shape in cross section as seen in the plan view fig4 according to the section line iv iv in fig3 which wall merges into the inner wall of the channel portion 5 in the area of an inner wall half lying opposite the mouth portion of the channel portion 4 in this way on the side directed away from the mouth portion of the channel portion 4 the flow guiding wall 13 forms a groove which extends coaxially or axially parallel with respect to the central longitudinal axis m the corresponding wall of the flow guiding wall 13 terminates flush and tight with the inner wall of the channel portion 5 on opposite sides over the entire height of the wall portion extending parallel to the inner wall of the channel portion 5 at a lower end area of the flow guiding wall 13 relative to the vertical orientation in the view according to fig2 and 3 the flow guiding wall 13 has a trough like curvature which extends to the remaining opposite inner wall portion of the channel portion 5 and merges there into the inner wall portion of the channel portion 5 below the mouth area of the channel portion 4 the trough portion thus formed accordingly extends in the shape of a portion of a ring underneath the vertically oriented and parallel v shaped wall portions of the flow guiding wall 13 and forms a catching basin for fluid flowing from the first channel portion 4 into the fluid flow chamber as is indicated by the dotted arrows in fig2 and 3 accordingly for the flow of fluid guided through the first channel portion 4 the flow guiding wall 13 forms a deflection surface by means of which the flow of fluid.