A thermal model for mixed hydrophilic and hydrophobic surfaces is used to maximize mass collection by optimizing hydrophilic nodule size and spacing.  Mixed hydrophilic and hydrophobic surfaces offer a means of sustaining dropwise condensation.  Depending on flow conditions, dropwise condensation has the potential to increase heat transfer by close to an order of magnitude over film heat transfer.  A hydrophobic surface creates a significant barrier for droplet nucleation.  A hydrophilic surface has a low energy barrier for nucleation but would sustain film heat transfer.  A mixed hydrophilic and hydrophobic surface minimized the energy barrier for nucleation and can sustain dropwise condensation.  A mixed surface also provides control of the droplet sites which allows you to maximize the diffusion rate. 

A thermal model has been developed by the present authors for modeling mixed hydrophilic and hydrophobic surfaces.  Each individual droplet is modeled and combined to establish a total surface performance.  The traditional thermal model is enhanced by the present authors by including an equivalent thermal resistance for diffusion.  The enhanced model permits predictions of surface performance based on the surface arrangement and environmental conditions.  The laminar flow regime will be a relevant part of any practical mixed surface application because it will be found on the leading edge of all heat transfer surfaces.  Surfaces are simulated using flow rates ranging from natural convection to the upper limit of laminar flow.  Nodule sizes are evaluated ranging from 0.1 mm to 3.7 mm.  The air has a dew point of 24.5°C, and the surface is maintained at a 10°C subcooling.

The model shows that there are three distinct optimum nodules sizes across the laminar flow regime.  The optimum nodule sizes are 3.1 mm, 2.3 mm, and 1.5 mm.  At every flow rate, one of the aforementioned nodules sizes will maximize mass transfer, and the other two nodule sizes will result in a local maximum when plotted against the nodule size.  As flow rates increase, the optimum nodule size shifts from 3.1 mm to 2.3 mm and eventually becomes 1.5 mm as flow rates approach the upper limit of laminar flow.  Under natural convection conditions, the simulations show that the mass transfer can be increased by over 20% compared to traditional film heat transfer.  At natural convection flow rates, a nodule size of 3.1 mm results in a slightly higher mass collection than a 2.3 mm nodule, and the 1.5 mm nodule is also present as a local maximum but is as effective as the 3.1 mm and 2.3 mm nodule size.


Optimization of Mixed Hydrophilic and Hydrophobic Surfaces Under Laminar Flow Conditions

Division / Sponsors: 
Heat Transfer Division (HTD), Fluids Engineering Division (FED)

Heat Transfer Summer Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/HT/HT2020/volume/83709

18th International Conference on Nanochannels, Microchannels, and Minichannels
Paper Proceedings: https://asmedigitalcollection.asme.org/ICNMM/ICNMM2020/volume/83693

Fluids Engineering Division Summer Meeting
Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2020/volume/83716

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