Radiation flux

Ideal Performance and Cooling Requirements. Eree carriers can be excited by the thermal motion of the crystal lattice (phonons) as well as by photon absorption. These thermally excited carriers determine the magnitude of the dark current,/ and constitute a source of noise that defines the limit of the minimum radiation flux that can be detected. The dark carrier concentration is temperature dependent and decreases exponentially with reciprocal temperature at a rate that is determined by the magnitude of or E for intrinsic or extrinsic material, respectively. Therefore, usually it is necessary to operate infrared photon detectors at reduced temperatures to achieve high sensitivity. The smaller the value of E or E, the lower the temperature must be. 

Sin e-Gas-Zone/Two-Surface-Zone Systems An enclosure consisting of but one isothermal gas zone and two gray surface zones can, properly specified, model so many industrially important radiation problems as to merit detailed presentation. One can evaluate the total radiation flux between any two of the three zones, including multiple reflec tion at all surfaces. 

Nx = / nowhere Ix is the radiation flux, n is the number of gas molecules in the path of the beam per cm of projected area, and a is the cross-section of absorption. Alternatively, the absorption coefficient, is defined drrough the Beer-Lambert equation... 

In addition to the effect of albedo on the amount of radiation that reaches the earth s surface, the angle of incidence of the radiation compared to the perpendicular to the surface affects the amount of radiation flux on an area. The flux on a horizontal surface is as follows ... 

Once a core material has been selected, the core style must be considered. Many different core styles are offered by the core manufacturers they fall basically into the categories shown in Figure D-6. Each has an advantage of size, cost, or shielding, and these factors should be considered in the light of the application. They fall basically into two styles toroid and bobbin style cores. Toroidal transformers are more expensive to build because of the special machinery needed to wind the turns onto the core, but they are superior in the amount of radiated flux escaping from the transformer. Bobbin cores are typi-... 

The heat flux, E, from BLEVEs is in the range 200 to 350 kW/m is much higher than in pool fires because the flame is not smoky. Roberts (1981) and Hymes (1983) estimate the surface heat flux as the radiative fraction of the total heat of combustion according to equation 9.1-32, where E is the surface emitted flux (kW/m ), M is the mass of LPG in the BLEVE (kg) h, is the heat of combustion (kJ/kg), is the maximum fireball diameter (m) f is the radiation fraction, (typically 0.25-0.4). t is the fireball duration (s). The view factor is approximated by equation 9.1-34. where D is the fireball diameter (m), and x is the distance from the sphere center to the target (m). At this point the radiation flux may be calculated (equation 9.1-30). 

Simple models for louvers and other solar protection devices are based on a statement of constant reduction of the solar radiation flux on the window. A common assumption is that a louver is controlled so that no direct light can penetrate into the room. 

Roberts (1982) also used the data of Hasegawa and Sato (1977) to correlate the measured radiation flux q received by a detector at a distance L (m) from the center of the fireball with the hydrocarbon fuel mass (kg) ... 

TABLE 6.9. Calculated Distances from Radiation Flux Given by CCPS (1989) for a BLEVE at San Juanico... 

The excited-state molecules may either undergo radiationless decay to the ground state leading to the formal generation of heat under conditions of high radiation flux or radiative decay (i.e., phosphorescence), thereby emitting light. 

It must, however, be taken into account that the radiation flux has not always been constant during the 4.6 billion years since the formation of the sun. The solar constant is in fact not a constant at all, as it depends on the state of the sun s surface. For prebiotic syntheses, it is important to consider the wavelengths which can be absorbed by small molecules such as CO2, CO, CH4, N2, NH3, H2O, H2S etc. The premise here, of course, is that most of the synthetic reactions occurred in the gas phase. 

The parametric effect of bed temperature is expected to be reflected through higher thermal conductivity of gas and higher thermal radiation fluxes at higher temperatures. Basu and Nag (1996) show the combined effect (Fig. 23) which plots heat transfer coefficients as a function of bed temperature for data from four different sources. It is seen that for particles of approximately the same diameter, at a constant suspension density (solid concentration), the heat transfer coefficient increases by almost 300% as the bed temperatures increase from 600°C to 900°C. 

Less obviously, perhaps, the second law of thermodynamics assures us that the intensity, 1(A), is also constant along the beam, for if this were not the case, then it would be possible to focus all the radiation from a hot body onto a part of itself, increasing the radiation flux onto that portion and raising its temperature of that portion without doing work - a violation of the second law. The constancy of beam energy and intensity has other consequences, some of which are familiar to most of us. If we solve equation 29-3 for the product (den da) we get ... 

Thermal burns result from the radiant heat emitted by a hydrogen fire and absorbed by a person, which is directly proportional to many factors including exposure time, burning rate, heat of combustion, size of the burning surface, and atmospheric conditions (mainly wind and humidity). For instance, thermal radiation flux exposure level of 0.95 W/cm2 may cause skin burns in 30 s. Cryogenic burns may result from contact with cold fluids or cold vessel surfaces. Exposure to large liquefied hydrogen spills could result in hypothermia, if proper precautions are not taken [17]. 

Turning now to the wavelength distribution of the starlight. The emission from a black body must, by definition, produce radiation at all wavelengths, i.e. a wavelength distribution. It turns out for a black body that the wavelength at which the maximum radiation flux occurs is characteristic of the temperature and is given by Wien s Law ... 

Use of the logarithmic scale relating radiation flux and magnitude difference... 

A fundamental assumption made for most dating with atmospheric radionuclides is that the cosmic radiation flux and hence, the natural production of the radionuclides has been constant with time. Various studies of this problem using 14C and tree-ring calibration have been made. Isotopic studies of meteorites have also been useful [17]. Considering the probable lack of basic accuracy of dating water, the problem of changes in cosmic ray flux is not serious. 

Vent-radiation factor. As shown by Figure 11.10, the radiation flux from the vent is reasonably represented as a blackbody at the gas temperature, T. Thus, the vent radiation loss can be estimated as... 

The convection heat flux is usually much smaller than radiation flux in significant fires. Preserving II5, Il6 and Ilg yields q" l1 2, which is inconsistent with the radiation flux (Equation (12.53)). However, if the same fuel is used in the model as in the prototype and no change occurs in k as k 7 then the radiation flux is... 

Nuclear reactors are a source of high radiation fluxes. 

The effects of the radiation flux in space on polymer materials is now of considerable importance due to the increasing use of communications satellites. Geosynchronous orbit corresponds to the second Van Allen belt of radiation, which comprises mainly electrons and protons of high energy. 

For semi-infinite layers the radiation fluxes simplify to single exponentials... 

If the surface temperature does not differ greatly from the surrounding temperature, the highly nonlinear surface boundary condition may be simplified by linearizing the expression for the radiation flux and the Clausius-Clapeyron equation to yield the approximation... 

Evaluate the convective heat flux to the surface, comparing it to an estimate of the radiation flux from the surface. Is wafer heating of cooling needed ... 

---