Experimental understanding on the dynamics of micro-explosion and puffing in ternary emulsion droplets

Madan Mohan Avulapati,Thanos Megaritis,Jun Xia,Lionel Ganippa

doi:10.1016/j.fuel.2018.11.112

Abstract

Dynamics of puffing and micro-explosion phenomena occurring in ternary fuel emulsion droplets under high temperature environment were explored using high speed backlight imaging technique. A single droplet composed of diesel-biodiesel-ethanol emulsion was placed at the tip of a 75 µm gauge thermocouple and introduced rapidly into a furnace maintained at 500 °C. Several interesting features such as oscillation of suspended droplets, physical transformations occurring within the droplet, vapour expulsion, puffing, micro-explosion, sheet formation, perforations, growth of perforations, sheet disintegration and rotation of secondary droplets were observed. High resolution image analysis revealed separation of emulsion components within the core of the suspended droplet, which appeared either as a single nucleus or multiple nuclei. Two distinct types of micro-explosion were identified. For droplets encountering a single nucleus at the core resulted in a stronger vapour expulsion followed by intense micro-explosion. For droplets having multiple nuclei at the core resulted in a weaker vapour expulsion and slower growth of droplet prior to micro-explosion. Both types of micro-explosion process resulted in a number of child droplets. For the case of strong vapour expulsion nearly 80% of its child droplets have their sizes distributed within 150 μm compared to 60% for weaker vapour expulsion. The child droplets that were generated from the primary events of both puffing and micro-explosion cascaded further into secondary and tertiary events of puffing and micro-explosion in freely suspended environment.

Highlights

Atomisation of liquid fuel produces a number of relatively smaller size droplets that leads to rapid evaporation, improved mixing and reduced soot formation in aircraft engines and in other power generation applications
When a ternary emulsion droplet consisting of diesel-biodieselethanol was introduced rapidly into high temperature environment at 500 °C, random and unpredictable chaotic events of either vapour expulsions or puffing or micro-explosion or a combination of these events were observed explicitly for these droplets
The observed cascading effect of puffing and micro-explosions occurring in primary, secondary and tertiary size droplets in our experiments indicate that micro-explosion is scalable and it has the potential to reduce PM and to reduce the time scale of combustion to curtail NOx emissions, which could be vital for emission control in larger size compression ignition engine

Summary

Introduction

Atomisation of liquid fuel produces a number of relatively smaller size droplets that leads to rapid evaporation, improved mixing and reduced soot formation in aircraft engines and in other power generation applications. When emulsion droplets are exposed to higher temperature it undergoes preferential evaporation, triggering puffing and micro-explosion to form a number of smaller size droplets that enhances mixing [1]. Puffing is a process where a part of emulsion droplet expels a mixture of liquid/vapour jet that disintegrates into smaller droplets due to rapid evaporation of higher volatile components of emulsion. Flash evaporation of lighter components in an emulsion droplet leads to micro-explosion, where the whole droplet fragments instantaneously to form a large number of smaller size droplets. During vaporisation or combustion of a multicomponent fuel droplet, higher volatile fractions tend to reach thermodynamically metastable superheat temperature, which leads to nucleation and bubble growth. Rapid expansion of vapour inside a droplet results in either total or partial disintegration of the liquid droplet

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Conclusion