Numerical And Experimental Study Of Flow/Heat/Mass Transport During The Pure/Binary Sessile Droplet Evaporation

Sessile droplet evaporation has been a complex physical problem all through.With the method of Arbitrary Lagrangian-Eulerian?ALE?locating the free interface,a fully-coupled transient numerical model was built in this paper to simulate the sessile droplet evaporation in constant contact radius?CCR?mode.It took account of the convective mass transfer in vapor transport to accurately calculate the evaporation rate.Meanwhile,the volume force term was added to the momentum control equation,which considered the general fluid mechanism,to guarantee the better simulation of droplet evaporation in gravity field and the flow structure inside droplet.Simulation results of transient effect and convective mass transfer of pure liquid droplets indicate that the stronger volatility of the droplet as well as the poorer conductivity of the substrate,the shorter the normalized transient stage last.In addition,with stronger droplet volatility and better substrate conductivity,the contribution of convective mass transfer to the total evaporation rate is greater.The further analysis of natural convection shows that it not only depends on the temperature gradient and vapor concentration vapor,but is also influenced by the vapor's molar mass compared with air's.In the spontaneous droplet evaporation,when the former is larger than the latter the mixture gas density near droplet surface will be greater than that of the far field which will just trigger the buoyancy flow along the interface.On the contrary,when vapor's molar mass is smaller than air's,with the weaker evaporation cooling,such as on the substrate having good heat conductivity,the mixture gas density near droplet surface will be lighter than that of far field,which intends to induce the buoyancy flow in the whole gas domain apart from the droplet surface.In this case,the buoyancy flow will play the most important role in convective mass transfer in contrast with other cases.Simulation results of flow structure evolution inside pure liquid droplets during evaporation show the main flow mode is the marangoni flow driven by the interface temperature gradient.However,due to inertia force,the flow vortex in the swallow region with small temperature gradient could be devoured by the neighbor-hood large scale vortex.In case of the large contact angle,for instance,the methanol droplet with?₀=150°,natural convection will also benefit the formation of multi vortices structure effectively.With increasing droplet volatility,Bi number will increase,which means the evaporation cooling gets obvious and results in greater Ma number.Thus more vortices are formed inside the droplet.In turn,the strong flow will give rise to Pe number which indicates the intensity of convective heat transfer and then uniform the temperature field inside droplet.As a result,the multi vortices stage will be shortened.Meanwhile,when initial contact angle become larger,Ma number will increase and the vortices number will show a step-growth.Due to the longer thermal path,the centers of vortices tend to move towards the droplet surface at the beginning of flow structure evolution which hinder the convective heat transfer in large scale.In contrast,the center of vortices will locate in the middle part of droplet with small contact angle in which convection is enable to accelerate the building of the stable temperature field inside though vortices decrease.Thus,with increasing?₀,the multi-vortex flow structure lasts for shorter period when?₀<90°while it lasts for longer when?₀>90°.In addition,the methanol aqueous solution droplets where the volume fractions of water?_w account for 100%,7%,60%,50%,25%and0,respectively,evaporating in the environment with high humidity of methanol vapor were experimentally studied.Through the simulation of a simple binary droplet evaporation-condensation model,it is demonstrated that when the water component evaporates,the methanol vapor will condensates onto the droplet surface in the experimental conditions.This result can be used to explain the first increase and then decrease phenomenon of droplet volumes during the“evaporation”.Because of the condensation of methanol vapor,the surface tension will decrease.Along with the first volume increasing,it leads to the droplet growth stage characterized by the spreading of the triple contact line and rapidly decrease of contact angle.When the methanol concentration is enhanced,the saturated partial pressure of methanol vapor near the droplet surface will be improved and thus relieves the condensation,dominated gradually by water evaporation.Then the binary droplet enters the stage of slow CCR evaporation.With the increasing initial volume fraction of water,the maximum volumes of droplets become larger along with the longer time it takes to reach the maximum volumes.