Abstract:
Walls that require cooling, including combustor walls of jet engines, having two members forming a waffle shaped structure are described. The dual member structure comprises passages for incoming air, which zigzag through part of the waffle structure. The air is then released to the combustor or exhausted elsewhere. Additionally, the need for a cool film on one side of the wall may be eliminated. The waffle shaped structure disclosed is relative easy to manufacture.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the cooling of thin walls for various aerospace applications and, more specifically, this invention relates to the cooling of thin walls using a waffle structure. 
     Various devices such as combustor liners, other exhaust structures or similar structures, and walls of rocket engine, need cooling. The air used for air/fuel mixtures can be used advantageously for that purpose. It is known that efficient burning of fuel is a goal for good turbine engine designs but the burning of fuel necessarily involves heat generation. Further, the burning of fuel also necessarily causes environmental problems necessitating the reduction of dangerous emissions. How to dissipate the generated heat and how to use the cooling air effectively becomes an important design issue. In addition, the difference in temperature in various parts of the thin walls challenges designers in that they need to consider both the efficiency of fuel burning and the structural integrity of the thin walls. Structural design of thin walls such as those for combustion liners is generally based on experience with previous systems. Various patents have addressed the issue. 
     U.S. Pat. No. 4,642,993, entitled Combustor Liner Wall, teaches a combustor liner wall with an interior wall, an exterior wall, and a honeycomb structure disposed therebetween. The honeycomb structure is formed with generally radially aligned cells. The patent teaches a structure known in the art as licolite. However, it does not address the issue of thermal growth differences between the hot and cold walls of a panel structure, which may result in high stresses for even moderately sized combustors. To solve the above problem, the walls are divided into small panels where spent cooling air is dumped. This may result in release of dangerous emissions. 
     U.S. Pat. No. 4,864,827, entitled Combustor, teaches a gas turbine engine combustor with a semi-spherical upstream wall comprised of two correspondingly shaped skins spaced apart by pedestals attached to one of the skins to define a space between them. In other words, this patent maintains a space between two walls by the use of pedestals. The two walls float, or move, relative to each other so as to relieve the geometrical changes that result from temperature variations. However, this does not allow a stable gap between the two walls. Further, for relatively large structures this patent may be impractical in that no stable gap can be maintained because the pedestals are not rigidly affixed to at least one of the walls. 
     U.S. Pat. No. 5,822,853, entitled Method for Making Cylindrical Structures with Cooling Channels, teaches a gas turbine having a double wall with a plurality of cooling channels therebetween. The cooling channels are formed between the inner member of the structure and the outer member thereof. In other words, the patent addresses a method of manufacturing, involving a sandwiched panel. However, the walls are not rigidly connected to each other and, therefore, the problem of thermal growth difference between the two walls is not resolved. Also, as can be appreciated, the fact that there is no rigid connection between the walls causes at least a reduction in efficiency with regard to heat transfer. Further, the lack of connection between the walls may make the concept impractical for larger structures. 
     As can be seen, there is a need for a method and apparatus to deal with thermal growth differences occurring between adjacently located hot and cold walls of panel structures, and which does not result in high stresses during operation. Further, it is desirable to have a simple structure that is easy to fabricate, and at the same time fulfills the need of cooling the apparatus. The waffle cooling satisfies that need. The second wall is an irregular non smooth surface that responds somewhat like a bellows, and expands and contracts with respect to the thermal expansion of the first surface thus reducing the stresses that are generated by the difference in temperatures between the two surfaces. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, there is disclosed an apparatus for cooling thin walls that comprises a first member, which has a smooth surface; a second member, which has an uneven surface disposed to form a plurality of cavities by rigidly affixing part of the uneven surface onto the smooth surface of the first member, and a plurality of openings connecting at least two cavities; and at least one opening in a first cavity for receiving incoming air, as well as at least one opening in a second cavity for disposing out-going air, whereby the temperature, as well as the temperature difference between the members, is reduced. 
     In another aspect of the present invention, there is disclosed an apparatus for cooling thin walls that comprises a first wall having a generally smooth shape with a first surface, and a second surface; a second wall having a waffle shape, wherein a plurality of cavities are formed, each cavity being defined by four sides with part of the cavity sides rigidly affixed to the first surface of the first wall; and a plurality of gaps formed at the sides of the cavities that are non-rigidly affixed to the first surface of the first wall, thereby forming a passage for air to flow from one end of the apparatus to the other through at least some cavities, whereby the first wall and the second wall are cooled. 
     In a further aspect of the present invention, a combustor is described comprising an inflow of air having a primary flow into an inner cylinder, and a secondary flow into an air casing defined and limited by the inner cylinder and an outer cylinder. The combustor further comprises a first wall that is part of the outer limit of the inner cylinder, which has a generally smooth shape, and which includes a first surface, and a second surface. A second wall is also provided that is part of the outer limit of the inner cylinder, which has a waffle shape within which there is a plurality of waffle shaped cavities. Further, each cavity is defined by four sides with part of the cavity sides being rigidly affixed to the first surface of the first wall, whereby a plurality of gaps is formed at the sides of the cavities that are non-rigidly affixed to the first surface of the first wall, thus forming a passage for air to flow from one end of the apparatus to the other end through at least some cavities. Thereby, the first wall and the second wall are cooled. 
     In yet another aspect of the present invention, a method for making a cooling device comprises the steps of providing a first member that has a generally smooth surface, providing a second member having a waffle shape with a plurality of trapezoidal shaped cavities each having four sides, and affixing at least some of the four sides onto the first member. 
    
    
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a schematic diagram of one embodiment of the present invention; 
     FIG. 2 depicts a perspective view of the present invention; 
     FIG. 3 depicts a schematic flow of a combustion chamber; and 
     FIG. 4 depicts an embodiment of the present invention for a can-type combustor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
     Referring to FIG. 1, numeral  100  denotes a waffle cooling apparatus embodying the present invention. A first or inner member  102  may be generally smooth in that it has at least one smooth (i.e., even or planar) surface made of flexible material, such as HA230 or HS188 made by Haynes International, such that the inner member  102  can be bent (not shown) to form a curve along with a second or outer member  103 . The outer member  103  may also be made of generally flexible material, such as HA230, HS188 or HastX made by Haynes International, but is not smooth (i.e., not even or planar) in form or shape. For example, the outer member  103  may have indentations such that a set of cavities can be formed utilizing the concave volume generated by the indentations. Thus, air can pass between the inner member  102  and the outer member  103 . 
     The outer member  103  can be formed by a plurality of airflow units of which at least one allows an airflow  114  therethrough. Each unit can comprise a plurality of cavities, with one representative cavity being intermediate cavity  104 . Intermediate cavity  104  may generally have four sides for ease of fabrication. However, intermediate cavity  104  may have a lesser or greater number of sides. Some considerations in determining the number of sides are ease of fabrication, and rigidity of the connection between the inner member  102  and the outer member  103  for efficient heat transfer between them. The intermediate cavity  104  can further comprise or define an air gap  106  such as for air intake into such cavity  104  when air flows out of one cavity and then flows into an adjacent cavity  104 . The intake gap  106  may be an opening having a circumference (not shown) formed by portions of the inner member  102  and the outer member  103 . 
     Intermediate cavity  104  can also have an air opening or gap  108  such as for air outflow from the cavity  104  when air flows out of one cavity  104  and into an adjacent downstream cavity  104 . Similarly arranged cavities can thereby define at least part of an air passage which will be described below. The shape and circumference of the air outflow gap  108  can be identical to that of the intake gap  106 , or it could be different. But the particular shape, size, and circumference of the openings or gaps  106  and  108  are based on the goal of efficient cooling of the walls or the inner and outer members  102 ,  103 , thus precluding undue stresses on the inner member  102  since a heat source (not shown) may cause the inner member  102  to be at a relatively higher temperature than that of the outer member  103 . It is evident that one wall may be cooler than the other and it is desirous to have a suitable structure that reduces this temperature difference as well as cools the inner and outer members  102 ,  103 . 
     Still referring to FIG. 1, and for purposes of illustration, incoming air  110  can form an initial airflow through an initial inflow gap  112 . Incoming air  110  can define an airflow  114  that traverses a plurality of cavities  104 . In other words, the cavities are interconnected such that the airflow  114  may traverse through the cavities via gaps on the sides of cavities such as at the intake gap  106  and air outflow gap  108 . Desirably, the airflow  114  follows a tortuous path as opposed to a straight one for greater cooling. 
     Outer member  103  may be subdivided into a plurality of independent airflow units. In each unit, an air passage can exist through which an airflow  114  can pass. For example, air passages may zigzag through units of similar shape and form as that of air passage  104 . Other than the gaps  106  and  108 , the sides of the cavities on the outer member  103  can be rigidly attached to inner member  102 . The inner and outer members  102  and  103  may be made of iron, aluminum, nickel, cobalt or related alloys. The inner and outer members  102  and  103  may be bonded to each other using bonding techniques know in the art and made of iron, aluminum, nickel, cobalt, or related alloys. A primary goal is to have a predetermined amount of air flowing through a particular area of the apparatus  100  that is comprised of inner member  102  and outer member  103 . Once the inner and outer members  102  and  103  are bonded to each other, the combined structure may be formed to achieve a desired shape. The bonding process increases the thermal unity between the inner member  102  and the outer member  103  because heat transfers more readily between the members. This is primarily because there are no other elements or materials existing between the inner member  102  and the outer member  103 . 
     Another feature of the invention is that air passage  104  may be adjusted in size and shape to achieve an optimal cooling effect. For example, the zigzag pattern of air passage  104  may be modified at different portions of the structure to address heat intensity variations. 
     Referring to FIG. 2, numeral  200  generally depicts a perspective view of the structure depicted in FIG.  1 . First or inner member  202  can comprise an inner surface  204  and an outer surface  206 . Generally, a heat source (not shown) may be located at the side of the inner surface  204 , Thereby, heat may be dissipated geometrically first to the inner surface  204 , then to the outer surface  206  of the inner member  202 , and then to a second or outer member  208 . A plurality of cavities, such as cavities  210 , can form part of an interconnection path (not shown) wherein an air passageway (not shown) is defined. It is contemplated that a plurality of interconnection paths may be formed. 
     Incoming air (depicted by arrows  212 ,  214  and  216 ) can flow through respective openings or gaps in the cavities  210 . For example, incoming air  212  may flow into gap  218 , incoming air  214  may flow into gap  220 , and incoming air  216  may flow into gap  222 . 
     The sides  224  of cavities  210  may be rigidly affixed to the outer surface  206 . For instance, the sides  224  of cavities  210  can be made out of the same material as the outer member  208 , and may be bonded onto the outer surface  206 , which may be made out of similar materials. As can be appreciated, some cavities  210  may be interconnected to each other, whereby an interconnect (not shown) is formed as a subset of all the cavities. This may be demonstrated by referring back to FIG. 1, wherein the outer member  103  comprises a plurality of independent zigzag sub-units or subsets. This necessarily requires openings on some sides of some cavities  210 . Hence, the opening portions (not shown) in between cavities of the sides  224  of cavities  210  are not rigidly affixed upon the outer surface  206 . This is also true for the inflow gaps  218 ,  220 ,  222  since the members are not rigidly affixed to each other as there is an opening in between. However, other than the inflow gaps  218 ,  220 , and  222 , and the openings on the sides (not shown), the sides may be affixed rigidly onto a smooth surface of the first member or the outer surface  206 . 
     The instant invention is described as waffle cooling because the cavities  210 , and the sides  224  of cavities  210 , together form a structure that is similar in configuration to a waffle, and its purpose is for cooling. However, the sides  224  of cavities  210  do not have to be equal in length, and the angle between the adjacent sides does not have to be ninety degrees or the same angle. In addition, since the finished waffle shaped structure may need to be bent in the shape of a curve, the resultant shape of the waffle cooling pattern may be trapezoidal, albeit on a non-flat plane. 
     As can be appreciated, the outer member  208  may comprise a plurality of units such the independent zigzag airflow patterns. Each unit can further comprise an interconnect wherein an air passage (not shown), such as air passage  104  of FIG. 1, is defined. Each unit may be independent in that no connections exist between units or, alternatively, each unit may be dependent and connected to each other. It is evident that more than one unit may exist in the apparatus. 
     An air outlet  226  can be provided at the end of each airflow passage  104 . The air inflow can be any one of the air inflows  212 ,  214 ,  216 . The function of this air outlet  226  may be to serve as an outlet for cooling air, as well as other purposes. For example, with a combustor, expended cooling air coming out of air outlet  226  may mix with fuel in a primary zone and be utilized for combustion purposes. 
     Since one the main purposes of the invention is cooling, it follows that the volume of air passing through any interconnect can increase, or reduce the cooling. A working definition of the interconnect is the space wherein each subset of cavities  210  is interconnected by openings. Therefore, the geometric configuration of the interconnect can determine the amount of airflow that passes through it. The geometric configuration includes, but is not limited to, the size of the inflow gaps  218 ,  220 ,  222 , and other openings (not shown) between cavities  210 . In addition, heat conduction characteristics of the material constituting the apparatus may also be significant. 
     FIGS. 1 and 2 illustrate only one embodiment of the instant invention. Variations in the heat intensity of various regions of the inventive waffle cooling apparatus may vary. It follows, therefore, that the amount of air passing through any interconnect can be increased or decreased depending on the extent of the cooling required. As a result, it may be desirable to modify some of the dimensional parameters of the inventive waffle cooling apparatus. Examples include increasing the opening of the initial airflow gap  112 , the incoming air flows  212 , the interconnecting intake gap  106 , and the air outflow gap  108 . 
     Referring now to FIG. 3, there is shown a schematic flow  300  of a combustion chamber wherein the instant invention can be used. A compressor (not shown) can discharge airflow at a velocity of about 490 ft/sec for purposes of illustration. This velocity may not be suitable for combustion and must be reduced. Air flowing into the combustion chamber can comprise primary airflow  302  and secondary airflow  304 . Upon reaching a suitable velocity, with a concomitant increase in pressure as well, the primary airflow  302  can enter the combustion chamber. The secondary airflow  304  may then flow into air casing  306  defined as the space between an inner cylinder having a radius r and an outer cylinder having a radius R. On the inner cylinder wall  308 , the structure of the instant invention may be applied. For example, the present invention may be applied at areas  310 ,  312 , and  314  which are located on regions  316 ,  318 , and  320 , respectively. It is known in the art that the primary airflow  302  contributes about 10-20% of the total air mass used for combustion. The balance comes from other sources including the secondary airflow  304 . Therefore, the waffle structure of this invention may be used for the combustion chamber at areas  310 ,  312 ,  314  that are located on regions  316 ,  318 , and  320  respectively. As can be appreciated, the secondary airflow  304  can be used both for combustion as well as for cooling purposes. 
     A typical combustion chamber receives 20% of its air from the primary airflow  302 , predominantly within region  316 . The balance of the air comes from the secondary airflow  304 , from which 10% goes into region  318  and about 70-80% goes into region  320 . Because different regions require different amounts of air, and there may be variations of pressure at various locations within the air casing  306 , the opening that takes the air in, for example areas  310 ,  312 ,  314  having the waffle structure for air intake and the opening that disposes of the air, needs to be suitably designed. For example, the dimension or specifically the diameter of the openings such as the inflow gaps  218 ,  220 ,  222 , the direction of the openings, and the size of the interconnect, need to be designed accordingly for optimal performance including sufficient air for sufficient cooling. The design must take into consideration various factors such as: sufficient air be fed into the combustion chamber at various locations or regions; air passing through be sufficient such that the walls  308  of the inner cylinder such as  204  and  206  of FIG. 2 be adequately cooled; the apparatus used be easy to fabricate; and the structure of the apparatus be sufficient for a limited air intake. Certain locations of the combustion chamber cylindrical wall  308  may need no cooling, and would not benefit from the use of the present invention. The present invention may accordingly be selectively applied on an as needed basis at locations where other forms of cooling are insufficient. 
     Referring now to FIG. 4, there is shown a can-type combustor  400 , wherein the present invention may be applied. An atomizer swirler assembly  402 , having a generally cylindrical shape, is located at a first end  408  of the can-type combustor  400 . The atomizer swirler assembly  402  comprises a coupling element  404  that circumferentially couples with a wall portion  406  that forms part of an inner cylindrical wall. The atomizer swirler assembly  402  typically receives primary airflow at a first end  408  at a high velocity, processes the air, and inputs the same into an inner cylindrical space  410  at a reduced velocity. Wall portion  406  extends to first member region  412 , wherein the inventive waffle cooling structure is desirable. 
     The waffle cooling structure comprises a second member  414  that has concave regions  413  disposed to form a plurality of cavities in combination with the outer smooth surface the first member  412 . It should be noted that the present invention is an improvement over prior art in that there is no need for any other structural component other than the first member  412  and the second member  414 . For example, there is no need for a third element between the first member  412  and the second member  414 . Air  416 ,  418  from secondary air source enters the waffle cooling structure via slots  422  and  420  respectively. 
     Air  416  enters slot  422  defining the beginning of a first passage  424  that traverses an interconnect comprised of a subset of a plurality of cavities interconnected together, slot  422 , and first passage  424 . First passage  424  comprises an opening  426  on wall portion  406  and an outlet channel  427 . Outlet channel  427  is limited by wall portion  406  and an extension member  428 . Expended air cools the first member  412  (and to some extent cools the second member  414 ) and comes out of the outlet channel  427 . The expended air, in turn, mixes with fuel in the primary zone of the inner cylindrical space  410 . At the end of the combustor, turbine transition duct  434  of a generally cylindrical shape is coupled to the end  436  of the first member  412 . 
     Similarly, air  418  enters slot  420  that defines a beginning of a second passage  425  that traverses an interconnect comprised of a subset of cavities interconnected together, slot  420 , and outlet  430 . The outlet  430  is comprised of an opening  432  on first member  412 . Expended air cools the first member  412 . The expended air also, to some extent, cools the second member  414 . Further, expended air comes out of the outlet  430  at the combustor exit segment. 
     Similar inventive waffle cooling structures may be provided at other locations or regions of the combustor where cooling is desired. The specific geometric configurations of such additional waffle cooling structures may be varied as necessary to provide the requisite cooling characteristics. 
     As can be appreciated, the present invention provides cooling to thin walls (e.g., about 0.015 to 0.100 inches thick) of structures such as, but without limitation, combustor liners. Cooling passages are formed by a subset of cavities between two bonded pieces of sheet metal. The first piece comprises at least one smooth surface and forms the structural wall that is to be cooled. The second piece is waffled to form cavities on at least one smooth surface. The cavities are interconnected with gaps to form a passage for air to zigzag from a first end to a second end of a panel as needed. In the present form of the invention, the narrow gap between the cavities accelerates the air, increases the turbulence, correspondingly increasing the heat transfer co-efficient. Air enters the passageways through gaps at one end of the passages and exits to the hot flow path at the other end. It should be noted that there may be one opening at the exit of each passage such as  426  or  430 , but may merge into a common slot such as  427 . 
     One notable feature of the present invention is that this type of cooling may allow walls such as the first member  412  of FIG. 4 to be cooled without a cold film on the hot side of the combustor. This may reduce hydrocarbon and carbon monoxide emissions. As can be appreciated, cool boundary layers along the walls are known to have the tendency to quench the combustion reaction, thus producing by-products of incomplete combustion such as CO and UHC. However, as an alternative, the cold film may be introduced in such cases to further enhance cooling, or if emissions are not of primary concern. 
     As a general matter, with regard to the waffle cooling of the present invention in relation to other types of cooling, the amount of air used should be comparable to that of effusion cooling. However, the need for the introduction of the cold film on the inner side of the wall or first member  412  may not be required. Furthermore, the first member generally a thin wall having a limited thickness of about 015 to 100 inches. 
     It should be understood, of course, that the foregoing relates to preferred embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims. For example, the present invention can be applied not only in a can-type combustor, but in an annular-type combustor and can-annular-type combustor as well. Furthermore, other exhaust system structures may benefit from use of this invention as well.