19th Int. Multidimensional Engine Modeling User’s Group Meeting at the SAE Congress, April 19, 2009, Detroit, MI
Development of a Computational Model for Heavy Fuel Oil for Marine Diesel Engine Applications Nikolaos Kyriakides, Christos Chryssakis* and Lambros Kaiktsis Dept. of Naval Architecture & Marine Engineering National Technical University of Athens (NTUA) Heroon Polytechniou 9, GR-15773 Zografou, Greece
ABSTRACT In the present work, a model with the thermophysical properties of Heavy Fuel Oil, typically used in marine diesel engines, has been developed, and implemented into the KIVA CFD code. The effect of fuel properties on spray atomization is investigated by performing simulations in a constant-volume high-pressure chamber, using the E-TAB and the USB breakup models. Two different nozzle sizes, representative of medium- and low-speed marine engines, have been considered. The simulations have been performed for two values of chamber pressure, corresponding to operation at partial and full load. The results indicate that, in comparison to a diesel spray, the Heavy Fuel spray is characterized by comparable values of penetration length, and larger droplet sizes. These findings are correlated to experimental results from the literature. With reference to medium-speed marine engines, a first experimental study performed by Fink et al. [1] suggests that, in comparison to automotive diesel sprays, HFO sprays exhibit small differences in terms of tip penetration, with the deviations in droplet sizes being significant. For large (low-speed) marine diesel engine conditions, a constant-volume combustion chamber has been developed [2], and experiments are currently underway. In 2006, Goldsworthy [3] presented a simplified model for heavy residual fuel, with constant viscosity, density and latent heat of evaporation, in which the surface tension was defined as a function of temperature. The model was used in CFD simulations for two representative fuels, and good agreement was reported between measured and computed data for ignition delay, burning rate, and spatial form of the spray and flame. More recently, Struckmeier et al. [4] adopted Goldsworthy’s approach, and elaborated it by adding multi-component droplet evaporation. This was achieved by applying different saturation pressures for light and heavy fuel components. The comparison of spray simulations with experimental measurements showed good agreement in terms of spray penetration, evaporation, and flame lift-off length. Motivated by similar studies in automotive engines, recent attempts in optimizing pollutant emissions and engine performance of marine diesel engines exploit advanced injection strategies with pilot injection [5-6]. Typi-
INTRODUCTION Current research efforts on optimizing diesel engine operation attempt in minimizing pollutant emissions without sacrificing fuel economy. To this end, Computational Fluid Dynamics (CFD) simulations are a valuable tool. The computed levels of pollutant concentrations and engine output are heavily dependent on a proper description of spray atomization processes. Currently used physical models of spray atomization are based on conditions representative of small automotive engines. In large marine diesel engines, the non-dimensional parameters affecting the spray dynamics differ substantially, due to both the larger size of injectors and the use of Heavy (or Residual) Fuel Oil (HFO). Roughly 2/3 of all merchant ships are operated with HFO. Commonly, the composition of HFO varies substantially, introducing many uncertainties in modeling. Reported values of the kinematic viscosity of HFO at 50oC can range between 50 cSt and 800 cSt, compared to approximately 3 cSt for automotive diesel fuel. Furthermore, surface tension values are substantially higher for HFO (by an order of 20%). It is expected that the differences in fuel properties affect the dynamics of spray atomization, droplet evaporation and fuel-air mixing in marine diesel engines. Thus, validation and further development of the existing models is necessary for modeling spray dynamics in these large engines. *
Corresponding author:
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1
the Weber number of the droplets, defined as:
cally, CFD studies as these of [5-6] rely on diesel oil properties for modeling spray dynamics. Thus, future studies should take into account the effects of marine fuel properties. To this end, a new fuel model is developed and tested in the present study. The new model accounts for the thermophysical properties of a representative heavy marine fuel. The model is implemented into a KIVA-based CFD code, and evaluated for a prototype fuel spray in a constant volume combustion chamber. Results for HFO are compared to computational results for a conventional diesel fuel, as well as with the experimental measurements of [1], in terms of spray breakup behavior.
We =
ρ GU 2 d o , σ
where ρG is the density of the ambient gas, U the droplet velocity, do the droplet initial diameter upon its creation, and σ the surface tension. For low Weber numbers (less than 12), atomization does not occur, and only droplet deformation takes place. For higher values of Weber number, the following regimes are identified in [11]: • Bag breakup, 12
