Abstract:
With highly loaded rotors and stators problems can occur with secondary flows sweeping low momentum fluid across the blades reducing efficiency. By provision of collector slots to collect the secondary air and direct that air to a return slot in a rotor hub it is possible to provide impetus to the collected secondary flow to an outlet slot such that there is dispersal of the secondary flow and therefore reduce the effects upon the overall performance of a gas turbine engine incorporating the arrangement.

Description:
The present invention relates to rotor arrangements and more particularly rotor arrangements utilised in gas turbine engines in the region of rotor hubs and stator shrouds. 
     BACKGROUND 
     Referring to  FIG. 1 , a gas turbine engine is generally indicated at  10  and comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate pressure compressor  13 , a high pressure compressor  14 , a combustor  15 , a turbine arrangement comprising a high pressure turbine  16 , an intermediate pressure turbine  17  and a low pressure turbine  18 , and an exhaust nozzle  19 . 
     The gas turbine engine  10  operates in a conventional manner so that air entering the intake  11  is accelerated by the fan  12  which produce two air flows: a first air flow into the intermediate pressure compressor  13  and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor  14  where further compression takes place. 
     The compressed air exhausted from the high pressure compressor  14  is directed into the combustor  15  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines  16 ,  17  and  18  before being exhausted through the nozzle  19  to provide additional propulsive thrust. The high, intermediate and low pressure turbines  16 ,  17  and  18  respectively drive the high and intermediate pressure compressors  14  and  13  and the fan  12  by suitable interconnecting shafts. 
     In view of the above it will be appreciated that control of flows through a gas turbine engine are important to achieve efficiency and operational performance. In such circumstances in order to extract propulsion and work blades secured upon rotor hubs are associated with stators to cause appropriate directionality with respect to flows through an engine. It will also be appreciated that the loads presented to the engine may vary over operational cycles such as takeoff, cruise, ascent, descent and landing if the engine is utilised for aircraft propulsion. It is found that highly loaded rotors and stators stall about the hub/shroud area due to secondary flows sweeping low momentum fluid from the annulus about the rotor hub and stator shroud onto the stator blades near to the peak suction point on the suction surface of the blades. In such circumstances subsequent operation is less efficient and effective over those blade suction surfaces. It would be advantageous to reduce or inhibit the effects of such secondary flows to improve engine efficiency and performance. 
     It will be appreciated that secondary flows relate to fluid flows through the engine blades near to the hub or shroud surface which are not part of or of lower momentum than the primary propulsion flows. 
     SUMMARY 
     In accordance with the present invention there is provided a rotor arrangement for a gas turbine engine, and a gas turbine engine including such a rotor arrangement, as set out in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the invention will now be described, by way of example, with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic part sectional view of a gas turbine engine of known type; 
         FIG. 2  is a schematic side illustration of a rotor and stator arrangement in accordance with the present invention; and, 
         FIG. 3  is a schematic part plan cross section of the rotor and stator arrangement as depicted in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     As indicated above secondary fluid flows about a stator hub can diminish operational efficiency and performance within a gas turbine engine. It will be appreciated that a rotor or compressor blade assembly generates fluid flow as it is turned and therefore any slower momentum fluid swept onto the blades from the annulus, that is to say the rotor hub, will diminish performance. 
     In accordance with the present invention an arrangement is provided to efficiently evacuate secondary flows when performance requires it. The driven sweeping action of the rotor hub is utilised in order to provide impulse to the secondary flow away from areas of too detrimental effect. It is this rotor movement that is used to pump this secondary flow. 
       FIG. 2  provides a schematic cross section of a rotor arrangement  30  in accordance with the present invention. The arrangement  30  comprises a rotor  31  secured upon a rotor hub  32 . The rotor hub  32  and associated blades or rotors  31  rotate about an axis X-X past a stator  33  in a set of stators associated with a stator shroud  34 . The primary function of the rotors  31  is to provide a powerful fluid flow in the direction of arrowheads A. As indicated above a particular problem is with respect to secondary flows about the hub  32  and shroud  34  sweeping low momentum fluid upwards in the direction of arrowheads E causing problems at the peak suction point on the suction surface of the rotors  31 . It will be appreciated that  FIG. 2  only illustrates one rotor  31  and one stator  33  but in a practical engine there will be a plurality of such rotors  31  and stators  33 . The rotors  31  on the hub  32  will sweep past the stators  33  for fluid flow control. 
     In accordance with the present invention, the stator shroud  34  includes a collector slot  35  and an outlet slot  36  angled relative to each other for flow coupling with a return slot  37  in the rotor hub  32 . In operation, secondary flow is collected at an inlet  38  of the collector slot  35  and passes along a collector passage towards the return slot  37 . The collected flow is projected and given impulse by rotation of the rotor hub  32  as it sweeps past the shroud  34 . Thus, the return slot  37  pumps the collected secondary flow towards the outlet passage  36  where it is taken sufficiently downstream and angled sufficiently such that there are no, or at least fewer, problems with regard to sweeping low momentum fluid upwards into the main fluid flow A. 
     Arrowheads C illustrate a notional secondary flow path through the slots  35 ,  36 ,  37  in accordance with the present invention. Thus, secondary flow C is initially collected and drawn into the collector slot  35  in the direction of arrowhead Ca then expelled across a gap  39  between the collector slot  35  and the return slot  37  in the direction of arrowhead Cb where that collected secondary flow is “pumped” and given impetus for return in the direction of arrowhead Cc, again across the gap  39  and into the outlet slot  36  where it is propelled along as flow Cd for release through the outlet  40 . 
     As will be appreciated, impetus to the collected flow is provided by rotation of the rotating hub  32 . This impetus is to stimulate appropriate displacement of the secondary flow along the slots  35 ,  36 ,  37 . However, it is also important that the relative sizes, angles and positioning of the slots  35 ,  36 ,  37  as well as the gap  39  are considered. 
     The slot  35 , through the inlet  38 , collects annulus fluid (secondary fluid) about an upper surface  41  of the stator shroud  34 . This collected secondary fluid Ca is drawn, possibly by a displacement effect, and moved towards the return slot  37 . In order to facilitate collection, generally the inlet  38  would be arranged to have a sharp turn into a collector passage forming the collector slot  35 . It will be appreciated that by appropriate angling such drawing and collecting of the annulus secondary flows into the collector slot  35  is achieved and facilitated. Generally the collector slot  35  is arranged to be in the direction of sweep or travel for the hub  32  as it rotates.  FIG. 3  illustrates the collector slot  35  angled towards the direction of rotation illustrated by arrowhead D. In such circumstances the windage and other effects caused by rotation of the rotor hub  32  will facilitate drawing of fluid through the inlet  38  and along the slot  35 . 
     The return slots  37  are generally rounded to turn and provide impetus to the collected secondary flows. The return slots  37  are generally half moon shaped and utilised as indicated to provide impetus to the secondary flow Cb taken from the collector slot  35 . As can be seen in the gap  39 , a secondary flow Cb is turned and given impetus projection in the direction of arrowheads Cc. It will be understood that the output from the collector passage  35  is projected across the gap  39  and is then incident upon a hub surface  32  where it will be collected and as indicated provided with impetus by the rounded shaping of the return slots  37 . The return slots  37  are generally half moon shaped in order to draw and project the collected secondary flow Cc towards the outlet passage  36 . 
     The outlet passage  36  is generally angled away from the direction of sweep rotation shown by arrowhead D. It will be noted that the respective levels of the outlet from the collector passage  35  and the inlet for the outlet passage  36  are generally presented at different heights such that the respective cross flows Cb, Cc do not directly impinge with each other. It will be understood that the impetus provided by the return slot  37  will at least partially collimate the returned collected secondary flow Cc such that it becomes incident upon the inlet for the outlet passage of the outlet slot  36 , if the arrangement is properly or desirably configured. 
     As indicated above, coupling of the flow C across the slots  35 ,  36 ,  37  and gap  39  is achieved. It will be understood that the relative sizing of the slots as indicated will depend upon operational conditions and speed of rotation in the direction of arrowhead D and gap width  39 . Typically, a consideration will be made of operational conditions and slot  35 ,  36 ,  37  configurations chose for optimised or desired performance. Nevertheless, such configuration will be chosen to facilitate desired flow coupling in accordance with the invention. In terms of facilitating flow as indicated angling of the slots  35 ,  36  will be a factor along with a sharp turn at the inlet  38  and a smooth curved exit  40 . Thus, once secondary flow is within the thrall of the arrangement in accordance with the invention there is a resistance to back flow caused by the sharp turn at the inlet  38  relative to the outlet  40 . Typically, the collector passage of the collector slot  35  will be narrower than the width of the outlet passage of the outlet slot  36  again to provide a “bias” or easier direction of flow towards the outlet slot  36  compared to back along the collector slot  35 . As the slots  35 ,  36 ,  37  in accordance with the invention will typically be round it will be appreciated that the outlet slot  36  will have a greater diameter than the collector slot  35 . 
     The size and dimensions as well as the angle of the return slots  37  will be such that they are arranged to receive the collected secondary flow Cb and provide impetus and projection for that collected secondary flow towards the outlet slot  36 . In such circumstance as indicated above generally, the return slots  37  will have a half moon shape in order to utilise the rotation of the hub  32  to generate impetus in the flow Cc toward the outlet slot  36 . It is possible that the return slots  37  may include a wider dimension towards a bottom edge  43  compared to an upper edge  44  in order to provide greater “nip” impetus on a “sling shot” projection basis of the collected secondary flow Cc towards the outlet passage  36 . 
     It will be understood that with the collected secondary flow Cd presented with impetus further downstream of the stator  33  as indicated above will mean there is less likelihood of causing displacement of low momentum fluid upwards into the main flow A and therefore diminishing performance. The number of slots  35 ,  36 ,  37  in the respective shroud  34  and the hub  32  will depend upon operational requirements and performance. 
     It will be understood that the present invention may also be utilised with regard to windage from a stator and stator shroud  34 . In such circumstance it will be appreciated that a stator upstream of a rotor may create secondary flows which could impinge upon the performance of that rotor downstream. Such windage would be in an area denoted  45  in  FIG. 2 . In such circumstances, this windage may be utilised with regard to providing slots extending in the opposite direction to those described in the shroud  34  as well as a return slot in the downstream rotor hub to facilitate and remove secondary air impinging upon the performance of the arrangement. 
     As can be seen the inlet  38  for the collector slots  35  as well as outlets  40  for the outlet slots  36  are generally in the upper surface  41  of the shroud  34 . By the present invention the outlet flow Cd as indicated has impetus and therefore is dispersed more readily to avoid forcing low momentum air to impinge upon the major flow A. 
     Modifications and alterations to the present invention will be appreciated by those skilled in the art. Thus, as indicated above the angles and dimensions of the slots  35 ,  36 ,  37  as well as their relative configuration may be adjusted to particular circumstances and each other. It will also be understood that the gap  39  is important with respect to the efficiency of coupling of the flows Cb and Cc. This gap  39  may vary in width dependant upon operational temperature and therefore the configuration, size and distribution of the slots arranged to reciprocate such variations in use. The configuration, size and distribution will be set for the gap at the normal operational temperature of the arrangement.