The impact surface (32) of the nozzle pipe (12,13) has indentations (34) arranged symmetrically enclosing the outlet opening (14) of the nozzle pipe. Between the indentations are radially outwardly aligned smooth sections. The indentations formed in the impact surface are designed as incisions (34) in the shape of circular ring segments spaced from each other and running in sections at right angles to each other or concentric with the outlet opening. The indentations can be formed by several spot-like conical imprints.