Brush seal

1. A brush seal for sealing between a first component and a second component which relatively rotate during normal use, comprising:

an annular housing which is attached to a first component in use;

a plurality of bristles arranged in multiple circumferential rows between an upstream side and a downstream side, each of the multiple circumferential rows extending around a circumference of the brush seal, each bristle of the plurality of bristles having a fixed end attached to the housing and a free end, wherein the free ends of at least some of the plurality of bristles contact the second component during normal use to provide a sealing surface,

wherein the sealing surface includes a plurality of discontinuities along at least one row of the multiple circumferential rows around the circumference of the brush seal for providing hydrodynamic lifting force in use,

wherein the at least one row of the plurality of bristles includes a first group which contact the second component in use, and a second group which are arranged to provide the discontinuity wherein the bristles of the first and second groups are randomly interspersed along the at least one row around an arcuate portion of the brush seal,

wherein the at least one row of the plurality of bristles is provided at either or both a most upstream side and a most downstream side of the plurality of bristles, and

wherein the bristles each have a longitudinal axis, an end face of the free ends of the bristles are at an angle relative to the longitudinal axis, the angles of the first group are at a common angle and at least some of the free end face angles of the second group of bristles are different to the free end face angles of the first group of bristles.

2. A brush seal as claimed in claim 1 , wherein the plurality of bristles are different lengths such that the free ends of the first group of bristles contact the second component in use, and the free ends of the second group do not contact the second component in use.

3. A brush seal as claimed in claim 1 , wherein the free end face angles of the second group include a plurality of free end face angles.

4. A brush seal as claimed in claim 1 wherein the bristles of either or both of the first or second groups have different diameters.

5. A brush seal as claimed in claim 2 , wherein the second group of bristles is shorter than the first group bristles by between 0.1 mm and 2 mm.

6. A brush seal as claimed in claim 2 , wherein the second group includes between 2.5% to 50% of the available bristles.

7. A brush seal as claimed in claim 2 wherein a minimum difference in length of the plurality of bristles in the first group and length of the plurality of bristles in the second group is greater than half a diameter of one of the bristles of the plurality of bristles.

8. A brush seal as claimed in claim 1 , wherein some of the second group of bristles contact the second component in use, the discontinuities being provided by angled end faces.

9. A brush seal for sealing between a first component and a second component which relatively rotate during normal use, comprising:

an annular housing which is attached to a first component in use;

a plurality of bristles arranged in multiple circumferential rows between an upstream side and a downstream side, each of the multiple circumferential rows extending around a circumference of the brush seal, each bristle of the plurality of bristles having a fixed end attached to the housing and a free end, wherein the free ends of at least some of the plurality of bristles contact the second component during normal use to provide a sealing surface,

wherein the sealing surface includes a plurality of discontinuities along at least one row of the multiple circumferential rows around the circumference of the brush seal for providing hydrodynamic lifting force in use,

wherein the at least one row of the plurality of bristles includes a first group which contact the second component in use, and a second group which are arranged to provide the discontinuity wherein the bristles of the first and second groups are randomly interspersed along the at least one row around an arcuate portion of the brush seal,

wherein the at least one row of the plurality of bristles is provided at either or both a most upstream side and a most downstream side of the plurality of bristles, and

wherein the bristles of the second group have a range of different lengths.