Study on evaporation and atomization characteristics of methanol in the side-wall air inlet pre-evaporation tube

Methanol holds significant promise as a sustainable fuel for turbojet engines in unmanned aerial vehicles (UAVs). To better adapt to methanol's properties and overcome the inherently poor atomization of the pre-evaporation tube, a novel pre-evaporation tube is developed with side-wall air inlets to enhance atomization performance and thereby improve the evaporation ratio. Utilizing backlit imaging, four distinct flow patterns—annular flow with continuous liquid film (AC), annular flow with semi-continuous liquid film (AS), annular flow with discrete liquid film (AD), and dispersed flow (DF) are identified. Further results show that increase of the ratio of the air and fuel (RAF) and temperatures suppress liquid films, with the modified design promoting DF dominance. A high-resolution composite defocus shadowgraph (CDS) technique which reaches the 3.57μm/pix is proposed and being used to quantify the evaporation ratio (ER) and droplet distributions. The results demonstrate that ER is quadratic dependence on RAF due to competing atomization enhancement and residence time reduction. The results also show that the modified pre-evaporation tube achieves higher ER than the traditional. CDS and background-oriented schlieren (BOS) integrated analysis delineates three spray regions (droplet core, transition zone, pure vapor) and shows linear radial decay of vapor concentration in the pure vapor area.