Enclosures natural convection

By forced convection The factors that can influence the temperature of the enclosure, installed outdoors are wind and snow, other than forced cooling. But their effect on actual cooling may be small. Sometimes this happens and sometimes not. It is better to ignore this effect when estimating various thermal effects. Natural convection and radiation will take account of this. 

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... 

NATURAL CONVECTIVE HEAT TRANSFER ACROSS A RECTANGULAR ENCLOSURE... 

Heat transfer by natural convection across an enclosed space, called an enclosure or, sometimes, a cavity, occurs in many real situations, see [34] to [67]. For example, the heat transfet between the panes of glass in a double pane window, the heat transfer between the collector plate and the glass cover in a solar collector and in many electronic and electrical systems basically involves natural convective flow across an enclosure. 

Some of the more commonly used methods of obtaining solutions to problems involving natural convective flow have been discussed in this chapter. Attention has been given to laminar natural convective flows over the outside of bodies, to laminar natural convection through vertical open-ended channels, to laminar natural convection in a rectangular enclosure, and to turbulent natural convective boundary layer flow. Solutions to the boundary layer forms of the governing equations and to the full governing equations have been discussed. 

Catton, I., Natural Convection in Enclosures , Proc. 6th. International Heat Trans. Conf., Toronto, Vol. 6, pp. 13-31, Hemisphere Publ, Washington, DC, 1978. 

Kamotani, Y., Wang, L.W., and Ostrach, S., "Experiments on Natural Convection Heat Transfer in Low Aspect Ratio Enclosures , Paper No. AIAA-81 -1066, AIAA 16th Thermophysics Conference, Palo Alto, California, June 23-25, 1981. 

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. 

Rubel, A. and Landis, R., Numerical Study of Natural Convection in a Vertical Rectangular Enclosure , Phys. Fluids, Suppl. II, Vol. 12-11, pp. 208-213, 1969. 

Wirtz, R. A. and Tseng, W.F., Natural Convection Across Tilted. Rectangular Enclosures of Small Aspect Ratio , Natural Convection in Enclosures, ASME publication HTD-Vol. 8, pp. 47-54, 1980. 

NATURAL CONVECTION IN POROUS MEDIA-FILLED ENCLOSURES... 

Heat transfer by natural convection across porous media-filled enclosures occurs in a number of oractical situations and will be considered in this sectif T 6] to f511... 

Oosthuizen, P.H. and Paul, J T.. "Natural Convection in a Square Enclosure Partly Filled with a Centrally Positioned Porous Layer and with a Partially Heated Wall", Proc. 2nd Thermal-Sciences and 14th VIT National Heat Trans. Conf, Rome. Italy, Vol. 2, 1996, pp. 851-856. 

Oosthuizen. P.H., Natural Convection in a Square Enclosure Partly Filled with Two Layers of Porous Material , Proc. 4th Int. Conf. on Advanced Computational Methods in Heat Trans., Advanced Computational Methods in Heat Transfer IV, Udine, Italy, 1996, pp. 63-72. 

Oosthuizen. PH. and Naylor. D.. Natural Convective Heat Transfer from a Cylinder in an Enclosure Partly Filled w ith a Porous Medium, Ini. J. Sumer. L riiods Heat and Fluid Flow. Vol. 6, No. 6. pp. 51-63. 1996. 

Natural Convection in Porous Media-Filled Enclosures 531... 

Free convection in inclined enclosures is discussed by Dropkin and Somerscales [12], Evans and Stefany [9] have shown that transient natural-convection heating or cooling in closed vertical or horizontal cylindrical enclosures may be calculated with... 

Warrington, R.O., and R. F.. Powe The Transfer of Heat by Natural Convection Between Bodies and Their Enclosures, Int. J. Heat Mass Transfer, vol. 28, p. 319, 1985... 

Mass How Rate ttirougn ttie Space beLV, een Plates 519 9-5 Natural Convection Inside Enclosures 521 effective Thermal Conductivity 522 Horizontal Rectangular Enclosures 523 Inclined Rectangular Enclosures 523 Vertical Rectangular Enclosures 524 Concentric Cylinders 524 Concentric Spheres 525 Combined Natural Convection and Radiation 525... 

The sensitive electronic circuitry of a power transistor at the junction is protested by its case, which is a rigid metal enclosure. Heat transfer characteristics of a power transistor are usually specified by the manufacturer in terms of the case-to-ambient thermal resistance, which accounts for both Ihe natural convection and radiation heat transfers. 

We stait this chapter with a discussion of the physical mechanism of natural convection and the Grashof number. We then present the correlations to evaluate heat transfer by natural convection for various geometries, including fmned surfaces and enclosures, (finally, we discuss simultaneous forced and natural convection. 

Examine natural conveclion from finned surfaces, and determine the optimum fin spacing, B Analyze natural convection inside enclosures such as double-pane windows, and... 

The characteristics of heat transfer through a horizontal enclosure depend ou whether the hotter plate is at the top or at the bottom, as shown in Fig. 9-22. When the hotter plate is at the top, no convection currents develop in the enclosure, since Ihe lighter fluid is always on top of the heavier fluid. Heat transfer in tlris case is by pure conduction, and we have Nu - 1. When the hotter plate is at the bottom, the heavier fluid will be on top of the lighter fluid, and there will be a tendency for Ihe lighter fluid to topple the heavier fluid and rise to the top, where it comes in contact with the cooler plate and cools down. Until that happens, however, heat transfer is still by pure couduc-tion and Nu — I. When Ra > 1708, the buoyant force overcomes the fluid resistance and initiates natural convection currents, which are observed to be in the form of hexagonal cells called BSnard cells. For Ra > 3 X 10, the cells break down and the fluid motion becomes turbulent. 

A Nussek number of 3 for an enclosure indicates that heat transfer through the enclosure by natural convection is three times that by pure conduction. 

Discussion Recall that a Nusselt number of Nu = 1 for an enclosure corresponds to pure conduction heat transfer through the enclosure. The air in the enclosure in this case remains still, and no natural convection currents occur in the enclosure. The Nusselt number in our case is 1.40, which indicates that heat transfer through the enclosure is 1.40 limes that by pure conduction. The increase in heat transfer is due to the natural convection currents that develop in (he enclosure. 

Discussion Note that the air in the spherical enclosure acts like a stationary fluid whose thermal conductivity is Aif/k = 0.1105/0.02566 = 4.3 times that of air as a result of natural convection currents. Also, radiation heat transfer 1 between spheres is usually significant, and should be considered in a complete 1 analysis. 

For a spherical enclosure, the rate of heat transfer through the space between the spheres by natural convection is expressed as... 

V. S. Arpaci, Two thermal microscales for natural convection and heat transfer correlations," Significant Questions in Buoyancy Affected Enclosure or Cavity Flows, ASME HTD-60,117,1986b. 

I. Catton, Natural convection in enclosures, Proc 6th Int Heat Transfer Conf, 6,13 Toronto, Canada, 1978. 

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