Internal Boiling to Flame Flickering: Insights into Various Spatio-temporal Dynamics of Nanofuel Droplet Combustion

Abstract

Addition of metal and metalloid particles (micron and sub-micron) to conventional fuels like Jet-A has been a ‘liquid-fuel extender’ technique i.e. increased specific energy for less amount of liquid fuels. One of the main advantages of metallic nanoparticles (Aluminium, Iron, and Boron, etc.) is the increased attainable combustion specific energy due to reduced temperatures of metal combustion. Furthermore, nanoparticles of metallic oxides, like cerium oxide, help in the reduction of soot emissions and pollutants like carbon monoxide and NOx because of their oxygen-carrying ability and their capability to oxidize soot particles in their precursor stage. Nanofuel droplets (conventional fuel +nanoparticles) show remarkable instabilities like internal boiling which lead to enhanced atomization of the droplets further leading to flame heat release augmentation. The research work is primarily focused the nanofuel droplet combustion; pathways of secondary atomization, flame dynamics, and the causality between the droplet shape and flame heat release. Here, the possible mechanisms responsible for internal boiling characteristics and its definitive effect on volumetric oscillations, shape deformations of droplets, and average flame heat release are articulated holistically under one umbrella. Being a multiphase system, nanofuel droplets exhibit heterogeneous nucleation leading to formation of vapor bubble mushrooms. These vapor bubbles further grow and eject from droplet free surface thereby opening a new pathway of parent droplet secondary atomization. Continuous disintegration of nanofuel droplets ensures homogeneous air-fuel mixture leading to increased combustion efficiency. Using time-resolved optical diagnostic techniques (High-speed PIV, High-speed shadow imaging, Chemiluminescence Imaging) the droplet shape and flame heat release coupling is investigated. The first part of the work encompasses the combustion dynamics for both low and high-vapour pressure nanofuel droplets in pendant mode where the droplet is suspended on a cross-wire arrangement. The studies emphasize the alterations in the physical arguments, combustion behavior, nucleation, and bubble mechanisms on a relative basis i.e. as compared to pure fuel droplets. For high-vapour pressure nanofuel droplets, a theoretical vaporization timescale is advocated which considers natural convection-based evaporation, mass loss due to daughter droplet ejections, and flow through porous media. In order to understand the sole effect of nanoparticle addition on internal ebullition while removing the wire-effects, the next set of experiments are conducted for acoustically levitated nanofuel droplets under external radiative heating. A theoretical non-dimensional time scale is coined to estimate the minimum value of droplet size necessary for exhibiting boiling. A unique three-dimensional regime map is proposed to correlate the breakup modes with droplet size, PLR, and heating rates. Further, droplet combustion is investigated in free-falling mode in a drop-tower arrangement. The course of droplet motion under gravitational force is characterized by its temporally varying velocity. Consequently, the droplet flame exhibits hydrodynamic topological self-tuning as a function of the transient Reynold number. Using a round-jet analogy, a linear relationship between the flame height and Reynolds number is established. Lastly, the acoustic-flame coupling for buoyant droplet diffusion flames is explored. It is shown that the spectral signature of a single droplet flame cannot be solely scaled using initial diameter rather it is transient in nature. The availability of the highest possible frequency is a function of instantaneous droplet diameter. The flame response is always associated with the low-frequency band ~ (6−22) Hz. Using bandpass filtering, the existence of multiple vertical length scales in the flame is isolated which are well correlated with kinematic scaling √(g/ℎ). The acoustic perturbations modulate the flame surface or heat release only at lower frequency bands by feeding energy into the natural vortical stability modes of a buoyant plume. Subsequently, it is established that the shedding length scale of the flame shortens by more than 25 % with increase in acoustic pressure. Theoretically, it is interesting to note that while the acoustic pressure amplitude is crucial for determining the convective length at which a flame rolls up, the frequency of excitation has no role to play.