Infra-Red Suppressor for Use With Turbo-Shaft Engine

Abstract

An infra-red radiation suppressor adapted to be positioned to alter the flow of the exhaust gases of a turbo-shaft engine and sized to block view into the engine outlet and including self-pumping ejector means to cool the visible surfaces of the suppressor. 1. Field of Invention This invention relates to infra-red radiation suppression and more particularly to an infra-red radiation suppressor which is adapted to be mounted to block the view into the outlet of a turbo-shaft engine and to intercept and alter the flow of exhaust gases thereof and which includes self-pumping ejector mechanisms to provide cooling of the exposed suppressor parts. 2. Description of the Prior Art During military combat it is common practice to utilize infra-red radiation seeking missiles and other destructive vehicles to seek out and destroy upon contact military flight vehilces which use propulsion mechanisms, such as turbo-jet or turbo-shaft engines or rockets, which operate at a sufficiently high temperature that the metal parts thereof emit significant infra-red radiation. The radiation emitted by the exhaust gases is largely attenuated by the atmosphere. In any case it has been found to be of less significance than that emitted by the exposed metal parts. The infra-red radiation threat is fully described in U.S. Pat. No. 3,210,934, to which reference may be made. Protection against this threat is accomplished by blocking the view into the engine outlet to prevent the infra-red radiation seeking devices from detecting the infra-red radiation emitting parts therewithin and by cooling exposed parts to prevent them from reaching the critical infra-red radiation emitting temperature which can be detected by the oncoming missile. In the prior art, such as in U.S. Pat. No. 3,210,934, this exposed part cooling function is performed by apparatus which either needs a pressure source for the cooling fluid or which is ram-air responsive. The disadvantage of such systems is that devices which require pressure sources for providing pressurized cooling fluid to the exposed vehicle parts either require a separate pumping source with its attendant added weight and complication to the flight vehicle system, or require that pressurized engine compressor bleed air be utilized for this purpose, thereby reducing the power generating capability of the engine so bled. The devices which are ram-air responsive must not only be capable of being positioned so that some part thereof is in impinging contact with atmosphere to develop the ram-air effect, and this is not possible or practical in power generating devices which are positioned internally within the flight vehicle and, further, such ram-air responsive devices are inoperative when the flight vehicle is not moving forward at a substantial velocity. Accordingly, such ram-air responsive devices could not be utilized if the flight vehicle were a helicopter, which is capable of operating in many directions other than forward and which is further capable of operating in the hover mode, which mode is very important in many military situations. It will therefore be seen that the prior art devices either add weight and complication to the overall system when a separate cooling fluid power source is utilized, or reduce the engine power generating capability when an engine generated pressure source, such as turbo-jet or turbo-shaft compressor, is bled to provide this cooling fluid. Further, ram responsive infra-red radiation suppressors are ineffective on helicopters during flight modes other than forward flight and the all-important military hover mode. In addition, it has been suggested in the prior art that infra-red radiation suppressor parts be made completely porous so as to be able to use transpiration cooling, that is, the forced passage of cooling air through the various pores in the suppressor wall members. While transpiration cooling is accepted as an efficient method of cooling from a theoretical thermodynamic standpoint, film cooling is only slightly less efficient and has many advantages. Transpiration cooling is particularly unsuited for helicopter operation. The helicopter often lands and takes-off from unprepared areas and foreign matter, including dust and debris, fill the air and are injected into the engine and pass therefrom into the suppressor. The debris will attempt to pass with the air through the small 40 micron pores of the transpiration cooled suppressor members, thereby clogging these pores with foreign matter. With transpiration cooling, even if a complete clogging of all the pores is not encountered, localized infra-red radiating hot spots in the walls would be encountered in the localized clogged portions. In helicopter use, an attempt to keep the pores of a transpiration cooled suppressor absolutely clean in service would present very substantial maintenance problems. A further disadvantage of transpiration cooling is that the porous parts involved cannot be fabricated by conventional methods, as can the solid sheet metal parts of our suppressor. In addition, in a transpiration cooled system, it would be necessary, in some suppressor stations, to be discharging cooling air into a high static pressure region, and this would require the use of a positive pressure source. Still further, the partially clogged pores, or the partially clogged porous wall, offers increased resistance to cooling air flow therethrough and hence a greater positive pressure is required to cause the cooling air to pass through these restricted pores, therefore, transpiration cooling produces the undersirable need of an extremely high capacity pressure source capable of generating high pressure. To complicate the problem, we are not able to produce an even cooling effect over the entire surface of the suppressor due to irregular pore clogging problems. Contrary to the clogging problems presented by the transpiration cooled suppressor, the ejector slots of our suppressor are sufficiently large that they are not subject to clogging. In addition, the prior art has taught infra-red radiation prevention by passing cooling air between closely positioned double walls of the suppressor parts to produce convection cooling thereof. Since there is substantial resistance to the flow of cooling air between two closely spaced walls of this type, very substantial air pressure is required to cause the air to flow therethrough with sufficient velocity to effectively cool the passage defining parts. Contrary to the transpiration cooling and the convection cooling of the prior art, we are using primarily film cooling. Since film cooling produces a cooling air film between the suppressor parts and the hot exhaust gases to prevent heating of the suppressor parts, less air is used in such a system because it requires less air to prevent heating of suppressor parts by film cooling than it does to convectively cool suppressor parts which are directly scrubbed by the hot exhaust gases. A primary object of the present invention is to provide an infra-red radiation suppressor for use with a turbine engine, such as a turbo-shaft engine, which both blocks view into the outlet of the turbine engine from all vantage points, which is mounted independently of the engine so as not to change the operating characteristics thereof and so as to be free from absorbing the vibrations and physical distortion therefrom, which includes no moving parts, and which includes a self-pumping, passive cooling system for the exposed suppressor parts to prevent them from reaching critical infra-red radiation emitting temperatures. In accordance with the present invention, an infra-red radiation suppressor is provided for use with a flight vehicle which is operable during all modes of operation of the flight vehicle and which requires no cooling fluid pressure generating mechanism, with its attendant added weight, added complication and reduced reliability, and which does not depend upon engine generated pressure to cool the suppressor. In accordance with the present invention, such a suppressor is provided and is made of sheet metal parts, all of which are capable of fabrication and repair by the use of conventional sheet metal fabricating methods and apparatus. In accordance with still a further aspect of the present invention, such a suppressor is provided which defines an annular exhaust gas passage for the engine exhaust gases, which passage increases in cross-sectional area from its forward or upstream end to its after or downstream end so as to constitute a diffuser. It is still a further object of the present invention to provide such an infra-red radiation suppressor which requires minimum cooling fluid flow and which is not prone to flow separation of the cooling fluid film and the adjacent exhaust gas flow from the suppressor parts over which it is flowing. In accordance with a further aspect of the present invention, such an infra-red radiation suppressor is provided and includes an inner body or central plug enveloped within an outer casing such that these parts are supported in spaced relation to define an annular exhaust gas passageway therebetween and such that the selected curvature of the outer surface of the central plug, the inner surface of the outer casing and the midstream path of the annular passageway interact to induce the flow of a cooling air blanket along the outer surface of the central plug and to prevent flow separation of that cooling air blanket and the adjacent exhaust gas flow from the outer surface of the central plug. In accordance with a further aspect of the present invention, an infra-red suppressor is provided which utilizes the ejector principle at at least one station in the suppressor. In accordance with still a further aspect of the present invention, such a suppressor is provided which includes a self-pumping boundary layer control, film cooling slot at substantially the maximum diameter station of the central plug, which slot is defined between wall members in the central plug which cooperate to define a converging passage culminating in the slot, the length of which passage is approximately 10 times the exit height of the slot, thereby cooling the convergent passage defining walls by convection cooling, and so that the cooling air passing through the slot is uniform and positively accelerated to prevent local flow separation of the cooling air from the slot walls upstream of the slot exit and from the central plug wall downstream of the slot exit.

Patent Drawings (2 sheets)

Sheet 1

Sheet 2