Criteria for recirculation in confined jet flames

Recirculation is an important phenomenon in furneces. This paper discusses two methods of predicting when recirculation will occur in a cylindrical confined-jet combustion chamber. One of these methods is based on the Thring-Newby number [1], and the other is based on an extension of the theory developed by Curtet [2] for confined jets without combustion. In each case, use of the criterion developed requires knowledge of the value of a parameter which is an empirical measure of the extent of heat transfer from the furnace. Experiments were carried out to test these criteria and determine the values of the empirical “furnace design factors” for the combustion chamber used in the tests. Temperatures were measured, and recirculation was detected by means of a flow visualization technique in which boron trifluoride was used to produce smoke. Both criteria were found to be valid, although the one based on the work of Curtet is expected to be applicable to a wider variety of flow conditions than that based on the Thring-Newby number.