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Heat transfer and breakup of emulsified fuel droplets

Fostiropoulos, S. R. (2020). Heat transfer and breakup of emulsified fuel droplets. (Unpublished Doctoral thesis, City, University of London)

Abstract

The current work investigates the breakup of a single emulsion droplet under pressure and temperature conditions realized in Diesel engine at the time of injection. The heating of immiscible heavy fuel oil-water droplets, termed as W/HFO emulsions, leads to explosive boiling of the water inside the surrounding fuel, due to their different boiling points; the resulting accelerated droplet breakup regimes are termed as either puffing or micro-explosion. The relevant processes are investigated here by numerical simulations based on the solution of the Navier-Stokes equations alongside with the energy conservation equation and transport equation of the formed interfaces using the Volume of Fluid (VoF) method. In contrast to past studies, which predefine the presence of vapor bubble inside the parent HFO droplet, this is modeled here with the aid of a phenomenological model based on the local temperature field and degree of superheat. Following their formation, the growth rate of the bubble is computed with the aid of the OCASIMAT phase-change algorithm. Simultaneously to internal boiling, the fuel droplet is also subjected to aerodynamic-induced deformation due to the surrounding air flow. Thus, the performed simulations quantify the relative time scales of the aerodynamic-induced and the emulsion induced breakup mechanisms. Initially, a benchmark case demonstrates the detailed mechanisms taking place, concluding that droplet fragmentation occurs only at a part of the fuel-gas interface, resembling characteristics similar to puffing. Next, a parametric study examining the effect of droplet Weber number is performed for both W/HFO emulsion and neat HFO droplets. It is observed that puffing process can speed up the breakup of the droplet relative to aerodynamic breakup for the specific range of conditions examined. As a next step, this model is further applied to a wide range of pressure, temperature, water droplet surface depth and Weber number. The obtained results from CFD model predictions are used to calibrate the parameters of a fitting model estimating the initiation breakup time of the W/HFO droplet emulsion with a single embedded water droplet. The model assumes that the breakup time can be split in two distinct temporal stages. The first one is defined by the time needed for the embedded water droplet to heat up and reach a predefined superheat temperature and a vapor bubble to form; while the succeeding stage accounts for the time period of vapor bubble growth, leading eventually to emulsion droplet break up. It is found that the fitting parameters are ±10% accurate in the examined range of conditions.

Publication Type: Thesis (Doctoral)
Departments: Doctoral Theses
Doctoral Theses > School of Mathematics, Computer Science and Engineering Doctoral Theses
School of Mathematics, Computer Science & Engineering > Engineering > Mechanical Engineering & Aeronautics
Date Deposited: 07 Apr 2021 15:32
URI: https://openaccess.city.ac.uk/id/eprint/25882
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