Current Issue : October - December Volume : 2012 Issue Number : 4 Articles : 7 Articles
A study of circular piezoelectric micro speakers is presented for applications in the audio frequency range, including values for\r\nimpedance, admittance, noise figures, transducer gain, and acoustic frequency responses. The micro speakers were modelled based\r\non piezoelectric micro ultrasonic transducer (pMUT) design techniques and principles. In order to reach the audio frequency\r\nrange, transducer radii were increased to the order of one centimetre, whilst piezoelectric layer thicknesses ranged the order of\r\nseveral �µm. The micro actuators presented might be used for a variety of electroacoustic applications including noise control,\r\nhearing aids, earphones, sonar, and medical diagnostic ultrasound. This work main contribution is the characterization of the\r\ndesign space and transducer performance as a function of transducer radius, piezoelectric layer thickness, and frequency range,\r\nlooking towards an optimized fabrication process....
The equivalent elastic moduli of asymmetrical hexagonal honeycomb are studied by using a theoretical approach. The deformation\r\nof honeycomb consists of two types of deformations. The first is deformation inside the unit, which is caused by bending,\r\nstretching, and shearing of cell walls and rigid rotation of the unit; the second is relative displacement between units. The equivalent\r\nelastic modulus related to a direction parallel to one cell wall of the honeycomb is determined from the relative deformation\r\nbetween units. In addition, a method for calculating other elastic moduli by coordinate transformation is described, and the\r\nelastic moduli for various shapes of hexagon, which are obtained by systematically altering the regular hexagon, are investigated.\r\nIt is found that the maximum compliance Cyy|max and the minimum compliance Cyy|min of elastic modulus Cyy in one rotation\r\nof the (x, y) coordinate system vary as the shape of the hexagon is changed. However, Cyy|max takes a minimum and Cyy|min takes\r\na maximum when the honeycomb cell is a regular hexagon, for which the equivalent elastic moduli are unrelated to the selected\r\ncoordinate system, and are constant with C11 = C22....
Infrared (IR) heating as alternative to forced air heating has been studied experimentally for 70 days in two identical, smallscale,\r\nexperimental greenhouses. The two heating options were implemented with four IR electric lamps and an electric forced\r\nair heater correspondingly. The microclimate in the greenhouses was monitored with thermocouples. Parameters characterizing\r\nthe outdoors macroclimate were also monitored. Lettuce was used as the test crop, and the two heating systems were operated\r\nautomatically to maintain a suitable reference temperature at the canopy. The investigation focused on night heating needs. Results\r\nindicated that the internal air temperature in the IR-heated greenhouse was always kept several degrees lower than the value\r\ntargeted for the canopy. This lower temperature resulted in 43% average energy savings for the IR-heated greenhouse, compared to\r\nthe conventionally heated one. A simple numerical model was used to estimate the potential energy savings in a production-scale\r\ngreenhouse....
Nanofluids are being considered for heat transfer applications; therefore it is important to know their thermophysical properties\r\naccurately. In this paper we focused on nanofluid specific heat capacity. Currently, there exist two models to predict a nanofluid\r\nspecific heat capacity as a function of nanoparticle concentration and material. Model I is a straight volume-weighted average;\r\nModel II is based on the assumption of thermal equilibrium between the particles and the surrounding fluid. These two models\r\ngive significantly different predictions for a given system. Using differential scanning calorimetry (DSC), a robust experimental\r\nmethodology for measuring the heat capacity of fluids, the specific heat capacities of water-based silica, alumina, and copper oxide\r\nnanofluids were measured. Nanoparticle concentrations were varied between 5wt% and 50wt%. Test results were found to be in\r\nexcellent agreement with Model II, while the predictions of Model I deviated very significantly from the data. Therefore, Model II\r\nis recommended for nanofluids...
We discuss briefly the constitutive modeling of the stress tensor for nanofluids. In particular, we look at the viscosity of nanofluids\r\ncontaining multiwalled carbon nanotubes (MWCNTs) stabilized by cationic chitosan. MWCNTs can be used either to enhance or\r\nreduce the fluid base viscosity depending on their weight fractions. By assuming that MWCNT nanofluids behave as generalized\r\nsecond-grade fluid where the viscosity coefficient depends upon the rate of deformation, a theoretical model is developed. A\r\nsimplified version of this model, similar to the traditional power-law model, is used in this study. It is observed that the theoretical\r\nresults agree well with the experimental data....
Nanofluids are being considered for heat transfer applications; therefore it is important to know their thermophysical properties\r\naccurately. In this paper we focused on nanofluid specific heat capacity. Currently, there exist two models to predict a nanofluid\r\nspecific heat capacity as a function of nanoparticle concentration and material. Model I is a straight volume-weighted average;\r\nModel II is based on the assumption of thermal equilibrium between the particles and the surrounding fluid. These two models\r\ngive significantly different predictions for a given system. Using differential scanning calorimetry (DSC), a robust experimental\r\nmethodology for measuring the heat capacity of fluids, the specific heat capacities of water-based silica, alumina, and copper oxide\r\nnanofluids were measured. Nanoparticle concentrations were varied between 5wt% and 50wt%. Test results were found to be in\r\nexcellent agreement with Model II, while the predictions of Model I deviated very significantly from the data. Therefore, Model II\r\nis recommended for nanofluids....
An experimental investigation on water-based nanofluids containing iron oxide (Fe2O3) in concentrations ranging between 5\r\nand 20% in mass is presented. The purpose of this study is to measure thermal conductivity and dynamic viscosity of these\r\nfluids, as a starting point to study the heat transfer capability. The stability of the nanofluids was verified by pH and Zeta\r\npotential measurements. A dynamic light scattering (DLS) technique was used to obtain the mean nanoparticle diameters. It was\r\nfound that thermal conductivity of these nanofluids improved with temperature and particles concentration. The temperature\r\nand nanoparticle concentration effects on viscosity were analyzed, obtaining a significant increase with respect to water. All the\r\nfluids exhibited a Newtonian behaviour. The experimental values were compared with some theoretical models for both thermal\r\nconductivity and dynamic viscosity....
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