Residence time distributions (RTD) in a confined, cold swirling how have been measured with a fast-response probe and helium as a tracer. The test-rig represented a scaled down version of a burner. The effect of variation of flow velocities and swirl angle on the flow pattern in the near-burner zone of the laboratory furnace-model were studied. RTD results have been used to derive a chemical reaction engineering model for the mixing process. The model is based on a combination of plug flow reactors and continuous stirred tank reactors, which represent the main how characteristics in regard of mixing in the near-burner zone. The model is well suited to handle mixing in complex flows with several zones of recirculation. Simulated RTD curves compared well with those of the experiment at the two swirl numbers studied. Modelling of the mixing is a prerequisite to handle the chemistry. A simplified, well characterised flow pattern makes it possible to investigate the importance of mixing intensity on the (pollution) chemistry in furnaces. The reactor model developed here will be the basis for the development of a chemical reaction engineering combustion model.