Radiation heated elements, example

Example 1 What Is the Correlation Between the Baking Time and the Weight of a Christmas Turkey We first recall the physical situation. To facilitate this we draw a sketch (Sketch 1). At high oven temperatures the heat is transferred from the heating elements to the meat surface by both radiation and heat convection. From there it is transferred solely by the unsteady-state heat conduction that surely represents the rate-limiting step of the whole heating process. 

The red glow given off by the heating element is an example of blackbody radiation. 

Example 5.11 The electrically heated oven in Fig. 5.63a is used for the surface treatment of thin, square metal plates that are covered on both sides. The oven has a square base of side length a = 1.50 m. The radiation emitting surface of the heating elements has emissivity s = 0.85 their distance from the metal plate is b = 0.25 m. 12.5 kW power is supplied to each of the two rows of heating elements being well insulated against the exterior. The non-insulated side walls of the oven have emissivity = 0.70. At steady-state the surface temperature of the heating elements reaches 750 K. Determine the temperature of the side walls and the temperature of the covered metal plate. 

The following examples show the mutual influences of individual parameters. Increasing the infrared transmission of the glass ceramic improves the heat transfer by radiation from the heating element through the glass ceramic so by this effect the heating-up time should be shortened. This, however, does... 

The spectra of atoms are different for the different elements and their observation can therefore identify the presence of particular elements. An important example is the Fraunhofer lines. These are observed as dark (that is, absorption) lines on the spectrum of the sun and the elements responsible for several of these lines were identified by noting their coincidence with the wavelengths emitted when the same elements were heated for example, in a Bunsen flame. In the sun, these lines appear in absorption because the background source is hotter than the medium through which the radiation is passing, whereas they are observed in emission from a flame since the background is cooler than the sample. 

All channel elements had an emissivity Sj = e = 0.6, j = 1, N, while examples of the calculated factors Fk-j are presented in Fig. 8.2 for two channel wall elements and the inlet channel enclosure. The inlet and outlet planes of the enclosure had emissivities equal to those of the channel wall surfaces, jj,j = squt — — 0.6, while the inlet and outlet exchange temperatures were set equal to the inlet mixture and outlet mixing cup temperatures, respectively. This arrangement mimics the tight space in microreactor systems, wherein the entry and outlet sections cannot usually be of large enough size to allow for a black body enclosure treatment. The outer horizontal wall of the microreactor channel was treated as adiabatic (see Fig. 8.1) nevertheless, the reactor itself was non-adiabatic due to radiation heat losses, primarily from the channel wall inner surface as well as from the vertical front solid wall face towards the colder inlet enclosure. 

Convection. Heat transfer by convection arises from the mixing of elements of fluid. If this mixing occurs as a result of density differences as, for example, when a pool of liquid is heated from below, the process is known as natural convection. If the mixing results from eddy movement in the fluid, for example when a fluid flows through a pipe heated on the outside, it is called forced convection. It is important to note that convection requires mixing of fluid elements, and is not governed by temperature difference alone as is the case in conduction and radiation. 

Net Ionic Equation a chemical equation that shows only the ionic species that actually take part in the reaction Neutralization process that occurs when an acid reacts with a base, a type of reaction involving an acid and base Newton SI unit for force equal to 1 kg-m/s Nonelectrolyte a substance that does not conduct current when it is dissolved in water Nonionizing Radiation electromagnetic radiation with insufficient energy to dislodge electrons and cause ionization in human tissue, for example, radio waves, microwave, visible light Nonmetal elements found on the right side of the periodic table that conduct heat and electricity poorly... 

Elements such as As, Se and Te can be determined by AFS with hydride sample introduction into a flame or heated cell followed by atomization of the hydride. Mercury has been determined by cold-vapour AFS. A non-dispersive system for the determination of Hg in liquid and gas samples using AFS has been developed commercially (Fig. 6.4). Mercury ions in an aqueous solution are reduced to mercury using tin(II) chloride solution. The mercury vapour is continuously swept out of the solution by a carrier gas and fed to the fluorescence detector, where the fluorescence radiation is measured at 253.7 nm after excitation of the mercury vapour with a high-intensity mercury lamp (detection limit 0.9 ng I l). Gaseous mercury in gas samples (e.g. air) can be measured directly or after preconcentration on an absorber consisting of, for example, gold-coated sand. By heating the absorber, mercury is desorbed and transferred to the fluorescence detector. 

Boundary conditions most commonly encountered in practice are the specified temperature, specified heat flux, convection, and radiation boundary conditions, and here we develop the finite difference fonnulations for them for the case of steady one-dimensional heat conduction in a plane wall of thickness L as an example. The node number at the left surface at. r = 0 is 0, and at the right surface at x = L it is M. Note that the width of Ihe volume element for either boundary node is Ax/2. 

The determination of metal purity and the elemental composition of alloys is of utmost importance to the metallurgical industry. Microwave-assisted digestion is often well-suited to metals and metallurgical samples that pose no difficulty and dissolve readily and safely with the aid of microwaves [148,186-196]. For example, hydrofluoric acid can be used in closed vessels to digest silicate matrices and stop the hydrolysis of refractory elements without loss of volatile fluorides or passivation. After cooling, boric acid can be added to complex unreacted hydrofluoric acid [14]. The solid sample itself may absorb microwave radiation, thus creating a heated surface on which the acid or acids can react. Microwave muffle furnaces are commercially available [197] based on oven linings made... 

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