Heat Transfer for Fusion Component Applications

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Abstract
A quasi-automated high heat flux flow boiling facility has been developed for the systematic study of critical heat flux (CHF), heat transfer, and two-phase pressure drop. High heat flux research is important in state-of-the-art electronics and fusion component design. For fusion applications, there are practically no low-pressure data for large values of coolant channel length-to-diameter (L/D) ratio (i.e., 100), channel diameters near 1.0 cm, and medium to high heat flux levels (i.e., 100 to 2000 W/cm2). A second step is provided to fill this void. Forced flow boiling (with water) quasi-steady experiments have been conducted on uniformly (resistively) heated horizontal copper tubes. The tubes were 1.02 cm in inside diameter and 117.87 cm long. The inlet water temperature was 20°C. For a 1.6-MPa exit pressure, measurements of the CHF varied from the annular flow regime (150 W/cm2) to the subcooled flow boiling regime (425 W/cm2). The mass velocity was varied from 0.63 to 3.5 Mg/m2·s. At 1.6 MPa, the transition between the annular and subcooled CHF regimes was measured to occur between 1.03 and 1.26 Mg/m2·s. Large axial variations in the Nusselt number were also measured. For example, at 1.7 Mg/m2·s, the Nusselt number varied from 120 at the channel’s entrance to 500 at the exit. The CHF data were compared with correlations developed by Bowring, Katto, and Merilo. Below 4.0 Mg/m2, all correlations overpredicted the CHF data. Merilo’s correlation, which was developed for high-pressure horizontal flows, predicted the CHF significantly above the present low-pressure data. The effects of orientation on the CHF data were small. Visual observations of the outside of the test section showed that burnout occurred simultaneously around the test section’s perimeter. Circumferential measurements of the outside wall temperature also showed negligible variations. Therefore, at low pressures, the following conditions reduced the effect of orientation: 1. high liquid Reynolds number 2. high inlet subcooling 3. moderate L/D 4. increased effects of surface tension relative to buoyant and viscous forces at higher pressures (i.e., low Bond and Ohnesorge numbers) 5. low value of buoyant forces relative to inertia forces.