Effects of Turbulence on Droplet Evaporation and Combustion: Droplet-Resolved Direct Numerical Simulation

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In most combustion engines used in automobiles and airplanes, fuel is injected in the form of the liquid. The evaporation of liquid fuel droplets and subsequent mixing with the surrounding air determine how fuel is ignited and burned. Turbulent fluid motions in the combustion engines enhance the rates of evaporation, mixing and burning. Better understanding of these processes will lead to more efficient engines. In this project, the effects of turbulence on the evaporation and combustion of fuel droplets will be investigated using a new computational method. Numerical simulations that resolve all relevant processes around and inside the droplets will be performed to advance the fundamental understanding of the droplet-turbulence-chemistry interactions under engine-relevant conditions. The advanced fundamental understanding will guide the improvement of engineering models and accelerate the development of combustion engines that meet future standards for fuel efficiency and pollutant emissions. The educational component includes the training of undergraduate and graduate students in computational combustion research and the K-12 outreach through an established program in the college.The project is to characterize the effects of turbulence on droplet evaporation rates, inter-droplet scalar mixing, ignition, and flame structures, when the size of droplets is comparable to or larger than the Kolmogorov scale, the smallest turbulence length scale. A new computational method, which can enable the accurate and stable computation of droplet evaporation and combustion in turbulent flows under engine-relevant conditions, will be developed. Direct numerical simulation (DNS) of the evaporation and combustion of single-component and bi-component fuel droplets in homogeneous isotropic turbulence and in temporally developing mixing layers will be performed using the developed method. The DNS data will be analyzed to delineate the conditions under which the existing laminar theory for the droplet evaporation rates can be used. The emphasis of the analysis will also be placed on identifying the key parameters that characterize the turbulence effects on the evaporation rates. The small-scale mixing statistics in the vicinity of evaporating and burning droplets will be analyzed to help improve spray combustion modeling.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.