Showing papers on "Forced convection published in 2007"

Determination of Critical Parameters in Electrical Machine Thermal Models

Abstract: In this paper, the authors discuss the problem concerning the determination of some thermal parameters which are very complex to compute. These parameters play an important role in thermal networks usually adopted for electrical machine thermal analysis. In particular, in this paper, the following thermal parameters are analyzed: equivalent thermal resistance between external frame and ambient, equivalent thermal conductivity between winding and lamination, forced convection heat transfer coefficient between end winding and end-caps, radiation heat transfer coefficient between external frame and ambient, interface gap between lamination and external frame, air cooling speed, and the bearings effective thermal resistance. While the information given in this paper is mainly related to induction motors, it is also of general use for the designers of other types of machine. The reported results have been validated using induction motors with power ranges from a few kilowatts up to 1 MW.

A numerical study of the steady forced convection heat transfer from an unconfined circular cylinder

Abstract: Forced convection heat transfer from an unconfined circular cylinder in the steady cross-flow regime has been studied using a finite volume method (FVM) implemented on a Cartesian grid system in the range as 10 ≤ Re ≤ 45 and 0.7 ≤ Pr ≤ 400. The numerical results are used to develop simple correlations for Nusselt number as a function of the pertinent dimensionless variables. In addition to average Nusselt number, the effects of Re, Pr and thermal boundary conditions on the temperature field near the cylinder and on the local Nusselt number distributions have also been presented to provide further physical insights into the nature of the flow. The rate of heat transfer increases with an increase in the Reynolds and/or Prandtl numbers. The uniform heat flux condition always shows higher value of heat transfer coefficient than the constant wall temperature at the surface of the cylinder for the same Reynolds and Prandtl numbers. The maximum difference between the two values is around 15–20%.

Numerical simulation of unsteady mixed convection in a driven cavity using an externally excited sliding lid

Abstract: A numerical investigation of unsteady laminar mixed convection heat transfer in a lid driven cavity is executed. The forced convective flow inside the cavity is attained by a mechanically induced sliding lid, which is set to oscillate horizontally in a sinusoidal fashion. The natural convection effect is sustained by subjecting the bottom wall to a higher temperature than its top counterpart. In addition, the two vertical walls of the enclosure are kept insulated. Discretization of the governing equations is achieved through a finite element scheme based on the Galerkin method of weighted residuals. Comparisons with previously reported investigations are performed and the results show excellent agreement. Temporal variations of streamlines, isotherms, and dimensionless drag force, and Nusselt number are presented in this investigation for various pertinent dimensionless groups. Fluid flow and heat transfer characteristics are examined in the domain of the Reynolds number, Grashof number and the dimensionless lid oscillation frequency such that: 10 2 ⩽ Re ⩽ 10 3 , 10 2 ⩽ Gr ⩽ 10 5 and 0.1 ⩽ ϖ ⩽ 5 . The working fluid is assigned a Prandtl number of 0.71 throughout this investigation. The obtained results reveal that the Reynolds number and Grashof number would either enhance or retard the energy transport process and drag force behavior depending on the conduct of the velocity cycle. Moreover, relatively small lid oscillation values are found to constrain the lid associated motion to a shallow depth from the sliding lid plane.

Integral ballast lamp thermal management method and apparatus

Abstract: A lamp having a lighting source, integral electronics, and a thermal distribution mechanism disposed in a housing. The thermal distribution mechanism may include a variety of insulative, radiative, conductive, and convective heat distribution techniques. For example, the lamp may include a thermal shield between the lighting source and the integral electronics. The lamp also may have a forced convection mechanism, such as an air-moving device, disposed adjacent the integral electronics. A heat pipe, a heat sink, or another conductive heat transfer member also may be disposed in thermal communication with one or more of the integral electronics. For example, the integral electronics may be mounted to a thermally conductive board. The housing itself also may be thermally conductive to conductively spread the heat and convect/radiate the heat away from the lamp.

Local Heat Transfer Coefficients Induced by Piezoelectrically Actuated Vibrating Cantilevers

Abstract: Piezoelectric fans have been shown to provide substantial enhancements in heat transfer over natural convection while consuming very little power. These devices consist of a piezoelectric material attached to a flexible cantilever beam. When driven at resonance, large oscillations at the cantilever tip cause fluid motion, which in turn results in improved heat transfer rates. In this study, the local heat transfer coefficients induced by piezoelectric fans are determined experimentally for a fan vibrating close to an electrically heated stainless steel foil, and the entire temperature field is observed by means of an infrared camera. Four vibration amplitudes ranging from 6.35 to 10 mm are considered, with the distance from the heat source to the fan tip chosen to vary from 0.01 to 2.0 times the amplitude. The two-dimensional contours of the local heat transfer coefficient transition from a lobed shape at small gaps to an almost circular shape at intermediate gaps. At larger gaps, the heat transfer coefficient distribution becomes elliptical in shape. Correlations developed with appropriate Reynolds and Nusselt number definitions describe the area-averaged thermal performance with a maximum error of less than 12%.

Lagrangian temperature, velocity, and local heat flux measurement in Rayleigh-Bénard convection.

Abstract: We have developed a small, neutrally buoyant, wireless temperature sensor. Using a camera for optical tracking, we obtain simultaneous measurements of position and temperature of the sensor as it is carried along by the flow in Rayleigh-Benard convection, at Ra approximately 10;{10}. We report on statistics of temperature, velocity, and heat transport in turbulent thermal convection. The motion of the sensor particle exhibits dynamics close to that of Lagrangian tracers in hydrodynamic turbulence. We also quantify heat transport in plumes, revealing self-similarity and extreme variations from plume to plume.

Entropy generation for microscale forced convection: effects of different thermal boundary conditions, velocity slip, temperature jump, viscous dissipation, and duct geometry

Abstract: Forced convection with viscous dissipation in a parallel plate channel filled by a saturated porous medium is investigated numerically. Three different viscous dissipation models are examined. Two different sets of wall conditions are considered: isothermal and isoflux. Analytical expressions are also presented for the asymptotic temperature profile and the asymptotic Nusselt number. With isothermal walls, the Brinkman number significantly influences the developing Nusselt number but not the asymptotic one. At constant wall heat flux, both the developing and the asymptotic Nusselt numbers are affected by the value of the Brinkman number. The Nusselt number is sensitive to the porous medium shape factor under all conditions considered.

Effects of viscous dissipation and boundary conditions on forced convection in a channel occupied by a saturated porous medium

Abstract: Forced convection with viscous dissipation in a parallel plate channel filled by a saturated porous medium is investigated numerically. Three different viscous dissipation models are examined. Two different sets of wall conditions are considered: isothermal and isoflux. Analytical expressions are also presented for the asymptotic temperature profile and the asymptotic Nusselt number. With isothermal walls, the Brinkman number significantly influences the developing Nusselt number but not the asymptotic one. At constant wall heat flux, both the developing and the asymptotic Nusselt numbers are affected by the value of the Brinkman number. The Nusselt number is sensitive to the porous medium shape factor under all conditions considered.

Gas Concentration Impedance of Solid Oxide Fuel Cell Anodes II. Channel Geometry

Abstract: This series of papers presents details of numerical studies of the nature of the impedance of solid oxide fuel cell (SOFC) anodes caused by gas-phase transport processes. The present part treats channel geometries where gases are transported parallel to the electrode surface. Two cases are investigated: (i) channel flow by forced convection, a typical situation in planar stack segments; and (ii) channel diffusion without convective flow, a typical situation in laboratory-scale single-chamber experiments using symmetrical cells. Current/voltage curves and electrochemical impedance spectra are simulated based on the Navier-Stokes transport equations and nonlinear electrochemistry models. Both channel flow and channel diffusion cause a capacitive behavior in the form of an resistance-capacitive (RC)-type semicircle in the Nyquist diagram. Its resistance and relaxation frequency strongly depend on operation parameters (gas concentration, flow rate, temperature, electrochemical polarization) and geometry (channel length and cross-sectional area). The model predictions are in good quantitative agreement with four different experimental studies published in the literature. The simulation approach thus allows a physically based assignment of observed gas concentration impedance processes.

The Modeling of Viscous Dissipation in a Saturated Porous Medium

Abstract: A critical review is made of recent studies of the modeling of viscous dissipation in a saturated porous medium, with applications to either forced convection or natural convection. Alternative forms of the viscous dissipation function are discussed. Limitations to the concept of fully developed convection are noted. Special attention is focused on the roles of viscous dissipation and work done by pressure forces (flow work) in natural convection in a two-dimensional box with either lateral or bottom heating.

Convective heat transfer in open cell metal foams

Abstract: Convective heat transfer in aluminum metal foam sandwich panels is investigated with potential applications to actively cooled thermal protection systems in hypersonic and reentry vehicles. The size eects of the metal foam core are experimentally investigated and the eects of foam thickness on convective transfer are established. Four metal foam specimens are utilized with a relative density of 0.08 and pore density of 20 ppi in a range of thickness from 6.4 mm to 25.4 mm in increments of approximately 6 mm. An exact-shapefunction nite element model is developed that envisions the foam as randomly oriented cylinders in cross o w with an axially varying coolant temperature eld. Our experimental results indicate that larger foam thicknesses produce increased heat transfer levels in metal foams. Initial FE simulations using a fully developed, turbulent velocity prole show the potential of this numerical tool to model convective heat transfer in metal foams. Metal foam sandwich panels have been proposed as alternative multi-functional materials for structural thermal protection systems in hypersonic and re-entry vehicles 1 . 2 This type of construction oers numerous advantages over other actively cooled concepts because of the unique properties of metal foams. These materials, when brazed between metallic face sheets, are readily suited to allow coolant passage without the addition of alien components that may compromise structural performance. Moreover, the mechanical properties can be varied to suit dieren t structural needs by varying the foam relative density. From a heat transfer point of view, these materials have been shown to be exceptional heat exchangers primarily due to the increased surface area available for heat transfer between the solid and uid phases. The thermo-mechanical response of metal foam sandwich panels has been recently studied and characterized. 2 In particular, it has been shown that using air as coolant at sucien tly high velocities, the strain due to buckling of these structures under thermo-mechanical loads can be virtually eliminated. The implementation of these materials in thermal protection systems, however, requires that a proper heat transfer model exists that allows the coupling between the thermo-mechanical and heat transfer problems to be properly analyzed. In other words, it is necessary to understand how dieren t foam properties such as relative density, pore density, and foam thickness will aect the heat loads that this type of structural component can remove. Heat transfer in metal foams has been a subject of active research in recent years. Lu et al. 3 developed an analytical model envisioning the foam as an array of mutually perpendicular cylinders subjected to cross-o w. In this study, a closed-form expression for the convective coecien t of a foam-lled channel with constant wall temperatures was presented based on foam geometry and material and uid properties. These authors reported that the simplifying assumptions used in their analysis were likely to lead to an over-prediction of the actual heat transfer level. This model has been partially validated by Bastawros and Evans 4 who performed forced convection experiments on aluminum foams adhered to silicon substrate face sheets. These authors reported that the predictions of Lu et al. 3 regarding the dependence of the convective coecien t on coolant velocity and strut diameter were qualitatively consistent with their observations, but that the foam thickness eects were not adequately modeled. In particular, they reported that the heat dissipation rate

Numerical Study of Laminar Forced Convection Fluid Flow and Heat Transfer From a Triangular Cylinder Placed in a Channel

Abstract: Computational study of two-dimensional laminar flow and heat transfer past a triangular cylinder placed in a horizontal channel is presented for the range 80≤Re≤200 and blockage ratio 1/12≤β≤1/3. A second-order accurate finite volume code with nonstaggered arrangement of variables is developed employing momentum interpolation for the pressure-velocity coupling. Global mode of cross-stream velocity oscillations predict the first bifurcation point increases linearly with blockage ratio with no second bifurcation found in the range of Re studied. The Strouhal number and rms of lift coefficient increase significantly with blockage ratio and Reynolds number while overall Nusselt number remains almost unchanged for different blockage ratios. At lower blockage ratios, flow is found to be similar to the unconfined flow and is more prone to wake instability. Instantaneous streak lines provide an excellent means of visualizing the von Karman vortex street.