Abstract:
An eductor assembly comprises a primary nozzle configured to discharge turbine exhaust gas therefrom. The eductor assembly further comprises a cooler plenum having an inlet and an outlet and a surge plenum at least partially surrounding the cooler plenum and the nozzle, the surge plenum for conducting a surge flow. Cooling air flows through a vent between the cooler plenum and the surge plenum when there is no surge flow.

Description:
TECHNICAL FIELD 
     Embodiments described herein relate to an exhaust eductor system, and more particularly, to a system and method for providing cooler air from an oil cooling plenum to the surge plenum of an eductor exhaust system. 
     BACKGROUND 
     Many modern aircraft are equipped with an auxiliary power unit (“APU”) that generates and provides electrical and pneumatic power to various parts of the aircraft for tasks such as environmental cooling, lighting, powering electronic systems, and main engine starting. Typically, such APUs are located in the aft section of the aircraft such as the tail cone and are isolated by a firewall. During operation, an APU produces exhaust gas that is directed through a nozzle and out of the aircraft through an exhaust opening. The nozzle may communicate with an eductor system that utilizes the APU exhaust gas to draw and direct other gases through the aircraft. 
     To achieve this, eductor systems have been developed that include a first plenum (i.e. the oil cooler plenum) for drawing gas across an oil cooler, and a second plenum (i.e. the surge plenum) for directing surge flow to an exhaust duct (i.e. air not required by the aircraft to satisfy its pneumatic requirements, commonly referred to as surge bleed flow). During normal operation with no surge flow, the surge plenum is a dead-headed cavity with its aft facing outlet exposed to the mixed eductor flow; i.e. the turbine exhaust at perhaps 1000° F. and the cooling air from the cooling plenum at approximately 200° F. Thus, the mixed eductor flow, which may be about 500° F., enters the surge plenum, circulating in and out of the surge plenum, and heating the surge plenum to approximately 500° F., exceeding the strict temperature limits (i.e. 450° F.) being imposed on the outer surfaces of the APU including the surge plenum. 
     In accordance with the forgoing, it would be desirable to provide a system and method for directing a cooling flow into the surge plenum to reduce the temperature of the surge plenum surfaces when there is no surge flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-section view of an aircraft including a power unit and eductor system disposed therein; 
         FIG. 2  is a cross-sectional view of an exemplary eductor oil cooler and surge flow plenum assembly that may be incorporated into the tail cone depicted in  FIG. 1  in accordance with the prior art; 
         FIG. 3  is a cross-sectional view of the exemplary eductor oil cooler and surge flow plenum illustrating the recirculation in the surge plenum when there is no surge flow; 
         FIG. 4  is a cross-sectional view of a first embodiment wherein cooler air is deflected from the oil cooler plenum into the surge plenum; 
         FIGS. 5A ,  5 B and  5 C are side, top and isometric views of a beveled tube for use in conjunction with the embodiment shown in  FIG. 4 ; 
         FIGS. 6A ,  6 B and  6 C are side, top and isometric views of a tube terminating with a scoop for use in conjunction with the embodiment shown in  FIG. 4 ; 
         FIG. 7  is a cross-sectional view of yet another embodiment wherein cooler air enters the surge plenum from the cooler plenum via a one-way flap; and 
         FIG. 8  illustrates yet another embodiment utilizing a plurality of tubes for deflecting cooler air from the cooler plenum into the surge plenum. 
     
    
    
     BRIEF SUMMARY 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid for determining the scope of the claimed subject matter. 
     In accordance with the foregoing, there is provided an eductor assembly comprising a primary nozzle configured to discharge turbine exhaust gas therefrom. The eductor assembly further comprises a cooler plenum having an inlet and an outlet, a surge plenum at least partially surrounding the cooler plenum and the nozzle, the surge plenum for conducting a surge flow, and a vent between the cooler plenum and the surge plenum through which gas flows from the cooler plenum into the surge plenum when there is no surge flow. 
     In accordance with the foregoing, there is also provided a method for cooling the external surfaces of a surge plenum in an eductor assembly in an APU of the type wherein the surge plenum at least partially surrounds a cooler plenum. The method comprises introducing cooler air from the cooler plenum into the surge plenum when there is no surge flow in the surge plenum. 
     An eductor assembly is also provided that comprises a primary nozzle configured to discharge turbine exhaust therefrom, a cooler plenum, a surge plenum at least partially surrounding the cooler plenum and the nozzle, the cooler plenum for conducting surge flow, a wall separating the cooler plenum and the surge plenum, and a passive valve in the wall for conducting cooler air from the cooler plenum to the surge plenum when there is no surge flow. 
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the disclosed embodiments or the application and uses thereof. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. 
     Turning now to the description,  FIG. 1  illustrates a housing  100  within which a power unit  102  and an eductor  104  are disposed. The housing  100 , which may be an aircraft tail cone or helicopter housing, is generally conical and has a sidewall  103  and inlet and exhaust openings  105 ,  106  that are formed therein. The power unit  102 , which may be an auxiliary power unit (“APU”), may be used to drive a number of non-illustrated devices, including, for example, a gearbox, a generator, or a load compressor, is mounted within the housing  100  and receives air from an inlet duct  107  that extends between the power unit  102  and the inlet opening  105 . The power unit  102  includes a nozzle  108  that communicates with the eductor  104 . It will be appreciated that the power unit  102  and eductor  104  may indirectly or directly communicate with each other. In any case, exhaust gas from the power unit  102  flows through the eductor  104  and an exhaust duct  109  and exits the aircraft via the exhaust opening  106 . 
     The eductor oil cooler and surge flow plenum assembly  104  is configured to employ the flow of high velocity exhaust gas to draw other gas through the aircraft. As shown in  FIG. 2 , the assembly  104  includes an oil cooler plenum  120  and a surge flow plenum  122  that are disposed along the longitudinal axis  116  and are circumferentially around the nozzle  114 . The oil cooler plenum  120  and surge flow plenum  122  are separated from each other and are each defined, in part, by a wall  126 . More particularly, the wall  126  has an inner surface  128  that, together with the nozzle  114 , forms the oil cooler plenum  120 . Wall  126 , together with an outer wall  132 , forms the surge flow plenum  122 . 
     The oil cooler plenum  120  includes a fluid inlet  134  and a fluid outlet  136 . The fluid inlet  134  communicates with an oil cooler duct  138  within which an oil cooler  140  is disposed. Preferably, the oil cooler plenum  120  surrounds an entire circumference of the nozzle  114  to maximize contact between high velocity APU exhaust gas that flows through the nozzle  114  and the gas that is pulled through the oil cooler plenum  120  to thereby increase pumping of gas through the fluid inlet  134 . To further increase pumping of gas through the fluid inlet  134 , the fluid outlet  136  is aligned with an end  144  of the nozzle  114 . Thus, gas flowing through the fluid outlet  136  will be entrained by the high velocity APU exhaust gas and both will flow together through the exhaust duct  109  (shown in  FIG. 1 ). 
     It will be appreciated that the volume of space needed to accommodate the cooled gas decreases as distance from the fluid inlet  134  increases, and that the gas in the oil cooler plenum  120  preferably flows around the circumference of the nozzle  114  at a substantially constant flow velocity. In this regard, the wall  126  may slope toward the longitudinal axis  116  forward to aft and is disposed nonconcentric thereto. As a result, the oil cooler plenum  120  includes a plurality of variously sized radial cross-sectional areas at different axial locations along the longitudinal axis  116  and a plurality of variously sized axial cross-sectional areas at different angular locations relative to the longitudinal axis  116 . The cross-sectional areas, which, as previously mentioned, preferably gradually decrease in size when the distance from the fluid inlet  134  increases, may be disposed axisymmetrically about the longitudinal axis  116 . 
     Returning to  FIG. 2 , the surge flow plenum  122  is partially defined by the walls  126 ,  132  and includes a fluid inlet  154  and a fluid outlet  156 . The fluid inlet  154  communicates with a surge bleed entry duct  158  that is coupled to or integrally formed as part of the outer wall  132 . The fluid outlet  156  is preferably axially aligned with and coterminous with the oil cooler plenum fluid outlet  136 . 
     Similar to the oil cooler plenum  120 , the surge flow plenum  122  preferably includes a plurality of variously sized axial cross-sectional areas at different angular locations relative to the longitudinal axis  116 . Most preferably, the areas of the axial cross-sections gradually decrease as the distance away from the surge flow fluid inlet section increases without overlapping the oil cooler plenum  120 . In other embodiments, the oil cooler plenum  120  may surround the first circumferential section of the nozzle  114  and the surge flow plenum  122  may surround the second circumferential section and a portion of the first circumferential section. 
     During operation, the APU  102  exhausts high velocity exhaust gas out of the nozzle  114 . When gas is needed to cool the oil cooler  140 , the gas enters the oil cooler  140 , travels through fluid inlet  134 , and flows through the oil cooler duct  138  into the oil cooler plenum  120 . When the gas exits the oil cooler fluid outlet  136 , it is pulled through the exhaust duct  109  by the high velocity APU exhaust gas. The pull of the APU exhaust gas causes additional gas to be pumped into the oil cooler plenum  120 . Occasionally, surge flow gas may be dumped into the surge bleed entry duct  158  and into the surge flow plenum  122 . The surge flow gas, which is already traveling at a high velocity, flows directly into the exhaust duct  109  and out the exhaust opening  106 . 
     As stated earlier, when there is no surge flow, the surge plenum becomes a dead-headed cavity with its aft outlet exposed to the mixed eductor flow which is about 500° F. This flow enters the surge plenum  122  and may continuously recirculate in and out of the surge plenum  122  heating the surge plenum surface to an unacceptable level. This situation is illustrated in  FIG. 3 . With no surge flow in to surge plenum  122 , heated mixed eductor flow  160  recirculates in and out of surge plenum  122  as is indicated by arrows  162  and  164 . 
     Embodiments described herein contemplate introducing cooler air from the oil cooler plenum into the surge plenum during normal APU operation when there is no surge flow. It is further contemplated that this may be accomplished by positioning at least one vent in a wall between the oil cooler plenum and the surge plenum. It is still further contemplated that the vent may be a tube that includes a scarfed or scooped end (e.g. a thumbnail scoop) that extends into the oil cooler plenum and is positioned to function as a diverter to scoop cooler air and direct it into the surge plenum. The air exit portion of the tube that extends into the surge plenum should be long enough so as to prevent surge flow from entering into the oil cooler plenum during a surge event; i.e. the tube protruding into the surge plenum has a high entry pressure loss. Thus, the arrangement acts as a passive one way valve. 
       FIG. 4  is a partial cross-sectional view in accordance with a first embodiment. As can be seen, a hollow tube  170  is shown extending through a wall of surge plenum  122  at first and second locations each of which will be discussed separately. In the first location, tube  170  extends through an inclined wall  126  of surge plenum  122  to an area of the cooler plenum characterized by lower speed oil cooler flow. In the second location, tube  170  extends through wall  128  of surge plenum  122  to an aft area characterized by a higher speed (e.g. 80-110 feet/second) oil cooler flow. In each location, tube  170  preferably extends substantially perpendicularly through walls  126  and  128 , respectively, as the case may be. 
     Tubes  170  may be manufactured from a variety of materials but are preferably made from a heat resistant weldable alloy. For maximum efficiency, the tubes should be manufactured so as to exhibit characteristics that discourage backflow through the tubes; i.e. flow from the surge plenum  122  to the oil cooler  120  during a surge event. To this end, the tubes may have a length between 0.25 inch and 2.0 inches and preferably between 0.75 inch and 1.25 inches. The diameter of the tube is preferably one-half to one-third the tube&#39;s length. These criteria will provide sufficient entry losses and friction losses to prevent flow back into the oil cooler plenum  120 . 
     In order to deflect cooler air flowing in the oil cooler plenum  120  into the surge plenum  122 , it is preferable to provide an obstruction at the end of the tube that resides within oil cooler plenum  120  and deflects cooler air in oil cooler plenum  120  through the tube and into the surge plenum  122 . 
     In a first embodiment, the obstruction may be a beveled portion of the tube. For example,  FIGS. 5A ,  5 B and  5 C are top, side, and isometric views, respectively, of a beveled tube  180 . The beveled portion  182  will extend into cooler plenum  120  and the bevel opening  184  will face the direction of air flow in cooler plenum  120 . Alternatively, in another embodiment,  FIGS. 6A ,  6 B and  6 C are top, side, and isometric views, respectively of a tube  190  that terminates with a scoop  192  which redirects air flowing in the cooler plenum  120  through tube  190  and into the surge plenum  122 . 
       FIG. 7  is a cross-sectional view of a further embodiment wherein cooler air is directed from the oil cooler plenum  120  into the surge plenum  122  through a flap  181  when there is no surge flow as indicated by arrow  183 . A surge flow will prevent flap  181  from opening. 
       FIG. 8  functionally illustrates an oil cooler plenum  200 , a surge plenum  202 , and an exhaust duct  204  in accordance with a still further embodiment. Also shown are a plurality of obstructions  206  (e.g. scoops) for deflecting air from cooler plenum  200  to surge plenum  202 . It is known that the air exits the oil cooler  140 , enters the air cooler plenum  200 , and circulates around the plenum resulting in velocity vectors  208  having different directions in different parts of cooler plenum  200 . Thus, the obstructions  206  in the form of bevels, scoops and the like as previously described are positioned such that their openings  210  are substantially perpendicular to the oil cooling flow velocity vectors  208  at their respective locations. The multiple scoops  206  provide substantially uniform cooling of surge plenum  202 . 
     Thus there has been provided a system and method for directing a cooling air flow into the surge plenum to reduce the temperature of the surge plenum surfaces when there is no surge flow. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.