Numerical Investigation of Evaporation Modelling for Different Diesel Fuels at High Temperature and Pressure in Diesel Engine

Abstract

Diesel engines are used widely in the world due to their capability of handling high torque and heavy loads efficiently. Fuel injection systems in the diesel engine have different processes that affect the complete burning of fuel in the combustion chamber. These include the primary and secondary breakup of liquid fuel droplets and evaporation. Diesel engines use the air as a working substance only. One of the processes is the evaporation of fuel droplets. Complete evaporation of diesel fuel liquid in the combustion chamber is a prerequisite for complete combustion. Hence evaporation of droplets has an impact on the efficiency of the engine and consequently on the output power and torque. In the present paper evaporation of two different diesel fuels is modeled numerically taking into account the turbulence effects present in the cylinder due to high temperature and pressure using the RANS turbulence model. Evaporation of n-heptane diesel fuel and n-decane is controlled by the numerical model that includes the conservation equations of mass, energy, momentum and species transport. During the evaporation phase heat is transferred to the droplet and due to evaporation from the surface of the droplet, the mass of evaporating droplets is decreased. These two processes are controlled by the equations of the Discrete Phase Model present in the Fluent. Then the results are plotted by varying the droplet diameter and temperature. Results include the decay in droplet diameter, increase in the droplet temperature effect on the velocity of droplet and temperature changes in the cylinder due to evaporation of fuel. There are different parameters which affect the rate of evaporation of fuel droplets inside the engine cylinder of an internal combustion engine. These parameters include the diameter of fuel droplets, initial temperature of fuel droplets, and the temperature inside the cylinder after the compression stroke and surface area of droplets.  Small droplets need a short time to evaporate while if the size of droplets is large then evaporation times is also increased.