Andrew D. Sommers

Creating micro-scale surface topology to achieve anisotropic wettability on an aluminum surface

A technique for fabricating micropatterned aluminum surfaces with parallel grooves 30 ?m wide and tens of microns in depth is described. Standard photolithographic techniques are used to obtain this precise surface-feature patterning. Positive photoresists, S1813 and AZ4620, are selected to mask the surface, and a mixture of BCl3 and Cl2 gases is used to perform the etching. Experimental data show that a droplet placed on the micro-grooved aluminum surface using a micro-syringe exhibits an increased apparent contact angle, and for droplets condensed on these etched surfaces, more than a 50% reduction in the volume needed for the onset of droplet sliding is manifest. No chemical surface treatment is necessary to achieve this water repellency; it is accomplished solely by an anisotropic surface morphology that manipulates droplet geometry and creates and exploits discontinuities in the three-phase contact line. These micro-structured surfaces are proposed for use in a broad range of air-cooling applications, where the management of condensate and defrost liquid on the heat transfer surface is essential to the energy-efficient operation of the machine.

A Review of Metal Foam and Metal Matrix Composites for Heat Exchangers and Heat Sinks

Recent advances in manufacturing methods open the possibility for broader use of metal foams and metal matrix composites (MMCs) for heat exchangers, and these materials can have tailored material properties. Metal foams in particular combine a number of interesting properties from a heat exchangers point of view. In this paper, the material properties of metal foams and MMCs are surveyed, and the current state of the art is reviewed for heat exchanger applications. Four different applications are considered: liquid–liquid, liquid–gas, and gas–gas heat exchangers and heat sinks. Manufacturing and implementation issues are identified and discussed, and it is concluded that these materials hold promise both for heat exchangers and heat sinks, but that some key issues still need to be solved before broad-scale application is possible.

Wetting phenomena on micro-grooved aluminum surfaces and modeling of the critical droplet size

The behavior of water droplets on aluminum surfaces with parallel grooves tens of microns in width and depth is considered, and a mechanistic model is developed for predicting the critical droplet size-droplets at incipient sliding due to gravity. The critical droplet size is nearly 50% smaller on micro-grooved surfaces than on the same surface without micro-grooves. The application of existing models fails to predict this behavior, and a new model based on empiricism is developed. The new model provides reasonable predictions of the critical droplet size for a given inclination angle, advancing contact angle, and maximum contact angle. When the grooves are aligned parallel to gravity, the maximum apparent contact angle does not occur at the advancing front but rather along the side of the droplet because of contact-line pinning. Droplets on these surfaces are elongated and possess a parallel-sided base contour shape. Novel data are provided for droplets in a Wenzel state, a Cassie-Baxter state, and combined state on micro-grooved surfaces, and the ability of the empirical model to handle these variations is explored. These findings may be important to a broad range of engineering applications.

Topography-Based Surface Tension Gradients to Facilitate Water Droplet Movement on Laser-Etched Copper Substrates

This paper describes a method for creating a topography-based gradient on a metallic surface to help mitigate problems associated with condensate retention. The gradient was designed to promote water droplet migration toward a specified region on the surface which would serve as the primary conduit for drainage using only the roughness of the surface to facilitate the movement of the droplets. In this work, parallel microchannels having a fixed land width but variable spacing were etched into copper substrates to create a surface tension gradient along the surface of the copper. The surfaces were fabricated using a 355 nm Nd:YVO4 laser system and then characterized using spray testing techniques and water droplet (2-10 μL) injection via microsyringe. The distances that individual droplets traveled on the gradient surface were also measured using a goniometer and CCD camera and were found to be between 0.5 and 1.5 mm for surfaces in a horizontal orientation. Droplet movement was spontaneous and did not require the use of chemical coatings. The theoretical design and construction of surface tension gradients were also explored in this work by calculating the minimum gradient needed for droplet movement on a horizontal surface using Wenzels model of wetting. The results of this study suggest that microstructural patterning could be used to help reduce condensate retention on metallic fins such as those used in heat exchangers in heating, ventilation, air-conditioning, and refrigeration (HVAC&R) applications.

An exact solution to steady heat conduction in a two-dimensional annulus on a one-dimensional fin : Application to frosted heat exchangers with round tubes

The fin efficiency of a high-thermal-conductivity substrate coated with a low-thermalconductivity layer is considered, and an analytical solution is presented and compared to alternative approaches for calculating fin efficiency. This model is appropriate for frost formation on a round-tube-and-fin metallic heat exchanger, and the problem can be cast as conduction in a composite two-dimensional circular cylinder on a one-dimensional radial fin. The analytical solution gives rise to an eigenvalue problem with an unusual orthogonality condition. A one-term approximation to this new analytical solution provides fin efficiency calculations of engineering accuracy for a range of conditions, including most frosted-coated metal fins. The series solution and the one-term approximation are of sufficient generality to be useful for other cases of a low-thermal-conductivity coating on a high-thermal-conductivity substrate. DOI: 10.1115/1.2165210

Spontaneous movement of water droplets on patterned Cu and Al surfaces with wedge-shaped gradients

We report a simple technique for moving water droplets on a hydrophilic aluminum surface containing a hydrophobic copper background. A surface tension gradient due to a triangular-shaped wedge moves the droplets towards the end of the wedge which contains more Al surface area. Movement on both horizontal surfaces and surfaces oriented vertically against gravity has been observed. The mode of droplet motion was found to depend on the wedge angle, and the speed was found to depend on both the wedge angle and the droplet contact angles on the Al and Cu.

Defrosting performance on hydrophilic, hydrophobic, and micro-patterned gradient heat transfer surfaces

In the current article, differences in drainage rates and defrosting effectiveness were explored for surfaces of differing wettability. Both patterned and nonpatterned surfaces were explored. Seven surfaces were examined in all—an uncoated, untreated aluminum plate (Sample 1), an identical surface treated with a hydrophilic coating (Sample 2), a surface containing evenly spaced microchannels with and without a hydrophobic coating (Samples 3 and 4), and a surface containing a microstructural roughness gradient with and without a hydrophobic coating (Samples 5 and 6). Cyclical tests consisting of three frosting/defrosting events were performed on each sample. Each cycle consisted of 1 hour of frost growth, followed by 10 minutes of defrost and drainage. The frost layer was grown on the surface inside an environmental test chamber under controlled operating conditions. The surface temperature, air temperature, and relative humidity were recorded to ensure that constant conditions were maintained during each experiment. Overall, the surface defrosting effectiveness varied from 52%–77% across all surfaces depending on the test conditions, with one test showing slightly lower percentages. The present data show that only small differences were observed in the defrosting effectiveness between the samples. The gradient surfaces did, however, remove slightly more water from the surface during defrosting (as compared to the baseline) when the frost was grown at colder surface temperatures. The average increase in defrosting effectiveness was 2%–4% for Surface 6 versus Surface 1 at Tw = −12°C. Interestingly, when the frost was grown at warmer surface temperatures, the gradient surfaces did not perform as well. However, in almost all cases the defrosting effectiveness increased as the surface temperature during the frost growth period was decreased. This finding suggests that defrosting effectiveness is intrinsically linked to the thermophysical properties of the grown frost layer.

FRICTION STIR WELDING OF SC-MODIFIED AL-ZN-MG-CU ALLOY EXTRUSIONS

Small additions of scandium to Al-Zn-Mg-Cu 7000 series alloys can significantly improve mechanical properties and augment the strength retention at low and elevated temperatures. This research program evaluates the residual properties of Sc-modified Al-Zn-Mg-Cu alloy extrusions joined through friction stir welding (FSW). Mechanical and corrosion testing were performed on the baseline material and on panels friction stir welded at 175, 225, 250, 300, 350 and 400 RPM (all other weld parameters held constant). A thermal model of friction stir welding is developed that utilizes an energy-based scaling factor to account for tool slip. The proposed slip factor is derived from an observed, empirical relationship between the ratio of the maximum welding temperature to the solidus temperature and energy per unit length of weld. The thermal model successfully predicts the maximum welding temperatures over a range of energy levels, and the mechanical and corrosion behavior is correlated to the temperature distribution predicted by the model.Copyright

A Study of the Thermal-Hydraulic Performance and System-Level Effects of Aluminum Oxide-Propanol Nanofluid

The application of nanofluids in real systems could lead to smaller, more compact heat exchangers and reductions in material cost. However, few studies have been conducted which have carefully measured the thermo-physical properties and thermal performance of these fluids as well as examine the system-level effects of using these fluids in traditional cooling systems. In this work, dilute suspensions of 10 nm aluminum oxide nanoparticles in propanol (0.5 wt%, 1 wt%, and 3 wt%) were investigated. Changes in density, specific heat, and thermal conductivity with particle concentration were measured and found to be linear whereas changes in viscosity were nonlinear and increased sharply with particle loading. Nanofluid heat transfer performance data were generally commensurate with that measured for the baseline. For the 1 wt% concentration, a small but significant enhancement in the heat transfer coefficient was recorded for 1800 2800 which is attributed to an earlier transition to turbulent flow. In the case of high particle loading (i.e. 3 wt%), the thermal performance was observed to deteriorate with respect to the baseline case. Discoloration of the fluid was also observed after being cycled at high flow rates and increased temperature.Copyright

Methodology for Calculating the Volume of Condensate Droplets on Topographically Modified, Microgrooved Surfaces

Liquid droplets on micropatterned surfaces consisting of parallel grooves tens of micrometers in width and depth are considered, and a method for calculating the droplet volume on these surfaces is presented. This model, which utilizes the elongated and parallel-sided nature of droplets condensed on these microgrooved surfaces, requires inputs from two droplet images at ϕ = 0° and ϕ = 90°--namely, the droplet major axis, minor axis, height, and two contact angles. In this method, a circular cross-sectional area is extruded the length of the droplet where the chord of the extruded circle is fixed by the width of the droplet. The maximum apparent contact angle is assumed to occur along the side of the droplet because of the surface energy barrier to wetting imposed by the grooves--a behavior that was observed experimentally. When applied to water droplets condensed onto a microgrooved aluminum surface, this method was shown to calculate the actual droplet volume to within 10% for 88% of the droplets analyzed. This method is useful for estimating the volume of retained droplets on topographically modified, anisotropic surfaces where both heat and mass transfer occur and the surface microchannels are aligned parallel to gravity to assist in condensate drainage.