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

Abstract

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 emulsioninduced 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 ( ℎ = 0.9, < 200) shows that puffing process can speed up to 10 times the breakup of the droplet relative to aerodynamic breakup.

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

“…Τhe difference between 𝑡 𝑏𝑟 and 𝑡 𝑎𝑒𝑟𝑜 decreases as 𝑊𝑒 increases; however it is important to mention that emulsion breakup occurs 3-5 times faster than the aerodynamic breakup for the conditions examined. This difference is in agreement with the results of [32]. The relative duration of heating (𝑡 ℎ ; red dashed line) and growth (𝑡 𝑔𝑟𝑜𝑤 ; blue dash-dot line) times, for the range of 𝑊𝑒 numbers examined, is also shown in Figure 3.…”

“…As already mentioned, the CFD model of [32] is employed to examine a configuration where a single spherical water sub-droplet (𝐷 𝑤 = 10 μm) is located inside a fuel droplet (𝐷 𝑓 = 50 μm) as shown in Figure 1 (left panel); note that the figure is not in scale. Equations are solved in an axisymmetric domain where the left vertical axis is a velocity inlet boundary that imposes the velocity of the stream flow, while the rest boundaries are open and the velocity gradient is set to zero.…”

“…On the contrary, the variation of the 𝑊𝑒 number is relevant since it controls the aerodynamic-induced deformation of the parent droplet. Demonstration of the temporal evolution of emulsion droplet heating and fragmentation for an indicative case is illustrated in the right panel of Figure 1; At 𝑡 = 3 μs, the appearance of a water-vapor bubble is observed at the proximity of the HFO-water interface after the criteria for the water-vapor generation have been fulfilled [32]. Then the bubble grows and at some time instant (𝑡 = 4.7 μs) it reaches the HFOgas interface, allowing it to escape into the surrounding ambient gas.…”

“…The heating process of emulsion droplets has been studied by [31], concluding that the flow inside the droplet and the fuel Peclet number define the time needed for the water droplets to become superheated and start boiling. Recently, in the CFD work [32] from the authors, the droplet heating up to water-vapor expansion and droplet fragmentation were simulated and the corresponding time needed for those processes to occur was predicted. The latter was observed to be faster compared to the aerodynamicinduced breakup of a neat HFO droplet exposed to the same surrounding gas flow conditions, suggesting that fuel emulsification is beneficial for viscous fuels, such as HFO.…”

Effect of the Addition of Petrochemicals onto the Atomization and Ignition of the Coal-Water Slurry Prepared from the Wastes

“…The evaporation of water resulting the drying of different flying droplets observed in [18,19]. Problems of droplet fragmentation under extremal thermal conditions are investigated in [20,21]. Some interesting observation of the slurry injection into the working burner were done in [22,23].…”