CRITICAL HEAT-FLUX DATA IN A VERTICAL TUBE AT LOW AND MEDIUM PRESSURES

AECL Research and Ecole Polytechnique have been cooperating on the val idation of the critical heat flux (CHF) look-up table (D.C. Groeneveld et al., Heat Transfer Eng. 7(1-2) (1986) 46-62). For low and medium p ressures the values in the table have been obtained by extrapolation a nd curve fitting; therefore, errors could be expected. To reduce these possible extrapolation errors, CHF experiments are bring carried out in water cooled 8 mm internal diameter (ID) tubes, at conditions where the data are scarce. This paper presents some of the experimental CHF data obtained for vertical up flow in an 8 mm ID test section, for a wide range of exit qualities (5-70%) and the exit pressure ranging fro m 5 to 30 bar. The experiments were carried out for heated lengths of 0.75, 1, 1.4 and 1.8 m. In general, the collected data show parametric trends similar to those described in the open literature. However, it was observed that for low pressure conditions CHF depends on the heat ed length; this dependence begins to disappear for exit pressure of ab out 30 bar. The CHF data have also been compared with predictions of w ell-known correlations (L. Biasi et al., Energia Nucl. 14(9) (1967) 53 0-536; R. Bowring, Br. Report AEEW-R789, Winfrith, UK, 1972; Y. Khatto and H. Ohno, Int. J. Heat Mass Transfer 27 (1984) 1641-1648) and thos e of the look-up table given by Groeneveld et al. For low pressures an d low mass fluxes the look-up table seems to yield better predictions of the CHF than the correlations. However, for medium pressures and ma ss fluxes the correlations perform better than the look-up table; amon g those tested, Katto and Ohno's correlation gives the best results.