In this study, the effects of swirl intensity on the heating process of a gas stream, which is introduced into an inner hot gas region of a swirling tubular flame, have been investigated. In order to obtain fundamental knowledge on the heating system with a tubular flame, a burner with variable swirl intensities has been made and the effects of the swirl number on the flame characteristics and the temperature fields have been experimentally investigated. In addition, using a PIV system, the flow fields are visualized in detail under combustion conditions as well as under cold flow conditions to discuss the heating process in detail. Main results obtained are (1) The tubular flame can be established for a wide range in the equivalence ratio and the flow rate of the combustible mixture. Even when a cold air stream is introduced into the tubular flame, the flame is stable and the air stream is heated gradually along the axis of the burner. (2) The temperature rise on the axis of the burner is slow for the swirl numbers 0.35 and 0.7, whereas it is fast for the swirl number of 1.4. (3) Detailed flow visualization under combustion conditions has shown that, in contrast to cold flow conditions, the central air stream is laminarized due to the axial gas expansion of the burned gas, which suggests that the enhancement of the heating rate with an increase of the swirl number can be explained mainly through an increase of the heat transfer area between the two streams induced by the rotational motion of the swirl burner.

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