Optimization of Nodule and Height Sizes for Mixed Hydrophilic and Hydrophobic Surfaces

Open cooling towers spray moisture into the air to cool the circulating water.  The cooling is performed sensibly by warming up the air, but the primary means is by evaporating some of the circulating water through the cooling tower.  During this process, droplets are entrained in the airflow and drift out of the cooling tower with the humid air.  The drift loss is one of the sources that require makeup to the circulating water system.  The drift losses increase the operating costs of the cooling tower due to the need to make up unneeded water and treat the water.  Because the drift is not entirely evaporated, it can carry microorganisms with the flow.  The most commonly known organism is legionella.

Drift elimination can be enhanced by including mixed hydrophilic and hydrophobic surfaces on the exhaust flow to capture the drift losses.  This practice is already enacted in nature by the Stenocara beetle of the Namib desert.  The beetle uses a mixed hydrophilic and hydrophobic surface on its wing cases to capture drinking water from the morning dew being blown over the sand dunes.  Using the same principles, we can extract moisture from the exhaust air of cooling towers.

A drop following the thermal model has been developed by Frymyer et. al.[1] for use on mixed hydrophilic and hydrophobic surfaces.  The model evaluates the heat transfer of each droplet to predict overall surface performance.  The model includes the thermal resistance of the surface, conduction through the droplet, and the capillary effect.  The model also consists of an equivalent thermal resistance for diffusion. 

Surface heights of 0.5 m, 1 m, and 2 m are evaluated to examine the impact of surface height on optimum nodule size.  The surfaces are evaluated at flow rates of 10 m/s, 25 m/s and 50 m/s for an examination of the impact due to flow rate.  The surface will be maintained with a 10°C subcooling below saturation.  The air is flowing from the bottom to top of the surface so that the turbulent boundary layer at the top aids in droplet removal from the laminar flow regime at the bottom. 

It has been shown that for small surfaces under turbulent flow conditions, the optimum square nodule size is 0.2 mm[2].  It has also been shown that under laminar flow conditions, the optimum square nodule sizes are 1.5 mm, 2.3 mm, and 3.1 mm nodule size and has an inverse relationship with the flow rate[3].  Initially, the larger nodules have better heat transfer due to the short drop height.  The larger nodules require larger droplets to overcome surface tension, which increases the thermal resistance due to conduction through the droplet.  Optimum nodule size is expected to be between 0.2 mm and 1.5 mm and be dependent on surface height and flow rate.  A nodule size in this region will balance the difficulty in removing the droplet from the surface with the initial heat transfer capabilities of larger nodules.

[1]       Frymyer, B., Vahedi, N., Neti, S., and Oztekin, A., 2020, “Thermal Modeling with Diffusion for Dropwise Condensation of Humid Air,” Int. J. Heat Mass Transf., 151, p. 119413.

[2]       Frymyer, B., 2020, “Thermal Modeling of Mixed Hydrophilic and Hydrophobic Surfaces,” Ph.D. Dissertation, Lehigh University.

[3]       Frymyer, B., and Oztekin, A., 2020, “Optimization of Mixed Hydrophilic and Hydrophobic Surfaces Under Laminar Flow Conditions,” SHTC2020-12052, pp. 1–9.