Showing papers by "Anthony M. Jacobi published in 2014"

Hydrophobic and oleophobic re-entrant steel microstructures fabricated using micro electrical discharge machining

Abstract: This paper presents the fabrication of metallic micro-mushroom re-entrant structures and the characterization of their hydrophobicity and oleophobicity. Five different microstructure geometries are introduced, with typical feature sizes in the range of 10–100 μm. These microstructures are realized in steel, and are fabricated over the cm-scale using micro electrical discharge machining (mEDM). The liquid repellency of these surfaces is characterized using droplets of either water (surface energy γlg = 72.4 mN m−1), RL-68H oil (γlg = 28.6 mN m−1), or Isopropanol (IPA) (γlg = 21.7 mN m−1). The water droplets form nearly perfect spheres, with contact angles in the range 146–162°, and contact angle hysteresis of 19–35°. The oil droplet contact angles are in the range 106–152° and the IPA contact angles are in the range 75–123°. Strong re-entrant features and close spacing are necessary to support a fully non-wetting state for use with oil and IPA. Water forms the highest contact angles with narrow, post-like, and widely spaced micro-mushroom geometries.

A Thermodynamic Basis for Predicting Falling-Film Mode Transitions

Abstract: Horizontal-tube, falling-film heat exchangers are used in many air-conditioning and refrigeration systems. Depending on the tube diameter and spacing, the flow rate, and fluid properties, when a liquid film falls over a series of horizontal tubes three distinct flow patterns can be manifested. These flow patterns are the droplet mode, the jet mode, and the sheet mode. A thermodynamic analysis is undertaken to predict the transitions between these modes. By seeking the condition corresponding to thermodynamic equilibrium between two neighboring modes, a scaling relation is developed for the transitional Reynolds number. This theoretical framework is used to provide the first explanation for the relationship between the transitional Reynolds number and modified Galileo number which has been previously based solely on experimental observations. This approach offers insight into the prevailing physics, and it suggests a tube-spacing effect on the mode transitions which has not previously been anticipated. Using limited data and prior results from the literature it is found that this effect is likely to exist. The implications of this thermodynamic approach to predicting two-phase flow patterns are discussed in terms of entropy generation minimization and transition hysteresis, as is its incompleteness.

Nanoparticle Deposition by Boiling on Aluminum Surfaces to Enhance Wettability

Abstract: Surface wettability of materials is important in a myriad of HVAC&R processes. This study is focused on how to manipulate wettability by nanofluid boiling nanoparticle deposition on aluminum surfaces. The focus is on aluminum, because it is commonly used as the material for heat transfer in air conditioning and refrigeration systems. The boiling deposition process occurs under atmospheric pressure, in a reservoir large compared to the sample size. The effect of nanoparticle concentration, boiling duration and surface initial roughness are studied by varying parameters one at a time while controlling the others. Al2O3 nanoparticles aredeposited on the aluminum surface, as confirmed by microscopy. The static contact angle of water is measured using a goniometer to characterize the wetting behavior. It is observed that the layer of Al2O3 nanoparticle deposited on aluminum surfaces enhances the wettability on the surface, and the higher the concentration of nanoparticle in the fluid during boiling deposition, the better the wetting of the surface. Surface roughness before and after the nanofluid boiling process is also investigated. The relationship between the surface topography and water wettability is discussed.

Determination of Transmission Loss in Slightly Distorted Circular Mufflers Using a Regular Perturbation Method

Abstract: A perturbation-based approach is implemented to study the sound attenuation in distorted cylindrical mufflers with various inlet/outlet orientations. Study of the transmission loss (TL) in mufflers requires solution of the Helmholtz equation. Exact solutions are available only for a limited class of problems where the method of separation of variables can be applied across the cross section of the muffler (e.g., circular, rectangular, elliptic sections). In many practical situations, departures from the regular geometry occur. The present work is aimed at formulating a general procedure for determining the TL in mufflers with small perturbations on the boundary. Distortions in the geometry have been approximated by Fourier series expansion, thereby, allowing for asymmetric perturbations. Using the method of strained parameters, eigensolutions for a distorted muffler are expressed as a series summation of eigensolutions of the unperturbed cylinder having similar dimensions. The resulting eigenvectors, being orthogonal up to the order of truncation, are used to define a Green's function for the Helmholtz equation in the perturbed domain. Assuming that inlet and outlet ports of the muffler are uniform-velocity piston sources, the Green's function is implemented to obtain the velocity potential inside the muffler cavity. The pressure field inside the muffler is obtained from the velocity potential by using conservation of linear momentum. Transmission loss in the muffler is derived from the averaged pressure field. In order to illustrate the method, TL of an elliptical muffler with different inlet/outlet orientations is considered. Comparisons between the perturbation results and the exact solutions show excellent agreement for moderate (0.4∼0.6) eccentricities.

Transient pressure drop correlation between parallel minichannels during flow boiling of R134a

Abstract: There is significant interest in the boiling perfor mance of refrigerants in miniand microchannels, especially in flow geometries relevant to compact h eat exchangers for air-conditioning and refrigerati on applications. Pressure drop ( ΔP) characteristics during flow boiling of refrigera nt R134a have been studied extensively over the past decade; however, in most research ΔP is measured over a single channel or multiple parallel channels (manifold to manifold). There has been no work examining the individual pressure dro p in each channel in multiple channel design. Moreover, corre lations or relationships between the instantaneous ΔP in individual minichannels operating in parallel have not been reported. In this work, an investigation of the effect of hea t flux, mass flux and inlet vapor qualities on the flow patterns and pressure drop for flow boiling of R134 a in 0.54 mm square parallel minichannels is report ed. In particular, flow boiling experiments are conducted at flow rates between 0.1 and 0.51 g/s and heat fl uxes from 0 to 36 kW/m. The heat flux input among a set of four horizonta l, p rallel minichannels is individually varied and controlled in each test. The focus of the work is o n the investigation of correlations between flow bo iling of R134a in parallel minichannels based on flow visualizatio n and pressure drop measurements in each channel in depe dently.

Self-healing, Slippery Surfaces for HVAC&R Systems

Abstract: Enhancing water shedding behavior on aluminum surfaces is important in the design of energy-efficient heat exchangers. In this work, a method for fabricating oil-infused aluminum surfaces for HVAC&R systems is described for the purpose of exploiting the slippery nature of such surfaces, thereby improving the overall surface wettability. A microstructured, porous aluminum fin stocks with heterogeneous hydrophobic coating are infused with a secondary liquid acting as a lubricant that enhances slippery, liquid repellant and self-healing behavior. The objective of this work is to study the feasibility of using these surfaces to more effectively manage condensate/frost formation on heat exchangers. The effects of the underlying oil-infused microstructure and hydrophobic coating on the behavior of droplets are studied. Although the slippery surfaces are observed to decrease the contact angle of droplets, they promote mobility by reducing the interfacial energy and friction force. From preliminary experiments, critical inclination angles of small droplets (volume =30 µl) are reduced by more than 40° compared to baseline surfaces. Moreover, slippery surfaces delay the frost formation, and have only one fourth of the baseline water retention after self-defrosting. Therefore, such properties provide potential for improving the water drainage behavior for HVAC&R systems.

Interactions Between Parallel Minichannels During Flow Boiling of R134A

Abstract: This work focuses on the study of flow boiling of R134a in 054 mm square parallel minichannels, with a particular focus on the transient pressure drop of individual channels and their interaction The individual pressure drop in each passage was analyzed to establish the inter-channel relationship; additionally, the effect of heat and mass flux and the inlet vapor qualities on the flow patterns of each channel was studied based on flow visualization and pressure drop measurements The mass flux and heat flux in the experiments were varied up to 800 kg/m2s and 10 kW/m2 respectively The heat flux was controlled and varied independently in each channel Results illustrate that interaction between channels exists, and the correlation degree depends on the flow boiling dynamics in each passage The pressure drop oscillation in each channel affects the flow redistribution between channels A channel subjected to the least heat flux tends to correlate the most with greater heated channels because of the mass flux fluctuations caused by boiling phenomena in other channelsCopyright © 2014 by ASME

Mass Diffusion Coefficient Of Desiccants For Dehumdification Applications: Silica Aerogels And Silica Aerogel Coatings On Metal Foams

Abstract: Silica aerogels prepared by the sol-gel process are often used as solid desiccants in enthalpy wheels for dehumidifying ventilation air in air-conditioning systems. These hygroscopic materials have good moisture adsorption and desorption characteristics due to their porous structure. The current study is focused on the evaluation of the mass diffusivity of solid silica aerogels and silica aerogel coatings on substrates, which determines the rate at which a dehumidification process can be performed. The mass diffusivity of silica aerogels is affected by their porous structure which depends on the synthesis technique used to prepare the silica aerogels. The sol-gel process is used to prepared silica aerogels using various basic (ammonium hydroxide, sodium hydroxide, potassium hydroxide) and acidic (hydrofluoric acid, steric acid, hydrogen peroxide) catalysts with the same precipitator (tetra methyl orthosilicate-TMOS) and solvent (methanol). Scanning electron microscopy is used to analyze the microstructure of supercritically dried aerogels. The dynamic vapor sorption method is used to determine the effective mass diffusivity for the different silica aerogels. It is found that the mass diffusivity is related to the microstructure of silica aerogels, which depends on the catalysts used in the sol-gel process; however, the value for mass diffusivities for solid desiccants and desiccant coatings are similar. In addition, a parametric study is conducted to determine the effect of relative humidity and temperature on the adsorption and desorption mass diffusivity.