Natural convection within enclosures

When both bottom and top surfaces are maintained at constant temperatures and there is internal generation, there is a superposition of the horizontal layer problem discussed in the section on natural convection within enclosures and the internal generation problem previously described. These are characterized by the external Rayleigh number defined in the section on natural convection within enclosures and the internal Rayleigh number defined in Fig. 4.40a. The dependence of the layer stability on these parameters has been discussed by Ning et al. [208]. The heat transfer at the top and bottom surfaces has been estimated for these conditions by Baker et al. [13], Suo-Anttila and Catton [276], and Cheung [51],... 

Newell, M.E. and Schmidt, F.W., Heat Transfer by Laminar Natural Convection Within Rectangular Enclosures , J. Heat Transfer, Vol. 92, pp. 159-168, 1970. 

Enclosure problems (Fig. 4.1c) arise when a solid surface completely envelops a cavity containing a fluid and, possibly, interior solids. This section is concerned with heat transfer by natural convection within such enclosures. Problems without interior solids include the heat transfer between the various surfaces of a rectangular cavity or a cylindrical cavity. These problems, along with problems with interior solids including heat transfer between concentric or eccentric cylinders and spheres and enclosures with partitions, are discussed in the following sections. Property values (including P) in this section are to be taken at Tm = (Th+ TC)I2. 

We have estimated the likely heat that may be generated by a particular size of conductor and enclosure for a certain current rating and then have counterchecked whether the conductor and the enclosure so chosen can dissipate this heat by radiation and natural convection, and reach a state of thermal stability within permissible limits or we may have to increase the size of the conductor... 

Since accidental fire spread mostly occurs under natural convection conditions within buildings and enclosures, some examples of configurations leading to opposed or wind-aided types of spread are illustrated in Figure 8.3. Flame spread calculations are difficult... 

The passive systems, known as gravity or natural draft type, rely on convection and the wind for air movement. The active systems are mechanical and generally provide a more reliable means for ventilation. The layout of the system should depend upon the physical properties of the gases or liquids stored. Gases that are lighter than air will rise within the enclosure and should be ventilated at the top of the enclosure. Conversely, heavier than air gases and vapors will require low level ventilation for removal. 

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