Radiation heat flux

Radiation heat flux is strongly time dependent, because both the flame surface area and the distance between the flame and intercepting surfaces vary during the eourse of a flash fire. The path of this curve ean be approximated by calculating the radiation heat flux at a sufficient number of discrete points in time. 

Thus, the final steps for calculating the radiation heat flux are as follows ... 

Figure 8.3. Graphical presentation for sample problem of the radiation heat flux as a function of time.

Radiation heat flux is graphically represented as a function of time in Figure 8.3. The total amount of radiation heat from a surface can be found by integration of the radiation heat flux over the time of flame propagation, that is, the area under the curve. This result is probably an overstatement of realistic values, because the flame will probably not bum as a closed front. Instead, it will consist of several plumes which might reach heights in excess of those assumed in the model but will nevertheless probably produce less flame radiation. Moreover, the flame will not bum as a plane surface but more in the shape of a horseshoe. Finally, wind will have a considerable influence on flame shape and cloud position. None of these eflects has been taken into account. 

In this case, all of the radiation loss from the flame is accounted for by XT, including that received by the surface of the condensed phase. Therefore flame radiation heat flux does not show up here. 

An instantaneous snapshot of the jet showing soot volume fraction contours and radiation heat flux vectors is shown in Fig. 10.3. The soot forms immediately downstream of the jet exit as a result of the mixing controlled soot formation model. The soot appears in thin streaks in physical space which is consistent with previous experimental observations [2]. The radiation heat flux vectors are seen... 

Figure 10.3 Instantaneous soot volume fraction contour plot with radiation heat flux vectors...

The radiation heat fluxes are exchanged between the internal and external surfaces of the cell slice and the surrounding surfaces. Moreover, fuel should not be considered as a transparent gas because of the significant partial pressures of CO2 and H2O. This means that radiation heat fluxes are exchanged between the external surface of the cell slice and the fuel volumes. The total net flux exiting the ith cell slice can be expressed as (see Figure 7.21) ... 

For a fire thermal radiation, heat flux, and the duration of exposure determine the severity of injury or damage. 

A blackbody enclosure at 1000°C has a small aperture into the environment. Determine (i) the blackbody radiation intensity emerging from the aperture, and (ii) the blackbody radiation heat flux from the blackbody. 

Fig. 3.25. Heat flux histories following an ELM of 1MJ/m2 with a power flux triangular waveform (curve 1) with ramp-up and ramp-down phases lasting 300 ds each on a 10mm thick W target under an inter-ELM power flux of 10MWm 2. Curves. (1) incident heat flux load (2) conducted heat flux into the material (3) heat flux spent in melting of the material (the evaporation and black-body radiation heat fluxes are comparatively small and not shown). Curve (4) shows the surface target temperature and (5) shows the temperature of the melt layer. Curve (6) shows the vaporized thickness (amplified of a factor of 1000) and (7) the melt layer assuming that no losses of molten material occur during the ELM [3]...

Note that the standard temperature (77°F or T0 = 298.15 K) is used in this definition. cp i is the specific heat and Ah p is the enthalpy of formation at the standard state, both for species i. The heat flux, q, includes contributions from conduction, radiation, differential diffusion among component species, and concentration gradient-driven Dufour effect. For combustion applications, the most important contributions come from conduction and radiation. As discussed in Section 4.3, conduction heat flux follows Fourier s law (Equation 4.27) and radiation heat flux is related to the local intensity as... 

A liquid enters the bottom of a large insulated pipe of diameter D and length L at a temperature T0, flows slowly upward with velocity V, and then spills over the top in a thin layer (Fig. 2P-17). A radiation heat flux is incident on the upper surface of the liquid and is absorbed within the liquid such that the flux at any point within the liquid is given... 

Such gauges are used for measurement of convection and/or radiation heat fluxes Although ideally the relationship between the heat flux and sensor temperature difference can be analyzed, calibration of the gauge is almost always necessary. More information is available in Refs 115 and 116. 

Brajuskovic, Matovic, and Afgan [84] and Brajuskovic, and Afgan [85] describe a blow-off heat flux sensor that uses gas flowing through a porous sintered metal disc to measure hemispherical radiation heat flux in a dirty environment that includes high particulate loads. The gas acts to blow off fhe boundary layer at the surface. It also cools the instrument, prevents fouling of the porous... 

In the thermoforming process, the temperature of the ceramic or metal radiant heaters is typically 400 °C (673 K). These can be treated as black bodies at an absolute temperature T, for which the radiation heat flux from an area A is... 

During burning, dripping may distort the result. These values fall sharply with increasing incident radiation (heat flux) on the specimen. 

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