The row of air guide members comprises a nozzle-mounted air guide member having around the air guide opening a plurality of air ejection holes, a first group of air guide members located downstream of the nozzle-mounted member and positioned between the nozzle-mounted member and the point at or about which the inboard boundary of the auxiliary air flow from each air ejection hole intersects the axis of the air guide channel of the row of the air guide members, and a second group of air guide members located downstream of the first group of air guide members. The air guide openings of the first group guide members are larger than the air guide opening of the nozzle-mounted member and gradually reduced in diameter with increase of distance from the nozzle-mounted member. The air guide openings of the second group members are identical to each other and to the air guide opening of the nozzle-mounted air guide member.