Radiation heat transfer shields

Now consider a radiation shield placed between these two plates, as shown in Fig. 13-30. Let the emissivities of the shield facing plates 1 and 2 be 3 j and E, 2, respectively. Note that the emissivity of different surfaces of the shield may be different. The radiation network of this geometry is constructed, as usual, by drawing a surface resistance associated with each surface and connecting these surface resistances with space resistances, as shown in the figure. The resistances are connected in series, and thus the rate of radiation heat transfer is... 

Therefore, when all emissivities are equal, 1 shield reduces the rale of radiation heat transfer to one-half, 9 shields reduce it to one-ienlh, and 19 shields reduce it to onc-twenlielh (or 5 percent) of whal it was when there were no shields. 

Radiation shields used to reduce the rate of radiation heat transfer between concentric cylinders and spheres can be handled in a similar manner. In case of oue shield, Eq. 13 42 can be used by taking 13 21 - 1 for both cases and by replacing lhe<4 s by the proper area relations. 

A thin aluminum sheet with an emissivity of 0.1 on both sides is placed between two very large parallel plates that are maintained at uniform temperatures Ti = 800 K and T2 = 500 K and have emissivities ci = 0.2 and et 0.7, respectively, as shown in Fig. 13-32. Determine the net rate of radiation heat transfer between the two plates per unit surface area of the plates and compare the.result to that without the shield. 

SOLUTION A thin aluminum sheet is placed betvreen tvro large parallel plates maintained at uniform temperatures. The net rates of radiation heat transfer between the two plates with and without the radiation shield are to be determined. 

Analysis The net rate of radiation heat transfer between these two plates without the shield was determined in Example 13-7 to be 3625 W/m. Heat transfer in the presence of one shield is determined from Eq. 13-43 to be... 

Discussion Note that the rate of radiation heat transfer reduces to about one-fourth of v/hat it was as a result of placing a radiation shield between.the two parallel plates. 

Radiation heat transfer between two surfaces can be reduced greatly by inserting belsveen the two siirface.s thin, high-reflectivity (low emissivity) sheets of material called radiation shields. Radiation heat transfer between two large parallel plates separated by N radiation shields is... 

Two parallel disks of diameter D = 1 m separated by E = 0.6 m are located directly on top of each other. The disks are separated by a radiation shield whose emissivity is 0.15. Both disks arc black and are maintained at temperatures of 650 K and 400 K, respectively. The environment that the disks are in can be considered to be a blackbody at 300 K. Determine the net rate of radiation heat transfer through the shield under steady conditions. Ansmt 268 W... 

Two coaxial cylindei.s of diameters D = O.IO 111 and O3 = 0.30 in and emissivities e, = 0.7 and 63 = 0.4 aie maintained at uniform temperatures of T = 750 K and = 500 K, respectively. Now a coaxial radiation shield of diameter D3 = 0.20 m and emissivity 63 = 0.2 is placed between tlie two cylinders. Determine the net rate of radiation heat transfer between the two cylinders per unit length of the cylinders and compare the result with that without the shield. 

Two thin radiation shields with emissivilies of Cj = 0.10 and = 0.15 on both sides are placed between two very large parallel plates, which arc maintained at uniform temperatures 7 = 600 K and Tj = 300 K and have cmissivities e, = 0.6 and 62 — 0.7, respectively. Deteiinine the net rates of radiation heat transfer between the two plates with and without (he shields per unit surface area of the plates, and the temperatures of the radiation shields in steady operation. 

Radiation shields [90] can be used to isolate a probe from a distant medium so that there will be relatively little radiation heat transfer to it at the same time, they do not interfere with good thermal contact between the probe and the surrounding fluid. Designs of thermometer probes for gas temperature measurement are described in Refs. 91 and 92. Analyses to account for some uncertainties in probe measurement can be found in Ref. 93. 

The effect of test chamber wrapping and the presence of CO2 on the heat transfer to the calorimeter at 76°K is shown in Fig. 5. The curves are independent of pressure where radiation heat transfer is dominant, and then break upward to become a function of pressure when gaseous conduction is appreciable. The single-wrap insulation acts as a semifloating radiation shield, with a corresponding decrease in unit heat transfer. Figure 5 shows that the solid CO2 has no effect on the heat transfer and that the foil surface has not been blackened by solid CO2. 

The heat transport due to conduction and that due to radiation are not readily separable from the experimental data. Curve A of Fig. 4 shows the measured temperature distribution through a typical sample containing 29 shields per inch. Curve B shows the temperature distribution expected if each sheet of aluminum foil were a floating radiation shield. These results were obtained from Fig. 1. Curve C shows the temperature distribution througji an ideal sample, whose thermal conductivity would be independent of temperature. The observed result is probably a combination of radiation heat transfer and the change in thermal conductivity of the insulation with temperature. The thermal conductivity of most disordered dielectrics is approximately proportional to the first power of the temperature, but the temperature dependence of multiple contacts is not well understood. The fact that the temperature distribution for a sample of this type can be accounted for by a temperature-dependent thermal conductivity is sufficient justification for using Eq. (3), a particular solution of the Fourier equation, rather than Eq. (1), the heat flux equation for radiant heat transport, to represent our results. 

Vacuum Radiation Furnaces. Vacuum furnaces are used where the work can be satisfactorily processed only in a vacuum or in a protective atmosphere. Most vacuum furnaces use molybdenum heating elements. Because all heat transfer is by radiation, metal radiation shields ate used to reduce heat transfer to the furnace casing. The casing is water-cooled and a sufficient number of radiation shields between the inner cavity and the casing reduce the heat flow to the casing to a reasonable level. These shields are substitutes for the insulating refractories used in other furnaces. 

The rate of heat transfer by radiation between two surfaces may be reduced by inserting a shield, so that radiation from surface 1 does not fall directly on surface 2, but instead is intercepted by the shield at a temperature Tsh (where 7, > T,h > T2) which then reradiates to surface 2. An important application of this principle is in a furnace where it is necessary to protect the walls from high-temperature radiation. 

Neglecting any temperature drop across the shield (which has a surface emissivity esh), then in the steady state, the transfer rate of radiant heat to the shield from the surface 1 must equal the rate at which heat is radiated from the shield to surface 2. 

In practice, as a result of introducing the radiation shield, the temperature T2 will fall because a heat balance must hold for surface 2, and the heat transfer rate from it to the surroundings will have been reduced to q h. The extent to which 72 is reduced depends on the heat transfer coefficient between surface 2 and the surroundings. 

After the flue gas leaves the combustion chamber, most furnace designs extract further heat from the flue gas in horizontal banks of tubes in a convection section, before the flue gas is vented to the atmosphere. The temperature of the flue gases at the exit of the radiant section is usually in the range 700 to 900°C. The first few rows of tubes at the exit of the radiant section are plain tubes, known as shock tubes or shield tubes. These tubes need to be robust enough to be able to withstand high temperatures and receive significant radiant heat from the radiant section. Heat transfer to the shock tubes is both by radiation and by convection. After the shock tubes, the hot flue gases flow across banks of tubes that usually have extended surfaces to increase the rate of heat transfer to the flue gas. The heat transferred in the radiant section will usually be between 50 and 70% of the total heat transferred. 

Three zones arc identified in a typical heater such as that of Figure 8.19(a). In the radiant zone, heat transfer is predominantly (about 90%) by radiation. The convection zone is out of sight of the burners although some transfer occurs by radiation because the temperature still is high enough, most of the transfer here is by convection. The application of extended surfaces permits attainment of heat fluxes per unit of bare surface comparable to those in the radiant zone. Shield section is the name given to the first two rows or so leading into the convection section. On balance these tubes receive approximately the same heat flux as the radiant... 

One way of reducing radiant heat transfer betwen two particular surfaces is to use materials which are highly reflective. An alternative method is to use radiation shields between the heat-exchange surfaces. These shields do not deliver or remove any heat from the overall system they only place another resistance in the heat-flow path so that the overall heat transfer is retarded. Consider the two parallel infinite planes shown in Fig. 8-30a. We have shown that the heat exchange between these surfaces may be calculated with Eq. (8-42). Now consider the same two planes, but with a radiation shield placed between them, as in Fig. 8-306. The heat transfer will be calculated for this latter case and compared with the heat transfer without the shield. 

Multiple-radiation-shield problems may be treated in the same manner as that outlined above. When the emissivities of all surfaces are different, the overall heat transfer may be calculated most easily by using a series radiation network with the appropriate number of elements, similar to the one in Fig. 

If the emissivities of all surfaces are equal, a rather simple relation may be derived for the heat transfer when the surfaces may be considered as infinite parallel planes. Let the number of shields be n. Considering the radiation network for the system, all the surface resistances would be the same since the emissivities are equal. There would be two of these resistances for each shield and one for each heat-transfer surface. There would be n + I space resistances, and these would all be unity since the radiation shape factors are unity for the infinite parallel planes. The total resistance in the network would thus be... 

Two very large parallel planes with emissivities 0.3 and 0.8 exchange heat. Find the percentage reduction in heat transfer when a polished-aluminum radiation shield ( = 0.04) is placed between them. 

Two large parallel planes having emissivities of 0.3 and 0.5 are maintained at temperatures of 800 K and 400 K, respectively. A radiation shield having an emissivity of 0.05 on both sides is placed between the two planes. Calculate (a) the heat-transfer rate per unit area if the shield were not present, (b) the heat-transfer rate per unit area with the shield present, and (c) the temperature of the shield. 

A long cylindrical heater 2.5 cm in diameter is maintained at 650°C and has a surface emissivity of 0.8. The heater is located in a large room whose walls are at 25°C. How much will the radiant heat transfer from the heater be reduced if it is surrounded by a 30-cm-diameter radiation shield of aluminum having an emissivity of 0.2 What is the temperature of the shield ... 

Two long concentric cylinders have diameters of 4 and 8 cm, respectively. The inside cylinder is at 800°C and the outer cylinder is at 100°C. The inside and outside emissivities are 0.8 and 0.4, respectively. Calculate the percent reduction in heat transfer if a cylindrical radiation shield having a diameter of 6 cm and emissivity of 0.3 is placed between the two cylinders. 

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