Radiation heat transfer blackbody

From Karlsson and Quintiere [1], it can be shown that for an enclosure with blackbody surfaces (ew = 1), the radiation heat transfer rate out of the vent of area A0 is... 

The processes of scattering and absorption of radiation in the atmosphere so significantly alter the spectral distribution that any similarity to extra terrestrial radiation is almost coincidental. Experiments with radiation between surfaces have shown that blackbody radiation theory can be extended successfully to many radiation heat transfer situations. In these situations the strict equilibrium requirements of the initial model have so far not proved to be necessary for practical designs. Most importantly the concept of temperature has proved useful in non-equilibrium radiation flux situations(3). 

Radiation heat transfer involves a different physical mechanism—that of propagation of electromagnetic energy. To study this type of energy transfer we introduce the concept of an ideal radiator, or blackbody, which radiates energy at a rate proportional to its absolute temperature to the fourth power. 

At high. surface temperatures (typically above 300°C), heat transfer acros.s the vapor film by radiation becomes significant and needs to be considered (fig. 10-12). Treating the vapor film as a transparent medium sandwiched between two large parallel plates and approximating the liquid as a blackbody, radiation heat transfer can be determined from... 

The direction of the net radiation heat transfer depends on the relative magnitudes of 7, (the radiosity) and (, (the emissive power of a blackbody at the teinpeiature of the surface). It is from the surface if > 7,- and to the surface if 7f > ft),-. A negative value for ft indicates that heat transfer is to the surface. All of this radiation energy gained must be removed from the other side of the surface through some mechanism if the surface temperature is to remain constant. 

The surface resistance to radiation for a blackbody is zero since e, = 1 and f -- E, The net rale of radiation heat transfer in this case is determined directly from Eq. 13-23. 

Consider a cylindrical furnace with r = W = 1 m, as shosvn in Fig. 13-27. The lop (surface 1) and the base (surface 2) of the furnace have emissivities ei = 0.8 and 2 = 0.4, respectively, and are maintained at uniform temperatures T 700 K and T2 - 500 K. The side surface closely approximates a blackbody and is maintained at a temperature of = 400 K. Determine the net rate of radiation heat transfer at each surface during steady operation and explain hovr these surfaces can be maintained at specified temperatures. 

Two parallel disks of diameler D = 0.6 m separated by i. = 0.4 m are located directly on top of each other. Both disks are black and are maintaiiied at a temperature of 450 K. The back sides of the disks are insulated, and the environment that Ihe disks are in can be considered to be a blackbody at 300 K. Deierinine the nel rate of radiation heat transfer from the disks to the environment. Answer 781 W... 

Consider a circular giill whose diameter is 0,3 m. I he bottom of the grill is covered with hot coal bricks at 950 K, while the wire mesh on top of the grill i.s covered wilh steaks initially at 5°C. The distance between the coal bricks and the steaks is 0.20 in. Treating both the steaks and the coal bricks as blackbodies, determine the initial rate of radiation heat transfer from the coal bricks to the steaks. Also, determine the initial rate of radiation heat transfer to the steaks if the side opening... 

T vo long parallel 20-cni-diaraeter cylinders are located 30 cm apart from each other. Both cylinders are black, and are maintained at temperatures 425 K and 275 K, The surroundings can be treated as a blackbody al 300 K. For a I-m-long section of the cylinders, determine the rales of radiation heat transfer between the cylinders and between the hot cylinder and the surroundings. 

A special case that occurs frequently in engineering practice involves radiation exchange between a small surface at Tg and a much larger, isothermal surface that completely surrounds the smaller one. The surroundings could be a furnace whose temperature Tgur differs from that of an enclosed surface ( sur — Tg). For such a condition, the irradiation may be approximated by emission from a blackbody at Tsud in which case G = crT. If the surface is assumed to be one for which a = e (a gray surface), the net rate of radiation heat transfer from the surface, is ... 

In conclusion, radiation effects can be neglected for large aspect ratios typical of microreactors. Blackbody radiation may be used to approximate radiation effects with reduced computational complexity of surface-to-surface radiation. However, for an aspect ratio of 10 or lower, blackbody radiation gives the upper limit of the radiation effect and the net radiation equation needs to be solved to obtain the radiation heat transfer. Heat transfer along the wall is the most critical heat transfer mechanism and needs to be accounted for. 

Radiation heat transfer in a system of blackbodies then becomes... 

We assume that the furnace sides and bottom can be approximated by black-bodies. Radiation heat transfer is presumed to predominate. Furthermore, the only heat loss will be through the opening to the surroundings. Because the surroundings are great in extent, we can also assume that the opening behaves as a blackbody. 

R. Slegal and J. R. Howell, "The Blackbody, Electromagnetic Theory and Material Properties", Thermal Radiation Heat Transfer, Vol. I (NASA SP. 164), NASA Office of Technical Utilization, 1968, pp. 1-37. 

Total heat transfer consists of radiation at different frequencies. The distribution of radiation energy in a spectrum and its dependency on temperature is determined from Planck s law of radiation. M ,and are the spectral radiation intensities for a blackbody ... 

FIGURE 4.33 Heat transfer factor representing blackbody radiation for various mean temperatures and temperature differences. 

A calorimetric method may be used where an electric heater is imbedded in the object of interest, and the power dissipated by the element is accurately calculated from voltage and current. Once steady state is established and the object is at constant temperature, the body must emit radiation at the same rate at which it is supplied. As long as conduction and convection are eliminated as mechanisms of heat transfer (e.g. vacuum conditions), the blackbody temperature is known by Rt = o"T4. The emittance can then be determined after py-rometric measurements of the brightness temperature of the object. 

Two very large parallel planes having surface conditions which very nearly approximate those of a blackbody are maintained at 1100 and 425°C, respectively. Calculate the heat transfer by radiation between the planes per unit time and per unit surface area. 

Equation 7.12 shows that the rate of energy emitted by a blackbody increases in proportion to the absolute temperature to the fourth power, so that radiation will generally be the dominating heat transfer mode at high absolute temperatures. 

Temperature can be measured from heat transfer by conduction, convection, or radiation. Household thermometers use either the expansion of metals or other substances or the increase in resistance with temperature. Thermocouples measure the electromotive force generated by temperature difference. Pyrometers measure infrared radiation from a heat source. Spectroscopic thermometry compares the spectrum of radiation against a blackbody spectrum. Temperature-sensitive paints and liquid crystals change intensity of radiation in certain wavelengths with temperature. 

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). 

Estimate the temperature at which ice will form on a clear night (sky effective radiation temperature is -73.3°C). The convective heat transfer coefficient is 28.4 W/m. Neglect water s heat of vaporization. Assume water is a blackbody. 

BGO crystals with eulithine structure have a very small absorption coefficient in the wavelength band where the main part of the blackbody radiation is concentrated. In contrast to this, the absorption coefficient of sillenite crystals reaches 0.3-0.4cm in the wavelength range from 2 to 6 tm. One can expect that absorption and emission of radiation by a crystal appreciably influence the heat-transfer process. Two growth processes were simulated. The first was performed in the setup used for the growth of BGO eulithine crystals, while the second was carried out in the Laboratory of Crystal Growth of the Autonomic University of Madrid. Results of simulation are described in Refs. [38, 45], respectively. Here, we focus on the second process. 

S. L. Chang and K. T. Rhee, Blackbody Radiation Functions, International Communications in Heat and Mass Transfer, vol. 11, p. 451,1984. 

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