Natural convection effect on the droplet ignition in high pressure condition

Abstract

A simulation of the droplet combustion in the high-pressure environment is very important in understanding the fundamental phenomena of liquid rocket, gas turbine and diesel engine combustion. Therefore, a more reliable ignition model of a droplet under high pressure condition is highly desirable in developing a useful tool for design. In this paper, the ignition model of the n-heptane droplet is suggested for high pressure environment and solved for the ignition delay. Its variations with the change in the ambient pressure and temperature are discussed for the cases with/without gravity so that the natural convection effects on the droplet ignition delay time are also examined. The Redlich-Kwong equation of state is used for calculating the phase equilibrium at the droplet surface under high pressure. The natural convection effects are considered by introducing so-called the time scale correction method. The results show that the logarithmic ignition delay time linearly decreases with the inverse of the ambient temperature, and its decreasing slope is independent of the ambient pressure. Thereby, the overall activation energy is found not to be affected by the pressure variation. As the pressure increases, the ignition delay time gets shorter since the droplet vaporization rate is enhanced. But, at a higher ambient pressure, the reduction rate in the ignition delay time becomes smaller. The natural convection effect is also found to make the ignition delay time shorter due to the improved heat feedback to the droplet.