TECHNICAL FIELD 
     The present application relates to gas turbine engines, and more particularly to controlled gap seals used in gas turbine engines. 
     BACKGROUND OF THE ART 
     Controlled gap seals, such as carbon controlled gap seals, are commonly used in gas turbine engines, generally to seal bearing compartments. These seals are designed to run with a few thousands of an inch of clearance between a stationary carbon element and a rotating seal runner or shaft. As the temperature of the bearing area heats and cools, the seal is designed to react to temperature variations and keep the seal clearance or gap relatively constant. This may be done by having a shrink band on the carbon element. The shrink band is a metal ring that is in a tight-fitting engagement onto the carbon element. The shrink band is heated and cooled by the surrounding air, thus controlling the expansion and contraction of the carbon element. In some transient temperature excursions, the shrink band may not be sufficiently responsive as it may not be directly exposed to surrounding air. This may cause seal rub that may eventually lead to increased leakage during steady-state running of the gas turbine engine. 
     Accordingly, there is a need to provide an improved thermally responsive controlled gap device. 
     SUMMARY 
     In one aspect, there is provided a controlled gap seal device adapted to surround a shaft, the device comprising: an annular seal element; a ring positioned on an outer surface of the seal element, the ring adapted to modify a diametrical dimension of the seal element by thermally expanding/contracting as a function of temperature variations; and a housing assembly having an interior enclosing the seal element and the ring, with the seal configured to be generally stationary in the interior, the housing assembly having at least one air inlet and at least one air outlet in fluid communication with a surrounding environment for directing a flow of gas from the surrounding environment onto the ring to controllably cool and shrink the ring. 
     In a second aspect, there is provided a method for modifying a diameter of a controlled gap seal relative to the shaft, comprising: inletting air/gases from a surrounding environment into a housing assembly enclosing a seal element; exposing a ring positioned on the seal element to the air/gases in the housing assembly to modify a diameter of the seal element by thermally expanding/contracting as a function of a temperature of the air/gases; and outletting the air/gases to the surrounding environment. 
     Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures, in which: 
         FIG. 1  is a sectional view of a thermally responsive controlled gap seal device in accordance with the present disclosure; 
         FIG. 2  is an exploded view of the thermally responsive controlled gap device of  FIG. 1 ; and 
         FIG. 3  is a sectional perspective view of the thermally responsive controlled gap device of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1 to 3 , there is illustrated a thermally responsive controlled gap seal device  10  in accordance with the present disclosure. The thermally responsive controlled gap seal device  10  is used between a shaft and a structural component, such as seal runner A and bearing housing B. According to an embodiment, the thermally responsive controlled gap seal device  10  may be adjacent to a bearing (part of which is the bearing housing B) supporting the seal runner A, with the seal runner A rotating about its longitudinal axis. The thermally responsive controlled gap seal device  10  is positioned about the seal runner A to reduce the amount of air/gases reaching the bearing. A gap is defined between the thermally responsive controlled gap seal device  10 , such that the thermally responsive controlled gap seal device  10  generally remains stationary while the shaft rotates. 
     The thermally responsive controlled gap seal device  10  may have a housing assembly  12 , a seal  14  and a shrink band  16 . 
     The housing assembly  12  interfaces the thermally responsive controlled gap seal device  10  to the bearing housing B, or to any other structural component. 
     The seal  14  performs the sealing between the thermally responsive controlled gap seal device  10  and its supporting structure (e.g., bearing housing B), and the seal runner A, or other shaft or rotating component. The seal  14  is made of carbon, or any other appropriate sealing materials. 
     The shrink band  16  is a ring that surrounds the seal  14  and reacts to temperature changes to modify diametrical dimensions of the seal  14 , by expanding/contracting thermally. 
     The housing assembly  12  is shown comprising a housing body  20 . The housing body  20  is typically cup-shaped and therefore comprises an outer annular wall  21  and a radial end wall  22 . The outer wall  21  is sized so as to be received in an appropriate cavity in the bearing housing B (e.g., force fit, interference fit, etc). The radial end wall  22  defines one of the radial ends of the thermally responsive controlled gap seal device  10 . Therefore, the outer wall  21  and the radial end wall  22  concurrently form an annular cavity of the housing body  20 . 
     One or more axial channels  23  (i.e., slots) are defined in an outer surface of the outer wall  21 . The axial channels  23  are in fluid communication with a space C adjacent to the bearing housing B. Alternatively, the outer wall  21  may be continuous, with axial channels being defined in the bearing housing B. Moreover, air passages  24  are defined in the housing body  20  (e.g., in the outer wall in  FIGS. 1 to 3 ) and are in fluid communication with an interior of the housing body  20  and with the axial channels  23 , whereby air may flow out of the annular cavity of the housing body  20 , through the air passages  24 , the axial channels  23  and out to the space C. 
     In an end opposed to the radial end wall  22 , an annular channel  25  may be defined in an inner surface of the outer wall  21 . The annular channel  25  is sized so as to receive an outer washer  30 , and hold it captive. The outer washer  30  closes the housing body  20  to encapsulate various components therein. The outer washer  30  could be connected to the housing body  20  in other ways, such as being threadingly engaged to the housing body  20 , etc. Air scoops  31  or like air inlets are circumferentially disposed at various locations on the outer washer  30 . The air scoops  31  will direct surrounding swirling air from an exterior of the housing assembly  12  to an interior thereof. The air scoops  31  may project into the environment C. 
     An inner washer  32  is within the housing body  20  and in contact with the seal  14 . The inner washer  32  also comprises air passages  33 . In the illustrated embodiment, the air passages  33  are cutouts in the outer peripheral edge of the inner washer  32 . The cutouts  33  may have a semi-circular shape, although other configurations are considered as well. 
     A spring  35  (such as a wave spring) is positioned between the outer washer  30  and the inner washer  32  and therefore presses the inner washer  32  against the seal  14 . Other biasing means could be used as alternatives to the wave spring  35 , such as coil springs, leaf springs, etc. In an embodiment, the spring  35  is directly in contact with the seal  14 . 
     The seal  14  may have abutments  40  projecting in opposed axial directions. According to an embodiment, the abutments  40  are annular. The abutments  40  will be in contact with the inner washer  32  and the radial end wall  22 , respectively. Therefore, the biasing force of the spring  35  will axially load the seal  14  against the radial end wall  22 , thereby maintaining its position within the housing body  20 . The seal  14  may be without such abutments  40 , and instead have its radial surfaces directly in contact with the spring  35  and the radial end wall  22 . In yet another embodiment, the seal  14  is directly in contact with the outer washer  30 , with the outer washer  30  effecting the axial loading of the seal  14  against the radial end wall  22 . In yet another embodiment, the spring  35  is between the radial end wall  22  and the seal  14 . 
     The seal  14  has an inner diameter  41  that is sized to be slightly greater than an outer diameter of the seal runner A, so as to define the gap therebetween. An outer diameter  42  of the seal  14  is sized so as to receive thereon the shrink band  16 . The shrink band  16  has an annular body  60  that is made of material with a coefficient of thermal expansion proportional to an expansion of the shaft (i.e., the seal runner A in the illustrated embodiment). For instance, the shrink band  16  is metallic ring, that it in a tight-fitting engagement on the seal  14 . The outer surface of the annular body  60  may have heat transfer fins  61  projecting radially outwardly therefrom, to increase a surface of the shrink band  16  that is exposed to thermal conditioning air. By the presence of the heat transfer fins  61  and the exposure of the shrink band  16  to air/gases circulating within the housing body  20 , the shrink band  16  will react to temperature changes and will cause a pressure on the seal  14  proportional to a variation in diameter of the seal runner A. Hence, the seal  14  adjusts its size as a function of temperature variations in the gas turbine engine. 
     In operation, air/gases in the environment C will penetrate the thermally responsive controlled gap seal device  10  via the air passages  31  of the outer washer  30 . In an embodiment, the air/gases are in a turbulent condition (e.g., swirling), whereby the air scoops  31  may increase the amount of air/gases entering the housing body  20 . The air scoops  31  may be oriented/aligned with flow direction to collect more air/gases. The resulting pressure increase in the housing body  20  causes a flow of the air/gases through the air passages  33  of the inner washer  32 , to the air passages  24 , thereby flowing over and across the shrink band  16 . The air/gases sucked by the air passages  24  will return to the environment C via the air channels  23 —the air channels  23  and air passages  24  forming outlets. The flow of air/gases in the housing assembly  12  will expose the shrink band  16  to temperatures generally equivalent to that to which the seal runner A is exposed. Hence, the shrink band  16  will exert/release pressure on the seal  14 , to maintain the gap between the seal  14  and the seal runner A The presence of air scoops  31  (and their number), as well as the heat transfer fins  61  may reduce the reaction time of the shrink band  16  to temperature variations. 
     It is observed that the combination of inner washer  32 , spring  35  and abutments  40  generally prevent air/gases leakage of the thermally responsive controlled gap seal device  10 , other than through the air channels  23  and air passages  24 . 
     The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. For example, the thermally responsive controlled gap seal device  10  may be used in different applications in addition to gas turbine engines. Controlled gap seals using materials besides carbon may have the present teachings applied, as well. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.
A controlled gap seal device is adapted to surround a shaft with an annular seal element. A ring is positioned on an outer surface of the seal element, the ring adapted to modify a diametrical dimension of the seal element by thermally expanding/contracting as a function of temperature variations. A housing assembly has an interior enclosing the seal element and the ring, with the seal configured to be generally stationary in the interior. The housing assembly has an air inlet and air outlet in fluid communication with a surrounding environment for directing a flow of gas from the surrounding environment onto the ring to controllably cool and shrink the ring. A method for modifying a diameter of a controlled gap seal relative to the shaft is also provided.