Simulating the Thermal Behavior an Earth-Stationed Satellite Terminal

Lumped parameter modeling using finite differencing, form the basis for a C language simulation of a thermal system. The study of the earth-stationed satellite terminal includes an introduction to the system, derivation of the governing equation, development of a simulation, and a comparison of the results to empirical data. The analysis is undertaken with emphasis on the worst-case thermal environment, and with determination of the cooling loads and transient response as primary goals. Comparing the predicted cooling loads to empirical data, and previous analyses, indicates an accurate simulation has been constructed. The computer models are offered as an alternative method in thermal design and analysis of electronic systems typical to the earth-stationed satellite terminal

Nanoparticle Deposition by Boiling on Aluminum Surfaces to Enhance Wettability

Surface wettability is known to be important in boiling, condensation, frosting/defrosting, liquid desiccant flows in falling-film devices, and a myriad of other HVAC&R processes. Research has shown that surfaces treated with nanofluid boiling nanoparticle deposition exhibit radical changes in wettability, because a layer of nanoparticle coating is formed on the metal surface during microlayer evaporation at the base of the vapor bubble inboiling. Wettability is changed because surface chemical composition, surface roughness and porosity can be modified by the nanoparticle layer. This study is focused on how to manipulate wettability by nanoparticle deposition on aluminum surfaces, since this metal 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 heat flux, boiling duration and surface initial roughness are studied by varying parameters one at a time while controlling the others. Al2O3 nanoparticles of an average size of 40nm are deposited on a 20mm x 20mm aluminum surface. After the surface is created, contact-angle measurements with water are conducted using a goniometer to measure the advancing, receding, and static contact angles so as to characterize the wetting behavior. The surface morphology is characterized through scanning electron microscopy (SEM) and profilometry. It is observed that the layer of Al2O3 nanoparticle deposited on aluminum surfaces enhances the wettability on the surface. This result is explained using Wenzel’s model, in which an increasing areal roughness factor leads to a decrease in the apparent contact angle. The boiling time ranges from five to thirty minutes. It is observed that the longer the boiling time the lower the contact water contact angle. The impact of surface contamination is also investigated, and it is shown that nanoparticle deposited surfaces manifest enhanced wettability superior to that of a bare aluminum surfaces even after exposure to laboratory air

Self-healing, Slippery Surfaces for HVAC&R Systems

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 for HVAC&R systems is described for the purpose of exploiting the slippery nature of such surfaces, thereby improving the overall surface wettability. The goal of this work is to determine the feasibility of using these surfaces to more effectively manage condensate/frost formation on the heat exchanger. A microstructured, porous aluminum fin stocks with heterogeneous polyfluoroalkyl silane coating are infused with a secondary liquid acting as a lubricant that enhances slippery, liquid repellant and self-healing behavior. 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 of droplets by reducing the oil-water 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 fifth of the baseline water retention after self-defrosting. Therefore, such properties provide potential for improving the water drainage behavior for HVAC&R systems

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

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

Adsorption and Desorption Isotherms Of Desiccants for Dehumidification Applications: Silica Aerogels and Silica Aerogel Coatings on Metal Foams

Silica aerogels are frequently employed as solid desiccants in enthalpy wheels for dehumidifying the supply stream in air-conditioning systems. These desiccant materials possess good moisture adsorption and desorption characteristics due to their porous structure. Analysis of adsorption and desorption isotherms is critical for performance characterization and is often performed to evaluate the capacity and transient performance of desiccant-based dehumidification systems. The current study is focused on the adsorption and desorption isotherms of solid silica aerogels and silica aerogel coatings on open-cell metal-foam substrates. The sol-gel process is adopted to synthesize silica aerogels using different 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 characterize the microstructure of super-critically dried aerogels and adsorption/desorption isotherms for the different samples are obtained by the dynamic vapor sorption method. The steady-state moisture adsorption and desorption capacity of silica aerogels is affected by their porous structure, which depends on the synthesis technique used to prepare the silica aerogels. For the silica aerogel coatings on metal foams, the substrate structure and surface area also play an important role. The effect of the substrate surface area on adsorption/desorption capacity is analyzed by comparing the isotherms for solid silica aerogel samples, and silica aerogels coatings on flat plates and on metal foams with different pore sizes

Flow Visualization of Two-Phase R-245fa at Low Mass Flux in a Plate Heat Exchanger near the Micro-Macroscale Transition

Two-phase R-245fa flow in a plate heat exchanger is experimentally investigated to understand the unique flow regimes found during operation at low refrigerant mass flux. A transparent plate heat exchanger replica with 3.4 mm hydraulic diameter is 3D-printed for flow visualization using high-speed videography. Observed flow regimes support that the thermofluidic characteristics peculiar to plate heat exchanger (PHE) operation are due to the macro-microscale transitional two-phase flow from the coexistence of fluid inertial force and surface tension effects, corresponding to the operation conditions. Maximum stable bubble diameter is bigger at low mass flux than at high mass flux, and the bubbles can become big enough to be fully confined in the millimeter-scale PHE channel to be deformed or elongated. This represents the main thermo-physical characteristics of two-phase flow in mini- and microchannels, which is different from turbulent mixing flow easily found at high-mass-flux operation or in channels of conventional macroscale. Flow morphology involving complex bubble coalescence and breakup dynamics is captured and analyzed in relation to the fluid properties and geometric obstructions provided by the plate heat exchanger channel. While there exist previous studies, and even heat transfer coefficient correlations, suggesting the potential microscale flow regimes in PHEs, this is the first time presenting evidences via flow visualization

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

There is significant interest in the boiling performance of refrigerants in mini- and microchannels, especially in flow geometries relevant to compact heat exchangers for air-conditioning and refrigeration applications. Pressure drop (?P) characteristics during flow boiling of refrigerant R134a have been studied extensively over the past decade; however, in most research ?P is measured over a single channel or multiple parallel channels (manifold to manifold). There has been no work examining the individual pressure drop in each channel in multiple channel design. Moreover, correlations or relationships between the instantaneous ?P in individual minichannels operating in parallel have not been reported. In this work, an investigation of the effect of heat flux, mass flux and inlet vapor qualities on the flow patterns and pressure drop for flow boiling of R134a in 0.54 mm square parallel minichannels is reported. In particular, flow boiling experiments are conducted at flow rates between 0.1 and 0.51 g/s and heat fluxes from 0 to 36 kW/sq.m.. The heat flux input among a set of four horizontal, parallel minichannels is individually varied and controlled in each test. The focus of the work is on the investigation of correlations between flow boiling of R134a in parallel minichannels based on flow visualization and pressure drop measurements in each channel independently