Study of burnout phenomena on a cylindrical heater tube simulating nuclear fuel rod

An experimental investigation was performed to determine the effects of fluid velocity, heater tube diameter, and the blockage factor (ratio of heater diameter to the width of the channel) on pool and flow boiling heat transfer rates as well as on the critical heat flux (CHF) for a single horizontal tube in a cross-flow. The fluid used was Freon (R-113), and its upward velocity across the tube ranged from 0 to 0.52 m/s.

Four different tubular heater diameters (7.94, 9.53, 12.70, and 19.05 mm) with the blockage factor ranging from 0.31 to 0.75 were prepared and tested. Boiling heat transfer data were obtained for all four tubes, while critical heat flux (CHF) data were only obtained for 7.94 and 9.53 mm tubes over the complete range of fluid velocities.

For a fixed blockage ratio and inlet fluid temperature, the boiling heat transfer coefficient increased with the fluid velocity at heat fluxes lower than 30 kW/m². However, at heat fluxes greater than 30 kW/m², the effect of the fluid velocity was negligibly small for all heater tube diameters. Furthermore, the functional dependence of the boiling heat transfer coefficient on the heat flux was found to be consistent with that obtained by Hwang and Yao (1984), and little effect of heater tube diameter was observed for diameters between 12.7 mm and 19.05 mm.

The critical heat flux (CHF) data were slightly greater than the available data obtained by others under similar conditions but agreed with the widely accepted Zuber's correlation in pool boiling (UDD=0). In cross-flow boiling, the CHF values were also affected by the increasing fluid velocity in the present data, but the effect was not as significant as in previous data. The critical heat flux data for much greater blockage factors tested in this work showed an increasing trend, while the previous work for smaller blockage factors found a decreasing trend.

The CHF data in cross-flow boiling were compared to several existing correlations and the available data.

The correlation of Katto and Haramura (1983) was found to agree with the present CHF data within ±10%, while the Lienhard and Eichorn's (1976) correlation yielded poorer predictions with more than 20% deviation from the data.