prevent on 
the one hand unheated air coming directly from the air source and, on the 
other hand, possibly contaminated air coming from the heating system 13 
from mixing with said hot air flow. 
It will furthermore be noted that the nozzle 21 has, in its central part 
29, a decrease in diameter so as to allow removal of air in a forced 
stream. 
Said hot air flow B available at the outlet of said nozzle 21 can be used, 
for example, for testing the stability of materials or for aerodynamic 
tests or for validation of wall cooling systems. However, said hot air 
flow is preferably used for developing ramjets, for which it has suitable 
characteristics, as previously mentioned, the device 1 according to the 
invention being associated for this purpose with the test rig used for 
this development.