Thermal radiation heat transfer

Would it be feasible to use a magnesia insulation which will not stand temperatures above 615 K and has a thermal conductivity 0.09 W/m K for an additional layer thick enough to reduce the outer surface temperature to 370 K in surroundings at 280 K Take the surface coefficient of heat transfer by radiation and convection as 10 W/m- IC... 

The thermal conductivity of materials has been examined in Chapter 2 and Chapter 3. As we shall see in this chapter, in many cases, at very low temperatures, the heat conduction is not limited by the bulk thermal resistivity of the material but by the contact thermal resistance appearing at the interface of two materials. This is a particularly severe problem, below IK, in the case of the heat transfer between liquid He and a solid (see Section 4.3). Heat transfer by radiation will be considered in Section 53.2.2. 

Conductive and Convective Heat Transfer, Thermal Explosion by- There are three fundamental types of heat transfer conduction, convection radiation. All three types may occur at the same time, but it is advisable to consider the heat thransfer by each type in any particular case. Conduction is the transfer of heat from one part of a body to another part of the same body, or from one body to another in physical contact with it, without appreciable displacement of the particles of either body. Convection is the transfer of heat from one point to another within a fluid, gas or liquid, by the mixing of one portion of the fluid with another. In natural convection, the motion of the fluid is entirely the result of differences in density resulting from temp differences in forced convection, the motion is produced by mechanical means. Radiation is the transfer of heat from one body to another, not in contact with it, by means of wave motion thru space (Ref 5)... 

The heat transfer in a foam, as in any other physical system, occurs through thermal conductivity, heat radiation and convection [87]. It was established that in disperse systems the heat transfer through radiation is only significant at high temperature (> 100°C) and in the presence of large pores, while convection is effective only if the particles (bubbles in the foam) are large (> 1 mm). This means that thermal conductivity is the basic mechanism of heat transfer at not very high temperatures. 

SOLUTION The base plate of an iron is considered. The variation of temperature in the plate and the surface temperatures are to be determined. Assumptions 1 Heat transfer is steady since there is no change with time. 2 Heat transfer is one-dimensional since the surface area of the base plate is large relative to its thickness, and the thermal conditions on both sides are uniform. 3 Thermal conductivity is constant. 4 There is no heat generation in the medium. 5 Heat transfer by radiation is negligible. 6 The upper part of the iron is well insulated so that the entire heat generated in the resistance wires is transferred to the base plate through its inner surface. 

Assumptions 1 Steady operating conditions exist. 2 The heat transfer coefficient is uniform over the entire fin surfaces. 3 Thermal conductivity is constant. 4 Heat transfer by radiation is negligible. 

Repeat Prob. 7-97, assumJng the inner surface of the lank to be at 0°C but by taking the thermal resistance of the tank and heat transfer by radiation into consideration. Assume the average surrounding surface temperature for radiation exchange to be 25°C and Ihe outer surface of the tank to have an emissivity of 0.75. Arrswers (a) 10,530 W, (b) 2727 kg... 

Here A is the thermal conductivity of the solid (not the fluid ) at the wall. Eq. (2.23) stipulates a linear relationship between the temperature t)w and the slope of the temperature profile at the surface, this is also known as the 3rd type of boundary condition. As in (2.18), d d/dn is the derivative in the normal direction outward from the surface. The fluid temperature t F, which can change with time, and the heat transfer coefficient a must be given for the solution of the heat conduction problem. If a is very large, the temperature difference (t w — i9F) will be very small and the boundary condition (2.23) can be replaced by the simpler boundary condition of a prescribed temperature (i9w = i9F). The boundary condition in (2.23) is only linear as long as a is independent of w or ( w — f F), this factor is very important for the mathematical solution of heat conduction problems. In a series of heat transfer problems, for example in free convection, a changes with (i w — F), thereby destroying the linearity of the boundary condition. The same occurs when heat transfer by radiation is considered, as in this... 

Thermal radiation differs from heat conduction and convective heat transfer in its fundamental laws. Heat transfer by radiation does not require the presence of matter electromagnetic waves also transfer energy in empty space. Temperature gradients or differences are not decisive for the transferred flow of heat, rather the difference in the fourth power of the thermodynamic (absolute) temperatures of the bodies between which heat is to be transferred by radiation is definitive. In addition, the energy radiated by a body is distributed differently over the single regions of the spectrum. This wavelength dependence of the radiation must be taken as much into account as the distribution over the different directions in space. 

All the considerations that follow are only valid for radiation that is stimulated thermally. Radiation is released from all bodies and is dependent on their material properties and temperature. This is known as heat or thermal radiation. Two theories are available for the description of the emission, transfer and absorption of radiative energy the classical theory of electromagnetic waves and the quantum theory of photons. These theories are not exclusive of each other but instead supplement each other by the fact that each describes individual aspects of thermal radiation very well. 

Thermal radiation is the subject of the fifth chapter. It differs from many other presentations in so far as the physical quantities needed for the quantitative description of the directional and wavelength dependency of radiation are extensively presented first. Only after a strict formulation of Kirchhoff s law, the ideal radiator, the black body, is introduced. After this follows a discussion of the material laws of real radiators. Solar radiation and heat transfer by radiation are considered as the main applications. An introduction to gas radiation, important technically for combustion chambers and furnaces, is the final part of this chapter. 

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... 

Finally, it is important to mention the effect of porosity. Since the thermal conductivity of air is negligible compared to the solid phases, the addition of large (>25 percent) volume fractions of pores can significantly reduce Ath. This approach is used in the fabrication of firebrick. As noted above, the addition of large-volume fractions of porosity has the added advantage of rendering the firebricks thermal-shock-tolerant. Note that heat transfer by radiation across the pores, which scales as has to be minimized. Hence for optimal thermal resistance, the pores should be small and the pore phase should be continuous. 

To reduce the values of the Rayleigh number, Ra, a small gap distance between cylinders d = (0.97 0.03) x 10 m was used. This way the risk of convection was minimized. Convection could develop when the Ra exceeds a certain critical value Ra, which for vertical coaxial cylinders is about 1000 (Gershuni, 1952). The absence of convection can be verified experimentally by measuring the thermal conductivity with different temperature differences AT across the measuring gap and different power Q transferred from inner to outer cylinder. Since heat transfer by radiation is proportional to 4r AT, we would expect radiation losses to substantially increase as a function of the cell temperature. This kind of correction is included in the calibration procedure. The emissivity of the walls was small and Qrad, estimated by Equation (5.7) is negligible 0.164 W) by comparison with the heat transfer (13.06 W) by conduction in the temperature range up to 600 K. 

Different types of heat transfer processes are called modes. The main modes of heat transfer are convection, radiation, and conduction. For a temperature gradient that exists between a surface and a moving fluid, one should use the term convection. The radiation mode of heat transfer is driven by electromagnetic waves emitted from all surfaces of finite temperature, so there is a net heat transfer by radiation between two surfaces at different temperatures. When a temperature gradient exists in a stationary medium, heat fiows under the law of conduction heat transfer. In the case of solid materials, such as polymers, conduction is the dominant mechanism for heat transfer, involving mainly lattice vibrations and, in few cases, the transfer of kinetic thermal energy from one electron to another. 

Heat transfer. A number of options exist for the heat exchanger/chemical reactor. Examples of alternative interfaces include traditional heat exchangers, radiation heat transfer (thermal infrared between tube banks), duplex tubes (tubes constructed of two metals), and intermediate heat exchanger loops. It is not clear what the preferred option is. The very high temperatures does create new options such as the use of a heat exchanger that operates on radiation heat transfer (Fig. 5). Such options provide very high degrees of separation between the nuclear and chemical facilities. 

---