Abstract:
A pulsed-jet active flowfield control actuation system enhances the rate of heat transfer and heat removal in a heat exchanger for better management of thermal loads. The pulsed jet actuators impart an unsteady component of velocity to the working fluid of the heat exchanger. This design increases the convective heat transfer, and avoids increases in heat exchanger volume and weight for a given performance value.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates in general to heat exchangers and, in particular, to an improved system, method, and apparatus for pulsed-jet-enhanced heat exchangers. 
     2. Description of the Related Art 
     Emerging and next-generation air vehicles will be required to manage unprecedented quantities of thermal, mechanical, and electrical power. This need arises from the demands for advanced propulsion, aerodynamics, sensor, and weapon/payload capability necessary to defeat threats, perform with greater fuel efficiency, reduce noise and emissions, and decrease life cycle cost. Specific technologies of interest, such as advanced engines, high power sensors, directed energy weapons, and enhanced electronic actuation, will require much more power than today&#39;s systems. While demand for power management functionality grows, the space and weight available for such capability continues to shrink. Since heat transfer systems (i.e., heat exchangers) are a critical element of power management, this capability must also improve. However, advancement of heat exchanger technology is not keeping up with demand. 
     At least three methods have been used to increase the rate of heat removal from heat exchangers. One method is simply to physically increase or scale the size of the heat exchanger to increase its wall surface area. Another method is to use materials that have greater thermal conductivity to allow more heat to pass through the heat exchanger for a given temperature difference. A third method characterizes the state-of-the-art, which is to use micro channel technology to configure a given volume of heat exchanger with more surface area without increasing its overall size. Although each of these solutions is workable, an improved solution that overcomes the limitations of the prior art and which meets the current and future design challenges is needed. 
     SUMMARY OF THE INVENTION 
     Embodiments of a system, method, and apparatus for rapidly enhancing the rate of heat transfer and removal in a heat exchanger used to manage thermal loads are disclosed. The invention increases the rate of heat transfer with a pulsed-jet-based active flowfield control actuation system that boosts the convective heat transfer coefficient. The pulsed-jet actuators are used to impart an unsteady or pulsatile component of velocity to the working fluid of the heat exchanger. This design transiently increases (1) the wall spatial velocity gradient ΔU/ΔY, and (2) the temporal velocity gradient ΔU/Δt, both of which are related to the rate of convective heat transfer. Methods such as those disclosed herein that impose transients to the working fluid increase the rate of heat transfer. This invention is suitable for many different applications including, for example, air vehicles, automobiles, missiles, electronics, HVAC, commercial, etc. 
     The pulsed-jet heat exchanger increases the convective heat transfer efficiency (i.e., related to thermal efficiency) without the need to increase the heat exchanger surface area design parameter which can increase fluid pressure loss, as well as heat exchanger volume and weight. In addition, pulsed-jet heat exchangers increase the rate of both impingement-based and tangential components of convective cooling. For example, in one embodiment, the average heat transfer coefficient in a heat sink is 2.5 times greater with a pulsed-jet actuation source than with a steady flow at the same Reynolds number. The invention enables designers such as systems integrators to utilize more substantial heat management capability in a given weight or volume. Conversely, the invention significantly reduces the weight and volume required for a given heat exchanger capacity. 
     The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the present invention, taken in conjunction with the appended claims and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features and advantages of the present invention, which will become apparent, are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof that are illustrated in the appended drawings which form a part of this specification. It is to be noted, however, that the drawings illustrate only some embodiments of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
         FIG. 1  is an isometric view of one embodiment of an aircraft constructed in accordance with the invention; 
         FIG. 2  is a schematic sectional view of one embodiment of a heat management system for the aircraft of  FIG. 1  and is constructed in accordance with the invention; 
         FIGS. 3A and 3B  are sectional views of one type of pulsed-jet actuator utilized by the system of  FIG. 2 ; 
         FIGS. 4A and 4B  are sectional views of another type of pulsed-jet actuator utilized by the system of  FIG. 2 ; 
         FIG. 5  is a sectional view of one embodiment of a pulsed ejector strut and pulsed-jet actuator constructed in accordance with the invention; 
         FIG. 6  is a sectional view of another embodiment of struts and pulsed-jet actuator constructed in accordance with the invention; and 
         FIG. 7  is an isometric view of an embodiment of a heat exchanger configuration of pulsed-jets constructed in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1-7 , embodiments of a system, method and apparatus for managing thermal loads are disclosed. The invention is well suited for use in conjunction with heat exchangers to improve the performance thereof. For example, as shown in  FIG. 1 , the invention may be incorporated into other systems, such as an aircraft  11 . In the embodiment shown, aircraft  11  is provided with an inlet  13  and an outlet  15 , such as a flush screened exit. 
     Referring now to  FIG. 2 , a passage  21  is formed in the aircraft  11  for directing a first working fluid  23  (e.g., ram air) therethrough. The passage  21  of the illustrated embodiment extends between the ram air inlet  13  for ingress of the first working fluid  23  and the flush screened exit  15  for egress of the first working fluid  23 . 
     A heat exchanger  25  is mounted in the passage  21  as shown, and may be configured such that all of the flow of first working fluid  23  passes therethrough. The heat exchanger  25  has a conduit  27  (see, e.g.,  FIG. 6 ) with an inlet port  29  for communicating a second working fluid  31  (e.g., liquid jet fuel) into the heat exchanger  25 . An outlet port  33  communicates the second working fluid  31  out of the heat exchanger  25 . A fluid conducting path (e.g., conduit  27 ) extends from the inlet port  29  to the outlet port  33  for directing the second working fluid  31  through the heat exchanger  25 . The heat exchanger also may comprise a plurality of fins  35  (see, e.g.,  FIGS. 6 and 7 ) for transferring heat from the conduit  27 . The fins  35  may be configured to align in a direction of flow of the first working fluid  23 . 
     Referring now to  FIGS. 2-7 , the invention also comprises various embodiments of pulsed excitation devices, such as zero mass flux jet actuators or pulsed-jet arrays. For example, as shown in  FIG. 2  an array  41  of pulsed-jets  43  may be mounted in the passage  23  upstream from the heat exchanger  25 . In addition, the pulsed-jet arrays  41  may be mounted to an inlet surface (see, e.g.,  FIG. 7 ) upstream of the heat exchanger  25 , or in an interior thereof (see, e.g.,  FIG. 6 ) such as between the conduit  27  and fins  35 . In another embodiment, an array  42  of pulsed-jets  44  may be mounted to a sidewall  46  of the passage  21 . 
     Each pulsed-jet actuator array may comprise a plurality of actuators  43  (see, e.g.,  FIGS. 3-6 ) for producing pulsed-jets  45  of the first working fluid that impart an unsteady component of velocity to the first working fluid. The pulsed-jets  45  may be directed at the heat exchanger  25  for rapidly enhancing a rate of heat transfer and removal in the heat exchanger. 
     As shown in  FIGS. 3A ,  4 A, and  6 , the pulsed-jets  45   a  may be emitted in a same direction of flow as the first working fluid  23 . Alternatively (see, e.g.,  FIGS. 3B ,  4 B, and  6 ), the pulsed-jets  45   b  may be emitted in a direction that is perpendicular to a direction of flow of the first working fluid  23 . Each of the pulsed-jets  43  may comprise an enclosure  51 , an orifice  53  and a vibrating diaphragm  55  for moving the first working fluid into and out of the orifice  53  to produce the pulsed-jets  45 . Alternatively ( FIGS. 4A and 4B ), the pulsed-jets maybe provided with enclosures  51 , orifices  53  and electrodes  57  between which pass electric arcs  59  for forming the pulsed-jets of fluid. 
     The pulsed-jet arrays comprise active flowfield control actuation systems that increase a convective heat transfer coefficient of the heat exchanger  25 . The pulsed-jet arrays transiently increase a wall spatial velocity gradient ΔU/ΔY of either or both of the working fluids, and a temporal velocity gradient ΔU/Δt of either or both of the working fluids. In one embodiment, the pulsed-jet arrays transiently increase a wall spatial velocity gradient ΔU/ΔY of the first working fluids, and a temporal velocity gradient ΔU/Δt of the first working fluid. 
     Referring now to  FIGS. 5 and 6 , the pulsed-jets  43  also may be mounted in struts  61  (e.g., aerodynamic members) that are located in the passage  21  to enhance fluid flow therethrough. As shown in  FIG. 5 , at least some of struts  61  may be configured as ejectors having inner strut housings  61  that contain the pulsed-jets  43 , and outer strut housings  63  that are spaced apart from the inner strut housings  61  for permitting flow of the first working fluid  23  between the inner and outer strut housings  61 ,  63  to the pulsed-jets  43 . In one embodiment, the pulsed-jets may comprise orifices having an opening size (e.g., diameter) of 0.1 to 1.0 inches. 
     While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.