Radiation combined with conduction-convection

Radiation may be important in heat transfer to outside tube surfaces. Inside tubes, the surface cannot see surfaces other than the inside wall of the same tube, and heat flow by radiation does not occur. Outside tube surfaces, however, are necessarily in sight of external surfaces, if not nearby, at least at a distance, and the surrounding surfaces may be appreciably hotter or cooler than the tube wall. Heat flow by radiation, especially when the fluid is a gas, is appreciable in comparison with heat flow by conduction and convection. The total heat flow is then a sura of two independent flows, one by radiation and the other by conduction and convection. The relations given in the remainder of this section have to do with conduction and convection only. Radiation, as such and in combination with conduction and convection, is discussed in Chap. 14. 

Finally, conductive heat protection is required for textiles that may come into direct contact with a heat source other than a flame. Major threats here include those met by metal industrial workers, who risk contact with hot metal tool handles and molten metal splashes. For this reason, not only must the thermal insulating characteristics of the textile be paramount, but also surfaces which minimise contact, for instance by resisting wetting by molten metals, must be considered. However, in many thermally hazardous environments, a combination of conduction, convection (or flame) and radiation may be operating in concert, and usually the last two are associated with flame sources in particular. 

In any operation in which a material undergoes a change of phase, provision must be made for the addition or removal of heat to provide For the latent heat of the change of phase plus any other sensible heating or cooling that occurs in the process. Heat may be transferred by any one or a combination of the three modes—conduction, convection, and radiation. The process involving change of phase involves mass transfer simultaneous with heat transfer. 

Contact temperature measurement is based on a sensor or a probe, which is in direct contact with the fluid or material. A basic factor to understand is that in using the contact measurement principle, the result of measurement is the temperature of the measurement sensor itself. In unfavorable situations, the sensor temperature is not necessarily close to the fluid or material temperature, which is the point of interest. The reason for this is that the sensor usually has a heat transfer connection with other surrounding temperatures by radiation, conduction, or convection, or a combination of these. As a consequence, heat flow to or from the sensor will influence the sensor temperature. The sensor temperature will stabilize to a level different from the measured medium temperature. The expressions radiation error and conduction error relate to the mode of heat transfer involved. Careful planning of the measurements will assist in avoiding these errors. 

Engineering thermal design of heat transfer equipment is concerned with heat flow mechanisms of the following three types—simply or in combination (1) conduction, (2) convection, and (3) radiation. Shell and tube exchangers are concerned primarily with convection and conduction whereas heaters and furnaces involve convection and radiation. 

Extensive experimental determinations of overall heat transfer coefficients over packed reactor tubes suitable for selective oxidation are presented. The scope of the experiments covers the effects of tube diameter, coolant temperature, air mass velocity, packing size, shape and thermal conductivity. Various predictive models of heat transfer in packed beds are tested with the data. The best results (to within 10%) are obtained from a recently developed two-phase continuum model, incorporating combined conduction, convection and radiation, the latter being found to be significant under commercial operating conditions. 

If heat is transferred solely by convection from a hot gas, the surface temperature of the granulation will approach or reach the wet-bulb temperature. If conduction and/or radiation contribute to heat transfer, the surface temperature of the material will reach a temperature between the wet-bulb temperature and the boiling point of the liquid. Thus, combining convection with conduction and/or radiation may allow an increase in the rate of heat transfer (with a resultant higher drying rate) provided that the vapor pressure gradients can be maintained. 

The present section deals with a number of examples combining radiation with conduction and/or convection. Most problems involving more than one mode of heat transfer are relatively involved, as they yield nonlinear differential equations and/or boundary conditions whenever radiation is included. They are usually solved after a linearization of the Stefan-Boltzmann law. During this process, however, the quantitative nature of a problem gets lost. 

There are many applications where radiation is combined with other modes of heat transfer, and the solution of such problems can often be simplified by using a thermal resistance Rq, for radiation. The definition of Rth is similar to that of the thermal resistance for convection and conduction. If the heat transfer by radiation, for the example in Fig. 1.10, is written... 

In drying, the heat of phase transformation, q, must be delivered by conduction, convection, or radiation. Sometimes combinations of these possibilities are used. If the heat is supplied by conduction, this is called contact drying. The drying good either rests on a heated surface or is moved over it. The heat flux may be calculated with the following equation ... 

Conduction usually occurs in conjunction with convection, and if the temperatures are high, they also occur with radiation. In some practical situations where radiation cannot be readily estimated, convection heat transfer coefficients can be enhanced to include the effect of radiation. Combined conduction and convection led to the concept of thermal resistances, analogous to electrical resistances, which can be solved similarly. 

A building s flat black roof has an emissivity of 0.9, along with an absorptivity of 0.8 for solar radiation. The sun s energy transfer is 946 W/m. The temperature of the air and surroundings is 26.7°C. Combined conduction-convection heat transfer is given by q/A — 0.38(A7 ) where the AT is the difference between the roof and the air. Find the roof temperature (assume that the blackbody temperature of space is —70°C). 

The slow, stable response of the GT-MHR to internally or externally initiated transients, in combination with passive safety features, provides a strong defence against internal or external threats to the plant. These characteristics arise from the inert coolant, large reactor heat capacity, low power density, strong negative reactivity feedback coefficients and passive decay heat removal by conduction, radiation and convection. 

The first term on the right side of Eq. (5-179) is so nearly dominant for most furnaces that consideration of the main features of chamber performance is clarified by ignoring the loss terms and Lr or by assuming that they and have a constant mean value. The relation of a modified chamber efficiency T g(1 o) lo modified firing density D/(l — and to the normahzed sink temperature T = T-[/Tp is shown in Fig. 5-23, which is based on Eq. (5-178), with the radiative and convective transfer terms (GSi)/ja(TG — T ) -i- hiAijTc Ti) replaced by a combined radiation/conduction term (GS,) ,a(T - T ). where (GS])/ = (GS])/ + /jiA]/4oTgi Tg is adequately approximated by the arithmetic mean of Tg and T. ... 

Consider a 3-m-diameter spherical tank that is initially filled with liquid nitrogen at 1 atm and I96°C. The tank is exposed to ambient air at I5°(. with a combined convection and radiation heal transfer coefficient of 35 W/m °C. The temperature of the thin-shellcd spherical tank is observed to be almost the same as the temperature of the nitrogen inside. Determine the rate of evaporation of the liquid nitrogen in the tank as a result of the he.ii transfer from the ambient air if the tank is (<7) not insulated, h) insulated with 5-cm-thick fiberglass insulation k = 0.035 W/m C), and (c) insulated with 2-cm-lhick superinsulation which has an effective thermal conductivity of 0.00005 W/m C. 

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