Finding resonant frequencies for high loss dielectrics in cylindrical cavities

This article proposes the use of argument principle method (APM) to find all complex resonant frequencies in a three layer cylindrical cavity. APM guarantees that no root is lost and frequencies can be associated with the resonant mode. The roots can be used to find permittivity of a material inside a cavity. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:530–535, 2015.     

Heat transfer in microchannel devices using DSMC

The heat transfer characteristics of supersonic flows in microchannels is studied using direct simulation Monte Carlo (DSMC) method. The velocity components and the spatial coordinates of the simulated particles are calculated and recorded by using a variable-hard-sphere (VHS) collision model. The effects of Knudsen number (Kn) on the heat transfer of the microchannel flows are examined. The results show that the magnitude of the temperature jump at the wall increases with increasing Kn. The heat transfer to the isothermal wall is found to increase significantly with Kn. The possible causes for the increase of wall heat transfer are discussed     

Heat transfer process in an elliptical channel

This paper addresses the problem of heat transport in an elliptical channel in the presence of a temperature gradient parallel to its axis. The Williams equation is used as the basic equation describing the kinetics of the process and a model of diffusive reflection is used as the boundary conditions on the channel wall. The deviation of the gas condition from the equilibrium is assumed to be small. In order to find a linear correction to the local equilibrium function of distribution, a boundary problem consisting of a linear homogeneous partial differential equation of the first order with a homogeneous boundary condition has been built. The solution of the built boundary value problem has been found by the method of characteristics. The value of the heat flow through the cross section of the channel is found by using numerical procedures implemented by the computer algebra Maple 17 system. The results were compared with the analogous results found in the open press.     

Fabrication of silicon and oxide membranes over cavities usingion-cut layer transfer

Silicon and oxide membranes were fabricated using an ion-cut layer transfer process, which is suitable for sub-micron-thick membrane fabrication with good thickness uniformity and surface micro-roughness. After hydrogen ions were implanted into a silicon wafer, the implanted wafer was bonded to another wafer that has patterned cavities of various shapes and sizes. The bonded pair was then heated until hydrogen-induced silicon layer cleavage occurred along the implanted hydrogen peak concentration, resulting in the transfer of the silicon layer from one wafer to the other. Using this technique, we have been able to form sealed cavities and channels of various shapes and sizes up to 50-μm wide, with a 1.6-μm-thick silicon membrane. As a process variation, we have also fabricated silicon dioxide membranes for optically transparent applications     

Entanglement and quantum state transfer between two atoms trapped in two indirectly coupled cavities

Quantum Information Processing - We propose a one-step scheme for implementing entanglement generation and the quantum state transfer between two atomic qubits trapped in two different cavities...     

Fluid flow and heat transfer past two spheres in a cylindrical tube

The fluid flow and the heat transfer for a row of spheres in a cylindrical tube is modelled by considering the flow past two spheres in a long tube. The problem is solved numerically by a finite element method, using a velocity-pressure formulation for the Navier-Stokes equations.Results are obtained for Reynolds number up to 200, with Prandtl numbers of 0.72 and 7.0, for a range of sphere sizes and sphere separations. It was found that as the distance between the spheres was decreased a circulatory region of flow appeared between the spheres for a given Reynolds number. This eddy led to poor heat transfer in this region. Increasing the Reynolds number was found not to improve the situation as the eddy grew in size and caused poorer heat transfer. This was found to be true with even the widest gap sizes considered.     

Heat transfer blockage in small scale combustion of polymers

Flame heat transfer blockage occurs as fuel vapors,soot and products of combustion near a burning fuel surface block much of the heat feedbacks(including external radiative heat flux)to the fuel surface of a burning object.Blockage clearly affects burning rates and heat release rates of fires.This needs to be included when calculating flame heat transfer in fire growth models.An understanding of burning of materials in small scale fires is of broad and vital importance for predicting their burning performan...     

On the flow between two rotating shrouded discs

Numerical solutions to the full Navier Stokes equations for the flow enclosed between two rotating discs and a sidewall are presented. The sidewall remains fixed throughout, while we consider examples in which: (i) the bottom disc is rotating and the top disc is fixed; (ii) the two discs rotate in the same sense; (iii) the two discs rotate in opposite senses. We present solutions up to a Reynolds number of 1200, using second order central finite differencing.     

Enhancing the quantum state transfer between two atoms in separate cavities via weak measurement and its reversal

Taking the advantage of weak measurement and quantum measurement reversal, we propose a scheme to enhance the fidelity of transferring quantum state from one atom trapped in cavity to another distant one trapped in another cavity which is coupled by an optical fiber. It is turned out that the fidelity can be greatly improved even when the system is under serious dissipation. Moreover, the scheme works in both the strong-coupling and weak-coupling regimes. It is also robust to the ratio of the coupling constant between the atoms and the cavity modes to the coupling constant between the fiber and cavity modes. The underlying mechanism can be attributed to the probabilistic nature of weak measurements.     

Numerical solution of laminar recirculating flow between shrouded rotating disks

In this paper we develop a method to obtain the numerical solution of the problem of recirculating flow between shrouded rotating disks. The major difficulty of this type of problem is in obtaining convergence at high Reynolds numbers. With the technique developed in this paper we have obtained convergence for Reynolds numbers up to 10,000. The procedure can be extended to higher Reynolds numbers if desired. The contours of the stream function, vorticity function and angular velocity are presented for Reynolds numbers of 500, 2000, 5000 and 10000. The method is applicable to any problem which has similar equations of motion.     

Heat,mass and momentum transfer across a wavy boundary

This paper describes the application of a numerical procedure to the computation of the hydrodynamics and heat/mass transfer processes for the shearing flow of a fluid over a wavy boundary. The flow situation is treated as elliptic in nature to cover speculations of previous authors that a circulating eddy is generated in the troughs. The turbulence model used comprises two differential equations, one for the kinetic energy of turbulence and the other for its dissipation rate. The present analysis needs none of the usual wave assumptions and is applicable to flows of any Reynolds number over waves of any shape and characteristics and of any type (solid and rigid, solid but flexible, interfaces between fluids). Predictions are obtained for flow of air over water standing in a channel, and these predictions agree satisfactorily with the observed experimental data.     

Heat transfer enhancement using flow-induced vibration of a microfin array

Advanced computers are facing thermal engineering challenges from both high heat generation due to rapid performance improvement and the reduction of an available heat removal surface due to large packaging density. Efficient cooling technology is desired to provide reliable operation of microelectronic devices. This paper investigates the feasibility of heat transfer enhancement in laminar flow using the flow-induced vibration of a microfin array. The microfins are initially bent due to the residual stress difference. In order to characterize the dynamics of the microfin flow-induced vibration, a microfin sensor is fabricated. Increase in air velocity provides larger vibrating deflection, while the vibrating frequency of the microfin is independent of the air velocity. The thermal resistances are measured to evaluate the thermal performance of the microfin heat sink and compared with those of a plain-wall heat sink. For a fluid velocity of 4.4 m/s, the thermal resistance of the microfin array heat sink is measured to be 4.45°C/W and that of the plain-wall heat sink to be 4.69°C/W, which indicates a 5.5% cooling enhancement. At a flow velocity of 5.5 m/s, the thermal resistance of the microfin array heat sink is decreased by 11.5%. From the experimental investigations, it is concluded that the vibrating deflection plays a key role in enhancing the heat transfer rate.     

Heat and mass transfer computations for laminar flow in an axisymmetric sudden expansion

Detailed heat or mass transfer rate predictions are made using a finite difference computer model for laminar flow in an axisymmetric sudden expansion. The structure of the recirculation zone and the distribution of mass transfer rate downstream of the expansion are calculated as functions of Reynolds number, inlet velocity profile, geometry and Schmidt number. It is shown that a Couette flow analysis of the appropriate scaled equations gives the essential details of the mass transfer behaviour.     

Fluid flows in microchannels with cavities

Pressure-driven gas and liquid flows through microchannels with cavities have been studied using both experimental measurements and numerical computations. Several microchannels with cavities varying in shape, number and dimensions have been fabricated. One set of microdevices, integrated with sensors on a silicon wafer, is used for flow rate and pressure distribution measurements in gas flows. Another set of microdevices, fabricated using glass-to-silicon wafer bonding, is utilized for visualization of liquid flow patterns. The cavity effect on the flow in the microchannel is found to be very small, with the mass flow rate increasing slightly with increasing number of cavities. The flow pattern in the cavity depends on two control parameters; it is fully attached only if both the reduced Reynolds number and the cavity number are small. A flow regime map has been constructed, where the critical values for the transition from attached to separated flow are determined. The numerical computations reveal another control parameter, the cavity aspect ratio. The flow in the cavity is similar only if all three control parameters are the same. Finally, the vorticity distribution and related circulation in the cavity are analyzed. [1546].     

Heat and mass transfer under laser sintering of a powder mixture

A model of heat and mass transfer during the sintering process of a two-component powder mixture under the action of laser beam irradiation was formulated and numerically investigated. The model accounts for the movement of solid particles in a shrinkage caused by the changed density of the powder mixture, as well as for convective fluxes caused by surface forces and gravity. The flow of the liquid phase of the low-melting component of the mixture was described by the generalized Darcy law. The space-inhomogeneous and non-steady distributions of the phase saturations and the temperature field were estimated to predict the dynamics of the phase change process depending on the properties of the powder mixture (porosity, penetration, and thermal properties of components), and depending on the parameters of the laser beam. The effect of surface subsidence of the powder mixture has been considered.     

Heat balance and transfer in men and women exercising in hot-dry and hot-wet conditions*

Sex-related differences in heat balance and transfer were studied in nine female and ten male heat-acclimatized subjects exposed to two hot-dry (HD) conditions(49°C, 20% rh; 54°C, 10% rh) and three hot-wet (H W) conditions (32°C, 80% rh; 35°C, 90% rh; 37°C, 80%rh). Exposures lasted 120min: l0min rest, 50min walk, l0min rest, 50min walk. Walking speed was 1 34 ms^-1 (level), and for 49°C, 20% rh, in addition, 1-34ms^-l, 5% grade. No sex-related differences were found in metabolic heat production (M), nor in heat exchange by radiation and convection (R + C)or evaporation (E), when expressed per unit body weight (wt). However, E per unit body surface area (A _D) was lower in females by 9–13% (P<0 05 in all HD conditions and for the 32°C, 80% rh condition) due to their lower M/A _D, and 4-6% lower (R + C)/A _D in HD. Core-to-periphery heat conductance was similar in both sexes despite a lower core-to-skin temperature gradient for women in HD. It was suggested that women have an advantage over men in heat transfer particularly in HW because of their higher A _D/wt. The disadvantage of a high A _D/wt at high HD environmental temperatures is diminished by a higher skin temperature, thus reducing (R + C) heat gain. The net effect is to require lower evaporative cooling for women in both HW and HD environments.     

Symbolic finite element analysis using computer algebra: Heat transfer in rectangular duct flow

The problem of heat transfer in fully developed laminar flow in a rectangular duct is solved using a symbolic finite element method. The Nusselt number is obtained as a power series of the aspect ratio of the duct. The solution procedure here differs from the conventional finite element method, in that the aspect ratio remains in symbolic form throughout the computation. Part of the computation is done using the computer algebra system Mathematica. However, the most computational intensive part which involves a Gauss elimination in symbolic form is implemented using an ordinary computer program without resorting to a computer algebra system. The agreement between the results from the present work and those from exact numerical procedures is reasonable.     

Oscillating bubbles in teardrop cavities for microflow control

Microstreaming generated from oscillating microbubbles has great potential in microfluidic applications for localized flow control. In this study, we explore the use of teardrop-shaped cavities for trapping microbubbles. Upon acoustic actuation, these microbubbles confined in teardrop cavities can be utilized to generate a directional microstreaming flow. We further show that by altering the acoustic excitation frequency, a flow-switch for altering flow direction in microfluidic environments can be achieved using two oppositely arranged teardrop cavities with different sizes. In the end, we show that an array of such bubble-filled teardrop cavities can act as a fixated microfluidic transport system allowing for on-chip particle manipulation in complex flow patterns. This inexpensive method to create flows to switch and transport elements based on teardrop cavities can be widely employed for microfluidic applications such as drug delivery systems.     

LIGA-fabricated compact mm-wave linear accelerator cavities

 Millimeter-wave rf cavities for use in linear accelerators, free-electron lasers, and mm-wave undulators are under development at Argonne National Laboratory. Typical cavity dimensions are in the 1000 μm range, and the overall length of the accelerator structure, which consists of 30–100 cavities, is about 50–100 mm. An accuracy of 0.2% in the cavity dimensions is necessary in order to achieve a high Q-factor of the cavity. To achieve this, these structures are being fabricated using deep X-ray lithography, electroforming, and assembly (LIGA). The first prototype cavity structures are designed for 108 GHz and 2π/3-mode operation. Input and output couplers are integrated with the cavity structures. The cavities are fabricated on copper substrates by electroforming copper into 1 mm-thick PMMA resists patterned by deep x-ray lithography and polishing the copper down to the desired thickness. These are fabricated separately and subsequently assembled with precision spacing and alignment using microspheres, optical fibers, or microfabricated spacers/alignment pieces. Details of the fabrication process, alignment, and assembly work are presented in here. Received: 25 August 1997/Accepted: 20 October 1997