TED-AJ03-163 A MODEL OF LIUID-SOLID CONTACT IN HIGH HEAT-FLUX BOILING

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There are a number of models for the critical heat flux of pool boiling (CHF) such as the well known hydrodynamic instability models, the macrolayer-dryout models and the hot spot models, but sufficient understanding of the CHF mechanism has not been made because vigorous generation of bubbles obstacles our view of the real boiling structure on/near the boiling surface. Therefore, using single-crystal sapphire, which is a material of comparatively high thermal conductivity as the heat transfer surface, we carried out visualization experiments to observe liquid-solid contacts from the back side of the heat transfer surface and also to observe simultaneously liquid-solid contacts and bubble behavior. From these experiments, we obtained the following results : The liquid-vapor structure at the heat transfer surface close to the critical heat flux has a simple structure, in which vapor bubbles of various sizes are directly attached to the heat transfer surface and grow as a result of repetitive mutual coalescing in the horizontal direction; the liquid-solid contact is restricted to the narrow gaps between vapor bubbles, and its area-fraction is as low as 0.3; the boiling heat flux near the CHF point closely relates to the contact-line-length density which is defined as the total length of the boundary between dry and wetted areas for unit boiling surface. In the present paper, based on the results mentioned above, we propose the following model for the distribution of dry areas which can predict the area-fraction of liquid-solid contact, the contact-line-length density, and the CHF of high heat-flux pool-boiling. For simplicity, the following assumptions are employed in the model; all the bubbles attached on the boiling surface are semispherical and the thermal growth of bubbles is mainly driven by evaporation near the contact line of bubbles. As the superheat increases, an increase in the nucleation site density brings about an increase in the number density of bubbles attached on the surface (or dry spots) and an increase in the evaporation rate near the contact line brings about an increase in the average diameter of dry spots. In the model, the nucleation site density is estimated by the number density of active sites at a moment. The evaporation rate at the contact line was estimated by Labuntsov's model. As a result of the above, there will be repeated violent coalescence in sideways between the densely populated bubbles (or densely populated dry spots) and the coalesced bubbles start growing rapidly while being attached directly to the heat transfer surface. For increasing the heat flux, since the resident-time of coalesced bubbles increases, the diameter of the largest dry area on the surface increases. The increase in nucleation site density results in an increase of the number density of small dry spots, and the increase in the maximum diameter of dry areas brings about an increase of the number density of large dry regions. The former brings about an increase in the contact-line-length density, but the latter a decrease in the contact-line-length density. Consequently, there exists a certain superheat at which the increase in the contact-line-length density resulting from the increase in the nucleation sites density becomes equal to the decrease resulting from the increase in the number of coalesced dry areas. In the present paper, this superheat is regarded as that at the critical heat flux. The coalescing process mentioned above is close to the image of that in dropwise condensation on a horizontal flat surface facing downward. Therefore, in the present work, based on the theoretical work on dropwise condensation, the coalescing process of dry areas is modeled to predict the distribution of dry areas. The contact-line-length density calculated from the model reaches a maximum at a superheat. This superheat is larger than the experimental data of the superheat because of the simplification of the model, and a more detailed model is necessary to predict real values of the distribution of dry areas, the contact-line-length density, and the CHF.
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TED-AJ03-163 A MODEL OF LIUID-SOLID CONTACT IN HIGH HEAT-FLUX BOILING

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