Absence of Marangoni convection at Marangoni numbers above 27,000 during water evaporation.
Two mechanisms by which Marangoni convection can be produced at the interface of water with its vapor are: (1) by imposing a temperature gradient parallel to the water-vapor interface, and (2) by imposing a temperature gradient perpendicular to the interface that results in the liquid becoming unstable. A series of evaporation experiments conducted with H2O and with D2O maintained at the mouth of a stainless-steel funnel indicated the presence of Marangoni convection, but the mechanism producing the convection was unclear. We have investigated the mechanism using a funnel constructed with a polymethyl methacrylate that has a small thermal conductivity relative to that of water and repeating the evaporation experiments. Marangoni convection was eliminated with this funnel even though the Marangoni number, Ma, was in the range 8277< or =Ma< or =27 847 . A comparison of the assumptions made in the theories available to predict the onset of Marangoni convection with the observations made in this study indicates some of the assumptions are invalid: although generally neglected, energy transport through the vapor to the interface of evaporating water is significant; there is an interfacial temperature discontinuity, but it is in the opposite direction of that assumed in the existing theories: the interfacial-vapor temperature is greater than that of the liquid during evaporation; and the prediction of the critical Marangoni number is based on an arbitrarily chosen value of the heat-transfer coefficient. When the temperature gradient is perpendicular to the water-vapor interface, these invalid assumptions indicate present theories do not apply to volatile liquids.
Thompson
Ian
I
Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Canada M5S 3G8.
Duan
Fei
F
Ward
C A
CA
D018481:Convection; D015233:Models, Statistical; D010825:Physics / Q000379:methods*; D011312:Pressure; D012997:Solvents; D013193:Stainless Steel; D013696:Temperature; D014867:Water / Q000737:chemistry
D012997:Solvents; D014867:Water; D013193:Stainless Steel