External Natural Convection

Sparrow, E. M., and M. A. Ansari A Refutation of King s Rule for Multi-Dimensional External Natural Convection, Int. J. Heat Mass Transfer, vol. 26, p. 1357, 1983. 

Discussion of Correlation for Specific Shapes. The equation for elliptical cylinders fits the approximate analysis of Raithby and Hollands [224] but has not been verified by experiment except in the limiting cases of a vertical plate (C/L = 0) and a circular cylinder (C/L = 1.0). The vertical plate predictions by Eq. 4.48 are slightly different than those based on the more accurate specialized equations given in the section on external natural convection in flat plates. 

A. V. Hassani and K. G. T. Hollands, Prandtl Number Effect on Natural Convection on External Natural Convection Heat Transfer From Irregular Three-Dimensional Bodies, Int. J. Heat Mass Transfer (32/11) 2075-2080,1989. 

The LFR system provides for ambient pressure single-phase primary coolant natural circulation heat transport and removal of core power under all operational and postulated accident conditions. The high boiling temperature of the Pb coolant enables heat transport by natural circulation of the primary coolant at significantly higher temperatures than with traditional liquid metal cooled reactors. External natural convection driven passive air-cooling of the guard/containment vessel is always in effect and removes power at decay heat levels. 

External natural convection driven passive air-cooling over the guard/containment vessel (surrounding the reactor vessel) that is always in effect and removes decay heat power levels. 

Convection is the movement of a species due to external mechanical forces. This can be of two types natural convection, which arises from thennal gradients or density differences within the solution, and forced convection, which can take the fomi of gas bubbling, pumping or stirrmg. The fomier is undesirable and can occur m any solution... 

Convective heat transfer is classified as forced convection and natural (or free) convection. The former results from the forced flow of fluid caused by an external means such as a pump, fan, blower, agitator, mixer, etc. In the natural convection, flow is caused by density difference resulting from a temperature gradient within the fluid. An example of the principle of natural convection is illustrated by a heated vertical plate in quiescent air. 

Free circulation of the coolant from the machine to the surrounding medium 0 Free convection No external power source is essential. Fleat dissipation is achieved through natural convection like a surface cooled motor... 

Natural convection is self-induced and is created by the density differences, which are temperature related the boiling of water in a kettle is an example of free convection. Forced convection is caused by an external force being applied by mechanical means such as a fan or pump the cooling of a warm bottle in cool flowing water is an example of forced convection. 

Convection is classified according to the motivating flow. When the flow takes place because of density variations caused by temperature gradients, the motion is called natural convection. When it is caused by an external agency such as a pump or a fan the process is called forced convection. 

In forced convection, circulating currents are produced by an external agency such as an agitator in a reaction vessel or as a result of turbulent flow in a pipe. In general, the magnitude of the circulation in forced convection is greater, and higher rates of heat transfer are obtained than in natural convection. 

If a beaker containing water rests on a hot plate, the water at the bottom of the beaker becomes hotter than that at the top. Since the density of the hot water is lower than that of the cold, the water in the bottom rises and heat is transferred by natural convection. In the same way air in contact with a hot plate will be heated by natural convection currents, the air near the surface being hotter and of lower density than that some distance away. In both of these cases there is no external agency providing forced convection currents, and the transfer of heat occurs at a correspondingly lower rate since the natural convection currents move rather slowly. 

In the pores of the electrodes, practically no natural convection of the liquid takes place. Reactants dissolved in the liquid can be supplied in two ways from the external surface to the internal reaction zones (and reaction products transported away in the opposite direction) (1) by diffusion in the motionless liquid diffusion electrode),... 

When a fluid is heated, the hot less-dense fluid rises and is replaced by cold material, thus setting up a natural convection current. When the fluid is agitated by some external means, then forced convection takes place. It is normally considered that there is a stationary film of fluid adjacent to the wall and that heat transfer takes place through this film by conduction. Because the thermal conductivity of most liquids is low, the main resistance to the flow of heat is in the film. Conduction through this film is given by the usual relation (74), but the value of h is not simply a property of the fluid but depends on many factors such as the geometry of the system and the flow dynamics for example, with tubes there are significant differences between the inside and outside film coefficients. 

In electrochemical reactors, the externally imposed velocity is often low. Therefore, natural convection can exert a substantial influence. As an example, let us consider a vertical parallel plate reactor in which the electrodes are separated by a distance d and let us assume that the electrodes are sufficiently distant from the reactor inlet for the forced laminar flow to be fully developed. Since the reaction occurs only at the electrodes, the concentration profile begins to develop at the leading edges of the electrodes. The thickness of the concentration boundary layer along the length of the electrode is assumed to be much smaller than the distance d between the plates, a condition that is usually satisfied in practice. 

Positive deviations of it1/2 with increasing time can also be evidence for convection within an electrochemical cell. Convection can be caused by external vibrations or by density gradients created by the local concentration differences resulting from the electrochemical perturbation. While the influence of external vibrations can be largely eliminated by isolation of the cell with a damped table, the natural convection due to unequal densities of O and R is an unavoidable consequence of the experiment, the importance of which depends on the particular species involved. The effect of natural convection at planar electrodes is most serious when the surface is mounted vertically. It is therefore desirable to carry out electrochemical experiments at surfaces facing up or down whenever possible. 

The studied BCAP0350 DLC has a D-cell battery shape factor which is defined in the standard with a 33 mm outside diameter and a 61.5mm length. The total external surface is about 80cm2. The production of losses inside the DLC is assumed to be uniform in the volume. In the case of a 30 A charge/discharge current the dissipated power is equal to 2.88 W. The measurements have been performed at room temperature 7 = 20°C which was constant during the experiment. The DLC is only cooled with a slowly moving airflow due to the natural convection. 

Here, L is the material thickness, T is the tlame temperature, v, is the velocity of the incoming oxidant flow if there is no external supply, v, is controlled by natural convection. A solid fuel which is heated throughout its depth prior to flame arrival is said to be thermally thin. If the thickness (L) of the material is more than that of the layer heated prior to flame arrival the material is said to be thermally thick. 

Since the reaction rate is relatively low in the flame edge when there is no external heat source ( 2 = 0), the flame stability may be essentially affected by even small fluxes due to natural convection. 

As reported in Ref. , the spread rate of a flame moving up a vertical surface of a sufficiently thick PMMA sheet increases under the effect of an external heat radiation. Depending on the heat radiation intensity and exposure time, various effects on the flame spread rate are observed. Additional heating of the polymer surface by a radiative flux results, first of all, in a decrease of the temperature dilTerence (T — Tp) and, in accordance with Eq. (2.19), in an increase of v. The experimental relationship v (T — To)" at T = 363 °C is close to that predicted by theory. According to Femandez-Pello , an increase of the initial polymer surface temperature, Tp, cause a parallel enhancement of the natural convection in the boundary heat layer and heat radiation by the surface, leading to its partial cooling. Therefore, when the intensity of the external radiative heat flux is low, the flame spread rate increases with time, but only up to a certain constant value. 

Convection is called forced convection if Ihe fluid is forced to flow over the surface by external means such as a fan, pump, or the wind. In contrast, convection is called natural (or free) convection if the fluid motion is caused by buoyancy forces that are induced by density differences due to the variation of temperature in the fluid (Fig. 1 33). For example, in the absence of a fan, heat transfer from the surface of the hot block in Fig. 1-32 is by natural convection since any motion in the air in this case is due to the rise of Ihe warmer (and thus lighter) air near the surface and the fall of the cooler (and thus heavier) air to fill its place. Heat transfer between the block and the surrounding air is by conduction if the temperature difference between Ihe air and the block is not large enough to overcome the resistance of air to movement and thus to initiate natural convection currents. 

Convection is classified as natural (or free) and forced convection, depend ing on how the fluid motion is initiated. In forced convection, the fluid is forced to flow over a surface or in a pipe by external means such as a pump or a fan. In natural convection, any fluid motion is caused by natural means such as the buoyancy effect, which manifests itself as the rise of warmer fluid and the fall of the cooler fluid. Convection is also classified a.s external and internal, depending on whether the fluid is forced to flow over a surface or in a pipe. 

In Chapters 7 and 8, we considered heat transfer by forced convection, where a fluid was forced to move over a surface or in a tube by external means such as a pump or a fan. In this chapter, we consider natural convection, where any fluid motion occurs by natural means such as buoyancy. The fluid motion in forced convection is quite noiicenhle, since a fan or a pump can transfer enough momentum to the fluid to move it in a certain direction. Tlie fluid motion in natural convection, however, is often not noticeable because of the low velocities involved. 

The magnitude of the natural convection heal transfer between a surface and a fluid is directly related to the flow rate of the fluid. The higher the flow rate, tbe higher the heat transfer rate. In fact, it is the very high flow rales that increase the heat transfer coefficient by orders of magnitude when forced convection is used. In natural convection, no blowers are used, and therefore the flow rale cannot be controlled externally. The flow rale in this case is established by the dynamic balance of buoyancy and friction. 

When a surface is subjected to external flow, the problem involves both natural and forced convection. The relative importance of each mode of heat transfer is determined by the value of the coefficient Gr /Ref Natural convection effects are negligible if GiJRel 1, free convection dominates and the forced convection effects are negligible if Gri/Re > 1, and both effects are significant atid must be considered if Grt/Re = 1. 

Heat is transferred from or to a region by the motion of fluids and the phenomenon of convection. In natural convection, the movement is caused by buoyancy forces induced by variations in the density of the fluid these variations are caused by differences in temperature. In forced convection, movement is created by an external agency such as a pump. 

Note that the distributions are cardinally different from those over smooth surfaces like in the previous cases. Again, two parts of the profiles have to be discussed. Over the top SCS s level z = h = 6 m, the wind velocity distributions grow monotonically in the case of a strong wind the temperature diminishes, as a rule. Few cases where the wind velocity diminishes over the SCS are characterized by a weak external wind so that the horizontal forced convection is perhaps comparative with the intense natural convective motion rising up from the heated and wetted air layer within SCS. 

Studies on the effect of hydrodynamics on localized corrosion and electrochemical etching processes have been reviewed by West et al. Much of the work has been performed by Alkire and co-workers." They have used FIDAP, a commercial FEM code, to investigate the influence of fluid flow on geometries relevant to etching and to pitting corrosion. In most cases, Stokes flow was considered. The Stokes flow approximation is frequently valid inside the cavity because its characteristic dimension is small. However, the flow outside the cavity may not be in the Stokes flow regime. Since it is the external fluid motion that induces flow inside the cavity, under many (especially unsteady) situations, the use of the Stokes flow approximation may be problematic. Some of the work of Alkire and co-workers has been extended hy Shin and Economou, " who simulated the shape evolution of corrosion pits. Natural convection was also considered in their study. 

Another distinction among flows is whether the flow is forced by an external means such as a pump (termed forced convection) or whether the flow arises as a result of a density difference developed in the fluid circuit as a result of the heat transfer (termed natural convection or thermosiphon action). Some cases include both mechanisms. 

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