Showing papers on "Nusselt number published in 2000"

Forced Convection in High Porosity Metal Foams

Abstract: We report an experimental and numerical study of forced convection in high porosity (e∼0.89-0.97) metal foams. Experiments have been conducted with aluminum metal foams in a variety of porosities and pore densities using air as the fluid medium. Nusselt number data has been obtained as a function of the pore Reynolds number. In the numerical study, a semi-empirical volume-averaged form of the governing equations is used. The velocity profile is obtained by adapting an exact solution to the momentum equation. The energy transport is modeled without invoking the assumption of local thermal equilibrium. Models for the thermal dispersion conductivity, k d , and the interstitial heat transfer coefficient, h sf , are postulated based on physical arguments. The empirical constants in these models are determined by matching the numerical results with the experimental data obtained in this study as well as those in the open literature. Excellent agreement is achieved in the entire range of the parameters studied, indicating that the proposed treatment is sufficient to model forced convection in metal foams for most practical applications

Turbulent convection at very high Rayleigh numbers

Abstract: Turbulent convection occurs when the Rayleigh number (Ra)--which quantifies the relative magnitude of thermal driving to dissipative forces in the fluid motion--becomes sufficiently high. Although many theoretical and experimental studies of turbulent convection exist, the basic properties of heat transport remain unclear. One important question concerns the existence of an asymptotic regime that is supposed to occur at very high Ra. Theory predicts that in such a state the Nusselt number (Nu), representing the global heat transport, should scale as Nu proportional to Ra(beta) with beta = 1/2. Here we investigate thermal transport over eleven orders of magnitude of the Rayleigh number (10(6) < or = Ra < or = 10(7)), using cryogenic helium gas as the working fluid. Our data, over the entire range of Ra, can be described to the lowest order by a single power-law with scaling exponent beta close to 0.31. In particular, we find no evidence for a transition to the Ra(1/2) regime. We also study the variation of internal temperature fluctuations with Ra, and probe velocity statistics indirectly.

Analysis of Variants Within the Porous Media Transport Models

Abstract: An investigation of variants within the porous media transport models is presented in this work. Four major categories in modeling the transport processes through porous media, namely constant porosity, variable porosity, thermal dispersion, and local thermal non-equilibrium, are analyzed in detail. The main objective of the current study is to compare these variances in models for each of the four categories and establish conditions leading to convergence or divergence among different models. To analyze the effects of variants within these transport models, a systematic reduction and sensitivity investigation for each of these four aspects is presented. The effects of the Darcy number, inertia parameter, Reynolds number, porosity, particle diameter, and the fluid-to-solid conductivity ratio on the variances within each of the four areas are analyzed. For some cases the variances within different models have a negligible effect on the results while for some cases the variations can become significant. In general, the variances have a more pronounced effect on the velocity field and a substantially smaller effect on the temperature field and Nusselt number distribution

Local Heat Transfer and Flow Structure on and Above a Dimpled Surface in a Channel

Abstract: Experimental results, measured on and above a dimpled test surface placed on one wall of a channel, are given for Reynolds numbers from 1,250 to 61,500 and ratios of air inlet stagnation temperature to surface temperature ranging from 0.68 to 0.94. These include flow visualizations, surveys of time-averaged total pressure and streamwise velocity, and spatially-resolved local Nusselt numbers, which are measured using infrared thermography, used in conjunction with energy balances, thermocouples, and in situ calibration procedures. The ratio of channel height to dimple print diameter is 0.5. Flow visualizations show vortical fluid and vortex pairs shed from the dimples, including a large upwash region and packets of fluid emanating from the central regions of each dimple, as well as vortex pairs and vortical fluid which form near dimple diagonals. These vortex structures augment local Nusselt numbers near the downstream rims of each dimple, both slightly within each depression, and especially on the flat surface just downstream of each dimple. Such augmentations are spread over larger surface areas and become more pronounced as the ratio of inlet stagnation temperature to local surface temperature decreases. As a result, local and spatially-averaged heat transfer augmentations become larger as this temperature ratio decreases. This is due to the actions of vortical fluid in advecting cool fluid from the central parts of the channel to regions close to the hotter dimpled surface.Copyright © 2000 by ASME

Natural convection in enclosures with localized heating from below and symmetrical cooling from sides

Abstract: Natural convection of air in a two‐dimensional, rectangular enclosure with localized heating from below and symmetrical cooling from the sides has been numerically investigated Localized heating is simulated by a centrally located heat source on the bottom wall, and four different values of the dimensionless heat source length, 1/5, 2/5, 3/5 and 4/5 are considered Solutions are obtained for Rayleigh number values from 103 to 106 Local results are presented in the form of streamline and isotherm plots as well as the variation of local Nusselt number on the heated wall Finally, the average Nusselt number at the heated part of the lower wall, \overline Nu, was shown to increase with an increase the Rayleigh number, Ra, or of the nondimensional heat source thickness, e

Heat transfer and pressure drop during condensation of refrigerants inside horizontal enhanced tubes

Abstract: This paper presents a critical review of correlations to compute heat transfer coefficients and pressure drop, for refrigerants condensing inside commercially available tubes with enhanced surfaces of various types, and a theoretical analysis of the condensation phenomenon. Predictions from some of the above equations are compared with experimental data. In addition, information is presented about the influence of small amounts of compressor oil on the condensation of refrigerants in enhanced tubes.

Laminar natural convection in isosceles triangular enclosures heated from below and symmetrically cooled from above

Abstract: A numerical study of natural convection in an isosceles triangular enclosure with a heated horizontal base and cooled upper walls is presented. Nearly every previous study conducted on this subject to date has assumed that the geometric plane of symmetry is also a plane of symmetry for the flow. This problem is re-examined over aspect ratios ranging from 0.2 to 1.0 and Grashof numbers from 10 3 to 10 5 . A pitchfork bifurcation occurs at a critical Grashof number for each of the aspect ratios considered, above which the symmetric solutions are unstable to finite perturbations and asymmetric solutions are instead obtained. Results are presented detailing the occurrence of the pitchfork bifurcation in each of the aspect ratios considered, and the resulting flow patterns are described. A flow visualization study is used to validate the numerical observations. Computed local and mean heat transfer coefficients are also presented and compared with results obtained when flow symmetry is assumed. Differences in local values of the Nusselt number between asymmetric and symmetric solutions are found to be more than 500 percent due to the shifting of the buoyancy-driven cells

Double diffusive convection in a cubic enclosure with opposing temperature and concentration gradients

Abstract: A three-dimensional numerical study has been performed to investigate double-diffusive, natural convection in a cubic enclosure subject to opposing and horizontal gradients of heat and solute. The flow is driven by buoyancy forces due to temperature and solutal gradients. Constant temperature and concentration are imposed along the two vertical side walls of the cubic enclosure, while the remaining walls are impermeable and adiabatic. The numerical simulations presented here span a wide range of thermal Rayleigh number (10.0

Similarity solutions for hydromagnetic mixed convection heat and mass transfer for Hiemenz flow through porous media

Abstract: The problem of coupled heat and mass transfer by mixed convection in a linearly stratified stagnation flow (Hiemenz flow) in the presence of an externally applied magnetic field and internal heat generation or absorption effects is formulated. The plate surface is embedded in a uniform Darcian porous medium and is permeable in order to allow for possible fluid wall suction or blowing and has a power‐law variation of both the wall temperature and concentration. The resulting governing equations are transformed into similarity equations for the case of linearly varying wall temperature and concentration with the vertical distance using an appropriate similarity transformation. These ordinary differential equations are then solved numerically by an implicit, iterative, finite‐difference scheme. Comparisons with previously published work are performed and excellent agreement between the results is obtained. A parametric study of all involved parameters is conducted and a representative set of numerical results for the velocity and temperature profiles as well as the skin‐friction parameter, local Nusselt number, and the local Sherwood number is illustrated graphically to elucidate interesting features of the solutions.

Hydromagnetic combined heat and mass transfer by natural convection from a permeable surface embedded in a fluid‐saturated porous medium

Abstract: The problem of coupled heat and mass transfer by natural convection from a vertical, semi‐infinite flat plate embedded in a porous medium in the presence of an external magnetic field and internal heat generation or absorption effects is formulated. The plate surface is maintained at either constant temperature or constant heat flux and is permeable to allow for possible fluid wall suction or blowing. The resulting governing equations are non‐dimensionalized and transformed using a non‐similarity transformation and then solved numerically by an implicit, iterative, finite‐difference scheme. Comparisons with previously published work are performed and excellent agreement is obtained. Useful correlations containing the various physical parameters for both isothermal and isoflux wall conditions are reported. A parametric study of all involved parameters is conducted and a representative set of numerical results for the velocity, temperature and concentration profiles as well as the skin‐friction parameter, Nusselt number, and the Sherwood number is illustrated graphically to show typical trends of the solutions.

Modeling of heat transfer for flow across tube banks

Abstract: A calculation procedure for two-dimensional elliptic flow is applied to predict the pressure drop and heat transfer characteristics of laminar and turbulent flow of air across tube banks. The turbulence model used involves the solution of two partial differential equations, one for the kinetic energy of the turbulence and the other for its dissipation rate. These differential equations are solved simultaneously with those for the conservation equations of mass, momentum and energy using an implicit finite volume procedure. The numerical methodology utilizes the stepped boundary technique to approximate the tube surface which is kept at constant temperature. The computations are extended to cover the case of two rows of tubes undergoing cross flow with in-line and staggered tube arrangements besides the case of a single row. Thereby, Reynolds number (Re) as well as the normal and parallel tube spacing-to-diameter ratios are varied. Effects of the flow and the geometry parameters on the friction factor and the local and global Nusselt number are presented. Moreover, velocity vector diagrams and temperature contours as well as axial flow velocity and turbulence kinetic energy profiles along the flow field upstream, over and downstream the tubes are also given. The theoretical results of the present model are compared with previously published experimental data of different authors. Satisfactory agreement is demonstrated.

Forced Convection Heat Transfer to Phase Change Material Slurries in Circular Ducts

Abstract: Conclusions Expressions for the analytical temperature distribution and efŽ ciency of a heat pipe Ž n were derived and compared with experimental data. Two heat pipe Ž n cases were studied,  ush mounted and inserted into the object. The derived expressions are useful as a design tool. They allow the designer to compare heat pipe Ž ns to standard Ž ns before doing a detailed heat pipe design. Further researchcould focus on variations in heat pipe design and geometry.Thesecouldincludenonconstantpropertiesandboundary conditions.For this work, the wall cross-sectionalarea, the internal convectioncoefŽ cient, and the external convectioncoefŽ cient were assumed constant. The impact of these assumptions on accuracy should be investigated. It would also be interesting to extend the analysis to Ž ns in radiation environments.

Heat Transfer and Fluid Flow in a Square Duct With 12 Different Shaped Vortex Generators

Abstract: Detailed local Nusselt number distributions in the first pass of a sharp turning two-pass square channel with various configurations of longitudinal vortex generator arranged on one wall were measured using transient liquid crystal thermography. Flow patterns and friction factors were measured by the use of laser-Doppler velocimeter and pressure transducer, respectively. The Reynolds number, based on channel hydraulic diameter and bulk mean velocity, was fixed at 1.2 × 10 4 . The vortex generator height-to-hydraulic diameter ratio and pitch-to-height ratio were 0.12 and 10, respectively. Comparisons in terms of heat transfer augmentation and uniformity and friction loss are first performed on 12 configurations of single longitudinal vortex generator. The fluid dynamic mechanisms and wall confinement relevant to heat transfer enhancement are then documented for three-selected vortex generator models. In addition, the differences in fluid flow and heat transfer characteristics between a single vortex generator and a vortex generator array are addressed for the delta wing I and 45 deg V (with tips facing upstream) models which provide better thermal performance among the 12 configurations examined