Natural convection to air

TABLE 11.9 Constants for Average Nusselt Numbers for Natural Convection and Simplified Equations for Average Heat Transfer Coefficients he (W/m -degree) for Natural Convection to Air over External Flat Surfaces (at... 

Let us just consider the piloted ignition case. Then, at Tpy a sufficient fuel mass flux is released at the surface. Under typical fire conditions, the fuel vapor will diffuse by turbulent natural convection to meet incoming air within the boundary layer. This will take some increment of time to reach the pilot, whereby the surface temperature has continued to rise. 

The experimental setup, described in Example 8.1, for calculating the bias in a dynamic environment will be used here to discuss the parameter estimation methodology. In this case both the surface heat transfer coefficient (h) and the thermal conductivity (A) of the body in the condition of natural convection in air are considered (Bortolotto et al., 1985). 

This burner, a butane lighter, a candle, a burning log, and a match (after ignition) are aU examples of diffusion flames where one generally provides the fuel and relies on natural convection of air to provide the oxidant... 

Removal of heat from the containers is of prime concern in designing such a facility. Several options are being considered, including cooled water basin, natural convection in air, and forced convection in air. Particular attention will be given to designing a system that will continue to maintain container temperature at acceptable levels even in the event of postulated accidents or temporary abandonment of the facilities under emergency conditions. 

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). 

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 ... 

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. 

M. K. El-Riedy, Analogy Between Heat and Mass Transfer by Natural Convection From Air to Horizontal Tubes, Int. J. Heat Mass Transfer (24) 365-369,1981. 

The CERTI concept corresponds to a hillside storage in a geological medium consisting of hard rock (Motuelle, 2000). A series of wells are filled with waste containers, around which a flow of air is induced by natural convection to cool the containers (see fig.l). 

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... 

Perhaps most ingenuity has been exercised in the provision of heat removal systems from the containment. Heat can be removed either through the walls of the containment itself or by means of a heat exchanger arrangement. An example would have condensing surfaces within the containment, to condense steam released from the reactor pressure vessel, and cooled by water circulating by natural convection to an air cooler. The latter system allows the use of a full double containment as is required in some countries. 

The heat transfer to air depends on the temperature difference, the surface of the resistor, and the speed and direction of the air flow. Air flow along the resistor may be due to external causes but it can also be caused by natural convection. The air that is heated by the resistor becomes lighter than the surrounding air and, therefore, it tends to rise. The actual formulas for the transfer of heat to air are complicated as they depend strongly on the geometry of the resistor and its surroundings. 

In case of a failure of air circulators in the secondary circuit, a relative share of the heat removed directly from the vessel by natural convection of air through the gap between the walls of the double vessel will increase, while the gas turbine circuit would remove the remaining heat, owing to the natural circulation of air. 

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. 

The secondary cooling system Under normal operating conditions three secondary coolant loops transfer decay heat to the steam generators by forced convection, which are cooled by natural convection of air within their compartments. Just one of the three loops is sufficient for the decay heat removal function. 

Decay heat removal s stem - 4 X 8 MWt Na/Na HX Natural convection to atmospheric air... 

The heat pipe has properties of iaterest to equipmeat desigaers. Oae is the teadeacy to assume a aeady isothermal coaditioa while carrying useful quantities of thermal power. A typical heat pipe may require as Htfle as one thousandth the temperature differential needed by a copper rod to transfer a given amount of power between two poiats. Eor example, whea a heat pipe and a copper rod of the same diameter and length are heated to the same iaput temperature (ca 750°C) and allowed to dissipate the power ia the air by radiatioa and natural convection, the temperature differential along the rod is 27°C and the power flow is 75 W. The heat pipe temperature differential was less than 1°C the power was 300 W. That is, the ratio of effective thermal conductance is ca 1200 1. 

Open Tube Sections (Air Cooled) Plain or finned tubes No shell required, only end heaters similar to water units. Condensing, high level heat transfer. Transfer coefficient is low, if natural convection circulation, but is improved with forced air flow across tubes. 0.8-1.8... 

The fan drive components are less accessible for maintenance, which may have to be done in the hot air generated by natural convection. 

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