Thin film evaporation is important in the design of heat pipes, desalination, lubrication, air conditioning, and medical devices. The existing theories suggest that with smaller film thickness, the local evaporation mass flux will increase due to an increased interfacial temperature that results from lower conduction thermal resistance in the liquid. However, this mass flux will sharply decrease to zero as the film gets even thinner due to the increased interface forces that inhibit the escape of interfacial vapor molecules.

In this project, we present the first experimental effort to locally measure the evaporation mass flux in a liquid meniscus. The microscale lateral extent of the meniscus and sub-Kelvin liquid-vapor temperature differences (superheats) challenge conventional thermometry. Here, frequency domain thermoreflectance (FDTR), a non-contact laser-based method with a lateral resolution of micrometers, is used. Water serves as the working fluid, and the liquid meniscus is generated inside a sealed chamber between the water and the vertical gold wall where the lasers are absorbed and reflected. By moving the sample downwards, the horizontal lasers can easily scan from the thick liquid film region to the pure vapor region. 

A non-monotonic experimental signal change is observed in the thin film portion of the meniscus, where a high evaporation heat flux is expected. This experimental technique has widely been employed with micron-scale lateral resolution in thin solid films, but not when evaporation is present. Therefore, a new model equation needs to be built. To extract the evaporation heat transfer coefficient in the meniscus region from the obtained signal, we modify the standard heat diffusion equation used to analyze FDTR data to account for the phase change. This modification provides a new paradigm for investigating a heat flux discontinuity in any thermal model. Besides, finite element simulation using transient thermal modules in ANSYS are performed to emulate the FDTR in the provided situation, accounting for the non-uniformity of the meniscus thickness. The laser is simulated as a given heat flux whose intensity is following a Gaussian decay from the center, and the evaporation is simulated as a convection boundary condition on the liquid meniscus surface. The ANSYS simulations are first validated against the modified FDTR equation with models having uniform liquid thicknesses and fair agreement has been observed. Afterwards, the experimental and ANSYS modeling results are combined into a machine learning framework that uses gradient descent to extract the spatial dependence of the evaporation heat transfer coefficient and water film thickness in the meniscus.

 


Local Study of the Evaporation Mass Flux in a Thin Liquid Film Using Thermoreflectance Experiments and Numerical Methods

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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