Title:
MECHANICALLY MODULATED COMBUSTION HEATED INFRARED RADIATION SOURCE
United States Patent 3771731


Abstract:
A combustion chamber is closed at one end by a highly transparent window. A volatile gas and oxygen are introduced into the combustion chamber. After ignition the gaseous flame heats the combustion chamber to a temperature slightly below the melting point of the chamber. Infrared radiation emitted by the heated combustion chamber walls and passing through the window is mechanically modulated by one or more motor driven shutters.



Inventors:
Dyner, Harry B. (Newton Centre, MA)
Hill, Jacques A. F. (Lincoln, MA)
Application Number:
05/282523
Publication Date:
11/13/1973
Filing Date:
08/21/1972
Assignee:
SANDERS ASS INC,US
Primary Class:
Other Classes:
250/495.1, 431/356
International Classes:
F21S8/00; H01J61/84; H01J61/94; (IPC1-7): F21M/
Field of Search:
240/41R,11
View Patent Images:



Primary Examiner:
Moses, Richard L.
Parent Case Data:


This is a continuation, of U.S. Pat. application Ser. No. 85,065, filed Oct. 29, 1970 now abandoned.
Claims:
Having described what is new and novel and desired to secure by Letters Patent, what is claimed is

1. A combustion heated source of modulated radiant energy comprising

2. Apparatus as recited in claim 1 further including means for cooling said window.

3. Apparatus as recited in claim 2 wherein

4. Apparatus as recited in claim 3 wherein

5. Apparatus as recited in claim 1 wherein

6. Apparatus as recited in claim 1 wherein

7. Apparatus as recited in claim 6 wherein

8. Apparatus as recited in claim 1 wherein

9. Apparatus as recited in claim 8 wherein

10. Apparatus as recited in claim 1 wherein

11. Apparatus as recited in claim 1 further including

12. Apparatus as recited in claim 11 wherein

13. Apparatus as recited in claim 12 wherein

14. Apparatus as recited in claim 1 wherein

15. Apparatus as recited in claim 3 wherein

16. Apparatus as recited in claim 15 wherein

17. Apparatus as recited in claim 3 wherein

18. Apparatus as recited in claim 17 wherein

19. Apparatus as recited in claim 18 wherein

20. Apparatus as recited in claim 17 further including

21. Apparatus as recited in claim 1 wherein

22. Apparatus as recited in claim 5 further including

23. Apparatus as recited in claim 22 wherein

24. Apparatus as recited in claim 1 further including

25. Apparatus as recited in claim 24 further including

26. Apparatus as recited in claim 24 further including

27. Apparatus as recited in claim 3 further including

28. Apparatus as recited in claim 3 further including

29. Apparatus as recited in claim 28 wherein

Description:
BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of infrared radiation sources and more particularly to a new and novel combustion heated infrared radiation source having a mechanical modulation means.

2. Description of the Prior Art

Prior to the present invention the primary sources of infrared radiation have been the electrically powered blackbody and the arc lamp. The primary drawback with such devices has been their inefficient use of relatively high levels of electrical input power. In some applications where electrical power is at a premium this inefficiency becomes prohibitive. Furthermore, modulation of the infrared radiation output of the arc lamp has presented non-trivial problems in that the lamp electrodes must be prevented from appearing in the field of view of the device in order to provide adequate depth of modulation. To provide complex modulation of the arc lamp output also requires modulator electronics of undue complexity. Modulation of blackbody sources has generally been provided through the use of rotating choppers which have proven satisfactory for small apertures. When it is sought, however, to use large aperture sources the rotating chopper becomes too large for practical use.

OBJECTS AND SUMMARY OF THE INVENTION

From the foregoing it will be understood that among the various objectives of the present invention are:

To provide a new and novel modulated source of infrared energy.

To provide apparatus of the above-described character which requires minimal electrical power.

To provide apparatus of the above-described character having an output aperture of increased area.

To provide apparatus of the above-described character which is of particular adaptability to mobile operation.

These and other objectives are achieved by providing an insulated high temperature, high emissivity ceramic combustion chamber closed at one end by a transparent window. A combustible fuel is introduced together with oxygen or air into the combustion chamber and ignited. The flame heats the ceramic chamber walls to slightly under their melting temperature and the combustion products are exhausted through a mixing chamber where they are mixed with cooling air. The heated ceramic walls emit infrared radiation which is transmitted through the window. A motor driven rotating shutter modulator is disposed in front of the exit aperture which provides efficient amplitude modulation of the output radiation.

The foregoing as well as other objects, features and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a combustion heated infrared radiation source of utility in the practice of the present invention.

FIG. 2 is a schematic elevation view of a rotating shutter modulator suitable for use with the radiation source of FIG. 1.

FIG. 3 is a schematic cross-sectional view of a mechanically modulated combustion heated infrared radiation source particularly adapted for use in a mobile application.

FIG. 4 is a schematic block diagram of a complete amplitude modulated infrared transmitter incorporating the apparatus of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

Turning now to FIG. 1 there is schematically illustrated in cross section simplified combustion heated infrared radiation source in accordance with the present invention. The source comprises a high temperature combustion chamber 10 formed of a high emissivity material and closed at one end by a window 12 which is transparent to radiation over a preselected spectral range. Fuel is introduced into the combustion chamber 10 via a burner orifice 14 preferably in a direction relative to the combustion chamber 10 such that rapid mixing and combustion are achieved. Since the window 12 is in direct contact with the hot combustion gases it is preferred that the oxygen or air necessary to support combustion of the fuel be introduced at a low temperature through a thin cylindrical slit 16 in the source faceplate 18 at the periphery of the window 12. The window 12 is sealed to the faceplate 18 by a window mounting bracket 20. In order to minimize radial heat losses from the combustion chamber 10 it is wrapped in a layer of thermal insulation 22. The heat transfer from the hot combustion gases is maximized and the combustion chamber walls thus comprise the infrared radiating surface.

The combustion products which must be exhausted from the chamber 10 via exhaust nozzle 24 are quite hot and must be cooled before exiting the source. Exhaust cooling air is introduced through ducts 26 to an exhaust gas mixing chamber 28. The exhaust products are thus cooled to a desired temperature determined by the mixture ratio of air to combustion products.

It has been found by the Applicants that the various forms of zirconium oxide are particularly useful in the fabrication of the combustion chamber although any material capable of withstanding the high combustion temperatures is acceptable in the practice of the invention. It is preferred that the combustion chamber be laid up on a mandrel with zirconium oxide yarn and impregnated with a zirconium oxide cast in a similar manner to that employed with glass-reinforced resin plastics. In this way complex shapes optimized to withstand thermal shock may be easily fabricated.

It is also preferred in the practice of the present invention that a fuel providing as high a flame temperature as possible be employed, however, considerations such as fuel availability and toxicity are factors bearing on practical choices. For example the temperature of an oxy-acetylene flame is 3,360° K and thus acetylene would be a desirable fuel. This fuel is, however, explosive and in some applications, particularly those which are mobile, it is necessary to dissolve the acetylene in acetone which increases the storage weight per pound of usefuel fuel. A relatively new fuel is the so called MAPP gas, a mixture of Methyl acetylene, Allene, Propane and Propylene. This is more stable than acetylene and thus may be stored as a liquid under its own vapor pressure without danger of explosion. Another readily available fuel is liquid propane.

The selection of an operating temperature for the radiation source of FIG. 1 may be made based upon minimum specific fuel consumption consistent with minimum source size and optical aperture. In one embodiment fabricated by the Applicants an operating temperature of 2,400° K for a propane fired device requires a combustion chamber diameter of 1.0 inch. Propane consumption was found to be 0.58 pounds/hour and oxygen consumption 2.11 pounds/hour. The device thus fabricated produced a total radiation output of 325 watts.

FIG. 2 schematically illustrates a rotating shutter mechanical modulator which is useful in combination with the radiation source of FIG. 1 in the practice of the present invention. This modulator comprises at least one bank of continuously rotatable blades 30 each mounted between mounting brackets 32 in bearing assemblies 34. The blades 30 are driven in rotation by a motor 36 through an output gear 38 and reduction gear 40. Adjacent blades are driven in opposite directions by the train of substantially identical gears 42. In order to eliminate twisting of the individual blades 30 each is driven from both ends. In addition to driving the gears 42 the motor 36 also drives gear 44 which is coupled by shaft 46 to a gear train substantially identical to that illustrated at 42 and 44 but disposed on the outside of the opposite mounting bracket 32. To further minimize twisting of the blades 30 the bearing assemblies 34 may be slightly preloaded thus placing the blades 30 under some tension. It has been found by the Applicants that a typical size 15 motor developing 0.5 in-oz of torque at 24,000 rpm and 0.8 in-oz of stall torque is adequate to drive a bank of nine blades 0.315 inch wide disposed on 0.300 inch centers. The bearing assemblies 34 are of standard commercially available type and the gears may be for example standard 20° pressure angle gears. It will be readily apparent that any of a wide variety of bearing assemblies and gears suitable to meet a given modulation frequency requirement could be used.

Also illustrated in FIG. 2 is a second drive motor 46 and an associated gear train 48 identical to that associated with blades 30 and discussed above. This second motor 46 and gear train 48 are operative to drive a second bank of rotatable blades (obscured by blades 30 and thus not shown). With such a double arrangement of independently driven shutter blades it is possible to modulate the radiation output of an infrared or other optical source with more complex functions. For example drive motor 36 may be operated at a constant speed and thus blades 30 modulate the output radiation at some preselected fixed frequency. The second drive motor 46 may be variable in speed and operate to drive the second bank of shutter blades such as to amplitude modulate the fixed frequency radiation. To achieve the variable speed motor drive for the second bank shutter blades the motor 46 may be driven by a voltage controlled oscillator 50 coupled to a power supply 52. It will be further understood that either or both drive motors 36 and 46 may be constant or variable speed depending upon the application in which the modulated infrared source is to be used.

With reference now to FIG. 3 there is illustrated in schematic cross-section a mechanically modulated infrared radiation source which is particularly adapted for use in a mobile application. In this embodiment the radiating cavity 60 is a circular cone of double-wall, 62 and 64, construction in order to minimize thermal losses from the back side of the inner wall 62. The inner wall 62 includes a plurality of flow passages 66 for the hot combustion gases to pass from the combustion chamber into the space 68 between the walls 62 and 64 and through a sonic exhaust nozzle 70 at the base of the radiation source.

It would be preferable in the practice of the present invention to insulate the radiating cavity with a single layer of high temperature, high efficiency insulation material. A problem which is encountered, however, is that the most efficient insulating materials generally cannot withstand very high temperatures. Thus it is necessary with high temperature sources to use a layer 72 of high temperature insulation such as zirconium oxide felt next to the outer cavity wall 64. A sufficient thickness is used to keep the maximum temperature at the outer surface of the layer 72 at a level which is tolerable by a layer 74 of insulation such as silicon dioxide glass which is of higher insulating efficiency but lower temperature capacity than the layer 72 of high temperature insulation. Similarly, a third or outer layer 76 of very high insulating efficiency but low temperature capacity insulation such as MIN-K a very low conductivity insulating material of utility at temperatures less than 1,200° K and commercially available from the Johns-Manville Co. may be used if necessary or desired. The insulated radiating cavity is disposed in a case structure 78 which is provided with a plurality of longitudinal heat exchange fins 80.

The radiating cavity 60 is closed at its output end by a window 82 which is transparent over the output wavelength range of interest. For example for a radiation output in the visible wavelength band a quartz window would be a logical choice based upon low cost and optimum resistance to thermal shock. Obviously, other window materials would be preferred for other regions of the spectrum. In order to avoid devitrification of the quartz window, the oxygen or air necessary to support combustion within the cavity 60 is introduced via an oxygen supply line 84 into a cylindrical slit 86 at the periphery of the window 82. The cold oxygen thus passes over the window 82 and serves to keep the window temperature within acceptable limits. It has been found by the Applicants that this technique is adequate to maintain a window temperature of less than 900° C while the radiating cavity walls were heated to 2,600° K. Further, the radiative output of the source has been found to be substantially unaffected when most of the oxygen is injected over the window 82 as long as combustion is completed within the cavity 60.

The fuel is introduced through fuel supply line 88 to a small orifice 90 disposed tangentially to the inner wall 62 of the radiating cavity 60. For mobile applications propane is a desirable fuel in accordance with the fuel selection criteria discussed above with reference to FIG. 1.

The simplest ignition system of utility in the practice of the present invention is an electrical spark generated by discharging a condenser across a spark gap. To this end copper electrodes 92 and 94 are inserted into the radiation cavity 60 through a high temperature gasket 96 such as a replaceable zirconium oxide insert. The electrodes 92 and 94 are coupled at their opposite ends through the housing structure 78 to an electrical connector block 98 of the type known in the art and the internal details of which are thus not specifically illustrated. Once combustion is initiated within the cavity the electrodes 92 and 94 will begin to melt and ignition will be maintained by the hot cavity wall 62.

Radiation from the heated cavity wall 62 is passed by window 82 and mechanically modulated by a rotating shutter modulator 100 as described with reference to FIG. 2. The electrical coupling to the modulator drive motor(s) is illustrated schematically at 102. If desired a reflective light funnel 104 of any preselected geometry may be used at the output side of the modulator 100 to provide a particular spatial distribution of the modulated output energy. A plurality of heat exchanging fins 106 may also be disposed about the circumference of the light funnel 104. Additionally if required in a given application a suitable filter 108 may be disposed over the light funnel 104 output aperture.

Cooling of the radiation source in the mobile environment is provided by ducted air. The exhaust gases issue from the exhaust nozzle 70 into a mixing chamber 110 formed as a part of the housing structure 78. To prevent erosion of the housing structure 78 and to reduce heating of the electrical connector block 98 a layer 112 of high temperature resistance material may be placed opposite the nozzle 70. Cooling air is admitted via opening 114 in the mixing chamber 110, mixed with the hot exhaust gases and vented to the environment by means of coupling 116. It has been found that the exhaust gases from a propane fired radiation source may be reduced to a temperature of less than 250° F by mixing with ram air at a mass ratio of about 35.

Cooling air is also admitted via air scoops 118 which are formed as a part of an outer shroud 120. This air passes over the heat exchange fins 106 on light funnel 104, past the drive motors of modulator 100 and over the longitudinal heat exchange fins 80 on the housing structure 78. This air may be vented to the environment either directly or via the vent coupling 116.

A thermal switch 121 may be disposed in the exhaust mixing chamber 110 to continuously monitor the exhaust gas temperature. If, for example, the exhaust gas temperature has not risen to a preselected level within a given period of time after an ignition spark across the electrodes 92 and 94 a reignition sequence may be initiated in the manner to be described hereinbelow. A second thermal switch 122 may be diposed at any convenient position near the output window 82. In the event that the window were to break combustion gases would quickly enter the optical cavity with an attendant temperature rise. Thus the rise in temperature may be used to initiate a shut-off sequence to be presently described.

Turning finally to FIG. 4 there is illustrated in schematic block form a complete mechanically modulated combustion heated infrared radiation transmitter in accordance with the principles of the present invention. The radiating cavity combustion chamber 124 is coupled by supply lines 126 and 128 to an oxygen and fuel supplies 130 and 132 respectively; each supply line being provided with a solenoid valve 134 and 136. The oxygen supply 130 may be a tank of liquid oxygen or could be a chemical oxygen generator whereby oxygen is generated chemically from sodium chlorate. Such chemical oxygen generators are known in the art and one such device is available from Life Support Systems of Santa Ana, California.

Combustion of the fuel heats the combustion chamber 124 which radiates the infrared. This radiation passes through the combustion chamber window 138 and is modulated by the above-described mechanical modulator 140. The modulator drive motors 142 are driven by any suitable power source 144.

The combustion products are coupled from the combustion chamber 124 to the exhaust mixing chamber 146 where they are mixed with cooling air which is provided via air intake 148. The cooled gases are vented to the environment via exhaust coupling 150.

The ignition spark gap 152 is coupled via control logic 154 to the power source 144. The control logic 154 also receives an inputs the electrical signals from the exhaust gas temperature sensor 156, optics temperature sensor 158, and starting means 160. In operation an initial starting signal applied from the starting means 160 to the control logic 154 serves to apply power from source 144 to the modulator drive motors and to initiate the ignition sequence. This sequence includes charging of the ignition capacitor 162 and opening the solenoid valves 134 and 136 in the oxygen and fuel supply lines 126 and 128 respectively. Once the ignition capacitor 162 is fully charged it is discharged across the spark gap 142 in the combustion chamber 124. Assuming proper ignition the temperature of the exhaust gases in the exhaust mixing chamber will rise quickly to an operating level. If this temperature has not risen to a given value within a selected time the exhaust gas temperature sensor 156 will so indicate and the ignition sequence may be repeated. In such cases the oxygen and fuel solenoid valves 134 and 136 are closed for a period sufficient to vent any combustible gases from the combustion chamber 124, and the above-described ignition sequence repeated. If desired a predetermined number of unsuccessful ignition attempts may serve as the criterion for inactivating the entire system. An automatic inactivation sequence may also be initiated if the optics temperature sensor 158 detects the conditions indicative of a broken window 138. In such event the oxygen and fuel solenoid valves 134 and 136 would be closed and power removed from the modulator drive motors 142.

It will thus be seen that the Applicants have provided a new and novel mechanically modulated combustion heated infrared radiation source wherein the objectives set forth hereinabove are efficiently met. Since certain changes in the above construction will become apparent to those skilled in the art without departing from the scope of the invention it is intended that all matter contained in the foregoing description or shown in the appended drawings shall be interpreted as illustrative and not in a limiting sense.