An experimental study of flow boiling chf with porous surface coatings and surfactant solutions

Abstract

The boiling crisis or critical heat flux (CHF) phenomenon is an enormously studied topic of the boiling heat transfer. The great interest in the CHF is due to practical motives, since it is desirable to design an equipment (heat exchanger or boiler, etc) to operate at as high a heat flux as possible with optimum heat transfer rates but without the risk of physical burnout. This study consists of two parts of flow boiling CHF experiment; with porous surface coated tubes and by using surfactant solutions as working fluid. In first part, the effect of micro- and nano-porous inside surface coated vertical tubes on the CHF was determined for flow boiling of water in vertical round tubes at atmospheric pressure. CHF was measured for a smooth and three different coated tubes, at mass fluxes of 100~300 kg/m2s and two inlet subcooling temperatures (50oC and 75oC). Greater CHF enhancement was found with microporous coatings. Al2O3 microporous coatings with particle size <10 &micro;m and coating thickness of 50 &micro;m showed the best CHF enhancement. The maximum increase in the CHF was about 25% for microporous Al2O3. A wettability test was performed to study the physical mechanism of increase of CHF with microporous coated surfaces and contact angle was measured for smooth and coated surfaces. Pressure drop measurements were also performed across the coated tubes using the DP-cell apparatus. In second part, surfactant effect on the CHF was determined for water flow boiling at atmospheric pressure in a closed loop filled with solution of tri-sodium phosphate (TSP, Na3PO4.12H2O). The TSP is usually added to the containment sump water to adjust pH level during accident in nuclear power plants. The CHF was measured for four different surfactant solutions of water in vertical tubes, at different mass fluxes (100 ~ 500 kg/m2s) and two inlet subcooling temperatures (50oC and 75oC). Surfactant solutions in the range of 0.05%~0.2% at low mass flux (~100 kg/m2s) showed the be...