Convection natural, definition

The diffusion-layer concept is an artifice for handling the flux arising from what would be, if treated in a proper hydrodynamic way, a complicated space variation of concentration at the interface. There is always some gradient of concentration at the interface there is an initial region in which the concentration changes linearly with distance, but there is, in the real case, no sharply defined layer of definite thickness, even when convection (natural or forced) produces a steady-state concentration... 

Dimensional analysis shows that, in the treatment of natural convection, the dimensionless Grashof number, which represents the ratio of buoyancy to viscous forces, is often important. The definition of the Grashof number, Gr, is... 

Thus, unlike k (which is a thermal property), A is merely a definition and depends on flow (conditions). That is, unlike thermal conductivity, the heat transfer coefficient cannot be tabulated and needs to be determined for each flow condition. Accordingly, Chapters 5 and 6 are devoted to elaboration of Eqs. (1.60) and (1.61) and the solution of convection problems in teams of a heat transfer coefficient. Here, for some appreciation, an order-of-magnitude range of each heat transfer coefficient corresponding to natural or forced convection in different fluids is given in Table 1.2. The order-of-magnitude difference between the A values for natural convection and forced convection resulting from flow of the same fluid should be noted. 

NA%TOAL CONVECTION TO AIR FROM VERTICAL SHAPES AND HORIZONTAL PLANES. Equations for heat transfer in natural convection between fluids and solids of definite geometric shape are of the form ... 

Each of the four segments of the graph in Fig. 13.5 corresponds to a definite mechanism of boiling. In the first section, at low temperature drops, the mechanism is that of heat transfer to a liquid in natural convection, and the variation of h with AT agrees with that given by Eq. (12.78). Bubbles form on the surface of the... 

The heat transfer coefficient defined by Eq. 1.12 is sensitive to the geometry, to the physical properties of the fluid, and to the fluid velocity. However, there are some special situations in which h can depend on the temperature difference NT = T - Tf. For example, if the surface is hot enough to boil a liquid surrounding it, h will typically vary as AT2 or in the case of natural convection, h varies as some weak power of AT— typically as AT1 4 or AT1 3. It is important to note that Eq. 1.12 as a definition of h is valid in these cases too, although its usefulness may well be reduced. 

Equations are presented in this section for evaluating the heat transfer by natural convection from the external surfaces of bodies of various shapes. The correlation equations are of the form described in the section on the heat transfer correlation method, and the orientation of the surface is given by the surface angle defined in Fig. 4.4. Supporting experimental evidence for each such equation set is outlined after each equation tabulation. The correlations are in terms of Nu, Ra, and Pr, parameters that involve physical properties, a length scale, and a reference temperature difference. Rules for the evaluation of property values are provided in the nomenclature, and the relevant length scale and reference temperature difference are provided in a separate definition sketch for each problem. 

FIGURE 4.6 Definition sketch for natural convection on a vertical plate with uniform wall temperature or uniform heat flux. 

FIGURE 4.9 Definition sketch for natural convection from plates of various planform. 

Circular Isothermal Fins on a Horizontal Tube. Tsubouchi and Masuda [269] measured the heat transfer by natural convection in air from circular fins attached to circular tubes, as in the configuration shown in Fig. 4.23/ Correlations for the heat transfer from the tips of the fins (see the figure for definition), and from the cylinder plus vertical fin surfaces, were reported separately. 

The value of E in relation to like the value of gj, indicates the evaporative nature of the surface so E < E q or E/Ee, < 1 reflects surface dryness or stomatal closure as well as the balance of energy exchange between the atmosphere and the underlying surface. By definition, E > Ef, can be caused only by advection. As implied above with respect to the partitioning of temperature, this may also result from the entrainment of dry air from above the convective boundary layer that develops daily over the earth surface. To further illustrate the relation between E and Ef, in terms of surface characteristics, it is helpful to write the Penman-Monteith equation (Monteith and Unsworth, 1990),... 

There is, however, a definite analogy between boiling and condensation this has been shown by Bromley, who derived a natural convection film boiling heat transfer equation by analogy with film condensation.. 

As highlighted by Shah and London [2], a natural tendency exists to use in convection problems a large number of different sets of dimensionless groups based on the analyst s particular normalization of the differential equations and boundary conditions. An effort to standardize the definitions of dimensionless groups for laminar flows through channels was made by Shah and London [2] some years ago. In this section, the normalization of the convection problems proposed by Shah and London will be followed. 

Many formulations based upon these assumptions can be derived. One formulation can be converted into another using the definitions of density, internal energy and the ideal gas law. Though equivalent analytically, these formulations differ in their numerical properties. Each formulation can be expressed in terms of mass and enthalpy flow. These rates represent the exchange of mass and enthalpy between zones due to physical phenomena such as plumes, natural and forced ventilation, convective and radiative heat transfer, and so on. For example, a vent exchanges mass and enthalpy between zones in connected rooms, a fire plume typically adds heal to the upper layer and transfers entrained mass and enthalpy from the lower to the upper layer, and convection transfers enthalpy from the gas layers to the surrounding walls. 

In this section definitions are given of the main kinematic tensors (see ref. 5, chapter 9) that are needed for the continuum mechanics description of viscoelastic materials as well as for the presentation of kinetic theory results. Some of these tensors are defined naturally in terms of the velocity field of the material, whereas others are defined easily in terms of the displacement functions that describe the motion of fluid particles. Inasmuch as the velocity field and the displacement functions are themselves interrelated, it is possible to interrelate the two groups of kinematic tensors. Here the emphasis is on working definitions of the kinematic tensors and not on their derivation from the motion of a convected coordinate system, which is a standard starting point for the discussion of continuum mechanics an important basic reference for the kinematics and dynamics of continuous media is a paper by Oldroyd. ... 

These emergency condensers do not need any power and no valves are needed to start the operation they are passive by definition. Therefore, a test should be performed to evaluate the capability of these condensers to remove heat produced in the core region to the outside pool by natural convection and, in addition, at ambient pressure, as it will be during long term shutdown. 

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