holder the feared failure of mechanical flame holders due to overheating followed possibly by serious failures of machine sets is therefore prevented in addition the flame does not lose any heat to the cold walls except by radiation this also aids in homogenizing the flame temperature and therefore contributes to lower noxious emissions and good combustion stability according to the invention measures are now provided to increasingly deform the swirl flow 9 inside the premix section 13 radially it is preferred that this deformation takes place symmetrically however this is not mandatory an important characteristic hereby is that this deformation is brought about by reducing the free flow cross section 18 the wall 21 of the chamber 6 has in a downstream part 20 at least one section 22 that has a smaller slope with respect to the burner axis 4 than an upstream part 19 this means that the contour 21 of the burner chamber 6 u2014 which when seen over its cross section is approximately circular u2014 is provided with sections 22 that are distributed over the circumference and deviate from the circular shape of the chamber contour 21 towards the center axis 4 i e constrict the chamber 6 as is shown in the longitudinal section schematically shown in fig2 a 2 d in this context it was found to be advantageous after all that the deformation of the flow is.