TED-AJ03-294 ULTRA HIGH CRITICAL HEAT FLUX DURING FORCED FLOW BOILING HEAT TRANSFER WITH AN IMPINGING JET :

An ultra high critical heat flux (CHF) has been challenged with a highly subcooled liquid jet impinging on a small rectangular heated surface. Major objective of this study is to achieve an ultra high heat flux cooling as large as an ultimate maximum heat flux proposed by Gambill and Lienhard. The ultimate maximum heat flux is attained on the assumptions that vapor molecules leave a liquid-vapor interface at the average speed of a Boltzman-Maxwellian gas and any molecules returning to the interface are not permitted. This value reaches up to 223MW/m^2 at atmospheric pressure. However, the existing CHF data are as low as about 10% of the ultimate heat flux due to a flow limitation of liquid and vapor. An external flow boiling with an impinging jet is one of the attractive methods to solve it. For development of components suffering high heat flux like a diverter of a fusion reactor, it is worth to know how large CHF can be obtained practically. The experiments were carried out over the experimental range; a fixed jet diameter of 2mm, jet velocity of 5-35m/s, degree of subcooling of 80-170K and system pressure up to 1.0MPa. The rectangular heated surface with a thin nickel foil of 0.03-0.3mm in thickness, 5 and 10mm in length, and 4mm in width and heated by a direct current within 1000A. Since the thin heated surface had to be used owing to the constraint of the current, the effect of wall thickness on the CHF was first examined. As decreasing in the wall thickness, the CHF decreases about 50% and the effect of thickness appears to be demised for the wall thickness over 0.2mm and this trend is quite similar with the well-known character of thickness in pool boiling. Consequently, we measured most of data with the wall thickness of 0.1mm compromising with current, while the CHF is about 10% lower than that of 0.3mm. Relationship between the CHF and subcooling is shown in Fig. A-1 where the solid and dashed lines denote the predicted CHF for the heater length 5 and 10mm with the generalized correlation of subcooled CHF proposed by two of the authors. Figure A-1 indicates that the CHF increases with subcooling of jet and the effect of subcooling on of the CHF well agrees with the correlation. The correlation was applicable for the heater length of 10mm within ±20%, while in the case of 5mm it overstates the experimental data especially at low subcooling and higher velocity. The maximum CHF of 212 MW/m^2 was achieved at the subcooling of 151K, the jet velocity of 35m/s and system pressure of 0.5MPa. As shown in Fig. A-2,the maximum CHF approaches to 48% of the ultimate maximum heat flux at atmospheric pressure and this value is the largest of the data, which has ever reported.[figure]