The present invention relates to an assembly for providing a laminar gas flow serving, e.g., to form a fluidized mixture of solids with a gas, said assembly comprising a plenum chamber (3) forming a closed space containing a gas infeed pipe (5) for blowing gas into the plenum chamber and a feeder pipe (1) into which said laminar flow is to be formed and whose one end (2) is adapted into said plenum chamber. The lower end (2) of said feeder tube (1) adapted into said plenum chamber (3) is formed into a diffuser mouth (8) of rotationally symmetrical shape having its inner diameter flared in a horn-like manner so that the inner diameter of the tube end is expanded smoothly toward the rim of its diffuser mouth facing the bottom of said plenum chamber (3), and the bottom (10) of said plenum chamber (3) at the area facing said mouth (8) is adapted at a constant distance from the rim of said mouth (8) over the area facing the perimeter of the mouth.