Showing papers on "Convection published in 1981"

High-performance heat sinking for VLSI

Abstract: The problem of achieving compact, high-performance forced liquid cooling of planar integrated circuits has been investigated. The convective heat-transfer coefficient h between the substrate and the coolant was found to be the primary impediment to achieving low thermal resistance. For laminar flow in confined channels, h scales inversely with channel width, making microscopic channels desirable. The coolant viscosity determines the minimum practical channel width. The use of high-aspect ratio channels to increase surface area will, to an extent, further reduce thermal resistance. Based on these considerations, a new, very compact, water-cooled integral heat sink for silicon integrated circuits has been designed and tested. At a power density of 790 W/cm2, a maximum substrate temperature rise of 71°C above the input water temperature was measured, in good agreement with theory. By allowing such high power densities, the heat sink may greatly enhance the feasibility of ultrahigh-speed VLSI circuits.

Double-diffusive convection

Abstract: In this paper we present a rather personal view of the important developments in double-diffusive convection, a subject whose evolution has been the result of a close interaction between theoreticians, laboratory experimenters and sea-going oceano-graphers. More recently, applications in astrophysics, engineering and geology have become apparent. In the final section we attempt to draw some general conclusions and suggest that further progress will again depend on a close collaboration between fluid dynamicists and other scientists.

A heat balance method for measuring water flux in the stem of intact plants

Abstract: This paper describes the method and apparatus for measuring the flow rate of water in intact plant stems, on the basis of heat balance of a stem segment. Under stationary conditions, the heat energy supplied continuously to a segment of plant stem is partitioned into three components such as conduction, mass flow and convection [see Eq. (1)]. By predetermining both heat losses due to conduction in the stem and convection from the segment surface into ambient air, it is possible to evaluate the heat loss due to mass flow of water in the stem, that is, the water flow rate equivalent to the transpiration stream. The water flow rate evaluated by this method is compared with the transpiration loss of water determined directly by weighing potted soybean and sunflower plants and measured by using a chamber method. The comparison shows there is a good agreement between them. This indicates that the newly developed method can be applied for determining transpiration rates of intact plants under laboratory and field conditions.

Large-scale flow generation in turbulent convection

Abstract: In a horizontal layer of fluid heated from below and cooled from above, cellular convection with horizontal length scale comparable to the layer depth occurs for small enough values of the Rayleigh number. As the Rayleigh number is increased, cellular flow disappears and is replaced by a random array of transient plumes. Upon further increase, these plumes drift in one direction near the bottom and in the opposite direction near the top of the layer with the axes of plumes tilted in such a way that horizontal momentum is transported upward via the Reynolds stress. With the onset of this large-scale flow, the largest scale of motion has increased from that comparable to the layer depth to a scale comparable to the layer width. The conditions for occurrence and determination of the direction of this large-scale circulation are described.

Oceans and continents: Similarities and differences in the mechanisms of heat loss

Abstract: The principal objective of this paper is to present a simple and self-consistent review of the basic physical processes controlling heat loss from the earth To accomplish this objective, we give a short summary of the oceanic and continental data and compare and contrast the respective mechanisms of heat loss In the oceans we concentrate on the effect of hydrothermal circulation, and on the continents we consider in some detail a model relating surface heat flow to varying depth scales for the distribution of potassium, thorium, and uranium From this comparison we conclude that the range in possible geotherms at depths below 100 to 150 km under continents and oceans overlaps and that the thermal structure beneath an old stable continent is indistinguishable from that beneath an ocean were it at equilibrium Oceans and continents are part of the same thermal system Both have an upper rigid mechanical layer where heat loss is by conduction and a lower thermal boundary layer where convection is dominant The simple conductive definition of the plate thickness is an oversimplification The observed distribution of area versus age in the ocean allows us to investigate the dominant mechanism of heat loss which is plate creation This distribution and an understanding of the heat flow through oceans and continents can be used to calculate the heat loss of the earth This heat loss is 1013 cal/s (42 × 1013W) of which more than 60% results from the creation of oceanic plate The relation between area and age of the oceans is coupled to the ridge and subducting slab forces that contribute to the driving mechanism for plate motions These forces are self-regulating and maintain the rate of plate generation required to achieve a balance between heat loss and heat generation

Propagating substorm injection fronts

Abstract: It is argued that a series of two-satellite observations leads to a clarification of substorm plasma injection, in which boundary motion plays a major role. Emphasis is put on a type of event characterized by abrupt, dispersionless changes in electron intensity and a coincident perturbation that consists of both a field magnitude increase and a small rotation toward more dipolar orientation. Comparing plasma observations at two points, it is found that in active, preinjection conditions the two most important features of the plasma sheet are: (1) the low-energy convection boundary for near-zero energy particles, determined by the magnitude of the large-scale convection electric field; and (2) the precipitation-flow boundary layer between the hot plasma sheet and the atmospherically contaminated inner plasma sheet.

Compressible convection in a rotating spherical shell. I - Anelastic equations. II - A linear anelastic model. III - Analytic model for compressible vorticity waves

Abstract: We derive anelastic equations for convection of a compressible fluid in a deep rotating spherical shell. Our motive is to develop equations the solution of which can help us understand what role the large density variation present in the solar convection zone plays in the maintenance of the solar differential rotation through angular momentum transports by global scale convection. As such, the model equations represent a generalization of a Boussinesq system that has been studied extensively with the solar differential rotation problem in mind.

Convection and magnetic fields in stars

Abstract: Recent observations have demonstrated the unity of the study of stellar and solar magnetic fields. Results from numerical experiments on magnetoconvection are presented and used to discuss the concentration of magnetic flux into isolated ropes in the turbulent convective zones of the Sun or other late-type stars. Arguments are given for siting the solar dynamo at the base of the convective zone. Magnetic buoyancy leads to the emergence of magnetic flux in active regions, but weaker flux ropes are shredded and dispersed throughout the convective zone. The observed maximum field strengths in late-tpe stas should be comparable with the field (8..pi..p)/sup 1/2/ that balances the photospheric pressure.

On some consequences and possible causes of layered mantle convection

Abstract: Laboratory and numerical experiments are used to study convection confined to discrete layers. The laboratory experiments, which use the water content of a predominantly glycerine solution as a way of establishing ‘chemical’ density variations, show that convection will remain confined to superimposed layers when the chemical density contrast between layers is greater than the density change associated with the greatest temperature difference within the convecting system. The numerical experiments show a double thermal boundary layer at the interface between layers, a feature that would result in an average temperature increase of about 500°C in the mantle near 700-km depth if the lower mantle is isolated as suggested by recent geochemical models. The possible causes of a layered mantle convection system include major element chemistry variations, the combined effect of major element chemistry and a phase change, or a phase change alone if it has a sufficiently negative Clapeyron slope.

Basic entrainment equations for the atmospheric boundary layer

Abstract: The parameterization of penetrative convection and other cases of turbulent entrainment by the atmospheric boundary layer is reviewed in this paper. The conservation equations for a one-layer model of entrainment are straightforward; all modeling problems arise in the context of the parameterization of various terms in the budget of turbulent kinetic energy. There is no consensus in the literature on the parameterization of shear production and of dissipation. Unfortunately, field experiments are not sufficiently accurate to guide the selection of suitable hypotheses. Carefully designed laboratory experiments are needed to settle the problems that remain.

A theoretical study of the high-latitude winter f region at solar minimum for low magnetic activity

Abstract: A simple plasma convection model is combined with an ionospheric-atmospheric composition model in order to study the high-latitude winter F region at the solar minimum for low magnetic activity. The high latitude ionospheric features, such as the main trough, the ionization hole, the tongue of ionization, the aurorally produced ionization peaks, and the universal time effects are a natural consequence of the competition between the various chemical and transport processes known to be operating in the high-latitude ionosphere. In the polar hole, the F region peak electron density is below 300 km, and the dominant process at 300 km for NO(+) ions is diffusion.

Nonlinear periodic convection in double-diffusive systems

Abstract: We study two examples of two-dimensional nonlinear double-diffusive convection (thermohaline convection, and convection in an imposed vertical magnetic field) in the limit where the onset of marginal overstability just precedes the exchange of stabilities. In this limit nonlinear solutions can be found analytically. The branch of oscillatory solutions always terminates on the steady solution branch. If the steady solution branch is subcritical this occurs when the period of the oscillation becomes infinite, while if it is supercritical, it occurs via a Hopf bifurcation. A detailed discussion of the stability of the oscillations is given. The results are in broad agreement with the largeramplitude results obtained previously by numerical techniques.

Numerical calculation of thermally driven two-dimensional unsteady laminar flow in cavities of rectangular cross section

Abstract: Numerical results are reported for thermally driven laminar flow in two-dimensional rectangular geometries with one plane, the aperture plane, removed. Finite-difference expressions are derived from a set of approximated transport equations in which large temperature and density variations are allowed but high-frequency pressure oscillations are not. The approach allows small time step limitations to be removed from the calculation procedure. A second-order accurate quadratic upstream interpolation technique is used for the finite differencing of convection terms in the transport equations, thus reducing numerical diffusion error. Parameters varied in the calculations were cavity aspect ratio and inclination angle with respect to gravity, inside wall temperature, and Grashof number. A value of Prandtl number corresponding to air was fixed (Pr = 0.73). For the conditions studied, flow and temperature fields within the cavity are determined mainly by local heat transfer events.

Structures of atmospheric precipitation systems: A global survey

Abstract: A survey of atmospheric precipitation systems, ranging from mid-latitude cyclones and thunderstorms to tropical cloud clusters, hurricanes, and monsoons, shows that all these systems are well described in terms of the rather traditional concepts of stratiform and convective precipitation. In stratiform precipitation, ice particles grow as they drift downward from high levels and pass through a well-defined melting layer. In convective precipitation, particles begin growing at low levels and are carried upward by strong updrafts and fall out in intense vertically oriented showers. Modern observations show that all the major types of precipitation observed over the globe can be and often are combinations of these two basic types of precipitation. Extratropical cyclonic precipitation is basically stratiform. However, it is typically intensified in regions called rainbands. Some rainbands are highly convective features which move through the basic stratiform precipitation. In other rainbands, shallow convective cells occur aloft and help to enhance the basic stratiform precipitation. Mid-latitude thunderstorms and tropical precipitation systems are basically convective. However, stratiform precipitation can develop in the middle to late stages of development. This type of stratiform precipitation, which can become quite extensive in both tropical and mid-latitude systems, apparently arises as groups or successions of active convective cells leave ice particles aloft to settle downward gradually after the cells' updrafts die out.

Pattern Selection in Rayleigh-Bénard Convection near Threshold

Abstract: A correct long-wavelength theory for convection near onset with free-slip boundary conditions requires two fields and reversible couplings. The wavelengths for which stable rolls exist are dramatically modified when the generation of vertical vorticity is taken into consideration. For small Prandtl numbers and rigid boundaries, the skewedvaricose instability of Busse and Clever is recovered by a plausible but nonrigorous modification of our free-slip equations.

Corotating magnetospheric convection

Abstract: The longitudinal asymmetry of the Io plasma torus, as predicted by the magnetic-anomaly model and observed by Earth-based optical astronomy, provides a driving mechanism for a corotating convection system in Jupiter's magnetosphere. Here we deduce some qualitative properties of this convection system from the general equations that govern a steady state corotating convection system (although we expect that time-dependent effects may also have to be included for a complete description of Jovian convection). The corotating convection system appears capable of providing both the dominant radial transport mechanism (with a time scale possibly as short as a few rotation periods) and the dominant mechanism for extracting energy from Jupiter's rotation (at a rate approx.10/sup 15/ W) for driving a wide variety of magnetospheric phenomena. A similar corotating convection system may occur in other rotation-dominated magnetospheres, for example, those of pulsars and Saturn.

South Atlantic thermocline ventilation

Abstract: South Atlantic Central Water (SACW) is carried poleward within a narrow longitude interval off the Argentine coast by the Brazil Current after confluence with the Malvinas (Falkland) Current. Hydrographic data obtained from the R.V. Atlantis II in December 1979 and January 1980 indicate that within the SACW poleward extension, convective processes are important in ventilating the main thermocline to the north. A number of relatively warm, salty intrusions 100 m thick are found within the 12 to 17°C layer of the thermocline along 38°S. These features are about 0.2 × 10−3 more saline than the regional potential temperature-salinity (θ–S) curve of the SACW thermocline and represent local oxygen maxima and nutrient minima. The characteristics of the intrusion are similar to thick remnant winter-mixed layers within various pockets (warm-core eddies) of SACW found further south. The observations support McCartney's 1977 (A Voyage of Discovery, M. Angel, editor, pp. 103–119, Pergamon) suggestion that the Brazil-Malvinas confluence region is the source of a thermostad in the South Atlantic thermocline. The present data suggest that a family of winter altered, relatively salty mixed layers and intrusions are formed in the poleward extension of the SACW thermocline waters, which then spread northward into the main thermocline. The number of family members and their range of θ–S characteristics depend on the distribution of warm eddies and meanders during the winter period. The lower boundary of the intrusions have density ratios (R = αΔT/βΔS) of 1.2 to 1.3, making them unstable to salt-finger activity. Salt fingers are likely to be the primary process that integrates the excess salt of the intrusions into a broader, but less extreme positive salinity anomaly (relative to the South Atlantic thermocline T-S curve), which is associated with the 12 to 17°C weak thermostad of the southwest Atlantic. It is suggested that deep winter convective processes within the SACW extension not only transfer salt and oxygen to mid-thermocline depths, but it also induces significant downward salt flux by salt-finger activity at mid-thermocline depths. Similar processes may be important in all poleward edges of the world thermocline, particularly along the western sectors.

A Martian general circulation experiment with large topography

Abstract: A three-layer general circulation model of the Martian atmosphere is described, and the assumptions governing the model are discussed. The simulated, zonally averaged circulation is found to have only limited sensitivity to differences between this model and an earlier general circulation model; this circulation compares reasonably well with observations. It is also found that the meridional mass flow produced by the seasonal condensation of CO2 in the winter polar region has a major influence on the circulation; owing to the weak influence of atmospheric heat transport, however, the mass flow is governed almost entirely by radiation. Quasi-barotropic stationary waves, which are forced kinematically by the topography and which resemble topographically forced terrestrial planetary waves, are generated by the model in the winter hemisphere region of strong eastward flow, while baroclinic stationary waves are thermally forced by topography in the tropics and summer subtropics. It is also concluded that transient baroclinically unstable waves, of somewhat lower dominant wavenumber than those found on the earth, are generated in winter midlatitudes; their amplitudes, wavenumbers, and phase speeds closely agree with what has been deduced from the Viking lander observations.

Thermal convection in an enclosure due to vibrations aboard spacecraft

Abstract: This paper discusses thermal convection in an enclosure induced by spacecraft vibrations (#-jitter). Under normal circumstances (no maneuvers, no intentional spinning of the spacecraft) the ^-jitter generates predominantly oscillatory velocity and temperature fields with zero time-mean values. The ^-jitter can also generate secondary flows with nonzero mean, but they are of much smaller order. Some implications of the g- jitter on materials processing in space are discussed. NE of the primary advantages foreseen for processing of materials in space is the reduction of natural convection associated with the Earth's gravity. However, there have been some indications (e.g., Apollo 14 experiments 1'2) that spacecraft vibrations might cause appreciable thermal con- vection. Such convection may be important in fluids ex- periments and also affect the quality of crystals grown in space. Therefore, to study the effectiveness of spacecraft vibrations in generating fluid flows, the present work theoretically investigates a case in which a fluid-filled con- tainer with differentially heated walls is subjected to spacecraft vibrations. Implications of the results for space processing are discussed. In an attempt to evaluate the effects of g-jitter on fluid motions Spradley el al. 3 made a numerical analysis for various configurations. They considered three g-jitter profiles, sinusoidal, absolute sinusoidal and saw tooth, and compared the flowfields with that for constant g level. They found that if the g-jitter is decomposed into a time mean part and an oscillatory part, the mean part is more important than the oscillatory part in determining the flowfield and heat- transfer rate. A somewhat related work has been done by Forbes4 where the effect of sinusoidal vibrations on natural convective heat transfer in a rectangular enclosure is studied experimentally as well as numerically under 1-g conditions. The results indicate that the vibrations have very little effect on the heat transfer when the flow is laminar. The present study is more comprehensive than the work by Spradley et al.,3 and is meant to give a better physical insight into the g-jitter in space. Some of the material presented herein is taken from the work by Prasad and Ostrach.5 Formulation of the Problem g-Jitter Consider an experimental setup which is in a spaceship orbiting the Earth (Fig. 1). The coordinates (X0,Y0,Z0) are fixed at the center of the Earth with origin 0 (fixed frame of reference), and the coordinates (X'f Y',Z') are attached to the spacecraft with origin 0' at its center of mass. The coor-

Layered double-diffusive convection in porous media

Abstract: In this paper it is shown that layered double-diffusive convection of a fluid within a porous medium is possible. A thin ‘diffusive’ interface was observed in a Hele Shaw cell and in a laboratory porous medium, with salt and sugar or heat and salt as the diffusing components. Heat–salt and salt–sugar fluxes through two-layer convection systems were measured and are compared with predictions of a model. For the thermohaline system the salt and heat buoyancy fluxes are approximately in the ratio r ≃ eτm½, where e is the porosity and Tm is the appropriate ratio of diffusivities. The behaviour of the heat flux is explained in terms of a coupling between purely thermal convection within each convecting layer and diffusion through the density interface. Salinity gradients are important only within the interface. The presence of a ‘diffusive’ interface in the Wairakei geothermal system is postulated. The ratio of heat and salt fluxes (that can be estimated from existing observations) through this convection system is consistent with the laboratory flux ratio.

A laboratory and theoretical study of the boundary layer adjacent to a vertical melting ice wall in salt water

Abstract: In an experimental and theoretical study we model the convection generated in the polar oceans when a fresh-water ice wall melts in salt water of uniform far-field temperature T∞, and salinity S∞. Our laboratory results show that there are three different flow regimes which depend on T∞ and S∞. First, when T∞ and S∞ lie between the maximum density curve and the freezing curve, the flow is only upward. Secondly, for the oceanic case 30 [les ] S∞ [les ] 35‰ and T∞ 20°C, the flow reverses: at the top of the ice there is a laminar bidirectional flow above a downward turbulent flow. To model the turbulent upward flow theoretically, we numerically solve the governing equations in similarity form with a spatially varying eddy diffusivity that depends on the density difference between the ice-water interface and the far-field. The laboratory data then allows us to evaluate the dependence of eddy diffusivity on T∞ and S∞. The results show that the magnitude of the eddy diffusivity is of the same order as the molecular viscosity and that both mass injection at the interface and opposed buoyancy forces must be included in a realistic flow model. Finally, we use an integral approach to predict the far-field conditions that yield the high-temperature flow reversal and obtain a result consistent with our observations.

The influence of oxygen concentration on fuel parameters for fire modeling

Abstract: The influence of oxygen concentration on fuel parameters in pool fires with areas of about 0.007 m 2 and 0.07 m 2 is described for methanol, polyoxymethylene (POM), polymethyl-methacrylate (PMMA), heptane, polypropylene (PP) and polystyrene (PS). Fuel parameters include mass loss rate, combustion efficiency, convective and radiative fractions of heat of complete combustion, and yields of CO 2 , CO, and soot and low-vapor-pressure liquid products. For combustion in excess air, the combustion efficiency and the yield of CO 2 are not very sensitive to changes in m 02 and fuel areas. The fuel mass loss rate, yield of unburnt soot, flame radiative heat flux, and radiative fraction of heat of complete combustion increase and flame convective heat flux and convective fraction of heat of complete combustion decrease as m 02 is increased; for m 02 values much higher than 0.233, all the parameters approach their respective asymptotic values. There appears to be an attenuation of flame radiation by cold vapors near the surface as well as a change in the flame shape as m 02 is varied.

The amplitude equation near the convective threshold: application to time-dependent heating experiments

Abstract: High-resolution measurements have been performed of the convective heat current as a function of time when a Rayleigh-Benard cell is swept through its threshold with a specified time-dependent heat input. The results are interpreted in terms of the amplitude equation which exactly describes the slow variations in space and time of hydrodynamic quantities near the threshold. A phenomenological forcing field is added to this equation, and its form and magnitude are fitted to the onset time of the convective heat current. A deterministic model in which the field is an adjustable constant yields a good fit to the data for both a step and a linear ramp in the heat input. An alternative stochastic model, in which the field is a Gaussian variable with zero mean and a white-noise spectrum, is adequate for the ramp experiments, but cannot fit the step data for any value of the mean-square field. The systematics of the field and onset time versus ramp rate are studied in both the deterministic and stochastic models, and attempts are made to interpret the field in terms of physical mechanisms. When the data for long times are analysed in terms of the amplitude equation, it is found that the state first excited at onset is not the roll pattern which is stable in steady state. Instead, the system goes first to an intermediate state, which we tentatively identify as a hexagonal configuration. The decay of this state is governed by a further adjustable field in the amplitude equation.

Chapter 2 Measurement of Thermal Balance of Man

Abstract: Publisher Summary The thermal environment of man begins at the skin surface and extends outward to the surrounding media, which consist of the air that he breathes; the clothing that he wears; man-made sources of heat and cold necessary for health and comfort; heat, cold, and humidity caused by weather; and exposure to solar radiation All these factors are characterized by temperature, or they in some way affect the heat transfer from the skin surface by radiation, convection, conduction, or evaporation Heat balance equations describing human heat exchange by radiation, convection, evaporation, and conduction from the skin surface with the thermal environment are discussed in the chapter The environmental variables in the basic heat balance equation that must be measured are the ambient air temperature, the mean radiant temperature or effective radiant field, the ambient water vapor pressure or humidity, air movement as it affects the convective and evaporative heat loss, and clothing insulation The physiological variables in the heat balance equation are skin temperature, skin wettedness, mean body temperature, metabolic energy consumption, and the rate of external work

Low-Prandtl-number convection in a layer heated from below

Abstract: Steady solutions in the form of two-dimensional rolls are obtained numerically for convection in a horizontal layer of a low-Prandtl-number fluid heated from below. Prandtl numbers in the range 0·001 [les ] P [les ] 0·71 are investigated for Rayleigh numbers between the critical value, R = 1708, and R = 20,000 in the case of rigid boundaries. The calculations reveal that the convective heat transport is relatively independent of the Prandtl number at Rayleigh numbers greater than a finite critical value R2 of the order of 5 × 103. At R = 10,000 the convective heat transport varies by only about 30% for Prandtl numbers in the range investigated. As the Rayleigh number is increased above the critical value R2, the streamlines of the convection flow become circular, independent of the horizontal wavelength as long as the latter is larger than or about equal to twice the height of the layer.