Patent Publication Number: US-11035250-B2

Title: Gas turbine engine fluid cooling systems and methods of assembling the same

Description:
BACKGROUND 
     The application described herein relates generally to gas turbine engines, and more specifically to fluid cooling systems for gas turbine engines. 
     Gas turbine engines typically include an inlet, a fan, low and high pressure compressors, a combustor, and at least one turbine. The compressors compress air which is channeled to the combustor where it is mixed with fuel. The mixture is then ignited for generating hot combustion gases. The combustion gases are channeled to the turbine(s) which extracts energy from the combustion gases for powering the compressor(s), as well as producing useful work to propel an aircraft in flight or to power a load, such as an electrical generator. 
     During engine operation, significant heat is produced which raises the temperature of engine systems to unacceptable levels. Various lubrication systems are utilized to facilitate lubricating components within the gas turbine engine. The lubrication systems are configured to channel lubrication fluid to various bearing assemblies within the gas turbine engine and to at least one external generator. During operation, heat generated by components like bearings and seals within the engine and generator is transmitted to the lubrication fluid. To facilitate reducing the operational temperature of the lubrication fluid, at least one known gas turbine engine utilizes one of surface coolers or brick coolers to cool the fluid circulating within. 
     At least some known turbine engines include surface coolers that are designed and sized to cool engine fluid during various predetermined operating conditions. Specifically, when used in an aircraft engine, at least some surface coolers are engineered to operate during standard day, hot day, and extreme hot day operation to ensure proper cooling of the engine fluid. Generally, the hotter the temperature of the engine fluid, the larger the surface cooler must be to reduce the temperature of the engine fluid. As such, extreme hot day conditions require a relatively large surface cooler as compared to the size of the surface needed for standard day or hot day operation. Because the size of the surface cooler is not variable, at least some known turbine engines include surface coolers sized to reduce engine fluid temperature to a predetermined temperature during extreme hot day conditions. 
     However, only approximately 0.1% of the turbine engine operating time occurs during extreme hot day conditions. Accordingly, at least some known turbine engines include surface coolers that are larger than actually required a vast majority of the time. Such overcapacity increases the weight of the aircraft and, therefore, reduces the fuel efficiency. In addition, the larger than necessary surface coolers occupy space within the turbine engine that may be used for a different purpose. 
     BRIEF DESCRIPTION 
     In one aspect, a fluid cooling system for use in a gas turbine engine including a core gas turbine engine having an axis of rotation and a fan casing substantially circumscribing the core gas turbine engine is provided. The fluid cooling system includes a heat source configured to transfer heat to a heat transfer fluid and a primary heat exchanger coupled in flow communication with the heat source. The primary heat exchanger is configured to channel the heat transfer fluid therethrough and is coupled to the fan casing. The fluid cooling system also includes a secondary heat exchanger coupled in flow communication with the primary heat exchanger. The secondary heat exchanger is configured to channel the heat transfer fluid therethrough and is coupled to the core gas turbine engine. The fluid cooling system also includes a bypass mechanism coupled in flow communication with the secondary heat exchanger. The bypass mechanism is selectively moveable based on a temperature of a fluid medium to control cooling airflow through the secondary heat exchanger. 
     In another aspect, a method operating a gas turbine engine is provided. The method includes transferring heat from a heat source to a heat transfer fluid and channeling the heat transfer fluid through a primary heat exchanger coupled in flow communication with the heat source. The method also includes determining a temperature of a fluid medium and controlling a bypass mechanism to selectively position a secondary heat exchanger in a cooling airflow based on the temperature of the fluid medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an exemplary gas turbine engine. 
         FIG. 2  is a schematic illustration of exemplary engine fluid cooling system that may be utilized with the gas turbine engine shown in  FIG. 1 . 
         FIG. 3  is an enlarged view of a portion of gas turbine engine shown in  FIG. 1  illustrating an exemplary brick cooler mechanism. 
         FIG. 4  is a schematic illustration of an alternative engine fluid cooling system that may be utilized with gas turbine engine shown in  FIG. 1 . 
         FIG. 5  is an enlarged view of a portion of gas turbine engine shown in  FIG. 1  illustrating an alternative brick cooler mechanism. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to present embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. The detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of the invention. 
     The singular forms “a”, “an”, and “the” include plural references unless the context clearly dictates otherwise. 
     “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. 
     Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, “approximately”, and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged; such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. 
     As used herein, the terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components. The terms “upstream” and “downstream” refer to the relative direction with respect to fluid flow in a fluid pathway. For example, “upstream” refers to the direction from which the fluid flows, and “downstream” refers to the direction to which the fluid flows. 
     As used herein, the terms “axial” and “axially” refer to directions and orientations that extends substantially parallel to a centerline of the turbine engine. Moreover, the terms “radial” and “radially” refer to directions and orientations that extends substantially perpendicular to the centerline of the turbine engine. 
     The exemplary systems and methods described herein relate to oil cooling systems that overcome at least some disadvantages of known systems and methods for cooling lubrication oil circulating through a gas turbine engine. Moreover, the systems and methods described herein include both a surface cooler and a brick cooler that cooperate to cool the lubrication oil. More specifically, the surface cooler described herein is designed to satisfy the cooling requirements of the lubrication fluid during engine operating conditions up to and including hot day conditions. As such, when the operating condition is determined to be an extreme hot day condition, further cooling of the lubrication oil is required. The brick cooler is positioned in the undercowl region of the engine and is activated upon a determination that the current operating condition of the engine is extreme hot day conditions. More specifically, the bypass mechanism is selectively moveable based on the temperature of a fluid medium to control the cooling airflow through the brick cooler. 
     Advantages of the oil cooling system and methods described herein include the reduction in size of the surface cooler leads to a reduction in overall weight of the engine. Furthermore, the smaller surface cooler has a smaller footprint in the engine and interacts with a correspondingly smaller amount of the inlet air. As such, not only does the smaller surface cooler allow more space within the engine for additional components, but the pressure losses of the fan air is reduced due to less air interacting with the smaller surface cooler. The reductions in weight, and also the reduction in pressure losses, result in an improved specific fuel consumption rate, which increases the efficiency of the engine and reduces overall operating costs. 
     Embodiments disclosed herein relate to surface coolers and more particularly to enhanced surface coolers for use in a nacelle of an engine such as an aircraft engine. The exemplary surface coolers may be used for providing efficient cooling. Further, the term “surface coolers” as used herein may be used interchangeably with the term “heat exchangers”. As used herein, the surface coolers are applicable to various types of turbomachinery applications such as, but not limited to, turbojets, turbo fans, turbo propulsion engines, aircraft engines, gas turbines, steam turbines, wind turbines, and water turbines. In addition, as used herein, singular forms such as “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. 
       FIG. 1  is a schematic illustration of an exemplary gas turbine engine assembly  10  having a longitudinal axis  11 . Gas turbine engine assembly  10  includes a fan assembly  12 , and a core gas turbine engine  13 . Core gas turbine engine  13  includes a high pressure compressor  14 , a combustor  16 , and a high pressure turbine  18 . In the exemplary embodiment, gas turbine engine assembly  10  may also include a low pressure turbine  20 . Fan assembly  12  includes an array of fan blades  24  extending radially outward from a rotor disk  26 . Engine  10  has an intake side  28  and an exhaust side  30 . Gas turbine engine assembly  10  also includes a plurality of bearing assemblies (not shown in  FIG. 1 ) that are utilized to provide rotational and axial support to fan assembly  12 , compressor  14 , high pressure turbine  18  and low pressure turbine  20 , for example. 
     In operation, air flows through fan assembly  12  and is split by an airflow splitter  44  into a first portion  50  and a second portion  52 . First portion  50  of the airflow is channeled through compressor  14  wherein the airflow is further compressed and delivered to combustor  16 . Hot products of combustion (not shown in  FIG. 1 ) from combustor  16  are utilized to drive turbines  18  and  20  and thus produce engine thrust. Gas turbine engine assembly  10  also includes a bypass duct  40  that is utilized to bypass a second portion  52  of the airflow discharged from fan assembly  12  around core gas turbine engine  13 . More specifically, bypass duct  40  extends between an inner wall  43  of a fan casing or shroud  42  and an outer wall  45  of splitter  44 . 
       FIG. 2  is a schematic illustration of exemplary engine fluid cooling system  100  that may be utilized with gas turbine engine  10  (shown in  FIG. 1 ). In the exemplary embodiment, fluid cooling system  100  includes a heat source  102 , a fuel cooled air-oil cooler (FCOC) heat exchanger  104 , a primary air-oil cooler (ACOC) heat exchanger  106 , and a secondary air-oil cooler (ACOC) heat exchanger  108  all coupled in flow communication to each other via an engine fluid conduit  110 . Heat source  102  includes core gas turbine engine  13 , which transfers heat to an engine fluid, such as lubrication oil. More specifically, heat source  102  includes any of a generator, a gearbox, a pump, or bearing within core gas turbine engine  13  that generates heat during operation and requires lubrication oil for cooling purposes. Alternatively, heat source  102  in system  100  includes any component of engine  10  that facilitates operation of system  100  as described herein. The engine lubricating fluid flows though system  100  via conduit  110  and returns to heat source  102 . The temperature of the lubrication oil is reduced as it flows through heat exchangers  104 ,  106 , and  108 . 
     Although cooling system  100  is described herein to cool lubrication fluid for engine  10 , it may alternatively or simultaneously cool other fluids. For example, it may cool a fluid used to extract heat from actuators used on the engine. It may also be used to cool fluids which extract heat from electronic apparatus such as engine controls. In addition to cooling a wide variety of fluids utilized by a gas turbine engine assembly, it should be realized that cooling system  100 , and the methods described herein illustrate that cooling system  100  may also cool an apparatus that is mounted on the airframe, and not part of the engine. In other applications, cooling system  100  may be mounted remotely from gas turbine engine  10 , for example on an external surface of the aircraft. 
     In the exemplary embodiment, fuel cooled heat exchanger  104  coupled in flow communication with heat source  102 . Heat exchanger  104  includes a circuit of relatively cool fuel flowing therethrough that reduces the temperature of the lubrication oil as it also flows through heat exchanger  104 . 
     In the exemplary embodiment, primary heat exchanger  106  includes an air cooled surface cooler positioned within a recess formed in inner surface  43  of fan casing  42 . Surface cooler  106  includes a plurality of fins  107  that are thermally coupled to receive thermal energy from the oil and transfer the energy to another medium such as air. More specifically, surface cooler  106  is flush mounted to fan casing  42  such that fins  107  extend into an inlet airflow  48  within intake side  28  of engine  10 . Surface cooler  106  transfers the thermal energy from the oil into inlet airflow  48  that removes the heat from the oil. The cooled oil is channeled from surface cooler  106  through conduit  110  to secondary heat exchanger  108  for additional cooling if required, as described in further detail below. 
     As shown in  FIG. 1 , surface cooler  106  is coupled to inner wall  43  of fan casing  42  downstream from fan assembly  12 , such that air channeled into intake side  28  is first channeled through fan assembly  12  prior to being supplied to surface cooler  106  aft of outlet guide vanes  150  to facilitate reducing the operating temperature of the engine fluids channeled through surface cooler  106 . Generally, surface cooler  106  is positioned anywhere along the axial length of inner wall  43  of fan casing  42 , or along radially outer surface  45  of splitter  44  within bypass duct  40 . In the exemplary embodiment, efficiency is increased when surface cooler  106  is positioned adjacent engine intake side  28 , where a diameter of fan assembly  12  is largest. 
     Typically, air-oil coolers are sized at extreme hot day conditions where the ambient temperature is approximately 131.degree. F., as shown in Table 1. In the exemplary embodiment, surface cooler  106  is smaller in size than conventional surface coolers that are sized for extreme hot day conditions. More specifically, surface cooler  106  is sized for operation up to and including standard day operation or up to and including hot day operating conditions based on a temperature of a fluid medium as defined in Table 1 below. Although Table 1 describes the fluid medium as an ambient airflow, in other embodiments, the fluid medium may be the lubrication oil itself. As such, smaller size surface cooler  106  satisfies cooling requirements of the lubrication oil during operating conditions up to and including either standard day operation or hot day operation. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Operating Condition 
                 Ambient Temp 
               
               
                   
                   
               
             
            
               
                   
                 Extreme Hot Day 
                 131° F. (55° C.)  
               
               
                   
                 Hot Day 
                  105° F. (40.5° C.) 
               
               
                   
                 Standard Day 
                 59° F. (15° C.) 
               
               
                   
                 Cold Day 
                 34° F. (1° C.)  
               
               
                   
                 Extreme Cold Day 
                 −65° F. (−54° C.) 
               
               
                   
                   
               
            
           
         
       
     
     Table 1 above describes the possible conditions under which engine  10  operates. As described above, known surface coolers are engineered to operate during standard day, hot day, and extreme hot day operation to ensure proper cooling of the lubrication oil. Generally, the higher the ambient temperature, the higher the temperature of the lubrication oil, the larger the surface cooler must be to reduce the temperature of the lubrication oil. As such, extreme hot day conditions require a relatively large surface cooler as compared to the size of the surface needed for standard day or hot day operation. However, only approximately 0.1% of the turbine engine operating time occurs during extreme hot day conditions. Accordingly, at least some known turbine engines include surface coolers that are larger than actually required 99.9% of the time. Such overcapacity increases the weight of the aircraft and, therefore, reduces the fuel efficiency. In addition, the larger than necessary surface coolers occupy space within the turbine engine that may be used for a different purpose. 
     In the exemplary embodiment, surface cooler  106  is sized for operation during either standard day or hot day operating conditions as defined in Table 1 below. That is, smaller size surface cooler  106  satisfies cooling requirements of the lubrication oil during operating conditions up to and including either standard day or hot day operation. As such, when the operating condition is determined to be an extreme hot day condition, further cooling of the lubrication oil is required. More specifically, when surface cooler  106  is sized for standard day operation, additional cooling is needed to cool the lubrication oil to a desired temperature when the engine is operating at hot day conditions or above. Similarly, when surface cooler  106  is sized for hot day operation, surface cooler  106  provides sufficient cooling for standard day operation, but additional cooling is needed to cool the lubrication oil to a desired temperature when the engine is operating at extreme hot day conditions. 
     In the exemplary embodiment, secondary heat exchanger  108  is coupled in flow communication with surface cooler  106  such that lubrication fluid is channeled therethrough and includes a brick cooler coupled to core turbine engine  13 . More specifically, brick cooler  108  is coupled to outer surface  45  and selectively exposed to bypass flow  52  based on the operating condition of engine  10 . Similar to surface cooler  106 , brick cooler  108  is also smaller in size than a conventional brick cooler. More specifically, brick cooler  108  is sized to operate in combination with surface cooler  106  such that surface cooler  106  and brick cooler  108  together reduce the temperature of the lubrication oil flowing therethrough at least as much as a conventional size surface cooler or brick cooler. As shown in  FIG. 2 , surface cooler  106  and brick cooler  108  are coupled in series with one another. In an alternative embodiment, surface cooler  106  and brick cooler  108  are coupled in parallel (as shown in  FIG. 4 ). As described herein, brick cooler  108  is only exposed to a cooling airflow during extreme hot day operating conditions. 
     Referring now to  FIGS. 2 and 3 , cooling system  100  also includes a temperature sensor  112  and a bypass mechanism  114  coupled in flow communication with brick cooler  108 . In the exemplary embodiment, bypass mechanism  114  is selectively moveable between a first position  116  and a second position  118  (shown in broken lines in  FIG. 3 ) based on a temperature of the fluid medium, as measured by sensor  112 , to control a cooling airflow  120  through brick cooler  108 . More specifically, bypass mechanism  114  includes a flap or a door that is selectively moveable to expose brick cooler  108  to cooling airflow  120  in response to the temperature of the fluid medium being above a predetermined threshold temperature. Even more specifically, when the threshold temperature as measured by sensor  112  is exceeded, indicating either of hot day or extreme hot day conditions, bypass mechanism  114  is controlled to move into first position  116  to expose brick cooler  108  to cooling airflow  120 . Similarly, when the measured temperature is below the threshold temperature, indicating an operating condition that is not either hot day or extreme hot day condition, bypass mechanism  114  is controlled to move into second position  118  to isolate brick cooler  108  from cooling airflow  120 . 
     In the exemplary embodiment, bypass mechanism  114  includes a flap or a door that extends into bypass airflow  54  when the threshold temperature is exceeded and either hot day or extreme hot day conditions are determined. In such a configuration, bypass mechanism  114  channels a portion of bypass airflow  52  through brick cooler  108  as cooling airflow  120  before channeling cooling airflow  120  back to bypass airflow  52  downstream of brick cooler  108  Although  FIG. 2  illustrates sensor measuring a temperature of bypass airflow  52  as the fluid medium on which the position of bypass mechanism  114  is based, in another embodiment, the fluid medium being measured is one inlet flow  48  or the lubrication oil itself flowing though conduit  110  of system  100 . Generally, the fluid medium on which the position of bypass mechanism  114  is based includes any engine fluid that is indicative of an operating condition of engine  10  as set forth in Table 1. In the exemplary embodiment, the threshold temperature of the ambient airflow fluid medium is based on the size of surface cooler. For example, when surface cooler  106  is sized for standard day operation, the threshold temperature may be approximately 60.degree. F. (15.5. degree. C.) such that the lubrication fluid is channeled through brick cooler  108  upon the determination of the temperature being greater than the standard day conditions, indicating one of hot day and extreme hot day operation. Similarly, when surface cooler  106  is sized for hot day operation, the threshold temperature may be approximately 105. degree. F. (40.5. degree. C.) such that the lubrication fluid is channeled through brick cooler  108  upon the determination of the temperature being greater than the hot day conditions, indicating extreme hot day operation. Alternatively, the threshold temperature may be any temperature that facilitates operation of system  100  as described herein. 
     In an alternative embodiment, mechanism  114  is permanently in the first position  116  as shown in solid lines in  FIG. 3  and bypass mechanism  114  includes an additional flap or door (not shown) that selectively covers an inlet defined between bypass mechanism  114  and outer wall  45 . As such, the flap is selectively movable based on a temperature of the fluid medium, as measured by sensor  112 , to control cooling airflow  120  through brick cooler  108 . 
     In the exemplary embodiment, the lubrication oil is channeled through brick cooler  108  in all operating conditions, including when bypass mechanism is in the second position and brick cooler  108  is not exposed to a cooling flow. Alternatively, cooling system  100  includes a lubrication oil bypass mechanism and associated lubrication oil bypass conduit (neither shown) that operate to channel the lubrication oil around brick cooler  108  during certain operating conditions. More specifically, the lubrication oil bypass mechanism and associated lubrication oil bypass conduit channel the lubrication oil around brick cooler  108  during operating conditions when the measured temperature is below the threshold temperature, indicating an operating condition that is not either hot day or extreme hot day condition. Similarly, cooling system  100  includes a second lubrication oil bypass mechanism and associated second lubrication oil bypass conduit (neither shown) that operate to channel the lubrication oil around surface cooler  106  during certain operating conditions. More specifically, the second lubrication oil bypass mechanism and associated second lubrication oil bypass conduit channel the lubrication oil around surface cooler  106  during operating conditions when the lubrication oil does not require cooling, such as extreme cold day conditions. 
     In operation, heat source  102  generates heat and transfers the thermal energy to the lubrication oil, which is then channeled through conduit  110  to fuel cooled oil cooler for reducing the temperature of the lubrication oil. The lubrication oil is then channeled through surface cooler  106  coupled to fan casing  42 . Surface cooler  106  is exposed to inlet airflow  48  such that thermal energy from the lubrication oil is transferred to inlet airflow. As described herein, in the exemplary embodiment, surface cooler  106  is smaller in size than conventional surface coolers and is designed to satisfy the cooling requirements of the lubrication oil for operating conditions of engine  10  up to and including hot day conditions. Sensor  112  then measures the temperature of a fluid medium, such as one of airflows  48  or  52  or the lubrication oil itself. The measured temperature of the fluid medium is then compared to a predetermined threshold temperature that is associated with the operating condition of engine  10 . When the measured temperature is below the threshold temperature, engine  10  is not operating in one of hot day or extreme hot day conditions and bypass mechanism  114  is positioned in second position  118  to allow bypass airflow  51  to bypass brick cooler  108 . Alternatively, when the measured temperature is at or above the threshold temperature, engine  10  is operating in one of hot day or extreme hot day conditions and bypass mechanism  114  is positioned in first position  116  to allow a portion of bypass airflow  51 , i.e. cooling airflow  120 , to flow through brick cooler  108  and transfer thermal energy from the lubrication oil therein to cooling airflow  120 . 
       FIG. 4  is a schematic illustration of an alternative engine fluid cooling system  200  that may be utilized with gas turbine engine  10  (shown in  FIG. 1 ). Fluid cooling system  200  includes the same components as cooling system  100 , only arranged in parallel rather than in series. As such,  FIG. 4  uses like reference numerals for components in system  200  that are identical to those in system  100 . Heat source  102 , fuel cooled air-oil cooler (FCOC), heat exchanger  104 , and primary air-oil cooler (ACOC) heat exchanger  106 , are all coupled in flow communication to each other via an engine fluid conduit  210 . Cooling system  200  includes a bypass conduit  216  that couples secondary air-oil cooler (ACOC) heat exchanger  108  in parallel with heat exchanger  106 . The engine lubricating fluid flows though system  200  via conduit  210 , and selectively  216 , and returns to heat source  102 . The temperature of the lubrication oil is reduced as it flows through heat exchangers  104 ,  106 , and  108 . 
       FIG. 5  illustrates an alternative bypass mechanism  414  coupled in flow communication with brick cooler  108 . Similar to bypass mechanism  114  above, bypass mechanism  414  is selectively moveable between a first position  416  and a second position  418  (shown in broken lines in  FIG. 5 ) based on a temperature of the fluid medium, as measured by sensor  112 , to control a cooling airflow  120  through brick cooler  108 . More specifically, bypass mechanism  414  includes two flaps or doors that are selectively moveable to expose brick cooler  108  to cooling airflow  120  in response to the temperature of the fluid medium being above a predetermined threshold temperature. Even more specifically, when the threshold temperature as measured by sensor  112  is exceeded, indicating either of hot day or extreme hot day conditions, bypass mechanisms  414  are controlled to move into first position  416  to expose brick cooler  108  to cooling airflow  120 . Similarly, when the measured temperature is below the threshold temperature, indicating an operating condition that is not either hot day or extreme hot day condition, bypass mechanism  414  are controlled to move into second position  418  to isolate brick cooler  108  from cooling airflow  120 . 
     In one embodiment, bypass mechanisms  414  includes one flap or door upstream of brick cooler  108  and a second flap or door downstream of brick cooler  108 . Flaps  414  move to enable bypass airflow  54  to become cooling airflow  120  through brick cooler  108  when the threshold temperature is exceeded and either hot day or extreme hot day conditions are determined. In such a configuration, bypass mechanism  414  channels a portion of bypass airflow  52  through brick cooler  108  as cooling airflow  120  before channeling cooling airflow  120  back to bypass airflow  52  downstream of brick cooler  108 . 
     Although cooling system  200  is described herein to cool lubrication fluid for engine  10 , it may alternatively or simultaneously cool other fluids. For example, it may cool a fluid used to extract heat from actuators used on the engine. It may also be used to cool fluids which extract heat from electronic apparatus such as engine controls. In addition to cooling a wide variety of fluids utilized by a gas turbine engine assembly, it should be realized that cooling system  200 , and the methods described herein illustrate that cooling system  200  may also cool an apparatus that is mounted on the airframe, and not part of the engine. In other applications, cooling system  200  may be mounted remotely from gas turbine engine  10 , for example on an external surface of the aircraft. 
     In the exemplary embodiment, fuel cooled heat exchanger  104  coupled in flow communication with heat source  102 . Heat exchanger  104  includes a circuit of relatively cool fuel flowing therethrough that reduces the temperature of the lubrication oil as it also flows through heat exchanger  104 . 
     In the exemplary embodiment, primary heat exchanger  106  includes an air cooled surface cooler positioned within a recess formed in inner surface  43  of fan casing  42 . Surface cooler  106  includes a plurality of fins  107  that are thermally coupled to receive thermal energy from the oil and transfer the energy to another medium such as air. More specifically, surface cooler  106  is flush mounted to fan casing  42  such that fins  107  extend into an inlet airflow  48  within intake side  28  of engine  10 . Surface cooler  106  transfers the thermal energy from the oil into inlet airflow  48  that removes the heat from the oil. 
     As shown in  FIG. 1 , surface cooler  106  is coupled to inner wall  43  of fan casing  42  downstream from fan assembly  12 , such that air channeled into intake side  28  is first channeled through fan assembly  12  prior to being supplied to surface cooler  106  aft of outlet guide vanes  150  to facilitate reducing the operating temperature of the engine fluids channeled through surface cooler  106 . Generally, surface cooler  106  is positioned anywhere along the axial length of inner wall  43  of fan casing  42 , or along radially outer surface  45  of splitter  44  within bypass duct  40 . In the exemplary embodiment, efficiency is increased when surface cooler  106  is positioned adjacent engine intake side  28 , where a diameter of fan assembly  12  is largest. 
     Referring now to  FIG. 4 , cooling system  200  also includes a temperature sensor  212  and a bypass mechanism  214  coupled in along conduit  210 . Bypass mechanism  214  is selectively moveable based on a temperature of the fluid medium, as measured by sensor  212 , to control lubrication oil flow through brick cooler  108 . More specifically, when the threshold temperature as measured by sensor  212  is exceeded, indicating either of hot day or extreme hot day conditions, bypass mechanism  114  is controlled to move to channel lubrication oil through brick cooler  108 . Similarly, when the measured temperature is below the threshold temperature, indicating an operating condition that is not either hot day or extreme hot day condition, bypass mechanism  114  is controlled to restrict lubrication oil flow through only surface cooler  106 . 
     In operation, heat source  102  generates heat and transfers the thermal energy to the lubrication oil, which is then channeled through conduit  210  to fuel cooled oil cooler  104  for reducing the temperature of the lubrication oil. The lubrication oil is then channeled along conduit  210  where it reaches sensor  212  and bypass mechanism  214 . Sensor  212  determines the temperature of the lubrication fluid and controls bypass mechanism  214  based on the temperature determination. In cases where the temperature is below a predetermined threshold, bypass mechanism  214  is actuated to channel all of the lubrication oil through surface cooler  106  and onward to heat source  102 . However, in cases where the temperature is determined by sensor  212  to be above the threshold, then bypass mechanism  214  is controlled to split the flow of lubrication oil such that a portion is channeled through surface cooler  106  and a portion is channeled through brick cooler  108 . After the lubrication oil exits the coolers  106  and  108 , it is combined and channeled to heat source  102  for use. 
     The exemplary apparatus and methods described herein overcome at least some disadvantages of known systems and methods for cooling a lubrication oil through a gas turbine engine. Moreover, the systems and methods described herein include both a surface cooler and a brick cooler that cooperate to cool the lubrication oil. More specifically, the surface cooler described herein is designed to satisfy the cooling requirements of the lubrication fluid during engine operating conditions up to and including hot day conditions, as specified in Table 1 above. As such, when the operating condition is determined to be an extreme hot day condition, further cooling of the lubrication oil is required. The brick cooler is positioned in the undercowl region of the engine and is activated upon a determination that the current operating condition of the engine is extreme hot day conditions. More specifically, the bypass mechanism is selectively moveable based on the temperature of a fluid medium to control the cooling airflow through the brick cooler. 
     A technical effect of the above described oil cooling system is that the reduction in size of the surface cooler leads to a reduction in overall weight of the engine. Furthermore, the smaller surface cooler has a smaller footprint in the engine and interacts with a correspondingly smaller amount of the inlet air. As such, not only does the smaller surface cooler allow more space within the engine for additional components, but the pressure losses of the fan air is reduced due to less air interacting with the smaller surface cooler. The reduction in weight and also the reduction in differential pressure result in an improved specific fuel consumption rate, which increases the efficiency of the engine and reduces overall operating costs. 
     Exemplary embodiments of oil cooling systems are described above in detail. The oil cooling systems, and methods of operating such systems and devices are not limited to the specific embodiments described herein, but rather, components of systems and/or steps of the methods may be utilized independently and separately from other components and/or steps described herein. For example, the methods may also be used in combination with other systems requiring oil cooling systems, and are not limited to practice with only the turbine engine system and methods as described herein. 
     Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.