Method and system for a leakage controlled fan housing

1. A fan assembly comprising a fan casing including a longitudinal centerline axis extending from an upstream inlet opening to a downstream outlet opening, said assembly comprising:

a rotor comprising a hub and a plurality of rotor blades extending radially outward from said hub, said plurality of rotor blades each comprising a leading edge and a trailing edge in a direction of fluid flow through said fan casing, and a blade tip at a radially distal end of said blade, said tips of said plurality of rotor blades defining a rotor diameter, said rotor configured to rotate about said centerline axis;

a first substantially cylindrical casing segment extending circumferentially about and substantially axially aligned with said blade tips, said first casing segment comprising a first inner diameter that is greater than said rotor diameter;

a second substantially cylindrical casing segment configured to extend axially in a direction opposite a flow of fluid through said fan casing, said second casing segment comprising a second inner diameter, said second inner diameter less than said first inner diameter and said rotor diameter; and

a third casing segment extending between said first and second casing segments, said third casing segment comprising the first inner diameter along a first circumferential edge, the second inner diameter along a second circumferential edge, and a surface extending therebetween, said surface is spaced apart from the leading edge of the plurality of blades to form a leakage gap that is less than a difference between said first inner diameter and said rotor diameter.

2. A fan assembly in accordance with claim 1 , wherein said fan casing is fabricated unitarily using a casting process.

3. A fan assembly in accordance with claim 1 , wherein said fan casing is fabricated from at least one of aluminum and an aluminum alloy.

4. A fan assembly in accordance with claim 1 , wherein said surface is conical.

5. A fan assembly in accordance with claim 1 , wherein said surface is at least one of convex and concave.

6. A fan assembly in accordance with claim 1 , wherein said surface is arcuate.

7. A method of forming a casing for a rotatable member, said method comprising:

forming a first radially inner surface of a first casing segment that is substantially cylindrical about an axis of rotation of the rotatable member and having a first inner diameter, said first inner diameter selected to be greater than an outer diameter of the rotatable member;

forming a second radially inner surface of a second casing segment that is substantially cylindrical about an axis of rotation of the rotatable member and having a second inner diameter, said second inner diameter selected to be less than the outer diameter;

forming a step segment including a first circumferential edge adjacent the first casing segment and a second circumferential edge adjacent the second casing segment and a surface extending between the first edge and the second edge, the first edge having the first inner diameter and the second edge having the second inner diameter; and

machining the surface to provide a predetermined gap between the surface and an adjacent leading edge of the rotatable member that is less than a difference between the first inner diameter and the outer diameter of the rotatable member.

8. A method in accordance with claim 7 , wherein said forming comprises forming the first, second, and step segments as a single integral unit using a casting process.

9. A method in accordance with claim 7 , wherein said forming comprises forming the first, second, and step segments using at least one of an aluminum and an aluminum alloy casting process.

10. A method in accordance with claim 7 , further comprising at least one of increasing the first inner diameter of the first casing segment and reducing the outer diameter of the rotatable member to increase a clearance gap between the first casing segment and the rotatable member.

11. A method in accordance with claim 7 , wherein forming a step segment comprising machining the surface to a conical contour.

12. A method in accordance with claim 7 , wherein forming a step segment comprising machining the surface to an arcuate contour.

13. A fan casing including a longitudinal centerline axis extending from an upstream inlet opening to a downstream outlet opening, said system comprising:

a first substantially cylindrical casing segment configured to extend axially and radially outboard of a path of a blade tip, said first casing segment comprising a first inner diameter;

a second substantially cylindrical casing segment configured to extend axially in a direction opposite a flow of fluid through the fan casing, said second casing segment comprising a second inner diameter, said second inner diameter less than said first inner diameter; and

a third casing segment comprising a surface extending between said first and second casing segments, said third casing segment comprising the first inner diameter along a first circumferential edge and the second inner diameter along a second circumferential edge, said surface is spaced apart from the leading edge of the plurality of blades to form a leakage gap that is less than a difference between said first inner diameter and said rotor diameter.

14. A fan casing in accordance with claim 13 , wherein said fan casing is fabricated unitarily using a casting process.

15. A fan casing in accordance with claim 13 , wherein said fan casing is fabricated from at least one of aluminum and an aluminum alloy.

16. A fan casing in accordance with claim 13 , wherein said surface is at least one of arcuate, stepped, and straight in profile.