Analysis of liquid-repellent surfaces within heat exchanger environments

Singh, Navdeep Sangeet (2025). Analysis of liquid-repellent surfaces within heat exchanger environments. University of Birmingham. Ph.D.

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Abstract

Steam condensation is omnipresent, and inevitable in many industrial processes. A classic example is seen within condensers that are used to liquify various gasses. In consequence, water-vapor is eventually deposited upon the exterior of heat pipes that passively cool the gas to form a thin film of liquid. This macroscopic film is responsible for the degradation of heat transfer efficiencies at the tubes’ solid-vapour interface. Liquid repellent surfaces can microscopically manipulate the hydrodynamics of formulating condensate to transition toward dropwise/jumping-droplet condensation. This effect will save operating and maintenance costs of steam condensers within power-plants, subsequently reducing emissions, operating power, and fuel consumption. The same can be said for an absorbertype (or evaporator) heat exchanger where a hot binary liquid falls over multiple horizontal subcooled heat pipes to be diluted with condensed water-vapour. Conversely, however, in this case, a liquid-adhesive surface is favourable as the expansion of the liquids’ surface area increases the rate of dilution and heat transfer rate whilst maintaining a small liquid film thickness. Nonetheless, within the literature, there has been little numerical insight that can adequately demonstrate how surface wettability can alter the flow behaviour of such heat exchanger tubes. Not to mention the effects of droplet impaction which can severely impact the heat transfer rate and flow behaviour. Therefore, for this thesis, various multiphase flow/phase-change simulations are presented for condenser/absorber applications.

Firstly, a two-dimensional numerical simulation of condensation of water upon a heat pipe is presented. By compiling numerous numerical strategies within the literature, the hydro/thermodynamics of the condensate is validated against the Nusselt film theory and experimental data. The tubes’ intrinsic contact angle is then altered to signify the how the flow behaviour is manipulated and its’ thermal implications. Then, the contact angle hysteresis of the tube is altered at various intrinsic contact angles to establish its consequence on the tubes’ heat transfer rate.

Using this as a foundation, the simulations’ domain is then extended to the 3rd dimension where additional horizontal heat pipes are allocated to establish falling-film flow of subcooled water. Again, the results were calibrated through systematic and experimental means within literature. Through meticulously refining the mesh within the vicinity of the falling liquid, a phenomenon known as the Rayleigh-Plateau instability of the liquid jet was depicted which radically altered the flow behaviour and heat transfer coefficients upon the tubes below at various contact angles. This was due to the emergence of additional impacting satellite droplets from the liquid jet which induced film waviness and mixing.

As droplet impaction was found to be the predominant factor for adjusting the heat transfer efficiencies of heat pipes, the effects of impacting droplets on functional surfaces were analysed through numerically accessing the droplet-repellency performance of microscopic doubly-re-entrant pillars. The performance of these pillars were compared to single re-entrant pillars and straight pillars to illustrate the importance of adjusting the overhang angle of the pillars against liquid-repellency. Furthermore, the key dimensions of the doubly re-entrant pillars are adjusted to again demonstrate their significance on droplet repellence. These results give further perception on how to design real-world functional surfaces by revealing which particular surface wettability parameters is able to provide the most dominance to enhance the performance of heat exchanger tubes further.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):