Heat transfer, radiative

Investigator Type of correlation Phases involved Model associated Whitaker [39] Particle-to-bed radiative heat transfer (radiative flux) Fluid-solid Treating both particle and bed as gray bodies... 

The temperature of the fiber at any axial position is of course determined by the extrusion temperature, convective heat transfer, radiative heat transfer, and heat liberation due to crystallization. Heat generated by viscous dissipation is negligible. 

When measuring the thermal conductivity with this technique, some corrections should be made for (1) radiation-induced heat transfer (radiative correction) (2) parasitic heat transfer from the inner to the outer cylinder through a central solid pintle, electric wires, and thermocouples (3) convective heat transfer and (4) effects of possible temperature jumps at the interface between the liquid layer and the cylinder surface. 

Conduction heat transfer Convective heat transfer Radiative heat transfer Momentum diffusivityfkinematic viscosity) Molecular diffusivity Convective mass transfer Molecular diffusion... 

Kakimoto and Liu [10] developed a partly 3D global model that takes into account feasible 3D global modeling with moderate requirements of computer memory and computation time. AH convective and conductive heat transfers, radiative heat exchanges between diffuse surfaces and the Navier-Stokes equations for the melt are all coupled and solved simultaneously by a finite-volume method in a 3D configuration. 

The relationship between heat transfer and the boundary layer species distribution should be emphasized. As vaporization occurs, chemical species are transported to the boundary layer and act to cool by transpiration. These gaseous products may undergo additional thermochemical reactions with the boundary-layer gas, further impacting heat transfer. Thus species concentrations are needed for accurate calculation of transport properties, as well as for calculations of convective heating and radiative transport. 

Third, design constraints are imposed by the requirement for controlled cooling rates for NO reduction. The 1.5—2 s residence time required increases furnace volume and surface area. The physical processes involved in NO control, including the kinetics of NO chemistry, radiative heat transfer and gas cooling rates, fluid dynamics and boundary layer effects in the boiler, and final combustion of fuel-rich MHD generator exhaust gases, must be considered. 

In fossil fuel-fired boilers there are two regions defined by the mode of heat transfer. Fuel is burned in the furnace or radiant section of the boiler. The walls of this section of the boiler are constmcted of vertical, or near vertical, tubes in which water is boiled. Heat is transferred radiatively from the fire to the waterwaH of the boiler. When the hot gas leaves the radiant section of the boiler, it goes to the convective section. In the convective section, heat is transferred to tubes in the gas path. Superheating and reheating are in the convective section of the boiler. The economizer, which can be considered as a gas-heated feedwater heater, is the last element in the convective zone of the boiler. 

Specialized heat transfer nomenclature used for radiative heat transfer is defined in the subsection Heat Transmission hy Radiation. Nomenclature for mass transfer is defined in the subsection Mass Transfer. ... 

An equation representing an energy balance on a combustion chamber of two surface zones, a heat sink Ai at temperature T, and a refractory surface A assumed radiatively adiabatic at Tr, inmost simply solved if the total enthalpy input H is expressed as rhCJYTv rh is the mass rate of fuel plus air and Tp is a pseudoadiabatic flame temperature based on a mean specific heat from base temperature up to the gas exit temperature Te rather than up to Tp/The heat transfer rate out of the gas is then H— — T ) or rhCp(T f — Te). The... 

A pseudo-convective heat-transfer operation is one in which the heating gas (generally air) is passed over a bed of solids. Its nse is almost exchisively limited to drying operations (see Sec. 12, tray and shelf dryers). The operation, sometimes termed direct, is more aldu to the coudnctive mechanism. For this operation, Tsao and Wheelock [Chem. Eng., 74(13), 201 (1967)] predict the heat-transfer coefficient when radiative and conductive effects are absent by... 

Radiative Heat Transfer Heat-transfer equipment using the radiative mechanism for divided solids is constructed as a table which is stationary, as with trays, or moving, as with a belt, and/or agitated, as with a vibrated pan, to distribute and expose the burden in a plane parallel to (but not in contacl with) the plane of the radiant-heat sources. Presence of air is not necessary (see Sec. 12 for vacuum-shelf dryers and Sec. 22 for resubhmation). In fact, if air in the intervening space has a high humidity or CO9 content, it acts as an energy absorber, thereby depressing the performance. 

For the radiative mechanism of heat transfer to solids, the rate equation for parallel-surface operations is... 

In rotary devices, reradiation from the exposed shelf surface to the solids bed is a major design consideration. A treatise on furnaces, including radiative heat-transfer effects, is given by Ellwood and Danatos [Chem. Eng., 73(8), 174 (1966)]. For discussion of radiation heat-transfer computational methods, heat fliixes obtainable, and emissivity values, see Schornshort and Viskanta (ASME Paper 68-H 7-32), Sherman (ASME Paper 56-A-III), and the fohowing subsection. 

Here R=r is the parameter for radiative heat transfer in K units, p is a heat of reaction term, in K/atm units tj is the fluid temperature in the j-th axial position e is the particle emissivity 1 is the celt dimension in m 6 is the clock time in minutes... 

Simulation by the improved Euler method has shown that a significant radiative heat transfer must be present before reaction zone migration can be demonstrated. 

In the case of a given surface temperature, the amount of energy released is determined by the parameters for the convective and radiative heat exchange. As far as convection is concerned, these are the temperatures ol the heat source surface and room air, respectively, and the heat transfer coefficient. The radiative heat exchange is determined by the view factors and the temperatures of the surrounding surfaces. 

Radiative heat transfer coefficient The heat-transfer coefficient wholly attributed to radiative heat transfer. 

Love, T. J. 1968. Radiative heat transfer. Cincinnati, OH C. E. Merrill. 

Radiative heat transfer is perhaps the most difficult of the heat transfer mechanisms to understand because so many factors influence this heat transfer mode. Radiative heat transfer does not require a medium through which the heat is transferred, unlike both conduction and convection. The most apparent example of radiative heat transfer is the solar energy we receive from the Sun. The sunlight comes to Earth across 150,000,000 km (93,000,000 miles) through the vacuum of space. FIcat transfer by radiation is also not a linear function of temperature, as are both conduction and convection. Radiative energy emission is proportional to the fourth power of the absolute temperature of a body, and radiative heat transfer occurs in proportion to the difference between the fourth power of the absolute temperatures of the two surfaces. In equation form, q/A is defined as ... 

A window consisting of a single piece of clear glass can also he treated with R-value analysis. As with the wall, there is convective and radiative heat transfer at the two surfaces and conductive heat transfer through the glass. The resistance of the window is due to the two surface resistances and to the conductive resistance of the glass, For typical window glass, R = 0.003 (W/ni -°C)" (0.02 (Btu/h-ft -°F) ) so the total resistance of the window is = (0.12 + 0.003 + 0.04) (W/m -- C) ... 

Heat transfer processes besides pure radiative transfer are involved in control of the temperature of the air, especially below the effective emission height of 6 km. Referring back to Chapter 7, we see that vertical motions of air in the troposphere are a main factor dictating that temperature decreases as altitude increases - air loses internal energy... 

At present there is no small-scale test for predicting whether or how fast a fire will spread on a wall made of flammable or semiflammable (fire-retardant) material. The principal elements of the problem include pyrolysis of solids char-layer buildup buoyant, convective, tmbulent-boundary-layer heat transfer soot formation in the flame radiative emission from the sooty flame and the transient natme of the process (char buildup, fuel burnout, preheating of areas not yet ignited). Efforts are needed to develop computer models for these effects and to develop appropriate small-scale tests. 

The heat transfer term envisions convection to an external surface, and U is an overall heat transfer coefficient. The heat transfer area could be the reactor jacket, coils inside the reactor, cooled baffles, or an external heat exchanger. Other forms of heat transfer or heat generation can be added to this term e.g, mechanical power input from an agitator or radiative heat transfer. The reactor is adiabatic when 7 = 0. 

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