Net radiant heat

Two parallel black plates 0.5 by 1.0 m are spaced 0.5 m apart. One plate is maintained at 1000°C and the other at 500°C. What is the net radiant heat exchange between the two plates ... 

Zone Method Let a zone of a furnace enclosure be an area small enough to make all elements of itself have substantially equivalent views of the rest of the enclosure. (In a furnace containing a symmetry plane, parts of a single zone would lie on either side of the plane.) Zones are of two classes source-sink surfaces, designated by numerical subscripts and having areas Ai, Aa,. . ., and emissivities El, and surfaces at which the net radiant-heat flux is zero (ful-... 

Solids radiate heat, even at low temperatures. The net radiant heat actually transferred to a receiver is the difference between radiant heat received from a souree and the radiant heat re-emitted from the receiver to the source. The net radiant heat flux between a hot body (heat source) and a cooler body (heat receiver) can be ealculated by any of the following Stefan-Boltzmann equations. 

A furnace has a 0.102-m diameter sight hole. If the furnace is at 482°C and the surroundings are at 26.7°C, what is the net radiant heat loss from the hole Assume blackbody behavior. 

When a combustible material is exposed to an external radiant heat source, its surface temperature starts to rise. The temperature inside the solid also increases with time, but at a slower rate. Provided the net heat flux into the material is sufficiently high, the surface temperature eventually reaches a level at which thermal decomposition begins. The fuel gases and vapors generated emerge through the exposed surface and mix with air in the gas phase. Under certain conditions, this mixture exceeds the lower flammability limit and ignites. 

Some specific studies on the measurement of heat losses and indoor temperatures in buildings deserve attention. In his review of the relative importance of thermal comfort in buildings, McIntyre considered that the mean radiant temperature was the most important parameter, followed closely by the "radiation vector," which is defined as the net radiant flux density vector at a given point and measures the asymmetry of the thermal radiation field in a room (97). Benzinger et al. characterized the mean radiant temperature, and asymmetric radiation fields, using a scanning plane radiometer, which maps the plane radiant temperature in a given space indoors (98). 

A simple radiation problem is encountered when we have a heat transfer surface at temperature T, completely enclosed by a much larger surface maintained at T2. We will show in Chap. 8 that the net radiant exchange in this case can be calculated with... 

The engineer is usually interested in the net rate of heat interchange between two bodies. Some of the radiated energy indicated by Eq. (5) may be returned to the source by reflection from the receiver, and the receiver, of course, emits radiant energy which can be partly or completely absorbed by the source. Equation (5), therefore, must be modified to obtain the net rate of radiant heat exchange between two bodies. The general steady-state equation is... 

The radiative source term is a discretized formulation of the net radiant absorption for each volume zone which may be incorporated as a source term into numerical approximations for the generalized energy equation. As such, it permits formulation of energy balances on each zone that may include conductive and convective heat transfer. For K—> 0, GS —> 0, and GG —> 0 leading to S —> On. When K 0 and S = 0N, the gas is said to be in a state of radiative equilibrium. In the notation usually associated with the discrete ordinate (DO) and finite volume (FV) methods, see Modest (op. cit., Chap. 16), one would write S /V, = K[G - 4- g] = Here H. = G/4 is the average flux... 

The radiative source term is a discretized formulation of the net radiant absorption for each volume zone which may be incorporated as a source term into numerical approximations for the generalized energy equation. As such, it permits formulation of energy balances on each zone that may include conductive and convective heat transfer. Eor K—> 0, GS —> 0, and GG —> 0 leading to S —> On. When and... 

A principal factor governing the operating cycle of ethylene steam crackers (ESC) is coke formation on the inside surfaces of the radiantly heated pyrolysis tubes. Steam is used as the carrier for the hydrocarbon feedstock as it is known empirically to minimise this coking. It is probable that the observed deposition is a net process representing the difference between formation and removal, primarily by thermal oxidation. A fundamental requirement of any detailed understanding of the overall processes involved, therefore, is knowledge of the oxidation behaviour of such deposits. Although several studies have been undertaken on various carbons considered to simulate ESC pyrolysis tube coke (e.g. ( )) no relevant information has been published for plant material. To provide these data, therefore, the oxidation behaviour of a coke formed on an ESC tube has now been examined in water vapour. 

Carbonaceous deposition during steam cracking is the net result of steady state formation and removal processes. If the measured oxidation rates in water vapour did represent the removal of the deposit in situ, then this would be an extremely rapid process over the temperature range at which deposition on radiantly heated process tubes is most significant. Thus, 1 mm thickness of deposit would be oxidised by 362 mm Hg water partial pressure in 300 h at 800°C, 33 h at 900°C and 5 h at 1000°C. If a hydrocarbon, or its decomposition products, enhanced the oxidation rate these times could be decreased. Coke removal by thermal oxidation cannot be ignored, therefore, although its extent would depend on specific plant operating conditions. 

In this equation, q",.dA is the net radiative heat flux at the moving material surface imposed by external sources such as radiant burners/heaters or electric resistance heaters. Both parabolic, boundary layer [80], and full, elliptic [61,81] problem solutions have been reported. Because of the nature of the problem, the heat transfer results can t be given in terms of correlations. The interested reader is referred to Refs. 62 and 79 for citation of relevant references. 

The heating in the bottom of cells A and B is driven by radiant energy from the flame, as demonstrated by a plot of the incident radiant flux in Figure 11.20. However, from this plot it can be deduced that peak heating near the center of the tube bank is not caused by radiation. It must, instead, be the result of convection. The net convective heat flux plot and the isosurface plot... 

The prereformer reactor is shown in Figure 5 [13]. An additiona] coil is added to the fired reformer to reheat the prereformer product to the inlet temperature of the primary reformer. This additional radiant heat in the primary reformer needed to reheat the prereformer product is supplied at the expense of excess steam generation. The net result is an overall improvement in plant efficiency. 

For a particular gas, the width of the absorption or emission bands depends on the pressure and also the temperature. If an absorbing gas is heated, it radiates energy to the cooler surroundings. The net radiation heat-transfer rate between surfaces is decreased in these cases because the gas absorbs some of the radiant energy being transported between the surfaces. 

Although the WBGT index considers the portion of heat stress that results from the net effects of dry air temperature, radiant heat transfer, and humidity, it does not sufficiently reflect the air move-... 

Example 5 Radiation in a Furnace Chamber A furnace chamber of rectangular paraUelepipedal form is heated hy the combustion of gas inside vertical radiant tubes hningthe sidewalls. The tubes are of 0.127-m (5-in) outside diameter on 0.305-m (12-in) centers. The stock forms a continuous plane on the hearth. Roof and end walls are refractory. Dimensions are shown in Fig. 5-20. The radiant tubes and stock are gray bodies having emissivities of 0.8 and 0.9 respectively. What is the net rate of heat transmission to the stock by radiation when the mean temperature of the tube surface is SIG C (1500 F) and that of the stock is 649 C (1200 F) ... 

There are several measure sets with immediate payback SE, SF, SH, SI. Measures SE and SF are radiant panel systems with displacement ventilation. These systems have a similar cost to the base case, but they offer energy savings. Furthermore, significant sizing reductions, mainly in the cooling tower and chiller sizes, offset the incremental cost of the envelope and heat recovery measures. Because the elevator efficiency measures offer a net savings in capital cost, the capital cost of the other measures is further offset. 

Two 10 by 30 cm rectangular plates are spaced 10 cm apart and connected by four insulated and re-radiating walls. The plate temperatures are uniform at 1000 and 300°C, and their emissivities are 0.6 and 0.4, respectively. Using the numerical method, determine the net heat transfer under the assumptions that (a) the four re-radiating surfaces act as one surface and have uniform radiosity and (b) the four re-radiating surfaces have radiosities determined from the radiant balance with all other surfaces. Assume that the 1000 and 300°C surfaces have uniform radiosity. Also calculate the temperatures for the re-radiating surfaces for each case above. 

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