An investigation of thermal modulation effect on liquid sheet breakup

The instability and breakup of free liquid sheets is of considerable scientific and technological importance and has been extensively studied due to its application to atomization and spray combustion. In previous studies, both theoretical and experimental investigations on the stability of planar liquid sheets have studied extensively. These studies have typically considered only the instability regime that is created when aerodynamic forces act on a planar liquid sheet. The breakup length of a liquid sheet has been determined to be the wavelength where the maximum growth rate exists due to aerodynamic instabilities.;In the current study, another source of instability, that due to the presence of thermal modulation in a liquid sheet, is considered. Both linear and nonlinear stability analyses of the liquid sheet with and without thermal modulation have been conducted. In the linear study, it is found that the thermal modulation results in additional sources of instability along with those generated due to aerodynamic forces. The results from this dissertation identify the conditions necessary for the dominance of either type of instability. When the combination of the two instabilities are considered, thermal modulation seems to cause the instability necessary for liquid sheet breakup at low Weber numbers while aerodynamic forces seem to create the instability conditions of the liquid sheet at high Weber numbers. In the nonlinear study, a spectral method is employed in order to obtain results associated with the fluid's nonlinearities. The linear analysis was used to validate the nonlinear methodology for flow conditions where the fluid's nonlinearities are insignificant; the breakup length of liquid sheet predicted in the nonlinear study agrees well with that determined by the linear analysis.;With the appropriate conditions, thermal modulation decreases the breakup length of the liquid sheet, which subsequently reduces the size of the spray droplets. This concept was employed to predict fuel evaporation enhancement at cold engine start. Using the decreased droplet size, predicted in the thermal-modulation analysis, as input to a CFD package (Star-CD), a simulation of cold start in an internal combustion engine was simulated. It is predicted that if a very high frequency (100 kHz) thermal modulation could be applied to the fuel jet in cold-start conditions, the amount of fuel droplet evaporation could be increased by a factor of three over that of the non-thermally-modulated fuel jet.