Near Field Condensation

Dropwise condensation represents the upper limit of condensation heat transfer. Promoting dropwise condensation relies on surface chemical functionalization, and is fundamentally limited by the maximum droplet departure size. A century of research has focused on active and passive methods to enable the removal of ever smaller droplets. However, fundamental contact line pinning limitations prevent gravitational and shear-based removal of droplets smaller than 250 µm. Here, we break this limitation through near field condensation. By de-coupling nucleation, droplet growth, and shedding via droplet transfer between parallel surfaces, we enable the control of droplet population density and removal of droplets as small as 20 µm without the need for chemical modification or surface structuring. We identify droplet bridging to develop a regime map, showing that rational wettability contrast propels spontaneous droplet transfer from condensing surfaces ranging from hydrophilic to hydrophobic. To demonstrate efficacy, we perform condensation experiments on surfaces ranging from hydrophilic to superhydrophobic. The results show that near field condensation with optimal gap spacing can limit the maximum droplet sizes and significantly increase the population density of sub-20 µm droplets. Theoretical analysis and direct numerical simulation confirm the breaking of classical condensation heat transfer paradigms through enhanced heat transfer. Our study not only pushes beyond century-old phase change limitations, it demonstrates a promising method to enhance the efficiency of applications where high, tunable, gravity-independent, and durable condensation heat transfer is required.

Experimental Investigation of Wall Nucleation Characteristics in Flow Boiling

Wall nucleation experiments have been performed in a vertical annulus test section for investigation of the bubble departure diameter and bubble departure frequency. The experimental data in forced convective subcooled boiling flow is presented as a parametric study of the effect of wall heat flux, local bulk liquid subcooling, liquid flow rate, and system pressure. The data are shown to extend the database currently available in literature to a wider range of system conditions. Along with the current database in forced convective flow, the available models for bubble departure size and frequency are reviewed and compared with the existing database. The prediction of the bubble departure frequency is shown to require accurate modeling of the bubble departure diameter which has poor agreement with the experimental database.

Experimental Investigation of Bubble Departure Diameter and Bubble Departure Frequency in Subcooled Flow Boiling in a Vertical Annulus

Wall nucleation experiments have been performed in a vertical annulus test section for investigation of the bubble departure diameter and bubble departure frequency. The experimental data in forced convective subcooled boiling flow is presented as a parametric study of the effect of wall heat flux, local bulk liquid sub-cooling, liquid flow rate, and system pressure. The data is shown to extend the database currently available in literature to a wider range of system conditions. Along with the current database in forced convective flow, the available models for bubble departure size and frequency are reviewed and compared with the new data. The prediction of the bubble departure frequency is shown to require accurate modeling of the bubble departure diameter which has poor agreement with the experimental database.

Modeling Bubble Behavior on a Heated Surface With Surface Pores and Sub-Surface Channels

Bubble behavior and pressure fluctuation during boiling are modeled for enhanced surfaces with surface pores and sub-surface channels. The hydraulic diameter of the channels is about 0.42 mm. It is assumed that the latent heat flux is solely generated by thin-film evaporation on the channel surface. The evaporation rate is given by a semi-empirical correlation including effects of surface tension and liquid viscosity. The activation pressure and the bubble departure size are different for different pores. The vapor pressure inside the bubble is related to the channel pressure by the orifice equation. The bubble growth rate is determined by a modified Rayleigh equation. The vapor mass in a given channel volume is given by the mass conservation equation. Thus the instantaneous channel pressure and vapor volume can be determined which, in turn, govern the dynamic processes of bubble initiation and growth on various pores. This model demonstrates for the first time the dynamic picture of boiling phenomena on enhanced surfaces, e.g. the instantaneous activation, growth and detachment of bubbles on various pores, the fluctuation of channel pressure and vapor mass etc.. The model has been tested for boiling of propane and isobutane on an enhanced tube. The predictions agree reasonably well with the experiments.

A Study of Bubble Departure in Forced-Convection Boiling

Visual observations in a boiling liquid flow indicate that, as the flow velocity is increased, the size of bubbles leaving the heating surface decreases. The purpose of this investigation is to arrive at a criterion for bubble departure in forced-convection nucleate boiling. Photographic studies indicate that a little before departure a “neck” is formed joining the almost spherical bubble to the heating surface. From a consideration of the hydrodynamic stability of the bubble-and-neck model, the departure-size-to-velocity relationship may be predicted. Measured departure radii in a forced-convection boiling water system taken by means of high-speed photography are satisfactorily correlated with the results derived analytically.