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

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

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

Channel-to-channel heat flux variation in compact minichannel heat exchangers due to the effect of louvered fins

A typical minichannel evaporator used in AC&R applications is made of stacked parallel aluminum extruded multiport plates and louvered fins placed between them. In this configuration cold refrigerant flows inside the ports, and hot air passes though the louvered fins. Heat flux variation between neighboring minichannels, in the direction from the leading to the trailing edge, may be expected since warmer air enters the louvered fins domain and its temperature is reduced at the outlet. The possible nonuniformity of the heat flux from channel to channel was studied numerically using ANSYS Fluent as a threedimensional time-dependent heat transfer problem of louvered fins bounded with multiport aluminum plates. While the fin geometry was kept constant in all simulations, two different multiport plate configurations (11 round ports, D= 1.2 mm; and 22 square ports, 0.54 0.54mm2) were analyzed at air face velocities from 1 m/s to 5 m/s. The wall temperature of all channels was set to be constant 10 oC, which corresponds to the typical saturation temperature of refrigerants used in AC&R applications. The incoming air flow temperature considered was 20 oC and 30 oC. Results illustrate that both air velocity and temperature play a profound role on heat flux variation from the leading to the trailing edge of the multichannel plate. The heat flux varies drastically in the case of the slower incoming air flow due to the significant change in the driving potential along the air flow, and it varies less at higher air velocities due to the heat transfer recovery effect behind the turning louver along with the smaller driving temperature difference between mixing cup and saturation temperature. The overall heat flux difference between the leading channel and the trailing one reaches almost 94% at free stream air velocity 1 m/s and 69% at air velocity of 5 m/s. This numerical modeling of the conjugate heat transfer problem proves the presence of heat flux difference among channels which was overlooked in the literature. Understanding of the channel-to-channel heat flux variation is valuable for understanding the flow boiling behavior in parallel non-uniformly heated minichannels and the two-phase flow maldistribution.Papers presented at the 13th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Portoroz, Slovenia on 17-19 July 2017 .International centre for heat and mass transfer.American society of thermal and fluids engineers

Channel-to-channel heat flux variation in compact minichannel heat exchangers due to the effect of louvered fins

A typical minichannel evaporator used in AC&R applications is made of stacked parallel aluminum extruded multiport plates and louvered fins placed between them. In this configuration cold refrigerant flows inside the ports, and hot air passes though the louvered fins. Heat flux variation between neighboring minichannels, in the direction from the leading to the trailing edge, may be expected since warmer air enters the louvered fins domain and its temperature is reduced at the outlet. The possible nonuniformity of the heat flux from channel to channel was studied numerically using ANSYS Fluent as a threedimensional time-dependent heat transfer problem of louvered fins bounded with multiport aluminum plates. While the fin geometry was kept constant in all simulations, two different multiport plate configurations (11 round ports, D= 1.2 mm; and 22 square ports, 0.54 0.54mm2) were analyzed at air face velocities from 1 m/s to 5 m/s. The wall temperature of all channels was set to be constant 10 oC, which corresponds to the typical saturation temperature of refrigerants used in AC&R applications. The incoming air flow temperature considered was 20 oC and 30 oC. Results illustrate that both air velocity and temperature play a profound role on heat flux variation from the leading to the trailing edge of the multichannel plate. The heat flux varies drastically in the case of the slower incoming air flow due to the significant change in the driving potential along the air flow, and it varies less at higher air velocities due to the heat transfer recovery effect behind the turning louver along with the smaller driving temperature difference between mixing cup and saturation temperature. The overall heat flux difference between the leading channel and the trailing one reaches almost 94% at free stream air velocity 1 m/s and 69% at air velocity of 5 m/s. This numerical modeling of the conjugate heat transfer problem proves the presence of heat flux difference among channels which was overlooked in the literature. Understanding of the channel-to-channel heat flux variation is valuable for understanding the flow boiling behavior in parallel non-uniformly heated minichannels and the two-phase flow maldistribution

Nonlinear Integer Programming

Research efforts of the past fifty years have led to a development of linear integer programming as a mature discipline of mathematical optimization. Such a level of maturity has not been reached when one considers nonlinear systems subject to integrality requirements for the variables. This chapter is dedicated to this topic. The primary goal is a study of a simple version of general nonlinear integer problems, where all constraints are still linear. Our focus is on the computational complexity of the problem, which varies significantly with the type of nonlinear objective function in combination with the underlying combinatorial structure. Numerous boundary cases of complexity emerge, which sometimes surprisingly lead even to polynomial time algorithms. We also cover recent successful approaches for more general classes of problems. Though no positive theoretical efficiency results are available, nor are they likely to ever be available, these seem to be the currently most successful and interesting approaches for solving practical problems. It is our belief that the study of algorithms motivated by theoretical considerations and those motivated by our desire to solve practical instances should and do inform one another. So it is with this viewpoint that we present the subject, and it is in this direction that we hope to spark further research.Comment: 57 pages. To appear in: M. J\"unger, T. Liebling, D. Naddef, G. Nemhauser, W. Pulleyblank, G. Reinelt, G. Rinaldi, and L. Wolsey (eds.), 50 Years of Integer Programming 1958--2008: The Early Years and State-of-the-Art Surveys, Springer-Verlag, 2009, ISBN 354068274