Self-Assembled Monolayer (SAM) Coatings for Condensation Enhancement are used to generate a low surface energy, non-wetting surface. Through surface modification, dropwise condensation can promote the condensation heat transfer coefficient to five-twenty times (5-20x) higher than standard filmwise condensation.
Dropwise condensation has demonstrated the capability to increase condensation heat transfer coefficients by an order of magnitude over filmwise condensation. Through self-assembled monolayer (SAM) promotion of rough micro/nano-textured surfaces, higher heat transfer efficiency can be achieved. An appropriately chosen low-thermal resistance coating for a given surface material can generate an ultra-low surface energy that is appreciably lower than the surface tension of the surrounding fluid. This ensures a non-wetting surface with a defined advancing and receding contact angle. The applications of dropwise condensation in two-phase thermal management include heat pipes, vapor chambers, power plant air-cooled and closed-loop condensers. A few examples of previous work in this area are shown below.
Microscale Heat Transfer Enhancement
To promote high heat transfer efficiency via dropwise condensation enhancement, research has been conducted at ACT to generate low surface energy coatings on microporous copper powder. The heat transfer enhancement exhibited by these textured surfaces is evidenced by the departing droplet size on the surface. The textured microporous surface can enhance heat transfer by changing the wetting state and contact angle hysteresis to allow droplets to depart the surface at a smaller size. For more detailed information, please refer to the journal and conference papers on this subject matter:
- Dropwise Condensation on Hydrophobic Microporous Powder and the Transition to Intrapowder Droplet Removal
- Dropwise Condensation on Superhydrophobic Microporous Wick Structures
For dropwise condensation phenomenon, the thermal conductivity of the condenser material influences the rate of heat transfer. In other words, the nonuniformity of surface heat flux for low thermal conductivity materials creates a narrowing of heat flow at the condensing surface. This is evidenced by the existence of large departing droplets on low thermal conductivity materials.
Long Life Coatings
The use of film-forming amine coatings is currently being investigated for replenishable, long-life coatings for steam surface condensers. For more information, please contact our R&D department.
Another rough surface of interest involves a wettability gradient, which is defined as a surface with locally varying surface energy or wettability. With dropwise condensation on a wettability gradient, heat transfer efficiency is further increased by the fast removal of small droplets on the surface. Unlike filmwise and traditional dropwise condensation, the gradient surface does not require gravity or forced convection to remove liquid from the condensing surface. The gradient surface causes a droplet to experience decreasing contact angles as the droplet travels along the path of increasing surface energy, as shown in Figure 3. The difference in contact angle is due to locally varying properties of the condensing surface, controlled by varying surface concentrations of molecules with low surface energy. The difference in contact angle on opposite sides of a droplet condensing on a gradient surface provides a driving force in the direction of decreasing contact angle. The motion is sustained until the droplet reaches the end of the gradient (e.g. near an evaporator).
The surface gradient is created by varying the concentration of a monomer with hydrophobic surface properties along the surface. In one method of fabricating surface gradients, the concentration on the surface is controlled by a diffusion controlled reaction of an organosilane molecule on the surface. An illustration of the surface modification process is shown in Figure 4. A liquid droplet of the organosilane molecule is suspended above the silanized copper surface. The molecules evaporate from the droplet and diffuse through air, approaching the copper/silicon surface. Once the organosilane molecules reach the surface, the reactive end of the molecule forms a chemical bond with the silicon surface. The low surface energy end of the molecule points directly away from the surface. Since there is a gradient in the bulk concentration (air/SAM mixture) above the surface, a gradient in the surface concentration of the molecules is developed. This gradient in bulk concentration is due to the diffusion of the SAM molecules from the concentrated liquid SAM droplet through the dilute air phase.
ACT has acquired data at a saturation temperature of 100°C without non-condensable gas (i.e. not in open atmosphere). Data were acquired at four input powers (25W, 50W, 75W and 100W) over the 2.54cm by 1.27cm heat input section (condenser heat fluxes ranged between 7.75 W/cm2 and 31 W/cm2). Figure 5 shows data acquired from a surface oriented vertically. The baseline filmwise mode condensation data agreed well with the Nusselt filmwise condensation model. The near perfect agreement validates the test section and method of heat transfer coefficient measurement for this study. Dropwise condensation on the non-wetting surface produced heat transfer coefficients that were 5-8 times higher than filmwise condensation, a significant improvement. By condensing on the gradient surface, a 35% further improvement in the heat transfer coefficient was observed as compared to a typical dropwise condensation surface. The expected improvement in the vertical orientation was not expected to be excessive, since gravitational force is able to pull condensing droplets on a vertical surface. The gradient adds some additional force to move the droplets in addition to gravity, resulting in increased performance.
Another test was performed with the condenser operating in a horizontal mode with the surface oriented about 5° against gravity (condenser below evaporator); see Figure 6. In this orientation the droplet has to be moved uphill in order to reach the end of the condensing section. The condensation heat transfer coefficients measured were very high (~10,000W/m2K), similar to that of filmwise condensation on a vertical surface.
- Advanced Coatings
- Applied Nanoscale Corrosion Erosion Resistant (ANCER™) Coatings
- Boiling Enhancement/Micro-porous Coatings
- SAM Coatings for Condensation Enhancement
- Plasma-Enhanced Chemical Vapor Deposition (PECVD) Coatings