Numerical investigation of heavy fuel oil droplet breakup enhancement with water emulsions

The heating and explosive boiling leading to fragmentation of immiscible heavy fuel oil-water droplets, termed as W/HFO emulsions, is predicted numerically by solving the incompressible Navier-Stokes and energy equations alongside with a set of three VoF transport equations separating the interface of co-existing HFO, water liquid and water vapour fluid phases. Model predictions suggest that explosive boiling of the water inside the surrounding HFO, ought to their different boiling points, accelerates droplet breakup; this process is termed as either puffing or micro-explosion. In contrast to past studies which predefine the presence of vapor bubbles inside the water droplet, this is predicted here with a phenomenological model based on local temperature and superheat degree. Following their formation, the growth rate of the bubbles is computed with OCASIMAT phase-change algorithm. Moreover, the fuel droplet is simultaneously subjected to convective air flow which further contributes to its deformation. As a result, the performed simulations quantify the relative time scales of the aerodynamic-induced and the emulsion-induced breakup mechanisms. The conditions examined refer to a highly viscous emulsified heavy fuel oil droplet in a gas phase having fixed temperature and pressure equal to 1000 K and 30 bar, respectively. Initially, a benchmark case demonstrates the detailed mechanisms taking place, concluding that droplet fragmentation occurs only at a part of the fuel-air interface, resembling characteristics similar to puffing. Next, a parametric study with Weber number (Oh=0.9,We<200) shows that puffing process can speed up to 10 times the breakup of the droplet relative to aerodynamic breakup

Global potential energy surface for the O2 + N2 interaction. Applications to the collisional, spectroscopic, and thermodynamic properties of the complex

A detailed characterization of the interaction between the most abundant molecules in air is important for the understanding of a variety of phenomena in atmospherical science. A completely {\em ab initio} global potential energy surface (PES) for the O$_2(^3\Sigma^-_g)$ + N$_2(^1\Sigma^+_g)$ interaction is reported for the first time. It has been obtained with the symmetry-adapted perturbation theory utilizing a density functional description of monomers [SAPT(DFT)] extended to treat the interaction involving high-spin open-shell complexes. The computed interaction energies of the complex are in a good agreement with those obtained by using the spin-restricted coupled cluster methodology with singles, doubles and noniterative triple excitations [RCCSD(T)]. A spherical harmonics expansion containing a large number of terms due to the anisotropy of the interaction has been built from the {\em ab initio} data. The radial coefficients of the expansion are matched in the long range with the analytical functions based on the recent {\em ab initio} calculations of the electric properties of the monomers [M. Bartolomei et al., J. Comp. Chem., {\bf 32}, 279 (2011)]. The PES is tested against the second virial coefficient $B(T)$ data and the integral cross sections measured with rotationally hot effusive beams, leading in both cases to a very good agreement. The first bound states of the complex have been computed and relevant spectroscopic features of the interacting complex are reported. A comparison with a previous experimentally derived PES is also provided

A simple model for breakup time prediction of water-heavy fuel oil emulsion droplets

Immiscible heavy fuel-water (W/HFO) emulsion droplets inside combustion chambers are subjected to explosive boiling and fragmentation due to the different boiling point between the water and the surrounding host fuel. These processes, termed as either puffing or micro-explosion, are investigated with the aid of a CFD model that solves the Navier-Stokes and energy conservation equations alongside with three sets of VoF transport equations resolving the formed interfaces. The model is applied in 2-D axisymmetric configuration and it is valid up to the time instant of HFO droplet initiation of disintegration, referred to as breakup time. Model predictions are obtained for a wide range of pressure, temperature, water droplet surface depth and Weber number; these are then 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 We < 220, T < 2000 K, P < 140 bar and δ < 0.15. © 202

Numerical investigation of heavy fuel oil droplet breakup enhancement with water emulsions

The heating and explosive boiling leading to fragmentation of immiscible heavy fuel oil-water droplets, termed as W/HFO emulsions, is predicted numerically by solving the incompressible Navier-Stokes and energy equations alongside with a set of three VoF transport equations separating the interface of co-existing HFO, water liquid and water vapour fluid phases. Model predictions suggest that explosive boiling of the water inside the surrounding HFO, ought to their different boiling points, accelerates droplet breakup; this process is termed as either puffing or micro-explosion. In contrast to past studies which predefine the presence of vapor bubbles inside the water droplet, this is predicted here with a phenomenological model based on local temperature and superheat degree. Following their formation, the growth rate of the bubbles is computed with OCASIMAT phase-change algorithm. Moreover, the fuel droplet is simultaneously subjected to convective air flow which further contributes to its deformation. As a result, the performed simulations quantify the relative time scales of the aerodynamic-induced and the emulsion-induced breakup mechanisms. The conditions examined refer to a highly viscous emulsified heavy fuel oil droplet in a gas phase having fixed temperature and pressure equal to 1000 K and 30 bar, respectively. Initially, a benchmark case demonstrates the detailed mechanisms taking place, concluding that droplet fragmentation occurs only at a part of the fuel-air interface, resembling characteristics similar to puffing. Next, a parametric study with Weber number (Oh=0.9,We<200) shows that puffing process can speed up to 10 times the breakup of the droplet relative to aerodynamic breakup. © 2020 Elsevier Lt