The influence of swirl angles on flame stability in pilot-scale plant

The article presents observations on the behaviour of coal dust-air flames imposed in pilot-scale plant by variations in the angle of swirl of the secondary air stream. It suggests some explanations for the existence of limits of stability, with the most probable for the lower one being the increase in dwell time of mixed eddies in the furnace, mitigating any stochastic increase in delays in mixing or in chemical ignition or, most likely, a combination of both. Two possible interpretations are offered for the occurrence of an upper limit. One is an increase in heat losses from the flame to the cooler furnace walls and the second is based on the concept of flame stretch.Some of the contrasts between the probable behaviour in pilot- and full-scale plant are emphasised and suggestions made, where present results might be applied to the latter.