Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019


The pressure-velocity coupling was treated using the PISO (Pressure Implicit Splitting


of


Operators)


procedure. The equations were discretized by the QUICK interpolation and the temporal treatment was solved by an implicit method.


3. RESULTS


The numerical model developed in the present work was employed to study the influence of several parameters such


as the alkalinity and temperature.


droplet diameter, SO2 concentration, The ranges


analyzed


correspond to typical concentrations obtained in diesel engine exhaust when burning heavy fuel oil (Kuiken, 2008; Woodyard, 2009; Lamas & Rodriguez, 2012; and Lamas & Rodriguez, 2018). Results are indicated below.


3.1 Figure 4. Computational mesh. Figure. 5 shows the mass fraction of HCO− 3


As indicated previously, several tests were performed in order to determine the adequate extent of the numerical domain in such a way to eliminate any potential effect of the outer boundaries (theoretically at infinity) on the flow close to the droplet. In addition, the solution was checked for refinement sensibility on both the mesh size and time step. Four meshes and time steps were tested to verify that the results are independent of these parameters. The number of elements of these meshes was 1414 (Mesh 1), 1821 (Mesh 2), 2112 (Mesh 3), and 2412 (Mesh 4), while the time steps were 2.5∙10-4 s (Time step 1), 5∙10-5 s (Time step 2), 10-5 s (Time step 3), and 2∙10-6 s (Time step 4). The SO2 absorbed by a droplet is indicated in Table 1. As can be seen, the results using Mesh 3 and Time step 3 are appropriate as refining mesh size or time step does not modify the error obtained. Therefore, Mesh 3 and Time step 3 were chosen for the present work.


Table 1. S absorbed by a droplet (mol/droplet) using several mesh sizes and time steps. 200 µm initial droplet diameter, 600 ppm SO2 concentration, 2,400μmol/kg alkaline concentration, 400ºC initial gas temperature and 20ºC initial droplet temperature.


Mesh 1 1.62∙10-14 Mesh 2 1.65∙10-14 Mesh 3 1.73∙10-14 Mesh 4 1.71∙10-14


Time step 1 Time step 2 Time step 3 Time step 4 1.67∙10-14 1.73∙10-14 1.77∙10-14 1.77∙10-14


1.69∙10-14 1.77∙10-14 1.77∙10-14 1.77∙10-14


1.73∙10-14 1.77∙10-14 1.77∙10-14 1.77∙10-14


Figure 5. Mass fraction of HCO− field.


3 overlaid with velocity


Figure. 6 indicates the droplet diameter reduction for 1,000, 500 and 200μm initial diameters. As expected, all droplets experiment a reduction of the diameter due to the evaporation effect implemented in the model. The time was represented between 0 and 3 seconds because


©2019: The Royal Institution of Naval Architects A-339 (0.00133 kg of SO2/kg overlaid


with the velocity field for 0.5s using 600ppm SO2 concentration


temperature, of H2O),


2,400μmol/kg alkaline concentration (0.000146 kg of alkaline/kg of H2O), free stream initial velocity 2m/s (1 m/s liquid droplet velocity and 1m/s gas flow velocity), 400ºC initial gas


and HCO− 3 20ºC initial liquid


temperature and 1,000μm initial droplet diameter. This figure shows that an internal vortex is formed due to drag


is consumed first at the interphase and then


at the core of the droplet. SO2 first accumulates at the droplet surface and then is transported from the interface to the core of the droplet by three mechanisms: radial diffusion, internal circulation and chemical reactions.


EFFECT OF THE INITIAL DROPLET DIAMETER


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