CFD and experimental studies of heavy fuel oil sprays for marine engine applications

Abstract

A major development for experimental studies in the context of marine Diesel engines was achieved in the course of the EU funded research project HERCULES (High-Efficiency R&D on Combustion with Ultra Low Emissions for Ships). In particular, a state-of-the-art experimental test facility, dedicated to spray and combustion studies under conditions similar to those of large marine Diesel engines, was developed by Wärtsilä Switzerland and ETH Zürich. This Spray Combustion Chamber (SCC) facility enables detailed flow, spray, combustion and emission formation studies. Thus, valuable experimental data can be attained for characterizing spray and combustion, as well as for validating and supporting model development for Computational Fluid Dynamics (CFD) studies. Heavy Fuel Oil (HFO) is the predominant fuel in the marine industry. Recently, a detailed model predicting the thermophysical properties of marine HFO of arbitrary composition was developed by the authors. The model is based on a one-component approach, and requires an input of four values of fuel properties, commonly measured at bunkering, namely: (a) density at a given temperature, (b) kinematic viscosity values at two temperatures, and (c) sulfur content. The model predicts a large set of fuel properties relevant for CFD studies, such as molecular weight, carbon-to-hydrogen ratio, enthalpy of formation, pseudo-critical pressure and temperature values, as well as the temperature dependence of density, viscosity, vapor pressure, heat of evaporation, sensible enthalpy, surface tension and mass diffusion coefficient. In the present study, the model is utilized for extensive CFD studies of HFO spray dynamics in the SCC, and results are compared against experiments. Here, the model is utilized to predict the thermophysical properties of the HFO used in the experiments. Spray modeling is based on a proper adaptation of the Cascade Atomization and drop Break-up (CAB) model. Simulations are performed with a KIVA-3 based CFD code for non-reactive evaporating conditions; the computational results are found to accurately reproduce the experimental data for spray penetration length and cone angle. The effects of HFO preheating on spray development are demonstrated. Further, spray propagation under non-evaporating conditions is investigated, and computational results are found in good agreement with experimental data reported in literature. Finally, first experimental and computational reactive spray studies in the SCC are reported, aiming at a characterization of reactive flow in terms of ignition delay and location; a good agreement between experiments and computations is demonstrated. The present investigation builds a strong basis for further combined computational/experimental studies of non-reactive and reactive HFO sprays under conditions relevant for large marine engines.