Characterization of the Spray for Twin-Fluid Atomizer for Inert Gas Generator

Abstract

In this era, the world is still hinged upon fossil fuels for energy production & power generation. However, stringent regulations are imposed upon industries by regulatory bodies. The highly efficient combustor systems are designed along with cleaner fuel usage to provide a sustainable source of energy with fewer pollutants emissions (NOx, CO). The major process that dominates the liquid fuel combustors is the atomization quality of the spray. In marine systems, LNG tankers use inert gas generators (IGG) to inert the surroundings around the combustible cargo. For economic purposes, it is advantageous to couple the IGG and combustion units such that liquid fuel such as diesel will burn according to the loadings (in terms of flow rates). Thus, the combined system must produce as little CO, NOx etc in low or high fuel flow rates such that it cause less damage to the tanks. For this, twin-fluid atomizers were studied for sheet breakup and spray characterization in terms of macroscopic and microscopic parameters.

In this experimental work, the annular sheet breakup dynamics study was performed for the simpler atomizer designs with two distinct configurations— converging and converging-diverging (CD). Different breakup regimes were observed at different fluid flow rates combination. The bursting phenomenon was also observed at higher ALR values. The breakup length and spray angle was quantified for the 3.0 mm diameter atomizers with 280 µm sheet thickness. It was found that the airflow (shock waves) pattern affects the sheet breakup mechanism for both types of atomizers. The spray dynamics study was conducted with different airjet diameters atomizers. The mean drop size (SMD) was found to be smaller in the case of CD atomizers than converging atomizers for the same working conditions. The droplet size distribution (DSD) was found to be narrower for CD atomizers with more uniform DSD at higher ALR values. The sheet breakup regimes and spray characteristics were predicted according to the non-dimensional numbers which correspond to different fluid flow rates using the acoustic chemometrics approach employing sensors data and multivariate analysis (PCA, PLS-R).

The spray was also characterized by utilizing bluff body atomizers with variations in the bluff body (cone) distances (Lc) and air-jet diameters (D) for various fluid flow rates. SMD increases radially away from the spray centreline due to milder aerodynamic interaction and spray-bluff body impact with a large fraction of low excentricity (near-spherical) droplets. The relative span factor (Δ) follows a reverse trend with decreasing value as we move radially away from the spray centreline due to low droplet number density at far-radial locations. The drop size distribution (DSD) becomes less narrow as we move towards the spray periphery. DSD broadens with the increase in air-jet diameters atomizer due to the low-pressure airflow, even though airflow rates are higher.