Heat by natural convection

Fig. 14. (a) Workpiece heated by natural convection (b) workpiece heated by forced convection (80). 

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. 

A horizontal cylinder having a diameter of 5 cm and an emissivity of 0.5 is placed in a large room, the walls of which are maintained at 35°C. The cylinder loses heat by natural convection with an h of 6.5 W/m2 °C. A sensitive thermocouple placed on the surface of the cylinder measures the temperature as 30°C. What is the temperature of the air in the room ... 

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... 

If we analyze the case of the rectification column which loses heat by natural convection, then we change the list of variables by considering the specific ascension force, gPtAt, as an important variable and by removing the fluid velocity, w. In this case, the application of the pi theorem shows that ... 

Therefore, this heat sink can dissipate heat by natural convection at a rate of 29.4 W. 

A large tank of water is heated by natural convection from submerged horizontal steam pipes. The pipes are 3-in Schedule 40 steel. When the steam pressure is atmospheric and the water temperature is 80°F, what is the rate of heat transfer to the water in Btu per hour per foot of pipe length ... 

Sodium flows through the shell, which is not insulated to facilitate removal of decay heat by natural convection of air in the casing in case of ofif-site power failure/station blackout. SG is a critical equipment in fast reactors due to highly reactive nature of sodium when it comes in contact with feed water/steam in case of a SG tube leak. The tube volume is minimized to... 

NCM - removal of decay heat by natural convection, through main coolant loops... 

Decay heat removal loops. The normal route for the removal of decay heat in a fast reactor is via the secondary sodium circuits and the steam plant. Should this route not be available, decay heat in PFR could be rejected by one or more of three thermal syphon loops, each filled with eutectic sodium/potassium alloy. Each loop extracted heat through an immersed coil, intercq>ting some of the primary sodium as it flowed from the core towards an intermediate heat exchanger, and delivered the heat by natural convection to the outside atmosphere through a sodium-potassium/air heat exchanger built into the wall of the secondary containment building. 

This shutdown was preceded by two safety tests in September which allowed, on the one hand, to confirm the ability of Superphenix to evacuate decay heat by natural convection of the primary circuit and the secondary loops after reactor shutdown, and on the other hand, to demonstrate the absence of risk of a reactivity accident, caused by a passage of argon gas in the core following depressurization of an intermediate heat exchanger argon-filled bell. 

Another possible transient is that due to a loss of power to all the gas blowers, causing complete loss of forced circulation. In this case, natural convection should be adequate to remove the decay heat. Heat removal is aided by the large thermal capacity of the graphite, which acts as a heat sink for the fuel. In the event of a loss of circulation coupled with a depressurization accident, the heat removal capacity of the low-pressure gas is no longer adequate to remove heat by natural convection, and in this case the forced circulation must be restored. The slow depressurization rate of the PCPV allows a reasonable time margin for the institution of emergency cooling. 

Metzner and Friend [Ind. Fng. Chem., 51, 879 (1959)] present relationships for turbulent heat transfer with nonnewtouiau fluids. Relationships for heat transfer by natural convection and through laminar boundaiy layers are available in Skelland s book (op. cit.). 

For example, vaporization may occur as a result of heat absorbed, by radiation and convection, at the surface of a pool of hquid or as a result of heat absorbed by natural convect ion from a hot wall beneath the disengaging surface, in which case the vaporization takes place when the superheated liquid reaches the pool surface. Vaporization also occurs from falling films (the reverse or condensation) or from the flashing of hquids superheated by forced convec tion under pressure. 

TABLE 10.7 Heat Loss Coefficients (h by Natural Convection ... 

Elsherbiny, S. M. Raithby, G. D. and Hollands, K. B. T. (1982). Heat Transfer by Natural Convection Across Vertical and Inclined Air Layers. ASME Transactions 204 96. 

The small condensing surface required by a domestic appliance such as a deep-freeze may allow the use of the outside metal skin of the appliance itself as a surface condenser. In such a construction, the condenser tube is held in close mechanical contact with the skin, so that heat is conducted through to the outside air, where it is lost by natural convection. This system is restricted to a few hundred watts. 

Gr Grashof l3p2flgAT P2 Heat transfer by natural convection... 

In interface evaporation, the bubbles of vapour formed on the heated surface move to the vapour-liquid interface by natural convection and exert very little agitation on the liquid. The results are given by ... 

The differential equation describing the temperature distribution as a function of time and space is subject to several constraints that control the final temperature function. Heat loss from the exterior of the barrel was by natural convection, so a heat transfer coefficient correlation (2) was used for convection from horizontal cylinders. The ends of the cylinder were assumed to be insulated. The equations describing these conditions are ... 

The mechanism of heat transfer from a submerged surface to a pool of liquid depends on the temperature difference between the heated surface and the liquid Figure 12.54. At low-temperature differences, when the liquid is below its boiling point, heat is transferred by natural convection. As the surface temperature is raised incipient boiling occurs, vapour... 

For simple jackets without baffles, heat transfer will be mainly by natural convection and the heat transfer coefficient will range from 200 to 400 Wm 2oC 1. 

Consider the combustion of a coal pile of diameter D and height H. Before ignition the air flow is driven by wind, and after ignition by natural convection. The heat of combustion of carbon is -94 kcal/mole. Set up a qualitative energy balance (no numbers) to explain the dependence of combustion on weather and on D and H. What size and shape of coal piles do you recommend to minimize spontaneous combustion ... 

Coefficients of heat transfer by natural convection from bodies of various shapes, chiefly plates and cylinders, are correlated in terms of Grashof, Prandtl, and Nusselt numbers. Table 8.9 covers the... 

Industrial reactors are thermally insulated for safety reasons (hot surfaces) and for economical reasons (heat losses). Nevertheless, at higher temperatures, heat losses may become important. Their calculation may become tedious, since heat losses are often due to a combination of losses by radiation and by natural convection. If an estimation is required, a simplified expression using a global overall heat transfer coefficient (a) may be useful ... 

Agitation of the reaction mass may also be critical in such a situation without agitation, cooling being provided by natural convection only, leads to a considerable reduction of the heat transfer coefficients (see Section 9.3.5). Generally, by natural convection, the heat transfer coefficient is reduced to 10% of its value with stirring [14], Nevertheless, this is only valid when natural convection is established, that is, for smaller vessels and contents with moderate viscosity (see Section... 

Both rooms had heating furnaces that needed to be fired from the vestibule, which, according to Pressac, was allegedly also used as a gas chamber (for the heating, see Fig. 56). No ventilation installation is known to have existed. Pressac assumes ventilation by natural convection.279 Franciszek Piper, Director of the Auschwitz Museum agrees 164... 

The temperature of the tube ends of the upper insulation module was higher than that of the tube ends in the lower insulation module, which is a result of additional heat transfer from the reaction zone by natural convection in the upper module. In both cases, the heat loss is sufficient to prevent the rotary valves next to the heat exchanger modules from being damaged by overheating. 

What would be the cost of steam saved by coating the pipe with a 50 mm thickness of 85% magnesia lagging of thermal conductivity 0.07 W/m K, if steam costs 0.5/100 kg The emissivity of both the surface of the bare pipe and the lagging may be taken as 0.85, and the coefficient h for the heat loss by natural convection is given by ... 

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