Department of Mechanical Engineering

ME Seminar - Thermocapillary Convection During Evaporation

 

Mechanical Engineering Seminar
Faculty Candidate
Wednesday, March 21, 2007
11:00 A.M., 3004/3006 Black Engineering Building
 

Thermocapillary Convection During Evaporation
By
Dr. Fei Duan
Department of Mechanical and Industrial Engineering
University of Toronto, Canada

ABSTRACT

Thermocapillary convection was investigated in water (H2O) and heavy water (D2O) during steady-state evaporation. A method was established to identify the transition to thermocapillary convection by energy transport analysis at the interface. The transition is parameterized by the Marangoni number (Ma). When Ma<~100, the interface was quiescent, and thermal conduction to the interface provided sufficient energy for evaporation. When ~100<Ma<22,000, at progressively increasing evaporation rates, the thermal conduction accounted for a progressively smaller portion of the energy transport to evaporate the liquid, reducing to 60%. It was suggested that the additional energy was transported by thermocapillary convection. By applying the Gibbs dividing-surface approximation, a new interfacial property of liquid, the surface-thermal capacity, was introduced to take the energy transport by thermocapillary convection into account quantitatively. If it was assigned a value of 30.6kJ/m2K for H2O or 32.5kJ/m2K for D2O, the energy required to maintain evaporation was balanced by the summation of thermocapillary convection and thermal conduction. When Ma>22,000, the interfacial flow became turbulent, and the viscous dissipation was significant. With the local evaporation flux and the measured interfacial temperatures in the liquid and vapor phases, statistical rate theory (SRT) was developed and applied to predict the vapor-phase pressure. The mean predicted vapor-phase pressure agreed with that measured for each of the H2O and D2O experiments. Since the energy transferred by Marangoni convection at the interface is significant in evaporation, a new evaporator has been designed. An air flow was allowed to pass along the water surface perpendicular to the heat elements. It was found that the evaporation flux increased with an increase of the air flow rate and the temperatures of heating elements. The measurements showed that the interfacial liquid temperature was higher close to the heating elements than that at the centerline between the heating elements. The evaporation energy efficiency could reach 80% in the new evaporator.

BIOGRAPHY

Dr. Fei Duan is a postdoctoral research associate at Thermodynamics and Kinetics Lab of Department of Mechanical & Industrial Engineering in University of Toronto, Canada. He finished his B. Sc. & Eng. in chemical engineering at Dalian Institute of Light Industry, China; and M. Sc. & Eng. in materials science at Wuhan University of Technology, China. Following over four years working as a lecturer in Beijing Graduate School of Wuhan University of Technology, he joined the Department of Mechanical & Industrial Engineering at University of Toronto for his doctoral degree study. The focus of Dr. Duan’s study was on thermodynamics and fluid mechanics. During his doctoral study, he visited Institute of Fluid Mechanics (Friedrich-Alexander-University, Erlangen-Nuremberg, Germany. His current research involvement involves the design of high efficiency evaporators.

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