M.Sc Thesis

M.Sc StudentKhosid David
SubjectTheoretical Investigation of Evaporating Polydisperse Fuel
Sprays in Jet Flows
DepartmentDepartment of Aerospace Engineering
Supervisor PROFESSOR EMERITUS Yoram Tambour


Liquid fuel injectors produce sprays of a wide range of drop-sizes in jet flows, where droplets of each size-group travel at a different velocity. The droplets and their vapors spread into the adjacent stream in which the oxidizer is contained and feed a spray diffusion flame. Effects of drop-size dependent, spray-injection velocity on the lateral evolution in drop-size histograms, on fuel vapor profiles and on flame location are analyzed.

Nonlinear, coupled partial differential equations that characterize the evaporating multisize spray flow are solved here employing a similarity transformation in which simplifying assumptions are used. In the case of a diffusion spray flame, the governing equations are transformed into the known Schvab-Zeldovich type equations for which new solutions are presented here. The present similarity equations still remain coupled via source terms and are solved here analytically and numerically.

In the current research we use a sectional approach to describe the spray. For receiving similarity equations for the spray and the vapors, we use the method developed by Katoshevski for shear-layer flows.

In the first part of our work, two models of the jet (two-dimensional and axisymmetrical) with polydisperse spray of evaporating droplets suspended in it are considered.  Similarity solutions are presented for the evolution in droplet-size distributions and for the profiles of vapor concentrations across the laminar jet flows in the far-field and the near-field.

The present solution for spray fluxes across the circular jet was compared with experimental results from a recent publication concerning polydisperse spray evolution by Sommerfeld & Qiu. Good qualitative agreement between our solution and their experimental results was found. The analytical solution of the spray similarity equations also predicts the sharpened edge of the mass fractions near the axis of symmetry. This effect was also observed in the experiments of Sommerfeld & Qiu.

Further research focuses on jet-spray-diffusion-flames. A two-dimensional spray diffusion flame which results from an evaporating polydisperse spray of fuel droplets suspended in the jet flow with an initial amount of fuel vapor is analyzed. The geometry and location of the flame is obtained here via the solution of Schvab-Zeldovich type equations. New similarity equations are presented for a flame similarity function across the jet, which take into account effects of various drop size distributions. An analytical solution for this equation was obtained. As a special case of this new analytical solution in the absence of droplets, the gaseous flame solution known from the literature is obtained.