A numerical analysis of a correlation of swirl number in a coannular swirl injector

Abstract

Gas turbine combustors using lean-premixed combustion often utilizes a swirling flow to generate a central-toiroidal recirculation zone (CTRZ) for flame stabilization. The characteristics of a CTRZ, such as the volume of a CTRZ, can be predicted by the swirl number at the combustor inlet (or the exit of the swirl injector). In the present study, a new numerical model for the swirl number is derived to estimate the swirl number at the end of a swirl-injector pipe, especially for the pipe with a variable cross-sectional area, where existing formulas cannot be applied accurately. First, a new one-dimensional model to predict the swirl decay rate is proposed to estimate the swirl intensity at the end of the swirl injector. Next, the proposed model is validated with the CFD simulations in various swirl injectors. As a result of validation, the proposed model is confirmed to be in good agreement with the results in CFD simulations. In Chapter 4, the variation of decay rate of swirl with the factors affecting the swirl is analyzed through the proposed model. It is found that the decay rate of swirl decreases as the inlet Reynolds number increases and initial swirl intensity decreases. It is also has been shown that a straight annular pipe can minimize the decay rate of swirl when having a specific the ratio of inner radius to outer radius of a swirl injector. This value is estimated as about $0.35$ according to the proposed model. It is expected that the proposed model of variation of swirl number is very useful for the selection of appropriate swirler to obtain the desired swirl number in swirl injectors.