of the peripheral wall 8 a depending on how many walls 14 a there are in the secondary channel 8 concerning the rim 19 of the inlet 11 and the edge 18 of the opening 17 their arrangement in the secondary channel 8 must also conform with the need to form a baffle for the air flow 3 b since the inlet 11 and the first opening 17 do not change position the rim 19 and the edge 18 are at different heights as already described in this application the edge 18 u2032 of the second opening 17 u2032 must be at a height less than the height of the edge 18 of the first opening 17 in relation to the total height of the secondary channel 8 in the same way the n th edge 18 of the n th opening 17 must be at a height greater or less than the height of the n 1 th edge of the n 1 th opening 17 in order to form a baffle for the air flow 3 b lastly the outlet 12 must be positioned so as to respect the baffle structure in relation to the number of walls 14 a and therefore of openings 17 present in the secondary channel 8 in a variant of the previous embodiment the inlet 11 to the secondary channel 8 is situated in a cavity 25 as shown in fig1.