Experimental and Numerical Study on Evaporation of Water at Low Pressures

  • Author / Creator
    Kazemi, Mohammad Amin
  • Although evaporation is considered to be a surface phenomenon, the rate of molecular transport across a liquid–vapor boundary is strongly dependent on the coupled fluid dynamics and heat transfer in the bulk fluids. Recent experimental thermocouple measurements of the temperature field near the interface of evaporating water into its vapor have begun to show the role of heat transfer in evaporation. However, the role of fluid dynamics has not been explored sufficiently. Here, a combined numerical and experimental study is performed to demonstrate how the simultaneous effects of heat transfer and fluid dynamics influence the evaporation of a liquid at low pressures. For this purpose, the liquid velocities near the interface during evaporation from two different geometries (a cylindrical tube and a rectangular cuvette) are measured using particle image velocimetry. The temperature profiles in the liquid and vapor near the interface along the centerline are also measured using a fine thermocouple. A mathematical model is developed to describe the coupling of the heat, mass, and momentum transfer in the fluids with the transport phenomena at the interface. The model is validated with the experimentally measured velocity fields, temperature profiles, and evaporation rates. Once demonstrating good agreement with the experimental data, the model is then used to understand the experimentally obtained velocity field in the liquid and temperature profiles in the liquid and vapor, in evaporation from a concave meniscus for various vacuum pressures. By using the validated model, it is shown that an opposing buoyancy flow in the liquid, even though it occurs at relatively small velocities, can suppress the thermocapillary flow in water during evaporation at low pressures. As such, in the absence of thermocapillary convection, the evaporation is controlled by heat transfer to the interface, and the predicted behavior of the system is independent of choosing between the existing theoretical expressions for evaporation flux. The possibility of occurrence of a thermocapillary convection at the interface is further explored numerically by increasing the thermal conductivity of the container that holds the liquid in the model. By doing so, it is found that thermocapillary convection at the interface can occur at higher thermal conductivities and increases the evaporation flux significantly. However, the occurrence of a thermocapillary flow does not guarantee that the heat transfer limitations to the evaporation are removed completely. The influence of the liquid thin film formed in the corner of the rectangular cuvette is studied experimentally and numerically. Both confirmed that the contribution of the thin films is negligible in the total evaporation rates. The temperature discontinuity at the interface is also investigated and it is confirmed that the discontinuity strongly depends on the heat flux from the vapor side, which depends on the geometrical shape of the interface. The reliability of the thermocouples in measuring the interfacial temperature discontinuities is also studied and discussed extensively.

  • Subjects / Keywords
  • Graduation date
    Fall 2017
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.