Natural heat convection

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

There are two basic types of watertube boilers assisted and natural circulation. Assisted circulation might apply where heat is from convection rather than a radiation source such as a waste heat application. Natural circulation is more suited where radiant heat and high gas temperatures are present. 

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. 

Convective heat exchange, natural or forced Radiation heat transfer, e.g. in furnaces Evaporation Condensation... 

Convective heat exchange, natural or forced Radiant heat transfer, e.g. furnaces Evaporation, e.g. in evaporators Condensation, e.g. in shell and tube heat exchanges Heat transfer to boiling liquids, e.g. in vaporizers, boilers, re-boilers ... 

Reviews covering ignition of cellulosic solids by Kanury (13), Beall and Eickner (15), Browne (3), and Matson et al. (14) report a wide range of ignition temperatures obtained on wood, and dependence on radiant or convective nature of heat. For radiant... 

Natural Convection. Natural convection arises from the differential thermal expansion of fluids at different temperatures. The resulting density differences give rise to buoyancy driven flows. Heat transfer coefficients would be expected to be a function of... 

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... 

Mixed Forced and Natural Convection Natural convection is commonly assisted or opposed by forced flow. These situations are discussed, e.g., by Mills (Heat Transfer, 2d ed., Prentice-Hall, 1999, p. 340) and Raithby and Hollands (Chap. 4 of Rohsenow, Hartnett, and Cho, Handbook of Heat Transfer, 3d ed., McGraw-Hill, 1998, p. 4.73). 

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. 

Oxidation tests have been carried out under xenon lamp heating and natural convection of air, where weight loss under constant temperature was monitored. The temperature was monitored by an infrared thermo-viewer and was calibrated with that of the tungsten-rhenium or platinum-rhodium thermo-couples. Temperature range examined was from 600 to 2300°C. Main features of this apparatus are that a large size specimen can be exposed and equilibrium constant temperature is rapidly attained (about 20s). 

Heat transfer to a laminar flow in an annulus is complicated by the fact that both the velocity and thermal profiles are simultaneously developing near the entrance and, often, over the length of the heated channel. Natural convection may also be a factor. It is usually conservative (i.e., predicted heat-transfer coefficients are lower than those experienced) to use equations for the fully developed flow. 

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 ... 

The first section presents some fundamental ideas that are frequently referred to in the remainder of the chapter. The next three sections deal with the major topics in natural convection. The first of these addresses problems of heat exchange between a body and an extensive quiescent ambient fluid, such as that depicted in Fig. 4.1a. Open cavity problems, such as natural convection in fin arrays or through cooling slots (Fig. 4.1fe), are considered next. The last major section deals with natural convection in enclosures, such as in the annulus between cylinders (Fig. 4.1c). The remaining sections present results for special topics including transient convection, natural convection with internal heat generation, mixed convection, and natural convection in porous media. 

FIGURE 15.46 Relative contributions of latent heat transport, natural convection, and enhanced convection in the saturated pool boiling of water at atmospheric pressure (from Paul et al. [97], with permission from ASME). 

Heat Transfer - The flow of heat from one area to another by conduction, convection, and/or radiation. Heat flows naturally from a warmer to a cooler material or space. 

There are two basic types of IR heaters electric IR and gas-fired IR (Figure 46.16). The most important element of the IR drying system is the IR emitter. These two types of IR heaters fit into three temperature ranges (1) 343°C-538°C (e.g gas and electric IR) (2) 538°C-1100°C (e.g gas and electric IR) and (3) 1100°C-2200°C (e.g., electric IR only). Electrically heated or natural-gas-heated units are used mainly to preheat the web, although these can be used in principle to carry out the entire drying process. IR temperatures are typically in the 650°C-1200°C range. Improved drying rates can be obtained by a combined radiation-convection system. 

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... 

We have to note that the calculated electric resistance of an electrodialysis compartment based on polarization curves is somewhat higher than experimental ones across all flow rates. This may be explained by influence of volume charge, concentration, heat, and natural convection on the polarization characteristic. 

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