Abstract:
A radial inlet assembly comprising a radial inlet adapted to be in fluid communication with a compressor, and a restricting member covering the radial inlet and receiving a circumferentially asymmetric airflow, the restricting member partially blocking the airflow around the radial inlet, the restricting member blocking a greater portion of the airflow where the airflow is greater to circumferentially redistribute the airflow in a more symmetric manner in the radial inlet.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to gas turbine engines and, more particularly, to compressor inlets of such engines. 
   2. Description of the Prior Art 
   An uneven airflow distribution through the inlet of a compressor of a gas turbine engine can decrease the engine performance, even leading to a stall of some of the engine blades. 
   This problem can be even more present in radial inlets, since they are often located in a plenum which is opened to the atmosphere only along part of its circumference. Because the distance to be covered by the airflow varies about the circumference of the inlet, the airflow through radial inlets can become substantially asymmetric. 
   A number of devices have been developed in an attempt to obtain a more uniform airflow distribution over the inlet. Most of these devices are meant for an axial inlet. For instance, it has been proposed to add a plenum around an inlet, the plenum being separated from the inlet by an evenly perforated wall. The air pressure in the inlet is evened by varying the air pressure in the plenum through active means such as a valve. However, these means represent an additional component of the engine that can potentially fail, adds weight to the engine and requires maintenance. 
   Accordingly, there is a need for a simple device which can provide a more uniform airflow for a radial inlet. 
   SUMMARY OF THE INVENTION 
   It is therefore an aim of the present invention to provide an improved radial inlet assembly for a compressor. 
   Therefore, in accordance with a general aspect of the present invention, there is provided a radial inlet assembly for a compressor, the assembly comprising a radial inlet adapted to be in fluid communication with the compressor, and a restricting member covering the radial inlet and receiving a circumferentially asymmetric airflow, the restricting member partially blocking the airflow around the radial inlet, the restricting member blocking a greater portion of the airflow where the airflow is greater to circumferentially redistribute the airflow in a more symmetric manner around the radial inlet. 
   In accordance with a further general aspect of the present invention, there is provided a restricting member for straightening an airflow in a radial inlet of a compressor in a gas turbine engine, the restricting member comprising an annular body adapted to cover the radial inlet such as to partially block the airflow, the annular body being separated in a plurality of regions having a same area, extending along a length of the body and covering a same angular portion of the body, such as to define a first region, a second region diametrically opposed to the first region, and a plurality of intermediary regions extending therebetween, and a plurality of openings in the annular body, the openings in each region defining an effective opening area, the effective opening area being minimal in the first region and becoming progressively greater in adjacent intermediary regions in a symmetrical manner such as to reach a maximum in the second region. 
   In accordance with a still further general aspect of the present invention, there is provided a radial inlet assembly for a compressor in a gas turbine engine, the assembly comprising first means for radially providing an airflow having a first circumferentially asymmetric distribution, second means for delivering the airflow to the compressor, third means for covering the second means such as to partially block the airflow, and openings provided in the third means, an effective area of the openings varying along the third means such that a blocked portion of the airflow is greater where the airflow is greater, so that the airflow enters the second means with a second distribution which is less circumferentially asymmetric than the first distribution. 
   In accordance with a still further general aspect of the present invention, there is provided a compressor inlet assembly comprising a radial inlet receiving a flow of incoming air, a perforated plate covering the radial inlet, the perforated plate having a variable open area over a length thereof, the open area being greater where the flow of air is weaker. 
   Further yet in accordance with a general aspect of the present invention, there is provided a method for increasing the uniformity of an airflow around a radial inlet of a compressor in a gas turbine engine, the method comprising the steps of evaluating the airflow along a circumference of the radial inlet to determine at least a first region where the airflow is greater and a second region where the airflow is weaker, providing a member covering at least the first region of the radial inlet, and variably obstructing the airflow along the circumference of the inlet with the member to redistribute the airflow in a more circumferentially symmetric manner around the radial inlet. 

   
     DESCRIPTION OF THE DRAWINGS 
     Reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment of the invention and in which: 
       FIG. 1  is a schematic side view of a gas turbine engine, in partial cross-section, to which an embodiment of the present invention is applied; 
       FIG. 2  is a partial cross-sectional view of the radial inlet assembly, used with the gas turbine engine of  FIG. 1 ; 
       FIG. 3A  is a partial side view, in cross-section, of the radial inlet assembly of  FIG. 2  used with an axial compressor; and 
       FIG. 3B  is a partial side view, in cross-section, of the radial inlet assembly of  FIG. 2  used with a radial compressor. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a turboprop engine  10  of a type preferably provided for use in subsonic flight to drive a propeller  12  via a reduction gear box (RGB)  14 . The engine  10  comprises a first rotating assembly consisting of a turbine  16  and a compressor  18  mounted on a common shaft  19 , and a second rotating assembly consisting of a power turbine  20  mounted on a power turbine shaft  22 . The first and second rotating assemblies are not connected together and turns at different speed and in opposite directions. This design is referred to as a “Free Turbine Engine”. It is understood that the present invention could be applied to other types of gas turbine engines as well. 
   The compressor  18  draws air into the engine  10 , increases its pressure and delivers it to a combustor  26  where the compressed air is mixed with fuel and ignited for generating a stream of hot combustion gases. The compressor turbine  16  extracts energy from the hot expanding gases for driving the compressor  18 . The hot gases leaving the compressor turbine  16  are accelerated again as they expand through the power turbine  20 . The power turbine  20  provides rotational energy to drive the propeller  12 . The RGB  14  reduces the power turbine  20  speed to one suitable for the propeller  12 . 
   The compressor  18  receives an airflow from a radial inlet assembly indicated at  30 . Referring to  FIGS. 2–3A , the inlet assembly  30  comprises a plenum  32  defined by a U-shaped wall  46  having an open top end  48 . The plenum  32  encloses a radial inlet  34  which is annular and disposed around the compressor shaft  19 . A restricting member, provided in the form of an annular perforated plate  36 , extends over the inlet  34  so as to cover it. A plurality of openings  38 , which are preferably circular holes, are defined in the plate  36 . The inlet  34  is connected to an axial conduit  44  ( FIG. 3A ) in fluid communication with an axial compressor stage  45 . 
   The plenum  32  receives atmospheric air from the open top end  48  and thus acts as a source distributing an airflow  40  around the plate  36  surrounding the inlet  34 . The plenum  32  shapes the airflow  40 . Near the open top end  48 , the airflow  40  hits the plate  36  directly. However, the air reaching a bottom region of the plate  36  has to travel a considerable distance and be progressively turned by the plenum wall  46 . As a consequence, the airflow  40  reaching a bottom part of the perforated plate  36  is substantially attenuated. Also, the angle of the airflow  40  reaching the plate  36  is influenced by the shape of a free space between the plenum  32  and the plate  36 . Generally, the airflow  40  becomes more inclined with respect to the plate  36  toward a bottom region of the plate  36 . 
   The airflow through a hole  38  located at a specific point of the plate  36  can be evaluated by considering the portion of the airflow at the hole that is normal to the plate  36  at that point. The airflow through a specific hole  38  therefore depends on the magnitude and angle of the airflow reaching that hole, or, in other words, on the location of that hole in the plate  36 . Thus, with equally distributed holes  38  all having the same surface area, the airflow reaching the inlet  34  would have a substantially asymmetrical distribution, with the airflow becoming generally progressively weaker toward a bottom end of the inlet  34 . 
   In order to correct the airflow distribution, an effective area of the holes  38  is varied around the plate  36  so that the portion of the airflow  40  that blocked by the plate  36  is greater where that airflow is greater. The effective area is defined as the sum of the areas of the holes  38  covering a region of the plate  36 . Letting a larger portion of the air reaching the plate  36  go through the holes  38  located where the airflow is weaker equilibrates the distribution of the airflow penetrating the plate  36 . An adequate effective area distribution will thus provide a uniform airflow around the inlet  34 . 
   In a preferred embodiment, the effective area is varied by varying the density of holes  38  with all holes  38  having a similar surface area. This is apparent in  FIG. 2  where the plate  36  is separated in six (6) regions extending along the length of the plate  36  and defining the same angle, thus having the same area, by the broken lines  50 . The region on top, labelled A, has the least number of holes  38 , and the region on the bottom, labelled B, has the most number of holes  38 . The number of holes  38  progressively increases in the intermediary regions C, D, E, F from the top to the bottom. One skilled in the art will of course understand that the plate  36  can be similarly separated in any number of regions, with six (6) being an exemplary embodiment. In another embodiment, it is also considered to vary the effective area by using a uniform distribution of holes  38 , i.e. the same number of holes in each region, but with holes having a larger surface area where the airflow  40  is weaker. Thus, the hole size would be progressively increased toward the bottom end of the plate  36 . 
   Although the radial inlet assembly  30  has been described as being used with an axial compressor, it can also be used with a radial compressor. Referring to  FIG. 3B , the inlet  34  delivers air to a radial compressor  47  adapted to redirect air from an axial direction to a radial direction. The function and components of the inlet assembly  30  in this case are the same as previously described. 
   The openings  38  have been illustrated as being circular holes, but other shapes could be used, including, but not limited to, slots, oblong holes and rectangular openings. Holes of various shapes could be used in various regions of the plate  36 . The plate  36  can be formed of a series of strips defining elongated spaces therebetween that act as the openings  38 . The plate  36  can also be used with other types of asymmetrically shaped inlets, and with other types of air devices requiring a more symmetric redistribution of an airflow. 
   The embodiments of the invention described above are intended to be exemplary. Those skilled in the art will therefore appreciate that the forgoing description is illustrative only, and that various alternatives and modifications can be devised without departing from the spirit of the present invention. Accordingly, the present is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.