but in each case the following relationship holds v ox u2266 2 u00d7 v s where v s is the sonic velocity of the primary oxidant stream usually air in the conditions of use the oxygen enriched zone is indicated in fig8 at 400 400 a 400 b and 400 c the angle u03b1 of each individual oxidant lance 100 100 a 100 b and 100 c again independently ranges from about 20 u00b0 to about 160 u00b0 again u03b1 is defined as the angle between the oxidant lance symmetry axis and the combustion wall surface another angle is defined in fig8 a angle u03b3 there being shown u03b3 1 u03b3 2 u03b3 3 and u03b3 4 angle 8 is defined as the angle between the oxidant lance symmetry axis and the combustion wall surface in a plane perpendicular to which the angle u03b1 is measured the angle u03b3 in each independent case ranges from about 20 u00b0 to about 160 u00b0 there is also defined in fig8 lengths l 1 l 2 l 3 and l 4 the length l i is the distance between the respective oxidant lance and its closest wall surface with the following relationship holding true l i lt l ch u00f7 2 where l ch represents a length or width of the combustion chamber 0045 fig9 a 9 b.