Thermocapillary Transport of Energy During Water Evaporation
Abstract
Recent studies [1] of water evaporation indicate that when water evaporates at high rates, thermal conduction to the interface does not provide enough energy to evaporate water at the observed rate, and that it is perhaps thermocapillary convection that transports the remaining energy. This possibility is examined by applying the Gibbs dividing surface approximation with the assumption of local equilibrium in each phase to develop an expression for the energy transported to the interface. When this energy transport rate is compared with that required to evaporate the liquid at the observed rate, it is found that a Gibbs excess property, the surface thermal capacity, can be evaluated. A series of 19 experiments have been conducted in which the evaporation rate was progressively increased by a factor of 34. For Marangoni numbers, Ma less than ∼100, the interface is quiescent and thermal conduction (the Stefan condition) correctly predicts the energy transport rate to the surface. For experiments with 100 < Ma < 22,000, thermocapillary convection is present, and the thermal condition does not fully account for the energy transport. However, if the surface thermal capacity is assigned a value of 30.6 ± 0.2 (kJ /m2K), then energy transport by thermal conduction and thermocapillary convection provides energy transport required to evaporate the liquid at the observed rate. For experiments with Ma > 22,000, the interfacial flow is turbulent, and viscous dissipation becomes important. [1] C. A. Ward and Fei Duan, Phys. Rev. E, 69, 056308 (2004).
- Publication:
-
APS Division of Fluid Dynamics Meeting Abstracts
- Pub Date:
- November 2004
- Bibcode:
- 2004APS..DFD.MP006D