Heat transfer radiant

There is ordinarily no measurable convection in cells of diameter less than about 4 mm (143). Theoretical arguments have been in general agreement with this work (151,191). Since most available cellular polymers have cell diameters smaller than 4 mm, convection heat transfer can be ignored with good justification. Studies of radiant heat transfer through cellular polymers have been made (143,151,191,196,197). 

The variation in total thermal conductivity with density has the same general nature for ah. cellular polymers (143,189). The increase in at low densities is owing to an increased radiant heat transfer the rise at high densities to an increasing contribution of k. ... 

Another design, shown ia Figure 5, functions similarly but all components are iaside the furnace. An internal fan moves air (or a protective atmosphere) down past the heating elements located between the sidewalls and baffle, under the hearth, up past the work and back iato the fan suction. Depending on the specific application, the flow direction may be reversed if a propeUer-type fan is used. This design eliminates floorspace requirements and eliminates added heat losses of the external system but requires careful design to prevent radiant heat transfer to the work. 

Simultaneous Loss by Radiation The heat transferred by radiation is often of significant magnitude in the loss of heat from surfaces to the surroundings because of the diathermanous nature of atmospheric gases (air). It is convenient to represent radiant-heat transfer, for this case, as a radiation film coefficient which is added to the film coefficient for convection, giving the combined coefficient for convection and radiation (h + hf In Fig. 5-7 values of the film coefficient for radiation are plotted against the two surface temperatures for emissivity = 1.0. 

In essence, one properly located low-emissivity shield can reduce the radiant heat transfer to around one-half of the rate without the shield, two shields can reduce this to around one-fourth of the rate without the shield, and so on. 

A horizontally fired burner is located at one end of the heater. The flame extends along the central longitudinal axis of the heater. In this way the wickets are exposed to the open flame and can be subjected to a maximum rate of radiant heat transfer. The tubes should be sufficiently far away from the flame to prevent hot spots or flame pinching. 

It should be noted that radiant heat transfer, which can be an important part of the heat flow in a building, has not been considered. 

Emissivity (e) The ability of a surface to emit radiant heat transfer. 

Radiant heat transfer had historically been the biggest heat transfer mechanism for windows. Low-e materials were developed and have historically been used to control for heat transfer. An example of the popularity of using metals to reflect heat to control for radiant heat transfer is the thermos bottle. Applying that new technology to windows, and getting materials that normally would affect transparency of the product to remain visually neutral, was a huge advance to the industry. 

PF burners and fluid beds best meet requirements for dual- and triple-fuel firing including solid fuel as one option. PF burners are particularly suitable, as no static grate exists to compromise the design. They also have a combustion geometry which is similar to gas and oil, and therefore the flame can be arranged to allow full development of flame shape and maximum radiant heat transfer surface utilization. 

Finally, there is the transfer of sensible heat from the bulk of the water to the surface area. This is so slight in terms of resistance that it is normally neglected. Radiant heat transfer is also ignored for all practical design purposes. 

If radiant heat transfer is taking place between two black surfaces. 1 and 2, then ... 

A flat-bottomed cylindrical vessel, 2 m in diameter, containing boiling water at 373 K, is mounted on a cylindrical section of insulating material, l m deep and 2 m ID at the base of which is a radiant heater, also 2 m in diameter, with a surface temperature of 1500 K. If the vessel base and the heater surfaces may be regarded as black bodies and conduction though the insulation is negligible, what is the rate of radiant heat transfer to the vessel How would this be affected if the insulation were removed so that the system was open to the surroundings at 290 K ... 

What do you understand by the terms black body and grey body when applied to radiant heat transfer Two large, parallel plates with grey surfaces are situated 75 mm apart one has an emissivity of 0.8 and is at a... 

Derive an expression for the radiant heat transfer rate per unit area between two large parallel planes of emissivities e and en and at absolute temperatures T and 73 respectively. 

The wall of the duct and the gas stream are at somewhat different temperatures. If the heat transfer coefficient for radiant heat transfer from the wall to the thermometer remains constant, and the heat transfer coefficient between the gas stream and thermometer is proportional to the 0.8 power of the velocity, what is the true temperature of the air system Neglect any other forms of heat transfer. 

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 ... 

Radiant heat transfer from a surface is governed by the Stefan-Boltzman equation, see Volume 1, Chapter 9. 

In such cases, radiant heat transfer is used from the combustion of fuel in a fired heater ox furnace. Sometimes the function is to purely provide heat sometimes the fired heater is also a reactor and provides heat of reaction. The special case of steam generation in a fired heater (a steam boiler) will be dealt with in Chapter 23. Fired heater designs vary according to the function, heating duty, type of fuel and the method of introducing combustion air. However, process furnaces have a number of features in common. A simple design is illustrated in Figure 15.19. The chamber where combustion takes place, the radiant section... 

When hot utility needs to be at a high temperature and/or provide high heat fluxes, radiant heat transfer is used from combustion of fuel in a furnace. Furnace designs vary... 

In general, gas-to-particle or particle-to-gas heat transfer is not limiting in fluidized beds (Botterill, 1986). Therefore, bed-to-surface heat transfer coefficients are generally limiting, and are of most interest. The overall heat transfer coefficient (h) can be viewed as the sum of the particle convective heat transfer coefficient (h ), the gas convective heat transfer coefficient (h ), and the radiant heat transfer coefficient (hr). 

The radiant heat transfer coefficient becomes important above about 600°C, but is difficult to predict. Baskakov et al. (1973) report that depending on particle size, hr increases from approximately 8% to 12% of the overall heat transfer coefficient at 600°C, to 20 to 33% of h at 800°C. 

Overall bed-to-surface heat transfer coefficient = Gas convective heat transfer coefficient = Particle convective heat transfer coefficient = Radiant heat transfer coefficient = Jet penetration length = Width of cyclone inlet = Number of spirals in cyclone = Elasticity modulus for a fluidized bed = Elasticity modulus at minimum bubbling = Richardson-Zaki exponent... 

Ozkaynak, T. F., Chen, J. C., and Frankenfield, T. R., An Experimental Investigation of Radiant Heat Transfer in High Temperature Fluidized Bed, Fluidization, Fourth International Conf. on Fluidization, pp. 371— 378, Engineering Foundation (1983)... 

The last point is worth considering in more detail. Most hydrocarbon diffusion flames are luminous, and this luminosity is due to carbon particulates that radiate strongly at the high combustion gas temperatures. As discussed in Chapter 6, most flames appear yellow when there is particulate formation. The solid-phase particulate cloud has a very high emissivity compared to a pure gaseous system thus, soot-laden flames appreciably increase the radiant heat transfer. In fact, some systems can approach black-body conditions. Thus, when the rate of heat transfer from the combustion gases to some surface, such as a melt, is important—as is the case in certain industrial furnaces—it is beneficial to operate the system in a particular diffusion flame mode to ensure formation of carbon particles. Such particles can later be burned off with additional air to meet emission standards. But some flames are not as luminous as others. Under certain conditions the very small particles that form are oxidized in the flame front and do not create a particulate cloud. 

The interfacial boundary condition may be written in terms of the convective heat transfer of sensible heat, latent heat transfer due to evaporation, and, if the surface temperature is high enough, radiant heat transfer. Mathematically, the surface boundary condition is... 

Radiant heat transfer can result in burns to personnel and can heat up unprotected process equipment and structural elements. If the heat is not dissipated by the application of cooling or conduction, the process equipment or structure may fail. 

The goal of the present study is to provide the information needed for design of a practical underwater propulsion system utilizing powdered aluminum burned with steam. Experiments are being conducted in atmospheric pressure dump combustors using argon/oxygen mixtures and steam as oxidizers. Spectrometer measurements have been made to estimate combustion temperatures and radiant heat transfer rates, and samples of combustion products have been collected to determine the composition and particle size distribution of the products. 

Other factors being constant, the rate of vaporization from a large exposed liquid surface is proportional to the area of the surface. This may also be true for droplets 67, 96) if radiant heat transfer is predominant 17). Under normal circumstances, however, the rate of vaporization of a droplet at rest with respect to its environment is proportional to the droplet diameter 3). This comes about because the vaporization rate is controlled by the rate of conduction of heat or of mass through the gas film surrounding the droplet. The appropriate equation is ... 

Smoke (carbon) formation, which apparently is due to incomplete combustion of portions of the fuel-air mixture (i.e., rich combustion), also can pose a serious public relations problem at civilian airports and, by radiant-heat transfer from incandescent carbon particles, can shorten the endurance life of combustion-chamber liners and adjacent parts (0). Smoke would also constitute a serious problem in the case of automotive gas turbines, because accumulation of carbon and other nonvolatile fuel components on the intricate passages of the heat exchanger could reduce turbine and heat-exchanger efficiency by reducing heat-transfer rate and increasing the pressure drop across the... 

Topper, L., Radiant Heat Transfer from Flames in Single Tubular Turbojet Com-... 

Radiant heat transfer in furnaces is roughly proportioned to the difference in the fourth power ol the absolute temperatures of the radiating and receiving surfaces. The water wall surface is approximately at boiler saturation lemperuture. while the superheater surfaee varies from this to somewhat above the temperature of the steam al the superheater outlet. However ihe mean radiating temperature of Ihe furnace gases is usually over 1204 C. The fourth power of the receiving surface temperature is thus seen to be small compared to the fourth power of the transmitting surface temperature consequently the latter controls the transmittance, and boiler lube temperature does not need to be considered a variable to be accounted for. 

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