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Showing papers on "Convection published in 1975"


Journal ArticleDOI
TL;DR: In this article, a simple empirical expression for the mean value of Nu over the cylinder for all Ra and all Pr is developed in terms of the model of Churchill and Usagi.

1,162 citations


Book ChapterDOI
TL;DR: In this paper, it was shown that the error due to heat conduction to the supports is particularly important with natural convection, especially where the heat loss and the temperature rise of the cylinder are calculated from the voltage drop across it.
Abstract: Publisher Summary Accurate knowledge of the overall convective heat transfer from circular cylinders is of importance in a number of fields, such as boiler design, hotwire anemometry, and the rating of electrical conductors. The wide dispersion in the published experimental data for the heat transfer from smooth circular cylinders by natural and forced convection is attributed to various factors associated with the experiments. The error due to heat conduction to the supports is particularly important with natural convection, especially where the heat loss and the temperature rise of the cylinder are calculated from the voltage drop across it. A common cause of error is the use of too small a space ratio, so that the temperature and velocity fields are distorted. To reduce this error to less than l%, the space ratio D c /D for natural convection or D T /D for forced convection should exceed 100. The error caused by blockage with wind tunnel measurements can be calculated depending on the type of tunnel. One of the greatest sources of error with forced convection is the failure to allow for the effect of stream turbulence.

761 citations


Journal ArticleDOI
TL;DR: In this paper, the absorption of radiation as it passes through the water, and derive equations for the resulting temperature range of the pond during year round operation, taking into account the heat that can be stored in the ground underneath the pond.

326 citations


Journal ArticleDOI
TL;DR: In this article, a computer model is used to simulate the winds and temperature variations in the thermosphere which result from auroral region electric currents during a large isolated magnetic substorm.
Abstract: A computer model is used to simulate the winds and temperature variations in the thermosphere which result from auroral region electric currents during a large isolated magnetic substorm. A disturbance propagates with a speed of 750 m/s poleward and equatorward, with an amplitude of about 200 m/s in the north-south velocity and about 100 K in the temperature at 400-km altitude. The amplitude decays relatively little before the disturbance reaches the equator. The time history of the disturbance is roughly that of a single sinusoid whose period increases with horizontal distance from the source and with decreasing altitude. East-west winds of over 400 m/s at 400-km altitude are created in the auroral region itself by the ion drag mechanism. The spatial distribution of these ion drag winds is significantly affected by momentum convection, so that a simple interpretation in terms of local ion drag forces is generally not sufficient. A residual electric field of about 5 mV/m remains after the substorm source is turned off, due to the dynamo effect of the ion drag winds. Vertical velocities up to about 40 m/s are produced inside the auroral region, primarily by the fact that the heated air is more buoyant than the air outside. Comparison of our simulation with numerous observations shows generally good agreement.

315 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that when the rotation rate exceeds a certain critical value ΩR (which depends on the acceleration of gravity, the shape and dimensions of the apparatus, the physical properties of the fluid and the distribution and intensity of the applied differential heating) Coriolis forces inhibit overturning motion in meridian planes and promote a completely different type of flow which has been termed "sloping convection" or "baroclinic waves".
Abstract: Laboratory experiments on thermal convection in a fluid which rotates about a vertical axis and is subject to a horizontal temperature gradient show that when the rotation rate Ω exceeds a certain critical value ΩR (which depends on the acceleration of gravity, the shape and dimensions of the apparatus, the physical properties of the fluid and the distribution and intensity of the applied differential heating) Coriolis forces inhibit overturning motion in meridian planes and promote a completely different type of flow which has been termed ‘sloping convection’ or ‘baroclinic waves’. The motion is then non-axisymmetric and largely confined to meandering ‘jet streams’, with trajectories of individual fluid elements inclined at only very small (though essentially non-zero) angles to the horizontal. The kinetic energy of the waves derives from the interaction of slight vertical motions with the potential energy field maintained by differential heating, and it is dissipated by friction arising largely...

314 citations


Book ChapterDOI
01 Jan 1975
TL;DR: In this paper, the discussion is phrased in terms of heat transfer but translation to pollutant-transfer problems is straightforward, using C, c for the mean and fluctuating parts of the concentration or temperature, for generality.
Abstract: As mentioned in Chapter 1 the equations governing heat transfer with small temperature differences also govern transfer of small concentrations of other passive scalar contaminants. Particle-laden flows (Chapter 7) are excluded unless the particles are so small that they move with the flow and are negligibly affected by gravity. For convenience the discussion below is phrased in terms of heat transfer but translation to pollutant-transfer problems is straightforward. We use C, c for the mean and fluctuating parts of the concentration or temperature, for generality.

262 citations


Journal ArticleDOI
TL;DR: In this paper, the authors considered two asymptotic states of convection: one where the whole motion is dominated by viscosity, and one where inertial effects dominate, and they derived the dependence of the current density ratio I/I0 on the stability parameter T = M2R = eϕ0/Kρν, and on 1/R = ν/Kϕ 0, which is an equivalent Prandtl number.
Abstract: The problem of electric charge convection in a dielectric liquid layer of high ionic purity, when subjected to unipolar injection, is in many ways analogous to that of thermal convection in a horizontal fluid layer heated from below, although no formal analogy can be established. The problem treated is intrinsically more nonlinear than the thermal problem. We consider two asymptotic states of convection: one where the whole motion is dominated by viscosity, and one where inertial effects dominate. In each state, two or three spatial regions are distinguished. From the approximate equations that hold in the different regions, information about the variation of the different quantities with distance from the injector is obtained, and further approximations permit us to establish the dependence of the current density ratio I/I0 (called the electric Nusselt number) on the stability parameter T = M2R = eϕ0/Kρν, and on 1/R = ν/Kϕ0, which is an equivalent Prandtl number (e is the permittivity, ρ the fluid density, K the mobility, ν the kinematic viscosity, and ϕ0 the applied voltage). In the viscous state, the analysis gives I/I0 ∞ T½; in the inertial state the law I/I0 ∞ (T/R)1/4 = M½ is obtained. Since M is independent of the applied voltage, the latter law shows the saturation in the electric Nusselt number observed in earlier experiments. The transition in the states is associated with a transition number (MR)T [gap ] 30, which is an electric Reynolds number, related to an ordinary Reynolds number of about 10.The experimental results, obtained in liquids of very different viscosities and dielectric constants, verify these theoretical predictions; further, they yield more precise numerical coefficients. As for the transition criteria, the experiments confirm that the viscous and inertial effects are of the same order when Re [gap ] 10. It was also possible to determine roughly the limits of the viscous and inertial states. The viscous analysis remains valid up to a Reynolds number of about 1; the inertial state can be considered valid down to a Reynolds number of 60. Schlieren observations show that the motion has the structure of very stable hexagonal cells at applied voltages just above the critical voltage, which are transformed into unstable filaments when the voltage is increased further. At even higher voltages, the motion finally breaks down into turbulence. It may be of interest to point out that, when M < 3, the electric Nusselt number approaches 1, which is equivalent to the situation in thermal convection at low Prandtl numbers.

196 citations


Journal ArticleDOI
01 Jan 1975
TL;DR: In this article, the upward spread and subsequent steady turbulent burning of a thermally thick vertical fuel surface is examined theoretically and experimentally, showing that the rate of upward spread increases exponentially with time.
Abstract: Two-dimensional upward flame spread and subsequent steady turbulent burning of a thermally thick vertical fuel surface is examined theoretically and experimentally. The upward spread rate for vertical PMM slabs is observed to increase exponentially with time. This result is predicted in terms of measured fuel thermophysical properties, flame heights and heat feedback to the fuel surface. The local steady burning rates established after completion of upward spread exhibit a minimum at a height of 18 cm from the bottom edge and increase continuously beyond this height, becoming 70% larger at a height of 140 cm. This increase is shown to be entirely attributable to increasing flame radiation. Individual measurements of the various energy transfer components during steady burning of the PMM slabs are obtained from radiant intensity measurements of (1) the surface alone and (2) flame plus surface. Above 76 cm flame radiation ranges from 75 to 80% of the total (radiation plus convection) heat transfer from the flames to the fuel surface. Surface heat transfer by convection decreases slightly with height.

169 citations


Journal ArticleDOI
TL;DR: In this paper, a numerical study carried out on a two-dimensional model in order to simulate this phenomenon shows that, apart from the influence of the Rayleigh number, the aspect ratio A (length/height) of a vortical cell is the most important parameter for the occurrence of this type of convection.
Abstract: Experimental investigations of natural convection in a porous layer placed between two horizontal and isothermal plane surfaces have revealed a new type of convection as the Rayleigh number Ra* increases: fluctuating convection. A numerical study carried out on a two-dimensional model in order to simulate this phenomenon shows that, apart from the influence of the Rayleigh number, the aspect ratio A (length/height) of a vortical cell is the most important parameter for the occurrence of this type of convection. These quasi-periodic fluctuations induce important variations in the temperature field and in the streamlines. The total heat transport, as defined by the Nusselt number Nu*, varies within limits which may be separated by 80% of the mean value. Using the Galerkin method it is possible to deduce the conditions for the onset of convection from a state of pure conduction and also to define the critical conditions for the development of fluctuating convection from another perturbed state. A physical interpretation of the results is given for each type of convection. The results seem to agree with the experimental and numerical results obtained by different authors.

139 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that the ratio of dissipative heating to the convected heat flux is approximately equal to c(d/HT), where the constant c is independent of the Rayleigh number.
Abstract: Dissipative heating is produced by irreversible processes, such as viscous or ohmic heating, in a convecting fluid; its importance depends on the ratio d/HT of the depth of the convecting region to the temperature scale height. Integrating the entropy equation for steady flow yields an upper bound to the total rate of dissipative heating in a convecting layer. For liquids there is a regime in which the ratio of dissipative heating to the convected heat flux is approximately equal to c(d/HT), where the constant c is independent of the Rayleigh number. This result is confirmed by numerical experiments using the Boussinesq approximation, which is valid only if d/HT is small. For deep layers the dissipative heating rate may be much greater than the convected heat flux. If the earth's magnetic field is maintained by a convectively driven dynamo, ohmic losses are limited to 5% of the convected flux emerging from the core. In the earth's mantle viscous heating may be important locally beneath ridges and behind island arcs.

137 citations


Journal ArticleDOI
TL;DR: In this article, the authors reported the first large-scale observations of a developing mixed layer in which only convection can be causing the deepening, and the results correspond to a mixed layer thickness 17% greater than that predicted for zero penetration in the case of a linear initial density profile, which qualitatively agrees with the laboratory work of Deardorff, Willis and Lilly who found that only a small fraction (about 0.015) was recovered in this way.
Abstract: This paper reports the first large-scale observations of a developing mixed layer in which only convection can be causing the deepening. The convection arises from solar heating of water beneath lake ice. Since the water is below its temperature of maximum density, heating due to the absorption of radiation passing through the ice results in a mixed layer which increases in thickness with time. Observations taken with an electronic bathythermograph and by recording thermistor chains have permitted the estimation of that fraction of kinetic energy made available by the gravitational instability, which is converted into potential energy by entrainment of stable fluid as the convecting layer advances. Twelve independent estimates vary from 0.003 to 0.113 with an average value of 0.036. This result qualitatively agrees with the laboratory work of Deardorff, Willis and Lilly (1969) who found that only a small fraction (about 0.015) was recovered in this way. However, relatively little penetration can have a significant effect on mixed layer growth; the results obtained in the present study correspond to a mixed layer thickness 17% greater than that predicted for zero penetration in the case of a linear initial density profile.

Journal ArticleDOI
TL;DR: In this paper, the dynamo equations which represent the longitudinally-averaged magnetohydrodynamical action of the global convection under the influence of the rotation in the solar convection zone are considered as an initial boundary-value problem.
Abstract: Extensive numerical studies of the dynamo equations due to the global convection are presented to simulate the solar cycle and to open the way to study general stellar magnetic cycles. The dynamo equations which represent the longitudinally-averaged magnetohydrodynamical action (mean magnetohydrodynamics) of the global convection under the influence of the rotation in the solar convection zone are considered here as an initial boundary-value problem. The latitudinal and radial structure of the dynamo action consisting of a generation action due to the differential rotation and a regeneration action due to the global convection is parameterized in accordance with the structure of the rotation and of the global convection. This is done especially in such a way as to represent the presence of the two cells of the regneration action in the radial direction in which the action has opposite signs, which is typical of the regeneration action of the global convection. The effects of the dynamics of the global convection (e.g., the effects of the stratification of the physical conditions in the solar convection zone) are presumed to be all included in those parameters used in the model and they are presumed not to alter the results drastically since these effects are only to change the structure of the regeneration action topologically. However, since the structure of the differential rotation is not known precisely, several typical cases of the differential rotation are examined. A nonlinear process is included by assuming that part of the magnetic field energy is dissipated away when magnetic-field strength exceeds some critical value, simulating the formation of active regions and subsequent dissipation.

Journal ArticleDOI
TL;DR: In this paper, a model based on fluid flow in fractures is proposed for modeling hot springs in continental geothermal areas, thermal springs which are not associated with known high-temperature areas, and hot springs associated with hydrothermal circulation on ocean ridge crests.
Abstract: Summary Hot springs in continental geothermal areas, thermal springs which are not associated with known high-temperature areas, and hot springs associated with hydrothermal circulation on ocean ridge crests are described by models based on fluid flow in fractures. A steady state model shows that fractures of the order of a few millimetres wide can carry a substantial convective flow and that the convective flow depends upon the third power of the fracture width. The steady state model also furnishes estimates of conductive temperature losses in springs and gives estimates of the depths of circulation for thermal springs in the southeastern United States which are in good agreement with available field data. In many cases the temperature and flow rate of springs is non-stationary. This is particularly true of hot springs in high temperature geothermal areas and it is expected to be true of springs on mid-ocean ridge crests. Time-dependent models for springs show that the main effect of the circulation is to lower the regional geothermal gradient. Non-stationary convection controlled by fractures can explain the variability of heat flow data obtained near ocean ridge crests. A numerical example shows that convection in a block 3 km wide, containing fractures 3 mm wide and 5 km deep, and circulating for 104 years gives rise to a hot spring with a temperature of 125 °C and a flow rate of 0.14 kg m−1 s−1. Such a spring discharging at the sea floor would give rise to an unmeasurably small temperature anomaly in the sea water. The convective heat transfer due to such a circulation system is roughly 200 times greater than the heat transfer that would have been achieved by conduction alone.

Journal ArticleDOI
TL;DR: The convective heat transfer resulting from a granular flow over a heated surface is investigated in this paper, where the specific type of flow considered is that in which adjacent material particles are in physical contact.

Book
01 Apr 1975
TL;DR: In this article, two-phase flow is used to transfer heat from a nucleate pool to a film, and then to a liquid mixture of liquid-vapor mixtures.
Abstract: 1: Introduction- I Conduction and Convection Heat Transfer- 2: Conductive Heat Transfer- 3: Convective Heat Transfer to Low-Temperature Fluids- II Two-Phase Phenomena- 4: Terminology and Physical Description of Two-Phase Flow- 5: Nucleate Pool Boiling- 6: Critical Heat Flux- 7: Film Boiling- 8: Minimum Film Boiling Heat Flux- 9: Vapor-Liquid Condensation on Cryogenic Surfaces- 10: Vapor-Solid Condensation- 11: Pressure Drop and Compressible Flow of Cryogenic Liquid-Vapor Mixtures- 12: Forced Convection Heat Transfer with Two-Phase Flow- 13: Transient Conditions in Boiling and Two-Phase Discharge- III Radiation and Helium II Heat Transport- 14: Radiative Properties- 15: Heat Transport in Liquid Helium II

Journal ArticleDOI
TL;DR: The theory of thermal convection was initiated by Rayleigh as mentioned in this paper, who assumed that the amplitude of the motion was infinitesimal such that the equations could be linearized and derived the critical temperature gradient for the onset of convection together with the wavenumber for the marginal stable mode.
Abstract: The topic of this. review article is thermal convection in thin horizontal fluid layers, uniformly heated from below and/or cooled from above. If the temperature difference between the two horizontal boundaries is sufficiently small, the heat will be transferred through the fluid by conduction alone. For greater temperature differences the conduction state becomes unstable and a convective motion is set up. The first experimental investigations on thermal convection date back to Thomson (1881) and Benard ( 1900). The experiments by Benard in particular have attracted great attention and are today considered classical in fluid mechanics. This is essentially due to the surprising and fascinating pattern of very regular hexagonal cells obtained for large values of time in his experiments. Benard studied convective motion in a very shallow layer of viscous fluid (molten spermaceti) and made the motion visible by graphite or aluminum powder. The theory of thermal convection was initiated by Rayleigh ( 1916). He assumed that the amplitude of the motion was infinitesimal such that the equations could be linearized. He thus derived the critical temperature gradient (or in modern language, the critical Rayleigh number) for the onset of convection together with the wavenumber for the marginal stable mode. The resemblance of the cell pattern in the Rayleigh theory to certain cloud forms was early noticed by meteorologists. For example, it was pointed out that often the observed ratio between the vertical height and the lateral extent in cumulus clouds is the same as found in the theory for hexagonal cells. The theory on cloud formation is perhaps best applied to the convective motion set up in an altostratus layer when it breaks up into altocumulus clouds because of radiative cooling at the top of the layer. This application is, however, obscured by the effect of the release of latent heat, which is not accounted for in the theory. The theory on thermal convection has become very important in the study of motion in the earth's interior (see Turcotte & Oxburgh 1972) and also in some branches of astrophysics (Spiegel 1971 , 1972). The great interest shown to the theory for the last 10--15 years is partly due to these and other applications. The main

Journal ArticleDOI
Abstract: The stability of a horizontal fluid layer when the thermal (or concentration) gradient is not uniform is examined by means of linear stability analysis. Both buoyancy and surface-tension effects are considered, and the analogous problem for a porous medium is also treated. Attention is focused on the situation where the critical Rayleigh number (or Marangoni number) is less than that for a linear thermal gradient, and the convection is not (in general) maintained. The case of constant-flux boundary conditions is examined because then a simple application of the Galerkin method gives useful results and general basic temperature profiles are readily treated. Numerical results are obtained for special cases, and some general conclusions about the destabilizing effects, with respect to disturbances of infinitely long wavelength, of various basic temperature profiles are presented. If the basic temperature gradient (considered positive, for a fluid which expands on heating, if the temperature decreases upwards) is nowhere negative, then the profile which leads to the smallest critical Rayleigh (or Marangoni) number is one in which the temperature changes stepwise (at the level at which the velocity, if motion were to occur, would be vertical) but is otherwise uniform. If, as well as being non-negative, the temperature gradient is a monotonic function of the depth, then the most unstable temperature profile is one for which the temperature gradient is a step function of the depth.

Journal ArticleDOI
TL;DR: In this paper, the critical Rayleigh numbers were determined as functions of the box diameter and the gap width respectively, and a distinct influence of side walls with different thermal conductivities was observed.
Abstract: Convective flow of the cellular type has been investigated in vertical cylinders of axisymmetric and annular shape. The critical Rayleigh numbers were determined as functions of the box diameter and the gap width respectively. A distinct influence of side walls with different thermal conductivities was observed.


Journal ArticleDOI
TL;DR: In this article, the authors present extensive linear numerical simulations of Boussinesq convection in a rotating spherical shell of finite depth, where the authors solve the marching equations on a staggered grid in the meridian plane for the amplitudes of the most unstable Fourier mode of longitudinal wavenumber m between 0 and 24, for Taylor number T between 0, 106, at a Prandtl number P=1, for a shell of depth 20% of the outer radius.
Abstract: We present extensive linear numerical simulations of Boussinesq convection in a rotating spherical shell of finite depth. The motivation for the study is the problem of general circulation of the solar convection zone. We solve the marching equations on a staggered grid in the meridian plane for the amplitudes of the most unstable Fourier mode of longitudinal wavenumber m between 0 and 24, for Taylor number T between 0 and 106, at a Prandtl number P=1, for a shell of depth 20% of the outer radius. Stress-free, fixed-temperature boundary conditions are used at the inner and outer bounding surfaces. Modes of two symmetries, symmetric and antisymmetric about the equator, are studied. The principal results are as follows: Increasing Taylor number T splits the most unstable solutions for each m into two classes: a broad band of high m solutions which peak at or near the equator, and a small number of low m solutions which peak at or near the poles. The equatorial modes are unstable at lower Rayleigh n...

Journal ArticleDOI
Fritz Wald1, Richard O. Bell1
TL;DR: In this article, a series of traveling heater experiments was performed on the crystal growth of CdTe from Te solution, and it was shown that acceleration rotation can increase the growth rate by a factor of 2, while the natural convection currents generated under the particular experimental conditions are already very strong.

Journal ArticleDOI
TL;DR: In this paper, the authors show that convection cells of aspect ratio as large as 8.6 are possible for variable viscosity convection in the upper mantle, which is due to the large contrast between the lithosphere and the asthenosphere.
Abstract: Two-dimensional numerical models of steady state convection show that convection cells of aspect ratio as large as 8.6 are possible for variable viscosity convection in the upper mantle. Our models include the effects of variable viscosity, viscous dissipation, internal heating, heat flow through the bottom, and the adiabatic gradient. The large aspect ratio of the convection cells is primarily due to the large viscosity contrast between the lithosphere and the asthenosphere. It appears possible for multiple convection cells to occur in a low-viscosity zone while the surface velocities give the appearance of a single cell. The details of the viscosity law relevant to mantle materials and conditions are presently uncertain but are of crucial importance; temperature, viscosity, and flow patterns are inextricably entwined. Convection decreases the overall temperature gradient; consequently, generally accepted temperatures for most of the mantle are too high. The controversies over plate-mantle decoupling and passive versus active plates are probably due to oversimplifications that disregard hydrodynamic concepts.


Journal ArticleDOI
TL;DR: In this paper, the Boussinesq equations for two-dimensional convection rolls and the assumption that the amplitude A of the convection and the Chandrasekhar number Q are small were analyzed.
Abstract: The interaction between convection in a horizontal fluid layer heated from below and an ambient vertical magnetic field is considered. The analysis is based on the Boussinesq equations for two-dimensional convection rolls and the assumption that the amplitude A of the convection and the Chandrasekhar number Q are small. It is found that the magnetic energy is amplified by a factor of order R½m, where Rm is the magnetic Reynolds number. The ratio between the magnetic and kinetic energies can reach values much larger than unity. Although the magnetic field always inhibits convection, this influence decreases with increasing amplitude of convection. Thus finite amplitude onset of steady convection becomes possible at Rayleigh numbers considerably below the values predicted by linear theory.

Journal ArticleDOI
TL;DR: Factors determining the convective flow patterns around the human head in ‘still’ conditions are discussed in relation to body posture and it is revealed that the head has a thicker ‘insulating’ layer of convecting air in the erect posture than in the supine position.
Abstract: 1. Factors determining the convective flow patterns around the human head in 'still' conditions are discussed in relation to body posture. 2. The flow patterns have been visualized using a schlieren optical system which reveals that the head has a thicker 'insulating' layer of convecting air in the erect posture than in the supine position. 3. Local convective and radiative heat transfer measurements from the head have been using surface calorimeters. These results are seen to be closely related to the thickness of the convective boundary layer flows. 4. The total convective and radiative heat loss from the head of a subject in the erect and supine position has been evaluated from the local measurements. For the head of the supine subject the heat loss was found to be 30% more than when the subject was standing.

Journal ArticleDOI
TL;DR: In this paper, the predictions of propagation velocity made by a linear model of moist convective instability are compared with actual case studies of severe local storms, and good agreement between observation and theory is found for a certain class of storms, namely those that propagate in a continuous fashion.
Abstract: The predictions of propagation velocity made by a linear model of moist convective instability are compared with actual case studies of severe local storms. Excellent agreement between observation and theory is found for a certain class of storms, namely those that propagate in a continuous fashion. The calculations suggest that such storms take the form of convectively forced internal gravity waves. The pattern of low-level convergence provided by the gravity wave organizes the convection, which in turn drives the wave.

Journal ArticleDOI
01 Jan 1975
TL;DR: In this article, a large-scale gas-supplied sintered-metal burner was used to study radiation and spatial orientation effects on steady turbulent fires over a range of mass transfer driving forces, B.
Abstract: A large-scale gas-supplied sintered-metal burner was used to study radiation and spatial orientation effects on steady turbulent fires over a range of mass transfer driving forces, B . Three principal burning modes are evident: (1) turbulent pool fires from θ =0° to θ =15°; (2) upward turbulent burning from θ ∼15° to θ ∼168°; and (3) cellular ceiling fires from θ ∼168° to θ =180°. Steady burning rates decrease rapidly with inclination from the horizontal within the pool regime, followed by a more gradual decreases with inclimation within the upward turbulent burning regime being minimum θ ∼168°, i.e., 12° from the horizontal ceiling orientation. This trend is ascribed to the decreasing direct gravitational generation of turbulent kinetic energy, causing a reduction in the turbulent flame thicknesses with their reduced radiant fluxes. Previous laminar burning studies showed opposite trends, with minimum burning rates in the “pool” orientation. Increased cellular flow mixing is accompanied by a sharp increase in burning rate as the fuel surface rotates from 168° to the horizontal ceiling fire. Radiometer comparison of outward and surface directed radiant flux for a vertical burning surface indicate at least 7% absorption by combustion products and intermediates near the surface. Radiation is found to exceed convective heat transfer to the fuel surface for B >1.0. At large B numbers the burning is increasingly radiation-dominated as convection decreases due to heat blockage.

Journal ArticleDOI
TL;DR: In this paper, local heat transfer data were obtained for turbulent natural convection on vertical and inclined upward and downward facing surfaces and the results showed the location of the transition to be a function of the plate angle.
Abstract: Local heat transfer data were obtained for turbulent natural convection on vertical and inclined upward and downward facing surfaces. The test surface consisted of a 1.83 m (6 ft) wide × 7.32 m (24 ft) high plate with a constant heat flux obtained by electrical resistive heating of a metal foil on the surface. The tests were conducted in air for modified Grashof numbers up to 1015 . Measurements were made of the local surface temperature for this constant heat flux condition, for the plate inclined at angles from 30 deg to the vertical (upward facing, unstable) through the vertical to 80 deg to the vertical (downward facing, stable). The results show the location of the transition to be a function of the plate angle. For the unstable case, the transition length decreases as the plate angle increases from the vertical while for the stable case the position of transition increases with the angle from the vertical. The laminar data for both orientations are correlated as: Nux = 0.55 (Grx*Pr)0.20 in which the gravity is the component along the surface, g cos θ. The turbulent natural convection data are correlated quite well by the relation: Nux = 0.17 (Grx*Pr)0.25 In the turbulent case the correlation is independent of angle for the unstable case, whereas for the stable case the data correlate best when the gravity is modified by cos2 θ, where θ is measured from the vertical. Thus, there is a significant influence of angle on the convective heat transfer for the stable turbulent region.

Journal ArticleDOI
17 Jan 1975-Science
TL;DR: Experiments conducted aboard Apollo 17 by astronaut Ronald E. Evans showed that in uncovered liquids convection driven by surface tension can occur at lower temperature gradients in low gravity than in 1g.
Abstract: Experiments conducted aboard Apollo 17 showed that in uncovered liquids, convection driven by surface tension can occur at lower temperature gradients in low gravity than in 1 g. In completely confined fluids (no liquid-gas interface), vibrations caused by spacecraft and astronaut movements increased the heat transfer considerably over the pure conduction case.

Book ChapterDOI
D.J. Carson1, F.B. Smith1
TL;DR: In this article, a combined dynamical and thermal approach to the parameterization of the entrainment process and the development of the convectively unstable boundary layer is presented. But the results of the thermal approach are limited.
Abstract: Publisher Summary The stability of the mixing layer is of primary importance in short-range pollution studies, and the ability to specify the diurnal cycle of boundary-layer evolution becomes important when dealing with pollutants tracked for several days on a regional scale. This chapter describes the general results and limitations of the simple thermal approach, and provides a combined dynamical and thermal approach to the parameterization of the entrainment process and the development of the convectively unstable boundary layer. Observations of the boundary layer in convective situations show that in general it is a diurnally evolving system with discontinuities around sunrise and sunset. Two main factors, which control the development of the convective layer, are (1) the flux of sensible eddy heat entering the boundary layer at the ground and (2) a turbulent mixing process occurring at the interface between the well-mixed boundary layer air and the nonturbulent air in the capping stable layer. Recent observations indicate that wind shear at the interface may be of fundamental importance not only to the entrainment of eddy momentum but also of eddy sensible heat.