Showing papers on "Heat transfer published in 1997"

Heat Transfer: A Practical Approach

Abstract: Part 1 Fundamentals: basic concepts of thermodynamics and heat transfer heat conduction steady heat conduction transient heat conduction numerical methods in heat transfer forced convection natural convection boiling and condensation radiation heat transfer heat exchangers mass transfer. Part 2 Applications: heating and cooling of buildings refrigeration and freezing of foods cooling of electronic equipment property tables and charts (SI units and English units) about the software.

Handbook of heat transfer

Abstract: Handbook of Numerical Heat Transfer Free Full Download Links from Multiple Mirrors added by DL4W on 2015-04-10 02:13:35. Handbook of heat transfer / editors, W.M. Rohsenow, J.P. Hartnett. Y.I. Cho. m 3rd ed. p. cm. Includes bibliographical references and index. ISBN 0-07053555-8. Students investigate heat transfer by conduction, convection and radiation. The analogy between heat and mass transfer is covered and applied in the analysis.

Heat transport in thin dielectric films

Abstract: Heat transport in 20–300 nm thick dielectric films is characterized in the temperature range of 78–400 K using the 3ω method. SiO2 and SiNx films are deposited on Si substrates at 300 °C using plasma enhanced chemical vapor deposition (PECVD). For films >100 nm thick, the thermal conductivity shows little dependence on film thickness: the thermal conductivity of PECVD SiO2 films is only ∼10% smaller than the conductivity of SiO2 grown by thermal oxidation. The thermal conductivity of PECVD SiNx films is approximately a factor of 2 smaller than SiNx deposited by atmospheric pressure CVD at 900 °C. For films <50 nm thick, the apparent thermal conductivity of both SiO2 and SiNx films decreases with film thickness. The thickness dependent thermal conductivity is interpreted in terms of a small interface thermal resistance RI. At room temperature, RI∼2×10−8 K m2 W−1 and is equivalent to the thermal resistance of a ∼20 nm thick layer of SiO2 .

The role of recoil pressure in energy balance during laser materials processing

Abstract: A theoretical analysis of the energy balance in the laser - metal interaction zone is carried out. The heat transfer due to the recoil-pressure-induced melt flow is taken into consideration. It is shown that, for the absorbed laser intensities typical in welding and cutting, the recoil pressure induces high-velocity melt-flow ejection from the interaction zone. This melt flow carries away from the interaction zone a significant portion of the absorbed laser intensity (about 70 - 90% at low laser intensities); thus, convection-related terms can be ignored neither in calculations of the energy balance in the interaction zone nor in calculations of the thermal field in the vicinity of the weld pool or cutting front.

Specification of eddy transfer coefficients in coarse resolution ocean circulation models

Abstract: Parametric representations of oceanic geostrophic eddy transfer of heat and salt are studied ranging from horizontal diffusion to the more physically based approaches of Green and Stone (GS) and Gent and McWilliams (GM). The authors argue for a representation that combines the best aspects of GS and GM: transfer coefficients that vary in space and time in a manner that depends on the large-scale density fields (GS) and adoption of a transformed Eulerian mean formalism (GM). Recommendations are based upon a two-dimensional (zonally or azimuthally averaged) model with parameterized horizontal and vertical fluxes that is compared to three-dimensional numerical calculations in which the eddy transfer is resolved. Three different scenarios are considered: 1) a convective ‘‘chimney’’ where the baroclinic zone is created by differential surface cooling; 2) spindown of a frontal zone due to baroclinic eddies; and 3) a wind-driven, baroclinically unstable channel. Guided by baroclinic instability theory and calibrated against eddy-resolving calculations, the authors recommend a form for the horizontal transfer coefficient given by 2 fM 2 2 k 5 a l 5 a l, N ˇRi where Ri 5 f2N2/M4 is the large-scale Richardson number and f is the Coriolis parameter; M2 and N2 are measures of the horizontal and vertical stratification of the large-scale flow, l measures the width of the baroclinic zone, and a is a constant of proportionality. In the very different scenarios studied here the authors find a to be a ‘‘universal’’ constant equal to 0.015, not dissimilar to that found by Green for geostrophic eddies in the atmosphere. The magnitude of the implied k, however, varies from 300 m2 s21 in the chimney to 2000 m2 s21 in the wind-driven channel.

Cabauw Experimental Results from the Project for Intercomparison of Land-Surface Parameterization Schemes

Abstract: In the Project for Intercomparison of Land-Surface Parameterization Schemes phase 2a experiment, meteorological data for the year 1987 from Cabauw, the Netherlands, were used as inputs to 23 land-surface flux schemes designed for use in climate and weather models. Schemes were evaluated by comparing their outputs with long-term measurements of surface sensible heat fluxes into the atmosphere and the ground, and of upward longwave radiation and total net radiative fluxes, and also comparing them with latent heat fluxes derived from a surface energy balance. Tuning of schemes by use of the observed flux data was not permitted. On an annual basis, the predicted surface radiative temperature exhibits a range of 2 K across schemes, consistent with the range of about 10 W m22 in predicted surface net radiation. Most modeled values of monthly net radiation differ from the observations by less than the estimated maximum monthly observational error (6 10 Wm 2 2). However, modeled radiative surface temperature appears to have a systematic positive bias in most schemes; this might be explained by an error in assumed emissivity and by models’ neglect of canopy thermal heterogeneity. Annual means of sensible and latent heat fluxes, into which net radiation is partitioned, have ranges across schemes of

Convection in mushy layers

Abstract: As a molten alloy or any multi-component liquid is cooled and solidified the growing solid phase usually forms a porous matrix through which the residual liquid can flow. The reactive two-phase medium comprising the solid matrix and residual liquid is called a mushy layer. Buoyancy forces, owing primarily to compositional depletion as one or more of the components of the alloy are extracted to form the solid phase, can drive convection in the layer. In this review, I present an account of various studies of buoyancy-driven convection in mushy layers, paying particular attention to the complex interactions between solidification and flow that lead to novel styles of convective behavior, including focusing of the flow to produce chimneys: narrow, vertical channels devoid of solid. I define an ‘ideal’ mushy layer and argue that chimneys are an inevitable consequence of convection in ideal mushy layers. The absence of chimneys in certain laboratory experiments is explained in terms of nonideal effects.

The interior structure of Mars: Implications from SNC meteorites

Abstract: Two end-member models of Mars' present interior structure are presented: the first model is optimized to satisfy the geochemical data derived from the SNC meteorites in terms of the bulk chondritic ratio Fe/Si = 1.71, while the second model is optimized to satisfy the most probable maximum value C = 0.366 x M p r p 2 of the polar moment of inertia factor. Hydrostatic equilibrium and stationary heat transfer are assumed, and the basic differential equations for the mechanical and thermal structure are solved numerically together with an isothermal Murnaghan-Birch type equation of state truncated in Eulerian strain at forth order. We obtain the radial distribution of mass, hydrostatic pressure, gravity, temperature, and heat flow density along with the corresponding density stratification, viscosity profiles, and the global seismic velocity structure of model Mars. The first model being consistent with the geochemical requirement produces C = 0.357 x M p r p 2 , whereas the second model commensurate with the geophysical constraint gives Fe/Si = 1.35. The calculated central pressure is about 40 GPa in both models, and the central temperature is in the 2000 to 2200 K range. The model calculations suggest a Fe-Ni-FeS core a little less than one half of the planetary radius in size surrounded by a silicate mantle subdivided into lower spinel and upper olivine layers and overlain by a 100- to 250-km thick basaltic crust and a surface heatflow density of 25 to 30 mW m -2 . In both models the pressure in the mantle is not sufficient for the spinel to perovskite transition to occur. The present thermal lithosphere is estimated to be about 500 km thick and to be subdivided into a 300-km-thick outermost rheological lithosphere and an underlying thermal boundary layer of mantle convection. Given the core sulfur content of 14 wt% as derived from SNC meteorites, the Martian core is found to be entirely molten, implying the nonoperation of a self-sustained dynamo due to the absence of sufficiently vigorous convection.

Spatial and seasonal variability in the components of the river heat budget

Abstract: Detailed hydrometeorological measurements have been used to establish the components of the river heat budget for 495 days covering 18 study periods and 11 study reaches in the Exe Basin, Devon, UK. Averaging the results across the whole data-set indicates that net radiation, friction, sensible heat transfer, condensation and bed conduction contributed 56.0, 22.2, 13.2, 5.8 and 2.8%, respectively, to the non-advective energy gains, whereas net radiation, evaporation, sensible heat exchange and bed conduction accounted for 48.6, 30.4, 10.6 and 10.4%, respectively, of the non-advective heat losses. Precipitation falling on the river channel had little impact on the river heat budgets, but energy advected in groundwater accounted for an average 5% of the heat storage in the river. The magnitude and importance of the river heat budget components were found to be variable in space and time. The influence of channel morphology, valley topography, riparian vegetation, substratum nature and hydrological conditions, especially the effects of river regulation, promoted inter-reach variability in the make up of the heat budget and caused significant differences in energy fluxes at a local scale. Heat budget components also exhibited considerable differences between seasons and varied from day to day for individual reaches. © 1997 by John Wiley & Sons, Ltd.

Performance of Different Soil Model Configurations in Simulating Ground Surface Temperature and Surface Fluxes

Abstract: This study compares three modifications to the one-dimensional planetary boundary layer scheme that is implemented in the σ–θ hybrid-b version of the Mesoscale Analysis and Prediction System (MAPS) and the Rapid Update Cycle (RUC). All three modifications are based on the incorporation of a simple soil model into the basic version to more accurately calculate the moisture and heat fluxes across the ground surface. The presented schemes are of increasing sophistication: the first model combines the soil model with heat and moisture budget equations for the ground surface and uses an explicit numerical scheme to compute the surface fluxes; the second model uses a more energy-conservative implicit solution for the latent and sensible surface fluxes and heat and moisture soil fluxes; the third model further incorporates a simple parameterization of the evapotranspiration process. The comparison includes the effect of different schemes on diurnal changes of surface temperature and soil heat flux. The ...

The Effects of Prandtl Numbers on Local and Average Convective Heat Transfer Characteristics in Helical Pipes

Abstract: To investigate the effects of the Prandtl number and geometric parameters on the local and average convective heat transfer characteristics in helical pipes, experiments with three different fluids-air, water, and ethylene glycol-were carried out on five uniformly heated helical pipes. The test sections were made from 22.9 mm I.D. and 10.2 mm I.D. 304 stainless steel pipes. The ratios of the pipe diameter and pitch to coil diameter (d/D and b/D) ranged from 0.0267 to 0.0884 and 0.20 to 2.56, respectively. The peripheral and average, fully developed Nusselt numbers were evaluated in the experiments. Experimental findings indicate that after two turns (X > 2) the temperature distributions along the wall are almost parallel to the linear fluid bulk temperatures, and all dimensionless peripheral wall temperatures are nearly identical, implying that both the flow and temperature distribution within the helical pipes are fully developed.

Heat Transfer Enhancement—The Encouragement and Accommodation of High Heat Fluxes

Abstract: This review considers the many techniques that have been developed to enhance convective heat transfer. After introducing the techniques, the applications to most of the modes of heat transfer (single-phase forced convection, including compound techniques, pool boiling, convective boiling/evaporation, vapor-space condensation, and convective condensation) are described. Comments are offered regarding commercial introduction of this technology and the generations of heat transfer technology : advanced enhancement represents third-generation heat transfer tehnology.

A Detailed Analysis of Film Cooling Physics: Part III — Streamwise Injection With Shaped Holes

Abstract: The physics of the film cooling process for shaped, streamwise-injected, inclined jets is studied for blowing ratio (M = 1.25, 1.88), density ratio (DR = 1.6), and length-to-diameter ratio (L/D = 4) parameters typical of gas turbine operations. A previously documented computational methodology is applied for the study of five distinct film cooling configurations: (1) cylindrical film hole (reference case); (2) forward-diffused film hole; (3) laterally diffused film hole; (4) inlet shaped film hole, and (5) cusp-shaped film hole. The effect of various film hole geometries on both flow and thermal field characteristics is isolated, and the dominant mechanisms responsible for differences in these characteristics are documented. Special consideration is given to explaining crucial flow mechanisms from a vorticity point of view. It is found that vorticity analysis of the flow exiting the film hole can aid substantially in explaining the flow behavior downstream of the film hole. Results indicate that changes in the film hole shape can significantly alter the distribution of the exit-plane variables, therefore strongly affecting the downstream behavior of the film. Computational solutions of the steady, Reynolds-averaged Navier-Stokes equations are obtained using an unstructured/adaptive, fully implicit, pressure-correction solver. Turbulence closure is obtained via the high-Reynolds-number {kappa}-{epsilon}more » model with generalized wall functions. Detailed field results as well as surface phenomena involving adiabatic film effectiveness {eta} and heat transfer coefficient (h) are presented. When possible, computational results are validated against corresponding experimental cases from data found in the open literature. Detailed comparisons are made between surface and field results of the film hole shapes investigated in this work; design criteria for optimizing downstream heat transfer characteristics are then suggested.« less

Effect of Squealer Tip on Rotor Heat Transfer and Efficiency

Abstract: Calculations were performed to simulate the tip flow and heat transfer on the GE-E 3 first-stage turbine, which represents a modern gas turbine blade geometry. Cases considered were a smooth tip, 2 percent recess, and 3 percent recess. In addition, a two-dimensional cavity problem was calculated. Good agreement with experimental results was obtained for the cavity calculations, demonstrating that the k-ω turbulence model used is capable of representing flows of the present type. In the rotor calculations, two dominant flow structures were shown to exist within the recess. Also areas of large heat transfer rate were identified on the blade tip and the mechanisms of heat transfer enhancement were discussed. No significant difference in adiabatic efficiency was observed for the three tip treatments investigated.

Concavity Enhanced Heat Transfer in an Internal Cooling Passage

Abstract: The present study evaluates an innovative approach for enhancement of surface heat transfer in a channel using concavities, rather than protruding elements. Serving as a vortex generator, a concavity is expected to promote turbulent mixing in the flow bulk and enhance the heat transfer. Using a transient liquid crystal imaging system, local heat transfer distribution on the surface roughened by an staggered array based on two different shapes of concavities, i.e. hemispheric and tear-drop shaped, have been obtained, analyzed and compared. The results reveal that both concavity configurations induce a heat transfer enhancement similar to that of continuous rib turbulators, about 2.5 times their smooth counterparts 10,000 ≤ Re ≤ 50,000. In addition, both concavity arrays reveal remarkably low pressure losses that are nearly one-half the magnitudes incurred with protruding elements. In turbine cooling applications, the concavity approach is particularly attractive in reducing system weight and ease of manufacturing.© 1997 ASME

Thermal conductivity and heat transfer in superlattices

Abstract: Understanding the thermal conductivity and heat transfer processes in superlattice structures is critical for the development of thermoelectric materials and optoelectronic devices based on quantum structures. This letter reports modeling of the heat transfer and thermal conductivity of superlattice structures based on solving the Boltzmann transport equation. Both diffuse and specular phonon scattering processes at interfaces are considered. The modeling results could explain recent experimental data on the cross-plane thermal conductivity of Si/Ge superlattices. Below the critical thickness, thermal conductivity is strongly influenced by diffuse interface scattering of phonons while above the critical thickness, dislocations are the dominant scattering centers in superlattices.