Abstract
The Navier–Stokes equations, energy and vapor transport equations coupled with the VOF methodology and a vaporization rate model are numerically solved to predict aerodynamic droplet breakup in a high temperature gas environment. The numerical model accounts for variable properties and uses an adaptive local grid refinement technique on the gas–liquid interface to enhance the accuracy of the computations. The parameters examined include Weber (We) numbers in the range 15–90 and gas phase temperatures in the range 400–1000K for a volatile n-heptane droplet. Initially isothermal flow conditions are examined in order to assess the effect of Weber (We) and Reynolds (Re) number. The latter was altered by varying the gas phase properties in the aforementioned temperature range. It is verified that the We number is the controlling parameter, while the Re number affects the droplet breakup at low We number conditions. The inclusion of droplet heating and evaporation mechanisms has revealed that heating effects have generally a small impact on the phenomenon due to its short duration except for low We number cases. Droplet deformation enhances heat transfer and droplet evaporation. An improved 0-D model is proposed, able to predict the droplet heating and vaporization of highly deformed droplets.
Highlights
Droplet breakup and evaporation are important physical processes controlling the efficiency of combustion systems; they are realised in medical and agricultural applications among others
Subsequent experimental studies aimed to clarify the physical mechanisms behind the breakup regimes [7, 8, 14] and provided useful information regarding the critical We numbers leading to different breakup regimes [15, 16], the temporal properties and the size distribution of the child droplets after the parent droplet disintegration [15, 17] and the gas flow structure during droplet breakup [18]
The droplet has an initial temperature of T0=300K which corresponds to Oh=0.01 and ensures that the breakup regime is not depending on the Oh number; the Oh number was kept constant for all cases examined
Summary
Droplet breakup and evaporation are important physical processes controlling the efficiency of combustion systems; they are realised in medical and agricultural applications among others Due to their importance they have attracted the scientific interest; but generally they have been studied independently. Except for the drop-gas relative velocity, other important parameters affecting the aerodynamic droplet breakup are the material (gas and liquid) properties and the droplet dimensions. All these can be grouped into dimensionless numbers, namely the Weber number (We), the Reynolds number (Re), the Ohnesorge number (Oh), the density ratio (ε) and the viscosity ratio (N), while under certain flow conditions the Mach number and the turbulence levels may become important: We =
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