## 1 Introduction
Although the temperature gradient and content gradient can cause Marangoni flow in the droplet, the measured results revealed that it is relatively weaker than the vapor-induced flow. These studies show that the vapor flow around the Leidenfrost droplet is usually the main factor to induce the internal flow in the droplet. In practical spray combustion process, droplets are dominantly burnt under the unconstrained environment. Different from sessile droplets and Leidenfrost droplets, which are strongly affected by the solid surface, droplets suspended on fibers are much closer to the combustion condition. Previous studies about internal flow in droplets suspended on fibers are limited [17; 18; 19]. [PERSON] et al. [17] studied the flow field in the suspended water/oil droplet with the upward vapor flow. They found that the values of flow velocities in the aqueous phase and oil phase are rather different, while the velocities both show increasing trends as the temperature of upward vapor flow increases. [PERSON] et al. [18] studied the internal flow fields in aviation kerosene droplets during combustion. They used the soot particles in the droplet as tracers to visualize the fluid motion in the droplet and also observed the dual-vortices flow field structure in the cross-section of the droplet originated from a toroidal vortex. They concluded that the Marangoni effect caused by the temperature gradient on the droplet surface is the main reason for the observed flow field structure. On the other hand, [PERSON] et al. [19] proposed that both thermal Marangoni effect and solutal Marangoni effect exist during the evaporation of multicomponent droplets. They put the droplet in a closed chamber to avoid any surrounding disturbance and observed an oscillating convection inside the droplet that was attributed to the competition between the thermal Marangoni effect and solutal Marangoni effect. It is obvious that these studiesusually did not consider all the factors that can influence the internal flow, especially the upward vapor flow during combustion and the gravitational effect caused by the content gradient. In particular, previous studies did not pay attention to the influence of internal flow on the mass transfer process and content gradient formation during the combustion of multicomponent droplets. Therefore, more studies are desired for a deep understanding of the internal flow in a suspended multicomponent droplet during combustion. In this work, micro-PIV is used to measure the internal flow fields in suspended binary solution droplets during combustion. Two \(\mathrm{PODE_{n}}\) components, which are among the major e-fuel families and have been recognized as promising alternative fuels and additives for combustion engines [20], i.e., \(\mathrm{PODE_{2}}\) and \(\mathrm{PODE_{4}}\), are used as the highly volatile component and weakly volatile component, respectively. The influential factors for the internal flow field structures are revealed. The spatially averaged flow velocity evolution and the velocity distribution in the flow field at the stable evaporation stage are derived. The influence of internal flow on the mass transfer process and content gradient formation in the binary droplets is also analyzed and discussed.