Showing papers on "Convection published in 1987"

Handbook of single-phase convective heat transfer

Abstract: Basics of Heat Transfer (S. Kakac & Y. Yener) External Flow Forced Convection (R. Pletcher) Laminar Convective Heat Transfer in Ducts (R. Shah & M. Bhatti) Turbulent and Transition Flow Convective Heat Transfer in Ducts (M. Bhatti & R. Shah) Convective Heat Transfer in Curved Ducts (R. Shah & S. Joshi) Convective Heat Transfer in Cross Flow (A. Zukauskas) Convective Heat Transfer over Rod Bundles (K. Rehme) Convective Heat Transfer in Liquid Metals (C. Reed) Convective Heat Transfer with Electric and Magnetic Fields (F. Kulacki et al.) Convective Heat Transfer in Bends and Fittings (S. Joshi & R. Shah) Transient Forced Convection in Ducts (Y. Yener & S. Kakac) Basics of Natural Convection (Y. Jaluria) Natural Convection in Enclosures (K. Yang) Mixed Convection in External Flow (T. Chen & B. Armaly) Mixed Convection in Internal Flow (W. Aung) Convective Heat Transfer in Porous Media (A. Bejan) Enhancement of Single-phase Heat Transfer (R. Webb) The Effect of Temperature-dependent Fluid Properties on Convective Heat Transfer (S. Kakac) Interaction of Radiation with Convection (M. Ozisik) Non-Newtonian Fluid Flow and Heat Transfer (T. Irvine, Jr & J. Karni) Fouling with Convective Heat Transfer (W. Marner & J. Suitor) Thermophysical Properties (P. Liley) Index.

On the role of sea surface temperature gradients in forcing low-level winds and convergence in the tropics

Abstract: The potential contribution of the SST gradient-driven flow to the low-level (p not less than 700 mb) convergence over tropical oceans is determined using a simple one-layer model of the trade cumulus boundary layer wherein surface temperature gradients are mixed vertically (consistent with the ECMWF analyzed data). The influence of the layers above 700 mb is intentionally suppressed. The results of the study demonstrate the importance of taking account of the fact that cumulonimbus convection takes a small but finite time to adjust to low-level convergence. Failure to consider this effect leads to unreasonably large equatorial convergence.

Empirical high-latitude electric field models

Abstract: The present analysis of electric field measurements from the Dynamics Explorer 2 satellite, which extends previous empirical models, emcompasses much data from polar crossings entering and exiting the high latitudes in all magnetic local time zones. The goal is to represent the typical distributions of convective electric fields with a minimum number of characteristic patterns. Significant large-scale revisions of the OGO 6 dawn-dusk measurement models are made. The deformations of the two-cell patterns lead to sunward convection in dayside polar regions, while maintaining the integrity of the nightside convection pattern.

Sea Surface Temperature, Surface Wind Divergence, and Convection over Tropical Oceans

Abstract: Large-scale convection over the warm tropical oceans provides an important portion of the driving energy for the general circulation of the atmosphere. Analysis of regional associations between ocean temperature, surface wind divergence, and convection produced two important results. First, over broad regions of the Indian and Pacific oceans, sea surface temperatures (SSTs) in excess of 27.5°C are required for large-scale deep convection to occur. However, SSTs above that temperature are not a sufficient condition for convection and further increases in SST appear to have little effect on the intensity of convection. Second, when SSTs are above 27.5°C, surface wind divergence is closely associated with the presence or absence of deep convection. Although this result could have been expected, it was also found that areas of persistent divergent surface flow coincide with regions where convection appears to be consistently suppressed even when SSTs are above 27.5°C. Thus changes in atmospheric stability caused by remotely forced changes in subsidence aloft may play a major role in regulating convection over warm tropical oceans.

An enthalpy method for convection/diffusion phase change

Abstract: An enthalpy formulation for convection/diffusion phase change is developed. The essential feature of this formulation is that latent heat effects are isolated in a source term. This formulation is applicable to a general convection/diffusion phase change, i.e. it is valid in the cases of evolution of latent heat either at an isothermal temperature or over a temperature range. Before implementation of the enthalpy formulation, a technique is required to ensure that velocities predicted to be in a solid region actually take the value zero. Three alternative schemes for achieving this are presented. The enthalpy formulation and velocity correction schemes are independent of the numerical technique. As an example of how the method can be implemented a control volume numerical discretization is chosen. This implementation is applied to two test problems: a solidification phase change in a cavity under conduction and the same phase change under conduction and natural convection. The natural convection problem is used to compare the performances of the various velocity correction schemes. The results of the problems are in good agreement with available analytical solutions and previous numerical solutions.

An Air-Sea Interaction Model of Intraseasonal Oscillations in the Tropics

Abstract: We present a linear model of intraseasonal oscillations produced by the interaction of an atmosphere on an equatorial Beta-plane with a fixed ocean. Convection is treated as a means of rapidly redistributing in the vertical heat acquired from the sea surface, rather than as a heat source in and of itself. The model produces a spectrum of equatorially trapped oscillating instabilities, among which is an eastward-propagating wavenumber 1 disturbance with an intrinsic phase speed in the range of 4–20 m s−1, depending on the mean zonal wind, the surface exchange coefficients, the air-sea equivalent potential temperature difference, and the difference of absolute temperature across the depth of the lower troposphere. The three-dimensional structure of this mode is in excellent agreement with observations and recent numerical experiments concerning the 30–60 day oscillation. The phase speed and growth rate of the disturbances depend only on conditions at the equator, while their meridional structure va...

Bedform-generated convective transport in bottom sediment

Abstract: Convective flow within bed sediment is an important mechanism enhancing the mobility of chemicals, both natural and anthropogenic, and thermal energy in this region of aquatic environments1,2. Experimental observations indicated that significant in-bed convection currents can be generated by water flowing over small obstructions on the surface of a porous bed. Significant porewater flow is induced by imbalances in pressure over distance, generated by differences in temperature, density and hydrostatic head3. We demonstrate here by laboratory simulation and a vignette model that flow over bedforms induces additional pressure imbalances which generate significant and complex convection currents within porous bed sediment. A model is proposed for estimating Peclet numbers for this effect/The results have particular application to chemical transport in the upper sediment layer that is often the recipient of high levels of chemical contamination. Although our analysis reflects river conditions, the results may have wider applications and include submarine currents moving over dune-like mega ripples on the ocean floor.

Origin of Low-Frequency (Intraseasonal) Oscillations in the Tropical Atmosphere. Part I: Basic Theory

Abstract: A theory of the origin of intraseasonal oscillations of the tropical atmosphere is presented and tested by simple model experiments. This study forces on the validation of the basic theory against key features of the observed 40–50 day oscillation. It is shown that the observed eastward propagation of intraseasonal oscillation in the tropical atmosphere arises as an intrinsic mode of oscillation resulting from an interaction of convection and dynamics via the so-called “mobile” wave-CISK mechanism. Through this mechanism, the heat source feeds on the east-west asymmetry of forced equatorial waves. As a result, Kelvin waves are selectively amplified, which in turn causes the heat source to propagate eastward. This mechanism also prevents small-scale waves from immediate destabilization, contrary to the results of traditional wave-CISK theory. The “mobile” wave-CISK establishes a new dynamics equilibrium state between convection and the wind field to form a wave packet or collective motion with rel...

The influence of the Coriolis force on flux tubes rising through the solar convection zone

Abstract: In order to study the effect of the Coriolis force due to solar rotation on rising magnetic flux, the authors consider a flux ring, azimuthally symmetric around the rotation axis, starting from rest at the bottom of the convection zone, and then follow the trajectory of the flux ring as it rises. If it is assumed that the flux ring remains azimuthally symmetric during its ascent, then the problem can be described essentially in terms of two parameters: the value of the initial magnetic field in the ring when it starts, and the effective drag experienced by it. For field strengths at the bottom of the convection zone of order 10,000 G or less, it is found that the Coriolis force plays a dominant role and flux rings starting from low latitudes at the bottom are deflected and emerge at latitudes significantly poleward of sunspot zones.

Forced convection in a duct partially filled with a porous material

Abstract: This paper presents a theoretical study of fully developed forced convection in a channel partially filled with a porous matrix. The matrix is attached at the channel wall and extends inward, toward the centerline. Two channel configurations are investigated, namely, parallel plates and circular pipe. For each channel configuration, both the case of constant wall heat flux and constant wall temperature were studied. The main novel feature of this study is that it takes into account the flow inside the porous region and determines the effect of this flow on the heat exchange between the wall and the fluid in the channel. The Brinkman flow model which has been proven appropriate for flows in sparsely packed porous media and for flows near solid boundaries was used to model the flow inside the porous region. Important results of engineering interest were obtained and are reported in this paper. These results thoroughly document the dependence of the Nusselt number on several parameters of the problem. Of particular importance is the finding that the dependence of Nu on the thickness of the porous layer is not monotonic. A critical thickness exists at which the value of Nu reaches a minimum.

Forced Convection Cooling Across Rectangular Blocks

Abstract: Conjugate heat transfer for two-dimensional, developing flow over an array of rectangular blocks, representing finite heat sources on parallel plates, is considered. Incompressible flow over multiple blocks is modeled using the fully elliptic form of the Navier-Stokes equations. A control-volume-based finite difference procedure with appropriate averaging for the diffusion coefficients is used to solve the coupling between the solid and fluid regions. The heat transfer characteristics resulting from recirculating zones around the blocks are presented. The analysis is extended to study the optimum spacing between heat sources for a fixed heat input and a desired maximum temperature at the heat source.

Convective transport within stable river sediments

Abstract: Local pressure variations of the order of 100–1000 N/m2 can be observed between the upstream and downstream faces of the typically triangular-shaped dunelike sediment structures that form at the sediment-water interface of rivers. Laboratory experiments were conducted examining the influence of this localized pressure variation on contaminant transport processes within the sediment. Numerical modeling of the in-bed flow via boundary element methods was also undertaken in order to predict convective transport under typical field conditions. The laboratory experiments and numerical simulation of the in-bed flow in several rivers verified that the pressure distribution observed on the sediment surface and the resulting interstitial fluid convection can control transport of chemically inert, nonsorbed contaminants in stable sediments. In-bed Peclet numbers were of the order of 100–1000, indicating the negligible influence of diffusion under the conditions examined.

Combined heat and mass transfer by natural convection in a vertical enclosure

Abstract: The phenomenon of natural convection caused by combined temperature and concentration buoyancy effects is studied analytically and numerically in a rectangular slot with uniform heat and mass fluxes along the vertical sides. The analytical part is devoted to the boundary layer regime where the heat and mass transfer rates are devoted to the boundary layer regime where the heat and mass transfer rates are ruled by convection. An Oseen-linearized solution is reported for tall spaces filled with mixtures characterized by Le = 1 and arbitrary buoyancy ratios. The effect of varying the Lewis number is documented by similarity solution valid for Le > 1 in heat-transfer-driven flows, and for Le < 1 in mass-transfer-driven flows. The analytical results are validated by numerical experiments conducted in the range 1 {le} H/L {le} 4, 3.5 {times} Ra {le} 7 {times} 10{sup 6}, {minus}11 {le} n {le} 9, 1 {le} Le {le} 40, and Pr = 0.7, 7. Massline patterns are used to visualize the convective mass transfer path and the flow reversal observed when the buoyancy ratio n passes through the value {minus}1.

Transient Response to Localized Episodic Heating in the Tropics. Part II: Far-Field Behavior.

Abstract: In Part I of this investigation, we described the stochastic, near-field behavior of disturbances excited by randomly evolving tropical heating. In the present paper, we examine how these disturbances are modified as they propagate through the far field in the presence of spatially-varying background states. Although the behavior can no longer be broken down into individual Hough modes, it can still be understood in terms of projection and barotropic components of the response. Responses to fast heating, as may be produced by daily fluctuations in convection, and to slow heating, evolving over seasonal time scales, are studied separately. For fast heating the projection response consists mainly of a spectrum of Kelvin waves which, in the lower stratosphere, is centered at frequencies corresponding to twice the effective depth of the heating. The spectrum shifts to higher frequency with increasing altitude due to differential damping. As a result, the slow, fast and ultrafast Kelvin waves identifi...

Recurring polynyas over the Cosmonaut Sea and the Maud Rise

Abstract: Two remarkable deep ocean polynyas observed in the Antarctic region during the winter of 1980, here referred to as the Cosmonaut polynya and the Maud Rise polynya, are discussed. It is proposed that both polynyas are products of deep-reaching convection which introduces warmer deep water into the surface layer. Hydrographic data at both sites indicate the existence of localized doming of the pycnocline. This brings warmer, saltier deep water close to the sea surface, which has been demonstrated to be an effective preconditioner for deep-reaching convection. A possible relationship between the two polynyas is sugggested in that both are in the eastern margins of the Weddell Sea.

Effects of non-Darcian and nonuniform porosity on vertical-plate natural convection in porous media

Abstract: This work examines analytically the effects of non-Darcian and nonuniform permeability conditions on the natural convection for a vertical plate in porous media. The non-Darcian effects, which include the no-slip and inertia effects, decrease the flow and heat transfer rate, while the nonhomogeneity effect enhances the heat transfer. For packed spheres, in particular, the nonhomogeneity in permeability due to the packing of spheres near the solid wall results in a strong flow-channeling effect that significantly increases the heat transfer. The effect of transverse thermal dispersion is also examined. This dispersion effect causes an increase in the heat transfer.

On the onset of convection in rotating spherical shells

Abstract: The linear problem of the onset of convection in rotating spherical shells is analysed numerically in dependence on the Prandtl number. The radius ratio η=r i/r o of the inner and outer radii is generally assumed to be 0.4. But other values of η are also considered. The goal of the analysis has been the clarification of the transition between modes drifting in the retrograde azimuthal direction in the low Taylor number regime and modes traveling in the prograde direction at high Taylor numbers. It is shown that for a given value m of the azimuthal wavenumber a single mode describes the onset of convection of fluids of moderate or high Prandtl number. At low Prandtl numbers, however, three different modes for a given m may describe the onset of convection in dependence on the Taylor number. The characteristic properties of the modes are described and the singularities leading to the separation with decreasing Prandtl number are elucidated. Related results for the problem of finite amplitude convec...

Compositional and thermal convection in magma chambers

Abstract: Magma chambers cool and crystallize at a rate determined by the heat flux from the chamber. The heat is lost predominantly through the roof, whereas crystallization takes place mainly at the floor. Both processes provide destabilizing buoyancy fluxes which drive highly unsteady, chaotic convection in the magma. Even at the lowest cooling rates the thermal Rayleigh number Ra is found to be extremely large for both mafic and granitic magmas. Since the compositional and thermal buoyancy fluxes are directly related it can be shown that the compositional Rayleigh number Rs (and therefore a total Rayleigh number) is very much greater than Ra. In the case of basaltic melt crystallizing olivine Rs is up to 106 times greater than Ra. However compositional and thermal buoyancy fluxes are roughly equal. Therefore thermal and compositional density gradients contribute equally to convection velocities in the interior of the magma. Effects of thermal buoyancy generated by latent heat release at the floor are included. The latent heat boundary layer at the floor of a basaltic chamber is shown to be of the order of 1 m thick with very low thermal gradients whereas the compositional boundary layer is about 1 cm thick with large compositional gradients. As a consequence, the variation in the degree of supercooling in front of the crystal-liquid interface is dominated by compositional effects. The habit and composition of the growing crystals is also controlled by the nature of the compositional boundary layer. Elongate crystals are predicted to form when the thickness of the compositional boundary layer is small compared with the crystal size (as in laboratory experiments with aqueous solutions). In contrast, equant crystals form when the boundary layer is thicker than the crystals (as in most magma chambers). Instability of the boundary layer in the latter case gives rise to zoning within crystals. Diffusion of compatible trace elements through the boundary layer can also explain an inverse correlation, observed in layered intrusions, between Ni concentration in olivine and the proportion of Ni-bearing phases in the crystallizing assemblage.

Forced convection over a flat plate submersed in a porous medium: Variable viscosity case

Abstract: This paper develops an analytical solution for the problem of forced convection of a variable viscosity fluid over an impermeable flat plate submersed in a saturated porous medium The conservation equations are coupled by the viscosity which is a function of temperature The problem is solved by using the boundary layer approximation The viscosity is taken as an inverse function of temperature For water and crude oil, this function is in excellent agreement with experimental data The results show that the variation of viscosity has significant influence on heat transfer Thus, they are applicable to both geothermal and petroleum flows

Plasma transport in the auroral return current region

Abstract: The classical and anomalous transport properties of a multifluid plasma consisting of H(+), O(+), and electron populations in the presence of auroral field-aligned return currents are investigated, using a multimoment fluid model with anomalous transport coefficients. The macroscopic effects of the electrostatic ion cyclotron (EIC) instability and of an EIC-related anomalous resistivity mechanism which heats the electrons are included in the present version of the model. The responses of the outflowing polar wind plasma to the application of current, with and without instabilities, are exhibited. The simulations show that the electron drift velocity corresponding to a return current of 0.65 micro-A/sq m is above the threshold for EIC waves. Downward electron heat flow competes with upward convection and adiabatic effects to determine the direction of the electron temperature anisotropy. Resistive electron heating lowers the critical drift velocity for marginal EIC stability and leads to enhanced ion heating.

Entropy generation in convective heat transfer and isothermal convective mass transfer

Abstract: The irreversible generation of entropy for two limiting cases of combined forced-convection heat and mass transfer in a two-dimensional channel are investigated. First, convective heat transfer in a channel with either constant heat flux or constant surface temperature boundary conditions are considered for laminar and turbulent flow. The entropy generation is minimized to yield expressions for optimum plate spacing and optimum Reynolds numbers for both boundary conditions and flow rigimes. Second, isothermal convective mass transfer in a channel is considered, assuming the diffusing substance to be an ideal gas with Lewis number equal to unity. The flow is considered to be either laminar or turbulent with boundary conditions at the channel walls of either constant concentration or constant mass flux. The analogy between heat and mass transfer is used to determine the entropy generation and the relations for optimum plate spacing and Reynolds number. The applicable range of the results for both limiting cases are then investigated by non-dimensionalizing the entropy generation equation.

Convection in a rotating cylindrical annulus. Part 2. Transitions to asymmetric and vacillating flow

Abstract: The instabilities of convection columns (also called thermal Rossby waves) in a cylindrical annulus rotating about its axis and heated from the outside are investigated as a function of the Prandtl number P and the Coriolis parameter η* When this latter parameter is sufficiently large, it is found that the primary solution observed at the onset of convection becomes unstable when the Rayleigh number exceeds its critical value by a relatively small amount Transitions occur to columnar convection which is non-symmetric with respect to the mid-plane of the small-gap annular layer Further transitions introduce convection flows that vacillate in time or tend to split the row of columns into an inner and an outer row of separately propagating waves Of special interest is the regime of non-symmetric convection, which exhibits decreasing Nusselt number with increasing Rayleigh number, and the indication of a period doubling sequence associated with vacillating convection