Abstract: The invention relates to variation of concentrations of abrasive particles in a stream of drilling fluid mixed with abrasive particles, passed as an abrasive jet through abrasive nozzle(s) of a drill bit along rotations thereof, to vary the erosive power of the stream along angular sections of the borehole for directional drilling. During subsequent time periods majorities of abrasive particles are alternately deflected into two parallel channels with a different flow resistance. A resulting velocity difference between said majorities makes that the subsequently deflected majorities recombine downstream of the channels, so that high concentration stream portions of the combined majorities are formed which alternate low concentration stream portions. Synchronising the frequency of the stream portions with the rotational velocity of the drill bit results in a consistently higher erosive power of the abrasive jet within a selected angular section of the borehole than outside thereof.

Abstract: The invention relates to increasing a concentration of particles (92) in a circumferentially enclosed stream (90) of a fluid mixed with the particles in a target area (90ct) of a cross-section (90c) of the stream transverse to a flow direction (90f) of the stream. Deflecting surfaces (31.1, 31.2, 31.3) are arranged in the stream to extend over respective angular zones (30?1, 30?2, 30?3) with respect to an axis (la) through the target part. The deflecting surfaces slant in the flow direction in a direction from the circumference of the stream towards the target area, and define angularly in between them multiple bypass openings (32.1, 32.2, 32.3) which extend over angular zones complementary (30?1, 30?2, 30?3) to the angular zones over which the deflecting surfaces extend, and which are larger than the particles. The deflecting surfaces deflect at least a part of the particles inwardly towards the axis.