Abstract:
A vernier duct blocker comprising a plurality of vanes each having a width and comprising a forward portion and an aft portion defining a plurality of gas paths each of the plurality of vanes being separated by a plurality of widths, and a rotatably movable ring interposed between the forward portion and the aft portion comprising a plurality of openings each having a width, wherein the width of one of the plurality of vanes differs from the width of another one of the plurality of vanes.

Description:
U.S. GOVERNMENT RIGHTS 
   The invention was made with U.S. Government support under contract N00019-02-C-3003 awarded by the U.S. Navy. The U.S. Government has certain rights in the invention. 

   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The present invention relates to an apparatus, and method for using such an apparatus, for controlling the flowpath area in a gas turbine engine. More specifically, the present invention relates to an apparatus for providing non-linear flowpath area control of a gas turbine engine through the use of vernier duct blocker. 
   (2) Description of the Related Art 
   When operating gas turbine engines, it is frequently desirable to control the amount of gas flowing through the secondary flowpath between the outer duct and the inner support structure. One common method of achieving such control is to install an apparatus for adjusting the area through which the gas may flow. Such flow blockers often include a rotatable member with a plurality of openings that can be rotated in order to control the size of an open area through which gas can flow. With reference to  FIG. 1 , there is illustrated one such apparatus. A number of vanes  21  are circumferentially attached about a ring or rings each vane separated from its neighbor by a uniform distance w. Each vane is formed of a forward portion  15  and an aft portion  13 , which, together, form an airfoil shaped vane  21 . Between each forward portion  15  and each aft portion  13 , there is positioned a rotatably movable ring  11 . Rotatably movable ring  11  has a series of openings  17  each of a width w and spaced so as to generally correspond to the widths w between each adjacent pair of vanes  21 . 
   With reference to  FIG. 4   a  there is illustrated a view of a portion of a flow blocker  10  looking aft and directly towards the leading edges  23  of the aft portions  13 . To assist in visualization, the forward portions  15  of each vane  21  are not shown. When the openings  17  of the rotatably movable ring  11  are aligned with the spaces between adjacent vanes  21 , the flow blocker  10  is in a fully open position whereby a maximum opening, consisting of the sum of all unblocked openings  17 , is created. With reference to  FIG. 4   b , it is evident that when rotatably movable plate  11  is rotated, the sum of the unblocked portions of all openings  17 , is substantially reduced. 
   It is most desirable for a flow blocker  10  to provide for complete, or nearly complete, blockage of gas flow when necessary while causing little if any blockage when needed. In addition, it is often the case that there are located several flow blockers arranged in series along a central axis  19  of a gas turbine engine. Depending on the flight envelope in which an engine is operating, differing flow blockers will be adjusted to provide for differing opening areas through which gas can flow. Unfortunately, there typically exists a linear relationship between the angular rotation of the rotatably movable ring  11  and the size of the resultant opening through which gas can flow. As a result, in instances wherein one wishes to finely control the area of an opening such that only a small area is provided through which gas can flow, small angular adjustments of the rotatably movable ring  11  result in relatively large differences in the opening area through which gas can flow. 
   What is therefore needed is a flow blocker  10 , and method for so using such a flow blocker, that permits a wide range of adjustable opening sizes through which gas can flow while allowing for fine control of the opening sizes when a small opening size is desired. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide an apparatus, and method for using such an apparatus, for controlling the flowpath area in a gas turbine engine. More specifically, the present invention relates to an apparatus for providing non-linear flowpath area control of a gas turbine engine through the use of vernier duct blocker. 
   In accordance with the present invention, a vernier duct blocker comprises a plurality of vanes each having a width and comprising a forward portion and an aft portion defining a plurality of gas paths each of the plurality of vanes being separated by a plurality of widths, and a rotatably movable ring interposed between the forward portion and the aft portion comprising a plurality of openings each having a width, wherein the width of one of the plurality of vanes differs from the width of another one of the plurality of vanes. 
   In further accordance with the present invention, a method of controlling gas flow through a gas flowpath comprises the steps of providing a plurality of vanes each having a width and comprising a forward portion and an aft portion defining a plurality of gas paths each of the plurality of vanes being separated by a plurality of widths, providing a rotatably movable ring interposed between the forward portion and the aft portion comprising a plurality of openings each having a width wherein the width of one of the plurality of vanes differs from the width of another one of the plurality of vanes, and rotating the rotatably movable ring about a central axis to at least partially block a flow of a gas through the plurality of gas paths. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  A perspective view of a split vane flow blocker known in the art. 
       FIG. 2  A side view of the vernier duct blocker of the present intention. 
       FIG. 3  A perspective view of the vernier duct blocker of the present invention. 
       FIG. 4   a  An illustration of a flow blocker known in the art shown in the fully open position. 
       FIG. 4   b  An illustration of a flow blocker known in the art shown in a partially closed position. 
       FIG. 5   a  An illustration of the vernier duct blocker of the present invention shown in the fully open position. 
       FIG. 5   b  An illustration of the vernier duct blocker of the present invention shown in a partially closed position. 
       FIG. 6  A graph illustrating the non-linear relationship between the open area of the vernier duct blocker of the present invention through which gas may flow versus radial displacement of the rotatably movable ring. 
   

   DETAILED DESCRIPTION 
   It is the teaching of the present invention to provide a vernier duct blocker comprised of varying width vanes and a rotatably movable ring to control the size of the area of duct blockage. A plurality of vanes is circumferentially disposed about a central axis of a gas turbine engine. Each vane is formed of an aft portion and a forward portion. Between the aft portion and the forward portion there is located a rotatably movable ring which contains openings through which gas can flow. Like the prior art, the openings in the rotatably movable ring can be aligned with the spaces between adjacent vanes so that gas can flow predominantly unimpeded between each of the vanes. However, unlike the prior art, the vernier duct blocker of the present invention is formed from vanes whose widths differ one from the other. As a result, the spaces between the vanes vary as opposed to the constant spacing between the vanes of the prior art. Such differing widths of the vanes and spaces between the vanes allows for a non-linear relationship between the rotation of the rotatably movable ring from a fully open position and the total area formed by the openings in the rotatably movable ring between which gas can flow. The widths of the vanes and the spaces between the vanes are chosen to provide this non-linear relationship in a fashion such that very fine control of the opening area is achieved when the duct blocker is operating in a restrictive mode. By restrictive mode, it is meant that the rotatably movable ring is positioned such that the exposed openings in the rotatably movable ring between the vanes is small relative to the sum total of the openings in the rotatably movable ring when positioned in a fully open position. 
   With reference to  FIG. 3 , there is illustrated a vernier duct blocker  31  of the present invention. Vernier duct blocker consists of numerous vanes  21  each formed of a forward portion  15  and an aft portion  13 . The forward portions  15  and the aft portions  13  are circumferentially disposed about a forward ring  33  and an aft ring  35 , respectively. Both forward ring  33  and aft ring  35  are of essentially identical diameters and are disposed about a central axis  19  of a turbine engine. As such, each vane  21  comprised of a forward portion  15  and an aft portion  13  is located in a secondary flowpath between an outer duct  29  and an inner support structure  27  of a gas turbine engine as shown in  FIG. 2 . Disposed between forward ring  33  and aft ring  35  is a rotatably movable ring  11 . Rotatably movable ring  11  is likewise circumferentially disposed about the central axis  19  of a gas turbine engine. Note that the vanes  21  of the vernier duct blocker  31  are of differing widths. For example, aft portion  13  and aft portion  13 ′ are of widths Y and Y′, respectively, Y and Y′ not being equal. Similarly, vanes  21  are separated by non-uniform distances. Note specifically that the distance between aft portion  13 ″ and aft portion  13 ′ is equal to a width of W′ while the distance between aft portion  13 ′ and aft portion  13  are separated by a distance of W, W not equal to W′. 
   Disposed between forward ring  33  and aft ring  35  is a rotatably movable ring  11  into which is fabricated a plurality of openings  17 . The width of individual openings  17  and the distance between such openings  17  are selected such that in at least one position, rotatably movable ring  11  may be rotated into a fully open position as illustrated in  FIG. 3 . By “fully open position” it is meant that in such a position the sum total of the area comprising each individual opening  17  not blocked by any forward portion  15  is at a maximum. Preferably, in a fully open position, the expanse formed between any two adjacent vanes  21  is predominantly in correspondence to a single opening  17  such that gas can flow virtually unimpeded between the vanes  21 . 
   With reference to  FIG. 5   a  there is illustrated a view of a portion of a vernier duct blocker  31  looking aft and directly towards the leading edges  23  of the aft portions  13 . To assist in visualization, the forward portions  15  of each vane  21  are not shown. When the openings  17  of the rotatably movable ring  11  are aligned with the spaces between adjacent vanes  21 , the flow blocker  10  is in a fully open position whereby a maximum opening, consisting of the sum of all unblocked openings  17 , is created. With reference to  FIG. 5   b , it is evident that when rotatably movable plate  11  is rotated, the sum of the unblocked portions of all openings  17 , is substantially reduced. Furthermore, it is evident that moving the rotatably movable ring  11  out of a fully open position results in a plurality of openings  17 ,  17 ′ of differing widths. As a result of these differing opening  17  widths, there results a non-linear relationship between the degree of rotation of the rotatably movable ring  11  and the sum total of the open area formed of each individual opening  17  through which gas may flow. 
   With reference to  FIG. 6 , there is illustrated an exemplary graph showing the relationship between the open area created from the openings  17  of the vernier duct blocker of the present invention versus the angular rotation of the rotatably movable ring  11 . The x axis represents the linear displacement of the rotatably movable ring  11  from the fully open position. When the rotatably movable ring  11  is in a fully open position, it has no displacement. In the illustrated example, when in the fully open position, the vernier duct blocker provides 180 units 2  of opening through which gas can flow. As the displacement of the rotatably movable ring  11  is increased through a rotation distance, the open area created by the openings  17  decreases. Note that this decrease is predominantly linear until a non-linear region  61  is reached. Non-linear region  61  is a region within which further displacement of the rotatably movable ring  11  away from the fully open position results in a slower decrease in the open area created by the openings  17 . As a result, as the total open area created by the openings  17  becomes small, relatively large rotational movements of the rotatably movable ring  11  result in small reductions in the open area through which gas can flow. This provides for fine control of the open area. In the embodiment pictured, the spacing between the vanes  21  and the openings  17  of the vernier duct blocker  31  are chosen such that, in its least open position, the open area through which gas can flow formed of the unblocked openings  17  does not approach zero, but rather tends towards a minimum open area value  65  observed during the minimum open area region  61 . 
   It is apparent that there has been provided in accordance with the present invention an apparatus for providing non-linear flowpath area control of a gas turbine engine which fully satisfies the objects, means, and advantages set forth previously herein. While the present invention has been described in the context of specific embodiments thereof, other alternatives, modifications, and variations will become apparent to those skilled in the art having read the foregoing description. Accordingly, it is intended to embrace those alternatives, modifications, and variations as fall within the broad scope of the appended claims.