Using Anisotropic Micro-Scale Topography to Manipulate the Wettability of Aluminum and Reduce the Retention of Water

A method is described for fabricating controlled micro-scale, topographical features on aluminum surfaces for the purpose of exploiting those features to affect the surface wettability. Using a photolithographic approach, a photoresist-masked surface is subjected to a plasma etch in a mixture of gaseous BCl3 and Cl2. Parallel grooves, microns to tens of microns in width, depth and spacing are studied, because this geometry is scaleable for mass production by roll-to-roll micro-embossing, and because the anisotropic nature of these features provides a directional change in wettability that can reduce the retention of water on the surface. Aluminum was studied because it is naturally hydrophilic and widely used in wet-surface heat exchanger applications, because of its low cost and excellent mechanical and thermal properties. Water droplets placed on a micro-grooved aluminum surface using a micro-syringe exhibit significantly increased apparent contact angles, and for water condensed onto an inclined, micro-grooved surface, the droplet volume at incipient sliding is reduced by more than 50% compared to droplets on a surface without micro-grooves. No chemical surface treatment is necessary to achieve this water repellency; it is accomplished solely through the anisotropic surface topography. The droplet geometry shows an elongated base contour relative to a surface without micro-grooves, and discontinuities in the three-phase contact line are also introduced by the grooves. A mechanistic model is presented for predicting the critical droplet size on micro-grooved surfaces. This model extends earlier work by accounting for the droplet geometry and contact-line changes caused by the micro-grooves. The model is validated through comparisons of predicted to measured critical droplet sizes, and it is then used to provide guidance for the development of surfaces with enhanced water drainage behavior. In a broad range of air-cooling applications, water retention on the air-side surface of metallic heat exchangers is problematic, because it can reduce the air-side heat transfer coefficient, increase core pressure drop, and provide a site for biological activity. In refrigeration systems, the accumulation of frost on metallic fins requires periodic defrosting and reduces energy efficiency. When water is retained on these surfaces following the defrost cycle, ice is more readily formed in the subsequent cooling period, and such ice can lead to shorter operation times before the next defrost is required. Thus the management and control of water droplets on heat-transfer and airhandling surfaces is vital to energy efficiency, functionality, and maintenance in air-cooling systems. The microstructured surfaces introduced in this work are proposed for use in air-cooling and dehumidifying applications, but they may have other applications where the management of liquids on a surface is important.Air Conditioning and Refrigeration Project 166Air Conditioning and Refrigeration Project 20

Heat Transfer and Pressure Drop in a Developing Channel Flow with Streamwise Vortices

Experiments to assess the heat transfer and pressure-drop effects of delta-wing vortex generators placed at the entrance of developing channel flows are reported in this study. The experimental geometry simulates common heat exchanger configurations and tests are conducted over a velocity range important to heating, air conditioning and refrigeration. An innovative liquid-crystal thermography technique is used to determine the local and average Nusselt numbers for an isoflux channel wall, and conventional methods are used to determine the Fanning friction factor. Vortex generators with aspect ratios of A = 2 and A = 4 are studied at attack angles of a. = 20?? to 45????. The results indicate that the streamwise vortices generated by a delta wing can enhance local Nusselt numbers by more than 200% in a developing channel flow. Under some conditions, the spatially average Nusselt number nearly doubled for a heat transfer area that was 37 to 63 times the wing area. The Fanning friction factor increased by a few percent to nearly 60%, depending on the Reynolds number.Air Conditioning and Refrigeration Project 4

Vapor-Liquid Equilibria for R-32 and R-410A Mixed With a Polyol Ester: Non-Ideality and Local Composition Modeling

Vapor-liquid equilibria (VLE) data were obtained over a wide range of mixture composition and saturation conditions for difluoromethane (R-32) mixed with a polyol ester oil (POE). These data were correlated using the following local composition models from the literature: Wilson, Heil, Wang and Chao, Tsuboka and Katayama, NRTL, and UNIQUAC. The results were used to evaluate the suitability of these models in predicting the saturation behavior of the R-32/POE mixture. The Heil model had the best performance, with a 2-a error of 4.81 % in predicted saturation pressure; UNIQUAC was the worst, with a 2-a pressure error of more than 12%. Using VLE results from the literature for pentafluoroethane (R-125) mixed with the same oil and model parameters for that mixture, and attempt was undertaken to make a priori predictions of the P-T-x behavior of a blend containing R-32, R-125 and the oil (R-410A/POE). Data were obtained for this blend, and the results indicate that the Heil model can make such predictions with a 2:' a pressure error of about 11 %.Air Conditioning and Refrigeration Project 5

Superheat Stability of an Evaporator and Thermostatic Expansion Valve

In some refrigeration applications, difficulties arise in establishing stable evaporator operating conditions, especially when using a thermostatic expansion valve. The unstable superheat signal, sometimes called hunting, of an evaporator was investigated by developing a mathematical model of a thermostatic expansion valve and a two-passage concentric-tube evaporator. The model was then used to study the dynamic response of the evaporator and valve in response to changes in the system operating conditions. The evaporator model was based on a two-passage concentric-tube heat exchanger configuration. Equations for the conservation of mass, momentum, and energy were used to simulate the flow and heat transfer, where differential equations for the length of the two-phase region and mean void fraction allowed the dynamic behavior of the evaporator to be investigated. The model also has the capability to examine the effects of refrigerant and heat flux maldistribution among the passages. The thermostatic expansion valve model takes into account the pressure forces on the diaphragm as well as the pressure drop across the orifice when predicting the refrigerant mass flow rate. The geometrical parameters that were varied in this study included the orifice size, obstructing pin-tip angle, and diaphragm area. The model also includes the effects of the spring constant, bulb time constant, and offset temperature-as determined by the force applied by the obstructing pin when the valve is closed. Superheat response was investigated by imposing suction line pressure oscillations that varied over a range of frequencies. Large superheat fluctuations were found to exist in a given frequency band, where the period was found to be on the order of 50 to 100 seconds, and pressure oscillations in this range should be avoided in operation. Disturbances outside of this frequency band did not produce significant superheat responses. Factors influencing the magnitude of the superheat response depend on the frequency of the perturbations: at high frequencies the valve does not respond to superheatfluctuations (feedback), but is very sensitive to the slope of the flow rate versus superheat curve as detennined by valve geometry; on the other hand, at low frequencies the valve behavior is dominated by the superheat feedback, and the flow rate versus superheat curve is insignificant. The effect of the valve parameters was also investigated by imposing a step increase of the suction line pressure and simulating the response of?? the evaporator superheat over time. This approach allowed comparison of the steady-state and transient behavior of superheat with different valve designs.Air Conditioning and Refrigeration Center Project 7

An Evaluation of Heat Exchangers Using System Information and PEC

This report describes analyses aimed at integrating component optimization and system design by developing heat-exchanger performance evaluation criteria (PEC) that account for the system-level performance impacts of heat exchanger design. It builds on earlier studies that used relatively simple PEC to capture some of the component-level tradeoffs, but which usually ignore the system impact of component design. This report evaluates four PEC-j/f, heat transfer/pumping power (8), heat transfer/(pumping + compressor power) (n), and system COP. It is shown that j/f and 8 are better used as comparison criteria for existing heat exchangers of equal heat duty rather than as design criteria. The other two PEC, n and COP, include the system effect of compressor efficiency and therefore can be used more effectively in heat exchanger and system design. Through a combination of PEC and system optimization techniques, a method is developed to evaluate and design heat exchangers for maximum system performance.Air Conditioning and Refrigeration Project 9

Experimental study on condensation, frost formation and condensate retention on microgrooved and plain brass surfaces under natural convection condition

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.In this study, frost was grown on microgrooved and baseline brass samples under specific operating conditions and a comparison of condensation, frosting and defrosting pattern on microgrooved and flat brass surfaces were carried out experimentally. The surfaces were fabricated by mechanical micro-machining process and no chemical alteration of the surface was conducted. It was found that the shape, size and distribution of condensed water droplets and subsequent frost structure are significantly affected by the micro-scale roughness on the surface. The condensed water droplets took an elongated shape and then coalesced along the pillars and grooves on grooved surfaces giving a parallel brick-like frosting pattern. The frost structure on the grooved surface was different than that on the flat surface and frost crystal exhibited more directional growth in the parallel to the surface direction, with numerous ice-flakes growing in the perpendicular and angular directions to the grooves. This non-uniform growth of the frost layer also gave the appearance of a spongy and loose frost structure and suggested the formation of less dense frost. Qualitative study of the spatial and temporal distribution of retained condensate on the grooved and plain brass surfaces after defrosting were carried out by analyzing thermal images of the sample surface during the defrosting period. Findings of this of this study can give significant insight about the frost properties and defrosting and condensate retention behaviour of heat transfer equipments with embedded microgrooves.pm201

Comparison of frosting, defrosting and condensate retention characteristics of vertical parallel microgrooved and plain brass surfaces in forced convection condition

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.In the present study, condensation, frosting and condensate (frost melt water) retention characteristics of brass surfaces with parallel microgrooves have been investigated experimentally and compared with the plain baseline surface. Parallel micro-scale surface features were obtained by a mechanical micromachining process (micro end-milling) without applying any chemical means to modify the surface energy. The surfaces exhibited anisotropic wettability with high static contact angles (SCA) of 1320 to about 1460 in the direction parallel to grooves. Frost was grown on sample surfaces (45 mm x 45 mm) inside a thermally controlled chamber, in the presence of very cold surrounding air (≈ -60C) under forced convection condition (air velocity of 0.25~1.0 m/s). Condensation and frosting pattern as well as condensate retention characteristics of the microgrooved surfaces were found to be significantly different than on the flat brass surfaces. Highly improved condensate drainage behavior was obtained for the microgrooved surfaces which drained up to 70% more condensate than the flat baseline. It was found that variation in the wettability (static contact angle) of the microgrooved surfaces significantly affects the condensate drainage characteristics. Improved condensate drainage was achieved for surfaces with higher static contact angle and lower wetting anisotropy. Variation of operating parameters (plate temperature, frost surface temperature etc.) during defrosting at different heating rate was also investigated. The findings of the present work provide valuable information on the frosting/defrosting characteristics of microgrooved surfaces signifying its possibility for better condensate management in a broad range of air conditioning, refrigeration and cryogenic applications.pm201

Flow Diagnostics and the Acoustic Behavior of a Fan-and-Coil Assembly

Project 84 concentrated on understanding the noise generating mechanisms of axial-flow fans with the intent of proposing methods of component design and system assembly by which noise generation is reduced or minimized. The project focused on the fan-coil unit typical to room air conditioners and many split-system applications. This report presents the accomplishments of the project, including the design, construction, and qualification of an anechoic chamber, the acquisition of acoustic, flow, and pressure data within a fan-coil unit over a typical operating range, and the development of an understanding of the flow-structure interactions responsible for noise generation in the fan-coil unit. This report focuses on the investigation of a method for measuring the dynamic axial force generated by a fan operating in a steady but spatially non-uniform flow field. Several variations of a measurement system that uses a cantilever beam were tested. Experimental results indicated, in all designs, that the measurement system introduced additional sources of axial motion, occurring at the frequencies of interest and at amplitudes much larger than the one to be measured. Recommendations for future work are given.Air Conditioning and Refrigeration Project 8

Surface embossing technique for condensate management in air-cooling heat exchangers

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.vk201