Numerical Investigation of Droplet Distribution from a Pre-filming Air-blast Atomizer

Abstract

The combustion characteristics and emission from a gas turbine greatly depends on the spray characteristics. Pre-filming air-blast atomizers are commonly used in gas turbines due to their high spray cone angle and shorter atomization axial distance when compared to direct air-blast atomizers. Various numerical models are available in the open literature for predicting droplet characteristics. In the present study commercial code FLUENT 6.3 is used to predict the spray characteristics. The numerical results are validated against experimental results and further the spray characteristics in terms of radial Sauter mean diameter (SMD), mass flux distribution at varied axial distance and swirl number (S) have been discussed. The variation of predicted SMD with swirl number demonstrated a decrease in its value for each level of increase in the swirl number. Smaller droplets start to appear further downstream in addition to significant increase in radial spread of droplets. Negligible mass contribution from the smaller droplets at the boundary compared to that from bigger droplets at the centre was observed. Recessing the liquid tube at the exit of air nozzles showed a slight decrease in SMD. Recessing the liquid tube in the exit air nozzles results in a slight in SMD values, hence, the recessing of liquid post in the atomizer within certain limit might be beneficial not only helping in flame holding but also resulting in finer spray. The lessons learnt from this study on use of CFD for simulating the atomization process is highlighted.