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HYDRODYNAMIC AND SURFACE EFFECTS ON THE PEAK HEAT FLUX IN POOL BOILING

Erich Hahne
Institut fur Thermodynamik und Warmetechnik, Universitat Stuttgart, 70550 Stuttgart, Germany

T. Diesselhorst
Kraftwerkunion, 8520 Erlangen, FRG

Abstract

Existing correlations for the prediction of peak heat fluxes do nut adequately describe the effects of liquid flow and heater surface conditions. In an experimental investigation such effects were systematically studied. Experiments were performed in a pool boiling arrangement an horizontal cylindric heaters of 1- 7.5 mm in diameter, of different materials, in distilled water at 1 bar.

As a result, the definite influence of the upstream velocity was quantitatively determined and it was shown that velocity fluctuations create an unavoidable scatter in the peak heat flux up to ± 10 %. Ðœoге pronounced effects are caused by changes in the surface wettability of the heater with the peak heat flux variing up to ± 50 %. Oxidation of the surface increased the wettability, nonmetallic deposits decreased it.

The comparison of the experimental results with existing burnout equations showed only satisfactory agreement for the effect of heater diameter.

A burnout mechanism, fully consistent with the test results is suggested.

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