The effect of the residence time distribution on the performance and efficiency of combustors

The fraction of unburned fuel as a function of residence time in a combustor was computed for two extreme cases: the perfectly-stirred and the plug-flow type combustors. It is shown that an “ideal” residence time distribution can be obtained by the combination of the two types of combustors in such a way that the perfectly stirred part is followed by the plug flow combustor in series. This combination of the two combustor types enables the highest volumetric rates of combustion to be obtained for a given combustion efficiency. Experiments were carried out both with a 1/10 scale water model of the IJmuiden furnace and also in the IJmuiden tunnel furnace. A salt solution was introduced as a tracer into the “burner fluid” of the water model and the residence time distribution was determined by measuring the decay of the salt concentration with time in the exit stream after cutting off the tracer flow. The form of the residence time distribution curves indicated that the combustor volume consisted of two parts, a well stirred and a plug flow type volume fraction in series. By introducing the “burner fluid” in the form of a swirling jet and by varying the degree of swirl it was possible to vary the ratio of the mean residence time in the perfectly stirred section to that in the whole combustor ( s / ) over a wide range. Residence time distributions were also determined in the IJmuiden furnace while burning pulverized anthracite. The tracer in this case was argon and the technique was the same essentially as in the water model experiments. The comparison of the residence time distributions in the cold model and in the furnace showed good agreement when the value of the dimensionless group G φ /G x ·ϑ was maintained the same for both model and prototype. ( G φ is the axial flux of the angular momentum of the swirling burner fluid, G x the axial flux of the linear momentum, and ϑ the burner radius.) Good agreement was found also between the experimental values of the unburned fraction of the fuel at the boundary of the two types of zones and those predicted from theory.