The transient heating by radiation of a stagnant layer of water is studied both analytically and experimentally. Solar radiation (insolation) was simulated by tungsten filament lamps in parabolic reflectors of known spectral characteristics. The volumetric rate of internal absorption of radiation was predicted by considering spectral absorption, scattering, and multiple forward scattering by the water according to the model of Viskanta and Toor (1972). The transient temperature distribution was predicted by solving the one‐dimensional energy equation analytically in closed form after first linearizing the boundary condition at the air‐water interface. The analytical model was verified on the basis of laboratory experimental data obtained from transient temperature measurements made in pure water contained in a glass wall test cell using a Mach‐Zehnder interferometer. Comparison of experimental data with the predicted temperature distributions showed good agreement, thus verifying the radiation and total energy models. It was determined that the surface boundary condition and internal radiant heating rate of water must be correctly specified in order to model thermal stratification of stagnant water by radiation properly.
Analysis is developed for the time dependent thermal stratification in surface layers of stagnant water by solar radiation. The transient temperature distribution is obtained by solving the one-dimensional energy equation for combined conduction and radiation energy transfer using a finite difference method. Experimentally, solar heating of water is simulated using tungsten filament lamps in parabolic reflectors of known spectral characteristics. The transient temperature distribution resulting from radiant heating of pure water in a glass-walled test cell is measured with a Mach-Zehnder interferometer. Measured and predicted temperature profiles show good agreement, thus verifying the radiation and total energy transfer models in stagnant water. It is found that the boundary condition at the air-water interface and internal radiant heating rate must be correctly specified in order to properly model stratification of water by radiation.
Analysis was performed to determine the thermal-hydraulic behavior in the electrically heated core simulator of the Semiscale Mod-1 system during the early stage of a simulated LOCA initiated by a large cold leg break. The calculated incore hydraulic behavior was used to obtain a better understanding of early CHF (480 to 700 ms after rupture) and the occurrence of rewet in some locations after CHF. Analysis indicated that shortly after rupture the flow in the upper core stagnated for 600 ms, and the core rapidly voided of coolant. In the center and lower regions of the core, the calculated fluid qualities were between 30 and 70 percent at the time of measured CHF. The high fluid qualities in the flow channels about the heater rods indicated that the mechanism of CHF was dryout of the heater rod surfaces. Critical heat flux did not happen at the location of instantaneous flow stagnation associated with the flow reversal; nor did CHF occur in the region of the prolonged flow stagnation. At about 700 ms after rupture the core flow completely reversed direction, and the influx of coolant from above the heated core was responsible for the measured rewets.
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