Experimental Investigation On Effect Of Horizontal Temperature Gradient On Marangoni Instabilities In An Evaporating Liquid Layer

Nonisothermal evaporative convections in horizontal thin layers give rise to both fundamental studies and practical applications such as heat pipe,distillation and preparation of thin films or coatings.Previous results show that surface tension driven Bénard-Marangoni(BM)cells appear in volatile liquid layers due to evaporation cooling when the vertical temperature gradient is greater than the critical value.In the meantime,a tangential temperature gradient caused by the meniscus and inhomogeneous evaporation could induce thermocapillary convection.In this research,further instabilities of thermocapillary convection during higher ambient temperature are investigated.The BM convection and thermocapillary convection are known as Marangoni convection and they are generated spontaneously in the evaporation process and coupled with each other.However,there is lack of work considering of the relationship between BM convection and thermocapillary convection during the evaporation process.Therefore,following works are carried out in a 0.76~1.68 mm liquid layer on different evaporation conditions.(1)In the first set of experiments,Marangoni convections in a rectangular pool(L×W=60×80mm)in different environment temperature are observed by visualization with the help of fine aluminum flakes added to the liquid.Critical conditions of longitudinal rolls and hydrothermal waves(HTWs)are measured for different depth layers ranged from 0.76 mm to 1.68 mm without additional tangential temperature gradient.The results show that evaporative cooling effect forms both vertical and tangential temperature gradient,BM convection and thermocapillary convection are generated,respectively.When the initial temperature reaches the critical condition,hydrothermal waves will form spontaneously.(2)When the depth of liquid layer is thicker than 1.27 mm,a large number of smaller cellular structures are observed at the end of thermocapillary convection.It is found that the dimensionless wavelength of these cellular structures equal to one.They are generating and traveling on the surface of liquid layers until eventually disappear at the border of thermocapillary convection and BM cells.Critical conditions of generating these surface drifting cells are measured from the extensive experiment.(3)Experimental research of involatile 1 cSt and 1.5cSt silicone oil are conducted to find critical Mac number of HTWs.Results show that the HTWs become hard to appear with the decrease of evaporation rate and increase of Pr number.The wavelength and propagation characteristics of BM convection and longitudinal rolls and HTWs of 0.65 cSt silicone oil are studied.