Computational fluid dynamics modelling of residual fuel oil combustion in the context of marine diesel engines

Abstract

A simplified model is presented for vaporization and combustion of heavy residual based fuel oil in high-pressure sprays, in the context of marine diesel engines. The fuel is considered as a mix of residual base and cutter stock. The model accounts for multiple fuel components as well as limited diffusion rates and thermal decomposition rates within droplets by the use of straight-line relationships for the saturation pressure of combustible fuel vapour at the droplet surface as functions of droplet temperature. The energy required for decomposition of heavy molecules is accounted for. Combustion is modelled using a timescale that is the sum of a kinetic timescale based on a single-step reaction and a turbulent timescale based on turbulent mixing rates. The ignition timescale is based on a simple three-equation model. Cellwise ignition is employed. The heavy fuel oil model is applied to two different constant volume chambers that are used to test ignition and combustion quality of marine heavy fuel oil, using the computational fluid dynamics code StarCD version 3.2. Good agreement is shown between trends in measured and computed data including ignition delay, burn rate and spatial distribution of spray and flame parameters. The model is tested for two representative fuels, one with good ignition and combustion properties and one poor. Essentially only two parameters need to be changed to set the fuel quality. These are the ignition delay factor and the activation energy for the high-temperature kinetics. Further tuning of the model to specific fuels is possible by modifying the saturation temperature relationships.