Insights into the dynamics of wake flame in a freely falling droplet

Abstract

The combustion of a freely falling dodecane droplet has been studied experimentally in a droptower-like facility under ambient conditions. A unique ignition mechanism is used by igniting the droplet in pendant mode and releasing it to fall freely. This unveils a different type of droplet wake flame behavior which is explored in this study. Initially, the droplet flame transitions from fully enveloped to a wake flame configuration due to forward extinction. The wake flame has similar characteristics as a laminar lifted triple-flame. As the droplet accelerates, the flame stand-off increases continuously. The change in wake flame topology and intensity occurs in two different regimes corresponding to different droplet diameters. A new non-dimensional parameter has been derived to account for the local balance between buoyancy and momentum diffusion that alters the fuel availability. To explain the flame topological evolutions and transitions for different droplet diameters and Reynolds numbers, a theoretical formulation has been proposed based on the momentum diffusion from surrounding due to relative motion. Further, at very high Reynolds number, flame stretching or shedding regime occurs, causing momentary spikes in flame intensity due to the interaction with asymmetric vortex shedding induced by the Bernard-Von Karman instability. Interestingly, the flame shedding height follows the buoyant flickering scaling, even for the momentum-dominant droplet wake flame. Additionally, the circulation build-up mechanisms are shown to be responsible for the flame shedding events for droplet wake flame at high Reynolds number.

Insights into the flame transitions and flame stabilization mechanisms in a freely falling burning droplet encountering a co-flow

“…The inner fuel-rich branch of the flame front has , whereas the outer fuel-lean branch has (Ko & Chung 1999; Vadlamudi et al. 2021), see figure 3( a ).

Figure 3.( a ) Schematic of edge-stabilized wake flame and edge-flame stabilization.

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“…Using the previous experimental data (Vadlamudi et al. 2021), the instantaneous flame height characteristics for the whole range of v rel were measured from the binary images obtained by performing Otsu's thresholding on the flame images, as explained in § 2. For this particular set of experiments, since the flame is stabilized in the far wake, the droplet velocity cannot be used for evaluating the relative bulk flow velocity perceived by the flame ( v rel ).…”

“…Because of this, unlike our previous studies, wherein a specific stable flame existed throughout the observation period, multiple flame regimes ranging from recirculation zone-stabilized wake flames to edge-stabilized flames, in both upright and reversed configurations, were noted, in addition to the enveloped flame structures around the droplet. The near-wake edge-stabilized flame was unique and was not explored in our previous works (Pandey et al 2020;Vadlamudi et al 2021). However, parallels could be drawn from the lifted flame configuration observed by Vadlamudi et al (2021) as, in both cases, the flame was stabilized locally by the same mechanism.…”

“…Some of these were addressed in the drop tower experiments conducted by Chen & Lin (2012), Pandey et al. (2020), Vadlamudi, Thirumalaikumaran & Basu (2021) and other researchers, wherein the enveloped and wake flame dynamics of freely falling burning fuel droplets was studied.…”

“…The suppression of the BVK instabilities at high Re was observed due to the high temperatures generated by the flame in the wake region. Vadlamudi et al (2021) used a unique ignition mechanism wherein the fuel droplet was ignited in pendant mode and then released to fall freely under gravity. As the droplet accelerated from rest, the flame transitioned from the enveloped to the wake configuration, and the flame was stabilized in the far wake with a tribrachial structure (see figure 1b).…”